ATRIOVENTRICULAR JUNCTIONAL TISSUE - DISCREPANCY BETWEEN HISTOLOGICALAND ELECTROPHYSIOLOGICAL CHARACTERISTICS

Background Previous work has demonstrated that cells with AV nodal-typ e action potentials are not confined to Koch's triangle but may extend along the AV orifices. The aim of this study was to examine the histo logical and electrophysiological characteristics of this tissue. Metho ds and Results Studies were performed in isolated, blood-perfused dog and pig hearts. Microelectrode recordings revealed cells with nodal-ty pe action potentials around the tricuspid and mitral valve rings. Thes e cells were found within 1 to 2 mm of the valve annuli. A zone of cel ls with intermediate action potentials, approximate to 1 cm wide, sepa rated cells with nodal-type action potentials from cells with atrial-t ype action potentials in the body of the atria. In cells with nodal-ty pe action potentials, adenosine caused a reduction in action potential amplitude (49+/-2 versus 33+/-2 mV, mean+/-SE; P<.001), upstroke velo city (2.5+/-0.2 ver sus 2.0+/-0.2 V/s, P<.05), and duration (150+/-4 v ersus 96+/-8 ms, P<.001). The light microscopic appearance of AV junct ional cells was similar to that of myocytes in the body of the atrium. A polyclonal antibody raised against connexin-43 bound to atrial and ventricular tissue but not to the AV junctional tissue or AV nodal reg ion. The absence of connexin-43 correlated with the sites of cells wit h nodal-like action potentials. With pacing techniques, the AV junctio nal tissue in the region of the posterior AV nodal approaches could be electrically dissociated from atrial, AV nodal, and ventricular tissu e. AV nodal echoes were induced with ventricular pacing in three dog h earts. In each case, retrograde conduction was through the slow pathwa y, and anterograde conduction was through the fast pathway. During ech oes, activation of AV junctional cells preceded atrial activation duri ng retrograde slow pathway conduction, but these cells were not activa ted during anterograde fast pathway conduction. Conclusions AV junctio nal cells around both annuli are histologically similar to atrial cell s but resemble nodal cells in their cellular electrophysiology, respon se to adenosine, and lack of connexin-43. The light microscopic appear ance of AV junctional cells is a poor guide to their action potential characteristics. The AV junctional cells in the posterior AV nodal app roaches appear to participate in slow pathway conduction. These cells may be the substrate of the slow ''AV nodal'' pathway.