Evidence that aldosterone influences transport in target tissues by dissimilar mechanisms

The present study was performed to answer the question: Is the action of aldosterone on electrolyte transport and electrical properties similar in all target tissues? Studies were performed in vivo in control animals and rats with secondary hyperaldosteronism, caused by a sodium-free diet, to compare the effects of hyperaldosteronism on distal colon with hormone-induced changes in proximal colon. In distal colon aldosterone increased net sodium absorption and potassium secretion approximately threefold. Transmural potential difference increased from -15 ± 2 to -83 ± 3 mV (lumen negative) and I(SC) rose from 167 ± 26 to 1,023 ± 17 μA·cm^-2. These aldosterone-induced responses were completey inhibited by 0.1 mM amiloride. In contrast, in proximal colon potential difference was unchanged or increased slightly in experimental animals and I(SC) increased only 28% above control, although increases in net sodium and potassium transport were similar to changes observed in distal colon. Amiloride did not reduce sodium absorption in proximal colon of animals with hyperaldosteronism; I(SC) was decreased by 43%. These studies demonstrate that rat proximal colon is an aldosterone-sensitive tissue, but that the mechanism by which aldosterone influences sodium transport is not identical in distal and proximal portions of colon.