In vitro effects of dexamethasone on sodium transport across rat colon.

1. The in vitro effects of dexamethasone on Na+ transport across the colon descendens from normal rats was investigated. Amiloride was used at two concentrations, 10 microM and 1 mM, to differentially inhibit the transport of Na+ across the colon. The colon descendens from each rat was divided into four segments and Na+ unidirectional fluxes before and 7 h after the addition of dexamethasone (10(-6) M) were determined under short-circuit conditions. 2. Base-line JnetNa (net flux of Na+) was twice as high in the proximal segment as in the distal segment. The two middle segments had intermediate rates of Na+ transport. JnetNa in control tissue was unaffected by 10 microM-amiloride but was completely inhibited by 1 mM-amiloride. In control tissue, amiloride at either 10 microM or 1 mM had no effect on the transmural potential difference (p.d.), the transmural conductance (Gt) or the short-circuit current (Isc). 3. Dexamethasone caused a time-dependent increase in the p.d. and in the Isc in all four segments of the colon. The increase in the p.d. and Isc was greatest in the most distal segment and less in each of the successive more proximal segments. This segmental difference along the colon was observed in tissue from all animals studied (n greater than 30). 4. The increase in p.d. and Isc caused by dexamethasone was accompanied by an increase in JnetNa to the same maximum rate of 14 mu equiv cm-2 h-1 in each segment.(ABSTRACT TRUNCATED AT 250 WORDS)