Relationship of Insulin-Like Growth Factor-I to Muscle Protein Synthesis in Fasted Chicks.

Abstract

The influence of 1 day fasting on muscle protein synthesis, hepatic insulin like growth factor-I (IGF-I) gene expression and plasma IGF-I concentration was investigated in chicks at 1 and 2 weeks of age. The fractional synthesis rate (Ks) of breast muscle protein was measured by a large dose injection of L-[2, 6-3 H] phenylalanine (Experiment 1). Hepatic IGF- I mRNA was measured by ribonuclease protection assay (Experiment 1) and plasma IGF-I concentration was determined by radioimmunoassay (Experiment 2). The Ks value for muscle protein was significantly decreased from 1 to 2 weeks of age. Fasting significantly decreased Ks and the amount of protein synthesized. The relative intensity of bands of the protected IGF- I mRNA fragment tended to increase, but not significantly, from 1 to 2 weeks of age. Whereas hepatic IGF-I mRNA levels were significantly reduced by fasting. Plasma IGF-I concentration significantly increased from 1 to 2 weeks of age. There was a significant reduction in plasma IGF-I level of fasted chicks. The relative intensity of hepatic IGF-I mRNA bands was significantly related to the amount of muscle protein synthesized. These results suggest that the influence of fasting and age on muscle protein synthesis in the chicken during early stages of posthatching growth may be, at least partially, regulated by changes in plasma IGF-I level resulted from alterations in hepatic IGF-I gene expression.