Abstract
KEY POINTS: Augmented inositol 1,4,5-trisphosphate (IP(3) ) receptor (IP(3) R2) expression has been linked to a variety of cardiac pathologies. Although cardiac IP(3) R2 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 IP(3) R2-mediated sarcoplasmic reticulum (SR)-Ca(2+) release in ventricular excitation-contraction coupling under IP(3) R2-overexpressing conditions by studying intracellular Ca(2+) events. We report that, upon IP(3) R2 overexpression in ventricular myocytes, IP(3) -induced Ca(2+) release (IP(3) ICR) modulates the SR-Ca(2+) content via "eventless" SR-Ca(2+) release, affecting the global SR-Ca(2+) leak. Thus, IP(3) R2 activation could act as a SR-Ca(2+) gateway mechanism to escape ominous SR-Ca(2+) overload. Our approach unmasks a so far unrecognized mechanism by which "eventless" IP(3) ICR plays a protective role against ventricular Ca(2+) -dependent arrhythmogenicity. ABSTRACT: Augmented inositol 1,4,5-trisphosphate (IP(3) ) receptor (IP(3) R2) function has been linked to a variety of cardiac pathologies including cardiac arrhythmias. The functional role of IP(3) -induced Ca(2+) release (IP(3) ICR) within ventricular excitation-contraction coupling (ECC) remains elusive. As part of pathophysiological cellular remodelling, IP(3) R2s are overexpressed and have been repeatedly linked to enhanced Ca(2+) -dependent arrhythmogenicity. In this study we test the hypothesis that an opposite scenario might be plausible in which IP(3) ICR is part of an ECC protecting mechanism, resulting in a Ca(2+) -dependent anti-arrhythmogenic response on the cellular scale. IP(3) R2 activation was triggered via endothelin-1 or IP(3) -salt application in single ventricular myocytes from a cardiac-specific IP(3) R type 2 overexpressing mouse model. Upon IP(3) R2 overexpression, IP(3) R activation reduced Ca(2+) -wave occurrence (46 vs. 21.72%; P < 0.001) while its block increased SR-Ca(2+) content (∼29.4% 2-aminoethoxydiphenyl borate, ∼16.4% xestospongin C; P < 0.001), suggesting an active role of IP(3) ICR in SR-Ca(2+) content regulation and anti-arrhythmogenic function. Pharmacological separation of ryanodine receptor RyR2 and IP(3) R2 functions and two-dimensional Ca(2+) event analysis failed to identify local IP(3) ICR events (Ca(2+) puffs). SR-Ca(2+) leak measurements revealed that under pathophysiological conditions, "eventless" SR-Ca(2+) efflux via enhanced IP(3) ICR maintains the SR-Ca(2+) content below Ca(2+) spark threshold, preventing aberrant SR-Ca(2+) release and resulting in a protective mechanism against SR-Ca(2+) overload and arrhythmias. Our results support a so far unrecognized modulatory mechanism in ventricular myocytes working in an anti-arrhythmogenic fashion.