THE STIFFNESS OF SKELETAL-MUSCLE IN ISOMETRIC CONTRACTION AND RIGOR -THE FRACTION OF MYOSIN HEADS BOUND TO ACTIN

Step changes in length (between -3 and +5 nm per half-sarcomere) were imposed on isolated muscle fibers at the plateau of an isometric tetan us (tension T-0) and on the same fibers in rigor after permeabilizatio n of the sarcolemma, to determine stiffness of the half-sarcomere in t he two conditions. To identify the contribution of actin filaments to the total half-sarcomere compliance (C), measurements were made at sar comere lengths between 2.00 and 2.15 mu m, where the number of myosin cross-bridges in the region of overlap between the myosin filament and the actin filament remains constant, and only the length of the nonov erlapped region of the actin filament changes with sarcomere length. A t 2.1 mu m sarcomere length, C was 3.9 nm T-0(-1) in active isometric contraction and 2.6 nm T-0(-1) in rigor. The actin filament compliance , estimated from the slope of the relation between C and sarcomere len gth, was 2.3 nm mu m(-1) T-0(-1). Recent x-ray diffraction experiments suggest that the myosin filament compliance is 1.3 nm mu m(-1) T-0(-1 ). With these values for filament compliance, the difference in half-s arcomere compliance between isometric contraction and rigor indicates that the fraction of myosin cross-bridges attached to actin in isometr ic contraction is not larger than 0.43, assuming that cross-bridge ela sticity is the same in isometric contraction and rigor.