Center for Cardiovascular Research, Department of Physiology and Biophysics, University of Illinois at Chicago, Chicago, Illinois, USA.
Biophys J. 2010 Nov 3;99(9):2978-86. doi: 10.1016/j.bpj.2010.09.003.
Length-dependent activation (LDA) is a prominent feature of cardiac muscle characterized by decreases in the Ca(2+) levels required to generate force (i.e., increases in Ca(2+) sensitivity) when muscle is stretched. Previous studies have concluded that LDA originates from the increased ability of (strong) cross-bridges to attach when muscle is lengthened, which in turn enhances Ca(2+) binding to the troponin C (TnC) subunit of the troponin complex. However, our results demonstrate that inhibition of strong cross-bridge attachment with blebbistatin had no effect on the length-dependent modulation of Ca(2+) sensitivity (i.e., EC(50)) or Ca(2+) cooperativity, suggesting that LDA originates upstream of cross-bridge attachment. To test whether LDA arises from length dependence of thin-filament activation, we replaced native cTnC with a mutant cTnC (DM-TnC) that is incapable of binding Ca(2+). Although progressive replacement of native cTnC with DM-TnC caused an expected monotonic decrease in the maximal force (F(max)), DM-TnC incorporation induced much larger increases in EC(50) and decreases in Ca(2+) cooperativity at short lengths than at long lengths. These findings support the conclusion that LDA arises primarily from the influence of length on the modulation of the Ca(2+) cooperativity arising from interaction between adjacent troponin-tropomyosin complexes on the thin filament.
长度依赖性激活（LDA）是心肌的一个显著特征，其特征是在肌肉拉伸时，产生力所需的 Ca（2+）水平降低（即 Ca（2+）敏感性增加）。先前的研究得出结论，LDA 源自于当肌肉伸长时（强）交联的附着能力增加，这反过来又增强了 Ca（2+）与肌钙蛋白复合物的肌钙蛋白 C（TnC）亚基的结合。然而，我们的结果表明，用 blebbistatin 抑制强交联的附着对 Ca（2+）敏感性（即 EC（50））或 Ca（2+）协同性的长度依赖性调节没有影响，这表明 LDA 起源于交联附着的上游。为了测试 LDA 是否源于细肌丝激活的长度依赖性，我们用一种不能结合 Ca（2+）的突变型 cTnC（DM-TnC）替代了天然的 cTnC。尽管用 DM-TnC 逐渐替代天然的 cTnC 导致最大力（F（max））预期的单调下降，但 DM-TnC 的掺入在短长度下引起 EC（50）的增加和 Ca（2+）协同性的降低比在长长度下更大。这些发现支持这样的结论，即 LDA 主要是由于长度对相邻肌钙蛋白-原肌球蛋白复合物之间相互作用引起的 Ca（2+）协同性的调节的影响而产生的。
