Expression and biological significance of Ca2+-activated ion channels in human keratinocytes

In whole-cell recording from HaCaT keratinocytes, ATP, bradykinin, and hist amine caused a biphasic change of the membrane potential consisting of an i nitial transient depolarization, followed by a pronounced and long-lasting hyperpolarization. Flash photolysis of caged IP3 mimicked the agonist-induc ed voltage response, suggesting that intracellular Ca2+ release and subsequ ent opening of Ca2+-activated ion channels serve as the common transduction mechanism. In contrast, cAMP- and PKC-dependent pathways were not involved in the electrophysiological effects of the extracellular signaling molecul es. The depolarization was predominantly mediated by a DIDS- and niflumic a cid-sensitive Cl- current, whereas a charybdotoxin- and clotrimazole-sensit ive K+ current underlay the prominent hyperpolarization. Consistent with th e electrophysiological data, RT-PCR showed that HaCaT keratinocytes express two types of Ca2+-activated Cl- channels, CaCC2 and CaCC3 (CLCA2), as well as the Ca2+-activated K+ channel hSK4. That the pronounced hSK4-mediated h yperpolarization bears significance on the growth and differentiation prope rties of keratinocytes is suggested by RNase protection assays showing that hSK4 mRNA expression is strongly down-regulated under conditions that allo w keratinocyte differentiation, hSK4 might thus play a role in linking chan ges in membrane potential to the biological fate of keratinocytes.