SARCOMERE RELAXATION IN HAMSTER DIAPHRAGM MUSCLE

We characterized instantaneous sar comere relaxation over the load con tinuum in isolated hamster diaphragm muscles by means of laser diffrac tion. In afterloaded twitches, sarcomere relaxation displayed two cons ecutive phases. The bulk of sarcomere lengthening occurred during the first phase and corresponded in time to muscle lengthening. The second phase of sarcomere relaxation was slower and corresponded in time to tension decay At initial muscle length, the peak velocity of sarcomere lengthening (SVL) was linearly related to both the maximum extent of sarcomere shortening (delta SL) and sarcomere length at peak shortenin g (SL(min); each P < 0.01). Varying preload modified the SVL vs. SL(mi n) relationship but not the SVL vs. delta SL relationship. At a given preload, muscle tension decay began at a similar sarcomere length, reg ardless of the afterload level. In conclusion, our results support the role played by sarcomere length in regulating the diaphragm muscle-le ngthening rate but not the rate of tension decline.