INTERACTION BETWEEN TITIN AND THIN-FILAMENTS IN INTACT CARDIAC-MUSCLE

A 'freeze break' technique and immunoelectron microscopy were used to study the elastic properties of cardiac titin filaments. Small bundles consisting of a few fibres from freshly prepared dog papillary muscle were quickly frozen and broken under liquid nitrogen to fracture sarc omeres in planes perpendicular to the filament axes. Breaks occurred a t each of several regions along the sarcomeres. The still-frozen speci mens were thawed during fixation to allow elastic filaments to retract . The broken muscle segments were then treated with monoclonal titin a ntibody 9D10 which labelled. a unique epitope in the I-band. In sarcom eres broken at the A-I junction, the titin filaments reacted toward th e Z-Iine, independently of the thin filaments. The retracted epitopes did not reach the Z-line; retraction stopped at the N-1-line level. In sarcomeres broken near the Z-line, the titin filaments retracted in t he opposite direction, to the tip of the thick filaments. When the bre ak occurred in the A-band, by contrast, the titin-epitope position was unaffected. On the basis of these results, and despite the reported i nteraction of titin and actin in vitro, it appears that cardiac titin molecules form elastic filaments that are functionally independent of the thin filaments. Near the Z-line, however, the titin filaments seem to associate firmly with the thin filaments.