11-BETA-HYDROXYSTEROID DEHYDROGENASE AND CORTICOSTEROID HORMONE RECEPTORS IN THE RAT COLON

In the rat kidney 11beta-hydroxysteroid dehydrogenase (11beta-HSD) maintains normal in vivo specificity for mineralo-corticoid receptor (MR) by converting the active steroid corticosterone to inactive 11-dehydrocorticosterone, leaving aldosterone to occupy the MR. Clinical observations support the hypothesis that 11beta-HSD also protects the distal colonic MR from glucocorticoid excess. We have measured 11beta-HSD mRNA and activity along the rat colon and have analyzed the distribution of 11beta-HSD, MR, and glucocorticoid receptor (GR) mRNA within rat distal colon using in situ hybridization. Levels of 11beta-HSD mRNA (1.7 and 3.4 kb) and activity were higher in distal vs. proximal colon, paralleling reported MR mRNA levels. Within the distal colon mucosa both 11beta-HSD immunoreactivity and mRNA was observed in cells in the lamina propria but not in epithelial cells. MR mRNA was present in surface epithelial cells, but was also colocalized with the same 11beta-HSD-expressing cells in the lamina propria. In contrast GR mRNA was more uniformly distributed. The localization of MR mRNA to nonepithelial cells in the lamina propria, possibly neuroendocrine cells, suggests that mineralocorticoid-regulated sodium transport across colonic epithelial cells may also involve a paracrine mechanism. As with the kidney, exposure of active mineralocorticoid to the MR in these cells in the lamina propria is dictated by 11beta-HSD in an autocrine fashion.