Anterior pituitary function in female dogs with mammary tumors: II. Prolactin.

Plasma prolactin (PRL) concentrations at rest and during dynamic tests were measured in dogs with benign or malignant proliferative mammary lesions and in age-matched healthy control dogs. Basal PRL concentrations of dogs with benign or malignant lesions were not found to be elevated in comparisons controlled for the effects of estrous cycle phase or progestin treatment. A statistically non-significant increase of the PRL response to thyrotropin-releasing hormone (TRH) was seen in separate groups of affected dogs as compared to matched controls. A tendency towards a significantly elevated response to TRH appeared in the combined (benign plus malignant disease) group of affected dogs in anestrus (P = 0.05) and in metestrus (P = 0.09). The PRL response to clonidine was very variable and did not differ between the 3 groups of dogs. The bromocriptine-induced suppression of PRL secretion did not vary significantly among the three groups. These results do not indicate an unequivocal association of mammary tumor development and altered PRL secretion. It is only warranted to conclude that in some dogs the occurrence of benign or malignant mammary lesions is associated with hyper-responsiveness of the PRL secretion to stimulation with TRH.