ARTICLE

Human kidney and liver gluconeogenesis: evidence for organ substrate selectivity

Abstract

To assess the contribution of the human kidney to gluconeogenesis (GN) and its role in conversion of glutamine and alanine to glucose, we used a combination of isotopic and organ balance techniques in nine normal postabsorptive volunteers and measured both overall and renal incorporation of these precursors into glucose before and after infusion of epinephrine. In the postabsorptive basal state, renal incorporation of glutamine (27 ± 2 μmol/min) and alanine (2.1 ± 0.5 μmol/min) into glucose accounted for 72.8 ± 3.3 and 3.9 ± 0.5% of their overall incorporation into glucose (37 ± 2 and 51 ± 6 μmol/min, respectively) and 19.0 ± 3.5 and 1.4 ± 0.2%, respectively, of overall renal glucose release. Infusion of epinephrine, which increased systemic and renal glucose release more than twofold (P< 0.001), increased overall glutamine and alanine incorporation into glucose (both P < 0.001) and increased renal GN from glutamine (P< 0.001) but not from alanine (P = 0.15). Renal glutamine GN now accounted for 90.3 ± 4.0% of overall glutamine GN (P = 0.01 vs. basal), whereas renal alanine GN still accounted for only 4.8 ± 1.7% of overall alanine GN (P = 0.36 vs. basal). With the assumption that kidney and liver are the only gluconeogenic organs in humans, these results indicate that glutamine GN occurs primarily in kidney, whereas alanine GN occurs almost exclusively in liver. Isotopic studies of glutamine and alanine incorporation into plasma glucose may provide a selective, noninvasive method to assess hepatic and renal GN.

the liver is generally considered to be the major, if not the exclusive, site for gluconeogenesis in postabsorptive humans (30, 70). However, measurements of splanchnic (liver plus gastrointestinal tissues) uptake of gluconeogenic precursors can account for only about one-half of gluconeogenesis (26, 59, 67). Moreover, despite the fact that glutamine and alanine are converted to glucose at similar rates (57, 72), there appears to be little or no net uptake of glutamine by the human liver (31, 32, 59).

There are several possible explanations for these discrepancies. Intestinal tissues could take up glutamine and convert it to alanine, which may then be converted to glucose in the liver (78). However, recent studies (57) indicating that only ∼5% of alanine is derived from glutamine cast doubt on the quantitative importance of this. A second possibility is that net balance measurements do not reflect actual hepatic uptake of glutamine and other precursors. The liver can both use and produce glutamine (35) as well as other gluconeogenic precursors (56). Consequently, calculations based on net balance measurements of these substrates would underestimate their hepatic uptake. A third possibility is that an organ other than the liver may be an important site of gluconeogenesis.

It has been long known from in vitro and animal studies that the kidney has a substantial capacity for gluconeogenesis and that glutamine is a major substrate for renal gluconeogenesis (65, 79). Nevertheless, the human kidney is, at present, generally not considered to be an important gluconeogenic organ in the postabsorptive state (10, 29, 70). This view is largely based on net balance experiments that did not take into consideration simultaneous renal uptake and release of glucose (1,5, 9). Recent studies in dogs (11, 53), rats (43), sheep (36), and humans (54, 71), using a combination of isotopic and balance techniques, which permits individual assessment of renal uptake and release of glucose, have provided evidence that the kidney may account for a considerable proportion of glucose released into the systemic circulation under postabsorptive conditions. Moreover, it seems likely that this renal release of glucose may be largely the result of gluconeogenesis, because the human kidney normally contains little glycogen (4), and cells that could potentially store glycogen lack glucose-6-phosphatase (65, 79).

The present studies were therefore undertaken to test the hypotheses that the kidney is a major site for glutamine and overall gluconeogenesis in postabsorptive humans and that renal use of other potential precursors of glucose, such as alanine, lactate, and glycerol, may explain the discrepancy between their splanchnic uptake and use for gluconeogenesis. In addition, because catecholamines have been shown to stimulate renal glucose release in humans (71), renal glutamine gluconeogenesis in vitro (65, 79), and hepatic alanine gluconeogenesis in vivo and in vitro (18, 64), we also compared the effects of epinephrine on renal gluconeogenesis from glutamine and alanine and their metabolism by the kidney.

METHODS

Subjects

Informed written consent was obtained from nine normal volunteers (7 men, 2 women) after the protocol had been approved by the University of Rochester Institutional Review Board. The subjects were 25 ± 2 (SE) yr of age, weighed 74 ± 5 kg (body mass index 24.0 ± 0.9 kg/m2), and had normal glucose tolerance tests according to World Health Organization criteria (81) and no family history of diabetes mellitus. For 3 days before the study, all had been on a weight-maintaining diet containing ≥200 g carbohydrate and had abstained from alcohol. Glucose turnover and renal glucose balance data of four of the nine subjects have been reported previously (71).

Protocol

Subjects were admitted to the University of Rochester General Clinical Research Center between 6:00 and 7:00 PM on the evening before experiments; they consumed a standard meal (10 kcal/kg, 50% carbohydrate, 35% fat, and 15% protein) between 6:00 and 8:00 PM and fasted overnight until experiments were completed.

At ∼5:30 AM, an antecubital vein was cannulated, and primed-continuous infusions of [6-3H]glucose (30 μCi, 0.3 μCi/min, n = 9), [U-14C]glutamine (25 μCi, 0.25 μCi/min, n = 7, Amersham International, Little Chalfont, UK), and [3-13C]alanine (170 mg, 1.7 mg/min, n = 6, Cambridge Isotope Laboratories, Andover, MA) were started.

Between 8:00 and 9:00 AM, a renal vein was catheterized through the right femoral vein under fluoroscopy, and the position of the catheter tip was ascertained by injecting a small amount of iodinated contrast material. The catheter was then continuously infused with a heparin-saline solution (1.5 units/ml) at a rate of 2.4 ml/min to maintain patency.

At 9:00 AM a dorsal hand vein was cannulated and kept in a thermoregulated Plexiglas box at 65°C for the sampling of arterialized venous blood (7), and an antecubital venous infusion ofp-aminohippuric acid (12 mg/min) was started for determination of renal blood flow. At ∼10:00 AM, after allowance of ≥4 h to achieve isotopic steady state, three blood samples were collected simultaneously from the dorsal hand vein and the renal vein at 30-min intervals (−60, −30, 0 min) for determination of glucose, glutamine, alanine, lactate, glycerol, insulin, glucagon, epinephrine andp-aminohippuric acid concentrations, glucose and glutamine specific activities, and glucose and alanine enrichments. At 0 min, a continuous infusion of epinephrine (270 pmol ⋅ kg−1 ⋅ min−1) was started via the antecubital infusion line, and blood was collected as described above at 30-min intervals for 3 h.

Analytic Procedures

Blood samples were collected for glucose, glutamine, alanine, lactate, and glycerol concentrations and specific activities (SAs) or enrichments in oxalate-fluoride tubes, for epinephrine in EGTA tubes, and for insulin and glucagon in EDTA tubes containing a protease inhibitor. Whole blood glucose was immediately determined in triplicate with a glucose analyzer (Yellow Springs Instrument, Yellow Springs, OH). For HPLC analysis of plasma alanine and glutamine concentrations as well as [14C]glutamine SAs, an internal standard (25 nmolp-fluorophenylalanine) was added to 4 ml of plasma, the pH was adjusted to 4.8–5.0, and samples were frozen for later analysis as described previously (38). For other determinations, samples were placed immediately in a 4°C ice bath, and plasma was separated within 30 min by centrifugation at 4°C. Plasma [3H]- and [14C]glucose SAs were determined in duplicate by HPLC (58). [13C]glucose and [13C]alanine enrichments were determined in duplicate by chemical ionization and selected ion monitoring gas chromatography-mass spectroscopy of the acetylbutylboronate (glucose) ort-butyldimethylsilyl (alanine) ester derivatives (66, 76). Plasma insulin and glucagon concentrations were determined by standard radioimmunoassays, as previously described (72). Plasma epinephrine concentrations were determined by a radioenzymatic method (22), plasmap-aminohippuric acid concentration by a colorimetric method (8), and plasma glycerol and lactate concentrations by standard microfluorometric assays (52, 77).

