Suchard S J, Lattanzio F A, Rubin R W, Pressman B C
J Cell Biol. 1982 Sep;94(3):531-9. doi: 10.1083/jcb.94.3.531.
The significance of intracellular Na+ concentration in catecholamine secretion of cultured bovine adrenal chromaffin cells was investigated using the monovalent carboxylic ionophore monensin. This ionophore, which is known to mediate a one-for-one exchange of intracellular K+ for extracellular Na+, induces a slow, prolonged release of catecholamines which, at 6 h, amounts of 75-90% of the total catecholamines; carbachol induces a rapid pulse of catecholamine secretion of 25-35%. Although secretory granule numbers appear to be qualitatively reduced after carbachol, multiple carbachol, or Ba2+ stimulation, overall granule distribution remains similar to that in untreated cells. Monensin-stimulated catecholamine release requires extracellular Na+ but not Ca2+ whereas carbachol-stimulated catecholamine release requires extracellular Ca2+ and is partially dependent on extracellular Na+. Despite its high selectivity for monovalent ions, monensin is considerably more effective in promoting catecholamine secretion than the divalent ionophores, A23187 and ionomycin, which mediate a more direct entry of extracellular Ca2+ into the cell. We propose that the monensin-stimulated increase in intracellular Na+ levels causes an increase in the availability of intracellular Ca2+ which, in turn, stimulates exocytosis. This hypothesis is supported by the comparable stimulation of catecholamine release by ouabain which inhibits the outwardly directed Na+ pump and thus permits intracellular Na+ to accumulate. The relative magnitudes of the secretion elicited by monensin, carbachol, and the calcium ionophores, are most consistent with the hypothesis that, under normal physiological conditions, Na+ acts by decreasing the propensity of Ca2+-sequestering sites to bind the Ca2+ that enters the cell as a result of acetylcholine stimulation.
利用单价羧酸离子载体莫能菌素，研究了细胞内钠离子浓度在培养的牛肾上腺嗜铬细胞儿茶酚胺分泌中的意义。已知这种离子载体介导细胞内钾离子与细胞外钠离子一对一交换，可诱导儿茶酚胺缓慢、持续释放，6小时时释放量占儿茶酚胺总量的75 - 90%；卡巴胆碱诱导儿茶酚胺快速分泌脉冲，释放量为25 - 35%。虽然卡巴胆碱、多次卡巴胆碱或钡离子刺激后分泌颗粒数量在质量上似乎减少，但总体颗粒分布仍与未处理细胞相似。莫能菌素刺激的儿茶酚胺释放需要细胞外钠离子但不需要钙离子，而卡巴胆碱刺激的儿茶酚胺释放需要细胞外钙离子且部分依赖于细胞外钠离子。尽管莫能菌素对单价离子具有高度选择性，但它在促进儿茶酚胺分泌方面比二价离子载体A23187和离子霉素更有效，后两者介导细胞外钙离子更直接地进入细胞。我们提出，莫能菌素刺激导致细胞内钠离子水平升高，进而使细胞内钙离子可用性增加，从而刺激胞吐作用。哇巴因抑制外向钠离子泵，从而使细胞内钠离子积累，它对儿茶酚胺释放的类似刺激支持了这一假说。莫能菌素、卡巴胆碱和钙离子载体引发的分泌相对幅度，与以下假说最为一致：在正常生理条件下，钠离子通过降低钙离子结合位点结合因乙酰胆碱刺激进入细胞的钙离子的倾向来发挥作用。
