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The large-conductance Ca 2+ -activated K +  channel, BK (KCNMA1), is expressed along the connecting tubule (CNT) and cortical collecting duct (CCD) where it underlies flow- and Ca 2+ -dependent K +  secretion. Its activity is partially under the control of the mechanosensitive transient receptor potential vanilloid type 4 (TRPV4) Ca 2+ -permeable channel. Recently, we identified three small-/intermediate-conductance Ca 2+ -activated K +  channels, SK1 (KCNN1), SK3 (KCNN3), and IK1 (KCNN4), with notably high Ca 2+ -binding affinities, that are expressed in CNT/CCD and may be regulated by TRPV4-mediated Ca 2+  influx. The K + -secreting CCD mCCDcl1 cells, which express these channels, were used to determine whether SK1/3 and IK1 are activated on TRPV4 stimulation and whether they contribute to Ca 2+  influx and activation of BK. Activation of TRPV4 (GSK1016790A) modestly depolarized the membrane potential and robustly increased intracellular Ca 2+ , [Ca 2+ ] i . Inhibition of both SK1/3 and IK1 by application of apamin and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34), respectively, further depolarized the membrane potential and markedly suppressed the TRPV4-mediated rise in [Ca 2+ ] i . Application of BK inhibitor iberiotoxin after activation of TRPV4 without apamin/TRAM-34 also reduced [Ca 2+ ] i  and further intensified membrane depolarization, demonstrating BK involvement. However, the BK-dependent effects on [Ca 2+ ] i  and membrane potential were largely abolished by pretreatment with apamin and TRAM-34, identical to that observed by separately suppressing TRPV4-mediated Ca 2+  influx, demonstrating that SK1/3-IK1 channels potently contribute to TRPV4-mediated BK activation. Our data indicate a direct correlation between TRPV4-mediated Ca 2+  signal and BK activation but where early activation of SK1/3 and IK1 channels are critical to sufficiently enhanced Ca 2+  entry and [Ca 2+ ] i  levels required for activation of BK.

american journal of physiology. renal physiology./american journal of physiology. renal physiology

Dynamic coupling between TRPV4 and Ca2+-activated SK1/3 and IK1 K+ channels plays a critical role in regulating the K+-secretory BK channel in kidney collecting duct cells