The effect of ouabain on intracellular activities of K⁺, Na⁺, Cl^-, H⁺ and Ca²⁺ in proximal tubules of frog kidneys

Using conventional and ion selective microelectrodes, the effect of ouabain (10^-4 mol/l) on peritubular cell membrane potential (PD_pt), on intracellular pH (pH_i) as well as on the intracellular ion activities of Cl^- (Cl_i^-), K⁺ (K_i⁺), Na⁺ (Na_i⁺) and Ca²⁺ (Ca_i²⁺) was studied in proximal tubules of the isolated perfused frog kidney. In the absence of ouabain (PD_pt=-57.0±1.9 mV), the electrochemical potential difference of chloride (apparent {ie6-1} and of potassium {ie6-2} is directed from cell to bath, of H⁺ {ie6-3}, of Na⁺ {ie6-4} and of Ca²⁺ {ie6-5} from bath to cell. Ouabain leads to a gradual decline of PD_pt, which is reduced to half (PD_{pt, 1/2}) within 31±4.6 min (in presence of luminal glucose and phenylalanine), and to a decline of the absolute values of apparent {ie6-6}, of {ie6-7}, {ie6-8} and {ie6-9}. In contrast, an increase of {ei6-10} is observed. At PD_{pt, 1/2} apparent Cl_i^- increases by 6.2±1.0 mmol/l, pH_i by 0.13±0.03, Ca_i²⁺ by 185±21 nmol/l, and Na_i⁺ by 34.2±4.6 mmol/l, whereas K_i⁺ decreases by 37.7±2.2 mmol/l. The results suggest that the application of ouabain is followed by a decrease of peritubular cell membrane permeability to K⁺, by an accumulation of Ca²⁺, Na⁺ and HCO₃^- in the cell and by a dissipation of the electrochemical Cl^- gradient.