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 concentr ations of ACTH. The 24-h secretory profiles of alpha-MSH, ACTH, and co rtisol were determined in eight dogs. Plasma samples were obtained at 10-min intervals via an indwelling jugular catheter during two 12-h pe riods. Pulsatile secretion of alpha-MSH was found in all dogs, with wi de variations in peak height. Plasma alpha-MSH levels were usually low (mean 15 pmol/l), but brief distinct periods of increased plasma a-MS H concentrations as high as 489 pmol/l were found. Analysis of pulse f requency 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 d examethasone on the 6-h secretory profiles and on the haloperidol-stim ulated secretion of alpha-MSH, ACTH, and cortisol was investigated. In these two studies, plasma ACTH was measured by a highly sensitive imm unoradiometric assay. Dexamethasone pretreatment significantly suppres sed the plasma concentrations of ACTH, cortisol, and alpha-MSH to 10.3 %, 3.9%, and 74.6% respectively. Dexamethasone pretreatment also signi ficantly reduced the haloperidol-stimulated secretion of ACTH and cort isol, but had no influence on the haloperidol-stimulated secretion of alpha-MSH. After the administration of haloperidol to the dexamethason e-pretreated dogs, there were small increases in the plasma concentrat ions of ACTH and cortisol, the latter being significant. These data de monstrate that alpha-MSH is secreted spontaneously in a pulsatile mann er in the dog and suggest that the canine PI contributes to circulatin g ACTH concentrations.