Kir4.1/Kir5.1 in the DCT plays a role in the regulation of renal K+ excretion.

The aim of this mini review is to provide an overview regarding the role of inwardly rectifying potassium channel 4.1 (Kir4.1)/Kir5.1 in regulating renal K+ excretion. Deletion of Kir4.1 in the kidney inhibited thiazide-sensitive NaCl cotransporter (NCC) activity in the distal convoluted tubule (DCT) and slightly suppressed Na-K-2Cl cotransporter (NKCC2) function in the thick ascending limb (TAL). Moreover, increased dietary K+ intake inhibited, whereas decreased dietary K+ intake stimulated, the basolateral potassium channel (a Kir4.1/Kir5.1 heterotetramer) in the DCT. The alteration of basolateral potassium conductance is essential for the effect of dietary K+ intake on NCC because deletion of Kir4.1 in the DCT abolished the effect of dietary K+ intake on NCC. Since potassium intake-mediated regulation of NCC plays a key role in regulating renal K+ excretion and potassium homeostasis, the deletion of Kir4.1 caused severe hypokalemia and metabolic alkalosis under control conditions and even during increased dietary K+ intake. Finally, recent studies have suggested that the angiotensin II type 2 receptor (AT2R) and bradykinin-B2 receptor (BK2R) are involved in mediating the effect of high dietary K+ intake on Kir4.1/Kir5.1 in the DCT.


INTRODUCTION
Potassium (K ϩ ) is critical for maintaining cellular function. Normal K ϩ balance is maintained over the short term through the uptake by cells, comprising a cellular storage reservoir (including muscle, liver, and red blood cells). Longer term balance, however, is dependent on excretion via the urine, during both potassium loading and depletion (13). The classic view of K ϩ excretion is focused on the endothelial sodium (Na ϩ ) channel (ENaC) and secretory potassium channels, including renal outer medullary K ϩ channel (ROMK) and calcium-dependent big-conductance potassium channels, in the aldosterone-sensitive distal nephron (ASDN), where Na ϩ absorption through ENaC provides the lumen negative driving force for K ϩ secretion. However, recent development in the field strongly indicates that NCC in the distal convoluted tubule (DCT) also plays a key role in regulating potassium homeostasis and renal K ϩ excretion (5,8,29). Furthermore, a large body of evidence has suggested that the basolateral potassium channel in the DCT, which is composed of Kir4.1 and Kir5.1, plays an important role in determining NCC activity (4,44). This notion is supported by the finding that the renal phenotype of loss-of-function mutations of Kir4.1 in the kidney is reminiscent of Gitelman syndrome, characterized by defective NCC function in the DCT (2). Thus understanding the regulation of Kir4.1/Kir5.1 in the kidney should shed light on the regulatory mechanism of overall K ϩ excretion.

Kir4.1/Kir5.1 HETEROTETRAMER IS A PREDOMINANT TYPE OF POTASSIUM CHANNEL IN THE BASOLATERAL MEMBRANE OF THE DCT
Kir4.1 is encoded by Kcnj10 and highly expressed in the brain, inner ear, eye, and kidney (10,27). In the kidney, Kir4.1 is expressed in the basolateral membrane of the late cortical thick ascending limb (cTAL), DCT, connecting tubule (CNT), and early cortical collecting duct (CCD) (2,20,26,43,44). In the cTAL and CCD, Kir4.1 contributes only partially to the basolateral potassium conductance because the deletion of Kir4.1 did not completely eliminate the basolateral potassium conductance (26,43). Furthermore, single-channel patch-clamp experiments have detected several types of potassium channels other than Kir4.1-composed potassium channels in the cTAL and CCD (15,26,43). In contrast, Kir4.1 is a necessary component of potassium channels in the entire DCT, since the deletion of Kir4.1 completely eliminated the basolateral potassium conductance in this segment (4,44).

Kir4.1/Kir5.1 IS ESSENTIAL FOR THE EFFECT OF DIETARY K ؉ INTAKE ON NCC
Regulation of NCC plays an essential role in modulating K ϩ excretion (5). For instance, high K-induced inhibition of NCC should augment Na ϩ delivery to the CNT and CCD, thereby stimulating K ϩ secretion. It has been reported that NCC dephosphorylation (an indication of inhibition) occurs within minutes in response to an acute rise in plasma K ϩ concentrations after a meal (25). Since high K ϩ intake should also stimulate aldosterone secretion and activate apical ENaCs, increased Na ϩ delivery and activated ENaC should enhance renal K ϩ excretion. Conversely, low dietary K ϩ intake should have an opposite effect on renal K ϩ excretion by increasing NCC activity in the DCT (33,35). Therefore, dietary K ϩ intake-induced modulation of NCC activity plays a key role in controlling K ϩ excretion in the ASDN and in maintaining potassium homeostasis. Terker et al. (28) have elegantly demonstrated the relationship between phosphorylated NCC (an indicator of NCC activity) and plasma K ϩ concentrations such that decreasing plasma K ϩ increased, while increased plasma K ϩ decreased, NCC activity. Furthermore, they have suggested that the cell membrane potential of the DCT may be responsible for the effect of dietary K ϩ intake on NCC (29). Indeed, we have observed that high K ϩ intake depolarized, while low K ϩ intake hyperpolarized, DCT membranes in the mouse kidney (35).
The negative membrane potential in the DCT is mainly determined by basolateral potassium conductance (permeability) and plasma K ϩ concentration. Although high K ϩ intake tends to increase the plasma K ϩ , the increase in plasma K ϩ concentration is modest under physiological conditions. Thus it is unlikely that change in plasma K ϩ concentrations alone would be mainly responsible for the depolarization of the DCT basolateral membrane observed in vivo. Therefore, it is conceivable that the change in basolateral K ϩ permeability is mainly responsible for dietary K ϩ intake-induced alteration of DCT membrane potential. This notion is confirmed by the finding that high K ϩ intake significantly inhibited, while low K ϩ intake increased, basolateral potassium permeability (35). The changes in basolateral K ϩ conductance and DCT membrane potential induced by dietary K ϩ intake completely depend on Kir4.1/Kir5.1 since the deletion of Kir4.1 not only abolished basolateral potassium conductance but also the effect of dietary K ϩ intake on membrane potential. In addition, dietary K ϩ intake failed to regulate NCC expression and activity in KS-Kcnj10 Ϫ/Ϫ mice, suggesting that Kir4.1/Kir5.1 is essential for the effect of dietary K ϩ intake on NCC. It is possible that high K ϩ intake-induced inhibition of Kir4.1/ Kir5.1 leads to inhibiting the chloride-sensitive WNK pathway, thereby inhibiting NCC.

