Obstruction of ventricular Ca2+-dependent arrhythmogenicity by inositol 1,4,5-trisphosphate-triggered sarcoplasmic reticulum Ca2+ release

Abstract

Key points: Augmented inositol 1,4,5-trisphosphate (IP3) receptor (IP3R2) expression has been linked to a variety of cardiac pathologies. Although cardiac IP3R2 function has been in the focus of research for some time, a detailed understanding of its potential role in ventricular myocyte excitation–contraction coupling under pathophysiological conditions remains elusive. The present study focuses on mechanisms of IP3R2-mediated sarcoplasmic reticulum (SR)-Ca2+ release in ventricular excitation–contraction coupling under IP3R2-overexpressing conditions by studying intracellular Ca2+ events. We report that, upon IP3R2 overexpression in ventricular myocytes, IP3-induced Ca2+ release (IP3ICR) modulates the SR-Ca2+ content via “eventless” SR-Ca2+ release, affecting the global SR-Ca2+ leak. Thus, IP3R2 activation could act as a SR-Ca2+ gateway mechanism to escape ominous SR-Ca2+ overload. Our approach unmasks a so far unrecognized mechanism by which “eventless” IP3ICR plays a protective role against ventricular Ca2+-dependent arrhythmogenicity. Abstract: Augmented inositol 1,4,5-trisphosphate (IP3) receptor (IP3R2) function has been linked to a variety of cardiac pathologies including cardiac arrhythmias. The functional role of IP3-induced Ca2+ release (IP3ICR) within ventricular excitation–contraction coupling (ECC) remains elusive. As part of pathophysiological cellular remodelling, IP3R2s are overexpressed and have been repeatedly linked to enhanced Ca2+-dependent arrhythmogenicity. In this study we test the hypothesis that an opposite scenario might be plausible in which IP3ICR is part of an ECC protecting mechanism, resulting in a Ca2+-dependent anti-arrhythmogenic response on the cellular scale. IP3R2 activation was triggered via endothelin-1 or IP3-salt application in single ventricular myocytes from a cardiac-specific IP3R type 2 overexpressing mouse model. Upon IP3R2 overexpression, IP3R activation reduced Ca2+-wave occurrence (46 vs. 21.72%; P < 0.001) while its block increased SR-Ca2+ content (∼29.4% 2-aminoethoxydiphenyl borate, ∼16.4% xestospongin C; P < 0.001), suggesting an active role of IP3ICR in SR-Ca2+ content regulation and anti-arrhythmogenic function. Pharmacological separation of ryanodine receptor RyR2 and IP3R2 functions and two-dimensional Ca2+ event analysis failed to identify local IP3ICR events (Ca2+ puffs). SR-Ca2+ leak measurements revealed that under pathophysiological conditions, “eventless” SR-Ca2+ efflux via enhanced IP3ICR maintains the SR-Ca2+ content below Ca2+ spark threshold, preventing aberrant SR-Ca2+ release and resulting in a protective mechanism against SR-Ca2+ overload and arrhythmias. Our results support a so far unrecognized modulatory mechanism in ventricular myocytes working in an anti-arrhythmogenic fashion.