Thermodynamic analysis of active sodium transport and oxidative metabolism in toad urinary bladder

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Abstract
Summary Measurements of electrical current and oxygen consumption were carried out concurrently under voltage clamp conditions in 11 toad hemibladders. Inhibition of active transport with amiloride then permitted evaluation of the passive conductance and the rate of basal oxygen consumptionJ br , allowing the simultaneous determination of the rates of active sodium transportJ aNa and suprabasal, oxygen consumptionJ sbr .J aNa andJ sbr were linear functions of the electrical potential difference over a range of ±80 mV. This allowed the comprehensive application of a linear nonequilibrium thermodynamic formalism, leading to the evaluation of the affinityA (negative free energy) of the metabolic reaction driving transport, all phenomenological coefficients, and the degree of couplingq relating transport to metabolism. Values ofA determined by two techniques wereA 1=56.0±5.8 andA 2=58.2±6.5 kcal per mole. Values ofq determined by two techniques agreed well and were less than 1, indicating incompleteness of coupling, and hence lack of fixed stoichiometry between Na transport and O2 consumption. The affinity and the electromotive force of sodium transportE Na are not closely correlated, reflecting the fact thatE Na comprises both kinetic and energetic factors.