Electrotonic interactions in delayed propagation and block within the guinea pig SA node

The influence of electrotonic interactions on propagation within the SA node was studied by recording transmembrane potentials simultaneously from two neighboring (less than 1 mm apart) subsidiary pacemaker cells within the sinoatrial (SA) node of the guinea pig. As single premature stimuli were delivered progressively earlier in diastole, retrograde propagation between cells was delayed progressively. Cells activated earlier displayed secondary depolarizations that were coincident with the depolarization of neighboring cells activated later. The secondary depolarizations increased action potential duration markedly. Rapid pacing elicited secondary depolarizations that resulted in a progressive increase in action potential duration and decrease in upstroke amplitude. These changes were associated with a progressive delay in retrograde propagation that led to intermittent block with Wenckebach periodicity. Exposure to tetrodotoxin (10(-5) g/ml) delayed antegrade propagation, resulting in electrotonically mediated secondary depolarizations and exit block with Wenckebach periodicity. It is concluded that delayed activation and electrotonically mediated interactions between cells can increase action potential duration and refractoriness. These changes contribute to progressive delays in propagation that may result in intermittent block with Wenckebach periodicity within the SA node.