Partial and Controlled Depletion of SR Calcium by RyR Agonists Prevents Calcium-dependent Arrhythmias

ABSTRACT Sympathetic stimulation of ventricular myocytes activates PKA, which phosphorylates L-type Ca2+ channels and phospholamban, among other substrates, to increase Ca2+ entry and SR Ca2+ uptake. This β- adrenergic-induced increase of intracellular Ca2+ (Ca2+ overload) is a natural and efficient mechanism to increase cardiac performance, inasmuch as the magnitude and force of cardiac contractions greatly depend on the amount of Ca2+ supplied to the myofilaments. However, Ca2+ overload also increases arrhythmia vulnerability because it brings ryanodine receptors (RyR2) closer to threshold for spontaneous Ca2+ release (SCR). SCR during diastole activates the Na+/Ca2+ exchanger of the sarcolemma and generates a depolarizing inward current (delayed after-depolarization or DAD) that may trigger extemporaneous actions potentials. Ca2+ overload and its subsequent SCRs are indeed the primary events that evolve in malignant cardiac arrhythmias, as observed in Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) and quite possibly in some forms of atrial fibrillation, long QT, and heart failure. To prevent these Ca2+-dependent arrhythmias, an emergent group of RyR2 blockers aim to stop SCR, despite the fact that the key process that spawns SCR, namely, Ca2+ overload, may not be affected or in some cases may be even exacerbated by the RyR2 blockers. We have found that imperacalcin, a high-affinity, membrane-permeable selective agonist of RyRs, paradoxically decreases arrhythmia burden and prevents Ca2+-dependent arrhythmias in animal models of CPVT. We hypothesize that this unexpected and remarkable anti-arrhythmic effect of imperacalcin is due to a partial and controlled depletion of SR Ca2+ load, which decreases the propensity of SCR events and thus dissipates the main arrhythmogenic substrate in CPVT. Our research program will systematically and rigorously test this hypothesis at the molecular, cellular, whole heart and intact animal levels using established (mouse) and novel (rabbit) models of CPVT. The first specific aim will determine the defining structural and functional characteristics that confer to imperacalcin and other members of the calcin family their capacity to permeate membranes and their anti-arrhythmic properties. In the second aim, we will use native and modified calcins on intact cardiomyocytes, Langendorff-perfused working hearts and intact animals for a rationalized design of a novel group of RyR2 ligands capable of preventing Ca2+-triggered arrhythmias. These studies use animal models of CPVT to develop a novel paradigm for the treatment of Ca2+ overload-generated cardiac arrhythmias; results may be applied to other cardiomyopathies where controlled unloading of SR Ca2+ may be desirable.