Lidofsky S D, Xie M H, Scharschmidt B F
Department of Medicine, University of California, San Francisco 94143.
Am J Physiol. 1990 Jul;259(1 Pt 1):G56-61. doi: 10.1152/ajpgi.1990.259.1.G56.
Plasma membrane Na(+)-Ca2+ exchange contributes importantly to the regulation of cytosolic Ca2+ concentration ([Ca2+]i) in excitable cells. Despite extensive study in excitable tissues, the role of this transporter in the regulation of [Ca2+]i in hepatocytes is unknown, and conflicting information has been reported regarding the presence of Na(+)-Ca2+ exchange in hepatocyte plasma membrane vesicles. We have therefore assessed the role of Na(+)-dependent Ca2+ transport in the regulation of [Ca2+]i in rat hepatocytes in primary culture under basal conditions and after exposure to vasopressin, a hormone that elevates [Ca2+]i. Ca2+ efflux, measured using 45Ca, did not differ in the presence or absence of extracellular Na+, either under basal conditions or in response to vasopressin. [Ca2+]i, measured using the Ca2(+)-sensitive dye fura-2, was not altered by transient or prolonged exposure to Na(+)-free media or by exposure to ouabain in concentrations sufficient to produce a five-fold elevation in intracellular Na+ concentration. The [Ca2+]i response to vasopressin was also unaffected by Na+ removal or ouabain. By contrast, in cultured rat cardiac myocytes, cells that possess Na(+)-Ca2+ exchange, transient or prolonged Na+ removal as well as ouabain exposure produced greater than fivefold increases in [Ca2+]i compared with controls. We conclude that Na(+)-Ca2+ exchange does not contribute to the regulation of [Ca2+]i in hepatocytes.
质膜钠钙交换在可兴奋细胞胞质钙浓度（[Ca2+]i）的调节中起着重要作用。尽管在可兴奋组织方面进行了广泛研究，但该转运体在肝细胞[Ca2+]i调节中的作用尚不清楚，并且关于肝细胞质膜囊泡中钠钙交换的存在也有相互矛盾的报道。因此，我们评估了在基础条件下以及暴露于血管加压素（一种可升高[Ca2+]i的激素）后，原代培养的大鼠肝细胞中钠依赖性钙转运在[Ca2+]i调节中的作用。使用45Ca测量的钙外流，在基础条件下或对血管加压素反应时，无论细胞外钠存在与否均无差异。使用钙敏感染料fura-2测量的[Ca2+]i，短暂或长时间暴露于无钠培养基或暴露于足以使细胞内钠浓度升高五倍的哇巴因浓度时均未改变。对血管加压素的[Ca2+]i反应也不受钠去除或哇巴因的影响。相比之下，在培养的大鼠心肌细胞（具有钠钙交换的细胞）中，短暂或长时间的钠去除以及哇巴因暴露导致[Ca2+]i比对照增加了五倍以上。我们得出结论，钠钙交换对肝细胞中[Ca2+]i的调节没有作用。
