Intracellular potassium activity in mammalian proximal tubule: effect of perturbations in transepithelial sodium transport.

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.