Department of Physiology and Biophysics, University of Illinois at Chicago School of Medicine , Chicago, IL, USA.
Charles E. Schmidt College of Medicine, Florida Atlantic University , Boca Raton, FL, USA.
J Gen Physiol. 2023 Apr 3;155(4). doi: 10.1085/jgp.202012821. Epub 2023 Mar 6.
Cardiac troponin I (cTnI) of higher vertebrates has evolved with an N-terminal extension, of which deletion via restrictive proteolysis occurs as a compensatory adaptation in chronic heart failure to increase ventricular relaxation and stroke volume. Here, we demonstrate in a transgenic mouse model expressing solely N-terminal truncated cTnI (cTnI-ND) in the heart with deletion of the endogenous cTnI gene. Functional studies using ex vivo working hearts showed an extended Frank-Starling response to preload with reduced left ventricular end diastolic pressure. The enhanced Frank-Starling response effectively increases systolic ventricular pressure development and stroke volume. A novel finding is that cTnI-ND increases left ventricular relaxation velocity and stroke volume without increasing the end diastolic volume. Consistently, the optimal resting sarcomere length (SL) for maximum force development in cTnI-ND cardiac muscle was not different from wild-type (WT) control. Despite the removal of the protein kinase A (PKA) phosphorylation sites in cTnI, β-adrenergic stimulation remains effective on augmenting the enhanced Frank-Starling response of cTnI-ND hearts. Force-pCa relationship studies using skinned preparations found that while cTnI-ND cardiac muscle shows a resting SL-resting tension relationship similar to WT control, cTnI-ND significantly increases myofibril Ca2+ sensitivity to resting tension. The results demonstrate that restrictive N-terminal deletion of cTnI enhances Frank-Starling response by increasing myofilament sensitivity to resting tension rather than directly depending on SL. This novel function of cTnI regulation suggests a myofilament approach to utilizing Frank-Starling mechanism for the treatment of heart failure, especially diastolic failure where ventricular filling is limited.
高等脊椎动物的心肌肌钙蛋白 I（cTnI）进化出了 N 端延伸结构，在慢性心力衰竭中，通过限制性蛋白水解发生 N 端缺失，以此作为代偿适应机制，增加心室舒张和每搏输出量。在此，我们通过在心脏中仅表达 N 端截断的 cTnI（cTnI-ND）的转基因小鼠模型来证明这一点，该模型中内源 cTnI 基因被删除。使用离体工作心脏的功能研究表明，与左心室舒张末期压力降低相关的前负荷的 Frank-Starling 反应得到了延长。增强的 Frank-Starling 反应有效地增加了收缩期心室压力的发展和每搏输出量。一个新的发现是，cTnI-ND 增加了左心室舒张速度和每搏输出量，而不增加舒张末期容积。一致地，cTnI-ND 心肌中用于最大力发展的最佳静息肌节长度（SL）与野生型（WT）对照没有差异。尽管 cTnI 中的蛋白激酶 A（PKA）磷酸化位点被删除，但β-肾上腺素能刺激仍然可以有效地增强 cTnI-ND 心脏的增强的 Frank-Starling 反应。使用去皮制备物进行的力-pCa 关系研究发现，尽管 cTnI-ND 心肌显示出与 WT 对照相似的静息 SL-静息张力关系，但 cTnI-ND 显著增加了肌球蛋白丝对静息张力的 Ca2+敏感性。结果表明，cTnI 的限制性 N 端缺失通过增加肌球蛋白丝对静息张力的敏感性来增强 Frank-Starling 反应，而不是直接依赖于 SL。cTnI 调节的这种新功能表明，利用 Frank-Starling 机制治疗心力衰竭，特别是心室充盈受限的舒张性心力衰竭，可以采用肌丝方法。
