Laprade R, Lapointe J Y, Breton S, Duplain M, Cardinal J
Groupe de recherche en transport membranaire, Université de Montréal, Quebec, Canada.
J Membr Biol. 1991 May;121(3):249-59. doi: 10.1007/BF01951558.
Intracellular potassium activity (alpha Ki) was measured in control conditions in mid-cortical rabbit proximal convoluted tubule using two methods: (i) by determination of the K+ equilibrium potential (EK) using Ba(2+)-induced variations in the basolateral membrane potential (VBL) during transepithelial current injections and (ii) with double-barrel K-selective microelectrodes. Using the first method, the mean VBL was -48.5 +/- 3.2 mV (n = 16) and the mean EK was -78.4 +/- 4.1 mV corresponding to alpha Ki of 68.7 mM. With K-selective microelectrodes, VBL was -36.6 +/- 1.1 mV (n = 19), EK was -64.0 +/- 1.1 mV and alpha Ki averaged 40.6 +/- 1.7 mM. While these last EK and VBL values are significantly lower than the corresponding values obtained with the first method (P less than 0.001 and P less than 0.01, respectively), the electrochemical driving force for K transport across the basolateral membrane (microK = VBL-EK) is not significantly different for both techniques (30.1 +/- 3.3 mV for the first technique and 27.6 +/- 1.8 mV for ion-selective electrodes). This suggests an adequate functioning of the selective barrel but an underestimation of VBL by the reference barrel of the double-barrel microelectrode. Such double-barrel microelectrodes were used to measure temporal changes in alpha Ki and microK in different experimental conditions where Na reabsorption rate (JNa) was reduced. alpha Ki was shown to increase by 12.2 +/- 2.7 (n = 5) and 14.1 +/- 4.4 mM (n = 5), respectively, when JNa was reduced by omitting in the luminal perfusate: (i) 5.5 mM glucose and 6 mM alanine and (ii) glucose, alanine, other Na-cotransported solutes and 110 mM Na. In terms of the electrochemical driving force for K exit across the basolateral membrane, microK, a decrease of 5.4 +/- 2.0 mV (P less than 0.05, n = 5) was measured when glucose and alanine were omitted in the luminal perfusate while microK remained unchanged when JNa was more severely reduced (mean change = -1.7 +/- 2.1 mV, NS, n = 5). In the latter case, this means that the electrochemical driving force for K efflux across the basolateral membrane has not changed while both the active influx through the Na-K pump and the passive efflux in steady state are certainly reduced. If the main pathway for K transport is through the basolateral K conductance, this implies that this conductance must have decreased in the same proportion as that of the reduction in the Na-K pump activity.
采用两种方法在对照条件下测量了兔肾皮质中部近端小管细胞内的钾活性（αKi）：（i）在跨上皮电流注入过程中，利用Ba(2+)诱导的基底外侧膜电位（VBL）变化来测定K+平衡电位（EK）；（ii）使用双管K+选择性微电极。使用第一种方法时，平均VBL为-48.5±3.2 mV（n = 16），平均EK为-78.4±4.1 mV，对应的αKi为68.7 mM。使用K+选择性微电极时，VBL为-36.6±1.1 mV（n = 19），EK为-64.0±1.1 mV，αKi平均为40.6±1.7 mM。虽然这些最后的EK和VBL值显著低于用第一种方法获得的相应值（分别为P＜0.001和P＜0.01），但两种技术中K跨基底外侧膜转运的电化学驱动力（microK = VBL - EK）并无显著差异（第一种技术为30.1±3.3 mV，离子选择性电极法为27.6±1.8 mV）。这表明选择性管功能正常，但双管微电极的参比管对VBL的估计偏低。这种双管微电极用于测量在不同实验条件下αKi和microK的时间变化，这些条件下钠重吸收率（JNa）降低。当通过在管腔灌流液中省略以下物质使JNa降低时，αKi分别增加了12.2±2.7（n = 5）和14.1±4.4 mM（n = 5）：（i）5.5 mM葡萄糖和6 mM丙氨酸；（ii）葡萄糖、丙氨酸、其他钠共转运溶质和110 mM钠。就K跨基底外侧膜流出的电化学驱动力microK而言，当管腔灌流液中省略葡萄糖和丙氨酸时，测量到microK降低了5.4±2.0 mV（P＜0.05，n = 5），而当JNa更严重降低时（平均变化=-1.7±2.1 mV，无显著性差异，n = 5），microK保持不变。在后一种情况下，这意味着K跨基底外侧膜流出的电化学驱动力没有改变，而通过钠钾泵的主动流入和稳态下的被动流出肯定都减少了。如果K转运的主要途径是通过基底外侧K+电导，这意味着该电导的降低比例与钠钾泵活性的降低比例相同。
