Penner R, Neher E
Max-Planck-Institut für Biophysikalische Chemie, Göttingen, FRG.
J Exp Biol. 1988 Sep;139:329-45. doi: 10.1242/jeb.139.1.329.
Secretion of vesicular contents by exocytosis is a common feature of excitable (neurones, chromaffin cells, beta cells) and non-excitable cells (platelets, neutrophils, mast cells). The simplistic view that the universal mechanism controlling secretion is elevation of [Ca2+]i--whatever the source of this second messenger may be--is no longer tenable in view of recent reports demonstrating secretion at basal or even reduced [Ca2+]i. It is nevertheless clear that in excitable cells an increase in [Ca2+]i is the triggering event that induces secretion. In non-excitable cells, secretion is presumably triggered by other second messengers, although [Ca2+]i appears to act as an important modulator of the rate of secretion. Conversely, these second messenger systems may serve a regulatory function in excitable cells. Given the relative importance of [Ca2+]i in the regulation of cellular functions in excitable and non-excitable cells, it is not surprising that several mechanisms are expressed in these cells to regulate intracellular calcium concentration. The major pathway for Ca2+ in excitable cells is by voltage-activated Ca2+ channels, but release of Ca2+ from intracellular stores, via second messengers, predominates in non-excitable cells, and may also be important in excitable cells. In addition, receptor-operated channels and second messenger-gated conductances may prove to be important. All of these pathways are subject to regulation by a variety of interactive second messenger systems, which provide necessary tuning for an appropriate control of intracellular calcium level.
通过胞吐作用分泌囊泡内容物是可兴奋细胞（神经元、嗜铬细胞、β细胞）和非可兴奋细胞（血小板、中性粒细胞、肥大细胞）的共同特征。那种认为控制分泌的普遍机制是细胞内钙离子浓度（[Ca2+]i）升高——无论这种第二信使的来源是什么——的简单观点，鉴于最近有报道表明在基础[Ca2+]i甚至降低的情况下也会发生分泌，已不再站得住脚。然而，很明显在可兴奋细胞中，[Ca2+]i的增加是诱导分泌的触发事件。在非可兴奋细胞中，分泌大概是由其他第二信使触发的，尽管[Ca2+]i似乎是分泌速率的重要调节因子。相反，这些第二信使系统可能在可兴奋细胞中发挥调节功能。鉴于[Ca2+]i在可兴奋细胞和非可兴奋细胞的细胞功能调节中的相对重要性，这些细胞中表达多种调节细胞内钙浓度的机制就不足为奇了。可兴奋细胞中钙离子的主要途径是通过电压激活的钙离子通道，但通过第二信使从细胞内储存库释放钙离子在非可兴奋细胞中占主导地位，在可兴奋细胞中也可能很重要。此外，受体操纵通道和第二信使门控电导可能也很重要。所有这些途径都受到各种相互作用的第二信使系统的调节，这些系统为适当控制细胞内钙水平提供了必要的调节。
