It is well known that in protein-starved chickens, small amounts of amino acid supplement, especially methionine, reduces nitrogen excretion and thereby improves nitrogen balance. On the other hand, excess intake of methionine causes growth depression and the growth-depressive effect of excess methionine can be alleviated by consumption of dietary glycine. Insulin-like growth factor-I (IGF-I) is one of various growth-promoting factors relating to the efficiency of animal production and is known to be very sensitive to changes in nutritional status. In the present study, the interactive effect of glycine on nitrogen sparing effect of methionine in protein-starved chickens was examined. In addition, the relation of IGF-I and its specific binding protein to the nitrogen sparing effect of supplemented methionine was also investigated. Two-days refeeding of methionine supplemented to protein-free diet could promptly alleviate body weight loss in protein-starved chickens, and the alleviation of body weight loss by methionine was not improved by glycine supplements. Moreover, such acute alleviation of body weight loss by dietary methionine was independent of the change in plasma IGF-I concentration.

The influence of varying leucine and fetal calf serum (FCS) concentrations in the culture medium on protein synthesis and degradation of chicken embryo myoblasts was examined. Medium+33 contained *, +, ,, / or +*ῌ FCS and ./1 (original concentration of leucine in Medium +33), ,,21 or ./1. mmol leucine/l. Protein synthesis was measured by incorporation of -H-phenylalanine. Protein degradation was measured by release of -H-phenylalanine once incorporated into myoblasts. There was an interaction between leucine and FCS concentrations on muscle protein synthesis. When FCS was not included in the medium, myoblast protein synthesis was increased with an increment of leucine concentration. At more than /ῌ of FCS, myoblast protein synthesis decreased by elevating leucine levels in the medium. Response surface of protein synthesis to varying leucine and FCS concentrations was calculated as follows: Protein synthesis (dpm)῏+-+1./ῌ.3/.1X῍,3.-X , ῌ.*.3Y῍+3.3XYῌ+..X , Y, where X is FCS concentration (ῌ), and Y is leucine concentration (῎./1.. mmol/l). FCS concentration not interacting with the e#ect of varying leucine concentrations on protein synthesis was calculated to be ,..3ῌ. No interaction between leucine and FCS concentrations on muscle protein degradation was observed. The e#ects of varying FCS and leucine levels on myoblast protein degradation showed quadratic response. When FCS increased from * to 1.,/ῌ, protein degradation dramatically decreased. However, when FCS increased from 1.,/ to +*ῌ, protein degradation tended to increase. The most e#ective leucine concentration suppressing protein degradation was calculated to be ,1/+ mmol/l. Thus, the influence of leucine on protein metabolism of chicken embryo myoblasts is modulated by varying serum concentrations in the medium.

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