The Relationship between Gonadotropin-Releasing Hormone-Stimulated Luteinizing Hormone Release and Inositol Phosphate Production: Studies with Calcium Antagonists and Protein Kinase C Activators*

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Abstract
The coupling between GnRH-stimulated phosphoinositide (PI) turnover and LH release has been investigated in rat pituitary cell cultures. Accumulation of [3H]inositol phosphates ([3H]IPs) formed by hydrolysis of PIs was measured in cells that had been preloaded with [3H]myo-inositol. GnRH stimulated both LH release and incorporation of [3H]inositol into total [3H]IPs with similar dose and time dependencies. [3H]IP production in response to GnRH could be blocked by a GnRH antagonist, but was stimulated by a compound that provokes receptor microaggregation. GnRH-stimulated IP production persisted in the presence of either the Ca2+ channel blocker D600 or the calmodulin antagonist pimozide at concentrations that reduced LH release to 60% and 20% of control, respectively. Stimulated [3H]IP production was inhibited at higher concentrations of D600. In 1-h incubations, GnRH-stimulated [3H]IP production, but not LH release, was markedly inhibited by the protein kinase C activators phorbol myristate acetate and 1,2-dioctanoylglycerol. These findings indicate that in the gonadotrope, 1) GnRH-stimulated LH release and [3H]IP production are closely coupled to receptor activation by an agonist; 2) Ca2+ antagonists uncouple stimulated LH release from [3H]IP production; and 3) protein kinase C activators uncouple stimulated PI turnover from LH release. Thus, GnRH-stimulated PI turnover from LH release. Thus, GnRH-stimulated production of PI metabolites, as measured by [3H]IP accumulation, is apparently not sufficient to support LH release in the absence of Ca2+. In addition, GnRH-stimulated LH release is apparently not dependent on full expression of the PI response.