Influence of porcine somatotropin administration and sex on glucose and endocrine responses in obese pigs

The endocrine and hyperglycemic responses to chronic administration of exogenous porcine somatotropin (pST) were investigated in genetically obese intact and castrate male (boars and barrows) and female (gilts) swine. Somatotropin was administered at a dose of 4 mg.pig-1/day by daily im injection. After a 26-hr fast on Day 22 of treatment, a loading dose of glucose (0.5 g/kg body wt) was administered iv, and blood samples were collected to determine parameters of glucose clearance and responses to a glucose bolus. Blood samples were assayed for glucose, Insulin, insulin-like growth factor (IGF)-I, IGF-II, and pST. On Day 24 of treatment, blood samples were similarly collected every 15 min for 8 hr, to examine metabolic and endocrine measures in the fed state. After collection of the 6-hr sample, porcine growth hormone-releasing factor (pGHRF, 0.5 microgram/kg body wt) was administered iv. Administration of pST reduced feed consumption, increased gain, and resulted in 50% greater efficiency of gain. Plasma glucose was increased with pST treatment in both fasted (44%) and fed (64%) pigs. After administration of glucose bolus, glucose distribution volume and half-life (t1/2) were approximately doubled and MCR was reduced 75% in pST-treated pigs. Peak glucose-induced Insulin concentrations were increased in pST-treated gilts and barrows but not boars. Concentrations of IGF-I were increased with pST treatment. Concentrations of pST during the sampling periods were influenced by sex and pST treatment. Control pigs responded to GHRF administration with increased plasma concentrations of pST, whereas, no pST response to GHRF administration was observed in the pST-treated pigs. The glucose and Insulin results indicate chronic administration of pST to gilts and barrows induces Insulin insensitivity and enhances Insulin responses to a glucose challenge. The differences among the sexes in pST concentrations in the pST-treated pigs suggest differences in sexual differentiation or activation of the mechanisms of pST clearance.