Structure and function of specific regions in the canine atrioventricular node.

The functional subregions of the rabbit atrioventricular (AV) node have been examined in detail, but it has not been possible to subject the normal canine AV node to such rigorous scrutiny. Arterial perfusion and beveled microelectrodes have now allowed us to explore the anatomic and functional regions of the isolated canine AV node. The atria and AV junctional tissues of 20 young dogs were excised and selectively perfused with physiological solution through the nutrient arteries to the sinus node, AV node, and His bundle. AV node action potentials had shapes similar to those of the rabbit AV node and were conducted slowly (56 +/- 7 mm/s); slowest conduction (16 +/- 14 mm/s) was observed in the distal AV node where diastolic depolarization was always recorded and where maximum upstroke velocity of the action potential was slowest. From subsequent anatomic studies of the same preparations we found that the proximal region of the canine AV node was predominantly composed of interwoven slender transitional cells and that the distal region contained bundles of transitional cells and small but conspicuous aggregations of P-cells. The proximal portion of the canine AV node appeared to be organized for triage of atrial input during sinus rhythm. Electron microscopic examinations revealed numerous relatively long (greater than 1.0 micrometers) gap junctions between proximal AV node cells. The large gap junctions may provide an anatomic basis for electrotonic interaction during summation and conduction of AV node inputs. The distal AV node is the site of slowest AV conduction and also where most forms of AV junctional rhythm originate.