Biosynthesis of Muscle Proteins in the Fasted Rat

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Abstract
A dual-isotope technique was used to compare relative rates of biosynthesis of rat cardiac, diaphragm and leg muscle sarcoplasmic and myofibrillar proteins in fed and fasted rats. Eight pairs of 120 g male rats, one of each pair fed ad libitum 48 hours (weight gain 26.2 ± 6.3 g) and other fasted 48 hours (weight loss, 35.1 ± 12.6 g) were injected intraperitoneally with 3H- and L-[14C]leucine, respectively. Eight hours after injection, the rats were killed and sarcoplasmic and myofibrillar extracts prepared. The mean isotope ratio for sarcoplasmic proteins was nearly the same as for myofibrillar proteins in each muscle, indicating the fractions were equally affected by the fast. Each fraction was also subjected to sodium dodecyluslfate polyacrylamide gel electrophoresis and the gels subsequently analyzed for radioactivity. Variations in the isotope ratios throughout the gels in each case were within the error of the method, again indicating the absence of a selective effect of fasting. Analyses of isotope ratios of isolated, purified contractile proteins of each muscle further indicated a non-specific fasting effect. Results were the same when experiments were repeated with or without isotope reversal or utilizing 4 or 6 hour initial labeling periods. We conclude that the synthesis of muscle proteins is equally affected by fasting.