Turnover of muscle protein in the fowl (Gallus domesticus). Rates of protein synthesis in fast and slow skeletal, cardiac and smooth muscle of the adult fowl

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Abstract
Rates of protein synthesis in skeletal, cardiac and smooth muscle of fully grown fowl (G. domesticus) were determined in vivo by the constant infusion method using [14C]proline. In the anterior latissimus dorsi, containing predominantly slow fibers, the average synthesis rate of non-collagen muscle proteins was 17.0 .+-. 3.1%/day, a value higher than that obtained for cardiac muscle (13.8 .+-. 1.3%/day) and for smooth muscle of the gizzard (12.0 .+-. 1.9%/day). In the posterior latissimus dorsi, containing predominantly fast fibers, synthesis rates were much lower (6.9 .+-. 1.8%/day). In each case these average rates for the non-collagen protein were similar to the average rate for the sarcoplasmic and myofibrillar protein fractions. The RNA concentration of these 4 muscles showed that relative rates of protein synthesis were determined mainly by the relative RNA concentrations. The rate of protein synthesis/unit of DNA (the DNA activity) was similar in the 2 skeletal muscles, but somewhat lower in cardiac muscle and gizzard, possibly reflecting the larger proportion of less active cell types in these 2 muscles. These quantitative aspects of protein turnover in the 2 skeletal muscles are discussed in terms of the determination of ultimate size of the DNA unit, and in relation to muscle ultrastructure.