Muscle protein synthesis rate is altered in response to a single injection of insulin-like growth factor-I in seven-day-old Leghorn chicks.

To determine if a single injection of insulin-like growth factor-I (IGF-I) can affect muscle protein synthesis in chickens, 7-d-old male Single Comb White Leghorn chicks were injected s.c. with physiological saline (control) or 35 microg of recombinant human IGF-I. After 2 h 30 min, or 6, 12, or 24 h the chicks were injected with 3H-phenylalanine and killed, and the fractional synthesis rate (Ks) of breast muscle protein was measured. The Ks of IGF-I-treated birds were lower (P = 0.03) than controls at 2 h 30 min post-injection, higher (P = 0.07) than controls at 6 h post-injection, but not different from controls at later times. A second experiment examined serum changes during the 6 h after chicks were injected with IGF-I or saline as in the first experiment. Serum IGF-I concentration increased relative to almost undetectable levels (1 ng/mL) of controls to 216 +/- 59 ng/mL at 20 min after IGF-I injection (P < 0.001) and decreased to 12 +/- 6 ng/ mL by 6 h. Serum glucose and nonprotein nitrogen concentrations were significantly decreased for all or most of the 3 h after IGF-I injection, respectively, but only glucose concentration was the same as controls by 6 h. Low serum glucose and nonprotein nitrogen during the first few hours after IGF-I injection may contribute to the inhibition of Ks at 2.5 h, but the mechanisms behind the increased Ks at 6 h are not clear. These results support a role for IGF-I in the posthatching muscle development of chicks.