Short-term effect of aldosterone on Na-Cl transport across equine colon

In ponies fed concentrated (pelleted) meals, postprandial increases of plasma aldosterone have been temporally associated with a decrease in colonic fluid volume that parallels the conclusion of postfeeding fermentation. To determine the significance of short-term increases of plasma aldosterone on the rate of colonic Na absorption, in vitro transport studies were conducted on the mucosae of three morphologically distinct colonic segments (i.e., ventral, dorsal, and small colons) from ponies infused with a high physiological concentration of aldosterone for an 8-h period. In control ponies, basal NaCl absorption across the proximal colon (ventral and dorsal colons) was amiloride-insensitive and electroneutral. In aldosterone-treated ponies, the rate of electroneutral Na absorption was doubled in both segments and a small, amiloride-sensitive current was detected in the dorsal colon. However, consistent with previous observations [Clarke and Argenzio. Am. J. Physiol. 259 (Gastrointest. Liver Physiol. 22): G62-G69, 1990], expression of electroneutral Na absorption in the ventral colon required pretreatment of the tissues with an inhibitor of prostaglandin synthesis, i.e., indomethacin. In the distal (small) colon, basal absorption was entirely electrogenic and amiloride-sensitive, and aldosterone treatment tripled the rate of absorption. The above findings are consistent with the notion that postprandial hyperaldosteronism can significantly increase colonic Na absorption and, thereby, may facilitate colonic fluid absorption during the concluding period of meal-induced fermentation (8-12 h postfeeding). However, in the ventral colon (i.e., the principal site of fermentation), mineralocorticoid action does not dominate control of electroneutral Na transport because accelerated absorption could be abolished by the antiabsorptive effect of local prostanoids.