Role of aldosterone and dietary potassium in potassium adaptation in the distal colon of the rat.

The present experiments were designed to differentiate the effects of aldosterone and chronic potassium loading in the mechanism of colonic potassium adaptation, using a recently described animal model that permits systematic manipulation of aldosterone and dietary potassium. Unidirectional 42K fluxes were performed under short-circuit conditions across isolated distal colonic mucosa from rats with intact adrenal glands and in adrenalectomized, hormone-replaced animals. Administration of a potassium-enriched diet reversed net potassium absorption (+0.43 +/- 0.10 muEq/h.cm2) in intact animals to net potassium secretion (-0.76 +/- 0.08 muEq/h.cm2). Infusion of aldosterone in adrenalectomized animals, in an amount equivalent to that produced by potassium loading in intact rats, reduced JnetK to zero (-0.03 +/- 0.06 muEq/h.cm2). Similarly, zero net potassium transport (-0.14 +/- 0.08 muEq/h.cm2) was observed when experimental rats were fed a potassium-enriched diet and maintained with basal aldosterone levels. Therefore, both elevated aldosterone levels and a potassium-enriched diet had an effect on net potassium transport, but neither produced the full effects of chronic potassium loading. An increase in net potassium secretion (-0.57 +/- 0.07 myEq/h.cm2) comparable to that of intact potassium-loaded animals did, however, occur when potassium loading and elevated aldosterone levels were combined in experimental animals. These studies demonstrate that (a) chronic potassium loading alters active potassium transport by an aldosterone-independent mechanism and (b) the mechanism of chronic potassium adaptation represents the additive effects of increased dietary potassium and aldosterone.