Ischaemia-induced changes in canine cardiac sarcoplasmic reticulum

Effects of myocardial ischaemia on sarcoplasmic reticulum (SR) of dog hearts were investigated. Regional ischaemia was produced by occlusion of the left circumflex artery, and a microsomal fraction enriched in vesicles of SR was isolated from subendocardium (Endo) and subepicardium (Epi) of control and ischaemic areas of the heart. No significant changes occurred in ischaemic Epi. A loss of in vitro activities (ie calcium transport and ATPase) was found for SR from ischaemic Endo which paralleled the changes in the histology of the tissue. At 5 min of coronary occlusion, Ca2+ binding and Ca2+-ATPase activities of SR from ischaemic Endo were normal. A decrease in the activities of SR was first evident at 15 min after the occlusion, decreased further at about 30 min and remained at that level at 60 min of ischaemia. The maximal rate of Ca2+ uptake did not parallel the Ca2+-binding and Ca2+-ATPase activities. The degree of cAMP-dependent phosphorylation by endogenous and exogenous protein kinase was not different between SR from control and ischaemic areas. A participatory role of SR in the ischaemic impairment of left ventricular systolic and diastolic performance is discussed.