Sodium transport and oxygen consumption in toad bladder. A thermodynamic approach

The relationship between active sodium transport and oxygen consumption was investigated in toad urinary bladder exposed to identical sodium-Ringer's solution at each surface, while controlling the transepithelial electrical potential difference delta phi. Rates of sodium transport and oxygen consumption were measured simultaneously, both in the short-circuited state (delta phi = 0) and when delta phi was varied. Under short-circuit conditions, when the rates of active sodium transport changed spontaneously or were depressed with amiloride, the ratio of active sodium transport to the estimated suprabasal oxygen consumption Na/O2 was constant for each tissue, but varied among different tissues. Only when delta phi was varied did the ratio Na+/O2 change with the rate of active sodium transport; under these circumstances dNa+/dO2 was constant but exceeded the ratio measured at short-circuit [(Na+/O2)delta phi = 0[. This suggests that coupling between transport and metabolism is incomplete. The results are analyzed according to the principles of nonequilibrium thermodynamics, and intepreted in terms of a simple model of the transepithelial sodium transport system.