Pulsatile Secretion of Alpha-MSH and the Differential Effects of Dexamethasone and Haloperidol on the Secretion of Alpha-MSH and ACTH in Dogs

This study was performed to determine whether, in the dog, there is at any time pulsatile release of alpha-MSH and whether secretion of ACTH from the pars intermedia (PI) contributes to the circulating concentrations of ACTH. The 24-h secretory profiles of alpha-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 alpha-MSH was found in all dogs, with wide variations in peak height. Plasma alpha-MSH levels were usually low (mean 15 pmol/l), but brief, distinct periods of increased plasma alpha-MSH concentrations as high as 489 pmol/l were found. Analysis of pulse frequency revealed a mean of 4.75 significant alpha-MSH peaks/24 h. The highest alpha-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 alpha-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 alpha-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 alpha-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 alpha-MSH is secreted spontaneously in a pulsatile manner in the dog and suggest that the canine PI contributes to circulating ACTH concentrations.