Chang J P, Wong A O, van der Kraak G, van Goor F
Department of Zoology, University of Alberta, Edmonton, Canada.
Gen Comp Endocrinol. 1992 Jun;86(3):359-77. doi: 10.1016/0016-6480(92)90061-n.
The relationship between drugs elevating intracellular cAMP levels and gonadotropin (GTH)-releasing hormone (GnRH) in the stimulation of GTH secretion in the goldfish was investigated using dispersed goldfish pituitary cells in primary culture. In static incubation experiments, activation of adenylyl cyclase by forskolin and the inhibition of cAMP phosphodiesterase by 3 isobutyl-1-methylxanthine (IBMX) increased cAMP release and stimulated GTH secretion. The addition of membrane permeant cAMP analogs, 8-bromoadenosine 3':5'-cyclic monophosphate (8Br-cAMP), and dibutyryl cAMP also increased GTH release, suggesting that elevation of cAMP levels can induce GTH secretion. In the goldfish, dopamine is a physiological inhibitor of GTH release. Application of the dopamine agonist apomorphine decreased the GTH responses to forskolin, 8Br-cAMP, and salmon GTH-releasing hormone (sGnRH). The ability of agents that elevate cAMP levels to mimic GnRH action on GTH release suggests that cAMP may mediate GnRH-stimulated GTH secretion in the goldfish; however, this possibility was not substantiated by results from further experiments. In 2-hr static incubation studies, the GTH responses to sGnRH and chicken GnRH-II (cGnRH-II) were enhanced by coincubations with forskolin, IBMX, and 8Br-cAMP. The magnitudes of these enhancements were at least additive, if not synergistic. The levels of cAMP released into the media were unaffected by treatment with sGnRH and cGnRH-II, either in the absence or in the presence of IBMX. Replacement of normal testing media with Ca(2+)-deficient media (without Ca2+ salts and in the presence of 0.1 mM EGTA) decreased sGnRH and cGnRH-II stimulation of GTH release but did not affect forskolin and 8Br-cAMP actions. These results indicate that sGnRH and cGnRH-II stimulation of short term (less than or equal to 2-h) GTH release in the goldfish is not mediated by cAMP. The kinetics of the interactions between sGnRH, forskolin, and IBMX were also investigated in cell column perifusion studies. Applications of 5-min pulses of forskolin and IBMX stimulated rapid increases in GTH release; the latencies of these responses were similar to that observed with sGnRH. The simultaneous applications of sGnRH with either forskolin or IBMX resulted in GTH responses that were of greater magnitude and longer duration than those in response to sGnRH alone. These results together indicate that elevation of cAMP levels can potentiate the GTH response to the native GnRHs by increasing the magnitude of the acute GTH release and by prolonging the duration of GnRH action; however, cAMP does not appear to be involved directly in mediating GnRH stimulation of GTH release.(ABSTRACT TRUNCATED AT 400 WORDS)
利用原代培养的分散金鱼垂体细胞，研究了能提高细胞内cAMP水平的药物与促性腺激素（GTH）释放激素（GnRH）在刺激金鱼GTH分泌中的关系。在静态孵育实验中，福司可林激活腺苷酸环化酶以及3-异丁基-1-甲基黄嘌呤（IBMX）抑制cAMP磷酸二酯酶可增加cAMP释放并刺激GTH分泌。添加膜通透性cAMP类似物8-溴腺苷3':5'-环磷酸（8Br-cAMP）和二丁酰cAMP也能增加GTH释放，这表明cAMP水平升高可诱导GTH分泌。在金鱼中，多巴胺是GTH释放的生理抑制剂。应用多巴胺激动剂阿扑吗啡可降低GTH对福司可林、8Br-cAMP和鲑鱼促性腺激素释放激素（sGnRH）的反应。能提高cAMP水平的药物模拟GnRH对GTH释放作用的能力表明，cAMP可能介导金鱼中GnRH刺激的GTH分泌；然而，进一步实验结果并未证实这一可能性。在2小时的静态孵育研究中，与福司可林、IBMX和8Br-cAMP共同孵育可增强GTH对sGnRH和鸡GnRH-II（cGnRH-II）的反应。这些增强作用的幅度至少是相加的，甚至可能是协同的。无论是否存在IBMX，用sGnRH和cGnRH-II处理均不影响释放到培养基中的cAMP水平。用缺钙培养基（不含钙盐并含有0.1 mM乙二醇双四乙酸）替代正常测试培养基可降低sGnRH和cGnRH-II对GTH释放的刺激，但不影响福司可林和8Br-cAMP的作用。这些结果表明，金鱼中sGnRH和cGnRH-II对短期（小于或等于2小时）GTH释放的刺激不是由cAMP介导的。在细胞柱灌流研究中还研究了sGnRH、福司可林和IBMX之间相互作用的动力学。5分钟脉冲式应用福司可林和IBMX可刺激GTH释放迅速增加；这些反应的延迟与sGnRH观察到的相似。sGnRH与福司可林或IBMX同时应用导致的GTH反应幅度更大、持续时间更长，比单独对sGnRH的反应更强。这些结果共同表明，cAMP水平升高可通过增加急性GTH释放的幅度和延长GnRH作用的持续时间来增强GTH对天然GnRHs的反应；然而，cAMP似乎并不直接参与介导GnRH对GTH释放的刺激。（摘要截选至400字）
