Basolateral membrane potential of a tight epithelium: Ionic diffusion and electrogenic pumps

Summary

The contribution of specific ions to the conductance and potential of the basolateral membrane of the rabbit urinary bladder has been studied with both conventional and ion-specific microelectrode techniques. In addition, the possibility of an electrogenic active transport process located at the basolateral membrane was studied using the polyene antibiotic nystatin. The effect of ion-specific microelectrode impalement damage on intracellular ion activities was examined and a criterion set for acceptance or rejection of intracellular activity measurements. Using this criterion, we found (K⁺)=72mm and (Cl⁻)=15.8mm. Cl⁻ but not K⁺ was in electrochemical equilibrium across the basolateral membrane. The selective permeability of the basolateral membrane was measured using microelectrodes, and the data analyzed using the Goldman, Hodgkin-Katz equation. The sodium to potassium permeability ratio (P _Na/P _K) was 0.044, and the chloride to potassium permeability ratio (P _Cl/P _K) was 1.17. Since K⁺ was not in electrochemical equilibrium, intracellular (K⁺) is maintained by active metabolic processes, and the basolateral membrane potential is a diffusion potential with K⁺ and Cl⁻ the most permeable ions. After depolarizing the basolateral membrane with high serosal potassium bathing solutions and eliminating the apical membrane as a rate limiting step for ion movement using the polyene antibiotic nystatin, we found that the addition of equal aliquots of NaCl to both solutions caused the basolateral membrane potential to hyperpolarize by up to 20 mV (cell interior negative). This popential was reduced by 80% within 3 min of the addition of ouabain to the serosal solution. This hyperpolarization most probably represents a ouabain sensitive active transport process sensitive to intracellular Na⁺. An equivalent electrical circuit for Na⁺ transport across rabbit urinary bladder is derived, tested, and compared to previous results. This circuit is also used to predict the effects that microelectrode impalement damage will have on individual membrane potentials as well as time-dependent phenomena; e.g., effect of amiloride on apical and basolateral membrane potentials.