Gesek F A, Friedman P A
Department of Pharmacology and Toxicology, Dartmouth Medical School, Hanover, New Hampshire 03755-3835.
Am J Physiol. 1995 Jan;268(1 Pt 2):F89-98. doi: 10.1152/ajprenal.1995.268.1.F89.
Sodium transport across apical membranes of distal convoluted tubules is thought to be mediated by Na-Cl cotransport and conductive Na entry. Immortalized mouse distal convoluted tubule cells were used to characterize Na entry pathways. Chlorothiazide, an inhibitor of Na-Cl cotransport, and amiloride, which blocks epithelial Na channels, reduced ouabain-suppressible oxygen consumption by 40 and 35%, respectively. In simple buffer solutions, free of bicarbonate, phosphate, or formate, chlorothiazide inhibited Na uptake by 44% and Cl uptake by 48%. Michaelis constants of 21 mM for Na and 14 mM for chloride were calculated. Amiloride inhibited Na uptake by 49% and had no effect on Cl uptake. The calculated Hill coefficient of 1.07 and the equivalence of chlorothiazide-sensitive Na and Cl uptake are consistent with the presence of Na-Cl cotransport. Na-Cl cotransport and amiloride-sensitive Na influx account for 85% of Na entry in distal convoluted tubule cells in the absence of phosphate and formate. The selective Na/H exchange inhibitor ethylisopropyl amiloride had no effect on Na uptake; however, it abolished formate-stimulated Na uptake. The anion exchange blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) inhibited Na uptake. These findings are consistent with parallel Na/H and Cl/formate exchange. Na uptake was inhibited 8% by the selective Na/Ca exchange inhibitor, dimethylbenzamil. An additional 7% of Na entry was phosphate dependent and was abolished by phosphonoformic acid, a competitive inhibitor of Na-Pi cotransport. In summary, the majority of Na entry into distal convoluted tubule cells occurs through Na-Cl cotransport and an amiloride-sensitive pathway (75% in presence of phosphate and formate). An additional 13% may enter by Na/H exchange, with the remainder mediated by Na/Ca exchange and Na-Pi cotransport.
远端曲小管顶端膜上的钠转运被认为是由Na-Cl共转运和钠的传导性内流介导的。永生化小鼠远端曲小管细胞被用于表征钠的内流途径。氯噻嗪是一种Na-Cl共转运抑制剂，而氨氯地平可阻断上皮钠通道，它们分别使哇巴因抑制的氧消耗降低了40%和35%。在不含碳酸氢盐、磷酸盐或甲酸盐的简单缓冲溶液中，氯噻嗪抑制钠摄取44%，抑制氯摄取48%。计算得出钠的米氏常数为21 mM，氯的米氏常数为14 mM。氨氯地平抑制钠摄取49%，对氯摄取无影响。计算得出的希尔系数为1.07，且氯噻嗪敏感的钠和氯摄取相当，这与Na-Cl共转运的存在一致。在没有磷酸盐和甲酸盐的情况下，Na-Cl共转运和氨氯地平敏感的钠内流占远端曲小管细胞钠内流的85%。选择性Na/H交换抑制剂乙基异丙基氨氯地平对钠摄取无影响；然而，它消除了甲酸盐刺激的钠摄取。阴离子交换阻滞剂4,4'-二异硫氰基芪-2,2'-二磺酸（DIDS）抑制钠摄取。这些发现与平行的Na/H和Cl/甲酸盐交换一致。选择性Na/Ca交换抑制剂二甲基苯氨氯地平抑制钠摄取8%。另外7%的钠内流依赖于磷酸盐，并且被磷甲酸（一种Na-Pi共转运的竞争性抑制剂）消除。总之，进入远端曲小管细胞的大部分钠是通过Na-Cl共转运和氨氯地平敏感途径（在有磷酸盐和甲酸盐存在时为75%）。另外13%可能通过Na/H交换进入，其余部分由Na/Ca交换和Na-Pi共转运介导。
