Localization and physiological functions of SRIF receptor and GHRH receptor in the rat pituitary

本研究分析了生长抑素（SRIF）和生长激素释放激素（GHRH）对原代培养的雄性大鼠垂体细胞的影响。在垂体细胞中，我们重点研究了生长激素细胞和滤泡星状细胞，首先，我们用穿孔膜片钳技术分析了这些肽对生长激素细胞的作用，发现了以下几点。GHRH增强电压门控钠电流。此外，GHRH在电位大于-60mV时可诱发持续性钠电流，前者可被河豚毒素完全阻断，后者则不受其影响。另一方面，SRIF对电压门控性钠电流无影响，但能减弱GHRH诱导的持续性钠电流。这些结果表明：（i）GHRH通过促进电压门控钠电流增加基础GH分泌，（ii）GHRH诱导的钠电流使生长激素细胞去极化，后者激活电压门控钙电流，从而促进GH分泌;（iii）SRIF不仅通过激活内向整流钾电流而且通过减弱GHRH诱导的钠电流来抑制GH分泌。我们通过钙成像方法分析了这些肽对卵泡星状细胞的作用，GHRH影响了总检查细胞中30%的细胞内钙浓度（[Ca^2+]i）。在响应细胞中，95%的细胞通过增加[Ca^<2+>]i而响应GHRH，其余细胞通过抑制[Ca^<2+>]i而响应GHRH。这两种反应都被细胞外溶液中的钙离子所阻断。SRIF在70%的细胞中抑制GHRH诱导的反应，而不影响其余30%的细胞。这些结果表明，相当大比例的卵泡星状细胞具有GHRH和SRIF的受体，并且肽调节它们的功能。

In the present research, we analyzed the effects of somatostatin (SRIF) and growth hormone releasing hormone (GHRH) on male rat pituitary cells in primary culture. Among pituitary cells we focused somatotrophs and folliculo-stellate cells.First, we analyzed the effects of these peptides on somatotrophs by a perforated patch clamp method and revealed following things. GHRH augmented the voltage-gated sodium current. In addition, GHRH elicited persistent sodium current at the potential more positive than-60mV.The former current was completely blocked by tetrodotoxin, whereas the latter current was not affected by that. On the other hand, SRIF had no effect on the voltage-gated sodium current, while that attenuated the GHRH-induced persistent sodium current. These results indicate (i) that GHRH augments basal GH secretion by facilitating the voltage-gated sodium current ; (ii) that GHRH-induced sodium current depolarizes somatotrophs which activates the voltage-gated calcium current, whereby facilitating GH secretion ; (iii) SRIF suppresses GH secretion not only by activating the inward rectifier potassium current but also attenuating the GHRH-induced sodium current.Second, we analyzed the effects of these peptides on folliculo-stellate cells by the method of calcium-imaging, GHRH affects intracellular calcium concentration ([Ca^<2+>]i) in 30% of cells in the total examined. Among responding cells, 95% of them responded to GHRH by augmenting [Ca^<2+>]i and remaining cells by suppressing [Ca^<2+>]i. Both responses were blocked by removing calcium from extracellular solution. SRIF suppressed the GHRH-induced responses in 70% of the cells without affecting remaining 30% of the cells. These results indicate that a substantial proportion of folliculo-stellate cells possesses the receptors for GHRH and SRIF and that the peptides regulate their function.