Regulation of alpha-melanocyte-stimulating hormone secretion from the parsintermedia of domestic cats

Objective-To identify factors regulating secretion of alpha-melanocyte-stim ulating hormone (alpha-MSH) from the pars intermedia (PI) of the pituitary gland of cats. Animals-28 healthy adult cats. Procedure-Indwelling catheters were placed in 1 jugular vein of each of 7 t o 10 cats, depending on treatment group. Sixteen hours later, 3 blood sampl es were collected for determination of baseline plasma hormone concentratio ns, and saline solution or a test substance (haloperidol, corticotropin-rel easing hormone, bromocriptine, isoproterenol, insulin, or dexamethasone) wa s administered via the catheter. Subsequent blood samples were collected at regular intervals for up to 240 minutes after injection. Concentrations of ACTH, cortisol, and alpha-MSH were measured in plasma by use of specific r adioimmunoassays. Cats were rested for at least 3 weeks between experiments . Results-Administration of haloperidol and isoproterenol resulted in increas ed, and bromocriptine and insulin in decreased, circulating concentrations of a-MSH from baseline. ACTH and plasma cortisol concentrations increased a fter administration of all test substances except dexamethasone. Dexamethas one injection resulted in decreased plasma concentrations of ACTH and corti sol. Conclusions-Secretion of alpha-MSH from the PI of cats appears to be inhibi ted by dopaminergic activity and stimulated by beta-adrenergic influences. Activation of secretion of alpha-MSH from the PI can be dissociated from ac tivation of secretion of other pro-opiomelanocortin-derived peptides, such as ACTH, arising from the pars distalis. Regulation of secretory activity o f the PI of cats resembles that of rats.