N(τ)-methylhistidine release: Contributions of rat skeletal muscle, GI tract, and skin

The relative contributions of skeletal muscle, gastrointestinal tract, and skin to urinary N(τ)-methylhistidine (MH) excretion were estimated during in vitro studies using the rat hemicorpus preparation. After 0.5 h of perfusion, MH release into the perfusate was linear for 3 h and averaged 29.8 nmol.h^-1.100 g hemicorpus^-1. In vivo, 24-h urinary MH excretion averaged 37.3 nmol.h^-1.100 g body wt^-1. The ratio of soft tissue to skin weight is equal (3.2:1) in the whole rat and in the hemicorpus. The gastrointestinal tract released 16.0 nmol.h^-1.100 g body wt^-1 or approximately 41% of the total urinary MH excretion. Preparations perfused with or without skin showed modest differences in the rate of MH release that were not statistically significant. Skeletal muscle contains 89.8% of total body MH content, whereas gastrointestinal tract and skin contain 3.8 and 6.4%, respectively. Gastrointestinal tract actomyosin turns over rapidly with a fractional catabolic rate of 24%/day versus 1.4%/day for skeletal muscle actomyosin.