Stehle Robert, Krüger Martina, Pfitzer Gabriele
Institute of Physiology, University of Cologne, Robert-Koch-Str. 39, D-50931 Köln, Germany.
Adv Exp Med Biol. 2003;538:469-79; discussion 479. doi: 10.1007/978-1-4419-9029-7_43.
The ability of force-generating cross-bridges to activate the thin filament in cardiac muscle was tested by studying the effects of initial force and [MgADP] on force relaxation kinetics in subcellular myofibrillar bundles prepared from left ventricles of the guinea pig. Relaxation was initiated by rapidly reducing the [Ca(2+)] from pCa 4.5 to 7.5. Initiating relaxation from lower force levels during pre-steady-state force development did not significantly accelerate the kinetics of the force decay compared to relaxations initiated from steady-state force development. This suggests that the force-generating cross-bridges which become formed during maximally Ca(2+)-activated steady-state contractions do not maintain thin filament activation for significant enough times after Ca(2+)-removal to exert a rate-limiting influence on force relaxation kinetics. Adding 2 mM MgADP to solutions slowed down relaxation kinetics approximately 4-fold. To differentiate whether these slower kinetics result from either (1) MgADP favoring accumulation of cross-bridges during the preceding contraction in a state of activating capability or (2) slow-down of cross-bridge turnover by the presence of the product MgADP during relaxation, the [MgADP] was either increased or removed at the time of Ca(2+)-removal. The addition of 2 mM MgADP to activating solutions (subsequent relaxation in the absence of MgADP) slowed-down the kinetics of the initial, slow, linear force decay following Ca(2+)-removal approximately 1.5-fold, suggesting that the high [MgADP] during contraction favors formation of cross-bridges which contribute in rate-limiting early relaxation kinetics by transiently sustaining thin filament activation. On the other hand, the addition of 2 mM MgADP to the relaxing solution (preceding Ca(2+)-activation in absence of MgADP) slowed-down the kinetics of the initial force decay approximately 3-fold, more similar to the kinetics observed in the continuous presence of 2 mM MgADP both before and after Ca(2+)-removal. This suggest that, despite some influence of cross-bridge activation, the main effect of MgADP on relaxation kinetics results from product inhibition of cross-bridge turnover. In summary, whereas under certain conditions (high [MgADP]) cross-bridge activation of the thin filament can weakly take part in rate-limiting relaxation kinetics induced by complete Ca(2+)-removal, cross-bridge activation does not influence relaxation kinetics under more physiologically normal conditions.
通过研究初始力和[MgADP]对豚鼠左心室亚细胞肌原纤维束中力松弛动力学的影响，测试了产生力的横桥激活心肌细肌丝的能力。通过将[Ca(2+)]从pCa 4.5快速降至7.5来启动松弛。与从稳态力发展开始的松弛相比，在预稳态力发展期间从较低力水平开始松弛并没有显著加速力衰减的动力学。这表明在最大Ca(2+)-激活的稳态收缩过程中形成的产生力的横桥在Ca(2+)-去除后不会维持细肌丝激活足够长的时间，从而对力松弛动力学产生限速影响。向溶液中添加2 mM MgADP使松弛动力学减慢约4倍。为了区分这些较慢的动力学是由于(1)MgADP有利于在前一次收缩期间在具有激活能力的状态下积累横桥，还是(2)在松弛期间由于产物MgADP的存在导致横桥周转减慢，在Ca(2+)-去除时增加或去除[MgADP]。向激活溶液中添加2 mM MgADP（随后在无MgADP的情况下松弛）使Ca(2+)-去除后初始的、缓慢的、线性力衰减的动力学减慢约1.5倍，这表明收缩期间的高[MgADP]有利于形成横桥，这些横桥通过短暂维持细肌丝激活对限速早期松弛动力学有贡献。另一方面，向松弛溶液中添加2 mM MgADP（在无MgADP的情况下进行Ca(2+)-激活之前）使初始力衰减的动力学减慢约3倍，更类似于在Ca(2+)-去除前后持续存在2 mM MgADP时观察到的动力学。这表明，尽管横桥激活有一定影响，但MgADP对松弛动力学的主要影响是由于产物对横桥周转的抑制。总之，在某些条件下（高[MgADP]），细肌丝的横桥激活可以微弱地参与由完全Ca(2+)-去除诱导的限速松弛动力学，而在更生理正常的条件下，横桥激活不影响松弛动力学。
