Ion transport by rabbit colon: II. Unidirectional sodium influx and the effects of amphotericin B and amiloride.

The unidirectional influx of Na from the mucosal solution into the epithelium of in vitro descending rabbit colon (JName) determined under short-circuit conditions, is comprised of two components: one represents entry of Na into transporting epithelial cells and is abolished by amiloride which also abolishes Na absorption (JNanet). The other represents diffusional Na entry into paracellular pathways traversing the epithelium. In all instances, exposure of the mucosal surface to amphotericin B increased tissue conductance and JName and elicited K secretion. Tissues showing a spontaneous Isc of approximately 4 microneq/cm2hr did not respond to amphotericin B with increased Isc and JNanet. However, in tissues characterized by a lower Isc under control conditions, amphotericin B increased Isc and JNanet to approximately 4 microneq/cm2 hr. These findings suggest that amphotericin increases JNanet and elicits K secretion by disrupting the normal perm-selectivity of the mucosal membrane. Under these conditions the extrusion of Na from cell-to-serosal solution becomes the rate limiting step in transepithelial Na transport. Finally, a close correlation between JName and JNanet was was observed when the rate of Na absorption varied either spontaneously or experimentally with amiloride, suggesting that the backflux of Na from cell-to-mucosal solution is undetectably small.