To maintain potassium homeostasis, kidneys exert flow-dependent potassium secretion to facilitate kaliuresis in response to elevated dietary potassium intake. This process involves stimulation of calcium-activated large conductance maxi-K (BK) channels in the distal nephron, namely the connecting tubule and the collecting duct. Recent evidence suggests that the TRPV4 channel is a critical determinant of flow-dependent intracellular calcium elevations in these segments of the renal tubule. Here, we demonstrate that elevated dietary potassium intake (five percent potassium) increases renal TRPV4 mRNA and protein levels in an aldosterone-dependent manner and causes redistribution of the channel to the apical plasma membrane in native collecting duct cells. This, in turn, leads to augmented TRPV4-mediated flow-dependent calcium ion responses in freshly isolated split-opened collecting ducts from mice fed the high potassium diet. Genetic TRPV4 ablation greatly diminished BK channel activity in collecting duct cells pointing to a reduced capacity to excrete potassium. Consistently, elevated potassium intake induced hyperkalemia in TRPV4 knockout mice due to deficient renal potassium excretion. Thus, regulation of TRPV4 activity in the distal nephron by dietary potassium is an indispensable component of whole body potassium balance.

Potassium Kir4.1/5.1 channels are abundantly expressed at the basolateral membrane of principal cells in the cortical collecting duct (CCD), where they are thought to modulate transport rates by controlling transepithelial voltage. Insulin and insulin-like growth factor-1 (IGF-1) stimulate apically localized epithelial sodium channels (ENaC) to augment sodium reabsorption in the CCD. However, little is known about their actions on potassium channels localized at the basolateral membrane. In this study, we implemented patch-clamp analysis in freshly isolated murine CCD to assess the effect of these hormones on Kir4.1/5.1 at both single channel and cellular levels. We demonstrated that K(+)-selective conductance via Kir4.1/5.1 is the major contributor to the macroscopic current recorded from the basolateral side in principal cells. Acute treatment with 10 μM amiloride (ENaC blocker), 100 nM tertiapin-Q (TPNQ; ROMK inhibitor), and 100 μM ouabain (Na(+)-K(+)-ATPase blocker) failed to produce a measurable effect on the macroscopic current. In contrast, Kir4.1 inhibitor nortriptyline (100 μM), but not fluoxetine (100 μM), virtually abolished whole cell K(+)-selective conductance. Insulin (100 nM) markedly increased the open probability of Kir4.1/5.1 and nortriptyline-sensitive whole cell current, leading to significant hyperpolarization of the basolateral membrane. Inhibition of the phosphatidylinositol 3-kinase cascade with LY294002 (20 μM) abolished action of insulin on Kir4.1/5.1. IGF-1 had similar stimulatory actions on Kir4.1/5.1-mediated conductance only when applied at a higher (500 nM) concentration and was ineffective at 100 nM. We concluded that both insulin and, to a lesser extent, IGF-1 activate Kir4.1/5.1 channel activity and open probability to hyperpolarize the basolateral membrane, thereby facilitating Na(+) reabsorption in the CCD.

Store-operated calcium entry (SOCE) is the mechanism by which extracellular signals elicit prolonged intracellular calcium elevation to drive changes in fundamental cellular processes. Here, we investigated the role of SOCE in the regulation of renal water reabsorption, using the inbred rat strain SHR-A3 as an animal model with disrupted SOCE. We found that SHR-A3, but not SHR-B2, have a novel truncating mutation in the gene encoding stromal interaction molecule 1 (STIM1), the endoplasmic reticulum calcium (Ca(2+)) sensor that triggers SOCE. Balance studies revealed increased urine volume, hypertonic plasma, polydipsia, and impaired urinary concentrating ability accompanied by elevated circulating arginine vasopressin (AVP) levels in SHR-A3 compared with SHR-B2. Isolated, split-open collecting ducts (CD) from SHR-A3 displayed decreased basal intracellular Ca(2+) levels and a major defect in SOCE. Consequently, AVP failed to induce the sustained intracellular Ca(2+) mobilization that requires SOCE in CD cells from SHR-A3. This effect decreased the abundance of aquaporin 2 and enhanced its intracellular retention, suggesting impaired sensitivity of the CD to AVP in SHR-A3. Stim1 knockdown in cultured mpkCCDc14 cells reduced SOCE and basal intracellular Ca(2+) levels and prevented AVP-induced translocation of aquaporin 2, further suggesting the effects in SHR-A3 result from the expression of truncated STIM1. Overall, these results identify a novel mechanism of nephrogenic diabetes insipidus and uncover a role of SOCE in renal water handling.