DISCONTINUOUS CONDUCTION AT PURKINJE-VENTRICULAR MUSCLE JUNCTION

Conduction through the cardiac syncytium varies from being nearly cont inuous, with very well coupled cells, to being clearly discontinuous, with significant conduction delays over very short distances. The Purk inje-ventricular muscle junction (PVJ) sites on the endocardial surfac e have characteristic delays of conduction and the presence of discret e groups of cells that suggest significant discontinuities of the cond uction process at PVJ sites, as compared with the more nearly continuo us conduction within either the Purkinje or the ventricular muscle lay ers of the papillary muscle. The purpose of the present study was to e xamine the relative sensitivity of conduction at PVJ sites versus cond uction within the Purkinje or the ventricular muscle layer of the cani ne 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 curre nt to test the hypothesis that discontinuous conduction at the PVJ sit es would be more sensitive to these agents than would continuous condu ction within either the Purkinje layer or the ventricular muscle layer of a canine papillary muscle. Conduction delay at the PVJ sites was s ignificantly increased by cadmium, with some PVJ sites reversibly beco ming nonjunctional at 200-400 mu M cadmium. Isoproterenol significantl y decreased PVJ delay, and this effect was attenuated by carbachol. Al l of the effects on conduction delay at the PVJ sites were much greate r than the effects for the same agents on conduction velocity within e ither the Purkinje or the ventricular muscle layer of the papillary mu scle.