Taniguchi S, Marchetti J, Morel F
Laboratoire de Physiologie Cellulaire, UA 219 CNRS, Collège de France, Paris.
Pflugers Arch. 1989 Jun;414(2):125-33. doi: 10.1007/BF00580953.
Cytosolic free Ca2+ ([Ca2+]i) was measured in single fragments of rat cortical collecting tubule (CCT) by using fura-2 and a tubule superfusion device. Under basal conditions, i.e. with 1 mM of external Ca2+ ([Ca2+]o), the average steady state [Ca2+]i was 179 +/- 16 nM (n = 44 tubules). Random alterations of [Ca2+]o between 0 mM and 4 mM led to corresponding variations in steady state [Ca2+]i levels, which were linearly correlated with [Ca2+]o (average slope 93 +/- 34 nM [Ca2+]i per 1 mM [Ca2+]o for six tubules). In contrast, [Ca2+]i was little affected by decreasing external Na+ concentration. Cell membrane depolarization with 100 mM of external K+ induced a sustained drop in [Ca2+]i (21% as an average). The data suggest that steady state [Ca2+]i in CCT cells resulted from a non-saturable passive entry of calcium ions across cell membranes balanced with an active extrusion by calcium ATPase (pump and leak mechanism). The passive component cannot be accounted for either by Na+/Ca2+ exchangers nor by voltage-dependent calcium channels; it is best explained by the presence of voltage-independent calcium channels in cell membranes.
利用fura-2和肾小管灌流装置，在大鼠皮质集合管（CCT）的单个片段中测量胞质游离Ca2+（[Ca2+]i）。在基础条件下，即细胞外Ca2+（[Ca2+]o）浓度为1 mM时，平均稳态[Ca2+]i为179±16 nM（n = 44个肾小管）。[Ca2+]o在0 mM至4 mM之间随机变化，导致稳态[Ca2+]i水平相应变化，且与[Ca2+]o呈线性相关（6个肾小管的平均斜率为每1 mM [Ca2+]o对应93±34 nM [Ca2+]i）。相比之下，细胞外Na+浓度降低对[Ca2+]i影响较小。用100 mM细胞外K+使细胞膜去极化，可诱导[Ca2+]i持续下降（平均下降21%）。数据表明，CCT细胞中的稳态[Ca2+]i是由钙离子通过细胞膜的非饱和被动内流与钙ATP酶的主动外排（泵和漏机制）平衡所致。被动成分既不能由Na+/Ca2+交换体解释，也不能由电压依赖性钙通道解释；最好的解释是细胞膜中存在电压非依赖性钙通道。
