Distribution and functional role of inositol 1,4,5-trisphosphate receptors in mouse sinoatrial node

Abstract

Rationale: Inositol 1,4,5-trisphosphate receptors (IP3Rs) have been implicated in the generation of arrhythmias and cardiac muscle nuclear signaling. However, in the mammalian sinoatrial node (SAN), where the heart beat originates, the expression and functional activity of IP3Rs have not been investigated. Objectives: To determine whether SAN express IP 3Rs and which isoforms are present. To examine the response of the SAN to IP3R agonists and antagonist, and the potential role played by IP3Rs in cardiac pacemaking. Methods and Results: The expression and distribution of IP3Rs were studied by reverse-transcription polymerase chain reaction, Western blotting, and immunolabeling. Ca2+ signaling and electric activity in intact mouse SAN were measured with Ca-sensitive fluorescent dyes. We found that although the entire SAN expressed three IP3R mRNA isoforms, the type II IP3R (IP 3R2) was the predominant protein isoform detected by Western blot using protein extracts from the SAN, atrioventricular node, and atrial tissue. Immunohistochemistry studies also showed that IP3R2 was expressed in the central SAN region. Studies using isolated single pacemaker cells revealed that IP 3R2 (but not IP3R1) was located with a similar distribution to the sarcoplasmic reticulum marker protein SERCA2a with some labeling adjacent to the surface membrane. The application of membrane-permeable IP3 (IP3-butyryloxymethyl ester) increased Ca 2+ spark frequency and the pacemaker firing rate in single isolated pacemaker cells. In intact SAN preparations, IP3R agonists, endothelin-1 and IP3-butyryloxymethyl ester both increased intracellular Ca2+ and the pacemaker firing rate, whereas the IP 3R antagonist, 2-aminoethoxydiphenyl borate decreased Ca2+ and the firing rate. Both of these effects were absent in the SAN from transgenic IP3R2 knockout mice. Conclusions: This study provides new evidence that functional IP3R2s are expressed in the mouse SAN and could serve as an additional Ca-dependent mechanism in modulating cardiac pacemaker activity as well as other Ca-dependent processes. © 2011 American Heart Association, Inc.