Age dependence of myosin heavy chain transitions induced by creatine depletion in rat skeletal muscle.

This study was designed to test the hypothesis that myosin heavy chain (MHC) plasticity resulting from creatine depletion is an age-dependent process. At weaning (age 28 days), rat pups were placed on either standard rat chow (normal diet juvenile group) or the same chow supplemented with 1% wt/wt of the creatine analogue beta-guanidinopropionic acid [creatine depletion juvenile (CDJ) group]. Two groups of adult rats (age approximately 8 wk) were placed on the same diet regimens [normal diet adult and creatine depletion adult (CDA) groups]. After 40 days (CDJ and normal diet juvenile groups) and 60 days (CDA and normal diet adult groups), animals were killed and several skeletal muscles were removed for analysis of creatine content or MHC distribution. In the CDJ group, creatine depletion (78%) was accompanied by significant shifts toward expression of slower MHC isoforms in two slow and three fast skeletal muscles. In contrast, creatine depletion in adult animals did not result in similar shifts toward slow MHC isoform expression in either muscle type. The results of this study indicate that there is a differential effect of creatine depletion on MHC transitions that appears to be age dependent. These results strongly suggest that investigators contemplating experimental designs involving the use of the creatine analogue beta-guanidinopropionic acid should consider the age of animals to be used.