Abstract-Atrial tissue expresses both connexin 40 (Cx40) and 43 (Cx43) proteins. To assess the relative roles of Cx40 and Cx43 in atrial electrical propagation, we synthesized cultured strands of atrial myocytes derived from mice with genetic deficiency in Cx40 or Cx43 expression and measured propagation velocity (PV) by high-resolution optical mapping of voltage-sensitive dye fluorescence. The amount of Cx40 and/or Cx43 in gap junctions was measured by immunohistochemistry and total or sarcolemmal Cx43 or Cx40 protein by immunoblotting. Progressive genetic reduction in Cx43 expression decreased PV from 34Ϯ6 cm/sec in Cx43 ϩ/ϩ to 30Ϯ8 cm/sec in Cx43 ϩ/Ϫ and 19Ϯ11 cm/sec in Cx43 Ϫ/Ϫ cultures. Concomitantly, the cell area occupied by Cx40 immunosignal in gap junctions decreased from 2.0Ϯ1.6% in Cx43 ϩ/ϩ to 1.7Ϯ0.5% in Cx43 ϩ/Ϫ and 1.0Ϯ0.2% in Cx43 Ϫ/Ϫ strands. In contrast, progressive genetic reduction in Cx40 expression increased PV from 30Ϯ2 cm/sec in Cx40 ϩ/ϩ to 40Ϯ7 cm/sec in Cx40 ϩ/Ϫ and 45Ϯ10 cm/sec in Cx40 Ϫ/Ϫ cultures. Concomitantly, the cell area occupied by Cx43 immunosignal in gap junctions increased from 1.2Ϯ0.9% in Cx40 ϩ/ϩ to 2.8Ϯ1.4% in Cx40 ϩ/Ϫ and 3.1Ϯ0.6% in Cx40 Ϫ/Ϫ cultures. In accordance with the immunostaining results, immunoblots of the Triton X-100 -insoluble fraction revealed an increase of Cx43 in gap junctions in extracts from Cx40-ablated atria, whereas total cellular Cx43 remained unchanged. Our results suggest that the relative abundance of Cx43 and Cx40 is an important determinant of atrial impulse propagation in neonatal hearts, whereby dominance of Cx40 decreases and dominance of Cx43 increases local propagation velocity. (Circ Res. 2006;99:1216-1224.)Key Words: atrial myocyte Ⅲ basic science Ⅲ cardiac gap junction connexins Ⅲ cardiovascular genomics Ⅲ cell culture Ⅲ conduction velocity Ⅲ connexin 40 Ⅲ connexin 43 Ⅲ mapping Ⅲ neonatal mouse cardiomyocytes Ⅲ optical mapping C onnexin (Cx) proteins enable the intercellular exchange of ions and small regulatory molecules and are important determinants of cardiac electrical propagation. 1 Three major connexins, Cx43, Cx40, and Cx45, are expressed in heart. 2,3 Cx43 is abundant in ventricular and atrial myocardium; Cx40 is expressed in atrial tissue and in the Purkinje system; Cx45 is present in the sinoatrial (SA) and atrioventricular (AV) nodes and colocalizes with Cx43 in ventricular myocardium. 4,5 Cx43 and Cx40 each form channels with relatively large pores, whereas Cx45 forms narrow channels (unitary channel conductances of 75, 130, and 30 pS, respectively). 6 Ventricular myocytes express abundant Cx43 and small amounts of Cx45. In contrast, atrial myocytes express large amounts of Cx43 and Cx40. In ventricular myocytes and transfected cells, Cx43 and Cx45 colocalize in gap junctions and may form heteromeric/heterotypic channels. 5,7-10 Heterotypic Cx43/Cx40 gap junction channels have been described in HeLa cell pairs, whereas the role of heteromeric Cx43/ Cx40 channels has not been fully clarified. 7,11 Changes in cell-to-cell...

Abstract-How vesicles are born in the trans-Golgi network and reach their docking sites at the plasma membrane is still largely unknown and is investigated in the present study on live, primary cultured atrial cardiomyocytes.