QUANTITATIVE EXCRETION OF 3-METHYLHISTIDINE IN URINE OF CATS AS A MEASURE OF IN-VIVO SKELETAL-MUSCLE PROTEIN CATABOLISM

The purpose of this study was to evaluate the use of urinary 3-methylh istidine excretion as art index of in vivo skeletal muscle protein deg radation in cats. The criterion for validation was the rapid and quant itative excretion of an intravenously administered dose of radiolabele d 3-methylhistidine (3-methyl-C-14). Four adult cats were maintained i n individual metabolism cages and allowed free access to a purified di et (43.5% protein) and water for 4 weeks. The cats were then injected intravenously with 740 kBq 3-[C-14]methylhistidine dihydrochloride dil uted in 1 mL of saline. Twenty-four-hour urine samples were collected for 7 days. Total radioactivity in each urine sample was determined by direct counting. Quench correction was determined by using an externa l standard. The mean (+/-SEM) cumulative urinary recovery of 3-[C-14]m ethylhistidine from the four cats was 94.9 +/- 3.5% at 48 hr following radioisotope injection. The mean (+/-SEM) cumulative urinary recovery of radioactivity from the four cars was 103.9 +/- 2.2% at 7 days foll owing radioisotope administration. There was no detectable radioactivi ty found in expired CO2 and negligible amounts (mean +/- SEM: 0.6 +/- 0.5%) in the feces. Chromatography of urinary amino acids and radioact ive urine metabolites revealed no significant radioactivity in any oth er peak besides 3-methylhistidine. Acid hydrolysis of urine resulted i n no increase in 3-methylhistidine content ol urine, indicating that t here is no significant acetylation of 3-methylhistidine in this specie s. On the basis of these results, 3-methylhistidine does not appear to be metabolized and should therefore be a valid index of in vivo skele tal muscle protein degradation in the cat. Urinary 3-methylhistidine e xcretion should be useful for studying how nutritional, hormonal, and other physiological or pathological factors cause losses or gains in s keletal muscle protein in this species.