Functional heterogeneity in somatotrophs isolated from the rat anterior pituitary

Somatotrophs in suspensions of anterior pituitary cells from adult male rats can be separated into 2 fractions by density gradient centrifugation. In addition to their different densities, somatotrophs in these 2 fractions can be distinguished morphologically by their staining characteristics and ultrastructure. Somatotrophs of lesser density (type I; approximately 1.068 g/cm3) have fewer secretory granules and a more extensive Golgi apparatus than the somatotrophs of greater density (type II; approximately 1.073 g/cm3). Responsiveness of type I and type II cells to secretory agents (i.e., dibutyryl cyclic adenosine monophosphate, somatostatin, thyroxine, and hydrocortisone) was evaluated by GH radioimmunoassay. Type I cells were consistently more responsive (% GH release) than type II cells. During 7 days in culture, type I cells produced more (approximately 200%) GH than they initially contained, whereas type II cells did not show evidence of increased GH production. Hydrocortisone significantly stimulated GH production in type I, but not type II cells. These results support the hypothesis that at least 2 functionally distinct populations of somatotrophs are present in the anterior pituitary gland of the adult male rat.