Inositol-1,4,5-Trisphosphate and Ryanodine-Dependent Ca<sup>2+</sup> Signaling in a Chronic Dog Model of Atrial Fibrillation

Abstract

Ca<sup>2+</sup> signaling regulation plays an important role in triggering and/or maintaining atrial fibrillation (AF). Little is known about the relationship of the inositol-1,4,5-triphosphate receptors (InsP<sub>3</sub>Rs) and ryanodine receptors (RyRs) in left atrium to chronic AF. In this study, we investigated the expression and function of InsP<sub>3</sub>R<sub>1</sub>, InsP<sub>3</sub>R<sub>2 </sub>and RyR<sub>2 </sub>in a chronic dog model of AF. AF was induced in 6 dogs by rapid right atrial pacing for 24 weeks, and a sham procedure was performed in 5 dogs (control group). The intact left atrial myocytes were used to examine the expression and function of InsP<sub>3</sub>Rs, RyRs by BODIPY<sup>O,R</sup> TR-X ryanodine, heparin-fluorescein conjugate, and were stimulated by caffeine, ATP to release Ca<sup>2+</sup> through RyRs, InsP<sub>3</sub>Rs separately. We also assessed the molecular components of left atrial tissue underlying the amount of RyR<sub>2</sub>, InsP<sub>3</sub>R<sub>1 </sub>and InsP<sub>3</sub>R<sub>2</sub> determined by RT-PCR, immunohistochemistry and Western blot analysis. In the chronic AF group, the Ca<sup>2+</sup> released through RyRs is not altered, but the Ca<sup>2+</sup> released through InsP<sub>3</sub>Rs increased significantly. RyR<sub>2</sub> distributed in cytosol of myocytes, cellular membrane; its expression significantly decreased in AF group compared to controls. InsP<sub>3</sub>R<sub>1</sub> distributed in cytosol, InsP<sub>3</sub>R<sub>2</sub> distributed not only in cytosol, cellular membrane, but also in nuclear envelope and intercalated discs. The InsP<sub>3</sub>R<sub>1</sub> and InsP<sub>3</sub>R<sub>2</sub> expression significantly increased in chronic AF group compared to controls. These results indicated that in a chronic dog model of AF, the expression and function of RyR<sub>2</sub> down-regulated; on the contrary, the expression and function of InsP<sub>3</sub>R<sub>1</sub>, InsP<sub>3</sub>R<sub>2</sub> up-regulated, and InsP<sub>3</sub>R<sub>2</sub> may be the major InsP<sub>3</sub>Rs, which regulate intracellular or even intercellular Ca<sup>2+</sup> signal transmission.