GLUCOCORTICOID METABOLISM IN THE CARDIAC INTERSTITIUM - 11-BETA-HYDROXYSTEROID DEHYDROGENASE-ACTIVITY IN CARDIAC FIBROBLASTS

The interstitial fibrosis seen in the heart and systemic organs in sta tes of primary or secondary mineralocorticoid excess suggests that fib roblasts are responsive to mineralocorticoid. In vitro studies demonst rating increased fibroblast collagen synthesis in response to MC are c onsonant with this view. The nicotinamide-adenine dinucleotide phospha te+-dependent enzyme 11beta-hydroxysteroid dehydrogenase converts the glucocorticoids corticosterone and cortisol to the inactive metabolite s II-dehydrocorticosterone and cortisone, respectively, conferring min eralocorticoid specificity to the cells within which it is active. We investigated the presence of 11beta-hydroxysteroid dehydrogenase in so nicates of cultured vascular endothelial cells and cardiac fibroblasts by incubating sonicates for 1 hour in the presence of 5 x 10(-9) mol/ L tritiated corticosterone or tritiated cortisol (1 muCi) and using re verse-phase high-performance liquid chromatography coupled to an on-li ne radioisotope detector for steroid separation and quantitation. Extr acts of bovine endothelial cells showed no enzymatic activity with eit her substrate, whereas extracts of rat cardiac fibroblasts readily con verted corticosterone to 11-dehydrocorticosterone, even in the absence of exogenous nicotinamide-adenine dinucleotide phosphate+ (10% conver sion). When 5 x 10(-4) mol/L nicotinamide-adenine dinucleotide phospha te+ was added to sonicated fibroblasts, conversion increased to 50%, c orresponding to 12 pmol II-dehydrocorticosterone formed/mg protein. Co nversion of cortisol to cortisone was not observed in fibroblast or en dothelial cell extracts. Significant levels of corticosterone to 11-de hydrocorticosterone conversion (0.14 pmol/10(6) cells/hour) were detec ted in intact fibroblasts, but no 11-dehydrogenation of corticosterone was observed in intact endothelial cells. Homogenates of rat heart al so possessed high levels of 11beta-hydroxysteroid dehydrogenase activi ty when incubated with corticosterone, effecting levels of conversion between 0.4 and 0.6 pmol/mg protein. Preincubation of intact fibroblas ts for 24 hours with 10(-7) mol/L corticosterone before sonication and incubation with tritiated corticosterone doubled the conversion to 11 -dehydrocorticosterone, indicating that 11beta-hydroxysteroid dehydrog enase activity was inducible. Addition of classic 11beta-hydroxysteroi d dehydrogenase inhibitors (5 x 10(-5) mol/L glycyrrhizic acid, glycyr rhetinic acid, or carbenoxolone) to fibroblast sonicates resulted in t otal inhibition of 11beta-hydroxysteroid dehydrogenase activity. Thus in cardiac fibroblasts, unlike in vascular endothelial cells, 11beta-h ydroxysteroid dehydrogenase plays a significant role in modulating cor ticosterone available to steroid receptors and may thereby confer mine ralocorticoid specificity to type I corticoid receptors in these mesen chymal cells.