Activation of K+ and Cl- channels by Ca2+ and cyclic AMP in dissociated kidney epithelial (MDCK) cells

In dissociated MDCK cells, activators of the cyclic AMP system cause depolarization detectable by changes in fluorescence of the membrane potential sensitive dye bisoxonol. Addition of forskolin (60 microM), vasopressin (2 microM), 8-bromo-cyclic AMP (0.5 mM) or 1-epinephrine (10 microM) depolarized the cells substantially in low Cl- (5 mM) but had little effect in high Cl- (140 mM) solution. These results are consistent with cyclic AMP activation of Cl- channels. The Ca2+-ionophore ionomycin (1 microM) produced a rapid hyperpolarization in low and high Cl- solutions, consistent with K+ channel opening. Using a clonal subline, MDCK-14, the magnitude of the ionomycin hyperpolarization was roughly proportional to the concomitant rise in [Ca2+]i as measured with the intracellular Ca2+ probe indo-I. Both l-epinephrine and isoproterenol appeared to activate the Cl- channels. However only l-epinephrine produced a [Ca2+]i rise and a transient hyperpolarization (due to K+ channel opening), which preceded the depolarization due to Cl- channel opening. The l-epinephrine-induced [Ca2+]i response of the heterogeneous MDCK cell population but not of the clonal subline MDCK-14 was inhibited by removal of extracellular Ca2+. In the latter only the slow secondary phase of the [Ca2+]i rise was affected by Ca2+ removal. It is concluded that l-epinephrine activates K+ and Cl- channels in a sequential manner in MDCK cells by Ca2+ and cAMP signals, presumably via alpha- and beta-adrenergic receptors located on the same cell.