Technical note: the use of a compartmental model to estimate the de novo production rate of N tau-methylhistidine in cattle.

Urinary N tau-methylhistidine (NMH) excretion has been used as an index of muscle protein breakdown in cattle. An alternative means to estimate muscle proteolysis in cattle is to estimate the de novo production of NMH from plasma kinetics isotopically. Three crossbred steers (average 229 kg) were given a 5.0-mg bolus intravenous injection of [methyl-2H3-N tau-methylhistidine (d3-NMH), after which 16 serial blood samples and three consecutive 24-h urine samples were taken. The enrichment of NMH in plasma was determined by gas chromatography-mass spectrometry, and compartmental analysis of the kinetic data was performed using the SAAM modeling program. The NMH production rates per day (NMHPR, micromoles per day) were 732, 782, and 725, and the fractional breakdown rates (FBR, percentage per day) were 1.61, 1.72, and 1.58 as determined by urinary excretion of NMH, by a three-pool catenary model (plasma kinetics, Model A), and by a more descriptive, three-pool model with two response curves (both plasma and urine kinetics, Model B), respectively. Model A and B estimates of NMHPR and FBR were similar (P greater than .25) to those of estimates obtained from urinary NMH excretion. Kinetic modeling also allows calculation of compartment mass and flux of NMH between compartments and indicates that when NMH exists the muscle pool it is rapidly excreted via the urine. In conclusion, kinetic modeling offers an alternative approach to estimating the NMH production rate.