Ambroz C, Fein H G, Smallridge R C
Department of Clinical Physiology, Walter Reed Army Institute of Research, Washington, DC 20307-5100.
Biochim Biophys Acta. 1990 Oct 19;1028(3):229-35. doi: 10.1016/0005-2736(90)90171-j.
Calcium is an important regulator of cell function, and may be influenced by the intracellular sodium content. In the present study, the Na(+)-ionophore, monensin, was used to investigate the interrelationship between changes in intracellular Na+ concentration ([Na+]i) and elevation of cytosolic Ca2+ concentration ([Ca2+]i) in FRTL-5 thyroid cells. Cytoplasmic Ca2+ levels were measured using the fluorescent dye, indo-1. Monensin induced a dose-dependent increase in [Ca2+]i in FRTL-5 cells. Inhibitors of intracellular Ca2+ release, TMB-8 and ryanodine, were unable to prevent the monensin effect on [Ca2+]i. The alpha 1-receptor antagonist, prazosin, did not block the monensin-stimulated increase in [Ca2+]i. In the absence of extracellular calcium there was a marked diminution in the monensin effect on [Ca2+]i, yet calcium channel antagonists (nifedipine, diltiazem and verapamil) did not inhibit the response. Replacement of Na+ by choline chloride in the medium depressed the monensin-evoked rise in [Ca2+]i by up to 84%. Furthermore, addition of the Na(+)-channel agonist, veratridine, elicited an increase in [Ca2+]i, even though less dramatic than that caused by monensin. Ouabain increased the resting cytosolic Ca2+ concentration as well as the magnitude of the monensin effect on [Ca2+]i. The absence of any effect on the Na(+)-ionophore evoked increase in [Ca2+]i upon addition of tetrodotoxin (TTX) excluded a possible involvement of TTX-sensitive Na+ channels. These data show that the rise in [Ca2+]i induced by increasing [Na+]i is largely dependent on both external Na+ and Ca2+. Calcium entry appears not to involve voltage-dependent or alpha 1-receptor sensitive Ca2+ channels, but may result from activation of an Na(+)-Ca2+ exchange system.
钙是细胞功能的重要调节因子，且可能受细胞内钠含量的影响。在本研究中，使用钠离子载体莫能菌素研究FRTL-5甲状腺细胞内钠离子浓度（[Na⁺]i）变化与胞质钙离子浓度（[Ca²⁺]i）升高之间的相互关系。使用荧光染料indo-1测量细胞质钙离子水平。莫能菌素在FRTL-5细胞中诱导[Ca²⁺]i呈剂量依赖性增加。细胞内钙离子释放抑制剂TMB-8和ryanodine无法阻止莫能菌素对[Ca²⁺]i的作用。α1受体拮抗剂哌唑嗪不能阻断莫能菌素刺激的[Ca²⁺]i增加。在无细胞外钙的情况下，莫能菌素对[Ca²⁺]i的作用显著减弱，但钙通道拮抗剂（硝苯地平、地尔硫䓬和维拉帕米）并未抑制该反应。培养基中用氯化胆碱替代Na⁺可使莫能菌素诱发的[Ca²⁺]i升高降低多达84%。此外，添加Na⁺通道激动剂藜芦碱可引起[Ca²⁺]i增加，尽管不如莫能菌素引起的明显。哇巴因增加了静息胞质钙离子浓度以及莫能菌素对[Ca²⁺]i的作用幅度。添加河豚毒素（TTX）后对钠离子载体诱发的[Ca²⁺]i增加无任何影响，排除了TTX敏感的Na⁺通道可能参与其中。这些数据表明，[Na⁺]i升高诱导的[Ca²⁺]i升高在很大程度上依赖于细胞外的Na⁺和Ca²⁺。钙离子内流似乎不涉及电压依赖性或α1受体敏感的Ca²⁺通道，而可能是由Na⁺-Ca²⁺交换系统的激活所致。
