Cinematographic studies on the A-band length changes during Ca-activated contraction in horseshoe crab muscle myofibrils.

Abstract

Cinematographic recordings of sarcomere shortening were performed on glycerinated horseshoe crab muscle myofibrils during Ca-activated contraction. When the preparations at slack length (sarcomere length, 7-9 microns) were locally activated with iontophoretically applied Ca ions, the A-band length did not change appreciably while the activated sarcomeres shortened linearly with a velocity similar to the maximum shortening velocity measured on intact muscle fibers. If, on the other hand, previously stretched preparations (sarcomere length, 11-14 microns) were locally activated, the A-band length first increased by 40-50% and then shortened to the initial length, while the activated sarcomeres continued to shorten. These results indicate that the thick filament shortening may not be associated with the physiological sarcomere shortening; the transient A-band lengthening with long initial sarcomere lengths may result from the transient misalignment of the thick filaments followed by their realignment, implying that the force exerted by the cross-bridge is not constant but may vary according to its past history.