Cx45 and Cx40, but not Cx43, were expressed in the central SA node, with a transitional zone on the endocardial side where Cx43 and Cx45 expression merged at the nodal-crista terminalis border.
The distinct spatial compartmentation of connexins in the rabbit SA node provides a morphological basis for its unique electrical coupling properties, restricting hyperpolarizing influence from atrial muscle while allowing impulse exit.
The pacemaker of the heart, the sinoatrial (SA) node, is characterized by unique electrical coupling properties. To investigate the contribution of gap junction organization and composition to these properties, the spatial pattern of expression of three gap junctional proteins, connexin45 (Cx45), connexin40 (Cx40), and connexin43 (Cx43), was investigated by immunocytochemistry combined with confocal microscopy. The SA nodal regions of rabbits were 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 applied to allow simultaneous visualization of the different connexins. Cx45 and Cx40, but not Cx43, were expressed in the central SA node. The major part of the SA nodal-crista terminalis border revealed a sharply demarcated boundary between Cx43-expressing myocytes of the crista terminalis and Cx45/Cx40-expressing myocytes of the node. On the endocardial side, however, a transitional zone between the crista terminalis and the periphery of the node was detected in which Cx43 and Cx45 expression merged. These distinct patterns of connexin compartmentation and merger identified suggest a morphological basis for minimization of contact between the tissues, thereby restricting the hyperpolarizing influence of the atrial muscle on the SA node while maintaining a communication route for directed exit of the impulse into the crista terminalis.
Coppen et al. (1999) studied this question. Connexin expression mapping was evaluated on Spatial pattern of expression of Cx45, Cx40, and Cx43. Cx45 and Cx40, but not Cx43, were expressed in the central SA node, with a transitional zone on the endocardial side where Cx43 and Cx45 expression merged at the nodal-crista terminalis border.
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