Effects of electrochemical gradients on active sodium transport in toad urinary bladder

To determine the concentration dependence of active sodium transport in toad urinary bladder, bidirectional fluxes of sodium were measured at three mucosal sodium concentrations (Cm) (114, 27, and 3 mM), before and after addition of sufficient ouabain to eliminate active transport. The results permit estimation of E, the electromotive force of the sodium pump, and its conductance, ga. E averaged 137 mV and was unaffected by depolarization or by reduction of Cm; ga fell by only 40%, suggesting that a Cm of 3 mM was still above the half-maximal value for the sodium pump. However, net open-circuit transport was estimated to become zero at Cm = 6 mM, owing to backflux through the shunt path, which did not vary with Cm. The flux ratio in the active path exhibited the type of abnormality first described as "single-file pore diffusion," which is attributed to relatively tight coupling between transport and metabolism.