Arrhythmogenic Action Of Endothelin-1_1-31 And Its Mechanism In Isolated Rat Hearts And Cultured Neonatal Rat Cardiomyocytes

Endothelin-11.31, a new member of the endothelin family, is generated from its precursor, big-ET-1, via a chymase originating from human mast cell. It has been reported that ET-11.31 induces constriction of vascular smooth muscle cell, promotes cell proliferation and increases [Ca2+], in human coronary artery smooth muscle cells.Our previous study demonstrated that endogenous ET-11.21 plays an important role in the pathogenesis of acute ischemic arrhythmia. In the present study, we attempted to determine if administration of ET-11.31 would result in arrhythmia in perfused isolated rat hearts, and observe the effect of ET-11.31 on [Ca2+]i in cultured neonatal rat cardiomyocytes. Seventy- two SD rats weighing 280-350g were anesthetized with sodium pentobarbital, the hearts were excised and connected rapidly to the aortic cannula of a Langendorff apparatus. The results are as follows.1. Perfusion with ET-11.31 at 2X10-11M, 2X10-10M and 2X10-9M result in a dose dependent manner, and perfusion with ET-11.31 2X10-9 M resulted in frequent ventricular ectopic beats (VEB) and ventricular tachycardia (VT), where overall VEBs was 128.0(66.0-1015.0) the arrhythmia score (AS)=2.18+0.87 (mean+SD, n=11),both of these significantly higher than those of control group (p<0.01).2. Pretreatment with perfusion of 2.5 X 10-10M , 2.5 X 10-9M, 2.5 X 10-8M BQ12 into isolated rat hearts 15 minutes before ET-11-31 2X10-9 M, markedly prevented the occurrence of VEB and VT. AS was 1.67 + 0.58,1.50 + 1.38 and 0.33 + 0.52,respectively.The AS in BQ123 2.5 X 10"8M group was significantly lower than that in ET-11.31 2 X 10-9M group(p<0.01).3. The arrhythmia induced by ET-11.312 X 10-9 M was partially and markably reduced by phosphoramidon(6 X 10-7M and 3 X 10-6M),a NEP/ECE inhibitor.4. In the group where the coronary flow (CF) was mimicked at at a low level parallel to the low CF produced by ET-11.31 2 X 10-9M perfusion, no severe arrhythmiawas observed where AS was 0.40 + 0.55(n=5), significantly lower than that in ET-11-312x10-9M group.5. Exposure of the cultured neonatal rat cardiomyocytes to ET-11.31 10-12M, 10-11 andl0-10M increased [Ca2+]j, and to ET-11-31 10-9~10-7M, increased diastolic [Ca2+]i and decreased systolic [Ca2+]i, and thereby terminated [Ca2+]i transient,changes of which were abolished by BQ123(3 X 10-6M) and attenuated by phosphoramidonThese observations lead us to speculate that ET-11.31 per se has a direct arrhythmogenic action; and this arrhythmogenic action seems to be mediated mainly by ETa receptor. Phosphoramidon partially blocked the arrhythmia and changes in [Ca2+]i transient induced by ET-11.31 suggesting a role of endothelin converting enzyme in the dynamic conversion of ET-11-31 to ET-11.21.