Abstract

In excitable cells, receptor-induced Ca²⁺ release from intracellular stores is usually accompanied by sustained depolarization of cells and facilitated voltage-gated Ca²⁺ influx (VGCI). In quiescent pituitary lactotrophs, however, endothelin-1 (ET-1) induced rapid Ca²⁺ release without triggering Ca²⁺ influx. Furthermore, in spontaneously firing and depolarized lactotrophs, the Ca²⁺-mobilizing action of ET-1 was followed by inhibition of spontaneous VGCI caused by prolonged cell hyperpolarization and abolition of action potential-driven Ca²⁺ influx. Agonist-induced depolarization of cells and enhancement of VGCI upon Ca²⁺ mobilization was established in both quiescent and firing lactotrophs treated overnight with pertussis toxin (PTX). Activation of adenylyl cyclase by forskolin and addition of cell-permeable 8-bromo-cAMP did not affect ET-1–induced sustained inhibition of VGCI, suggesting that the cAMP-protein kinase A signaling pathway does not mediate the inhibitory action of ET-1 on VGCI. Consistent with the role of PTX-sensitive K⁺ channels in ET-1–induced hyperpolarization of control cells, but not PTX-treated cells, ET-1 decreased the cell input resistance and activated a 5 mM Cs⁺-sensitive K⁺ current. In the presence of Cs⁺, ET-1 stimulated VGCI in a manner comparable with that observed in PTX-treated cells, whereas E-4031, a specific blocker of ether-a-go-go–related gene-like K⁺ channels, was ineffective. Similar effects of PTX and Cs⁺ were also observed in GH₃ immortalized cells transiently expressing ET_A receptors. These results indicate that signaling of ET_A receptors through the G_i/o pathway in lactotrophs and the subsequent activation of inward rectifier K⁺ channels provide an effective and adenylyl cyclase-independent mechanism for a prolonged uncoupling of Ca²⁺ mobilization and influx pathways.