Energetics of sodium transport in toad urinary bladder.

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Abstract
The ratio of the transepithelial Na transport rate JNA, across the isolated toad urinary bladder to the rate of transport-dependent metabolism, .**GRAPHIC**. was measured as a function of the transepithelial electrical voltage, .DELTA..psi.. The ratio remains constant with a mean value of 18-20 over the range of imposed voltages of 0 to +70 mV. With increasing hyperpolarization of the bladder, JNA decreases and the calculated electromotive force or apparent ENA of the Na pump increases. From thermodynamic and kinetic arguments the apparent ENA approaches the maximal electrochemical potential gradient, ENA, against which Na can be transported by this tissue only when JNA .apprxeq. 0. At this unique condition FENA (in which F is the Faraday constant) is the maximal free energy of the chemical reaction driving Na transport, equal to the maximal extramitochondrial phosphorylation potential, and maximal free energy of the mitochondrial respiratory chain within the transporting cells.