SECRETORY AND MORPHOLOGICAL HETEROGENEITY OF PORCINE SOMATOTROPES DURING POSTNATAL-DEVELOPMENT

Previous studies from our laboratory have demonstrated that porcine so matotropes can be separated into two subpopulations of low (LD) and hi gh density (HD) by centrifugation in a Percoll gradient. The two subse ts are present throughout porcine postnatal growth, although their rel ative proportions vary with age, In prepubertal animals, HD cells exhi bit higher secretory granule content and release more GH than LD cells under basal culture conditions, In the present study, we analysed the ultrastructure of separated LD and HD cells from neonate and mature f emale pigs, and quantified cell size as well as the relative abundance of several subcellular organelles on immunoidentified somatotropes, S ubsequently, GH release under basal conditions was assessed for cultur es of unseparated cells and also for LD and HD somatotropes obtained a t different stages of postnatal development, Results from the morphome tric study demonstrated that LD somatotropes were significantly smalle r in size, contained less secretory granules and displayed a more deve loped endoplasmic reticulum than their HD counterparts, regardless of the age of the pituitary donors, In terms of secretory ability, a sign ificant age-associated decrease in GH release was observed in monolaye r cultures of unseparated cells from prepubertal and mature pigs compa red to neonates. A similar decline in GH-releasing ability was detecte d for cultures of HD cells. For LD cells, GH secretion only decreased significantly in mature animals. In spite of the divergent pattern fol lowed by both subpopulations during growth, Ho somatotropes released s ignificantly more GH than LD somatotropes at the three ages studied, T aken together, our findings demonstrate that the population of porcine somatotropes is mainly composed of two subtypes, LD and HD, which dif fer in density, morphology and basal secretory activity, These differe nces are essentially maintained during porcine postnatal development. The progressive reduction in the secretory capacity of HD and LD somat otropes, coupled to the decrease in the relative abundance reported fo r the HD subpopulation, provides the cytological basis for a better un derstanding of the decline in GH release associated with age in pigs.