Effect Of Growth Hormone-Releasing Factor Infusion On Somatotropin, Prolactin, Thyroxine, Insulin, Insulin-Like Growth Factor-I And Blood Metabolites In Control And Somatostatin-Immunized Growing Pigs

The aim of this study was to characterize the effects of prolonged infusion of growth hormone-releasing factor (1-29)NH2 (GRF) on plasma concentrations of hormones and metabolites when administered to control pigs and pigs immunized against somatostatin (SRIF). In the first experiment, eight purebred Yorkshire boars averaging 113 +/- 2 kg BW were immunized against SRIF conjugated to bovine serum albumin (BSA) (n = 4) or BSA alone (n = 4). Somatotropin (ST) response to four rates of GRF infusion (0, 1.66, 5 and 15 ng/min/kg BW) for 6 hr was evaluated using a double balanced 4 x 4 Latin square design. During the 4 hr before infusion, SRIF-immunized animals tended (P = 0.06) to have a higher ST release (613 vs 316 ng . min/ml, SE = 232) than controls. During infusion, GRF elicited a dose-dependent increase in ST release in both squares; the ST response was not better in SRIF-immunized animals than in controls (P > 0.05) (1435 vs 880 ng . min/ml; SE = 597). In the second experiment, ten purebred Yorkshire boars (5 controls and 5 SRIF-immunized animals) averaging 69 +/- 2 kg BW were continuously infused with GRF at the rate of 15 ng/min/kg BW for six consecutive d. Under GRF infusion, ST concentrations increased (P < 0.05) from 805 to 4768 ng . min/ml (SE = 507) from day 1 to day 6 in both SRIF-immunized and control animals. Prolactin levels increased (P < 0.05) with GRF infusion; pattern of increase was different (P < .01) overtime in control and SRIF-immunized animals. Thyroxine levels increased from 2.53 to 3.45-mu-g/dl (SE = 0.16) after six d of infusion. Insulin-like growth factor I was higher (P < 0.05) before (139 vs 90 ng/ml; SE = 11) and during (222 vs 185 ng/ml; SE = 11) GRF infusion in SRIF-immunized animals. A transient increase (P < 0.05) in glucose and insulin was observed in both groups. Immuniation against SRIF had no effect on blood metabolites; however, GRF infusion increased free fatty acids from 157 to 204-mu-Eq/l (SE = 11) and decreased blood urea nitrogen from 4.1 to 3.5 mmol/l (SE = 0.2) from day 1 to day 6, respectively. In summary, active immunization against SRIF in growing pigs increased ST and IGF-I concentrations. Infusion of GRF continuously raised ST levels with days of infusion without any sign of decrease responsiveness.