Endocrinology, Vol 122, 1905-1914, Copyright © 1988 by Endocrine Society

ARTICLES

Enhanced responsiveness of gonadotropes after protein kinase-C activation: postreceptor regulation of gonadotropin releasing hormone action

CA McArdle, WR Huckle, LA Johnson and PM Conn
Department of Pharmacology, University of Iowa College of Medicine, Iowa City 52242-1109.

GnRH stimulates the release of LH from pituitary gonadotropes in a Ca2+- and calmodulin-dependent manner. Although GnRH also appears to activate protein kinase-C in gonadotropes, the role of this enzyme in GnRH action remains undetermined. In the present work we have assessed the effect of pretreatment of pituitary cell cultures with a protein kinase- C-activating phorbol ester on gonadotrope responsiveness to GnRH. Pretreatment for 6 h with phorbol 12-myristate 13-acetate (PMA) reduced the EC50 for GnRH-stimulated LH release approximately 8-fold without altering the maximum proportion of total cellular LH release. This increase in the potency of GnRH occurred in the absence of any measurable change in receptor affinity. Subsequent studies revealed that PMA pretreatment did not alter the EC50 for GnRH-stimulated [3H]inositol phosphate accumulation (an indicator of phosphoinositide hydrolysis), but did cause a modest reduction (approximately 2-fold) in the EC50 for LH release in response to the Ca2+ ionophore A23187 and the Ca2+ channel-activating compound maitotoxin. These observations demonstrate that the efficiency of coupling of the GnRH receptor to LH release can be regulated at a postreceptor locus by activation of protein kinase-C and that an increased responsiveness of Ca2+-regulated effector systems to mobilized Ca2+ appears to contribute to the observed effect.

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