ESTIMATION OF 3-METHYLHISTIDINE PRODUCTION IN PIGS BY COMPARTMENTAL ANALYSIS

Direct in vivo methodology is not available to accurately evaluate mus cle turnover in pigs. Urinary 3-methylhistidine (3MH) excretion, which is used as an in vivo marker of muscle protein breakdown in humans an d cattle, is not a valid indicator for pigs. The present study propose s that data from a single bolus dose of S-[methyl-H-2(3)]methylhistidi ne tracer can mathematically describe 3MH metabolism in pigs. Plasma c oncentration of the tracer is described by a linear time-invariant thr ee-compartment model by using the SAAM/CONSAM computer modeling progra m. The model defines masses and fluxes of 3MH within the pigs and, in particular, the intracellular de novo production of 3MH, which should reflect muscle proteolysis. The de novo production of 3MH as calculate d by the model was 621 mu mol/d, corresponding to a fractional breakdo wn rate of 2.28 %/d, which is similar to values reported by using indi rect methodology. These data also suggest that certain model compartme nts may be indicators of body muscle mass (mass of compartment 3, r = .59, P = .006). The mathematical model developed does not depend on ur ine collections and can be used to assess changes in muscle proteolysi s in vivo.