The relevance of non-excitable cells for cardiac pacemaker function

Area of Science:

• Cardiovascular Physiology
• Cardiac Electrophysiology
• Cell Biology

Background:

• Age-related changes in the sinus node show increased fibroblasts relative to cardiomyocytes.
• This architectural shift is implicated as a cause of sinus node disease.

Purpose of the Study:

• To investigate how non-excitable cells, specifically fibroblasts, influence the spontaneous activity of cardiomyocyte preparations.
• To elucidate the mechanisms by which fibroblasts modulate cardiac pacemaker function.

Main Methods:

• Utilized HL-1 cells and embryonic stem cell-derived cardiomyocytes as 2D and 3D cardiac pacemaker models.
• Measured spontaneous activity and conduction velocity (theta) using microelectrode arrays (MEAs).
• Assessed fibroblast influence in heterocellular cultures with varying cardiomyocyte-to-fibroblast ratios, including Cx43-deficient fibroblasts to evaluate gap junction coupling.

Main Results:

• Increased fibroblast concentration negatively correlated with beating frequency and conduction velocity in heterocellular cultures.
• Fibroblast interspersion in cardiomyocyte monolayers heightened beat-to-beat interval variability.
• Cx43-deficient fibroblasts showed reduced impact on conduction velocity compared to wild-type fibroblasts, but similar effects on beating frequency and variability, suggesting dual mechanisms.

Conclusions:

• Non-excitable cells modulate cardiac pacemaker excitability via distinct mechanisms: electrical coupling and physical separation.
• Electrical coupling allows fibroblasts to depolarize cardiomyocytes or act as a current sink.
• Physical separation by fibroblasts induces bradycardia by reducing frequency entrainment, independent of electrical coupling.