Measurement of 3-methylhistidine production in lambs by using compartmental-kinetic analysis.
The kinetics of 3-methylhistidine (3MH) metabolism in four crossbred lambs were studied. Each lamb was injected with an intravenous dose of 3-[2H3]methylhistidine (d3-3MH) and the stable isotope disappearance in plasma and appearance in both urine and muscle were measured. Immediately 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 whole 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 were 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 urinary 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 kinetics from biopsies of the longissimus dorsi were entered into either pool 2 or 3 using the plasma-urinary model. Steady-state mass and transport rate values were obtained for each model construct described, and a de novo production rate corresponding to a fractional breakdown rate of myofibrillar protein of approximately 5%/d was also calculated. The model predicted that only 15% of 3MH was excreted in urine as free 3MH, which is consistent with current knowledge of 3MH excretion in sheep. The simple three-pool plasma kinetic model, therefore, could be used to estimate, by a relatively simple injection-sampling technique, the extent of muscle protein turnover in lambs.
Rathmacher
J A
JA
Department of Animal Science, Iowa State University, Ames 50011.
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D000818:Animals; D003198:Computer Simulation; D005260:Female; D007275:Injections, Intravenous; D008297:Male; D008762:Methylhistidines / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics* / Q000652:urine; D008954:Models, Biological; D009132:Muscles / Q000378:metabolism*; D012756:Sheep / Q000378:metabolism*; D013997:Time Factors
D008762:Methylhistidines; C028118:3-methylhistidine