Mammosomatotropes are abundant in bovine pituitaries: influence of gonadal status

Mammosomatotropes--cells that secrete both GH and PRL--are a common pituitary cell type in a variety of mammalian species. Recently, immunogold electron microscopy was used to demonstrate that all acidophils (a generic term used here to denote cells which secrete GH and/or PRL) in cattle stain for both GH and PRL. This high degree of overlap, which is unique among the mammalian species examined in this regard, raises the question: do all acidophils actively release both hormones concurrently? In the present study, we addressed this question by employing reverse hemolytic plaque assays to measure GH and PRL release from individual bovine pituitary cells in culture in the presence of stimulatory secretagogues. Inconsistent with the previous reports on hormone storage, all bovine acidophils in gonad-intact and castrated males did not actively release both GH and PRL. In gonad-intact males approximately 9% of all pituitary cells released both hormones, whereas 21% released only GH and 45% PRL alone. In castrated males, mammosomatotropes comprised only 22% of the pituitary cell population, whereas GH-only cells accounted for 8% and PRL secretors 46%. This divergence in the relative proportions of GH-only cells and mammosomatotropes suggests that the hormonal milieu plays an important role in maintenance of the various acidophilic cell types. In addition, the reciprocal relationship between cells that released GH-only and dual hormone secretors in the two models studied supports the possibility that these cell types are functionally interconvertible.