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J Endocrinol. 1997 Jan;152(1):113-21.

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

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Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, The Netherlands.


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.

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