Prolactin and growth hormone levels in different inbred strains of mice: patterns in association with estrous cycle, time of day, and perphenazine stimulation.

These experiments were designed to compare prolactin (PRL) and growth hormone (GH) secretion in strains of mice with varying incidences of mammary tumors. In addition to the basal levels, PRL concentrations were compared after stimulation with perphenazine. Although pituitary concentrations of PRL and pituitary serum concentrations of GH appeared to be generally higher in strains with high incidence of mammary tumors, basal PRL levels in single decapitate serum samples seemed to have little correlation with the incidence of mammary tumors in different strains. However, PRL concentrations in sera after perphenazine injection followed a pattern characteristic of the mammary tumor incidence rate of the strain: C3H/St and CBA/St--the two high-incidence strains--had lower levels of PRL; C57BL/St and BALB/cST--the two low-incidence strains--had higher levels; and DBA/2St--the medium-incidence strain--had an intermediate level. PRL contents of the pituitary glands were depleted virtually equally in all strains except the BALB/cSt. These results suggested that the rate of metabolism of PRL in strains with high incidence of mammary tumors may be faster than in those with low incidences. Perphenazine had no influence on GH secretion in most mice. The strain-specific differences in PRL and GH concentrations were usually present even during cyclical and diurnal fluctuations. Serum PRL and GH levels were generally higher during the follicular phase and lower during the luteal phase of the estrous cycle in both C3H/St and C57BL/St strains. There were signs of episodic secretion of PRL together with evidences of circadian periodicity in the secretion of PRL and GH in mice of both strains. The levels of GH were usually high during the morning hours of those of PRL high during the evening hours. The data show a high degree of specificity in the secretion of PRL and GH im mice of different inbred strains, and it is possible that these strain-specific differences may be an important factor in the development of mammary tumors.