INHIBITION OF IMCD 11-BETA-HYDROXYSTEROID DEHYDROGENASE TYPE-2 BY LOWPH AND ACUTE ACID LOADING

Mineralocorticoid receptors in the inner medullary collecting duct (IM CD) are protected from glucocorticoid binding by an enzyme, 11 beta-hy droxysteroid dehydrogenase type 2 (11 beta-HSD2). To study the role of 11 beta-HSD2 in acid-base homeostasis, 11 beta-HSD2 activity was meas ured in rat IMCD-enriched cell suspensions. Homogenates of cell suspen sions were incubated in buffers ranging in pH from 6.00 to 8.15 in the presence of 1 mu Ci of H-3-corticosterone (CS) and 400 mu M NAD(+). E nzyme activity was expressed as the amount of H-3-CS converted to H-3- 11-dehydrocorticosterone (DHCS). IMCD 11 beta-HSD2 activity at pH 6.5 was 49% of activity at pH 7.5; 22.5 versus 11.0 fmol/mu g of protein p er h. Experiments also were performed on intact cell suspensions at pH 7.5 and 6.5. There was a 42% inhibition in the IMCD cell suspension c onversion rate of H-3-CS to H-3-11-DHCS at pH 6.5; 13.1 versus 7.6 fmo l/mu g per h (P < 0.005). In cell suspensions at pH 7.5, 1-day acid lo ading caused a 26% inhibition in conversion rate, 13.2 versus 9.9 fmol /mu g per h (P < 0.05), when compared with controls, These results sug gest that during acute metabolic acidosis, IMCD 11 beta-HSD2 is inhibi ted and may allow access to the mineralocorticoid receptors by glucoco rticoids.