Taglialatela M, Di Renzo G, Annunziato L
Institute of Pharmacology, 2nd School of Medicine, University of Naples, Italy.
Mol Pharmacol. 1990 Sep;38(3):385-92.
Ca2+ entrance in central nerve endings can occur through voltage-operated Ca2+ channels and/or through the Na(+)-Ca2+ antiporter. The aim of the present study was to evaluate, in brain synaptosomes, the possible contribution of these two Ca2+ entrance pathways in the process of 45Ca2+ uptake elicited by different extracellular ionic conditions. The decrease in extracellular Na+ concentration from 145 mM to 95 mM and its concomitant substitution with complemental concentration of K+ (5-55 mM) caused an increase in 45Ca2+ uptake, whereas an equimolar concentration of choline (50 mM), although in the presence of the same Na+ concentration (95 mM), failed to stimulate 45Ca2+ uptake. Only when the extracellular Na+ concentration was further lowered from 95 mM to 0 mM and substituted with equivalent amounts of choline (50-145 mM) did a dose-dependent stimulation of 45Ca2+ uptake occur. In addition, when the lowering of the extracellular Na+ concentration from 95 mM to 0 mM was compensated for by K+ concentrations higher than 55 mM (55-150 mM), 45Ca2+ uptake was higher than that elicited by Na+ ion substitution with equimolar amounts (50-145 mM) of choline. The amount of 45Ca2+ uptake induced by 55 mM K+ did not differ either in Na(+)-preincubated or in Na(+)-depleted synaptosomes. Synaptosomal membrane potential, monitored with the potential-sensitive fluorescent dye bis-(1,3-diethyltiobarbiturate)trimethineoxonol, showed a progressive depolarization when extracellular K+ concentrations were raised from 5 to 150 mM, reaching a plateau at 55 mM extracellular K+ concentration, whereas when choline (145 mM) completely substituted for extracellular Na+ ions, synaptosomal membrane potential did not show any depolarization. Collectively, these results demonstrate that 45Ca2+ uptake induced by 55 mM K+ ions occurs selectively through voltage-operated Ca2+ channels, whereas, in choline-substituted media, starting from 70 mM choline, Ca2+ ions seemed to utilize the Na(+)-Ca2+ antiporter to penetrate into synaptosomes. In contrast, when extracellular K+ concentrations are raised above 55 mM, 45Ca2+ entrance may occur through two cumulative mechanisms, the opening of Ca2+ channels that are activated by high K(+)-induced depolarization and the activation of the Na(+)-Ca2+ antiporter, which follows the reduction of the transmembrane Na+ electrochemical gradient.(ABSTRACT TRUNCATED AT 400 WORDS)
钙离子进入中枢神经末梢可通过电压门控钙离子通道和/或钠钙反向转运体。本研究的目的是评估在脑突触体中，这两种钙离子进入途径在不同细胞外离子条件引发的45Ca2+摄取过程中可能发挥的作用。细胞外钠离子浓度从145 mM降至95 mM，并同时用补充浓度的钾离子（5 - 55 mM）进行替代，导致45Ca2+摄取增加，而等摩尔浓度的胆碱（50 mM），尽管存在相同的钠离子浓度（95 mM），却未能刺激45Ca2+摄取。只有当细胞外钠离子浓度进一步从95 mM降至0 mM并用等量的胆碱（50 - 145 mM）进行替代时，才会出现45Ca2+摄取的剂量依赖性刺激。此外，当细胞外钠离子浓度从95 mM降至0 mM由高于55 mM（55 - 150 mM）的钾离子浓度进行补偿时，45Ca2+摄取高于用等摩尔量（50 - 145 mM）胆碱替代钠离子所引发的摄取。55 mM钾离子诱导的45Ca2+摄取量在预先用钠离子孵育的突触体和钠离子耗尽的突触体中并无差异。用电压敏感荧光染料双（1,3 - 二乙基硫代巴比妥酸）三甲苯并恶唑监测突触体膜电位，当细胞外钾离子浓度从5 mM升高至150 mM时，膜电位呈逐渐去极化，在细胞外钾离子浓度为55 mM时达到平台期，而当胆碱（145 mM）完全替代细胞外钠离子时，突触体膜电位未出现任何去极化。总体而言，这些结果表明，55 mM钾离子诱导的45Ca2+摄取选择性地通过电压门控钙离子通道发生，而在胆碱替代培养基中，从70 mM胆碱开始，钙离子似乎利用钠钙反向转运体进入突触体。相反，当细胞外钾离子浓度升高至55 mM以上时，45Ca2+进入可能通过两种累积机制发生，即由高钾诱导的去极化激活的钙离子通道开放以及随着跨膜钠电化学梯度降低而激活的钠钙反向转运体。（摘要截于400字）
