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

有报道称，重新饲喂可以迅速增加禁食鸡的蛋白质合成，血浆胰岛素样生长因子-I(IGF-I)浓度对营养状态的变化也非常敏感。在本研究中，我们研究了不同营养条件下血浆IGF-I浓度和肝脏IGF-I mRNA水平与组织蛋白质合成速率的关系。大剂量注射[~(3 H)]-苯丙氨酸测定组织蛋白质合成速率。放射免疫法测定血浆IGF-I浓度。用核糖核酸酶保护法测定肝脏IGF-I m RNA的含量。禁食雏鸡1~6周龄血浆IGF-I浓度升高，血浆IGF-I浓度下降。禁食1天后，1周龄和2周龄雏鸡的肌肉蛋白质合成和肝脏IGF-I mRNA水平均降低，且肌肉合成的蛋白质量与肝脏IGF-I mRNA水平的变化呈正相关。15日龄时，将雏鸡禁食2天，然后在饲料中添加蛋白质、碳水化合物和脂肪。复饲30min后，测定肌肉和肝脏蛋白质合成率及血浆IGF-I浓度。在肝脏和肌肉的蛋白质合成中，没有观察到各种营养物质的再投喂的交互作用。蛋白质合成速度因禁食而降低，但通过个体营养再摄食而提高。禁食后大鼠血浆IGF-I浓度明显降低，补充各种营养素30min后，血浆IGF-I浓度无明显变化。这些结果表明，禁食雏鸡肌肉蛋白质合成的急剧增加与血浆IGF-I浓度的变化无关。

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.