Thermodynamic analysis of active sodium and potassium transport in the frog corneal epithelium

The formalism of linear nonequilibrium thermodynamics for a three-flow system was applied to the isolated frog corneal epithelium to study the coupling between metabolism and the Na-K transport system across this layer. There is little or no net ion transport across the isolated frog corneal epithelium bathed in Na₂CO₄ Ringer. Addition of amphotericin B to the tear side solution increases apical membrane permeability, which results in a net Na transport (from tear to stroma) and a net K transport in the opposite direction. Corneas were mounted in a modified Ussing chamber that permitted the simultaneous measurements of electrical parameters and O₂ consumption by means of Clark-type oxygen electrodes. The overall degree of coupling, q̄, of the Na-K transport system to metabolism was calculated from measuring the suprabasal O₂ consumption rate at 'static head' and 'level flow' conditions and by a second independent technique. Measurements of electrical conductance used in conjunction with other previously measured parameters allowed the calculation of the affinity, A, of the metabolic reaction driving transport, all phenomenological coefficients, and the electromotive forces of sodium (E(Na)) and potassium transport (E(K)). Values of q̄ determined by the two techniques agreed (q̄ = 0.80 and 0.84, respectively). This indicates incomplete coupling and a variable stoichiometric relationship among O₂ consumption rate, net Na transport, and net K transport. The value calculated for A was 70.5 kcal.mol^-1, for E(Na) 142.5 mV, and for E(K) -34.9 mV.