Intercellular coupling in frog heart muscle. Electrophysiological and morphological aspects

Area of Science:

• Cardiovascular Physiology
• Cellular Electrophysiology
• Biophysics

Background:

• Understanding the electrical properties of cardiac tissue is crucial for comprehending heart function.
• Intercellular junctions play a vital role in electrical coupling between cardiac cells.
• The specific nature of junctions in amphibian atrial muscle requires further elucidation.

Purpose of the Study:

• To measure passive electrical parameters of bullfrog atrial trabeculae.
• To investigate the influence of external calcium depletion on internal longitudinal resistance.
• To characterize the structure and function of junctional complexes in frog atrial muscle.

Main Methods:

• Electrophysiological measurements using a single gap arrangement.
• Morphometric analysis via light and electron microscopy.
• Freeze-fracture electron microscopy to visualize junctional structures.
• Fluorescent dye (CTC) to probe membrane-bound calcium distribution.

Main Results:

• Specific internal resistance increased from 523 omega cm to 1140 omega cm upon external calcium depletion (EGTA).
• "Atypical gap junctions" were observed, with particle numbers per junction unaffected by EGTA, though junctional complexes converted to clusters.
• Junctional membrane resistance increased by approximately 230% with EGTA, suggesting a role in altered conductance.

Conclusions:

• External calcium depletion significantly increases internal resistance in bullfrog atrial trabeculae, impacting electrical signal propagation.
• The observed "atypical gap junctions" are functionally analogous to mammalian gap junctions, despite structural differences.
• The mechanism of EGTA-induced depression of junctional conductance is not directly related to cytoplasmic free calcium levels.