CONCENTRATIONS, METABOLIC-CLEARANCE RATES, PRODUCTION-RATES AND PLASMA-BINDING OF CORTISOL IN ANTARCTIC PHOCID SEALS

We have reported previously that plasma of the Weddell seal, a member of the phocid family, contains a very high concentration of cortisol. The present study was undertaken to determine whether high cortisol le vels were common to seals in the Antarctic environment, or to other ph ocidae, and to determine the mechanism of the hypercortisolaemia. High levels of cortisol (0.82-2.38 mumol/l) were found in 4 phocidae (Wedd ell, crabeater, leopard and Southern elephant seals), whereas levels i n a member of the otariid family (Antarctic fur seal) were similar to human values. Metabolic clearance rates (MCR) and production rates (PR ) of cortisol were determined in the field in Weddell (N = 1), crabeat er (N = 3) and leopard (N = 3) seals following bolus injections of [H- 3] cortisol. The MCR and PR did not differ between the three phocids, but whereas the MCR of 410-590 l/day was twice that of human values, t he PR of 460-1180 mumol.m-2.d-1 was up to 40-fold greater. The binding capacity of corticosteroid-binding globulin (CBG) was equal to or gre ater than the plasma concentrations of cortisol, resulting in relative ly low concentrations of free cortisol. We conclude that hypercortisol aemia is maintained in phocid seals mainly by a high production rate-t he highest (corrected for surface area) reported in any species. The r elatively low cortisol levels in otariid seals studied in the same env ironment suggest that the high PR in phocidae is unrelated to the hars h climatic conditions, but may be part of their adaptation for diving to extreme depths. The phocid seals and New World primates have simila rly high levels of cortisol and a high PR but CBG in the primates has low binding capacity and affinity and cortisol is mainly free.