MEASUREMENT OF 3-METHYLHISTIDINE PRODUCTION IN LAMBS BY USING COMPARTMENTAL-KINETIC ANALYSIS

The kinetics of 3-methylhistidine (3MH) metabolism in four crossbred l ambs were studied. Each lamb was injected with an intravenous dose of 3-H-2(3)!methylhistidine (d3-3MH) and the stable isotope disappearanc e in plasma and appearance in both urine and muscle were measured. Imm ediately after the administration of tracer there was a phase of rapid disappearance of tracer from the plasma, which was followed by a more gradual decrease in d3-3MH from the plasma during the last 4 d of the experiment. A minimum of three exponentials was required to describe the plasma decay curve adequately. The kinetic model of 3MH in the who le animal was constructed by using the SAAM/CONSAM computer modelling program. Two different configurations of a three-compartment model are described: (1) a simple three-pool model, in which plasma kinetics we re entered into pool 1 out of which they had one undefinable exit; (2) a plasma-urinary three-pool model with two exits, in which the urinar y kinetics were entered as an exit out of pool 1 and required a second exit out of pool 3 to produce an adequate fit. In addition, muscle ki netics from biopsies of the longissimus dorsi were entered into either pool 2 or 3 using the plasma-urinary model. Steady-state mass and tra nsport rate values were obtained for each model construct described, a nd a de novo production rate corresponding to a fractional breakdown r ate of myofibrillar protein of approximately 5 %/d was also calculated . The model predicted that only 15 % of 3MH was excreted in urine as f ree 3MH, which is consistent with current knowledge of 3MH excretion i n sheep. The simple three-pool plasma kinetic model, therefore, could be used to estimate, by a relatively simple injection-sampling techniq ue, the extent of muscle protein turnover in lambs.