The role of inositol 1,4,5‐trisphosphate receptors in Ca2+ signalling and the generation of arrhythmias in rat atrial myocytes

Abstract

Various cardio‐active stimuli, including endothelin‐1 (ET‐1), exhibit potent arrhythmogenicity, but the underlying cellular mechanisms of their actions are largely unclear. We used isolated rat atrial myocytes and related changes in their subcellular Ca 2+ signalling to the ability of various stimuli to induce diastolic, premature extra Ca 2+ transients (ECTs). For this, we recorded global and spatially resolved Ca 2+ signals in indo‐1‐ and fluo‐4‐loaded atrial myocytes during electrical pacing. ET‐1 exhibited a higher arrhythmogenicity (arrhythmogenic index; ratio of number of ECTs over fold‐increase in Ca 2+ response, 8.60; n= 8 cells) when compared with concentrations of cardiac glycosides (arrhythmogenic index, 4.10; n= 8 cells) or the β‐adrenergic agonist isoproterenol (arrhythmogenic index, 0.11; n= 6 cells) that gave similar increases in the global Ca 2+ responses. Seventy‐five percent of the ET‐1‐induced arrhythmogenic Ca 2+ transients were accompanied by premature action potentials, while for digoxin this proportion was 25 %. The β‐adrenergic agonist failed to elicit a significant number of ECTs. Direct activation of inositol 1,4,5‐trisphosphate (Ins P3) receptors with a membrane‐permeable Ins P3 ester (Ins P3 BM) mimicked the effect of ET‐1 (arrhythmogenic index, 14.70; n= 6 cells). Inhibition of Ins P3 receptors using 2 μM 2‐aminoethoxydiphenyl borate, which did not display any effects on Ca 2+ signalling under control conditions, specifically suppressed the arrhythmogenic action of ET‐1 and Ins P3 BM. Immunocytochemistry indicated a co‐localisation of peripheral, junctional ryanodine receptors with Ins P3 Rs. Thus, the pronounced arrhythmogenic potency of ET‐1 is due to the spatially specific recruitment of Ca 2+ sparks by subsarcolemmal Ins P3 Rs. Summation of such sparks efficiently generates delayed afterdepolarisations that trigger premature action potentials. We conclude that the particular spatial profile of cellular Ca 2+ signals is a major, previously unrecognised, determinant for arrhythmogenic potency and that the Ins P3 signalling cassette might therefore be a promising new target for understanding and managing atrial arrhythmia.