Protocol-dependence of equivalent circuit parameters of toad urinary bladder

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Abstract
Determinations of current-voltage relationships are widely employed in the characterization of epithelial sodium transport. In order to determine the protocol dependence of transport parameters in the toad urinary bladder, studies were carried out in the presence and absence of amiloride, an inhibitor of active sodium transport. With symmetric positive and negative perturbations of the transepithelial electrical potential difference Δψ (0→±100 mV) for 30 sec, the amiloride-sensitive current-voltage (i a -Δψ) relationship was near linear over the range −75→+100 mV, indicating constancy of the conductance κa and the apparent electromotive force “E Na”, lumped parameters of the standard electrical equivalent circuit model of the active transport system. With a reverse protocol (±100→0 mV) or 15 min perturbations thei a -Δψ relationships were highly nonlinear. Nonlinearity reflected voltage dependence of parameters: perturbations that increased active transport decreased “E Na” and increased κa, as evaluated from 10 sec perturbations of Δψ; slowing of active transport produced the converse changes. These effects are usefully analyzed in both quasi-steady states and true steady states by means of a detailed equivalent circuit incorporating the significant ionic currents across each plasma membrane. Precise understanding of the significance of κa and “E Na” will require characterization of the partial ionic conductances on perturbation of Δψ.