Changes in myosin heavy chain profile of mature regenerated muscle with endurance training in rat

The objective of the present study was to examine the response of fast-twit ch muscle to endurance training long after the muscle had regenerated from toxin injury. Seventeen male Wistar rats were randomly assigned to a sedent ary (S, n = 10) or a trained group (T, n = 7). Endurance training by treadm ill running (5 days week(-1), 30 m min(-1), 7% grade, 2 h day(-1) for 5 wee ks) was initiated 5 weeks after myofibre degeneration was induced in the ri ght extensor digitorum longus muscle (EDL) by two injections of 0.2 mL of t he unfractionated venom from Naja nigricollis snake. Gel electrophoresis an alyses showed that training alone resulted in a 140% increase in type IIX m yosin heavy chain (MHC) (P < 0.01) and a slight decrease in type IIB MHC (- 14% P < 0.05). Regeneration alone induced an increase in both type IIA and IIX MHC expression (103%, P < 0.05, and 131%. P < 0.01, respectively), and a concomitant decrease in the percentage of type IIB MHC (P < 0.05). The sh ift from type IIB toward type IIA MHC composition observed in regenerated m uscles of T rats resulted not only from an additive, but from a cumulative effect of training and regeneration. Immunohistochemical analysis of MHC co ntent in individual fibres showed similar changes. These data suggest that the impact of endurance training on fast-type MHCs was more marked in matur e regenerated muscles than in regenerating ones, and provide evidence of th e heightened plasticity of fully regenerated muscles to repeated exercise.