SOMATOTROPH FUNCTION IN THE NEONATAL PIG

The objective of this study was to evaluate developmental changes in s omatotroph function and related gene expression in neonatal pigs. Male piglets were sacrificed at 1, 7, 14, 21, 28, 35, and 42 d of age (8/a ge group) for the collection of tissue and blood. Serum concentrations of GH were determined. Quantitations of mRNA were performed for pitui tary Pit-1, GH, and GHRH receptor. Cultures of pituitary cells from ea ch pig were stimulated with 0, 0.1, 1, or 10 nM GHRH; 2 mM 8-Br-cAMP; or 100 nM phorbol myristate acetate. Elevated serum concentrations of GH were observed at 1 d of age, followed by a pronounced decrease to b asal levels thereafter (P < 0.0001). A mild transient increase in circ ulating GH occurred at Day 28. In vitro GH secretion was significantly stimulated by secretagogue treatments (P < 0.0001). Age-related decli nes in in vitro GH secretion were observed regardless of if the cells were stimulated by GHRH or by secretagogues that bypass the GHRH recep tor (P < 0.001). Similarly, cellular GH content varied with age (P = 0 .01). Levels of pituitary GH mRNA (P = 0.01) and GHRH receptor mRNA (P = 0.0002) de:creased with age. The quantity of GHRH receptor mRNA was correlated with GH mRNA levels (r = 0.55, P = 0.02), serum GH concent rations (r = 0.55, P = 0.02), and in vitro GH secretion (r = 0.66, P = 0.001). Pituitary Pit-1 mRNA levels at 7 and 14 d of age were signifi cantly elevated relative to all other sampling times (P = 0.0002). Lev els of Pit-1 and GH mRNAs were significantly correlated (r = 0.64, P = 0.003). These results demonstrate a strong developmental regulation o f somatotrophic function and related gene expression during the early neonatal period of the pig. Age-related decreases in secretory functio n may be mediated by concurrent mechanisms relating to the expression of the GHRH receptor and of GH. (C) Elsevier Science Inc. 1997.