Secretagogue-induced RVD in HSY cells is due to K⁺ channels activated by Ca²⁺ and protein kinase C

To maintain cell volume, absorptive epithelia must coordinate ion fluxes associated with transcellular transport with those required for volume regulation. K⁺ channels are thought to play a central regulatory role in this process. Electronic cell sizing was used to study the regulatory volume decrease (RVD) response of the human salivary ductal cell line HSY. Following a hypotonic challenge, RVD was markedly enhanced by the muscarinic agonist carbachol (half-maximal effect ~ 1.4 μM). Carbachol-induced RVD was mimicked by the K⁺ ionophore valinomycin and inhibited by K⁺ channel blockers, indicating that it is due to the activation of K⁺ channels. Carbachol-induced RVD was blocked by maneuvers that blunted the carbachol- induced rise in intracellular Ca²⁺ concentration ([Ca²⁺](i)) and mimicked by ionomycin-induced increases in [Ca²⁺](i), but concentrations of ionomycin (100 nM) yielding [Ca²⁺](i) well above carbachol-induced levels were required. However, when treatment with 2.5 nM ionomycin, which produced [Ca²⁺](i) in the carbachol-generated range, was combined with treatment with low concentrations of an active phorbol ester, an enhancement of RVD similar to that observed with carbachol alone was observed. These data provide strong evidence that carbachol-induced RVD in the HSY cells involves K⁺ channels that are tightly regulated by both [Ca²⁺](i) and protein kinase C.