Calculations

After 4 h of isotope infusion, isotopic steady state had been achieved for all infused isotopes, as evidenced by the fact that SAs and enrichments for plasma [3H]glucose, [14C]glutamine, and [13C]alanine at −60 min were not significantly different from those at 0 min (paired Student’s t-test). However, isotopic steady state was apparently not achieved for [14C]glucose and [13C]glucose until at least −30 min, as evidenced by the fact that values at −60 min, but not at −30 min, were significantly different from values at 0 min (paired Student’s t-test). Consequently, calculations involving [14C]glucose and [13C]glucose in the basal postabsorptive state used only the mean of values at −30 and 0 min. To the extent that isotopic steady state may not have been achieved, estimates of renal and systemic gluconeogenesis from glutamine and alanine would all be underestimated. However, because all should be affected to a comparable degree, this potential shortcoming would not be expected to materially influence relative comparisons.

Renal plasma flow (RPF) was determined by thep-aminohippuric acid clearance technique, and renal blood flow (RBF) was calculated as RPF/(1 − hematocrit). Fractional extraction (FX) of glucose across the kidney was calculated as ([6-3H]glucose SAart ⋅ glucoseart− [6-3H]glucose SArenal vein ⋅ glucoserenal vein)/([6-3H]glucose SAart ⋅ glucoseart), where the subscript art stands for arterial (14). Renal glucose uptake (RGU) was calculated as RBF ⋅ glucoseart ⋅ FX, and renal glucose net balance (NB) as RBF ⋅ (glucoseart− glucoserenal vein) (20). Renal glucose release (RGR) was calculated as RGU − NB (20). Analogous equations were used for glutamine, alanine, lactate, and glycerol, except that RPF was used for glutamine, alanine, and glycerol because these substrates’ tissue exchange essentially occurs via plasma (16, 61).

Systemic appearance (Ra) and removal (Rd) of glucose from the circulation were determined with steady-state equations under basal conditions (80) and subsequently during infusion of epinephrine with non-steady-state equations of DeBodo et al. (24, 71) by use of a pool fraction (p) of 0.65 and a volume of distribution (V) of 200 ml/kg for glucose. Hepatic glucose release (HGR) was calculated as the difference between the overall plasma Ra of glucose and RGR.

Glutamine and alanine Ra values were calculated using steady-state equations under basal conditions (57) and DeBodo’s equation (24) during the non-steady state. Values of 0.75 and 0.65 for p and of 430 and 500 ml/kg body weight for V were used for glutamine and alanine, respectively (23, 33, 48).

The proportion of systemic glucose appearance in the steady state due to whole body glutamine gluconeogenesis was calculated as ([14C]glucose SAart/[14C]glutamine SAart) ⋅ 100/1.2, by use of the standard precursor-product calculation (49). The division by 1.2 corrects for differences in carbons (i.e., glutamine has 5, glucose has 6 carbons). Total glutamine gluconeogenesis was calculated as the proportion of glucose Ra due to glutamine multiplied by glucose Ra. During the non-steady state, whole body glutamine gluconeogenesis was calculated using the equation of Chiasson et al. (15)

[RdGlc(t1)+RdGlc(t2)]/2{[14C]glucoseSAart(t1)+[14C]glucoseSAart(t2)}/2+pV
{[14C]glucoseSAart(t2)×glucoseart(t2)[14C]glucoseSAart(t1)×glucoseart(t1)}/30min
1.2{[14C]glutamineSAart(t1)+[14C]glutamineSAart(t2)}/2


wheret 1 andt 2 refer to the beginning and end of every 30-min time interval. The previously mentioned p and V for glucose were used. Respective rates for alanine were calculated by utilizing [13C]glucose and [13C]alanine mole percent excess (MPE) (80) instead of specific activities and by correcting for the difference in carbons by 2 instead of 1.2.

Renal gluconeogenesis from glutamine was calculated as {RBF ⋅ [14C]glucose SArenal vein ⋅ glucoserenal vein− (1 − FX) ⋅ [14C]glucose SAart ⋅ glucoseart}/(1.2 ⋅ [14C]glutamine SArenal vein) (11), where FX is the renal fractional extraction of glucose obtained from the [3H]glucose data as described above. Renal gluconeogenesis from alanine was calculated analogously using [13C]glucose MPEart and [13C]alanine MPEart instead of SAs and correcting by 2. Hepatic gluconeogenesis was calculated as the difference between systemic and renal gluconeogenesis for glutamine and alanine, respectively.

Statistical Analysis

Data are expressed as means ± SE. Unless stated otherwise, paired two-tailed Student’s t-tests were used to compare data obtained before and after infusion of epinephrine by using the mean of three baseline determinations and the mean of six determinations during the epinephrine infusion. AP value <0.05 was considered to be statistically significant.

RESULTS

Arterial and Venous Concentrations of Epinephrine, Insulin, Glucagon, Glucose, Glutamine, Alanine, Lactate, and Glycerol; Specific Activities or Enrichments of Plasma Glucose, Glutamine, and Alanine; Systemic Turnovers and Gluconeogenesis

Specific activities and enrichments of plasma glucose, glutamine, and alanine are given in Table 1. Plasma glucose, glutamine, alanine, lactate, glycerol, insulin, glucagon, and epinephrine concentrations are given in Table2.

Table 1. Specific activities and enrichments at baseline and during infusion of epinephrine

Minutes
−60 −30 030 60 90 120 150 180
Plasma [3H]glucose, dpm/mol
 Artery633 ± 33 647 ± 31 659 ± 36491 ± 26 478 ± 26 485 ± 29496 ± 29 513 ± 31 526 ± 31
 Renal vein 610 ± 32 628 ± 31638 ± 34 463 ± 26 457 ± 25464 ± 27 477 ± 28 493 ± 30505 ± 30
Plasma [14C]glucose, dpm/μmol
 Artery 65 ± 6 67 ± 672 ± 8 62 ± 5 70 ± 675 ± 7 80 ± 7 84 ± 883 ± 7
 Renal vein 70 ± 772 ± 7 76 ± 8 63 ± 672 ± 7 80 ± 7 84 ± 888 ± 8 87 ± 7
Plasma [13C]glucose, MPE
 Artery0.68 ± 0.19 0.72 ± 0.24 0.76 ± 0.270.69 ± 0.21 0.75 ± 0.19 0.82 ± 0.160.86 ± 0.15 0.91 ± 0.14 0.92 ± 0.17
 Renal vein 0.68 ± 0.18 0.75 ± 0.230.80 ± 0.26 0.71 ± 0.20 0.76 ± 0.190.82 ± 0.14 0.89 ± 0.15 0.88 ± 0.180.93 ± 0.17
Plasma [14C]glutamine, dpm/μmol
 Artery 1,391 ± 1471,403 ± 144 1,405 ± 142 1,261 ± 1211,232 ± 119 1,251 ± 122 1,255 ± 1171,211 ± 121 1,218 ± 130
 Renal vein1,354 ± 152 1,343 ± 138 1,362 ± 1331,213 ± 125 1,202 ± 132 1,215 ± 1231,215 ± 130 1,144 ± 123 1,143 ± 118
Plasma [13C]alanine, MPE
 Artery6.34 ± 0.94 6.17 ± 1.02 6.41 ± 0.983.88 ± 0.64 3.61 ± 0.65 3.55 ± 0.773.52 ± 0.89 3.25 ± 0.97 3.37 ± 1.09
 Renal vein 5.95 ± 0.82 5.65 ± 0.875.67 ± 0.84 3.45 ± 0.69 3.02 ± 0.813.06 ± 0.81 2.82 ± 0.90 2.94 ± 0.922.89 ± 1.01

Values are means ± SE. MPE, mole percent excess.

