The mean (se) basal plasma aldosterone concentrations were significantly lower in 31 dogs with pituitary-dependent hyperadrenocorticism (PDH) (75  pmol/litre) than in 12 healthy dogs (118  pmol/litre), whereas in five dogs with hyperadrenocorticism due to an adrenocortical tumour they were significantly higher (205  pmol/litre). The mean basal renin activity was not significantly different between the dogs with PDH (303  fmol/litre/second), the dogs with an adrenocortical tumour (141  fmol/litre/second), and the control dogs (201  fmol/litre/second). At three and four hours after the intravenous administration of 0.1 mg/kg dexamethasone, the concentrations of aldosterone decreased significantly to about 60 per cent of their initial values in the control dogs but did not change in the dogs with PDH or an adrenocortical tumour. In the dogs with PDH the renin activity increased significantly after the administration of dexamethasone.
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