Myosin MgADP release rate decreases at longer sarcomere length to prolong myosin attachment time in skinned rat myocardium

摘要

Cardiac contractility increases as sarcomere length increases, suggesting that intrinsic molecular mechanisms underlie the Frank-Starling relationship to confer increased cardiac output with greater ventricular filling. The capacity of myosin to bind with actin and generate force in a muscle cell is Ca~(2+) regulated by thin-filament proteins and spatially regulated by sarcomere length as thick-to-thin filament overlap varies. One mechanism underlying greater cardiac contractility as sarcomere length increases could involve longer myosin attachment time (t_(on)) due to slowed myosin kinetics at longer sarcomere length. To test this idea, we used stochastic length-perturbation analysis in skinned rat papillary muscle strips to measure t_(on) as [MgATP] varied (0.05-5 mM) at 1.9 and 2.2 mum sarcomere lengths. From this t_(on)-MgATP relationship, we calculated cross-bridge MgADP release rate and MgATP binding rates. As MgATP increased, t_(on) decreased for both sarcomere lengths, but t_(on) was roughly 70% longer for 2.2 vs. 1.9 mum sarcomere length at maximally activated conditions. These t_(on) differences were driven by a slower MgADP release rate at 2.2 mum sarcomere length (41 ± 3 vs. 74 ± 7 s~(-1)), since MgATP binding rate was not different between the two sarcomere lengths. At submaximal activation levels near the pCa_(50) value of the tension-pCa relationship for each sarcomere length, length-dependent increases in t_(on) were roughly 15% longer for 2.2 vs. 1.9 mum sarcomere length. These changes in cross-bridge kinetics could amplify cooperative cross-bridge contributions to force production and thin-filament activation at longer sarcomere length and suggest that length-dependent changes in myosin MgADP release rate may contribute to the Frank-Starling relationship in the heart.