BIOCHEMICAL CHARACTERIZATION OF PITUITARY-DEPENDENT HYPERADRENOCORTICISM IN THE DOG

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Abstract
The biochemical characterization of 22 cases of pituitary-dependent hyperadrenocorticism in the dog, is reported. The principal characteristics of the disease include excessive and non-rhythmic production of cortisol, decreased sensitivity of the hypothalamic-pituitary system to the suppressive effects of dexamethasone, decreased responsiveness of the pituitary-adrenocortical system to the stimulus of insulin-induced hypoglycaemia and increased responsiveness of the system to stimulation with lysine-vasopressin. From these observations it is concluded that pituitary-dependent hyperadrenocorticism in the dog is a valid model for study of the pathogenesis of the disease in man. For the diagnosis of hyperadrenocorticism itself, the measurement of the concentration of corticosteroids in a single sample of plasma obtained 8 h after intravenous injection of 0·01 mg dexamethasone/kg was sufficient. The level of 11-hydroxycorticosteroids was less than 140 nmol/l plasma in normal dogs, whereas higher values were found in dogs with hyperadrenocorticism. For purposes of differential diagnosis, measurement of the level of corticosteroids in the plasma both before and 4h after intravenous injection of 0·05 mg dexamethasone/kg is adequate: suppression is obtained only in cases of pituitary-dependent hyperadrenocorticism.