Adrenocortical function in a prototherian mammal, Tachyglossus aculeatus (Shaw)

The peripheral plasma concentrations and production rates of corticosterone and cortisol were measured in the conscious, unrestrained echidna (Tachyglossus aculeatus) under basal conditions and during maximal ACTH stimulation. Using Sephadex LH-20 column chromatography and radioligand assay, only cortisol and corticosterone could be detected in the peripheral blood plasma at very low concentrations of 0-07 +/- 0-03 (S.E.M.) mug/100 ml and 0-14 +/- 0-07 mug/100 ml respectively. Two-hourly sampling over periods of 36-48 h disclosed a diurnal periodicity in the combined plasma concentration of these corticosteroids, the high concentrations corresponding to periods of behavioural activity. Marked, short-term fluctuations in plasma corticosteroid concentration were also observed during periods of more frequent (20 min) sampling. Constant rate i.v. infusion of synthetic ACTH increased the plasma concentrations of both steroids to maximal values of 0-42 +/- 0-23 mug cortisol/100 ml and 1-06 +/- 0-56 mug corticosterone/100 ml at infusion rates of 1 i.u. ACTH/kg/h. This is approximately 1/160 of the potency of this ACTH in man. The production rates of corticosterone and cortisol, measured by isotope dilution during constant rate i.v. infusion of 3H-labelled tracers, were only 0-35 +/- 0-21 and 0-56 +/- 0-26 mug/kg/h respectively during saline infusion, and increased to 2-86 +/- 3-47 and 2-74 +/- 2-07 mug/kg/h during the infusion of 1 i.u. ACTH/kg/h. The metabolic clearance rate of cortisol was greater than that of corticosterone and both were depressed by ACTH. Plasma corticosteroid concentrations were increased after surgery during ether anaesthesia and in sick animals with heavy worm infestation. It is concluded that the adrenal cortex of echidnas responds to ACTH stimulation and stress in a similar way to eutherians, but the level of activity is much lower.