Impairment of Excitation-Contraction Coupling in Right Ventricular Hypertrophied Muscle with Fibrosis Induced by Pulmonary Artery Banding

Interstitial myocardial fibrosis is one of the factors responsible for dysfunction of the heart. However, how interstitial fibrosis affects cardiac function and excitation-contraction coupling (E-C coupling) has not yet been clarified. We developed an animal model of right ventricular (RV) hypertrophy with fibrosis by pulmonary artery (PA) banding in rats. Two, four, and six weeks after the PA-banding operation, the tension and intracellular Ca2+ concentration of RV papillary muscles were simultaneously measured (n = 33). The PA-banding rats were clearly divided into two groups by the presence or absence of apparent interstitial fibrosis in the papillary muscles: F+ or F- group, respectively. The papillary muscle diameter and size of myocytes were almost identical between F+ and F-, although the RV free wall weight was heavier in F+ than in F-. F+ papillary muscles exhibited higher stiffness, lower active tension, and lower Ca2+ responsiveness compared with Sham and F- papillary muscles. In addition, we found that the time to peak Ca2+ had the highest correlation coefficient to percent of fibrosis among other parameters, such as RV weight and active tension of papillary muscles. The phosphorylation level of troponin I in F+ was significantly higher than that in Sham and F-, which supports the idea of lower Ca2+ responsiveness in F+. We also found that connexin 43 in F+ was sparse and disorganized in the intercalated disk area where interstitial fibrosis strongly developed. In the present study, the RV papillary muscles obtained from the PA-banding rats enabled us to directly investigate the relationship between fibrosis and cardiac dysfunction, the impairment of E-C coupling in particular. Our results suggest that interstitial fibrosis worsens cardiac function due to 1) the decrease in Ca2+ responsiveness and 2) the asynchronous activation of each cardiac myocyte in the fibrotic preparation due to sparse cell-to-cell communication.

心肌间质纤维化（interstitial myocardial fibrosis）是导致心功能障碍的诱因之一。然而，心肌间质纤维化如何影响心脏功能及兴奋-收缩耦联（E-C coupling），目前尚未阐明。我们通过对大鼠实施肺动脉（PA）环扎术，构建了伴有纤维化的右心室（RV）肥厚动物模型。在肺动脉环扎术后第2、4、6周，同步测量大鼠右心室乳头肌的张力与细胞内钙离子浓度（n=33）。根据乳头肌是否存在明显的心肌间质纤维化，可将肺动脉环扎大鼠明确分为两组：分别为纤维化阳性组（F+）与纤维化阴性组（F-）。尽管F+组大鼠的右心室游离壁重量显著高于F-组，但两组的乳头肌直径与心肌细胞尺寸基本一致。与假手术组及F-组乳头肌相比，F+组乳头肌表现出更高的僵硬度、更低的主动张力与更弱的钙离子反应性。此外，我们发现，在右心室重量、乳头肌主动张力等多项参数中，钙离子达峰时间与纤维化百分比的相关系数最高。F+组的肌钙蛋白I（troponin I）磷酸化水平显著高于假手术组与F-组，这一结果佐证了F+组钙离子反应性更低的结论。我们还观察到，在间质纤维化严重的闰盘（intercalated disk）区域，F+组的连接蛋白43（connexin 43）表达稀疏且排列紊乱。本研究中，源自肺动脉环扎大鼠的右心室乳头肌，可用于直接探究纤维化与心功能障碍之间的关联，尤其是兴奋-收缩耦联的损伤机制。我们的研究结果表明，心肌间质纤维化会通过以下两种途径恶化心脏功能：1）降低钙离子反应性；2）由于细胞间通讯稀疏，导致纤维化组织内各心肌细胞的激活不同步。