REGULATION OF Kir4.1/Kir5.1
Dietary K ϩ intake plays an important role in the regulation of Kir4.1/Kir5.1 not only in the DCT but also in the CCD. However, unlike in the DCT, high K ϩ intake has been shown to stimulate Kir4.1/Kir5.1 in the CCD (30). One possible reason for this difference may be that aldosterone is more active in the CCD, where it plays a dominant role in regulating ENaC; thus the rise in aldosterone that occurs during dietary potassium loading may activate Kir4.1/Kir5.1 in the CCD but not in the DCT. The different responses of Kir4.1/Kir5.1 to high K ϩ intake between DCT and CCD are physiologically significant since high K ϩ -induced inhibition of Kir4.1/Kir5.1 in the DCT is essential for the inhibition of NCC, whereas high K ϩ -induced stimulation of Kir4.1/Kir5.1 in the CCD should increase the driving force for Na ϩ absorption, thereby stimulating K ϩ excretion. In addition, it has been reported Kir4.1/ Kir5.1 in the CCD is stimulated by insulin/IGF-1 and inhibited by dopamine (42).
Although the effect of dietary K ϩ intake on Kir4.1/Kir5.1 in the DCT is well established (35), the mechanism by which dietary K ϩ intake alters Kir4.1/Kir5.1 activity is not fully understood. Our recent studies have suggested the role of AT2R and BK2R in mediating the effect of high K ϩ intake on basolateral Kir4.1/ Kir5.1 in the DCT. We observed that AT2R inhibition significantly increased basolateral potassium conductance in the DCT, hyperpolarized the DCT membrane, and augmented NCC activity (37). Because the stimulatory effect of AT2R inhibition on NCC was absent in KS-kcnj10 Ϫ/Ϫ mice, it suggests the possibility that AT2R inhibition-induced activation of Kir4.1/Kir5.1 was required for the upregulation of NCC. Since high K ϩ intake has been reported to increase the expression of AT2R, it raises the possibility that AT2R may be involved in mediating the effect of high K ϩ intake on Kir4.1/Kir5.1 in the DCT. This notion was also supported by the finding that application of an AT2R agonist inhibited Kir4.1/Kir5.1 in the DCT. However, inhibition of AT2R may indirectly enhance AT1R activity, which has been reported to directly activate NCC (3,6,23,32,46). Further investigation is required to explore the role of angiotensin II in regulating Kir4.1/ Kir5.1 and NCC.
Three lines of evidence have suggested that BK2R is possibly involved in mediating the effect of high K ϩ intake on Kir4.1/Kir5.1 and NCC (45). First, bradykinin inhibited Kir4.1/Kir5.1 in the DCT and depolarized the DCT membrane, an effect blocked by the specific BK2R antagonist. Second, immunohistochemistry has shown that BK2R is highly expressed in the lateral and apical membrane of the DCT. Also, a previous study demonstrated that kininogen was detected in the early distal tubule and neighboring CCD (19). Third, bradykinin infusion inhibited NCC activity and increased Na ϩ excretion, an effect which was absent in KS-kcnj10 Ϫ/Ϫ mice. High K ϩ intake has been reported to increase renal BK2R expression and renal tissue kallikrein expression levels (11,34). Thus we speculate that BK2R may be involved in mediating the effect of high K ϩ intake on basolateral Kir4.1/Kir5.1 in the DCT. Accordingly, the stimulation of BK2R in the DCT is expected to inhibit NCC, thereby increasing Na ϩ delivery to the collecting duct and facilitating K ϩ excretion. In this regard, stimulation of BK2R has been shown to inhibit ENaC (41), a mechanism which should work in concert with BK2R-induced inhibition of NCC to stimulate K ϩ excretion and to prevent excessive Na ϩ absorption in the CCD. Thus activation of both AT2R and BK2R by high dietary K ϩ intake should play a key role in the regulation of basolateral potassium conductance in the DCT and NCC. intake on NaCl cotransporter (NCC) activity and how ion transport is altered in the response to dietary K ϩ intake to mediate sodium and potassium homeostasis. High K ϩ intake (green arrow or symbol) decreases Kir4.1/Kir5.1 channel activity, which subsequently inhibits the WNK-SPAK pathway and NCC activity in the early distal convoluted tubule (DCT1) cells. The sodium reabsorption and potassium excretion rates increase along the downstream segments. AT2R, angiotensin II type 2 receptor; BK2R, bradykinin type 2 receptor; MR, mineralocorticoid receptor. The dotted line indicates diminished effects.