The effects of changes in plasma concentration of cortisol (F) on the metabolic clearance rate (MCR) of this hormone were investigated in 11 normal adult males as follows: constant blood withdrawal was carried out using a portable pump attached to a non-thrombogenic catheter which itself was placed in the antecubital vein. Two pulse-injections of radioactive F were administered, first14C-F at 8 PM and then3H-F at 5 AM. Six-hr blood collections were started immediately before each of these injections. The integrated plasma concentrations (I.C.) of endogenous and radioactive F were determined in each 6-hr pool. The MCR was calculated as radioactive F injected/I.C. of radioactive F. The MCR values of the 5 AM-11 AM period (149 ±. 28 l/M2/24 hr) were significantly (p < 0.005) higher than those obtained during the 8 PM-2 AM period (91 ± 19 l/M2/24 hr). The I.C. of endogenous F were 10.9 ± 1.7 (SD) μg/100 ml and 3.3 ± 1.0 μg/100 ml for the 2 periods of study respectively. The marked changes in MCR invalidate the determinations of the blood production of F on the basis of a 24-hr integrated concentration of this steroid and the value of its MCR measured during a fraction of the day. When the production rate of F was calculated from the cumulative specific activities of urinary THF and THE glucuronides, it was found that the values derived from the14C-THE (14C-F being given at 8 PM) were about 1/4 less than those from 14C-THF. In contrast, when3H-F was injected at 5 AM, the urinary productions derived from3H-THF was slightly but significantly smaller than those from 3H-THE. However, when the production was calculated from the mean of THF and THE, there was only minimal effect of the time of injection of radioactive F.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism
- Clinical Biochemistry
- Biochemistry, medical