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-dependen t process. At weaning (age 28 days), rat pups were placed on either st andard rat chow (normal diet juvenile group) or the same chow suppleme nted with 1% wt/wt of the creatine analogue beta-guanidinopropionic ac id [creatine depletion juvenile (CDJ) group]. Two groups of adult rats (age similar to 8 wk) were placed on the same diet regimens [normal d iet adult and creatine depletion adult (CDA) groups]. After 40 days (C DJ and normal diet juvenile groups) and 60 days (CDA and normal diet a dult groups), animals were killed and several skeletal muscles were re moved 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 s keletal muscles. In contrast, creatine depletion in adult animals did not result in similar shifts toward slow MHC isoform expression in eit her muscle type. The results of this study indicate that there is a di fferential effect of creatine depletion on MHC transitions that appear s to be age dependent. These results strongly suggest that investigato rs contemplating experimental designs involving the use of the creatin e analogue beta-guanidinopropionic acid should consider the age of ani mals to be used.