Differences in I band structure, sarcomere extensibility, and electrophoresis of titin between two muscle fiber types of the perch (Perca fluviatilis L.)

I bands of perch muscle contain crosslinks that interconnect the thin filaments as well as the connecting filaments and link them to the membrane systems and to other intermyofibrillar material. In shortened sarcomeres cross connections seem to pile up in the I band and at the A/I band junction. Sarcomeres of red fibers, which in another study are reported to have steep passive stress-strain curves, were pulled apart when they were stretched by about 40% of their slack length. Sarcomeres of white fibers could be stretched by almost 100% and still display filament alignment. Sarcomeres of the red fibers also differed from those of white fibers in the distance over which connecting filaments could be discerned, the presence of an extra transverse filament in their NI lines, the electrophoretic mobility of titin, and the titin/actin ratio.