Activated Skeletal Muscle Myofibrils have Different Peak Stresses at Similar Sarcomere Lengths when Lengthened Beyond Myofilament Overlap

Abstract

We suggested previously that the free-spring region of titin in the I-band can be length-modulated; giving rise to dramatically elevated forces in actively compared to passively stretched myofibrils at sarcomere length (SL) beyond actin-myosin filament overlap (>4µm).[1]If peak stresses were to vary at very long SL (> 4µm) where titin is the sole contributor (in myofibrils) to force, then that would suggest that the free-spring length of titin can be length-modulated somehow. The result being that a shorter free-spring titin would generate higher stress at matched SL compared to a myofibril where titin length was not shortened. We stretched calcium activated (pCa+2 3.5 in all experiments) rabbit psoas myofibrils beyond myofilament overlap (>4µm) while measuring forces and mean SL. Samples were either fully activated (control) or were activated with 2mM BDM. All samples were lengthened (0.1µm/sarcomere/second) following activation. Initial activation stress at SL=2.2µm for BDM was 35.5 ±10.5nN/um2 compared to control 89.3nN/um2. The BDM activated samples had less peak stress (278±32nN/um2) compared to the control (407nN/um2) at similar mean SL 4.7±0.05µm. Previous data [1] for myofibrils lengthened passively showed less peak stress compared to all experiments here; for mean SL 4.6µm, the mean stress was 135±47nN/um2.Based on these results, we suggest that titin is a molecular spring whose stiffness may be regulated by changes in effective length and that regulation is not an “all-or-none” model. We suggest that titin is a molecular spring whose stiffness is regulated by changes in effective length controlled by force-dependent actin-titin interactions.