Table 2. Circulating levels of epinephrine, glucagon, insulin, glucose, glutamine, alanine, lactate, and glycerol at baseline and during infusion of epinephrine

Minutes
−60 −30 030 60 90 120 150 180
Epinephrine, pM
 Artery 254 ± 56 218 ± 30237 ± 36 3,411 ± 368 3,432 ± 3823,656 ± 381 3,566 ± 582 3,733 ± 4003,247 ± 592
Glucagon, pg/ml
 Artery113 ± 16 114 ± 16 114 ± 18134 ± 20 133 ± 21 125 ± 19123 ± 18 110 ± 18 107 ± 18
Insulin, pM
 Artery 37 ± 3 37 ± 338 ± 3 55 ± 5 63 ± 561 ± 4 60 ± 5 64 ± 567 ± 6
Glucose, mM
 Artery4.47 ± 0.09 4.45 ± 0.10 4.36 ± 0.115.97 ± 0.26 6.61 ± 0.23 6.81 ± 0.27.00 ± 0.23 7.05 ± 0.25 6.97 ± 0.29
 Renal vein 4.52 ± 0.09 4.51 ± 0.104.39 ± 0.11 6.22 ± 0.29 6.79 ± 0.256.96 ± 0.23 7.15 ± 0.24 7.19 ± 0.287.11 ± 0.30
P value<0.015 <0.028 <0.023 <0.001 <0.001<0.009 <0.016 <0.006 <0.004
Glutamine, mM
 Artery 0.66 ± 0.05 0.65 ± 0.050.66 ± 0.05 0.63 ± 0.03 0.62 ± 0.030.59 ± 0.05 0.59 ± 0.06 0.60 ± 0.070.56 ± 0.05
 Renal vein 0.64 ± 0.050.61 ± 0.05 0.62 ± 0.04 0.57 ± 0.040.55 ± 0.03 0.53 ± 0.04 0.53 ± 0.040.50 ± 0.04 0.51 ± 0.04
P value <0.011 <0.02 <0.02  <0.007 <0.001 <0.009 <0.05 <0.04  <0.02 
Alanine, mM
 Artery0.31 ± 0.02 0.29 ± 0.03 0.29 ± 0.020.33 ± 0.02 0.32 ± 0.01 0.32 ± 0.020.32 ± 0.01 0.33 ± 0.02 0.34 ± 0.02
 Renal vein 0.30 ± 0.03 0.29 ± 0.030.29 ± 0.02 0.32 ± 0.02 0.32 ± 0.020.34 ± 0.02 0.32 ± 0.02 0.34 ± 0.030.33 ± 0.03
P valueNS NS NS NS NS NS NS NS NS
Lactate, mM
 Artery 0.59 ± 0.040.58 ± 0.05 0.58 ± 0.05 1.43 ± 0.111.74 ± 0.17 2.00 ± 0.23 2.15 ± 0.262.41 ± 0.29 2.60 ± 0.36
 Renal vein0.43 ± 0.04 0.43 ± 0.03 0.45 ± 0.041.21 ± 0.12 1.58 ± 0.19 1.86 ± 0.241.94 ± 0.27 2.17 ± 0.28 2.33 ± 0.35
P value <0.003 <0.003<0.003 <0.002 <0.002 <0.001 <0.002<0.01  <0.004
Glycerol, mM
 Artery0.066 ± 0.006 0.076 ± 0.0080.066 ± 0.006 0.181 ± 0.0140.124 ± 0.008 0.096 ± 0.0080.093 ± 0.009 0.105 ± 0.0070.099 ± 0.009
 Renal vein 0.040 ± 0.0050.045 ± 0.005 0.038 ± 0.0040.105 ± 0.010 0.061 ± 0.0060.51 ± 0.005 0.048 ± 0.0050.056 ± 0.004 0.054 ± 0.005
P value <0.001 <0.001<0.002 <0.001 <0.001 <0.001 <0.001<0.001 <0.001

Values are means ± SE. NS, not significant.

Postabsorptive plasma glucose, glutamine, and alanine turnover (Table3) averaged 859 ± 41, 345 ± 14, and 344 ± 46 μmol/min, respectively. Systemic glucose production from glutamine (37 ± 2 μmol/min) accounted for 4.2 ± 0.2% of plasma glucose Ra. Systemic glucose production from alanine (51 ± 6 μmol/min) accounted for 6.3 ± 0.7% of plasma glucose Ra.

Table 3. Rates of systemic and renal release, uptake of glucose, and gluconeogenesis from glutamine and alanine

Minutes
−60 −30 030 60 90 120 150 180
Rates of appearance
 Glucose 878 ± 45 856 ± 43843 ± 40 1,467 ± 112 1,203 ± 751,141 ± 85 1,089 ± 70 1,046 ± 791,022 ± 79
 Glutamine 347 ± 14345 ± 18 343 ± 18 405 ± 23399 ± 35 383 ± 23 381 ± 24402 ± 20 393 ± 16
 Alanine342 ± 49 356 ± 49 335 ± 42544 ± 72 599 ± 89 640 ± 104670 ± 109 753 ± 123 795 ± 157
Renal glucose release 217 ± 23 195 ± 28187 ± 26 500 ± 83 410 ± 50374 ± 35 370 ± 47 358 ± 33391 ± 42
Rates of disappearance
 Glucose878 ± 45 856 ± 43 843 ± 40937 ± 79 1,002 ± 65 1,078 ± 701,032 ± 65 1,033 ± 80 1,051 ± 73
 Glutamine 347 ± 14 345 ± 18343 ± 18 416 ± 26 402 ± 35392 ± 18 379 ± 21 398 ± 21411 ± 26
 Alanine 342 ± 48356 ± 48 335 ± 42 533 ± 76604 ± 89 639 ± 101 667 ± 105754 ± 120 795 ± 154
Renal glucose uptake153 ± 16 119 ± 23 147 ± 26175 ± 34 188 ± 28 190 ± 31168 ± 41 169 ± 29 199 ± 24
Rates of gluconeogenesis
 Overall glutamine37 ± 2 38 ± 2 40 ± 346 ± 4 55 ± 4 58 ± 361 ± 3 65 ± 4
 Renal glutamine27 ± 2 27 ± 2 34 ± 441 ± 4 49 ± 6 52 ± 557 ± 6 63 ± 5
 Overall alanine51 ± 7 51 ± 7 63 ± 996 ± 17 124 ± 21 141 ± 24153 ± 26 176 ± 32
 Renal alanine1.3 ± 0.5 2.9 ± 0.7 3.0 ± 1.95.8 ± 3.4 7.6 ± 2.1 9.6 ± 4.94.9 ± 2.2 6.2 ± 3.9

Values are means ± SE in μmol/min.

Postabsorptive Renal Substrate Metabolism

RBF averaged 1,370 ± 84 ml/min (Table4). RGU (140 ± 16 μmol/min) accounted for 16.5 ± 1.8% of overall systemic glucose uptake. RGR (199 ± 14 μmol/min) accounted for 23.6 ± 1.7% of overall systemic glucose Ra.

