SOMATOSTATIN INCREASES GROWTH-HORMONE (GH) SECRETION IN A SUBPOPULATION OF PORCINE SOMATOTROPES - EVIDENCE FOR FUNCTIONAL AND MORPHOLOGICALHETEROGENEITY AMONG PORCINE GH-PRODUCING CELLS

Previous results demonstrate that porcine somatotropes can be separate d by density gradient centrifugation into low density (LD) and high de nsity (HD) subpopulations. In rat, two analog somatotrope subpopulatio ns differ morphologically and functionally. In an attempt to determine whether morphological differences were also present within LD and HD porcine somatotropes, we undertook a quantitative electron microscope study of the subcellular organelles of immunoidentified LD and HD soma totropes. In addition, to test for the existence of functional differe nces, cultures of separated HD and LD subpopulations were treated for 4 h with or without 10 nM GRF-(1-29) and/or 100 nM somatostatin (SRIF) , and porcine GTT release and intracellular content were evaluated usi ng a homologous enzyme immunoassay. Morphometric results demonstrate t hat LD somatotropes are smaller in size (P < 0.05) and contain fewer s ecretory granules (P < 0.05) and more rough endoplasmic reticulum (P < 0.05) than HD somatotropes. In terms of secretion, LD somatotropes sh owed a classical response; GRF increased GH release 1.7-fold (n = 6; P < 0.05) over the control value, whereas treatment with SRIF alone did not affect basal GH release in this subpopulation, but partially bloc ked GRF-induced GH release. HD somatotropes responded to GRF with a si milar 1.7-fold increase in GH release. However, SRIF administered alon e or in combination with GRF exerted a paradoxical stimulatory effect on HD somatotropes (2.15- and 2.12-fold over control value, respective ly; n = 6; P < 0.05). These results demonstrate that the porcine somat otrope population is composed of two major subpopulations that display a distinctive pattern of ultrastructural organization and a markedly divergent secretory response to in vitro SRIF treatment.