Pulsatile secretion of α-MSH and the differential effects of dexamethasone and haloperidol on the secretion of α-MSH and ACTH in dogs

Abstract This study was performed to determine whether, in the dog, there is at any time pulsatile release of α-MSH and whether secretion of ACTH from the pars intermedia (PI) contributes to the circulating concentrations of ACTH. The 24-h secretory profiles of α-MSH, ACTH, and cortisol were determined in eight dogs. Plasma samples were obtained at 10-min intervals via an indwelling jugular catheter during two 12-h periods. Pulsatile secretion of α-MSH was found in all dogs, with wide variations in peak height. Plasma α-MSH levels were usually low (mean 15 pmol/l), but brief, distinct periods of increased plasma α-MSH concentrations as high as 489 pmol/l were found. Analysis of pulse frequency revealed a mean of 4·75 significant α-MSH peaks/24 h. The highest α-MSH peaks were associated with definite changes in the plasma concentrations of ACTH. In separate studies, the influence of dexamethasone on the 6-h secretory profiles and on the haloperidol-stimulated secretion of α-MSH, ACTH, and cortisol was investigated. In these two studies, plasma ACTH was measured by a highly sensitive immunoradiometric assay. Dexamethasone pretreatment significantly suppressed the plasma concentrations of ACTH, cortisol, and α-MSH to 10·3%, 3·9%, and 74·6% respectively. Dexamethasone pretreatment also significantly reduced the haloperidol-stimulated secretion of ACTH and cortisol, but had no influence on the haloperidol-stimulated secretion of α-MSH. After the administration of haloperidol to the dexamethasone-pretreated dogs, there were small increases in the plasma concentrations of ACTH and cortisol, the latter being significant. These data demonstrate that α-MSH is secreted spontaneously in a pulsatile manner in the dog and suggest that the canine PI contributes to circulating ACTH concentrations. Journal of Endocrinology (1997) 152, 113–121