Cellular Mechanism of Ca2+−Dependent Arrhythmogenesis in Failing Myocytes of Aortic Banding Rats

Abstract

We have previously shown that myocyte t-tubular system undertakes dramatic remodeling in the failing hearts from rats after long term hypertension (Song et al, PNAS 2006). We further hypothesize that t-tubular remodeling plays an important mechanistic role in unstable Ca2+ homeostasis and therefore Ca2+−dependent arrhythmogenesis during heart failure (HF). To test this idea, we generated a HF model in Sprague-Dawley rats with thoracic-aortic banding (TAB) surgery. TAB rats developed HF in about 12-14 weeks, as confirmed by echocardiography. T-tubular structure and cellular Ca2+ function (Ca2+ sparks, waves, and field stimulated Ca2+ transients) were then examined with laser scanning confocal microscope in single isolated myocytes from TAB and sham operated rats. Confocal imaging of t-tubular system with Di-8-ANEPPS, a fluorescent membrane marker showed remarkable disarray and/or loss of t-tubular system in TAB failing myocytes. Power spectrum analysis indicated that myocytes from failing TAB rats displayed a significant reduction in the power of t-tubular organization (or the regularity of t-tubular distribution), as comparison to sham-operated controls (sham 2.0 vs TAB 1.1, n > 10, p<0.001). As a result, TAB myocytes exhibited a significantly slower rising phase of Ca2+ transients upon steady state field stimulation (93.5±4.8 ms vs sham control 38.5±2.9 ms at 3 Hz, p<0.01). Moreover, TAB myocytes had much higher probability of developing unstable Ca2+ release (Ca2+ waves) during field stimulation (48% vs 3% of control, at 3 Hz). These results further support our previous finding of t-tubular remodeling observed in a hypertensive HF model. In conclusion, t-tubular remodeling is a common structural alteration at the end stage of heart failure, responsible for Ca2+ release instability and Ca2+ dependent arrhythmogenesis during HF.