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

KEY POINTS

Augmented inositol 1,4,5-trisphosphate (IP ) receptor (IP R2) expression has been linked to a variety of cardiac pathologies. Although cardiac IP 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 R2-mediated sarcoplasmic reticulum (SR)-Ca release in ventricular excitation-contraction coupling under IP R2-overexpressing conditions by studying intracellular Ca events. We report that, upon IP R2 overexpression in ventricular myocytes, IP -induced Ca release (IP ICR) modulates the SR-Ca content via "eventless" SR-Ca release, affecting the global SR-Ca leak. Thus, IP R2 activation could act as a SR-Ca gateway mechanism to escape ominous SR-Ca overload. Our approach unmasks a so far unrecognized mechanism by which "eventless" IP ICR plays a protective role against ventricular Ca -dependent arrhythmogenicity.

ABSTRACT

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