Growth hormone (GH)-releasing hormone (GHRH) and the GH secretagogue (GHS), L692,585, differentially modulate rat pituitary GHS receptor and GHRH receptor messenger ribonucleic acid levels.
Growth hormone (GH)-releasing hormone (GHRH) and the GH secretagogue (GHS), L692,585, differentially modulate rat pituitary GHS receptor and GHRH receptor messenger ribonucleic acid levels.
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DOI：
10.1210/endo.140.8.6918
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发表时间：
1999-08
期刊：
影响因子：
4.8
通讯作者：
R. Kineman;J. Kamegai;L. Frohman
中科院分区：
文献类型：
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作者：
R. Kineman;J. Kamegai;L. Frohman
合成的GH促分泌素（GHS）在体内引起GH最大释放的能力依赖于完整的GH释放激素（GHRH）信号系统。GHRH在GHS诱导的GH释放中的作用主要归因于GHS从下丘脑神经元释放GHRH的能力。然而，GHS也直接在垂体水平释放GH。一些证据表明，GHRH是必要的，以维持垂体反应GHS刺激GHS受体（GHS-R）的合成。为了检验这一假设，用氯胺酮/甲苯噻嗪（其不改变脉冲式GH分泌）麻醉雄性大鼠（250-290 g），并静脉内输注GHRH类似物（[des-NH 2 Tyr 1，D-Ala 15]hGRF-（1-29）-NH 2; 10 μ g/h）或盐水4 h。检测血清GH水平，收集垂体组织，RT-PCR法检测GHS-R和GHRH受体（GHRH-R）mRNA水平。GHRH输注导致循环GH浓度增加10倍，同时GHS-R mRNA水平增加至生理盐水对照组的200%（P < 0.01）。而GHRH则使GHRH-R mRNA水平略有降低，但差异不显著（P < 0.07）。GHRH对GHS-R mRNA水平的刺激作用与生长抑素张力无关，因为用生长抑素抗血清预处理并不改变GHRH输注的有效性。相反，在基础条件下，阻断生长抑素作用上调GHRH-R mRNA水平，并揭示了GHRH对其自身受体mRNA的抑制作用。这些观察结果表明生长抑素对GHRH-R mRNA具有张力性抑制作用。GHRH对GHS-R mRNA水平的刺激作用不依赖于循环GH，因为在没有免疫检测到GH的自发侏儒大鼠中输注GHRH使GHS-R mRNA水平增加到生理盐水处理对照组的150%（P < 0.05）。为了确定这种效应是否通过对垂体的直接作用而发生，将正常大鼠垂体的原代细胞培养物与GHRH（0.01-10 nM）或毛喉素（10 μ M）孵育4小时。这些GH促分泌素在体外不改变GHS-R mRNA水平。而GHRH和forskolin使GHRH-R mRNA水平降低40%（P < 0.05）。为了确定GHS-R的合成是否像GHRH-R的合成一样由其自身配体负介导，向麻醉大鼠输注非肽基促分泌素L-692，585（100 μ g/h）4小时。L-692，585对GH和GHRH-R mRNA水平均无显著影响，但GHS-R mRNA水平降低50%（P < 0.05）。总之，这些结果表明，GHRH诱导的上调垂体GHS-R的合成在体内是间接的和独立的生长抑素和GH。他们还证明，GHS-R的合成，像GHRH-R一样，可以通过其自身的配体快速下调。
The ability of synthetic GH secretagogues (GHSs) to elicit a maximal release of GH in vivo is dependent on an intact GH-releasing hormone (GHRH) signaling system. The role of GHRH in GHS-induced GH release has been attributed primarily to the ability of GHS to release GHRH from hypothalamic neurons. However, GHS also releases GH directly at the pituitary level. Several lines of evidence suggest that GHRH is necessary to maintain pituitary responsiveness to GHS by stimulating GHS receptor (GHS-R) synthesis. To test this hypothesis, male rats (250-290 g) were anesthetized with ketamine/xylazine (which does not alter pulsatile GH secretion) and infused i.v. with a GHRH analog ([des-NH2Tyr1,D-Ala15]hGRF-(1-29)-NH2; 10 microg/h) or saline for 4 h. Serum was analyzed for GH, pituitaries were collected, and GHS-R and GHRH receptor (GHRH-R) messenger RNA (mRNA) levels were determined by RT-PCR. GHRH infusion resulted in a 10-fold increase in circulating GH concentrations that were accompanied by an increase in GHS-R mRNA levels to 200% of those in saline-treated controls (P < 0.01). In contrast, GHRH reduced GHRH-R mRNA levels slightly, but not significantly (P < 0.07). The stimulatory effect of GHRH on GHS-R mRNA levels was independent of somatostatin tone, as pretreatment with somatostatin antiserum did not alter the effectiveness of GHRH infusion. In contrast, blockade of somatostatin actions up-regulated GHRH-R mRNA levels under basal conditions and unmasked the inhibitory effects GHRH on its own receptor mRNA. These observations suggest GHRH-R mRNA is tonically suppressed by somatostatin. The stimulatory effect of GHRH on GHS-R mRNA levels was independent of circulating GH, as GHRH infusion in spontaneous dwarf rats, which do not have immunodetectable GH, increased GHS-R mRNA levels to 150% of those in saline-treated controls (P < 0.05). To determine whether this effect occurred by a direct action on the pituitary, primary cell cultures from normal rat pituitaries were incubated with GHRH (0.01-10 nM) or forskolin (10 microM) for 4 h. These GH secretagogues did not alter GHS-R mRNA levels in vitro. However, GHRH and forskolin reduced GHRH-R mRNA levels by 40% (P < 0.05). To determine whether the synthesis of the GHS-R, like that of the GHRH-R, is negatively mediated by its own ligand, anesthetized rats were infused with the nonpeptidyl secretagogue, L-692,585 (100 microg/h) for 4 h. Neither circulating GH (at 4 h) nor GHRH-R mRNA levels were significantly altered by L-692,585, whereas GHS-R mRNA levels were reduced by 50% (P < 0.05). Taken together, these results indicate that GHRH-induced up-regulation of pituitary GHS-R synthesis in vivo is indirect and independent of both somatostatin and GH. They also demonstrate that GHS-R synthesis, like that of GHRH-R, can be rapidly down-regulated by its own ligand.