The effect of diffusible ions on the peritubular membrane potential of proximal tubular cells in perfused bullfrog kidneys.

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Abstract
Effects of extracellular diffusible ions, such as K+, Cl- and HCO3- (pH), on the peritubular membrane potential (EM) and intracellular activities of K+, [K]i or Cl-, [Cl]i, were studied in the perfused proximal tubule of bullfrog kidneys with K+- or Cl--selective microeletrodes. In steady-state conditions, in which the peritubular and luminal sides were perfused with control Ringer solutions, the K+ equilibrium potential (EK) always exceeded the EM by .apprx. 19 mV and correlated well with the EM (correlation coefficient r = 0.78); no correlation was recognized between the equilibrium potential of Cl- (ECl) and the EM. In the quick peritubular perfusion experiments, in which the extracellular diffusible ions were changed, the [K]i and [Cl]i were maintained relatively stable. Several facts were observed: at constant EK, decreasing the peritubular chloride [Cl]e produced a small degree of hyperpolarization of the EM instead of depolarization; at constant ECl, increasing the [K]e depolarized the EM; at constant PCO2, the EM was depolarized with low HCO3- (acid) perfusions while it was hyperpolarized with high HCO3- (alkaline) perfusions. These results are in agreement with the views that intracellular K+ in the proximal tubule is maintained by an uphill uptake mechanism on the peritubular cell membrane, the ionic conductance of peritubular membrane is relatively high to K+ but low to Cl- and the pH gradient across the peritubular membrane can modulate the passive permeability to Na+ or K+.

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