Abstract
The gallbladder epithelium has been used extensively for more than 20 years as a model system for investigations of the mechanisms of isosmotic salt and water transport. In particular, the transporting gallbladder of Necturus macu losus has been employed successfully in intracellular microelectrode and quantitative microscopic studies. The main advantages of this preparation are its simple structure (a monolayered epithelium of only one cell type that can be mounted as a flat sheet) and the large size of the cells. Experimental studies in Necturus gallbladder have permitted the formulation of well-founded work ing hypotheses to explain the mechanisms of salt and water transport. The main purpose of this review is to summarize recent experimental results in support of the notion that apical membrane N aCI entry occurs via a double ion exchange mechanism, namely the operation of independent Na+ -H+ and Cl--HCOi exchangers. Since alternative entry mechanisms have been pro posed, we present and illustrate criteria to distinguish between double ex change and other NaCI entry models. In addition, we briefly discuss the effects of cyclic AMP (cAMP) on these transport mechanisms and thus on the regulation of saIt transport.
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