RECOVERY OF SYNERGISTIC SKELETAL-MUSCLE FUNCTION FOLLOWING ISCHEMIA

This study tests the hypothesis that if the fast twitch muscles in a synergistic group were more susceptible to ischemia/reperfusion injury, then the slow twitch muscle would compensate functionally during recovery. Rat hindlimb fast twitch gastrocnemius and plantaris muscles and slow twitch soleus muscle were studied. In the experimental (E) group of rats, the right hindlimbs had 2 hr of pressure-controlled (300 mmHg) tourniquet ischemia. The masses and the maximal isometric tetanic forces of the three muscles were evaluated at 1, 3, 5, and 7 weeks in E (n = 24) and a control (C) group of rats (n = 24). Gastrocnemius mass and plantaris mass were both reduced (at 1, 3, and 5 weeks and at 1 and 3 weeks, respectively), whereas there were no significant changes in the mass of the soleus. The maximal isometric tetanic forces (N) measured at 1 week of recovery were reduced to 52, 53, and 67% of C values for the gastrocnemius, plantaris, and soleus, respectively. However, at 1 week the normalized isometric tetanic forces (N/g) for all three muscles were reduced to 66-69% of the C values. By Week 3, the tetanic forces (N or N/g) of all muscles had recovered to control values. On average, the gastrocnemius, plantaris, and soleus muscles of the C groups contributed 68, 24, and 8%, respectively, of the total synergistic tetanic force. These values were unaffected by ischemia. From this experiment, it is concluded that 2 hr of tourniquet ischemia resulted in a selective decrement in mass of the fast twitch muscles. However, there were no differential effects of ischemia on muscle force that were due to muscle fiber type, and subsequently functional compensation by the slow twitch soleus muscle was not detected.