The role of inositol 1,4,5-trisphosphate receptors in Ca(2+) signalling and the generation of arrhythmias in rat atrial myocytes.
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 beta-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 beta-adrenergic agonist failed to elicit a significant number of ECTs. Direct activation of inositol 1,4,5-trisphosphate (InsP(3)) receptors with a membrane-permeable InsP(3) ester (InsP(3) BM) mimicked the effect of ET-1 (arrhythmogenic index, 14.70; n = 6 cells). Inhibition of InsP(3) receptors using 2 microM 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 InsP(3) BM. Immunocytochemistry indicated a co-localisation of peripheral, junctional ryanodine receptors with InsP(3)Rs. Thus, the pronounced arrhythmogenic potency of ET-1 is due to the spatially specific recruitment of Ca(2+) sparks by subsarcolemmal InsP(3)Rs. Summation of such sparks efficiently generates delayed after depolarisations 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 InsP(3) signalling cassette might therefore be a promising new target for understanding and managing atrial arrhythmia.
Mackenzie
Lauren
L
Laboratory of Molecular Signalling, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK.
Bootman
Martin D
MD
Laine
Mika
M
Berridge
Michael J
MJ
Thuring
Jan
J
Holmes
Andrew
A
Li
Wen-Hong
WH
Lipp
Peter
P
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001145:Arrhythmias, Cardiac / Q000139:chemically induced / Q000503:physiopathology*; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D002316:Cardiotonic Agents / Q000494:pharmacology; D004077:Digoxin / Q000494:pharmacology; D004558:Electric Stimulation; D004594:Electrophysiology; D019332:Endothelin-1 / Q000494:pharmacology; D006325:Heart Atria / Q000166:cytology; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D008856:Microscopy, Fluorescence; D018408:Patch-Clamp Techniques; D051381:Rats; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D013347:Subcellular Fractions / Q000187:drug effects / Q000378:metabolism
D015220:Calcium Channels; D002316:Cardiotonic Agents; D019332:Endothelin-1; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D004077:Digoxin; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium