INTRACELLULAR POTASSIUM ACTIVITY IN MAMMALIAN PROXIMAL TUBULE - EFFECT OF PERTURBATIONS IN TRANSEPITHELIAL SODIUM-TRANSPORT

Intracellular potassium activity (alpha-K1) was measured in control conditions in mid-cortical rabbit proximal convoluted tubule using two methods: (i) by determination of the K+ equilibrium potential (E(K)) using Ba2+-induced variations in the basolateral membrane potential (V(BL)) during transepithelial current injections and (ii) with double-barrel K-selective microelectrodes. Using the first method, the mean V(BL) was -48.5 +/- 3.2 mV (n = 16) and the mean E(K) was -78.4 +/- 4.1 mV corresponding to a alpha-K1 of 68.7 mM. With K-selective microelectrodes, V(BL) was -36.6 +/- 1.1 mV (n = 19), E(K) was -64.0 +/- 1.1 mV and alpha-K1 averaged 40.6 +/- 1.7 mM. While these last E(K) and V(BL) values are significantly lower than the corresponding values obtained with the first method (P < 0.001 and P < 0.01, respectively), the electrochemical driving force for K transport across the basolateral membrane (mu-K = V(BL) - E(K)) 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 V(BL) by the reference barrel of the double-barrel microelectrode. Such double-barrel microelectrodes were used to measure temporal changes in alpha-K1 and mu-K in different experimental conditions where Na reabsorption rate (J(Na)) was reduced. Alpha-K1 was shown to increase by 12.2 +/- 2.7 (n = 5) and 14.1 +/- 4.4 mM (n = 5), respectively, when J(Na) 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, mu-K, a decrease of 5.4 +/- 2.0 mV (P < 0.05, n = 5) was measured when glucose and alanine were omitted in the luminal perfusate while mu-K remained unchanged when J(Na) 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.