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

Summary

The unidirectional influx of Na from the mucosal solution into the epithelium ofin vitro descending rabbit colon (J ^Na _me ) 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 (J ^Na_net ). 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 andJ ^Na _me and elicited K secretion. Tissues showing a spontaneousI _sc of approximately 4 μeq/cm²hr did not respond to amphotericin B with increasedI _sc andJ ^Na_net . However, in tissues characterized by a lowerI _sc under control conditions, amphotericin B increasedI _sc andJ ^Na_net to approximately 4μeq/cm²hr. These findings suggest that amphotericin increasesJ ^Na_net and elicits K secretion by disrupting the normal permselectivity 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 betweenJ ^Na _me andJ ^Na_net 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.