Stoichiometry of Na+-HCO-3 cotransport in basolateral membrane vesicles isolated from rabbit renal cortex.
Stoichiometry of Na+-HCO-3 cotransport in basolateral membrane vesicles isolated from rabbit renal cortex.
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DOI：
10.1172/jci112948
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发表时间：
1987-04
期刊：
影响因子：
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通讯作者：
M. Soleimani;S. M. Grassi;P. Aronson
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文献类型：
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作者：
M. Soleimani;S. M. Grassi;P. Aronson
HCO 3-跨近曲小管细胞基底外侧膜转运的主要途径是产电Na+-HCO 3-共转运。在这项研究中，我们已经确定了化学计量的Na+-HCO 3-cotransporter系统的基底外侧膜囊泡，从兔肾皮质Percoll梯度离心分离。当膜电位近似为K+的能斯特电位时，如在存在K+离子载体缬氨霉素的情况下，平衡热力学预测当（Na）i/（Na）o = [（HCO 3）o/（HCO 3）i]n[（K）o/（K）i]n-1时，Na+-HCO 3-共转运系统应达到平衡，并且不介导净通量，其中n是HCO 3-：Na+的化学计量。我们的实验方法是施加跨膜Na+，HCO 3-，和K+梯度的大小和方向不同，然后在随后的3秒内测量Na+的净流量。这样，我们就可以确定输运系统达到平衡的条件，从而计算出n。这些实验的结果表明，n的值大于2.6且小于3.5，与3 HCO 3-：1 Na+的化学计量或化学当量过程一致。基于报告的细胞内电位和离子活性，化学计量的该值表明，内部负膜电位足以驱动HCO 3-排出，对抗在生理条件下存在于完整近端小管细胞的基底外侧膜上的HCO 3-和Na+的向内浓度梯度。
The major pathway for HCO3- transport across the basolateral membrane of the proximal tubule cell is electrogenic Na+-HCO3- cotransport. In this study, we have determined the stoichiometry of the Na+-HCO3- cotransport system in basolateral membrane vesicles that were isolated from rabbit renal cortex by Percoll gradient centrifugation. When the membrane potential is approximated by the Nernst potential for K+, as in the presence of the K+ ionophore valinomycin, equilibrium thermodynamics predicts that the Na+-HCO3- cotransport system should come to equilibrium and mediate no net flux when (Na)i/(Na)o = [(HCO3)o/(HCO3)i]n[(K)o/(K)i]n-1, where n is the HCO3-:Na+ stoichiometry. Our experimental approach was to impose transmembrane Na+, HCO3-, and K+ gradients of varying magnitude and direction, and then to measure the net flux of Na+ over the subsequent 3-s period. In this way, we could determine the conditions for equilibrium of the transport system and thereby calculate n. The results of these experiments indicate that the value of n is greater than 2.6 and less than 3.5, consistent with a stoichiometry of 3 HCO3-:1 Na+, or a thermodynamically equivalent process. Based on reported intracellular potentials and ion activities, this value for the stoichiometry indicates that the inside-negative membrane potential is sufficient to drive HCO3- exit against the inward concentration gradients of HCO3- and Na+ that are present across the basolateral membrane of the intact proximal tubule cell under physiologic conditions.