Changes Of Action Potential Profiles In Papillary Muscle From Mouse Hypertrophy And Failing Hearts

Cardiac hypertrophy and failure is the common complication for such diseases as hypertension, ischemic heart disease, and heart valve disease, etc. Despite advances in the medical treatment of heart failure in recent years, it's reported that annual mortality remains high, reaching up to 50%, about 50% of which die suddenly presumably due to ventricular tachyarrhythmias. Therefore, it is of significance to explore the molecular mechanism underlying the arrhythmias companying cardiac hypertrophy and failure. The electrophysiological remodeling in cardiac hypertrophy and failure has been documented. The prolongation of action potential duration is the most consistent electrophysiological abnormality in myocytes from hypertrophied and failing hearts. Delay of the repolarization process may initiate arrhythmias by causing abnormal impulse generation due to early after-depolarization and triggered activity or, when spatial or temporal inhomogeneity of repolarization ensues, to abnormal impulse conduction which promotes re-entrant arrhythmias. However, the exact mechanism of arrhythmias in cardiac hypertrophy and failure is still unknown. The mouse experimental models are being used increasingly, due to the ease with which molecular genetic strategies can be applied. Here, a mouse pressure over-loaded cardiac hypertrophy and failure model was established. It was observed by using electrophysiological recordings that changes of action potential profiles in papillary muscle from hypertrophied and failing hearts at the different time after operation. We also evaluated the role of calcineurin pathway in the prolongation of action potentials in cardiac hypertrophy and failure by treating mice with calcineurin inhibitor Cyclosporin A. Part 1. Establishing a mouse pressure over-loaded cardiac hypertrophy and failure model AIM Establishing a repeatable and stable animal model for the next study. METHOD Microsurgical techniques were used to band the transverse aortae creating pressure overload, as previously described by others. Briefly, Kunming male mice, with body weight 20±2g, were anesthetized and had the left thorax opened at the second intercostal space. A nylon suture ligature was tied around the transverse aorta against a needle with a diameter of 0.4mm. Aortic-banded mice were then maintained for 13 weeks. Hemodynamic parameters including LVSP, ±dp/dt, cardiac index, and cardiac histopathology were assessed and compared to the sham-operated animals at the different time after operation.RESULTS (1) Aorta-banded mice had mortality after 5 weeks, with a mortality of more than 60% for 13 weeks; (2) All hemodynamic parameters were significantly increased in banded mice compared to sham-operated mice (p<0.01) during the study time. They started to increase at week 2 with LVSP (21.8±7.3 Kp), +dp/dt (374±55), -dp/dt (381±56) and kept increasing to reach peak level at week 7, with LVSP (22.2±3.1Kp), +dp/dt (446±53), -dp/dt ( 497±73). Then, the parameters began to decrease at week 8 and kept decline to week 13. (3) The cardiac index in banded mice at week 1 was not different from that in sham-operated mice (p>0.05). However, it was significantly larger at week 2 with 4.2±0.2 and was kept gain to reach peak level at week 7, with 5.7±0.4. Afterward, it began to get small at week 8 and was kept decline to week 13. (4) Cardiac histopathology: Cardiac myocytes from sham-operated mice had well-arranged appearance, with similar sizes and clear transverse lines. There was little collagen between the cells. However, cardiac cells from banded mice at week 2 came out with bigger size and disorganized appearance. No denatured cells were observed. Moreover, the cells from banded mice at week 9 were lightly dyed and had ripple-like appearance, with blurry transverse lines. There were a lot of denatured cells with dark-dyed and blurry nucleus. CONCLUSION A mouse pressure over-loaded cardiac hypertrophy and failure model was established by aorta banding. The results shown that from 2-7 weeks hearts from banded micewere in a stage of compensatory hypertrophy manifested the increasing contraction function and was followed by a stage of heart failure characterized by declining contraction function. Part 2. Changes of action potential profiles in papillary muscle from mouse hypertrophy and failing hearts. AIM To observe the changes of action potential profiles in papillary muscle from mouse at the different time after operation. METHOD The action potentials in papillary muscle from banded or sham-operated mice at the different time (week 1, 2, 5, 7, 9, 13) were recorded by using glass microelectrode and parameters including the amplitude of action potentials (APA), resting potential (RP), maximal velocity of depolarization (Vmax), APD₅₀, and APD₉₀ were calculated. RESULTS APD₅₀ in banded mice was significantly prolonged at week 1 compared to sham-operated mice and it was gradually increased during the 13 weeks. However, APD₉₀ maintained unchanged from 1-7 weeks until 9 weeks and was kept prolonged to 13 weeks. There was no changes in all other parameters except APD₅₀ and APD₉₀. CONCLUSION It was in the early phase that action potentials started to be prolonged in hypertrophied heart. APD₅₀ was prolonged in hypertrophied as well as failing hearts, the prolongation of APD₉₀ was only seen in the failing hearts. Theresults suggest that there are different mechanisms contributing to the electrophysiological remodeling at different time. Part 3. The possible role of calcineurin pathway in causing prolongation of action potentials in hypertrophy and failing hearts. AIM To valuate the role of calcineurin signaling pathway in causing prolongation of action potentials in hypertrophy and failing hearts by using selective calcineurin inhibitor CsA. METHOD The action potentials in papillary muscle from banded or sham-operated mice were recorded by using glass microelectrode after mice were treated (subcutaneous injection) with CsA (25mg/kg) or vehicle, twice a day. Whole experiment included following parts: (1) Mice were treated with CsA for 1 or 2 weeks from the second day after operation, including four groups: Band 1w + CsA, Band 1w + Veh, Band 2w + CsA, Band 2w + Veh. (2) Mice were treated with CsA for 2 weeks from the 7th week after operation, including two groups: Band 9w + CsA, Band 9w + Veh. (3) To verify whether CsA selectively affect cardiac myocytes in phathological condition, we treated sham-operated mice as well, including Sham + CsA and Sham + Veh groups. RESULTS (1) There was no mortality in all mice. The cardiac index of Band 2w + CsA group was significantly decreased compared to Band 2w + Veh group, but not to the level in Sham 2w + Veh group. (2) Results from CsA early...