Does intracellular sodium modify membrane permeability to sodium ions?

Abstract

IN sodium transporting epithelia the asymmetrical cells have the apical face specialised for sodium entry while the basolateral membranes are concerned with sodium export, the accompanying anion passing passively. Epithelia exposed to various sodium concentrations may need to control sodium entry as a way of regulating the intracellular concentration. Some amphibian epithelia can show adaptive changes in their transporting capacity with changes in the ambient sodium concentration^1–3. In addition, rapid changes in permeability of the apical face occur with time constants of a few seconds when epithelia are exposed to step changes in sodium concentration⁴. While the slow adaptive changes may involve alterations in synthesis of membrane macromolecules important for membrane permeability, the rapid changes are likely to result from fast changes involving the existing membrane apparatus. One effector agent for the rapid reactions may be the intracellular sodium concentration and the experiments reported here were designed to test this hypothesis. We have manipulated frog skin epithelium in ways to increase the intracellular sodium content and found that the density of sodium channels in the apical face, measured by labelling with ¹⁴C-amiloride, is reduced in these circumstances.