Connexin45, a major connexin of the rabbit sinoatrial mode, is co-expressed with connexin43 in a restricted zone at the nodal-crista terminalis border

The pacemaker of the heart, the sinoatrial (SA) node, is characterized by u nique electrical coupling properties. To investigate the contribution of ga p junction organization and composition to these properties, the spatial pa ttern of expression of three gap junctional proteins, connexin45 (Cx45), co nnexin40 (Cx40), and connexin43 (Cx43), was investigated by immunocytochemi stry combined with confocal microscopy. The SA nodal regions of rabbits wer e dissected and rapidly frozen. Serial cryosections were double labeled for Cx45 and Cx43 and for Cx40 and Cx43, using pairs of antibody probes raised in different species. Dual-channel scanning confocal microscopy was applie d to allow simultaneous visualization of the different connexins. Cx45 and Cx40, but not Cx43, were expressed in the central SA node. The major part o f the SA nodal-crista terminalis border revealed a sharply demarcated bound ary between Cx43-expressing myocytes of the crista terminalis and Cx45/Cx40 -expressing myocytes of the node. On the endocardial side, however, a trans itional zone between the crista terminalis and the periphery of the node wa s detected in which Cx43 and Cx45 expression merged. These distinct pattern s of connexin compartmentation and merger identified suggest a morphologica l basis for minimization of contact between the tissues, thereby restrictin g the hyperpolarizing influence of the atrial muscle on the SA node while m aintaining a communication route for directed exit of the impulse into the crista terminalis.