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Protocol dependence of equivalent circuit parameters of toad bufo marinus urinary bladder



Protocol dependence of equivalent circuit parameters of toad bufo marinus urinary bladder



Journal of Membrane Biology 55(1): 53-68



Determinations of current-voltage relationships are widely employed in the characterization of epithelial Na transport. 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 Na transport. With symmetric positive and negative perturbations of the transepithelial electrical potential difference .DELTA.psi. (0 .fwdarw. .+-. 100 mV) for 30 s, the amiloride-sensitive current-voltage (ia - .DELTA.psi.) relationship was near linear over the range -75 .fwdarw. + 100 mV, indicating constancy of the conductance .kappa.a and the apparent electromotive force ENa, lumped parameters of the standard electrical equivalent circuit model of the active transport system. With a reverse protocol (.+-. 100 .fwdarw. 0 mV) or 15 min perturbations the ia - .DELTA.psi. relationships were highly nonlinear. Nonlinearity reflected voltage dependence of parameters: perturbations that increased active transport decreased ENa and increased .kappa.a, as evaluated from 10 s perturbations of .DELTA.psi.; 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 .kappa.a and ENa will require characterization of the partial ionic conductances on perturbation of .DELTA.psi.

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Accession: 006215280

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