Spatial Distribution of Connexin43, the Major Cardiac Gap Junction Protein, Visualizes the Cellular Network for Impulse Propagation From Sinoatrial Node to Atrium

Abstract

Myocytes are electrically coupled by gap junctions, which are composed of low-resistance intercellular channels. The major cardiac gap junction protein is connexin43 (Cx43). The distribution of Cx43 has been studied by immunofluorescence to visualize the electrical coupling between atrial tissue and sinoatrial node. From modeling studies, this coupling was inferred to be gradual in order to shield the sinoatrial node from the atrial hyperpolarizing influence. The actual Cx43 labeling pattern did not show the expected gradient but instead a rather black and white staining in a striking pattern of strands of cells. We used an immunohistochemical marker (anti-alpha-smooth muscle actin [alpha SMA]) that specifically cross-reacts with guinea pig sinoatrial node cells together with Cx43 antibody to stain previously electrophysiologically mapped sinoatrial nodes. We found that in the guinea pig sinoatrial 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 layers 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, whereas 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 region where alpha SMA-positive cells fill the whole space between endocardium and epicardium, ie, the putative primary pacemaker region). From these results, we conclude that the primary pacemaker is shielded from the hyperpolarizing influence of the atrium by a gradient in coupling brought about by tissue geometric factors rather than by a gradient of gap junction density.

Fibroblast Network in Rabbit Sinoatrial Node

“…As previously described, 12,24 we observed an abundance of well-organized Cx43 in the intercalated discs between atrial myocytes near the CT (Figure 4A). At the interface between the SAN and CT (Figure 4B), bundles of atrial muscle cells that express Cx43 protrude into the SAN.…”

Differential Distribution of Cardiac Ion Channel Expression as a Basis for Regional Specialization in Electrical Function

“…105 Cx43 protein is present on bovine, guinea pig, and human atrial myocytes, 106 -108 with a distinct transitional zone containing interdigitating Cx43-expressing atrial and Cx43-lacking nodal cells at the periphery of the SAN. 45,106,108 Canine and rabbit RA gap junctions contain mainly Cx40 and Cx43 and less Cx45. 39,109 Cx40 expression in the atrium is much stronger than in the ventricle (where it is virtually undetectable) in humans, rabbits, guinea pigs, and mice.…”

Atrio-Sinus Interaction Demonstrated by Blockade of the Rapid Delayed Rectifier Current

“…However, our previous data 25 suggest that the densities of major ionic currents in SA node rabbit cells are independent of cell size (membrane capacitance). In addition, accumulating data are available on the presence of (strands of) atrial cells in the SA node of rabbit, 26 guinea pig, 27 bovine, 28 and mouse. 29 In rabbit, we were able to isolate atrial cells from the primary pacemaker region.…”

“…In rabbit SA node, moving from the primary pacemaker center to the atrium, slowing of diastolic depolarization and increase in upstroke velocity correlates with an overall increase in myofibril density and organization. 4 In detailed immunohistochemical studies in rabbit, 26 guinea pig, 27 bovine, 28 and mouse 29 hearts, it was found that the very center of the node is surrounded by a region in which atrial strands protrude the nodal area. Furthermore, in rabbit SA node, a close apposition of typical nodal cells and cells with a high myofibril density and high degree of organization has been observed by electronmicroscopy.…”