| Abstract: | The mechanisms of increased aldosterone and cortisol metabolic clearance rates (MCR) following ACTH or cortisol administration were studied in 13 subjects undergoing cardiac catheterization and in 9 healthy controls. In control subjects, the MCR (plasma) of both steroids increased by 29% (aldosterone: from 936 +/- 57 to 1204 +/- 55 l/day/m2, cortisol: from 205 +/- 12 to 264 +/- 17 l/day/m2 +/- SE) after ACTH (12 units/h) for 1 to 4 h, and by 20 and 32%, respectively, after cortisol (12 mg/h) for 1 to 2 h. In contrast, aldosterone MCR (whole blood) did not change with ACTH or cortisol administration (from 1276 +/- 57 to 1330 +/- 59 l/day/m2), indicating that the plasma MCR increase results from a redistribution of aldosterone from plasma to red cells. Aldosterone splanchnic extraction was 92 +/- 1% (n = 12) with normal morning cortisol levels, and extraction was unchanged after ACTH administration. For cortisol, however, the splanchnic extraction increased from 8 +/- 0.8% to 17.8 +/- 5.0%, and the MCR (whole blood) likewise increased by 15 to 31% (from 295 +/- 23 to 357 +/- 30 l/day/m2), after ACTH or cortisol administration. In vivo and in vitro measurements (at 37 C) of tracer aldosterone concentration in plasma and in red cells showed an increase in distribution to red cells with increasing cortisol concentrations. The results suggest that a fraction of aldosterone is bound in plasma and displaced by cortisol into red cells. There is an increased aldosterone plasma MCR, but unaltered whole blood MCR, since the liver extracts aldosterone almost completely from both plasma and red cells. The increase in cortisol MCR (plasma) results from both an increased splanchnic extraction as plasma binding sites approach saturation and a redistribution into red cells. |
