Myofibrillar protein degradation was evaluated by measuring excreted urinary 3-methyl histidine (3-MH) (35), which is released during myofibrillar protein break- down without being either metabolized or reutilized for protein synthesis (36). In the rat, 3-MH is chiefly excreted in acetylated form. Samples were thus hydrolyzed with HCl (6 mol/L, vol/vol) at 1008C for 12 hours, then centrifuged (30 min, 2500 g, 48C) and filtered on 0.20-lm filters before quantification by ion-exchange chromatography with nin- hydrin detection (Hitachi L8500A; Tokyo, Japan) (37). Results are expressed in lmol 3-MH/mmol creatinine to
factor muscle mass into the evaluation of myofibrillar
protein breakdown.