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.