Gonadotropin secretion is primarily regulated by gonadotropin-releasing hormone (GnRH), and is also influenced by direct actions of oxytocin, GABA, and endothelin on pituitary gonadotrophs. Binding of GnRH to its receptors leads to rapid stimulation of phosphoinositide hydrolysis and calcium mobilization, with biphasic increases in cytoplasmic calcium ([Ca]i) and gonadotropin secretion. The maintenance of gonadotropin secretion depends on calcium influx through plasma-membrane channels, predominately L-type voltage-sensitive calcium channels (VSCC). Activation of L-type channels of gonadotrophs during depolarization by high potassium concentrations, and the inward calcium current measured by patch-clamp studies, were found to be closely correlated with the LH response to depolarization. The L- channels of gonadotrophs were shown to be highly susceptible to calcium- dependent inactivation during potassium-induced depolarization, which also attenuated agonist-induced calcium influx and LH release. Such calcium- dependent inactivation of L-channels also occurs during GnRH action, and initiates the process of desensitization of gonadotrophs. Single gonadotrophs were found to exhibit low-amplitude spontaneous fluctuations in [Ca]i, and prominent agonist-induced oscillations in [Ca]i that are generated by periodic release of intracellular Ca. GnRH elicited three types of [Ca]i responses: at low doses, subthreshold increases in [Ca]i; at intermediate doses, oscillations in [Ca]i with dose-dependent modulation of spiking frequency; and at high doses, biphasic elevations without oscillations. Thus, gonadotrophs possess both membrane and cytoplasmic oscillators, the latter being operative during agonist stimulation of hormone secretion. Activation of protein kinase C (PKC) by phorbol esters stimulated Ca influx in quiescent cells but inhibited influx when VSCC were already activated by agonist stimulation or depolarization. PKC is involved in the sustained phase of hormone secretion in GnRH-stimulated cells, and appears to amplify the effects of Ca, either positive or negative, on cell function. The stimulatory actions of endothelin on gonadotropin release were found to be accompanied by lesser stimulation of exocytosis in thyrotrophs, somatotrophs, and lactotrophs. In gonadotrophs, endothelin elicited the same patterns of single cell [Ca]i response as GnRH, but both [Ca]i and secretory responses to the vasoconstrictor peptide underwent extremely rapid desensitization. Endothelin exerts potent but transient stimulatory actions in several pituitary cell types, and is a potential regulator of gonadotropin secretion in vivo. The rapid desensitization of endothelin vs. GnRH action is critical in determining the cellular responses to the two peptides.
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