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

Area of Science:

• Cardiac Electrophysiology
• Cardiovascular Physiology
• Cellular Electrophysiology

Background:

• The sinoatrial (SA) node initiates the heartbeat, and its proper function is crucial for maintaining normal cardiac rhythm.
• Understanding signal propagation within the SA node is key to explaining cardiac arrhythmias.
• Electrotonic interactions, the electrical influence between adjacent cells, are proposed to play a role in SA node function.

Purpose of the Study:

• To investigate the influence of electrotonic interactions on signal propagation between subsidiary pacemaker cells in the SA node.
• To determine how premature stimuli and rapid pacing affect propagation and action potential characteristics.
• To elucidate the role of electrotonic coupling in the development of Wenckebach periodicity within the SA node.

Main Methods:

• Simultaneous recording of transmembrane potentials from two neighboring subsidiary pacemaker cells in the guinea pig SA node.
• Delivery of single premature stimuli at progressively earlier diastolic intervals.
• Application of rapid pacing protocols and tetrodotoxin to assess propagation under various conditions.

Main Results:

• Premature stimuli progressively delayed retrograde propagation between cells.
• Early activated cells exhibited secondary depolarizations coinciding with later activated neighbors, increasing action potential duration.
• Rapid pacing led to increased action potential duration, decreased upstroke amplitude, and progressive propagation delays, causing intermittent block with Wenckebach periodicity.
• Tetrodotoxin delayed antegrade propagation, inducing electrotonically mediated secondary depolarizations and exit block with Wenckebach periodicity.

Conclusions:

• Delayed activation and electrotonic interactions between SA node cells prolong action potential duration and refractoriness.
• These electrotonic effects contribute to progressive propagation delays.
• Such delays can result in intermittent block and Wenckebach periodicity within the SA node, offering insights into cardiac rhythm regulation.