RESPONSIVENESS TO CORTICOTROPIN-RELEASING HORMONE AND VASOPRESSIN IN CANINE CUSHINGS-SYNDROME

Corticotropin-releasing hormone (CRH) and vasopressin are the most imp ortant hypothalamic factors regulating adrenocorticotropic hormone (AC TH) secretion. In this study we have investigated the responsiveness o f the pituitary-adrenocortical axis to intravenous administration of C RH or lysine vasopressin (LVP) in 16 control dogs, 22 dogs with pituit ary-dependent hyperadrenocorticism and five dogs with hyperadrenocorti cism due to an adrenocortical tumor, using doses of CRH and LVP that c aused equivalent ACTH responses in the control dogs. After CRH adminis tration, the increment in plasma ACTH was significantly (p < 0.05) low er in dogs with pituitary-dependent hyperadrenocorticism (221 +/- 53 n g/l) than that in control dogs (279 +/- 41 ng/l). In the dogs with pit uitary-dependent hyperadrenocorticism, the relative increases in ACTH after CRH: were significantly (p < 0.05) lower than those after LVP. D espite the absence of an increase in ACTH following LVP administration in dogs with hyperadrenocorticism due to an adrenocortical tumor, the re was a significant increase in plasma cortisol, the increment (790 /- 238 nmol/l) being not statistically different from that in the cont rol 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 s ecretion by corticotropic cells in pituitary-dependent hyperadrenocort icism was relatively less sensitive to stimulation with CRH than with LVP. Adrenocortical tumors develop an aberrant sensitivity to LVP.