Evidence for Summation and Voltage Dependency in Rabbit Atrioventricular Nodal Fibers

Abstract

This study demonstrated summation in the rabbit AV node and suggested that the action potential rise time (dV/dt) of nodal cells was voltage dependent. Summation was produced in two ways. (1) The atrium and the floor of the coronary sinus were divided to establish two noncontiguous, anatomically separate atrial inputs to the AV node. Conduction to the His bundle occurred when both sides of the atrium were stimulated simultaneously at premature cycle lengths which produced local responses in the N region when either side was stimulated individually. (2) With the atrium intact, the atrium and the His bundle were stimulated together at intervals which allowed the simultaneous arrival of the advancing wave fronts at the impaled N cell. This collision resulted in an N action potential which occurred slightly earlier and had an increased dV/dt and amplitude, suggesting that the nodal cells tested, as well as other cardiac cells, were voltage dependent. Further evidence was provided by the influence of the N region on nodal action potential contours according to the direction of propagation. During orthograde conduction, cells close to N in the AN region had a larger dV/dt and cells in the NH region had a smaller dV/dt than these respective cells had during retrograde conduction. Since dV/dt is a major determinant of conduction velocity, during orthograde conduction rapid conduction within the AN region but relatively slow conduction where N cells supply excitatory current to NH cells occurred. In the retrograde direction, conduction from the His bundle to the midnodal region was prompt, but deceleration occurred where the N region feeds the upper nodal cells.