Table 4. Renal net balance, fractional extraction, and uptake and release of glucose, glutamine, and alanine at baseline and during epinephrine infusion

Glucose Glutamine Alanine
Net balance, μmol/min
 Baseline −60 ± 15 36 ± 811 ± 11
 Epinephrine −219 ± 4059 ± 12 1 ± 14
P vs. baseline 0.002 0.01 0.48
Fractional extraction, %
 Baseline2.3 ± 0.2 9.2 ± 1.2 8.2 ± 1.2
 Epinephrine 2.0 ± 0.2 15.1 ± 1.415.0 ± 1.2
P vs. baseline 0.08  0.001 0.02
Uptake, μmol/min
 Baseline 140 ± 16 63 ± 1227 ± 9
 Epinephrine 181 ± 2196 ± 13 39 ± 7
P vs. baseline 0.15  0.0050.04
Release, μmol/min
 Baseline 199 ± 1418 ± 2 20 ± 6
 Epinephrine401 ± 43 36 ± 4 75 ± 19
P vs. baseline 0.001 0.0010.06

Values are means ± SE; those for epinephrine are mean values of 6 time points during epinephrine infusion.

Renal glutamine uptake (63 ± 12 μmol/min) accounted for 18.3 ± 2.9% of overall systemic glutamine uptake (Table 4). Renal glutamine release (18 ± 2 μmol/min) accounted for 5.2 ± 0.6% of overall glutamine Ra in plasma.

Renal alanine uptake (27 ± 9 μmol/min) accounted for 7.3 ± 1.7% of overall systemic alanine uptake (Table 4). Renal alanine release (20 ± 6 μmol/min) accounted for 6.7 ± 2.3% of overall systemic alanine appearance.

Postabsorptive Renal and Hepatic Glutamine and Alanine Gluconeogenesis

Renal glucose production from glutamine (27 ± 2 μmol/min) was >10-fold greater than that from alanine (2.1 ± 0.5 μmol/min,P < 0.001) and accounted for 72.8 ± 3.3% of overall glutamine gluconeogenesis and 19.0 ± 3.5% of overall RGR. Renal glucose production from alanine accounted for 3.9 ± 0.5% of its gluconeogenesis and 1.4 ± 0.2% of overall RGR (Table 5). Renal glucose production from glutamine and alanine accounted for 60.3 ± 8.2 and 24.4 ± 6.7% of their respective uptake by the kidney.

Table 5. Comparison of renal and hepatic gluconeogenesis from glutamine and alanine

Gluconeogenesis
Glutamine Alanine
Renal Hepatic Renal Hepatic
Rate, μmol/min
 Baseline 27 ± 2 10.1 ± 1.42.1 ± 0.5 49 ± 6
 Epinephrine49 ± 4 4.8 ± 2.1 6.2 ± 1.6119 ± 20
P < 0.001 P = 0.01 P = 0.03 P < 0.001
% of Systemic gluconeogenesis
 Baseline 72.8 ± 3.327.2 ± 3.2 3.9 ± 0.5 96.1 ± 0.5
 Epinephrine 90.3 ± 4.0 9.7 ± 4.04.8 ± 1.7 95.2 ± 1.7
P = 0.01  P < 0.001P = 0.36 P = 0.48 
% of Renal glucose release
 Baseline 19.0 ± 3.51.4 ± 0.2
 Epinephrine 12.7 ± 1.71.4 ± 0.6
P = 0.13  P = 0.46
% of Hepatic glucose release
 Baseline 1.4 ± 0.17.9 ± 1.2
 Epinephrine 0.7 ± 0.320.7 ± 3.7
P = 0.02 P = 0.02 

Values are means ± SE.

Hepatic glucose production from alanine (49 ± 6 μmol/min) was nearly five times greater than that from glutamine (10.1 ± 1.4 μmol/min, P < 0.001) and accounted for 96.1 ± 0.5% of overall alanine gluconeogenesis (Table 5). Hepatic glutamine conversion to glucose accounted for 27.2 ± 3.2% of overall glutamine gluconeogenesis. Together, glutamine and alanine conversion to glucose accounted for ∼9% of overall hepatic glucose release.

Glutamine plus alanine accounted for only ∼20% of RGR. Consequently, other gluconeogenic precursors must be used by the kidney, because RGR is mainly due to gluconeogenesis. Renal NBs of lactate (209 ± 32 μmol/min) and glycerol (27 ± 3 μmol/min) were both positive, indicating net uptake of these potential gluconeogenic precursors. Their net uptake, if solely used for gluconeogenesis, could have accounted for 53 and 7%, respectively, of RGR. Thus, in the postabsorptive state, lactate (53%), glutamine (19%), and glycerol (7%) could account for nearly all (79%) of RGR via gluconeogenesis.

Effects of Epinephrine Infusion

Substrate and hormone concentrations.

During infusion of epinephrine, arterial epinephrine concentrations averaged 3,550 ± 387 pM (Table 2). Arterial glucose, lactate, glycerol, and insulin all increased significantly (all P < 0.001); arterial glutamine decreased slightly (P < 0.01), whereas concentrations of alanine and glucagon did not change significantly (bothP > 0.5).

Systemic turnovers and gluconeogenesis.

During infusion of epinephrine, plasma glucose Ra and Rd, glutamine Ra and Rd, alanine Ra and Rd, and systemic glucose production from glutamine and alanine all increased significantly (allP < 0.001).

Renal metabolism substrate.

During infusion of epinephrine, RBF decreased ∼10% to 1,235 ± 104 ml/min, P = 0.01. RGR increased approximately twofold (P = 0.001) and accounted for an increased proportion of systemic glucose appearance (35.1 ± 3.7 vs. 23.6 ± 1.7%,P < 0.01). Neither renal glucose FX (P = 0.08) nor RGU (P = 0.15) changed significantly.

Renal glutamine FX increased ∼60% (P = 0.001), and glutamine uptake increased nearly 80% (P = 0.005). Renal glutamine release increased about twofold (P = 0.001). Renal alanine FX increased nearly twofold (P = 0.02), and alanine uptake increased ∼50% (P = 0.04). Renal alanine release increased more than twofold but did not quite reach statistical significance (P = 0.06). Net renal uptake of lactate and glycerol both increased, averaging 384 ± 38 and 40 ± 5 μmol/min, respectively (bothP < 0.001).

Renal and hepatic glutamine and alanine gluconeogenesis.

During infusion of epinephrine, renal glucose production from glutamine increased nearly twofold (P< 0.001) and accounted for 90.3 ± 4.0% of systemic glutamine gluconeogenesis and 67.3 ± 7.9% of renal glutamine uptake during this interval. The contribution of renal glutamine gluconeogenesis to overall RGR remained unchanged (12.7 ± 1.7%,P = 0.13), suggesting that gluconeogenesis from other precursors was increased. Renal glucose production from alanine did not increase significantly during infusion of epinephrine (P = 0.15) and still accounted for only 4.8 ± 1.7% of systemic alanine gluconeogenesis, 1.4 ± 0.6% of overall RGR, and 31.3 ± 10.4% of renal alanine uptake.

Hepatic glucose production from glutamine decreased 50% during infusion of epinephrine (P = 0.01) and accounted for <10% of overall glutamine gluconeogenesis and <1% of overall hepatic glucose release during this period. Hepatic glucose production from alanine increased more than twofold during infusion of epinephrine (P < 0.001) and accounted for 95% of overall alanine gluconeogenesis and 21% of overall hepatic glucose release.

