[Effect and action mechanism of hypothermia to preserve the ischaemic myocardium (author's transl)].

The ability of hypothermia (34 degrees, 28 degrees) to preserve cardiac metabolism and performance during ischemia, was evaluated in the isolated Langendorff perfused rabbit heart. The hearts, isolated and perfused aerobically for 20', were made ischemic for 90' and their wall temperature maintained either at 37 degrees, 34 degrees and 28 degrees. The hearts were consequently reperfused at 37 degrees for 30'. Some of the hearts were frozen and assayed for ATP and CP. Others were homogenized and their mitochondria harvest, using either an EDTA free or an EDTA-containing extraction medium. The oxidative phosphorylating and ATP generating capacity of these mitochondria were established and their Ca++ content determined. The mechanical performance of the hearts, which were paced, was monitored by means of an intra-ventricular balloon filled with water and connected with a pressure transducer. The hearts that were made ischemic and maintained at 37 degrees were severely depleted in ATP and CP content, their mitochondria accumulated Ca++ and their oxidative phosphorylating activity was impaired. During reperfusion mitochondrial Ca++ was substantially increased, the capacity of the mitochondria to use O2 for state III respiration was further impaired and their ATP generating capacity reduced. Diastolic pressure increased and there was no recovery of the ability of the hearts to develop sistolic pressure. The hearts made ischemic and maintained at 28 degrees were protected. There was a less marked rise in mitochondrial Ca++ concentration after ischemia and during reperfusion; the mitochondria recovered the capacity of utilizing O2 and of generating ATP. That was coincident with and almost complete recovery of mechanical performance. Hypothermia at 34 degrees during ischemia provoked only a partial protection. These results are discussed in accordance with the hypothesis that hypothermia protects heart muscle against the deleterious effects of ischemia not only by reducing the metabolic requirement but also by maintaining intracellular homeostasis with respect to Ca++.