Interaction between titin and thin filaments in intact cardiac muscle

Abstract

A ’freeze break‘ technique and immunoelectronmicroscopy were used to study the elastic properties of cardiactitin filaments. Small bundles consisting of a few fibres fromfreshly prepared dog papillary muscle were quickly frozen andbroken under liquid nitrogen to fracture sarcomeres in planesperpendicular to the filament axes. Breaks occurred at each ofseveral regions along the sarcomeres. The still-frozen specimenswere thawed during fixation to allow elastic filaments toretract. The broken muscle segments were then treated withmonoclonal titin antibody 9D10 which labelled a unique epitope inthe I-band. In sarcomeres broken at the A-I junction, the titinfilaments reacted toward the Z-line, independently of the thinfilaments. The retracted epitopes did not reach the Z-line;retraction stopped at the N1-line level. In sarcomeres brokennear the Z-line, the titin filaments retracted in the oppositedirection, to the tip of the thick filaments. When the breakoccurred in the A-band, by contrast, the titin-epitope positionwas unaffected. On the basis of these results, and despite thereported interaction of titin and actin in vitro, it appears thatcardiac titin molecules form elastic filaments that arefunctionally independent of the thin filaments. Near the Z-line,however, the titin filaments seem to associate firmly with thethin filaments