DISCUSSION

Contribution of the Kidney and Liver to Overall Glutamine and Alanine Gluconeogenesis

To our knowledge, renal production of glucose from glutamine and alanine has not been previously assessed in humans. In our postabsorptive subjects, overall glutamine gluconeogenesis was responsible for ∼5% of systemic glucose appearance, and renal production of glucose from glutamine accounted for nearly 75% of all glucose derived from glutamine. This increased to over 90% during infusion of epinephrine. Overall alanine gluconeogenesis was responsible also for ∼5% of systemic glucose appearance in the postabsorpative state; however, in contrast to glutamine, renal production of glucose from alanine accounted for <5% of overall alanine gluconeogenesis in both the postabsorptive state and during infusion of epinephrine. Thus, under our experimental conditions, the kidney was the predominant site for glutamine gluconeogenesis, whereas alanine conversion to glucose was essentially limited to the liver.

These observations provide a possible explanation for the discrepancy between the reported limited hepatic uptake of glutamine (31, 32, 59) and the contribution of glutamine to gluconeogenesis. Moreover, they are consistent with previous in vitro and animal studies indicating that glutamine is a major substrate for renal gluconeogenesis in most species (36, 47, 65, 79). Glutamine uptake normally accounts for >60% of all amino acids taken up by the postabsorptive human kidney (9, 73). In the present study, renal uptake of glutamine was more than sufficient to account for all of the glucose produced from glutamine by the kidney, both in the postabsorptive state (63 vs. 27 μmol/min) and during infusion of epinephrine (96 vs. 49 μmol/min).

The present studies indicate that there appears to be little renal gluconeogenesis from alanine in humans and that conversion of alanine to glucose occurs largely in the liver. This is consistent with the observations of Bjorkman et al. (5), who found that infusion of alanine into 60-h-fasted normal volunteers increased net splanchnic glucose release but had no effect on net renal glucose release. The ability of the kidney to produce glucose from alanine has been shown to vary considerably among species (6, 25, 45, 60). Our finding that postabsorptive hepatic gluconeogenesis from alanine was approximately fivefold greater than that from glutamine is generally consistent with in vitro studies (2, 21, 40, 62) indicating that glucose production from alanine by perfused rat livers and isolated rat hepatocytes considerably exceeds that from glutamine.

Mechanism for Hepatic and Renal Selectivity

Renal uptake of alanine in the present study was approximately one-half that of glutamine, whereas its conversion to glucose was less than one-tenth that of glutamine. Thus differences in uptake or transport of alanine and glutamine do not explain the selectivity in the use of these amino acids for gluconeogenesis by the kidney. Proximal cortical tubules, where renal gluconeogenesis primarily takes place in the human kidney, are known to lack, or to have very little, alanine aminotransferase activity (12, 25, 34). This would severely limit conversion of alanine to pyruvate, an essential step in the formation of glucose from alanine. Thus this low alanine aminotransferase activity could explain the limited gluconeogenic use of alanine by the human kidney.

There are several possible explanations for the difference in renal and hepatic use of glutamine for gluconeogenesis. First of all, in terms of substrate supply, concentrations of glutamine presented to the kidney are likely to be greater than those presented to the liver, because portal venous glutamine levels are lower than arterial levels (3, 32,55) because of intestinal glutamine extraction (78), and portal venous blood flow is approximately three times greater than hepatic arterial flow (55). Second, glutamine is transported across the plasma membrane in liver specifically by the N system, whereas in kidney, it is mainly transported by the A system (17). There are several differences in these transport systems: for example, the A system, but not the N system, is stimulated by hormones such as glucagon and epinephrine (17,68). Third, although transport across the hepatocyte plasma membrane appears to be the rate-limiting step for metabolism of alanine (28, 50,69), there is evidence that glutaminase is the rate-limiting step for glutamine metabolism (44). Liver and kidney glutaminase differ in many respects (39), one of which is a relatively low glutaminase activity in liver (39). Thus a combination of differences in substrate supply, transport, and enzymatic activity of rate-limiting steps might explain differences in the use of glutamine by the liver and kidney for gluconeogenesis.

Implications For Gluconeogenesis in General

One of the aims of our study was to provide a possible explanation for the failure of splanchnic uptake of gluconeogenic substrates to account for overall gluconeogenesis. It has been estimated that splanchnic uptake of lactate, glycerol, alanine and other gluconeogenic amino acids could maximally account for ∼25% of systemic glucose release (26, 59, 67), whereas gluconeogenesis appears to be responsible for ≥50% of systemic glucose Ra (51,63). In the present studies, we found that postabsorptive RGR accounted for 20–25% of overall systemic glucose Ra, consistent with previous reports in rats (43), dogs (11, 53), and sheep (36). The human kidney normally has little glycogen (4), and cells that could potentially store glycogen lack glucose-6-phosphatase (10, 65, 79) and therefore cannot release free glucose via glycogenolysis. It is therefore likely that all or nearly all of the glucose released by the kidney is due to gluconeogenesis. If so, renal gluconeogenesis would account for about one-half of the overall gluconeogenesis.

Glucose produced from glutamine in the present studies accounted for <20% of all RGR. Consequently, if RGR was mainly the result of gluconeogenesis, there must have been uptake and conversion to glucose of other gluconeogenic substrates by the kidney. Indeed, we found that there was substantial net renal uptake of lactate and glycerol, which was comparable with that previously reported for splanchnic tissues (26, 67, 75). Thus renal uptake and incorporation into glucose of gluconeogenic substrates could provide an explanation for the failure of splanchnic substrate uptake to account for all gluconeogenesis.

Our findings also have practical implications regarding assessment of gluconeogenesis. Currently, the only way to assess the individual contributions of kidney and liver to gluconeogenesis in vivo involves invasive procedures, such as renal or hepatic vein catheterization. Because the present study indicates that, in humans, glutamine conversion to glucose occurs predominantly in the kidney, whereas alanine conversion to glucose occurs almost exclusively in the liver, isotopic assessment of glutamine and alanine incorporation into glucose may provide a noninvasive method to selectively evaluate renal and hepatic gluconeogenesis.

Effects of Epinephrine

In the present studies, epinephrine was used as a probe to assess the use of glutamine and alanine for gluconeogenesis by kidney and liver under a condition known to stimulate renal and hepatic gluconeogenesis (18, 64, 65, 71, 79). However, our results have additional implications for the mechanism by which epinephrine stimulates gluconeogenesis.

It is currently thought that epinephrine augments gluconeogenesis primarily by increasing delivery of precursors from peripheral tissues (13, 18, 64). We found that epinephrine increased renal uptake of both glutamine and alanine. This was wholly accounted for by increased FX of these amino acids, because it occurred in the absence of an increase in arterial glutamine and alanine concentrations, and RBF decreased. This increase in FX could reflect a direct effect of epinephrine on the kidney, because epinephrine has been shown to stimulate the A amino acid transport system or indirect effects due to changes in the availability of other substrates or changes in hormones (68). We cannot distinguish between these possibilities on the basis of our data.

Methodological Limitations

The present studies have used a combination of balance and isotopic techniques, each of which has certain shortcomings that need to be taken into consideration. First of all, the calculation of renal production of glucose from glutamine and alanine depends on several determinations (e.g., RBF, blood glucose concentrations, glucose and glutamine SAs, glucose and alanine enrichments), each of which involves some measurement error. Imprecision in any of these, although small, will lead to larger imprecision in the final calculation. Second, isotopic assessment of gluconeogenesis using a labeled precursor will underestimate the incorporation of that precursor into glucose because of dilution of the specific activity or enrichment of the labeled precursor in the tricarboxylic acid cycle and other pools (19, 37, 41,42, 74). Underestimation resulting from the tricarboxylic acid cycle carbon exchange, which occurs in both liver and kidney (41, 74), will depend on the location of the label in the precursor used and on the specific experimental conditions (41, 46). In postabsorptive humans, this underestimation has been calculated to be as great as 40% (19).

