We have studied the effect of a pharmacological dose of testosterone enanthate (3 mg.kg-1.wk-1 for 12 wk) on muscle mass and total-body potassium and on whole-body and muscle protein synthesis in normal male subjects. Muscle mass estimated by creatinine excretion increased in all nine subjects (20% mean increase, P less than 0.02); total body potassium mass estimated by 40K counting increased in all subjects (12% mean increase, P less than 0.0001). In four subjects, a primed continuous infusion protocol with L-[1-13C]leucine was used to determine whole-body leucine flux and oxidation. Whole-body protein synthesis was estimated from nonoxidative flux. Muscle protein synthesis rate was determined by measuring [13C]leucine incorporation into muscle samples obtained by needle biopsy. Testosterone increased muscle protein synthesis in all subjects (27% mean increase, P less than 0.05). Leucine oxidation decreased slightly (17% mean decrease, P less than 0.01), but whole-body protein synthesis did not change significantly. Muscle morphometry showed no significant increase in muscle fiber diameter. These studies suggest that testosterone increases muscle mass by increasing muscle protein synthesis.

We studied androgen receptor function in cultured scrotal skin fibroblasts from eight subjects with X-linked spinal and bulbar muscular atrophy (SBMA) (Kennedy's syndrome) from four families. The neuromuscular and endocrine features were similar in all patients. High-affinity dihydrotestosterone binding (Bmax) was decreased in three patients from one family (average, 11.1 fmol/mg) similar to values in subjects with androgen resistance syndromes. Bmax was normal in five SBMA patients from three other families (average, 26.0 fmol/mg). This finding provides direct evidence for abnormal androgen receptor function in some patients with SBMA. There was some correlation between severity of neuromuscular and endocrine dysfunction, providing further evidence that the two types of manifestations are related.

Because testosterone has an anabolic effect in myotonic dystrophy, we conducted a 12-month, randomized, double-blind therapeutic trial of testosterone enanthate (3 mg/kg/wk) in 40 men with myotonic dystrophy. We evaluated strength by manual muscle tests, quantitative myometry, pulmonary function, and quantitative functional assessment. A sustained, significant elevation of testosterone levels was produced but there was no effect on any measurement of muscle strength. Muscle mass as estimated by creatinine excretion and lean body mass (40K method) increased significantly. We conclude that testosterone does not improve strength in myotonic dystrophy despite increasing muscle mass.

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