Some functional aspects of canine corticotrophs

To examine the regulation and functional significance of canine pituitary pars intermedia corticotrophs, ACTH and cortisol responses to CRF were studied in healthy dogs before and after treatment with dexamethasone. In addition the effects of the dopamine agonist bromocriptine and the dopamine antagonist pimozide were investigated. In the latter two instances prolactin concentrations were also measured. Finally the pituitaries were studied immunocytochemically for ACTH and alpha-MSH. No response of ACTH or cortisol to bromocriptine was observed. Pimozide caused a slight rise in ACTH levels in some dogs. However, prolactin levels significantly decreased with bromocriptine and increased with pimozide. Injection of synthetic ovine CRF to dogs was followed by sharp increases in ACTH and cortisol values. These responses were obliterated by prior treatment with dexamethasone. In 1 of 4 dogs given dexamethasone before euthanasia, there were few pars distalis cells with ACTH(1-24) immunopositivity, although persistence of ACTH(1-24) reaction was noted within cells of the pars intermedia. The results indicate that none of the CRF-induced ACTH secretion in dogs is derived from pars intermedia corticotrophs. Dosages of bromocriptine and pimozide that clearly alter prolactin secretion do not consistently affect ACTH levels.