Relationship between the contents of high-energy phosphates and Regulation of muscular properties

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It is known that the properties of skeletal muscles are regulated in response to exercise training, environment, nutrition, growth and development, and/or aging. The energy source for muscle contraction is adenosine triphosphate (ATP), which is synthesized through aerobic and anaerobic energy metabolism. It is also influenced by the content of creatine (AJP Cell Physiol, ’94; Jpn J Physiol, ’94 & ’95). However, the precise mechanism responsible for the regulation of contractile and metabolic properties of muscles is still unclear. Therefore, the current study was performed to investigate the effects of creatine depletion, by feeding creatine analogue, β-guanidinopropionic acid (β-GPA) on the properties of plantaris muscle of C57BL6 mice. Three weeks old male mice were separated into two groups randomly. One group of mice were fed regular powdered diet and other group were supplied the same diet, but containing 1% β-GPA. The amount of food, pair -fed, was gradually increased following growth, but 1 g of food was given to each mouse daily after 3 weeks. Water was supplied ad libitum. At 4th and 8th week, blood and muscle samplings were performed. Blood was withdrawn from the jugular vein under anesthesia with i.p. injection of sodium pentobarbital. Serum was saved after centrifugation and creatine content was analyzed. Plantaris muscles were sampled bilaterally. Muscle fiber types were analyzed in the cross-section of right muscles. Gene expressions were determined in the left muscles. Significant depletion of creatine and shift of fibers toward slow-type were noted, especially after 8 weeks of β-GPA feeding. Increase in the expression of genes related to slow-type fibers and mitochondrial metabolism was noted even at 4th week. It is suggested that depletion of creatine causes the transformation of muscle fibers toward slow-twitch type.