Ouabain decreases apparent potassium-conductance in proximal tubules of the amphibian kidney

According to a previous study from this laboratory, the electrochemical gradient for potassium across the peritubular cell membrane of proximal tubules in the isolated perfused frog kidney increases following the application of ouabain. In order to test, if this phenomenon were due to a decrease of potassium conductance, the effects of ouabain on cell membrane resistances and the sensitivity of the peritubular cell membrane potential difference (PD_pt) to step changes of peritubular potassium and bicarbonate concentration were studied. In the absence of ouabain, PD_pt averaged –60±3 mV (n=25). A step increase of peritubular potassium concentration from 3 to 18 mmol/l (pH 8.07) depolarises PD_pt (PD_k) by +24±2 mV (n=8). An increase of bicarbonate from 20 to 40 mmol/l (pH 8.07) hyperpolarises PD_pt (PD_b) by –2.8±0.4 mV (n=9). The resistance of the luminal and peritubular cell membranes in parallel (R_m) amounts to 45±9 k cm (tubule length) (n=4) and the voltage divider ratio (VDR) to 1.4±0.2 (n=7). The resistance of the cellular cable (cellular core,R_c) approaches 131±37 M/cm (n=4). Peritubular application of 0.1 mmol/l ouabain leads to a gradual decline of PD_pt (t_1/2 approx. 30 min), to an increase ofR_m, a decrease of PD_k and an increase of PD_b. VDR andR_c are not changed significantly. The data point to a functional link between the sodium/potassium ATPase and the potassium conductance of the peritubular cell membrane.