Condrescu M, Gerardi A, DiPolo R
Instituto Venezolano de Investigaciones Científicas, Centro de Biofísica y Bioquímica, Caracas, Venezuela.
Biochim Biophys Acta. 1988 Dec 22;946(2):289-98. doi: 10.1016/0005-2736(88)90404-x.
The role of intracellular Ca2+ as essential activator of the Na+-Ca2+ exchange carrier was explored in membrane vesicles containing 67% right-side-out and 10% inside-out vesicles, isolated from squid optic nerves. Vesicles containing 100 microM free calcium exhibited a 2-fold increase in the initial rate of Na+i-dependent Ca2+ uptake as compared with vesicles where intravesicular calcium was chelated by 2 mM EGTA or 10 mM HEDTA. The activatory effect exerted by intravesicular Ca2+ on the reverse mode of Na+-Ca2+ exchange (i.e. Na+i-Ca2+o exchange) is saturated at about 100 microM Ca2+i and displays an apparent K 1/2 of 12 microM. Intravesicular Ca2+ produced activation of Na+i-Ca2+i exchange activity rather than an increase in Ca2+ uptake due to Ca2+-Ca2+ exchange. The presence of Ca2+i was essential for the Na+i-dependent Na+ influx, a partial reaction of the Na+-Ca2+ exchanger. In fact, the Na+ influx levels in vesicles loaded with 2 mM EGTA were close to those expected from diffusional leak while in vesicles containing Ca2+i an additional Na+-Na+ exchange was measured. The results suggest that in nerve membrane vesicles Ca2+ at the inner aspect of the membrane acts as an activator of the Na+-Ca2+ exchange system.
在从鱿鱼视神经分离出的膜囊泡中，研究了细胞内Ca2+作为Na+-Ca2+交换载体的必需激活剂的作用。这些膜囊泡中67%为外翻囊泡，10%为内翻囊泡。与囊泡内钙被2 mM乙二醇双四乙酸（EGTA）或10 mM羟乙基亚乙基二胺四乙酸（HEDTA）螯合的囊泡相比，含有100 microM游离钙的囊泡在依赖细胞内钠离子（Na+i）的Ca2+摄取初始速率上提高了2倍。囊泡内Ca2+对Na+-Ca2+交换逆向模式（即Na+i-Ca2+o交换）的激活作用在约100 microM细胞内钙离子（Ca2+i）时达到饱和，其表观半最大激活浓度（K 1/2）为12 microM。囊泡内Ca2+引起的是Na+i-Ca2+i交换活性的激活，而非由于Ca2+-Ca2+交换导致的Ca2+摄取增加。Ca2+i的存在对于Na+-Ca2+交换器的部分反应——依赖Na+i的Na+内流至关重要。实际上，装载2 mM EGTA的囊泡中的Na+内流水平接近扩散渗漏预期的值，而在含有Ca2+i的囊泡中则检测到额外的Na+-Na+交换。结果表明，在神经膜囊泡中，膜内侧的Ca2+作为Na+-Ca2+交换系统的激活剂。
