Tanner Bertrand C W, Breithaupt Jason J, Awinda Peter O
Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, Washington
Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, Washington.
Am J Physiol Heart Circ Physiol. 2015 Dec 15;309(12):H2087-97. doi: 10.1152/ajpheart.00555.2015. Epub 2015 Oct 16.
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 (ton) 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 ton as [MgATP] varied (0.05-5 mM) at 1.9 and 2.2 μm sarcomere lengths. From this ton-MgATP relationship, we calculated cross-bridge MgADP release rate and MgATP binding rates. As MgATP increased, ton decreased for both sarcomere lengths, but ton was roughly 70% longer for 2.2 vs. 1.9 μm sarcomere length at maximally activated conditions. These ton differences were driven by a slower MgADP release rate at 2.2 μm 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 pCa50 value of the tension-pCa relationship for each sarcomere length, length-dependent increases in ton were roughly 15% longer for 2.2 vs. 1.9 μm 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.
随着肌节长度增加，心脏收缩力增强，这表明内在分子机制是Frank-Starling关系的基础，可使心输出量随着心室充盈增加而增加。肌球蛋白与肌动蛋白结合并在肌肉细胞中产生力的能力受细肌丝蛋白对Ca(2+)的调节，并且随着粗肌丝与细肌丝重叠程度因肌节长度而异而在空间上受到调节。随着肌节长度增加心脏收缩力增强的一种机制可能涉及由于在较长肌节长度下肌球蛋白动力学减慢导致的肌球蛋白附着时间（ton）延长。为了验证这一想法，我们在去表皮的大鼠乳头肌条带中使用随机长度微扰分析，在1.9和2.2μm肌节长度下，测量随着[MgATP]变化（0.05 - 5 mM）时的ton。根据这种ton-MgATP关系，我们计算了横桥MgADP释放速率和MgATP结合速率。随着MgATP增加，两种肌节长度下的ton均降低，但在最大激活条件下，2.2μm肌节长度的ton比1.9μm肌节长度大约长70%。这些ton差异是由2.2μm肌节长度下较慢的MgADP释放速率（41±3对74±7 s(-1)）驱动的，因为两种肌节长度之间的MgATP结合速率没有差异。在每个肌节长度的张力-pCa关系的pCa50值附近的次最大激活水平下，2.2μm肌节长度的ton的长度依赖性增加比1.9μm肌节长度大约长15%。横桥动力学的这些变化可能会在较长肌节长度下放大横桥对力产生和细肌丝激活的协同贡献，并表明肌球蛋白MgADP释放速率的长度依赖性变化可能有助于心脏中的Frank-Starling关系。
