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In an investigation of the factors leading to the increase in the concentration of plasma free glucocorticoid, which results in immunosuppression and death after mating of all males in natural populations of a small shrew-like marsupial, the dusky antechinus (Antechinus swainsonii), the integrity of the glucocorticoid feedback control of the concentration of plasma cortisol was examined by use of dexamethasone-suppression tests. Injection of 0.2 mg dexamethasone/kg i.m. caused a marked fall in the concentration of plasma cortisol 17 h later, approximately 2 months and 2 weeks before the annual mating period in mid-July. However, the same dose had no significant effect on the increased concentration of plasma cortisol characteristic of the mid- to late July mating period. Injection of 100 i.u. ACTH/kg i.m. caused a significant increase in the concentration of plasma cortisol 6-7 h later on all occasions, indicating that the responsiveness of the adrenal cortex to ACTH did not change. Pretreatment with dexamethasone had no effect on the ACTH-stimulated cortisol concentration, ruling out a possible direct effect of dexamethasone on adrenocortical secretion in this species. Dexamethasone also reduced the concentration of plasma testosterone when the level was low, before the mating period, but not when the level was high, at the beginning of the mating period. It is concluded that, in association with a rapid increase in the concentration of plasma testosterone, an increase in aggression and intense mating activity, glucocorticoid feedback control of ACTH secretion is impaired.(ABSTRACT TRUNCATED AT 250 WORDS)

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SUMMARY An investigation was made of the interactions between insulin and cortisol on carbohydrate, fat and protein metabolism in the marsupial brush-tailed opossum Trichosurus vulpecula (Kerr). Intravenous injection of 0·15 i.u. regular insulin/kg caused a prompt fall in plasma glucose concentration to 33–54% of the control value, in the first 30 min, with complete recovery within 4 h. This was associated with a slow fall in plasma amino acid concentration and a moderate rise in plasma free fatty acid (FFA) concentration. Plasma cortisol concentration was increased 1·5 h after insulin injection to maximum values of 1·24–4·44 μg/100 ml, which were approximately proportional to the degree of hypoglycaemia. Pretreatment with five daily i.m. injections of 1 mg cortisol acetate/kg caused a marked reduction in insulin sensitivity of three out of four opossums, and increasing the dose to 5 mg/kg caused a similar reduction in insulin sensitivity of the remaining opossum. Cortisol pretreatment raised the control plasma amino acid and FFA concentrations and enhanced the effect of insulin injection on these variables. There was a linear relationship between the control plasma cortisol concentration, within the physiological range, and sensitivity to insulin. It is concluded that, in contrast to the red kangaroo, the interactions between insulin and glucocorticoids in Trichosurus resemble those reported for eutherian mammals. However, the unusual increase in plasma FFA after insulin injection is unexplained.