Electrochemical profile of K and Cl ions across the proximal tubule of bullfrog kidneys: a study using double-barreled ion-sensitive microelectrodes.

Micropuncture study was performed in the proximal segment of bullfrog nephrons with double-barreled ion-sensitive microelectrodes to determine the electrochemical profile of K and Cl across the individual borders of tubular epithelium. The mean peritubular potential obtained was -68.4 mV, and the K activity in the plasma and the cell interior was 2.64 and 61.6 mEq/liter, respectively. The calculated equilibrium potential for K was 79.3 mV. Therefore, the cell K must be maintained by some K uptake mechanism against an electrochemical gradient of 11 mV. Further, the K activity of the tubular fluid was 2.92 mEq/liter and the actually measured PD across the brush border membrane was -55.7 mV. Since the calculated Nernst K potential is 76.8 mV, the K entry from lumen to cell must be done against an electrochemical gradient of 21.1 mV in the net. These facts might suggest a possibility of bilateral existence of K uptake mechanism in both the peritubular and brush border membranes of proximal tubular cells. In contrast, the Cl activity was measured to be 9.30, 78.0, and 72.5 mEq/liter for the cell, tubular fluid, and plasma, respectively. The Nernst Cl potentials calculated were 53.6 and 51.7 mV for the brush border and the peritubular membrane, the data being less than the actually determined values, 55.7 and 68.4 mV, respectively. Thus, the Cl ion seemed to distribute passively throughout its reabsorptive process in the bullfrog proximal tubule.