OSMOREGULATION OF SYSTEMIC VASOPRESSIN RELEASE DURING LONG-TERM GLUCOCORTICOID EXCESS - A STUDY IN DOGS WITH HYPERADRENOCORTICISM

In 9 dogs with pituitary-dependent hyperadrenocorticism and in 6 dogs with hyperfunctioning adrenocortical tumours, the osmoregulation of arginine vasopressin (AVP) release was investigated by iv infusion of 20% NaCl for 2 h at a rate of 0.03 ml per kg body weight. The responses were analysed in terms of sensitivity and threshold of the osmoregulation of AVP secretion. The sensitivity was normal in 6 dogs and lowered in 9. In 4 of the latter dogs there was complete absence of a response to hypertonicity. The osmotic threshold of AVP release was raised in 9 dogs and normal in 2 dogs, whereas in the four dogs without any response the term threshold was not applicable. The results were not different for dogs with pituitary-dependent hyperadrenocorticism and dogs with hyperfunctioning adrenocortical tumour. It is concluded that corticosteroid excess per se induces a marked impairment of the osmoregulation of AVP secretion. The loss of reactivity of the osmoreceptor system may contribute to the corticosteroid-induced polyuria, which is also the result of resistance to AVP in the kidney.