We have assumed that tricarboxylic acid cycle exchange would comparably influence estimation of hepatic and renal gluconeogenesis from glutamine and alanine. To our knowledge there have been no direct comparisons of tricarboxylic acid cycle carbon exchange in liver and kidney. However, in vitro data of Vinay et al. (74) for glutamine incorporation into glucose by canine renal tubules and in vivo data of Hetenyi (37) for incorporation of lactate into glucose in dogs indicate comparable degrees of underestimation. These observations suggest that our assumption is probably reasonable. Regardless of the absolute rates of gluconeogenesis, if hepatic and renal carbon exchange were similar, our method of partitioning gluconeogenesis from glutamine and alanine between liver and kidney would provide a valid index of the relative incorporation of these precursors to glucose in liver and kidney.

Differences in isotopic dilution for glutamine and alanine would affect the absolute calculated rates for their incorporation into glucose in both liver and kidney but not their relative gluconeogenesis in each of these tissues. We used [U-14C]glutamine and [3-13C]alanine as tracers. Because uniformly labeled tracers are subject to greater dilution in the tricarboxylic acid cycle than those labeled only at C-3 (19), it is likely that glutamine gluconeogenesis in liver and kidney was underestimated to a greater extent than that of alanine. However, this underestimation would not affect our conclusion regarding the selective use of these precursors by liver and kidney.

Summary and Conclusions

Notwithstanding the above noted methodological considerations, the results of the present study indicate that glutamine gluconeogenesis occurs primarily in the kidney and that alanine gluconeogenesis occurs almost exclusively in the liver. Thus isotopic studies of glutamine and alanine incorporation into plasma glucose may provide a noninvasive method to selectively assess hepatic and renal gluconeogenesis.

We thank the staff of the University of Rochester General Clinical Research Center (GCRC) and the research volunteers, Sandy Webster, Mary Little, William Peifer, and Dave Robson, for their excellent technical help.

FOOTNOTES

  • The present work was supported in part by Division of Research Resources-GCRC Grant 5M01 RR-00044, National Institute of Diabetes and Digestive and Kidney Diseases Grants DK-20411 and DK-20579, and Grant Stu-192/1–2 (to M. Stumvoll) by the Deutsche Forschungsgemeinschaft, Germany. M. Kreider was a medical student at the University of Pennsylvania and spent a year in research at the University of Rochester.

