Ultrastructural changes in gap junctions associated with CO2 uncoupling in frog atrial fibres

The correlation between gap junction morphology and the state of electrical coupling was investigated in the frog auricle, which presents an atypical gap-junction organization. Electrical uncoupling of the tissue was achieved by perfusion with CO2-saturated Ringer medium. The tissue was fixed with glutaraldehyde and freeze-fractured before and during the application of CO2-saturated Ringer medium and after returning to the initial medium. The electrical tissue coupling was assayed by microelectrode recording just before fixation. At least 97% uncoupling was induced by CO2-saturated Ringer medium, as estimated from double sucrose gap experiments on several single atrial trabeculae. Several modifications were induced by CO2-saturated Ringer medium. A decrease in the number of particles per junctional assembly and dispersion of these assemblies in the plane of the membrane, indicate a decrease in the organization of the gap junctions. In parallel, loose clusters of particles became evident on the P-fracture face of the membrane. The size of the particles in these clusters was larger than the size of the background particles, and of the gap junction particles. They never corresponded with complementary pits on the E-fracture face. On return to the initial Ringer perfusion medium the cell coupling was reversed and the gap junction dispersion was also reversed. However, the gap junctions remained small. The relationship between these morphological modifications and the conducting state of the tissue is discussed.