Hardie R C
Department of Anatomy, University of Cambridge, UK.
Cell Calcium. 1996 Oct;20(4):315-27. doi: 10.1016/s0143-4160(96)90037-8.
It has been suggested that excitation in Drosophila photoreceptors may be mediated by the depletion of intracellular Ca2+ stores (capacitative Ca2+ entry). To investigate this hypothesis, simultaneous whole-cell recordings and Indo-1 Ca2+ measurements were made from dissociated Drosophila photoreceptors, whilst testing the effects of Ca2+ releasing agents. In Ca2+ free Ringer's solution, thapsigargin raised cytosolic Ca2+ by approximately 80 nM; subsequent application of ionomycin released further Ca2+ (approximately 100 nM). A possible third compartment was indicated by the ability of monensin to mobilize further Ca2+ after saturating doses of ionomycin. Under most conditions, none of these agents activated an inward conductance, and their effects on the light response were consistent with their effects on cytosolic Ca2+. However, in the absence of both external Ca2+ and Mg2+ (to relieve a Mg2+ block of the light-sensitive channels), and after loading cells with BAPTA buffering cytosolic free Ca2+ at approximately 10 nM, ionomycin (but not thapsigargin) activated inward currents of approximately 800 pA. The response to ionomycin was enhanced (10 nA) by buffering cytosolic Ca2+ at 250 nM. A similar current also developed after approximately 3 min in cells loaded with Ca-BAPTA without any ionomycin application. The current-voltage relationships of currents activated by Ca-BAPTA or ionomycin were indistinguishable from that of the light-activated conductance and were similarly affected by a null mutation of the transient receptor potential (trp) gene which is believed to encode a subunit of the light-sensitive channels. These experiments provide some evidence for the suggestion that the light-activated and trp-dependent conductance in Drosophila photoreceptors can be activated by depletion of internal stores. However, activation by Ca-BAPTA and ionomycin had an absolute requirement for cytosolic Ca2+ as no currents could be activated by ionomycin in cells loaded with BAPTA and no Ca2+.
有人提出果蝇光感受器中的兴奋可能由细胞内Ca2+储存的耗尽（容量性Ca2+内流）介导。为了研究这一假设，在测试Ca2+释放剂的作用时，对解离的果蝇光感受器进行了同步全细胞记录和Indo-1 Ca2+测量。在无Ca2+的林格氏液中，毒胡萝卜素使胞质Ca2+升高约80 nM；随后应用离子霉素释放出更多Ca2+（约100 nM）。莫能菌素在饱和剂量的离子霉素后能够动员更多Ca2+，这表明可能存在第三个区室。在大多数情况下，这些试剂均未激活内向电导，并且它们对光反应的影响与其对胞质Ca2+的影响一致。然而，在既无细胞外Ca2+又无Mg2+（以解除Mg2+对光敏感通道的阻断）的情况下，在用BAPTA将胞质游离Ca2+缓冲至约10 nM后，离子霉素（而非毒胡萝卜素）激活了约800 pA的内向电流。通过将胞质Ca2+缓冲至250 nM，对离子霉素的反应增强（10 nA）。在加载Ca-BAPTA但未应用任何离子霉素的细胞中，约3分钟后也出现了类似的电流。由Ca-BAPTA或离子霉素激活的电流的电流-电压关系与光激活电导的电流-电压关系无法区分，并且同样受到瞬时受体电位（trp）基因无效突变的影响，该基因被认为编码光敏感通道的一个亚基。这些实验为果蝇光感受器中光激活和trp依赖性电导可通过内部储存的耗尽而激活这一观点提供了一些证据。然而，Ca-BAPTA和离子霉素的激活对胞质Ca2+有绝对要求，因为在加载了BAPTA且无Ca2+的细胞中，离子霉素无法激活电流。
