Effect of prolonged hypothermic ischaemia on myocardial sarcoplasmic reticular calcium transport

Study objective – The aim of the study was to assess the effects of hypothermia on sarcoplasmic reticulum in myocardium subjected to prolonged ischaemia. Design – Calcium regulatory activity of myocardial sarcoplasmic reticulum was measured in hearts subjected to various periods of hypothermic ischaemia in comparison with hearts subjected to normothermic ischaemia. Subjects – Hearts (n = 5-9 per experiment) were obtained from male New Zealand white rabbits, 2.0-2.8 kg weight. Measurements and results – Calcium uptake and calcium dependent ATPase activity were measured in isolated sarcoplasmic reticulum vesicles after hypothermic ischaemia was produced by immersing hearts in saline at 4°C for 3,6, or 12 h. Normothermic hearts were immersed for 3 h at 37°C. Calcium uptake and calcium dependent ATPase (Ca-ATPase) activity were markedly inhibited by normothermic ischaemia. In hypothermic ischaemia, calcium uptake was only slightly depressed after 3 h, though longer periods of ischaemia resulted in significant depression of uptake. Ca-ATPase activity was unaffected after 6 h of hypothermic ischaemia. The ratio of calcium uptake to Ca-ATPase activity decreased after 3 h of hypotheric ischaemia. The phosphoenzyme concentration in sarcoplasmic reticulum was unaffected up to 6 h. The ratio of Ca-ATPase activity to phosphoenzyme concentration was not significantly altered until 12 h. Protein composition, examined by SDS-polyacrylamide gel electrophoresis, showed a decrease in 100 000 dalton polypeptide in normothermic ischaemia and after 12 h of hypothermic ischaemia. Conclusions – These results suggest that the depression of calcium uptake activity after 6 h of hypothermic ischaemia is likely to be due to uncoupling of calcium transport from ATP hydrolysis. Depressed Ca-ATPase activity at 12 h can be attributed to a reduction in the number of active calcium pump units. Hypothermia preserves function of myocardial sarcoplasmic reticulum during ischaemia for up to 3 h.