Structural changes in cardiac gap junctions after hypoxia and reoxygenation: a quantitative freeze-fracture analysis

Summary

Isolated rat hearts were subjected to increasing periods of hypoxia with or without subsequent reoxygenation and the gap-junctional particle configuration was followed quantitatively. Irregular contractions were prevented by K⁺-arrest; glucose, counteracting the effects of hypoxia, was omitted. Hyperkalemia alone and a maximum of 20 min of hypoxia do not produce reorganization of the gap-junctional particles normally forming multiple hexagonally packed arrays separated by smooth aisles. After 30 min of hypoxia, the aisles disappear in a proportion of the junctions, thereby increasing the particle density from 9400±800/μm² to 10200±900/μm². After 40 min of hypoxia, the normal configuration is no longer found and numerous junctions are arranged as uninterrupted hexagonal lattices. The particles are further condensed to 11600±900/μm². Following reoxygenation after both 30 and 40 min of hypoxia, the proportion of crystalline gap junctions dramatically augments and the mean particle density has further increased significantly. Corresponding thin sections show irreversible cell damage. When reoxygenation is performed with a control solution containing normal levels of K⁺ and glucose, the particle density does not increase substantially in comparison to the respective 30- and 40-min hypoxic periods. In both groups, the gap junctions display either a normal, a crystalline or an intermediate configuration with crystalline margins and loose centers. The gap-junctional reorganization during hypoxia essentially represents a particle condensation, while the mean center-to-center distances between the particles and pits remain constant. Furthermore, the reappearance of normal gap junctions after reoxygenation appears to depend on glucose availability.