Effects of vasopressin on electrolyte transport across isolated colon from normal and dexamethasone‐treated rats.

Vasopressin enhanced the absorption of Na+ and Cl‐ across the short‐circuited colon descendens from normal rats. This effect of vasopressin results from an increase in the mucosal to serosal movement of Na+ and Cl‐ and a decrease in the serosal to mucosal movement of Cl‐ and was accompanied with a decrease in the short‐circuit current (ISC). Neither the base‐line absorption of Na+ and Cl‐, the vasopressin‐induced increase in Na+ and Cl‐ absorption nor the decrease in ISC were inhibited by amiloride in the colon from normal rats. Colon descendens from rats treated for 3 days with dexamethasone had remarkably higher transmural potential difference (p.d.), tissue conductance (Gt) and ISC. The absorption of Na+ across the short‐circuited colon descendens from dexamethasone‐treated rats was increased 3‐fold when compared to colon from normal rats. The absorption of Cl‐ in normal rats was reversed to Cl‐ secretion in treated rats. Amiloride rapidly and reversibly decreased the p.d., Gt and ISC in colon from dexamethasone‐treated rats. The transport of Na+ was nearly completely inhibited by amiloride in treated rats. In contrast to its enhancing effects on Na+ absorption in colon from normal rats vasopressin did not enhance Na+ absorption in colon from dexamethasone‐treated rats. This enhancement of Cl‐ absorption by vasopressin was retained in colon from treated rats. This enhancement of Cl‐ transport was due solely to a decrease in the serosal to mucosal movement of Cl‐ and was accompanied with a decrease in ISC and Gt. The results support the hypothesis that vasopressin causes inhibition of the electrogenic secretion of Cl‐ in colon from dexamethasone‐treated rats. Furthermore, the results suggest that the increase in the mucosal to serosal movement of Na+ and Cl‐ and the decrease in the serosal to mucosal movement of Cl‐ in colon from normal rats are caused by independent effects of vasopressin.