Skeletal muscle accretion and turnover in lean and obese (ob/ob) mice

Abstract

Skeletal muscle growth in obese (ob/ob) and lean mice was estimated from changes in fat-free carcass weight during development. No differences were observed at 2 wk of age, but fat-free carcasses of obese mice weighed less than those of lean mice at 5 wk of age. Fat-free carcasses of the adult obese mice were only 78% as heavy as those of lean mice, although the obese mice weighed up to twice as much as the lean mice. Even though obese mice accumulated less muscle than lean mice, urinary creatinine output of the obese mice was equal to that of the lean mice. The amount of 3-methylhistidine (3MH) in the carcasses of the mice and urinary output of 3MH were used to calculate the fractional breakdown rate (FBR) of myofibrillar proteins. Accumulation of 3MH in the carcasses paralleled carcass weight gain; thus, obese mice accumulated less 3MH than lean mice. Urinary output of 3MH was measured from 5 to 37 wk of age and was as great in the obese as in the lean mice; consequently, the FBR of the myofibrillar proteins was faster in the obese than in the lean mice. The FBR averaged 8.5 ± 0.6%/day for obese mice and 5.0 ± 0.1%/day for lean mice. The rate of 3MH accumulation in the carcass and the FBR were subsequently utilized to calculate a fractional synthesis rate (FSR) for the myofibrillar proteins. The FSR was not reduced in obese mice. The results suggest that obese mice accumulate less muscle because the rate of muscle degradation is increased.