Pacemaker cell types in the rabbit sinus node: a correlative ultrastructural and electrophysiological study.

Area of Science:

• Cardiovascular Physiology
• Cell Biology
• Cardiac Electrophysiology

Background:

• The sinoatrial node is the heart's primary pacemaker, responsible for initiating the cardiac cycle.
• Cellular structure and function within the sinus node are crucial for understanding cardiac automaticity and conduction.
• Previous studies have suggested a link between cell morphology and electrical properties in pacemaker tissue.

Purpose of the Study:

• To investigate the relationship between the fine structure (myofilament density) and electrophysiological properties of rabbit sinus node cells.
• To characterize different cell types within the sinus node based on myofilament organization and organelle density.
• To correlate structural characteristics with pacemaker activity, action potential parameters, and impulse conduction.

Main Methods:

• Combined microelectrode recordings for electrophysiological identification with electron microscopy for ultrastructural analysis of rabbit sinus node tissue.
• Quantified myofilament density and assessed myofibril organization semi-quantitatively.
• Measured activation moment, rate of diastolic depolarization, and maximum rate of rise of action potentials at various sites.

Main Results:

• A gradual increase in myofilament density was observed from the center to the periphery of the sinus node.
• The rate of diastolic depolarization, a key indicator of automaticity, was inversely related to myofilament volume percentage.
• Cells in the central node with lower myofilament density exhibited more specialized pacemaker activity; conduction velocity increased with myofilament density towards the crista terminalis, but impulse conduction was blocked towards the interatrial septum.

Conclusions:

• Pacemaker action is directly related to cell type, with less developed central cells being the most specialized pacemakers.
• Myofilament density is a significant factor in determining sinus node automaticity.
• While myofilament density influences conduction velocity in certain directions, a direct correlation between cell type and impulse conduction across all pathways could not be established.