Objective: Intercellular coupling through gap junctions allows the morphologically and functionally heterogeneous sinoatrial node to synchronize and drive the atrial muscle. The purpose of this study was to identify the connexin isotypes expressed by sinoatrial node cells and to analyse the density of connexins in relation to cell size.
Methods: Labeling for the different connexins using isotype-specific antibodies was assessed in cells isolated from the rabbit sinoatrial node by immunoconfocal microscopy.
Results: Sinoatrial node cells with a cell projection area smaller than 800 microm(2) were devoid of immunolabeling for connexin43. Such small cells showed high levels of connexin45 labeling (compared to that in large cells) and low levels of connexin40 labeling. Sinoatrial node cells with a projection area between 800 and 1200 microm(2) had a lower amount of connexin45 label and again a small amount of connexin40 but an increased amount of connexin43 label. In the larger sinoatrial node cells, some colocalization of connexin45 and connexin43 immunolabeled spots was observed.
Conclusions: Rabbit sinoatrial node cells are heterogeneous in terms of connexin expression, and there is a clear cell size-dependence in pattern of connexin expression. Small (putative central) cells express connexin45 but not connexin43, whereas larger (putative peripheral) cells express both connexin45 and connexin43. The co-localization of connexin43 and connexin45 in larger cells raises the possibility that heterotypic or heteromeric connexin43/connexin45 channels could be present in gap junctions at the periphery of the sinoatrial node.