ALTERATIONS IN ANTERIOR-PITUITARY FUNCTION OF DOGS WITH PITUITARY-DEPENDENT HYPERADRENOCORTICISM

For the purpose of obtaining an integral picture of anterior pituitary function in canine pituitary-dependent hyperadrenocorticism (PDH), 47 dogs with PDH and eight control dogs received combined administration of four hypophysiotropic hormones (CRH, GHRH, GnRH and TRH) and measu rements were made of ACTH, cortisol, GH, LH, PRL and TSH. Basal plasma levels in 47 dogs with PDH were higher for ACTH, cortisol and PRL, lo wer for GH, and not different for LH (n=25 noncastrated dogs) and TSH compared with controls (n=8). In dogs with PDH the responses to combin ed hypophysiotropic stimulation, measured as increment and area under the curve (AUC), were not different for ACTH, lower for GH and TSH (in crements and AUC) and higher for cortisol (increments), LH (AUC, n=25 noncastrated dogs) and PRL (increments and AUC) than in controls. We c onclude that pituitary function is altered in several respects in dogs with PDH. 1) In spite of persisting hypercortisolemia and the neoplas tic transformation of the corticotropic cells, these cells usually rem ain responsive to combined hypophysiotropic stimulation. 2) Basal plas ma GH concentrations and GH responsiveness in the combined stimulation test are decreased, probably as a result of the glucocorticoid-induce d increase in somatostatin tone. 3) Plasma PRL concentrations and the PRL, response to stimulation are increased, probably as a result of co secretion with ACTH by the transformed corticotropic cells. 4) Despite the well known effect of glucocorticoids of decreasing circulating co ncentrations of gonadal steroids and thyroxine, the basal plasma conce ntrations of LH and TSH remain unchanged and there is a tendency to hy perresponsiveness to stimulation for LH and hyporesponsiveness for TSH . The most likely explanation for these changes is a dual effect of gl ucocorticoids: a direct effect on the gonads and thyroids and/or the t ransport and metabolism of their secretory products, and an influence on the sensitivity of the feedback control at the hypothalamic-pituita ry level.