A sensitive method for the determination of corticosteroids in 0·1 ml. or less of ovine plasma is described. The method uses the steroid-binding properties of corticosteroid-binding globulin (CBG) and gel filtration on small columns of Sephadex G-25 (fine) at 4° for separation of CBG-bound and free steroids. Cortisol was found to be the predominant corticosteroid in ovine plasma and accounts for about 90% of the value determined by this method. The corticosteroid concentration in peripheral plasma of unstressed sheep was in the range 0·1–1·0 μg./100 ml. In untrained animals, venipuncture increased corticosteroid concentration substantially; training reduced the effect. An infusion of cortisol sodium succinate (100 μg. cortisol/min.) increased the plasma corticosteroid level to 9·5 ± 0·49 μg./100 ml. Intravenous infusion of the synthetic adrenocorticotrophic preparation Synacthen at rates of 10 and 20 μg./hr. for 2 hr. increased peripheral corticosteroid concentrations to 8 μg./100 ml. Single i.v. injections of 0·2–0·8 μg. Synacthen also significantly increased peripheral corticosteroid concentrations 7–15 min. later. The injection of 0·05 and 0·1 μg. Synacthen significantly increased the corticosteroid concentration too, but the increase was not significantly greater than that produced by the injection of acidified saline diluent alone. Injection of insulin (0·25 unit/kg. body weight, i.v.) caused a fivefold increase in the corticosteroid concentration 30–60 min. later, in both adult sheep and lambs. Glucose (0·25 g./kg. body weight, i.v.) had no effect on corticosteroid concentration.
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Online ISSN: 1479-6805
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