Contractile protein breakdown in human leg skeletal muscle as estimated by [2H3]-3-methylhistidine: a new method

3-Methylhistidine urinary excretion and net balances across the leg or forearm have been used as markers of contractile protein breakdown in muscle tissue. Here we investigate whether infusion of labeled 3-methylhistidine and the measurement of the arteriovenous dilution of the tracer with unlabeled 3-methylhistidine will result in more consistent and precise measurements of 3-methylhistidine rates of appearance and consequently muscle contractile protein breakdown rates in comparison with conventional arteriovenous concentration difference measurements. Six healthy volunteers were studied in the postabsorptive state and received a primed continuous infusion of 3-[2H3-methyl]- methylhistidine and L-[ring-2H5]-phenylalanine for 4 hours. 2H3-3-methylhistidine reached an isotopic steady state after 210 minutes in all subjects. Arteriovenous differences of 3-methylhistidine, measured by high-performance liquid chromatography (HPLC), showed both uptake and release from skeletal muscle, which is theoretically not likely to occur. The enrichment of 2H3-3-methylhistidine was consistently lower in the femoral vein than in the artery, and therefore a constant net release of 3-methylhistidine from the leg was observed. The mean rates of appearance for 3-methylhistidine and phenylalanine were 0.44 +/- 0.30 nmol x min(-1) x 100 mL(-1) and 11.2 +/- 5.7 nmol x min(-1) x 100 mL(-1), respectively. In summary, arteriovenous difference measurement of 2H3-3-methylhistidine enrichment is more reliable than measurement of arteriovenous difference of unlabeled 3-methylhistidine. Consequently, measuring rates of appearance from leg muscle using labeled 3-methylhistidine resulted in more consistent and accurate values of contractile protein degradation rates in human skeletal muscle.