SPATIAL-DISTRIBUTION OF CONNEXIN43, THE MAJOR CARDIAC GAP JUNCTION PROTEIN, VISUALIZES THE CELLULAR NETWORK FOR IMPULSE PROPAGATION FROM SINOATRIAL NODE TO ATRIUM

Myocytes are electrically coupled by gap junctions, which are composed of low-resistance intercellular channels. The major cardiac gap junct ion protein is connexin43 (Cx43). The distribution of Cx43 has been st udied by immunofluorescence to visualize the electrical coupling betwe en atrial tissue and sinoatrial node. From modeling studies, this coup ling was inferred to be gradual in order to shield the sinoatrial node from the atrial hyperpolarizing influence. The actual Cx43 labeling p attern did not show the expected gradient but instead a rather black a nd white staining in a striking pattern of strands of cells. We used a n immunohistochemical marker (anti-alpha-smooth muscle actin [alpha SM A]) that specifically cross-reacts with guinea pig sinoatrial node cel ls together with Cx43 antibody to stain previously electrophysiologica lly mapped sinoatrial nodes. We found that in the guinea pig sinoatria l node the impulse originates in an alpha SMA-positive, virtually Cx43 -negative, region (primary pacemaker region). The impulse then travels obliquely upward to the crista terminalis through a region where laye rs of alpha SMA-positive cells alternate with layers of Cx43-positive SMA-negative cells. The layers of Cx43-positive cells appear to become broader and thicker in the direction of the crista terminalis, wherea s the layers of alpha SMA-positive cells become thinner and narrower. Lateral contacts between Cx43- and alpha SMA-positive cells were very sparse and only detected where the Cx43-positive strands ended (the re gion where alpha SMA-positive cells fill the whole space between endoc ardium and epicardium, ie, the putative primary pacemaker region). Fro m these results, we conclude that the primary pacemaker is shielded fr om the hyperpolarizing influence of the atrium by a gradient in coupli ng brought about by tissue geometric factors rather than by a gradient of gap junction density.