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

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