Discontinuous conduction at Purkinje-ventricular muscle junction

Conduction through the cardiac syncytium varies from being nearly continuous, with very well coupled cells, to being clearly discontinuous, with significant conduction delays over very short distances. The Purkinje-ventricular muscle junction (PVJ) sites on the endocardial surface have characteristic delays of conduction and the presence of discrete groups of cells that suggest significant discontinuities of the conduction process at PVJ sites, as compared with the more nearly continuous conduction within either the Purkinje or the ventricular muscle layers of the papillary muscle. The purpose of the present study was to examine the relative sensitivity of conduction at PVJ sites versus conduction within the Purkinje or the ventricular muscle layer of the canine papillary muscle to agents that modulate L-type calcium current. We have used cadmium as a relatively specific blocker of L-type calcium current and isoproterenol as an agent to increase L-type calcium current to test the hypothesis that discontinuous conduction at the PVJ sites would be more sensitive to these agents than would continuous conduction within either the Purkinje layer or the ventricular muscle layer of a canine papillary muscle. Conduction delay at the PVJ sites was significantly increased by cadmium, with some PVJ sites reversibly becoming nonjunctional at 200-400 microM cadmium. Isoproterenol significantly decreased PVJ delay, and this effect was attenuated by carbachol. All of the effects on conduction delay at the PVJ sites were much greater than the effects for the same agents on conduction velocity within either the Purkinje or the ventricular muscle layer of the papillary muscle.