Forearm 3-methylhistidine efflux in myotonic dystrophy.

Myotonic dystrophy is associated with progressive muscular atrophy. To define the mechanism of muscle wasting in this disease, we studied myofibrillar proteolysis in vivo in 8 men moderately affected with myotonic dystrophy, and compared the results with those of 10 normal men. Myofibrillar proteolysis was estimated by measuring the 3-methylhistidine arteriovenous difference (A-V) and efflux (Q) across the forearm in the postabsorptive state. Plasma 3-methylhistidine concentrations were determined by high-performance liquid chromatography with postcolumn o-phthalaldehyde derivatization and fluorescence detection. Plasma flow to the forearm muscles (F) was estimated to represent 85% of total forearm plasma flow as determined by the indicator-dilution technique. Forearm 3-methylhistidine efflux was calculated as: Q = F(A-V). Mean muscle mass (24-hour creatinine excretion), lean body mass, and forearm volume were decreased in the patients with myotonic dystrophy, confirming the presence of muscle atrophy. Mean forearm 3-methylhistidine arteriovenous difference and efflux were not significantly different in the two groups. We conclude that myofibrillar protein degradation is not increased in myotonic dystrophy, even when measured in a muscle compartment selectively affected by wasting. Muscle atrophy in myotonic dystrophy is probably the result of defective anabolism rather than accelerated catabolism.