The squirrel monkey (SM), a New World primate, has plasma cortisol (F) levels 10 times higher than Old World primates, such as Cynomologus (C), and man. The high F levels in SM are necessitated by F receptors of decreased affinity. We have tried to elucidate the mechanism by which SM maintains high plasma F levels. Two possible mechanisms are: increased F production rate (PR) or decreased F metabolic clearance rate (MCR). F MCR is about 62 L/M2/d in SM, half that of C (121 L/M2/d). F PR, however, is 5 times greater in SM than in C: 120 mg/M2/d vs. 24 mg/M2/d. The absence of adrenal gland hypertrophy in SM (500 mg adrenal/Kg body weight vs. 400 mg/kg body weight in C), suggests an increase in F biosynthetic efficiency. This was investigated by measuring the activity of 4 adrenal microsomal enzymes: 21 hydroxylase, 3 B hydroxysteroid dehydrogenase, 17 hydroxylase and 17, 20 desmolase.The pattern of enzyme activities observed, increased 21OH and decreased 17,20 desm, suggests an efficient use of 17 hydroxyprogesterone as substrate for F biosynthesis. We conclude that SM maintains high plasma F levels mainly by increased FPR,which may be facilitated by alterations in the activity of F biosynthetic enzymes.
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