ENZYME AND MINERALOCORTICOID RECEPTOR-CONTROLLED ELECTROGENIC NA+ ABSORPTION IN HUMAN RECTUM IN-VITRO

In vivo electrogenic Na+ absorption (J(Na)(e)) in the human rectum is controlled by acute variation of aldosterone in nanomolar concentration range. In this study we report both the induction of J(Na)(e), in human rectum epithelium by nanomolar aldosterone added in vitro and the enzymatic control of glucocorticoid action on J(Na)(e). J(Na)(e) was measured as amiloride-sensitive short-circuit current 8 h after addition of the respective steroid. Aldosterone (10 nM) caused J(Na)(e), of 5.7 +/- 1.4 mu mol . h(-1). cm(-2). Cortisol in the same concentration did not induce significant J(Na)(e). Because cortisol is readily inactivated by 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD), the true mineralocorticoid activity of cortisol was evaluated after inhibition of 11 beta-HSD by carbenoxolone. Carbenoxolone alone did not exhibit mineralocorticoid activity. If cortisol (10 nM) was given together with carbenoxolone (1 mu M), the resulting J(Na)(e) (4.5 + 0.4 +/- mu mol . h(-1). cm(-2)) was not significantly different from that after 10 nM aldosterone, indicating equal intrinsic mineralocorticoid activity of cortisol and aldosterone. The same mechanisms were found in rat late distal colon. Kinetic data of carbenoxolone at 10 nM cortisol resulted in a Michaelis constant of 0.3 mu M, maximal absorption of 8.4 mu mol . h(-1). cm(-2), and a Hill coefficient of 1.8. The effects of carbenoxolone and glycyrrhetinic acid did not differ. We conclude that J(Na)(e) is under complete control of mineralocorticoid action. ''Spontaneous'' J(Na)(e) in the beginning of the in vitro period can be explained by elevated steroid levels before tissue removal. The biological activity of 11 beta-HSD localized in rectum epithelium suffices to completely inactivate cortisol to cortisone in vitro for several hours, thus protecting the mineralocorticoid receptor from high glucocorticoid concentrations.