Staircase but not posttetanic potentiation in rat muscle after spinal cord hemisection

Spinal cord hemisection (SCH) results in atrophy of skeletal muscle and altered contractile properties. In this study our purpose was to assess staircase and posttetanic potentiation in the rat gastrocnemius muscle in situ, 1 week after SCH. Tetanic force was reduced by SCH, but twitch amplitude was not. The time course and magnitude of staircase during stimulation at 5 HZ (for 21 s) was similar in the control, sham-operated, and SCH groups. However, posttetanic potentiation observed after 100-, 500-, and 1000-ms tetanic (200 HZ) contractions was absent or drastically reduced after SCH. Twitch force increased 44+/-8.7%, 47+/-7.4%, and 15+/-2.8% for the control, sham, and SCH groups, respectively, after the 1000-ms tetanic contraction. After the 1000-ms tetanic contraction, twitch active force decreased in all groups and was significantly reduced at 5 min relative to pretetanic twitch. In the control and sham groups, but not SCH, the active force recovered to the pretetanic level by 15 min. Resting regulatory light chain (RLC) phosphorylation was 15.4+/-2.5% and 10.97+/-3.3% for the control and SCH groups, respectively. After the 1-s tetanic contraction, values were 41.6+/-2.8% and 9.3+/-2.9%, respectively. The potentiation observed in the SCH animals with 1000-ms contraction apparently occurred without increases in RLC phosphorylation. One week after SCH there were clear changes in the contractile properties typically associated with prior activation. It is concluded that activity-dependent potentiation can occur by a mechanism that is independent of RLC phosphorylation, and accelerated fatigue can mask the potentiating effects of prior activity.