Responsiveness to corticotropin-releasing hormone and vasopressin in canine Cushing's syndrome.

Corticotropin-releasing hormone (CRH) and vasopressin are the most important hypothalamic factors regulating adrenocorticotropic hormone (ACTH) secretion. In this study we have investigated the responsiveness of the pituitary-adrenocortical axis to intravenous administration of CRH or lysine vasopressin (LVP) in 16 control dogs, 22 dogs with pituitary-dependent hyperadrenocorticism and five dogs with hyperadrenocorticism due to an adrenocortical tumor, using doses of CRH and LVP that caused equivalent ACTH responses in the control dogs. After CRH administration, the increment in plasma ACTH was significantly (p < 0.05) lower in dogs with pituitary-dependent hyperadrenocorticism (221 +/- 53 ng/l) than that in control dogs (279 +/- 41 ng/l). In the dogs with pituitary-dependent hyperadrenocorticism, the relative increases in ACTH after CRH were significantly (p < 0.05) lower than those after LVP. Despite the absence of an increase in ACTH following LVP administration in dogs with hyperadrenocorticism due to an adrenocortical tumor, there was a significant increase in plasma cortisol, the increment (790 +/- 238 nmol/l) being not statistically different from that in the control dogs (412 +/- 37 nmol/l). We conclude that in spite of the changes inherent to pituitary-dependent hyperadrenocorticism, i.e. neoplastic transformation of corticotropic cells and hypercortisolism, there is persistence of responsiveness to hypophysiotropic hormones. The ACTH secretion by corticotropic cells in pituitary-dependent hyperadrenocorticism was relatively less sensitive to stimulation with CRH than with LVP. Adrenocortical tumors develop an aberrant sensitivity to LVP.