Effect of aldosterone on ion transport by rabbit colonIn vitro

Summary Segments of descending colon obtained from rabbits, that had been maintained on drinking water containing 25mm NaCl and an artificial diet which contains 1% Na and is nominally K-free, respond to aldosterone in vitro (after a 30 to 60-min lag period) with a marked increase in the short-circuit current (I _sc ), an equivalent increase in the rate of active Na absorption (J _net ^Na ) and a decline in tissue resistance (R _t ). Aldosterone also brings about a marked increase in the unidirectional influx of Na into the cells across the mucosal membrane (zero-time rate of uptake) which does not differ significantly from the increase inI _sc . Treatment of control tissues with amphotericin B brings about sustained increases inI _sc andJ _net ^Na to levels observed in aldosterone-treated tissues. However, addition of amphotericin B to the mucosal solution of aldosteronetreated tissues does not result in a sustained increase inI _sc orJ _net ^Na and these values do not differ markedly from those observed in control tissues treated with amphotericin B. These findings, together with other evidence that Na entry in the presence of amphotericin B is sufficiently rapid to saturate the active Na extrusion mechanism at the baso-lateral membrane, are consistent with the notion that the aldosterone-induced protein increases the permeability of the mucosal membrane to Na but does not increase the saturation level of the active Na pump within the time-frame of these studies (3 hr).Finally, aldosterone has no effect on the bidirectional or net transepithelial movements of K under short-circuit conditions, suggesting that the enhanced secretion of K observed in vivo is the result of increased diffusion of K from plasma to lumen via paracellular pathways in response to an increased transepithelial electrical potential difference (lumen negative).