Ojeda S R, Urbanski H F, Katz K H, Costa M E
Department of Physiology, University of Texas Health Science Center, Dallas 75235-9040.
Brain Res. 1988 Feb 16;441(1-2):339-51. doi: 10.1016/0006-8993(88)91412-6.
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.
前列腺素E2（PGE2）被认为是去甲肾上腺素诱导下丘脑释放促黄体生成激素释放激素（LHRH）的介质。本实验旨在检验以下假设：细胞内钙库中Ca2+的动员和环磷酸腺苷（cAMP）的形成参与了这一效应。将幼年大鼠正中隆起（ME）神经末梢置于无钙的碳酸氢盐缓冲Krebs-Ringer液中孵育，可降低但不能阻止PGE2对LHRH释放的刺激作用。5种钙调蛋白拮抗剂或钙调蛋白依赖性激酶的阻滞剂均未影响LHRH对PGE2的反应。相反，在无钙培养基中用TMB-8或丹曲林抑制细胞内Ca2+的动员可阻止PGE2的LHRH释放效应。与PGE2相似，Ca2+离子载体A 23187对LHRH释放的刺激作用不受钙调蛋白活性抑制的影响。然而，这阻止了离子载体诱导的PGE2形成的增加。PGE2在含钙培养基中引起cAMP积累的剂量相关增加，这种效应被细胞内Ca2+动员的阻滞剂和钙调蛋白拮抗剂所抑制。令人惊讶的是，去除细胞外Ca2+可增加孵育培养基中的基础cAMP水平，而不影响LHRH释放；PGE2诱导cAMP进一步增加，这可通过抑制细胞内Ca2+转运来阻止。用福司可林（F）刺激腺苷酸环化酶活性，无论有无细胞外Ca2+，cAMP水平都会有类似的增加。然而，F在无钙培养基中未能引起LHRH释放。结果表明：（a）PGE2对LHRH释放的刺激作用涉及细胞内Ca2+的动员，但不涉及钙调蛋白的参与；（b）PGE2自身的形成是钙调蛋白依赖性的；（c）PGE2通过一种需要细胞内（膜？）Ca2+转运的钙调蛋白依赖性机制刺激cAMP形成；（d）一群与对PGE2有反应的神经末梢不同的神经末梢的cAMP系统通常受到Ca2+依赖性抑制控制；（e）尽管PGE2是ME中cAMP形成的有效刺激物，内源性产生的cAMP可诱导LHRH释放，但在无细胞外Ca2+的情况下，PGE2以不依赖cAMP的方式刺激LHRH释放。
