X-Ray Diffraction Resolves How Actin-Myosin Spacing Explains the Differences of Two Muscles with Identical Steady State Properties

Abstract

The actin-myosin lattice underlying muscle is dynamic. Radial lattice spacing changes during activation and during periodic axial strain like muscle experiences during locomotion. Actin-myosin spacing in muscle affects crossbridge binding rates and the steady-state length-tension properties of muscle. The interaction of macroscopic strain, lattice spacing and binding kinetics raises the possibility that small changes in lattice spacing could have profound consequences for the dynamic behavior of muscle. To study the functional implications of lattice spacing, we examined two muscles in Blaberus discoidalis. These muscles have nearly identical length-tension and force-velocity relationships and share innervation. However, when periodically activated under identical cyclical length changes, one of them does net positive mechanical work while the other is dissipative. Using the BioCAT x-ray beamline at the Advanced Photon Source at Argonne National Lab, we measured lattice spacing during dynamic oscillations and isometric twitches. We found passive lattice spacing in the two muscles to be 50.01+/−1.54 nm and 50.51+/−.6 nm (95% CI) with the second muscle's spacing consistently higher (p≈10−5). We found that in the isometric twitch case, the first muscle's lattice spacing increased to 51.15+/−1.7 while the other did not change significantly (51.3+/−.65), meaning that the spacing difference vanishes under steady state, activated conditions (p≈.19). A difference in spacing under passive conditions, but not active, means that during cyclic length changes the lattice spacing changes were larger in one muscle. In the muscle with the larger dynamic range, lattice spacing correlates with the timing of increased stress and positive mechanical work. A 1 nm transient difference in actin-myosin spacing mediates the dynamic difference between the two muscles (motor vs. brake) but also predicts their identical quasi-static properties, allowing nanoscale structure to influence macroscopic muscle function.