Studies on the influence of nutrition on the rate of protein synthesis and insulin-like growth factor-I in the chicken

据报道，再喂养迅速增加蛋白质合成在禁食鸡和血浆胰岛素样生长因子-I（IGF-I）的浓度也是非常敏感的营养状况的变化。在本研究中，我们研究了血浆IGF-I浓度和肝脏IGF-I mRNA水平的组织蛋白质合成率在各种营养条件下的关系。通过注射大剂量的[^3H]-苯丙氨酸来测量组织蛋白质的合成速率。用放射免疫法测定血浆IGF-I浓度。采用核糖核酸酶保护试验定量肝脏IGF-Ⅰ mRNA。血浆IGF-I浓度从1周龄到6周龄呈上升趋势，禁食鸡血浆IGF-I浓度下降。禁食1 d后，1周龄和2周龄雏鸡肌肉蛋白质合成和肝脏IGF-I mRNA水平均下降，且肌肉蛋白质合成量与肝脏IGF-I mRNA水平变化呈正相关。在15日龄时，将雏鸡禁食2天，然后再喂以蛋白质、碳水化合物和脂肪的组合。再喂食30分钟后，测量肌肉和肝脏蛋白质合成速率以及血浆IGF-I浓度。在肝脏和肌肉蛋白质合成中没有观察到各种营养素的再喂养的相互作用。蛋白质合成的速度，这已经减少了禁食，增加了个别营养再喂养。空腹血浆IGF-I浓度降低，再喂各种营养素30分钟后血浆IGF-I无变化。这些结果表明，急性增加肌肉蛋白质合成的再喂养在禁食鸡是不相关的血浆IGF-I浓度的变化。

It has been reported that refeeding rapidly increases protein synthesis in fasted chickens and that plasma insulin-like growth factor-I (IGF-I) concentration is also very sensitive to the alteration in nutritional status. In the present study, we investigated the relationship of plasma IGF-I concentration and hepatic IGF-I mRNA level to the rate of tissue protein synthesis under various nutritional conditions. The rate of tissue protein synthesis was measured by a large dose injection of [^3H]-phenylalanine. Plasma IGF-I concentration was measured by radioimmunoassay. Hepatic IGF-I mRNA was quantified by ribonuclease protection assay. Plasma IGF-I concentration increased from 1 to 6 weeks of age, and the reduction of plasma IGF-I concentration was observed in fasted chicks. Both muscle protein synthesis and hepatic IGF-I mRNA level were decreased by 1-day fasting in 1-and 2-week-old chicks, and the amount of protein synthesized in muscles positively correlated to the change in hepatic IGF-I mRNA levels. On 15 days of age, chicks were fasted for 2 days and then refed with the combination of protein, carbohydrate and fat. After 30 minutes of refeeding the rates of muscle and liver protein synthesis and plasma IGF-I concentration were measured. No interaction of refeeding of various nutrients was observed in liver and muscle protein synthesis. The rate of protein synthesis, which has been decreased by fasting, was increased by individual nutrient refeeding. Plasma IGF-I concentration was decreased by fasting, and no change was observed in plasma IGF-I after 30 minutes of refeeding with various nutrients. These results suggested that acute increase in muscle protein synthesis by refeeding in fasted chicks is not associated to the change in plasma IGF-I concentration.