A diet of 1% beta-guanidinopropionic acid (beta-GPA) fed to rats for weeks results in decreased muscle adenosine triphosphate and creatine phosphate concentrations (J. Biol. Chem. 249: 1060-1063, 1974), increased activities of selected mitochondrial enzymes (Biochem. J. 232: 125-131, 1985), and atrophied type IIb fibers (Lab. Invest. 33: 151-158, 1975). The hypothesis of the present study was that chronic beta-GPA feeding would increase cytochrome c mRNA in muscle and would decrease alpha-skeletal actin mRNA in type IIb muscle. Data collected supported, in part, the hypothesis. After 22 days of a 1% beta-GPA diet, cytochrome c mRNA was increased 60-67% in muscles with inherently low cytochrome c mRNA but was not altered in muscles with higher cytochrome c mRNA levels. alpha-Skeletal actin mRNA was unchanged in muscles with low and high cytochrome c mRNA after 22 days of 1% beta-GPA. After 66 days of beta-GPA feeding, both cytochrome c mRNA and alpha-skeletal actin mRNA were decreased 18 and 26%, respectively, per unit of total RNA, in white quadriceps muscle. At the same time muscles composed of predominantly type II fibers atrophied 22%, whereas type I muscle size was unaltered. These data suggest that high-energy phosphate levels could play some role in adaptive changes in muscle composition.
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