REFERENCES

  • 1 Aber G., Morris L., Housley E.Gluconeogenesis by the human kidney.Nature212196615891590
    Crossref | PubMed | Web of Science | Google Scholar
  • 2 Aikawa T., Matsutaka H., Takezawa K., Ishikawa E.Gluconeogenesis and amino acid metabolism. I. Comparison of various precursors for hepatic gluconeogenesis in vivo.Biochem. Biophys. Acta2791972234244
    Crossref | PubMed | Web of Science | Google Scholar
  • 3 Aikawa T., Matsutaka H., Yamamoto H., Okuda T., Ishikawa E., Kawano T., Matsumura E.Inter-organal relations and roles of glutamine and alanine in the amino acid metabolism of fasted rats.J. Biochem.74197310031017
    PubMed | Web of Science | Google Scholar
  • 4 Biava C., Grossman A., West M.Ultrastructural observations on renal glycogen in normal and pathologic human kidneys.Lab. Invest.151966330356
    PubMed | Web of Science | Google Scholar
  • 5 Björkman O., Felig P., Wahren J.The contrasting responses of splanchnic and renal glucose output to gluconeogenic substrates and to hypoglucagonemia in 60-h-fasted humans.Diabetes291980610616
    Crossref | PubMed | Web of Science | Google Scholar
  • 6 Bowman R.Gluconeogenesis by the isolated perfused rat kidney.J. Biol. Chem.245197016041612
    PubMed | Web of Science | Google Scholar
  • 7 Brooks D., Black M., Arcangeli T., Aoki T., Wilmore D.The heated dorsal hand vein: an alternative arterial sampling site.J. Parenter. Enteral Nutr.131989102105
    Crossref | PubMed | Web of Science | Google Scholar
  • 8 Brun C.A rapid method for the determination of para-aminohippuric acid in kidney function tests.J. Lab. Clin. Med.371951955958
    PubMed | Google Scholar
  • 9 Brundin T., Wahren J.Renal oxygen consumption, thermogenesis, and amino acid utilization during iv infusion of amino acids in man.Am. J. Physiol.267Endocrinol. Metab. 301994E648E655
    Abstract | Web of Science | Google Scholar
  • 10 Castellano P., DeFronzo R. A.Glucose metabolism and the kidney.Semin. Nephrol.101990458463
    PubMed | Web of Science | Google Scholar
  • 11 Cersosimo E., Judd R., Miles J.Insulin regulation of renal glucose metabolism in conscious dogs.J. Clin. Invest.93199425842589
    Crossref | PubMed | Web of Science | Google Scholar
  • 12 Chan A., Perry S., Burch H., Fagioli S., Olvey T., Lowry O.Distribution of two aminotransferases and d-amino acid oxidase within the nephron of young and adult rats.J. Hist. Cyt. Chem.271979751755
    Crossref | PubMed | Web of Science | Google Scholar
  • 13 Cherrington A. D., Fuchs H., Stevenson R. W., Williams P. E., Alberti K. G. M. M., Steiner K. E.Effect of epinephrine on glycogenolysis and gluconeogenesis in conscious overnight-fasted dogs.Am. J. Physiol.247Endocrinol. Metab. 101984E137E144
    Abstract | Web of Science | Google Scholar
  • 14 Cherrington A. D., Williams P. E., Abou-Mourad N., Lacy W. W., Steiner K. E., Liljenquist J. E.Insulin as a mediator of hepatic glucose uptake in the conscious dog.Am. J. Physiol.242Endocrinol. Metab. 51982E97E101
    Abstract | Web of Science | Google Scholar
  • 15 Chiasson J., Liljenquist J., Lacy W., Jennings A., Cherrington A.Gluconeogenesis: methodological approaches in vivo.Federation Proc.361977229235
    PubMed | Google Scholar
  • 16 Chiasson J., Liljenquist J., Sinclair-Smith B., Lacy W.Gluconeogenesis from alanine in normal postabsorptive man: intrahepatic stimulatory effect of glucagon.Diabetes241975574584
    Crossref | PubMed | Web of Science | Google Scholar
  • 17 Christensen H.Role of amino acid transport and countertransport in nutrition and metabolism.Physiol. Rev.7019904377
    Link | Web of Science | Google Scholar
  • 18 Chu C. A., Singer D. K., Neal D. W., Cherrington A. D.Direct effects of catecholamines on hepatic glucose production in conscious dog are due to glycogenolysis.Am. J. Physiol.271Endocrinol. Metab. 341996E127E137
    Abstract | Web of Science | Google Scholar
  • 19 Consoli A., Kennedy F., Miles J., Gerich J.Determination of Krebs cycle metabolic carbon exchange in vivo and its use to estimate the individual contributions of gluconeogenesis and glycogenolysis to overall hepatic glucose output in man.J. Clin. Invest.80198713031310
    Crossref | PubMed | Web of Science | Google Scholar
  • 20 Consoli A., Nurjhan N., Reilly J. J., Bier D. M., Gerich J. E.Contribution of liver and skeletal muscle to alanine and lactate metabolism in humans.Am. J. Physiol.259Endocrinol. Metab. 221990E677E684
    Abstract | Web of Science | Google Scholar
  • 21 Cornell N., Lund P., Krebs H.The effect of lysine on gluconeogenesis from lactate in rat hepatocytes.Biochem. J.1421974327337
    Crossref | PubMed | Web of Science | Google Scholar
  • 22 Cryer P., Santiago J., Shah D.Measurement of norepinephrine and epinephrine in small volumes of human plasma by a single isotope derivative method: response to the upright posture.J. Clin. Endocrinol. Metab.39197410251029
    Crossref | PubMed | Web of Science | Google Scholar
  • 23 Darmaun D., Matthews D. E., Bier D. M.Glutamine and glutamate kinetics in humans.Am. J. Physiol.251Endocrinol. Metab. 141986E117E126
    Abstract | Web of Science | Google Scholar
  • 24 DeBodo R., Steele R., Dunn A., Bishop J.On the hormonal regulation of carbohydrate metabolism: studies with 14C glucose.Rec. Prog. Horm. Res.191963445448
    PubMed | Google Scholar
  • 25 DeRosa G., Swick R.Metabolic implications of the distribution of the alanine aminotransferase isoenzymes.J. Biol. Chem.2501197579617967
    Google Scholar
  • 26 Dietze G., Wicklmayer M., Hepp K., Bogner W., Mehnert H., Czempiel H., Henftling H.On gluconeogenesis of human liver: accelerated hepatic glucose formation by increased precursor supply.Diabetologia121976555561
    Crossref | PubMed | Web of Science | Google Scholar
  • 27 Earle J., Berliner R.A simplified clinical procedure for measurement of glomerular filtration rate and renal plasma flow.Proc. Soc. Exp. Biol. Med.621946262264
    Crossref | PubMed | Web of Science | Google Scholar
  • 28 Fafournoux P., Remesy C., Demigne C.Control of alanine metabolism in rat liver by transport or cellular metabolism.Biochem. J.2101983645652
    Crossref | PubMed | Web of Science | Google Scholar
  • 29 Felig P., Bergman M.The endocrine pancreas: diabetes mellitus.Endocrinology and Metabolism, Felig P., Baxter J., Frohman L.199511071250McGraw-HillNew York
    Google Scholar
  • 30 Felig P., Berman M.Integrated physiology of carbohydrate metabolism.Diabetes Mellitus Theory and Practice, Rifkin H., Porte D.19905252Ellenberg and RibkinsNew York
    Google Scholar
  • 31 Felig P., Wahren J., Karl I., Cerasi E., Luft R., Kipnis D.Glutamine and glutamine metabolism in normal and diabetic subjects.Diabetes221973573576
    Crossref | PubMed | Web of Science | Google Scholar
  • 32 Felig P., Wahren J., Lars R.Evidence for interorgan amino acid transport by red blood cells in humans.Proc. Natl. Acad. Sci.70197317751779
    Crossref | PubMed | Web of Science | Google Scholar
  • 33 Foster D. M., Hetenyi G., Berman M.A model for carbon kinetics among plasma alanine, lactate, and glucose.Am. J. Physiol.239Endocrinol. Metab. 21980E30E38
    Abstract | Web of Science | Google Scholar
  • 34 Guder W., Ross B.Enzyme distribution along the nephron.Kidney Int.261984101111
    Crossref | PubMed | Web of Science | Google Scholar
  • 35 Häussinger D., Sies H.Glutamine Metabolism in Mammalian Tissues.1984Springer VerlagNew York
    Google Scholar
  • 36 Heitmann R. N., Bergman E. N.Glutamine metabolism, interorgan transport, and glucogenicity in the sheep.Am. J. Physiol.234Endocrinol. Metab. Gastrointest. Physiol. 31978E197E203
    Abstract | Web of Science | Google Scholar
  • 37 Hetenyi G.Correction for the metabolic exchange of 14C for 12C atoms in the pathway of gluconeogenesis in vivo.Federation Proc.411982104109
    PubMed | Google Scholar
  • 38 Jenssen T., Nurjhan N., Perriello G., Bucci A., Toft I., Gerich J.Determination of [14C] glutamine specific activity in plasma.J. Liq. Chromatogr.17199413371348
    Crossref | Web of Science | Google Scholar
  • 39 Joseph S., McGivan J.The effects of ammonium chloride and bicarbonate on the activity of glutaminase in isolated liver mitochondria.Biochem. J.1761978837844
    Crossref | PubMed | Web of Science | Google Scholar
  • 40 Kaloyianni M., Freedland R. A.Contribution of several amino acids and lactate to gluconeogenesis in hepatocytes isolated from rats fed various diets.J. Nutr.1201990116122
    Crossref | PubMed | Web of Science | Google Scholar
  • 41 Katz J.Determination of gluconeogenesis in vivo with 14C-labeled substrates.Am. J. Physiol.248Regulatory Integrative Comp. Physiol. 171985R391R399
    Abstract | Web of Science | Google Scholar
  • 42 Kelleher J. K.Gluconeogenesis from labeled carbon: estimating isotope dilution.Am. J. Physiol.250Endocrinol. Metab. 131986E296E305
