Pressure-overload-induced angiotensin-mediated early remodeling in mouse heart

Our previous work on angiotensin II-mediated electrical-remodeling in canine left ventricle, in connection with a long history of other studies, suggested the hypothesis: increases in mechanical load induce autocrine secretion of angiotensin II (A2), which coherently regulates a coterie of membrane ion transporters in a manner that increases contractility. However, the relation between load and A2 secretion was correlative. We subsequently showed a similar or identical system was present in murine heart. To investigate whether the relation between mechanical load and A2-mediated electrical remodeling was causal, we employed transverse aortic constriction in mice to subject the left ventricle to pressure overload for short-term (1 to 2 days) or long-term (1 to 2 weeks) periods. Heart-to-body weight ratios and cell capacitance measurements were used to determine hypertrophy. Whole-cell patch clamp recordings of the predominant repolarization currents Ito,fast and IK,slow were used to assess electrical remodeling. Hearts or myocytes subjected to long-term load displayed significant hypertrophy, which was not evident in short-term load. However, short-term load induced significant reductions in Ito,fast and IK,slow. Incubation of these myocytes with the angiotensin II type 1 receptor inhibitor saralasin for 2 hours restored Ito,fast and IK,slow to control levels. The number of Ito.fast or IK,slow channels did not change with A2 or long-term load, however the hypertrophic increase in membrane area reduced the current densities for both channels. For Ito,fast but not IK,slow there was an additional reduction that was reversed by inhibition of angiotensin receptors. These results suggest increased load activates an endogenous renin angiotensin system that initially reduces Ito,fast and IK,slow prior to the onset of hypertrophic growth. However, there are functional interactions between electrical and anatomical remodeling. First, hypertrophy tends to reduce all current densities. Second, the hypertrophic program can modify signaling between the angiotensin receptor and target current.

本团队此前针对犬左心室中血管紧张素II（angiotensin II）介导的心肌电重构开展的研究，结合既往大量相关研究，提出如下假说：机械负荷增加可诱导血管紧张素II的自分泌分泌，进而协同调控一组膜离子转运体，以提升心肌收缩能力。然而，当时仅明确了负荷与血管紧张素II分泌之间的相关性。后续我们证实，小鼠心脏中存在相似乃至完全一致的调控系统。

为探究机械负荷与血管紧张素II介导的心肌电重构之间的关联是否为因果关系，我们通过小鼠主动脉弓缩窄术（transverse aortic constriction），使左心室承受短期（1~2天）或长期（1~2周）的压力超负荷（pressure overload）刺激。通过心脏重量与体重比值以及细胞电容测量来评估心肌肥厚（hypertrophy）情况。对主要复极电流快速一过性外向钾电流（Ito,fast）与缓慢延迟整流钾电流（IK,slow）开展全细胞膜片钳（whole-cell patch clamp）记录，以此评估心肌电重构程度。

接受长期负荷刺激的心脏或心肌细胞出现显著心肌肥厚，而短期负荷刺激组未观察到明显肥厚现象。但短期负荷刺激可使Ito,fast与IK,slow的电流幅值显著降低。将这些心肌细胞与血管紧张素II 1型受体（angiotensin II type 1 receptor）抑制剂沙拉赛辛（saralasin）共同孵育2小时后，Ito,fast与IK,slow的电流幅值可恢复至对照组水平。

Ito,fast与IK,slow通道的数量并未随血管紧张素II刺激或长期负荷刺激发生改变，但肥厚状态下细胞膜面积的增大，使得两种通道的电流密度均出现下降。仅针对Ito,fast而言，还存在额外的电流幅值降低现象，该现象可通过阻断血管紧张素受体得到逆转。

上述结果表明，负荷增加会激活内源性肾素-血管紧张素系统（renin angiotensin system），该系统在心肌肥厚发生初期即可使Ito,fast与IK,slow的电流幅值降低。但心肌电重构与解剖重构之间存在功能性交互作用：其一，心肌肥厚往往会降低所有离子通道的电流密度；其二，心肌肥厚的调控程序可改变血管紧张素受体与靶电流之间的信号转导通路。