Ultrastructure of the sarcoplasmic reticulum in the ischemic myocardium

Fine structural changes in the sarcoplasmic reticulum (SR) in myocardial ischemia, induced by occlusion of the anterior descending branch of the left coronary artery in the canine heart, were studied by the freeze-fracture technique in situ and in vitro and compared to the alterations in Ca2+-stimulated ATPase activity and sodium dodecyl sulfate gel electrophoresis of the isolated SR. Both SR in situ and the isolated SR exhibited the typical intramembranous particles of 70-90 A in freeze-fracture replicas, and the numbers of particles were more numerous in the concave face (PF) than in the convex face (EF). The numbers of particles in the PF were 2748/micron 2 on the average. In ischemia for 1-2 hr, a significant decrease in the numbers of the particles was found in SR in situ, and corresponding changes were noted in the isolated SR. Decreases in Ca2+-stimulated ATPase activity and in the major protein band of ATPase were recognized simultaneously. The close correlation of the changing patterns between the reduction in Ca2+-ATPase and that in the intramembranous particle density during ischemia supports the suggestion that a large part of the intramembranous particles represents ATPase protein itself. The decrease in the particles of SR membrane indicates the degradation of ATPase in the process of ischemic myocardial necrosis.