EFFECT OF THIN FILAMENT LENGTH ON THE FORCE-SARCOMERE LENGTH RELATION OF SKELETAL-MUSCLE

We studied a slow-and a fast-twitch muscle fiber type of the perch that have different thin filament lengths. The force-sarcomere length relations were measured, and it was tested whether their descending limbs were predicted by the cross-bridge theory. To determine the predicted relations, filament lengths were measured by electron microscopy. Measurements were corrected for shrinkage with the use of I-band and H-zone periodicities. Thick filament lengths of the two fiber types were found to be similar (1.63 +/- 0.06 and 1.64 +/- 0.10-mu-m for slow- and fast-twitch fibers, respectively), whereas the thin filament lengths were clearly different: 1.24 +/- 0.10-mu-m (n = 86) for the slow-twitch type and 0.94 +/- 0.04-mu-m (n = 94) for the fast type. The descending limbs of the two fiber types are therefore predicted to be shifted along the sarcomere length axis by approximately 0.6-mu-m. Sarcomere length was measured on-line by laser diffraction in a single region in the center of the fibers. The passive forcesarcomere strain relation increased much more steeply in the slow-twitch fibers. The descending limb of the active forcesarcomere length relation of fast twitch fibers was linear (r = 0.92), but was found at sarcomere lengths approximately 0.1-mu-m greater than predicted. The descending limb of the slow-twitch fibers was also linear (r = 0.87) but was now found at sarcomere lengths approximately 0.05-mu-m less than predicted. The difference in position of the descending limbs of the two fiber types amounted to 0.5-mu-m, approximately 0.1-mu-m less than predicted. The difference between measured and predicted descending limbs was statistically insignificant.