Myosin MgADP Release Rate Decreases with Greater Sarcomere Length and Reduced Thick-to-Thin Filament Spacing in Skinned Soleus Muscle Fibers from Rats

Abstract

The actin-myosin cross-bridge interaction uses chemical energy from MgATP hydrolysis to generate force and shortening in striated muscle. Previous studies have shown that increases in sarcomere length can reduce thick-to-thin filament spacing in skinned muscle fibers, thereby increasing force production at longer sarcomere lengths. However, it is unclear how changes in sarcomere length and lattice spacing affect cross-bridge kinetics at fundamental steps of the cross-bridge cycle, such as the MgADP release rate. We hypothesize that decreased lattice spacing, achieved through increased sarcomere length or osmotic compression of the fiber via Dextran T-500, could reduce MgADP release rate and increase cross-bridge attachment time (ton). To test this, we measured ton in skinned soleus fibers using stochastic length-perturbation analysis at 2.5 and 2.0 µm sarcomere lengths as pCa and [MgATP] varied. In the absence of Dextran, the force-pCa relationship showed greater Ca2+-sensitivity for 2.5 vs 2.0 sarcomere length fibers (pCa50=5.68±0.01 vs 5.60±0.01; pCa 4.8, 5.0 mM MgATP, 17° C). When fibers were compressed with 4% Dextran, the force-pCa relationships exhibited even greater Ca2+-sensitivities and the length-dependent increases in Ca2+-sensitivity were preserved (pCa50=5.84±0.03 vs 5.80±0.04). Without Dextran, longer sarcomere lengths also exhibited increased ton (86±5 vs 70±2 ms), resulting from a slower MgADP release rate at 2.5 vs. 2.0 sarcomere length (11.24±0.5 vs 13.14±0.7 s−1). Osmotic compression further increased ton at both sarcomere lengths (98±7 vs 73±4 ms) due to even slower MgADP dissociation from myosin (9.57±0.5 vs 12.6±0.5 s−1), yet length-dependent kinetic differences were preserved. These data suggest that skeletal muscle exhibits length-dependent changes in cross-bridge kinetics as a function of lattice spacing, which may impact overall muscle behavior as length varies.