Apical membrane permeability and kinetic properties of the sodium pump in rabbit urinary bladder.
Apical membrane permeability and kinetic properties of the sodium pump in rabbit urinary bladder.
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兔膀胱钠泵顶膜通透性和动力学特性。
DOI：
10.1113/jphysiol.1983.sp014799
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发表时间：
1983
期刊：
影响因子：
--
通讯作者：
Wills,NK
中科院分区：
文献类型：
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作者：
Lewis,SA;Wills,NK
先前的研究表明，醛固酮通过增加顶端膜对Na+的渗透性来刺激Na+穿过兔膀胱上皮的转运速率。有趣的是，离子敏感和常规微电极测量表明，醛固酮基本上不改变细胞内Na+活性aiNa+，即，无论Na+转运速率如何，aiNa+都是恒定的。本研究旨在解决这一明显的矛盾。使用Ussing型室和细胞内常规和离子敏感微电极，在体外研究了低Na+饮食（刘易斯& Diamond，1976）产生的升高的内源性醛固酮水平对膀胱Na+转运的影响。顶膜的选择性和动力学的Na+泵进行了评估作为激素刺激的函数。醛固酮刺激的Na+转运增加可通过顶端膜对Na+的相对选择性渗透性增加及其绝对Na+渗透性增加来解释。通过向细胞加载Na+（用Na+敏感微电极监测）或通过操纵Serbine溶液K+浓度并测量基底外侧膜电动势和电阻的变化，以电学方式评价Na+泵的动力学。根据这些测量，计算由泵产生的电流作为细胞内Na+或细胞外K+的函数。泵的动力学不被醛固酮改变。高度合作结合的模型估计Na+的Km为14.2 mM，K+的Km为2.3 mM。这些离子的希尔系数分别为2.8和1.8，与3 Na+至2 K+的泵化学计量一致。Na-K泵的动力学性质表明，aiNa+的生理水平处于阶跃动力学曲线的底部，该阶跃动力学曲线在能量上有利于Na+挤出。
Previous studies have shown that aldosterone stimulates the rate of Na+ transport across the rabbit urinary bladder epithelium by increasing the apical membrane permeability to Na+. Paradoxically, ion‐sensitive and conventional micro‐electrode measurements demonstrated that intracellular Na+ activity aiNa+ was essentially unchanged by aldosterone, i.e. aiNa+ was constant regardless of the rate of Na+ transport. The present study was designed to resolve this apparent contradiction. The effects of elevated, endogenous aldosterone levels produced by low‐Na+ diet (Lewis & Diamond, 1976) on urinary bladder Na+ transport were investigated in vitro using Ussing‐type chambers and intracellular conventional and ion‐sensitive microelectrodes. Apical membrane selectivity and kinetics of the Na+ pump were assessed as a function of hormone stimulation. The aldosterone‐stimulated increase in Na+ transport was accounted for by increases in both the relative selective permeability of the apical membrane to Na+ and an increase in its absolute Na+ permeability. The kinetics of the Na+ pump were evaluated electrically by loading the cells with Na+ (monitored with Na+‐sensitive micro‐electrodes) or alternatively by manipulating serosal solution K+ concentration and measuring changes in the basolateral membrane electromotive forces and resistance. From these measurements the current generated by the pump was calculated as a function of intracellular Na+ or extracellular K+. The kinetics of the pump were not altered by aldosterone. A model of highly co‐operative binding estimated Km for Na+ as 14.2 mM and 2.3 mM for K+. Hill coefficients for these ions were 2.8 and 1.8, respectively, consistent with a pump stoichiometry of 3 Na+ to 2 K+. The kinetic properties of the Na‐K pump indicate that physiological levels of aiNa+ are poised at the foot of a step kinetic curve which energetically favours Na+ extrusion.