Force--sarcomere-length relation and filament length in rat extensor digitorum muscle

Relations between sarcomere length (SL) and force (F) were studied in ten fiber bundles (six to twenty fibers) from rat extensor digitorum muscles. A bundle (60 micron by 200-300 microns) was mounted in a glass covered perfusion chamber containing modified Krebs Henseleit buffer at 25 degrees C, oxygenated with 95% O2, 5% CO2 and pancuronium bromide (8 mg/1). F ( Disa 51E 01 transducer) and SL (laser diffraction and light microscopy) were measured; the latter could be controlled by a servomotor system. 200-500 ms tetanic stimulus trains were applied via platinum electrodes parallel to the muscle with 20% above maximal intensity, 160 Hz frequency and 1 ms duration of pulses. Tetani were at 2 min intervals. F attained a steady value 100 ms after the start of the tetanus at 2.0-2.5 microns SL and 350 ms at 3.5 microns SL. Active force, measured during tetani in which sarcomere length was held constant, was maximal between SL = 2.15 microns and 2.65 microns and declined in linear fashion with SL to zero at SL = 3.90 microns. Active force at SL = 2.00 microns was 95% of maximal force. Passive force was manifest above SL = 3.10 microns and was 10% of maximal force at 3.80 microns. Eight similar bundles were processed conventionally for electron microscopy (Philips EM 201A ) while SL was measured during the processing steps. Measurements were made from micrographs of longitudinal sections. SL measured from the micrographs were consistent with the observed shrinkage (5%). Actin periodicity was 41.5 +/- 0.19 nm; twenty-seven periods per actin filament were found. Filament lengths were corrected for an assumed actin periodicity of 39 nm. Actin length was 1.13 +/- 0.013 micron; myosin length was 1.53 +/- 0.015 micron. Bare zone was 0.17 micron +/- 0.01 micron. These filament lengths would give optimum overlap at SL between 2.26 and 2.43 microns and a linear decrease to zero with increasing SL from 2.43 microns to 3.79 microns. Actual force was consistently higher than predicted by overlap and force was maintained to both the left and the right of the predicted plateau.