Influence of tension time on muscle fiber sarcolemmal injury in rat diaphragm

We hypothesized that the amount of sarcolemmal injury is directly related t o the total tension time (TTtot), calculated as mean tension x total stimul ation time. Diaphragm strips from Sprague-Dawley rats were superfused at op timal muscle length with Krebs containing procion orange to identify sarcol emmal injury. TTtot was induced by stimulation with 100 Hz for 3 min at dut y cycles of 0.02, 0.15, 0.3, and 0.6, or with continuous contractions at 0. 2, 0.4, 0.6, and 1.0 of maximal tension. A significant positive correlation between TTtot and the percentage of fibers with injured sarcolemma (r(2) = 0.63, P < 0.05) is seen. Stimulation (at 100 Hz, duty cycle 1) resulted in fast fatigue with low injury, likely caused by altered membrane conductivi ty. Stimulations inducing the largest injury are those showing progressive force loss and high TTtot, where injury may be due to activation of membran e degradative enzymes. The maximal tension measured at 20 min poststimulati on was inversely related to the number of fibers injured, suggesting loss o f force is caused by cellular injury.