Current-voltage analysis of apical sodium transport in toad urinary bladder: Effects of inhibitors of transport and metabolism

Summary The basal-lateral surface of the epithelium of the urinary bladder of the toad (Bufo marinus) was depolarized by exposure of the serosal surface to 85mm KCL and 50mm sucrose. The extent of depolarization appeared to be virtually complete, as evaluated by the invariance in the transepithelial electrical potential difference and conductance on addition of nystatin (a monovalent cation ionophore) to the serosal medium. The Na-specific current (I_Na) was defined as the current sensitive to the removal of Na from the mucosal medium or inhibitable by addition of amiloride to this medium. In the presence of the high K-sucrose serosal medium, rapid, serial, stepwise clamping of the transepithelial voltage (V) yielded a curvilinear dependence ofI_Na onV; which is taken to represent theI–V curve of the apical Na channels. The constant field equation (Goldman, D.E. 1943;J. Gen. Physiol.27:37) fits theI–V data points closely, allowing estimates to be made of the permeability to Na of the apical membrane (P_Na) and of the intracellular Na activity (Na_c). Exposure of the apical surface to amiloride (5×10^–7m) decreasedP_Na in proportion to the decrease inI_Na (i.e., 70%) but decreased Na_c only 25%. In contrast, an equivalent lent reduction inI_Na elicited by exposure of the basallateral surface to ouabain was accompanied by only a 20% decrease inP_Na and a sixfold increase in Na_c. The effects of amiloride onP_Na and ouabain on Na_c are consistent with the primary pharmacological actions of these drugs. In addition,P_Na appears to be under metabolic control, in that 2-deoxyglucose, a specific inhibitor of glycolysis, decreasedI_Na andP_Na proportionately, and lowered Na_c marginally, effects indistinguishable from those obtained with amiloride.