BASIS OF PASSIVE TENSION AND STIFFNESS IN ISOLATED RABBIT MYOFIBRILS

By examining the mechanical properties of isolated skeletal and cardia c myofibrils in calcium-free, ATP-containing solution, we attempted to separate the stiffness contribution of titin filaments hom that of we akly bound cross bridges. Efforts to enhance weak cross-bridge binding by lowering ionic strength were met, by clear contractile responses. Even at low temperature, myofibrils bathed in low-ionic-strength relax ing solution generated increased force and exhibited sarcomere shorten ing, apparently caused by active contraction. At normal ionic strength , myofibril stiffness, estimated from the force response to rapid sinu soidal oscillations, increased steadily with sarcomere extension up to a strain limit. No obvious stiffness contribution from weak cross bri dges was detectable. Instead, the stiffness response, which was freque ncy dependent at all sarcomere lengths, was apparently generated by th e viscoelastic titin filaments. During imposed stretch-hold ramps, bot h peak force/stiffness and the amount of subsequent stress relaxation increased with higher stretch rates, larger stretch amplitudes, and lo nger sarcomere lengths. We conclude that, for a truly relaxed myofibri l, both passive force and dynamic stiffness principally reflect the in trinsic viscoelastic properties of the titin filaments.