Synchronized Spontaneous Calcium Release Events Throughout The Intact Heart

Abstract

Intracellular calcium (Ca) dysregulation associated with cardiac disease (e.g. heart failure) has been linked to mechanisms of ventricular arrhythmias. Such arrhythmias can arise at the sub-cellular level from delayed afterdepolarizations that are due to spontaneous calcium release from the sarcoplasmic reticulum. The mechanisms that govern an aggregate of sub-cellular spontaneous calcium release events at the tissue level (i.e. an SCR) are not well understood. We hypothesize that in tissue, an SCR can be significantly influenced by ryanodine receptor (RyR) function. Methods: High resolution optical mapping of Ca (Indo-1AM) from the anterior surface of the Langendorff perfused guinea pig heart (n=4) was performed in hearts under high Ca conditions ([Ca2+]e=5.5mM), with and without caffeine (CAFF, 1mM) to enhance RyR open probability. Endocardial cryoablation was performed to eliminate Purkinje fibers and cytochalasin-D (7μM) was administered to remove motion artifact. Fifteen seconds of rapid pacing (400-160 ms cycle length) followed by a pause was used to induce SCR activity. Results: In all preparations, synchronized SCR activity was observed across the entire anterior surface of the heart with and without CAFF. SCR activity increased in magnitude and occurred earlier with decreasing pacing cycle length. With CAFF, the amplitude of SCR activity increased (+10.9%, p < 0.05) and occurred earlier (+15.6%, p < 0.05). CAFF also decreased the spatial heterogeneity of SCR onset across the mapping field (-37.4%, p < 0.05), suggesting that increased RyR open probability enhances the synchronization of SCR activity. Conclusions: These results demonstrate that sub-cellular spontaneous calcium release events (an SCR in tissue) occur over a broad region of the intact heart and are enhanced when RyR open probability is increased. SCR activity may be an important mechanism of arrhythmogenesis in heart disease associated with calcium dysregulation.