Creatine metabolism in skeletal muscle

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Abstract
Fourteen patient-volunteers were given tracer creatine -1-14C, and the flux of creatine through the body was followed by measuring urinary creatlnlne specific activity. Eleven patients from the present study and 7 patients from earlier studies yielded the following half-times expressed In days: 39.8 [plus or minus] 2.6 (mean [plus or minus] SD) for 4 patients with no disease of muscle, 48.6 [plus or minus] 13.8 for 5 patients with neurogenlc muscular atrophy, 39.0 [plus or minus] 7.0 for 4 patients with polymyosltls, and 18.9 [plus or minus] 5.1 for 5 patients with muscular dystrophy. Dissimilar defects In creatine metabolism In skeletal muscle can cause similar patterns of change In excretion and in tissue concentrations of creatine. At least 2 mechanisms operate to maintain creatine concentrations In skeletal muscle In man. These mechanisms are transport of creatine Into the cell and intra-cellular trapping. Each mechanism may be the composite of several Individual reactions. Creatine transport Into skeletal muscle Is reduced In neurogenlc muscular atrophy. Creatine trapping In skeletal muscle Is Ineffective In muscular dystrophy. In patients with muscular dystrophy and creatlnurla, this defect causes a short half-time for the decrease in urinary creatlnlne specific activity. A short half-time for the decrease in urinary creatlnlne specific activity is not pathognomonic for a specific type of muscular dystrophy. Changes in transport or In trapping could be caused by an abnormality In any one of several possible reactions in either mechanism; thus there may be several abnormalities that would reduce transport and several others that would Impair trapping. Present knowledge of creatine kinetics can be satisfactorily described by a mathematical model based on a multlcom-partmental flow diagram of creatine metabolism.