The hypothalamic control of reproductive function is expressed through the actions of GnRH on gonadotropin secretion after binding to high-affinity receptors in the plasma membrane of pituitary gonadotrophs. The mechanism of cellular activation by GnRH involves the integrated actions of several messenger systems, including phosphoinositide breakdown and mobilization of intracellular and extracellular calcium. GnRH stimulates the hydrolysis of phosphatidylinositol bisphosphate to diacylglycerol and inositol trisphosphate (InsP3). The role of diacylglycerol formation and activation of protein kinase C in gonadotropin secretion was indicated by the impaired action of GnRH in pituitary cells depleted by kinase C by prolonged treatment with phorbol esters. An extracellular Ca2+-independent component of GnRH action was defined by studies on the effects of Ca2+ channel agonist and antagonist analogs on GnRH-and K+-induced LH secretion from pituitary cells in normal and calcium-depleted incubation medium. The initiation of the secretory response to GnRH was found to be largely independent of calcium entry, whereas the prolongation of gonadotropin secretion was maintained by calcium influx, in part through voltage-sensitive calcium channels. The role of arachidonic acid metabolites in GnRH action is probably related to the calcium-independent component of GnRH-induced LH secretion. Since GnRH is secreted episodically and for short periods, much of its physiological action on pulsatile gonadotropin release could be independent of calcium influx from the extracellular fluid. Further evidence for this mechanism was obtained by analysis of cytosolic calcium concentration ((Ca2+)i) during GnRH stimulation of enriched gonadotrophs, in which rapid peak increases of (Ca2+)i and LH release were followed by sustained elevations of both (Ca2+)i and hormone secretion. Whereas the rapid peak of (Ca2+)i was largely attributable to mobilization of Ca2+ from intracellular stores by InsP3, and showed little dependence on extracellular Ca2+, the sustained increase in (Ca2+)i and LH release were highly dependent on intracellular Ca2+ and are partly mediated by influx through dihydropyridine- and voltage-sensitive calcium channels. The regulation of calcium release in pituitary microsomes was shown to be mediated by high-affinity InsP3 binding sites that were characterized in pituitary membranes and serve as receptors through which InsP3 triggers Ca2+ mobilization in the pituitary gland.
Showing the most recent 10 out of 31 publications
