Tomić Melanija, Van Goor Fredrick, He Mu-Lan, Zivadinovic Dragoslava, Stojilkovic Stanko S
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-4510, USA.
Mol Pharmacol. 2002 Jun;61(6):1329-39. doi: 10.1124/mol.61.6.1329.
In excitable cells, receptor-induced Ca(2+) release from intracellular stores is usually accompanied by sustained depolarization of cells and facilitated voltage-gated Ca(2+) influx (VGCI). In quiescent pituitary lactotrophs, however, endothelin-1 (ET-1) induced rapid Ca(2+) release without triggering Ca(2+) influx. Furthermore, in spontaneously firing and depolarized lactotrophs, the Ca(2+)-mobilizing action of ET-1 was followed by inhibition of spontaneous VGCI caused by prolonged cell hyperpolarization and abolition of action potential-driven Ca(2+) influx. Agonist-induced depolarization of cells and enhancement of VGCI upon Ca(2+) mobilization was established in both quiescent and firing lactotrophs treated overnight with pertussis toxin (PTX). Activation of adenylyl cyclase by forskolin and addition of cell-permeable 8-bromo-cAMP did not affect ET-1-induced sustained inhibition of VGCI, suggesting that the cAMP-protein kinase A signaling pathway does not mediate the inhibitory action of ET-1 on VGCI. Consistent with the role of PTX-sensitive K(+) channels in ET-1-induced hyperpolarization of control cells, but not PTX-treated cells, ET-1 decreased the cell input resistance and activated a 5 mM Cs(+)-sensitive K(+) current. In the presence of Cs(+), ET-1 stimulated VGCI in a manner comparable with that observed in PTX-treated cells, whereas E-4031, a specific blocker of ether-a-go-go-related gene-like K(+) channels, was ineffective. Similar effects of PTX and Cs(+) were also observed in GH(3) immortalized cells transiently expressing ET(A) receptors. These results indicate that signaling of ET(A) receptors through the G(i/o) pathway in lactotrophs and the subsequent activation of inward rectifier K(+) channels provide an effective and adenylyl cyclase-independent mechanism for a prolonged uncoupling of Ca(2+) mobilization and influx pathways.
在可兴奋细胞中，受体诱导的细胞内钙库释放Ca(2+)通常伴随着细胞的持续去极化以及电压门控Ca(2+)内流（VGCI）的增强。然而，在静止的垂体催乳素细胞中，内皮素-1（ET-1）诱导快速Ca(2+)释放，却不触发Ca(2+)内流。此外，在自发放电和去极化的催乳素细胞中，ET-1的Ca(2+)动员作用之后是自发VGCI的抑制，这是由细胞长时间超极化以及动作电位驱动的Ca(2+)内流的消除所导致的。在用百日咳毒素（PTX）处理过夜的静止和放电的催乳素细胞中，均证实了激动剂诱导的细胞去极化以及Ca(2+)动员时VGCI的增强。福斯可林激活腺苷酸环化酶以及添加细胞可渗透的8-溴-cAMP并不影响ET-1诱导的对VGCI的持续抑制，这表明cAMP-蛋白激酶A信号通路并不介导ET-1对VGCI的抑制作用。与PTX敏感的K(+)通道在ET-1诱导对照细胞超极化中的作用一致，而不是PTX处理的细胞，ET-1降低了细胞输入电阻并激活了一种5 mM Cs(+)敏感的K(+)电流。在存在Cs(+)的情况下，ET-1以与PTX处理细胞中观察到的方式类似的方式刺激VGCI，而醚-去极化相关基因样K(+)通道的特异性阻滞剂E-4031则无效。在瞬时表达ET(A)受体的GH(3)永生化细胞中也观察到了PTX和Cs(+)的类似作用。这些结果表明，催乳素细胞中ET(A)受体通过G(i/o)途径的信号传导以及随后内向整流K(+)通道的激活提供了一种有效且不依赖腺苷酸环化酶的机制，用于长时间解耦联Ca(2+)动员和内流途径。
