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

据报道，在禁食的鸡中，再饲喂会迅速增加蛋白质合成，血浆胰岛素样生长因子- i （IGF-I）浓度对营养状况的改变也非常敏感。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水平与组织蛋白合成速率的关系。通过大剂量注射[^3H]-苯丙氨酸来测定组织蛋白的合成速率。用放射免疫法测定血浆igf - 1浓度。采用核糖核酸酶保护法定量肝脏IGF-I mRNA。1 ~ 6周龄血浆IGF-I浓度升高，空腹雏鸡血浆IGF-I浓度降低。1、2周龄雏鸡禁食1 d肌肉蛋白质合成和肝脏IGF-I mRNA水平均降低，且肌肉蛋白质合成量与肝脏IGF-I mRNA水平变化呈正相关。15日龄时，先禁食2 d，再饲喂蛋白质、碳水化合物和脂肪组合饲料。再喂养30分钟后，测量肌肉和肝脏蛋白质合成率及血浆IGF-I浓度。在肝脏和肌肉蛋白质合成中未观察到各种营养物质复饲的相互作用。蛋白质的合成速率，在禁食时降低，在个别营养补充后提高。空腹后血浆IGF-I浓度降低，再喂各种营养物30分钟后血浆IGF-I无变化。上述结果表明，空腹雏鸡复饲后肌肉蛋白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.