Secretion of aldosterone in the monotreme mammal, Tachyglossus aculeatus

The secretion of aldosterone and its regulation by ACTH and angiotensin II were investigated in conscious, unrestrained echidnas with chronically implanted jugular catheters. Aldosterone was measured by radioimmunoassay after extraction and isolation by silica gel thin-layer chromatography. The mean concentration of aldosterone in blood plasma of five male and three female echidnas was only 5.4 +/- 1.3 (S.E.M.) pg/ml. During stress (surgery and anaesthesia) the mean concentration increased to 17.6 +/- 3.8 pg/ml. Infusion of beta 1-24 ACTH at a rate of 5 units/kg per h increased the plasma concentration of aldosterone to 53.8 +/- 9.8 pg/ml. Infusion of angiotensin II at rates of 100 and 500 ng/kg per h also increased aldosterone concentration, to 24.1 +/- 8.6 and 35.1 +/- 10.9 pg/ml respectively. Production and metabolic clearance rates were measured by the constant-rate infusion of [3H]aldosterone and found to be 5.0 +/- 2.2 ng/kg per h and 14.3 +/- 1.3 ml/kg per min respectively in the unstimulated state. Production rate was increased approximately sevenfold by the infusion of ACTH at 5 units/kg per h and fourfold and sixfold by infusion of angiotensin II at 100 and 500 ng/kg per h respectively. Metabolic clearance decreased following the infusion of ACTH or angiotensin II. Direct measurement of secretion rate by the collection of adrenal venous blood from three anaesthetized, laparotomized echidnas gave values of 9.4, 15.6 and 8.8 mg/kg per h. It is concluded that the adrenal secretion of aldosterone in the echidna is extremely low compared with that in other mammals but the response to stress, ACTH and angiotensin II indicates the presence of typical mammalian control mechanisms for its secretion.