The factors that change contractile properties of rat hindlimb muscles: Hindlimb suspension and feeding of creatine analogue

Changes in contractile properties of soleus and extensor digitorum longus in response to hindlimb suspension and/or altered high-energy phosphate contents were studied in rats. High-energy phosphates, especially phosphocreatine, in hindlimb muscles were lowered by feeding creatine analogue β-guanidinopropionic acid (β-GPA), but they were recovered after 10-d supplementation of creatine. The one-half relaxation time was increased by feeding β-GPA. Fatigue resistance was also improved in muscle treated with β-GPA. Such a shift was reversed by feeding creatine or by hindlimb suspension : However, the suspension induced shift of contracile properties toward fast-type was not prevented completely by β-GPA feeding. It is suggested that the levels of high-energy phosphates and tension production play important roles in the regulation of contractile properties.