Vol. 61, Issue 6, 1329-1339, June 2002

Ca²⁺-Mobilizing Endothelin-A Receptors Inhibit Voltage-Gated Ca²⁺ Influx through G_i/o Signaling Pathway in Pituitary Lactotrophs

Melanija Tomi, Fredrick Van Goor, Mu-Lan He, Dragoslava Zivadinovic, and Stanko S. Stojilkovic

Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland

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.