Sarcolemmal disruption during the calcium paradox.

Reperfusion of an isolated heart with calcium-containing solution after a short period of calcium-free perfusion may result in irreversible cell damage (calcium paradox). The ultrastructure of the sarcolemma of the rabbit heart during the calcium paradox was studied by using fast freezing devices. This method excluded ultrastructural changes induced by chemical fixation and cryoprotection. In addition, thin-section and conventional freeze-fracture electron microscopy were used. During reperfusion with calcium-containing solution disruption of the sarcolemma was observed, which was attended with formation of unilamellar and multilamellar vesicles and aggregation of intramembrane particles. These ultrastructural changes are explained in terms of calcium- and proton-induced lateral phase separation and fusion processes in the lipid bilayer of the sarcolemma.