REPEATED FORCE PRODUCTION AND METABOLITES IN 2 MEDIAL GASTROCNEMIUS-MUSCLE COMPARTMENTS OF THE RAT

The most proximal and distal motor nerve branches in the rat medial ga strocnemius innervate discrete muscle compartments dominated by fast-t witch oxidative and fast-twitch glycolytic fibers, respectively. The f unctional consequences of the difference in oxidative capacity between these compartments were investigated. Wistar rats were anesthetized w ith pentobarbital sodium (90 mg/kg ip). Changes in force of both compa rtments during 21 isometric contractions (train duration 200 ms, stimu lation frequency 120 Hz, 3 s between contractions) were studied in sit u with and without blood flow. Without blood flow, force and phosphocr eatine declined to a greater extent in the proximal than the distal co mpartment compared with the run with intact flow. After the protocol w ithout blood flow, when flow was restored, the time constants for forc e recovery (which were closely associated to the recovery of phosphocr eatine) were 37 +/- 7 (SD) (proximal compartment) and 148 +/- 20 s (di stal compartment). It was concluded that the proximal compartment had a four times higher oxidative capacity and, therefore, a superior abil ity for repeated force production.