Emerging Role of the Calcium-Activated, Small Conductance, SK3 K(+) Channel in Distal Tubule Function: Regulation by TRPV4

The Ca(2+)-activated, maxi-K (BK) K(+) channel, with low Ca(2+)-binding affinity, is expressed in the distal tubule of the nephron and contributes to flow-dependent K(+) secretion. In the present study we demonstrate that the Ca(2+)-activated, SK3 (K(Ca)2.3) K(+) channel, with high Ca(2+)-binding affinity, is also expressed in the mouse kidney (RT-PCR, immunoblots). Immunohistochemical evaluations using tubule specific markers demonstrate significant expression of SK3 in the distal tubule and the entire collecting duct system, including the connecting tubule (CNT) and cortical collecting duct (CCD). In CNT and CCD, main sites for K(+) secretion, the highest levels of expression were along the apical (luminal) cell membranes, including for both principal cells (PCs) and intercalated cells (ICs), posturing the channel for Ca(2+)-dependent K(+) secretion. Fluorescent assessment of cell membrane potential in native, split-opened CCD, demonstrated that selective activation of the Ca(2+)-permeable TRPV4 channel, thereby inducing Ca(2+) influx and elevating intracellular Ca(2+) levels, activated both the SK3 channel and the BK channel leading to hyperpolarization of the cell membrane. The hyperpolarization response was decreased to a similar extent by either inhibition of SK3 channel with the selective SK antagonist, apamin, or by inhibition of the BK channel with the selective antagonist, iberiotoxin (IbTX). Addition of both inhibitors produced a further depolarization, indicating cooperative effects of the two channels on Vm. It is concluded that SK3 is functionally expressed in the distal nephron and collecting ducts where induction of TRPV4-mediated Ca(2+) influx, leading to elevated intracellular Ca(2+) levels, activates this high Ca(2+)-affinity K(+) channel. Further, with sites of expression localized to the apical cell membrane, especially in the CNT and CCD, SK3 is poised to be a key pathway for Ca(2+)-dependent regulation of membrane potential and K(+) secretion.