The peripheral plasma concentration and secretion rate of cortisol in the conscious marsupial brush-tailed possum Trichosurus vulpecula (Kerr) was investigated under basal conditions and during maximal corticotrophin (ACTH) stimulation.
Cortisol concentration in peripheral plasma was 0·92 ± 0·48 (s.d.) μg/100 ml in males, and 1·01 ± 0·57 in females. It was lowest at 08.00 h, when the possums were asleep and not easily aroused. Although there was a high variance, a third degree polynomial fitted the data plotted against time over 24 h at the 95% level of confidence, indicating a diurnal periodicity.
Ether anaesthesia, porcine and synthetic (24 amino acids) ACTH caused plasma cortisol concentrations to rise to maxima of no more than 7 μg/100 ml. The dose-response curve to ACTH indicated a maximum response at 0·12 i.u./kg/h with a potency of approximately 1/6 that found in sheep and 1/20 that found in man.
Cortisol secretion rates in non-stimulated possums were 2·71 ± 1·01 μg/kg/h in males and 4·32 ± 1·74 in females. They increased during maximal ACTH stimulation to 6·24 ± 1·58 μg/kg/h in males, and 13·28 ± 7·82 in females. Metabolic clearance rates were lowest in animals not accustomed to repeated blood sampling and rose in subsequent experiments.
It is concluded that the adrenal cortex of Trichosurus, like that of eutherian mammals, responds to changes in activity, stressful stimuli and ACTH, but quantitatively, the responses to these stimuli are very much lower.
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