Gap junctions in the rabbit sinoatrial node

In comparison to the cellular basis of pacemaking, the electrical interacti ons mediating synchronization and conduction in the sinoatrial node are poo rly understood. Therefore, we have taken a combined immunohistochemical and electrophysiological approach to characterize gap junctions in the nodal a rea. We report that the pacemaker myocytes in the center of the rabbit sino atrial node express the gap junction proteins connexin (Cx) 40 and Cx46. In the periphery of the node, strands of pacemaker myocytes expressing Cx43 i ntermingle with strands expressing Cx40 and Cx46. Biophysical properties of gap junctions in isolated pairs of pacemaker myocytes were recorded under dual voltage clamp with the use of the perforated-patch method. Macroscopic junctional conductance ranged between 0.6 and 25 nS with a mean value of 7 .5 nS. The junctional conductance did not show a pronounced sensitivity to the transjunctional potential difference. Single-channel recordings from pa irs of pacemaker myocytes revealed populations of single-channel conductanc es at 133, 202, and 241 pS. With these single-channel conductances, the obs erved average macroscopic junctional conductance, 7.5 nS, would require onl y 30-60 open gap junction channels.