Components of Sodium and Chloride Flux Across Toad Bladder |
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| Authors: | Mackenzie Walser |
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| Abstract: | The effect of transepithelial potential difference (ψ) on Na and Cl flux across toad bladder was assessed by measuring isotopic flux between identical media at various values of ψ. The contribution of edge damage to ionic permeability was eliminated, resulting in relatively high spontaneous ψ (-97 ±4 mv) and low electrical conductance g. Bidirectional Na fluxes were measured simultaneously. Unidirectional Cl fluxes were measured in paired hemibladders at ψ = 0 mv or -97 mv. Net Na flux JNa, at ψ = 0 mv, was slightly less than short-circuit current (SCC). At ψ = -97 mv, JNa averaged 17% of SCC, and was sometimes zero. ΔJNa/Δψ (= g+) averaged 60% of g between -97 mv and +75 mv; at -150 mv, g+ fell, indicating rectification. Analysis of unidirectional Na fluxes indicates low passive conductance (1.5 μmho/mg wet weight), a bidirectional, electrically neutral flux of approximately 0.13 μa/mg, and relatively large conductance of the active transport path at ψ ≥ -97 mv. The absence of appreciable transstimulation of serosal (S)-to-mucosal (M) Na flux (in response to increasing mucosal Na concentration) indicates that the electrically neutral flux is not exchange diffusion in the usual sense. Analysis of Cl fluxes indicates similar values for passive conductance and neutral flux, suggesting linked neutral flux of Na and Cl. Either the electromotive force of the Na pump E, its conductance ga, or both are strong functions of ψ. The product of these two quantities, Ega, is a measure of the “transport capacity” at any given value of ψ, independent of the direct effect of ψ on JNa through the pump path. Ega varies with ψ. Hence estimation of the net Na flux or current at any one value of ψ, including ψ = 0, fails to reveal the maximal transport capacity of the pump, its resting electromotive force (when JNa = 0 through the pump), or the dependence of transport capacity on potential. |
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