    Abstract | Web of Science | Google Scholar
  • 43 Kida K., Nakago S., Kamiya F., Ttoyama Y., Takashi N., Nakagawa H.Renal net glucose release in vivo and its contribution to blood glucose in rats.J. Clin. Invest.621978721726
    Crossref | PubMed | Web of Science | Google Scholar
  • 44 Kovacevic Z., McGivan J.Mitochondrial metabolism of glutamine and glutamate and its physiological significance.Physiol. Rev.631983547605
    Link | Web of Science | Google Scholar
  • 45 Krebs H., Bennett A., DeGasquet P., Gascoyne T., Yoshida T.Renal gluconeogenesis, the effect of diet on the gluconeogenic capacity of rat kidney cortex slices.Biochem. J.8619632228
    Crossref | PubMed | Web of Science | Google Scholar
  • 46 Krebs H., Hems R., Weidemann M., Speake R.The fate of isotopic carbon in kidney cortex synthesizing glucose from lactate.Biochem. J.1011966242249
    Crossref | PubMed | Web of Science | Google Scholar
  • 47 Krebs H., Yoshida T.Renal gluconeogenesis. II. The gluconeogenic capacity of the kidney cortex of various species.Biochem. J.891963398400
    Crossref | PubMed | Web of Science | Google Scholar
  • 48 Kreider M. E., Stumvoll M., Meyer C., Overkamp D., Welle S., Gerich J.Steady-state and non-steady-state measurements of plasma glutamine turnover in humans.Am. J. Physiol.272Endocrinol. Metab. 351997E621E627
    Abstract | Web of Science | Google Scholar
  • 49 Kreisberg R., Pennington L., Boshell B.Lactate turnover and gluconeogenesis in normal and obese humans.Diabetes1919705363
    Crossref | PubMed | Web of Science | Google Scholar
  • 50 Kristensen L. Ø., Sestoft L., Folke M.Concentrative uptake of alanine in hepatocytes from fed and fasted rats.Am. J. Physiol.244Gastrointest. Liver Physiol. 71983G491G500
    Abstract | Web of Science | Google Scholar
  • 51 Landau B., Wahren J., Chandramouli V., Schuman W., Ekberg K., Kalhan S.Contributions of gluconeogenesis to glucose production in the fasted state.J. Clin. Invest.981996378385
    Crossref | PubMed | Web of Science | Google Scholar
  • 52 Lowry O., Passonneau J.Typical fluorimetric procedures for metabolic assays.A Flexible System for Enzymatic Analysis, Lowry O., Passonneau J.1972194199AcademicNew York
    Google Scholar
  • 53 McGuinness O. D., Fugiwara T., Murrell S., Bracy D., Neal D., O’Connor D., Cherrington A. D.Impact of chronic stress hormone infusion on hepatic carbohydrate metabolism in the conscious dog.Am. J. Physiol.265Endocrinol. Metab. 281993E314E322
    Abstract | Web of Science | Google Scholar
  • 54 Meyer C., Nadkarni V., Stumvoll M., Gerich J.Human kidney free fatty acid and glucose uptake: evidence for a renal glucose-fatty acid cycle.Am. J. Physiol.273Endocrinol. Metab. 361997E650E654
    Abstract | Web of Science | Google Scholar
  • 55 Miller B., Cersosimo E., McRae J., Williams P., Lacy W.Interorgan relationship of alanine and glutamine during fasting in the conscious dog.J. Surg. Res.351983310318
    Crossref | PubMed | Web of Science | Google Scholar
  • 56 Mitrakou A., Jones R., Okuda Y., Pena J., Nurjhan N., Field J. B., Gerich J. E.Pathway and carbon sources for hepatic glycogen repletion in dogs.Am. J. Physiol.260Endocrinol. Metab. 231991E194E202
    Abstract | Web of Science | Google Scholar
  • 57 Nurjhan N., Bucci A., Perriello G., Stumvoll M., Dailey G., Bier D., Toft I., Jenssen T., Gerich J.Glutamine: a major gluconeogenic precursor and vehicle for interorgan carbon transport in man.J. Clin. Invest.951995272277
    Crossref | PubMed | Web of Science | Google Scholar
  • 58 Nurjhan N., Kennedy F., Consoli A., Martin C., Miles J., Gerich J.Quantification of the glycolytic origin of plasma glycerol: implications for the use of the rate of appearance of plasma glycerol as an index of lipolysis in vivo.Metabolism371988386389
    Crossref | PubMed | Web of Science | Google Scholar
  • 59 Owen O., Reichle F., Mozzoli M., Kreulen T., Patel M., Elfenbein I., Golsorkhi M., Chang K., Rao N., Sue H., Boden G.Hepatic, gut, and renal substrate flux rates in patients with hepatic cirrhosis.J. Clin. Invest.681981240252
    Crossref | PubMed | Web of Science | Google Scholar
  • 60 Pitts R.Metabolism of amino acid by the perfused rat kidney.Am. J. Physiol.2201971862867
    Link | Web of Science | Google Scholar
  • 61 Pitts R. F., DeHaas J., Klein J.Relation of renal amino and amide nitrogen extraction to ammonia production.Am. J. Physiol.2041963187191
    Link | Web of Science | Google Scholar
  • 62 Ross B., Hems R., Krebs H.The rate of gluconeogenesis from various precursors in the perfused rat liver.Biochem. J.1021967942951
    Crossref | PubMed | Web of Science | Google Scholar
  • 63 Rothman D., Magnusson I., Katz L., Shulman R., Shulman G.Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR.Science2541991573576
    Crossref | PubMed | Web of Science | Google Scholar
  • 64 Sacca L., Vigorito C., Cicala M., Corso G., Sherwin R. S.Role of gluconeogenesis in epinephrine-stimulated hepatic glucose production in humans.Am. J. Physiol.245Endocrinol. Metab. 81983E294E302
    Abstract | Web of Science | Google Scholar
  • 65 Schoolwerth A., Smith B., Culpepper R.Renal gluconeogenesis.Miner. Electrolyte Metab.141988347361
    PubMed | Google Scholar
  • 66 Schwenk W., Berg P., Beaufrere B., Miles J., Haymond M.Use of t-butyldimethylsilylation in the gas chromatographic/mass spectrometric analysis of physiologic compounds found in plasma using electron-impact ionization.Anal. Biochem.1411984101109
    Crossref | PubMed | Web of Science | Google Scholar
  • 67 Sestoft L., Trap-Jensen J., Lyngsoe J., Clausen J., Holst J., Nielsen S., Rehfeld J., DeMuckadell O.Regulation of gluconeogenesis and ketogenesis during rest and exercise in diabetic subjects and normal men.Clin. Sci. Mol. Med.531977411418
    PubMed | Google Scholar
  • 68 Shotwell M., Kilberg M., Oxender D.The regulation of neutral amino acid transport in mammalian cells.Biochem. Biophys. Acta7371983367284
    Google Scholar
  • 69 Sips H., Groen A., Tager J.Plasma membrane transport of alanine is rate-limiting for its metabolism in rat-liver parenchymal cells.FEBS Lett.1191980271274
    Crossref | PubMed | Web of Science | Google Scholar
  • 70 Steinberg D.Regulation of carbohydrate metabolism.Physiological Basis of Medical Practice, Satterfield T.1991728739Williams & WilkinsBaltimore
    Google Scholar
  • 71 Stumvoll M., Chintalapudi U., Perriello G., Welle S., Gutierrez O., Gerich J.Uptake and release of glucose by the human kidney: postabsorptive rates and responses to epinephrine.J. Clin. Invest.96199525282533
    Crossref | PubMed | Web of Science | Google Scholar
  • 72 Stumvoll M., Perriello G., Nurjhan N., Bucci A., Welle S., Jansson P.-A., Dailey G., Bier D., Jenssen T., Gerich J.Glutamine and alanine metabolism in NIDDM.Diabetes451996863868
    Crossref | PubMed | Web of Science | Google Scholar
  • 73 Tessari P., Garibotto G., Inchiostro S., Robaudo C., Saffioti S., Vettore M., Zanetti M., Russo R., DeFerrari G.Kidney, splanchnic and leg protein turnover in humans: insights from leucine and phenylalanine kinetics.J. Clin. Invest.98199614811492
    Crossref | PubMed | Web of Science | Google Scholar
  • 74 Vinay P., Mapes J. P., Krebs H. A.Fate of glutamine carbon in renal metabolism.Am. J. Physiol.234Renal Fluid Electrolyte Physiol. 31978F123F129
    Link | Web of Science | Google Scholar
  • 75 Wahren J., Felig P., Hagenfeldt L.Effect of protein ingestion on splanchnic and leg metabolism in normal man and in patients with diabetes mellitus.J. Clin. Invest.571976987999
    Crossref | PubMed | Web of Science | Google Scholar
  • 76 Wiecko J., Sherman W.Boroacetylation of carbohydrates. Correlations between structure and mass spected behavior in monacetylhexose cyclic boronic esters.J. Am. Chem. Soc.98197676317637
    Crossref | Web of Science | Google Scholar
  • 77 Wieland O.Glycerol UV method.Methods of Enzymatic Analysis, Bergmeyer U.3197414041414AcademicNew York
    Google Scholar
  • 78 Windmueller H. G., Spaeth A. E.Uptake and metabolism of plasma glutamine by the small intestine.J. Biol. Chem.249197450705079
    Crossref | PubMed | Web of Science | Google Scholar
  • 79 Wirthensohn G., Guder W.Renal substrate metabolism.Physiol. Rev.661986469497
    Link | Web of Science | Google Scholar
  • 80 Wolfe R.Radioactive and Stable Isotope Tracers in Biomedicine: Principles and Practice of Kinetic Analysis.1992Wiley-LissNew York
    Google Scholar
  • 81 World Health Organization Expert CommitteeDiabetes Mellitus: A Second Report.1980180WHOGeneva, Switzerland (Tech. Rep. Ser. 646)
    Google Scholar

AUTHOR NOTES

  • Address for reprint requests: J. E. Gerich, Univ. of Rochester School of Medicine, 601 Elmwood Ave., Box MED/CRC, Rochester, NY 14642.