Delayed excretion of 3-methylhistidine in goats

The purpose of this study was to evaluate the use of 3-methylhistidine (3MH) as an index of muscle protein degradation in dairy goats. The criterion for validation was the rapid and quantitative excretion of radiolabeled 3MH. Three adult dry does and four bucks were confined to metabolism cages and injected with 5 ml of L-3-[14C]methylhistidine dihydrochloride (4 microCi/mL) diluted in normal saline. The does' urine was collected from catheters and the bucks' urine from stainless steel separators protected with fecal collection bags. The daily urine samples were analyzed for labeled amino acid. Urinary recovery of radioactivity from all does was less than 33% after 3 d and was proceeding at less than 5% d. After 9 d, total recovery was less than 50% of total dose for the does. Elimination rates were slightly higher and more variable for the yearling bucks (25-63% after 4 d). On the basis of these data, 3MH does not appear to be a valid index of muscle protein degradation in either male or female dairy goats.