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 my ofibrillar proteolysis in vivo in 8 men moderately affected with myoto nic dystrophy, and compared the results with those of 10 normal men. M yofibrillar proteolysis was estimated by measuring the 3-methylhistidi ne arteriovenous difference (A - V) and efflux (Q) across the forearm in the postabsorptive state. Plasma 3-methylhistidine concentrations w ere determined by high-performance liquid chromatography with postcolu mn o-phthalaldehyde derivatization and fluorescence detection. Plasma flow to the forearm muscles (F) was estimated to represent 85% of tota l forearm plasma flow as determined by the indicator-dilution techniqu e. Forearm 3-methylhistidine efflux was calculated as: Q = F(A - V). M ean muscle mass (24-hour creatinine excretion), lean body mass, and fo rearm volume were decreased in the patients with myotonic dystrophy, c onfirming the presence of muscle atrophy. Mean forearm 3-methylhistidi ne arteriovenous difference and efflux were not significantly differen t in the two groups. We conclude that myofibrillar protein degradation is not increased in myotonic dystrophy, even when measured in a muscl e compartment selectively affected by wasting. Muscle atrophy in myoto nic dystrophy is probably the result of defective anabolism rather tha n accelerated catabolism.