Force generation among cells in the relaxing heart

Area of Science:

• Cardiology
• Cell Biology
• Biophysics

Background:

• Mammalian heart cell force transmission is primarily attributed to intercalated discs.
• The precise mechanisms of force propagation within cardiac tissue remain an area of investigation.

Purpose of the Study:

• To investigate alternative pathways for force transmission in mammalian myocardium.
• To identify structural components responsible for intercellular force coupling beyond intercalated discs.

Main Methods:

• Isolated mammalian heart cell bundles were treated with EGTA, a calcium chelator, to permeabilize cell membranes and open intercalated discs.
• Ultrastructural examination of the myocardium was performed using electron microscopy.
• Force generation was measured following direct calcium activation of contractile proteins.

Main Results:

• EGTA treatment rendered cell membranes permeable, allowing direct calcium activation of contractile proteins.
• Direct calcium activation generated greater forces than electrical stimulation under physiological conditions.
• Ultrastructural analysis revealed a previously undescribed microfilament network connecting adjacent cell basement membranes and collagen fibers.

Conclusions:

• The intercalated discs are not the sole structures responsible for force transmission in mammalian myocardium.
• A novel, highly organized microfilament network likely serves as a primary pathway for force transmission during systole and relaxation.
• This microfilament network represents a significant force-bearing structure in cardiac tissue.