Possible role of cytosolic calcium and Na-Ca exchange in regulation of transepithelial sodium transport.

Emerging evidence in a number of different epithelia suggests that changes in cytosolic calcium ion levels play a critical role in the regulation of transepithelial sodium transport. Maneuvers believed to raise cytosolic calcium ion activity lead to an inhibition of net sodium transport in toad urinary bladder, frog skin, and isolated perfused proximal renal tubules. Regulation of the level of ionized calcium in the cytosol of the epithelial cells appears to involve a process of coupled Na-Ca exchange across the basolateral plasma membrane, energized, at least in part, by the sodium gradient. It is suggested that changes in cytosolic calcium ion levels, secondary to changes in Na-Ca exchange, in turn dependent in part on the activity of the sodium pump, constitute a link in a negative feedback mechanism. Through such a feedback mechanism, the rate of entry of sodium into the cell across the apical surface may be kept in step with its rate of extrusion across the basolateral surface.