Quantitation of urinary 3-methylhistidine excretion in growing dogs as an index of in vivo skeletal muscle catabolism

Purpose: To quantitate urinary 3-methylhistidine (3-mh) excretion as an index of in vivo muscle catabolism in dogs fed diets containing either normal or high protein levels. Methods: Twelve male, 5-month-old Beagle dogs were housed individually in metabolism cages and fed a non-meat, purified diet. They were divided into two diet groups of six dogs each, receiving 22.6% (NP) or 41.1% (HP) DM crude protein, respectively. Three dogs from each group received an intravenous injection of 385 ± 29 kBq [14C] 3-mh · HCl. Urine and feces were collected daily until radioactivity returned to background levels (17 days). Urinary 3-mh was measured using an amino acid analyzer and percentage of bound 3-mh was estimated via acid hydrolysis. Results: Results are reported as means ± SEM. 3-mh recovery in urine and feces of dogs were 263 ± 28 kBq and 50.7 ± 2.2 kBq and 327 ± 45 kBq and 25.9 ± 25.9 kBq for the NP and HP groups, respectively. The total cumulative 3-mh recoveries for the NP and HP groups were 81.8% ± 2.8 and 91.4% ± 2.7, respectively. Bound 3-mh accounted for 2.1 to 4.8% of urinary 14C-3-mh. Conclusions: Growing Beagle dogs excrete a higher percentage of 3-mh in feces (13.5% vs. 6.7%) when consuming the NP versus the HP diet. It appears that some of the 14C was lost in CO2 and/or re-circulated in the body, as reported for sheep and pigs. We conclude that urinary 3-mh does not appear to be a quantitative index of in vivo muscle catabolism in growing dogs.