THE SPECIFIC ACTIVITY OF PLASMA CORTISOL IN SHEEP AFTER INTRAVENOUS INFUSION OF [1,2-3H2]CORTISOL, AND ITS RELATION TO THE DISTRIBUTION OF CORTISOL

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Abstract
SUMMARY: Tritium-labelled cortisol was administered to sheep by intravenous infusion at constant rate for up to 4 hr. When the infusion was stopped, [3H] cortisol disappeared rapidly from plasma and its concentration could be described by a double exponential function. There was good agreement between the results from 50 experiments on 11 sheep. In pregnant ewes, there was no noticeable difference in the rate of disappearance of [3H]cortisol from plasma until about 2 weeks before lambing, when the rate became more rapid. These data were interpreted in terms of a two-compartment model of cortisol distribution. The central compartment contains about 42 μg. cortisol and may be identical with the cortisol contained in whole blood volume. The outer compartment contains about 130 μg. cortisol; less than half of this compartment may be in intercellular fluids, partly bound to protein, and the remainder in intracellular fluids. In pregnant ewes near term there is a decrease in plasma cortisol concentration which appears to result from expansion of plasma volume. The decrease in unbound cortisol concentration probably results in a decrease in the size of the outer compartment of cortisol. This may contribute to the observed increase in the rate of disappearance of [3H] cortisol from plasma, but this change may also coincide with the initiation of secretion of cortisol by the foetus, at about 1 or 2 μg./min.