Fluctuations in the Proportions of Growth Hormone- and Prolactin-Secreting Cells during the Bovine Estrous Cycle*

Abstract

Reverse hemolytic plaque assays were performed on monodispersed pituitary cells from cattle at various stages of the estrous cycle in an attempt to determine whether short term fluctuations in the gonadal steroid milieu influenced the proportions of pituitary cells that released GH and/or PRL. Phase of the estrous cycle was initially determined by gross ovarian morphology and later confirmed by determination of estradiol-17 beta and progesterone peripheral serum concentrations. Animals were subdivided into four groups according to phase of cycle: early luteal (EL; day 1-10), midluteal (ML; day 11-16), late luteal (LL; day 17-19), and follicular (F; day 20-21). Plaque assays demonstrated that the percentage of all pituitary cells that released PRL was greater in the EL phase than during the ML or F phases, whereas the relative abundance of GH-secreting cells remained unchanged. A more critical analysis of hormone-secreting subtypes revealed that the increase in total PRL secretors could be attributed almost exclusively to an increase in the abundance of those cells that released both GH and PRL (mammosomatotropes). Accompanying this augmentation of dual hormone-secretors was a decrease in the proportion of cells that released GH alone without a change in the abundance of cells that secreted only PRL. These results strongly suggest that during the estrous cycle there is a bidirectional interconversion among cells that release only GH and mammosomatotropes. Moreover, the relationship between ratios of acidophilic subpopulations and stage of reproductive cycle indicates that ovarian steroids may regulate this phenomenon.