Experimental Depletion of Creatine and Phosphocreatine from Skeletal Muscle

To evaluate the long term effects on skeletal muscle of feeding a competitive inhibitor of creatine transport, β-guanidinopropionic acid was fed to rats as 1% of their diet for 6 weeks or longer. Although these rats appeared healthy on casual inspection, the P-creatine concentration in their gastrocnemius muscles decreased from a mean value of 22.5 µmoles per g wet weight to 1.6. Instead of P-creatine, the muscles contained a new phosphorylated guanidino compound in concentrations as high as 30 µmoles per g wet weight. By chromatography on Sephadex QAE-A25 this new compound was indistinguishable from phosphorylated β-guanidinopropionate formed in vitro in a reaction mixture containing β-guanidinopropionate, rabbit muscle creatine kinase, and an ATP generating system. Hydrolysis of the new compound liberated β-guanidinopropionic acid and orthophosphate in a 1:1 molar ratio. When muscles from rats fed β-guanidinopropionic acid were caused to contract under anoxic conditions, the concentration of phosphorylated β-guanidinopropionate decreased dramatically, raising the possibility that it, like P-creatine, can serve in a system to regenerate ATP. On the other hand β-guanidinopropionate is a relatively ineffective substrate for creatine kinase in vitro, and animals fed β-guanidinopropionic acid have subnormal concentrations of glucose 6-phosphate, ATP, and ADP in resting muscle in vivo. These animals may prove useful in studies of the metabolic adaptations of skeletal muscle.