Prostaglandin E2 releases luteinizing hormone-releasing hormone from the female juvenile hypothalamus through a Ca2+-dependent, calmodulin-independent mechanism.
Prostaglandin E2 (PGE2) has been implicated as a mediator of norepinephrine-induced luteinizing hormone-releasing hormone (LHRH) release from the hypothalamus. The present experiments were undertaken to examine the hypothesis that mobilization of Ca2+ from intracellular stores and cyclic AMP (cAMP) formation are involved in this effect. Incubation of median eminence (ME) nerve terminals from juvenile rats in Ca2+-free Krebs-Ringer bicarbonate buffer reduced, but failed to prevent the stimulatory effect of PGE2 on LHRH release. None of 5 calmodulin antagonists or a blocker of calmodulin-dependent kinase affected the LHRH response to PGE2. In contrast, inhibition of intracellular Ca2+ mobilization with TMB-8 or dantrolene in Ca2+-free medium prevented the LHRH releasing effect of PGE2. Similarly to PGE2, the stimulatory effect of the Ca2+ ionophore A 23187 on LHRH release was not affected by inhibition of calmodulin activity. This, however, blocked the increase in PGE2 formation induced by the ionophore. PGE2 evoked a dose-related increase in cAMP accumulation in Ca2+-containing medium and this effect was inhibited both by blockers of intracellular Ca2+ mobilization and by calmodulin antagonists. Surprisingly, removal of extracellular Ca2+ increased basal cAMP levels in the incubation medium without affecting LHRH release; PGE2 induced a further increase in cAMP which was prevented by inhibition of intracellular Ca2+ translocation. Stimulation of adenylate cyclase activity with forskolin (F) resulted in similar increases in cAMP levels both in the presence and absence of extracellular Ca2+. However, F failed to evoke LHRH release in Ca2+-free medium. The results indicate that: (a) the stimulatory effect of PGE2 on LHRH release involves mobilization of intracellular Ca2+ but not the participation of calmodulin; (b) the formation of PGE2 itself is calmodulin-dependent; (c) PGE2 stimulates cAMP formation through a calmodulin-dependent mechanism that requires translocation of intracellular (membrane?) Ca2+; (d) the cAMP system of a population of nerve terminals different from that responsive to PGE2 is normally subjected to a Ca2+-dependent inhibitory control; and (e) although PGE2 is a potent stimulator of cAMP formation in the ME and endogenously produced cAMP can induce LHRH release, in the absence of extracellular Ca2+ PGE2 stimulates LHRH release in a cAMP-independent manner.
Ojeda
S R
SR
Department of Physiology, University of Texas Health Science Center, Dallas 75235-9040.
Urbanski
H F
HF
Katz
K H
KH
Costa
M E
ME
D000818:Animals; D002118:Calcium / Q000494:pharmacology / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D002147:Calmodulin / Q000502:physiology*; D000242:Cyclic AMP / Q000502:physiology; D015232:Dinoprostone; D004958:Estradiol / Q000494:pharmacology; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008473:Median Eminence / Q000187:drug effects / Q000378:metabolism*; D010672:Phenytoin / Q000494:pharmacology; D011458:Prostaglandins E / Q000494:pharmacology*; D047428:Protein Kinase Inhibitors; D051381:Rats; D011919:Rats, Inbred Strains
D002121:Calcium Channel Blockers; D002147:Calmodulin; D011458:Prostaglandins E; D047428:Protein Kinase Inhibitors; D007987:Gonadotropin-Releasing Hormone; D004958:Estradiol; D010672:Phenytoin; D000242:Cyclic AMP; D015232:Dinoprostone; D002118:Calcium