Somatotroph function in the neonatal pig

Abstract

The objective of this study was to evaluate developmental changes in somatotroph 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/age group) for the collection of tissue and blood. Serum concentrations of GH were determined. Quantitations of mRNA were performed for pituitary Pit-1, GH, and GHRH receptor. Cultures of pituitary cells from each 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 basal levels thereafter (P < 0.0001). A mild transient increase in circulating GH occurred at Day 28. In vitro GH secretion was significantly stimulated by secretagogue treatments (P < 0.0001). Age-related declines in in vitro GH secretion were observed regardless of if the cells were stimulated by GHRH or by secretagogues that bypass the GHRH receptor (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) decreased with age. The quantity of GHRH receptor mRNA was correlated with GH mRNA levels (r = 0.55, P = 0.02), serum GH concentrations (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 significantly elevated relative to all other sampling times (P = 0.0002). Levels of Pit-1 and GH mRNAs were significantly correlated (r = 0.64, P = 0.003). These results demonstrate a strong developmental regulation of somatotrophic function and related gene expression during the early neonatal period of the pig. Age-related decreases in secretory function may be mediated by concurrent mechanisms relating to the expression of the GHRH receptor and of GH.