Thyrotoxic myopathy in mice: accentuation by a creatine transport inhibitor.

To demonstrate the importance of creatine and phosphocreatine in skeletal muscle during periods of metabolic stress, thyrotoxicosis was induced in mice fed the creatine transport inhibitor, beta-guanidinopropionic acid (beta-GPA). Adding 2% of beta-GPA to the diet of normal mice inhibited weight gain and caused a 75% reduction of creatine and phosphocreatine concentrations in skeletal muscle. Addition of 0.25% or 2% of thyroid powder to the diet of normal mice was associated with hyperactivity, cardiomegaly, and a high mortality rate. Superimposing thyrotoxicosis on mice already depleted of creatine and phosphocreatine resulted in degeneration of muscle fibers. These results indicate that high concentrations of creatine and phosphocreatine are essential for the maintenance of muscle integrity during periods of metabolic stress.