The Stiffness of Skeletal Muscle in Isometric Contraction and Rigor: The Fraction of Myosin Heads Bound to Actin

Abstract

Step changes in length (between -3 and +5 nm per half-sarcomere) were imposed on isolated muscle fibers at the plateau of an isometric tetanus (tension T0) and on the same fibers in rigor after permeabilization of the sarcolemma, to determine stiffness of the half-sarcomere in the two conditions. To identify the contribution of actin filaments to the total half-sarcomere compliance (C), measurements were made at sarcomere lengths between 2.00 and 2.15 microm, 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 nonoverlapped region of the actin filament changes with sarcomere length. At 2.1 microm sarcomere length, C was 3.9 nm T0(-1) in active isometric contraction and 2.6 nm T0(-1) in rigor. The actin filament compliance, estimated from the slope of the relation between C and sarcomere length, was 2.3 nm microm(-1) T0(-1). Recent x-ray diffraction experiments suggest that the myosin filament compliance is 1.3 nm microm(-1) T0(-1). With these values for filament compliance, the difference in half-sarcomere compliance between isometric contraction and rigor indicates that the fraction of myosin cross-bridges attached to actin in isometric contraction is not larger than 0.43, assuming that cross-bridge elasticity is the same in isometric contraction and rigor.

Spatial and temporal traction response in human airway smooth muscle cells

“…Strain energy is a particularly useful measure of the contractile strength. For instance, a single actin-myosin interaction can generate a maximum energy of about 10 Ϫ8 pJ (9,21). The strain energy we measured in a single cell stimulated with 10 M histamine is of the order of 1 pJ.…”

Mechanism of force generation by myosin heads in skeletal muscle

“…We used a distributed filament compliance formalism 21,29,30 to calculate the force and strain distribution along the myosin and actin filaments. The strain in myosin heads during isometric contraction was assumed to be uniform along the filament overlap zone.…”

“…18). The discrepancy is reduced by increasing the value of kS A l, but would not be eliminated even if kS A l were twice the measured value 20,21 , in which case almost the whole compliance of the muscle sarcomere would be in the filaments, with virtually no contribution left for the myosin heads.…”

“…The filament compliances are usually expressed as the average strain in the actin or myosin filament, kS A l or kS M l, owing to the active isometric force (T 0 ). kS A l is 0.26%/T 0 in the conditions used here 20,21 . kS M l is the slope of the relationship between the myosin filament periodicity d and the normalized force T/T 0 .…”

“…The total instantaneous compliance of each set of overlapping myosin and actin filaments crosslinked by myosin heads during active isometric contraction is 5.1 nm/T 0 . Of this, roughly 40% is due to the compliance of the actin filaments, 20% to that of the myosin filaments, and 40% to that of the myosin heads 20,21 .…”