Wortmannin-Sensitive and -Insensitive Steps in Calcium-Controlled Exocytosis in Pituitary Gonadotrophs: Evidence That Myosin Light Chain Kinase Mediates Calcium-Dependent and Wortmannin-Sensitive Gonadotropin Secretion

In cultured rat pituitary cells, increases in the cytosolic calcium concentration ([Ca2+]i) and LH release are induced by activation of GnRH receptors as well as by nonreceptor-mediated stimuli. Treatment of pituitary cells with the myosin light chain kinase (MLCK) inhibitor, wortmannin, attenuated GnRH-induced LH release. Wortmannin also reduced the LH responses to nonreceptor-mediated elevation of [Ca2+]i by ionomycin and activation of voltage-sensitive Ca2+ channels by Bay K 8644 or high K+, as well as Ca2+-induced LH release in permeabilized pituitary cells. The [Ca2+]i responses to these stimuli were unaltered in wortmannin-treated pituitary cells, indicating that this compound inhibits a Ca2+-dependent step in exocytosis without affecting Ca2+ signaling. In perifused pituitary cells, the GnRH-induced early spike phase of LH release was not affected by wortmannin, whereas the subsequent plateau phase was almost completely inhibited. No significant changes in GnRH-induced phospholipase D activity and diacylglycerol production were observed in wortmannin-treated pituitary cells during the sustained phase of agonist stimulation. Wortmannin also had no effect on LH responses to the protein kinase C activator, phorbol 12-myristate 13-acetate, further indicating that the attenuation of agonist-induced LH release is not related to inhibition of the diacylglycerol/protein kinase C pathway. In addition, agonist-induced LH release was attenuated by two other MLCK inhibitors, MS-347a and KT5926. These data suggest that MLCK mediates the downstream effects of Ca2+ on exocytosis, an action supported by the finding of wortmannin-sensitive phosphorylation of a 20-kDa protein in pituitary cells and αT3–1 gonadotrophs treated with GnRH, K+, and Bay K 8644. This protein was coprecipitated from pituitary extracts with a specific antibody to nonmuscle myosin IIB and comigrated with 20-kDa smooth muscle myosin light chain on SDS-PAGE. These results demonstrate that Ca2+ controls exocytosis through an initial wortmannin-insensitive step and a sustained wortmannin-sensitive step and suggest that the latter event in the cascade of cellular responses is dependent on phosphorylation of nonmuscle myosin IIB light chain by MLCK.