Quadriceps muscle protein synthetic rate has been determined in healthy subjects in the post-absorptive (n = 18) and fed (n = 10) states and in patients with a variety of myopathies, by analysis of the enrichment of serial muscle biopsies taken during primed continuous infusion of L-[1-13C]leucine. 2. Quadriceps protein synthetic rates in normal subjects were (mean +/- SD) 0.046 +/- 0.012 and 0.075 +/- 0.014%/h in the post-absorptive and fed states respectively. These results are significantly lower than we previously reported (M. J. Rennie et al., Clinical Science, 1982, 63, 519-523 [1]) but show the same relative differences of direction and magnitude, confirming the effects of feeding previously reported. In patients with muscular dystrophy, muscle protein synthetic rate was, as previously reported [1], much lower in the fed state than in normal subjects. A new finding is that for patients with myotonic dystrophy the rate is also depressed in the post-absorptive state. 3. We suggest that the present estimates in post-absorptive and fed normal subjects be used as reference values for quadriceps mixed muscle protein synthetic rate.
