Currie K P, Fox A P
Department of Neurobiology, Pharmacology and Physiology, The University of Chicago, Chicago, Illinois 60637, USA.
J Neurophysiol. 2000 Mar;83(3):1435-42. doi: 10.1152/jn.2000.83.3.1435.
Histamine is a known secretagogue in adrenal chromaffin cells. Activation of G-protein linked H(1) receptors stimulates phospholipase C, which generates inositol trisphosphate leading to release of intracellular calcium stores and stimulation of calcium influx through store operated and other channels. This calcium leads to the release of catecholamines. In chromaffin cells, the main physiological trigger for catecholamine release is calcium influx through voltage-gated calcium channels (I(Ca)). Therefore, these channels are important targets for the regulation of secretion. In particular N- and P/Q-type I(Ca) are subject to inhibition by transmitter/hormone receptor activation of heterotrimeric G-proteins. However, the direct effect of histamine on I(Ca) in chromaffin cells is unknown. This paper reports that histamine inhibited I(Ca) in cultured bovine adrenal chromaffin cells and this response was blocked by the H(1) antagonist mepyramine. With high levels of calcium buffering in the patch pipette solution (10 mM EGTA), histamine slowed the activation kinetics and inhibited the amplitude of I(Ca). A conditioning prepulse to +100 mV reversed the kinetic slowing and partially relieved the inhibition. These features are characteristic of a membrane delimited, voltage-dependent pathway which is thought to involve direct binding of G-protein betagamma subunits to the Ca channels. However, unlike virtually every other example of this type of inhibition, the response to histamine was not blocked by pretreating the cells with pertussis toxin (PTX). The voltage-dependent, PTX insensitive inhibition produced by histamine was modest compared with the PTX sensitive inhibition produced by ATP (28% vs. 53%). When histamine and ATP were applied concomitantly there was no additivity of the inhibition beyond that produced by ATP alone (even though the agonists appear to activate distinct G-proteins) suggesting that the inhibition produced by ATP is maximal. When experiments were carried out under conditions of low levels of calcium buffering in the patch pipette solution (0.1 mM EGTA), histamine inhibited I(Ca) in some cells using an entirely voltage insensitive pathway. We demonstrate that activation of PTX insensitive G-proteins (most likely Gq) by H(1) receptors inhibits I(Ca). This may represent a mechanism by which histamine exerts inhibitory (in addition to previously identified stimulatory) effects on catecholamine release.
组胺是肾上腺嗜铬细胞中一种已知的促分泌素。G蛋白偶联的H(1)受体激活会刺激磷脂酶C，后者生成三磷酸肌醇，导致细胞内钙库释放，并通过储存操纵性通道和其他通道刺激钙内流。这种钙会导致儿茶酚胺释放。在嗜铬细胞中，儿茶酚胺释放的主要生理触发因素是通过电压门控钙通道（I(Ca)）的钙内流。因此，这些通道是调节分泌的重要靶点。特别是N型和P/Q型I(Ca)会受到异三聚体G蛋白的递质/激素受体激活的抑制。然而，组胺对嗜铬细胞中I(Ca)的直接作用尚不清楚。本文报道组胺抑制培养的牛肾上腺嗜铬细胞中的I(Ca)，且这种反应被H(1)拮抗剂美吡拉敏阻断。在膜片钳微管溶液中进行高浓度钙缓冲（10 mM乙二醇双乙醚二胺四乙酸）时，组胺减缓了I(Ca)的激活动力学并抑制其幅度。一个+100 mV的预处理脉冲逆转了动力学减缓并部分缓解了抑制作用。这些特征是一种膜限定的、电压依赖性途径的特征，该途径被认为涉及G蛋白βγ亚基与钙通道的直接结合。然而，与几乎所有其他这种类型抑制的例子不同，用百日咳毒素（PTX）预处理细胞并没有阻断对组胺的反应。与ATP产生的PTX敏感性抑制相比（28%对53%），组胺产生的电压依赖性、PTX不敏感性抑制作用较小。当同时应用组胺和ATP时，抑制作用没有超过单独ATP产生的抑制作用的叠加（尽管这两种激动剂似乎激活不同的G蛋白），这表明ATP产生的抑制作用是最大的。当在膜片钳微管溶液中低浓度钙缓冲（0.1 mM乙二醇双乙醚二胺四乙酸）的条件下进行实验时，组胺在一些细胞中通过完全电压不敏感的途径抑制I(Ca)。我们证明H(1)受体激活PTX不敏感的G蛋白（最可能是Gq）会抑制I(Ca)。这可能代表了组胺对儿茶酚胺释放发挥抑制作用（除了先前确定的刺激作用）的一种机制。
