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Dogs with spontaneous pituitary-dependent hyperadrenocorticism were divided into two groups, one with normal plasma concentrations of -MSH (normal -MSH dogs, n = 26) and the other with high plasma concentrations of -MSH (high -MSH dogs, n= 14), on the presumption that high -MSH concentrations indicated a parent cell of pars intermedia origin.

The urinary corticoid/creatinine ratios of the high -MSH dogs were significantly higher than those of the normal -MSH dogs. The percentage decrease of the corticoid/creatinine ratios following dexamethasone administration was significantly higher in the normal -MSH dogs than in the high -MSH dogs. Dexamethasone resistance occurred in both the normal -MSH dogs (4 out of 26) and the high -MSH dogs (7 out of 14), indicating a relative rather than an absolute difference.

The short-term effect of orally administered bromocriptine, at a dose (10 µg/kg body weight) known to be effective in lowering prolactin concentrations in dogs, was investigated by measuring concentrations of cortisol, ACTH and -MSH in plasma at 4, 6 and 8 h after administration. Significant decreases were observed for cortisol in both groups and for -MSH only in the high -MSH dogs.

The effect of 5 days of bromocriptine administration (10 µg at 12-h intervals) was assessed by measurements of urinary corticoid/creatinine ratios. Considering both groups as a whole, only the corticoid/creatinine ratios of the high -MSH dogs decreased significantly on the first day of treatment. In each group there was one dog in which bromocriptine administration lowered the corticoid/creatinine ratios for several days, but the values stayed above the reference range and the animals did not recover.

It is concluded that high -MSH concentrations are not unambiguously associated with the theoretically expected characteristics of pars intermedia disease. The results indicate that the pituitary lesions causing hyperadrenocorticism may not maintain the regulation characteristics of the lobe of origin.

J. Endocr. (1988) 118, 271–277

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