Hyde Gregory N, Seale Andre P, Grau E Gordon, Borski Russell J
Department of Zoology, North Carolina State University, Box 7617, Raleigh, NC 27695-7617, USA.
Am J Physiol Endocrinol Metab. 2004 Apr;286(4):E626-33. doi: 10.1152/ajpendo.00088.2003. Epub 2003 Dec 2.
Cortisol was previously shown to rapidly (10-20 min) reduce the release of prolactin (PRL) from pituitary glands of tilapia (Oreochromis mossambicus). This inhibition of PRL release by cortisol is accompanied by rapid reductions in (45)Ca(2+) and cAMP accumulation. Cortisol's early actions occur through a protein synthesis-independent pathway and are mimicked by a membrane-impermeable analog. The signaling pathway that mediates rapid, nongenomic membrane effects of glucocorticoids is poorly understood. Using the advantageous characteristics of the teleost pituitary gland from which a nearly pure population of PRL cells can be isolated and incubated in defined medium, we examined whether cortisol rapidly reduces intracellular free calcium (Ca(i)(2+)) and suppresses L-type voltage-gated ion channel activity in events that lead to reduced PRL release. Microspectrofluorometry, used in combination with the Ca(2+)-sensitive dye fura 2 revealed that cortisol reversibly reduces basal and hyposmotically induced Ca(i)(2+) within seconds (P < 0.001) in dispersed pituitary cells. Somatostatin, a peptide known to inhibit PRL release through a membrane receptor-coupled mechanism, similarly reduces Ca(i)(2+). Under depolarizing [K(+)], the L-type calcium channel agonist BAY K 8644, a factor known to delay the closing of L-type Ca(2+) channels, stimulates PRL release in a concentration-dependent fashion (P < 0.01). Cortisol (and somatostatin) blocks BAY K 8644-induced PRL release (P < 0.01; 30 min), well within the time course over which its actions occur, independent of protein synthesis and at the level of the plasma membrane. Results indicate that cortisol inhibits tilapia PRL release through rapid reductions in Ca(i)(2+) that likely involve an attenuation of Ca(2+) entry through L-type voltage-gated Ca(2+) channels. These results provide further evidence that glucocorticoids rapidly modulate hormone secretion via a membrane-associated mechanism similar to that observed with the fast effects of peptides and neurotransmitters.
先前的研究表明，皮质醇可迅速（10 - 20分钟）减少罗非鱼（莫桑比克罗非鱼）垂体中催乳素（PRL）的释放。皮质醇对PRL释放的这种抑制作用伴随着（45）Ca（2+）和cAMP积累的迅速减少。皮质醇的早期作用通过不依赖蛋白质合成的途径发生，并且可被一种不能透过细胞膜的类似物模拟。介导糖皮质激素快速、非基因组膜效应的信号通路尚不清楚。利用硬骨鱼垂体的有利特性，从中可以分离出几乎纯的PRL细胞群体并在特定培养基中培养，我们研究了皮质醇是否能迅速降低细胞内游离钙（Ca(i)(2+)）并抑制L型电压门控离子通道活性，从而导致PRL释放减少。与Ca(2+)敏感染料fura 2结合使用的显微分光荧光测定法显示，皮质醇在数秒内可逆地降低分散垂体细胞中的基础和低渗诱导的Ca(i)(2+)（P < 0.001）。生长抑素是一种已知通过膜受体偶联机制抑制PRL释放的肽，同样会降低Ca(i)(2+)。在去极化的[K(+)]条件下，L型钙通道激动剂BAY K 8644是一种已知可延迟L型Ca(2+)通道关闭的因子，它以浓度依赖的方式刺激PRL释放（P < 0.01）。皮质醇（和生长抑素）阻断BAY K 8644诱导的PRL释放（P < 0.01；30分钟），这完全在其作用发生的时间范围内，与蛋白质合成无关且发生在质膜水平。结果表明，皮质醇通过迅速降低Ca(i)(2+)来抑制罗非鱼PRL释放，这可能涉及通过L型电压门控Ca(2+)通道减少Ca(2+)内流。这些结果进一步证明，糖皮质激素通过类似于肽和神经递质快速作用所观察到的膜相关机制来快速调节激素分泌。
