Hysteresis and Efficiency in Passive Skeletal Muscle Myofibrils

Abstract

Determining mechanical properties of titin is difficult but the passive properties of single myofibrils are mostly attributed to titin. We investigated whether myofibril behaviour mirrors that of single titin molecules. Single myofibrils were subjected to three passive stretch-shortening cycles of up to 3.5μm/sarcomere at a speed of 0.1 μm/sarcomere/second.Stretched-shortened myofibrils (Figure 1) show reduced force during stretch for a given sarcomere length (SL) and reduced peak force for cycles 2 and 3 compared to cycle 1. Force-SL curves for all 3 cycles during the shortening are similar. We see increased efficiency (shortening energy/lengthening energy) with repeated cycles (37%, 48% and 53%). These properties are in general agreement with results observed in single titin preparations (Kellermayer et al., 1997). Titin properties can be studied using single myofibrils. In the future, we would like to study titin properties in calcium activated myofibrils in which active (actin-myosin based cross-bridges forces) are eliminated either by chemical inhibition or by deletion of regulatory proteins on actin.Figure 1. Force and mean sarcomere length for a myofibril subjected to 3 sequential stretch-shortening cycles. Insert shows hysteresis behaviour of a single titin molecule (Kellermayer et al., 1997; Science:276-1112-1116).View Large Image | View Hi-Res Image | Download PowerPoint Slide