ADAPTATION SPEED OF CARDIAC MITOCHONDRIAL OXYGEN-CONSUMPTION DECREASES WITH HIGHER HEART-RATE

The purpose of the present study was to determine whether the mean res ponse time of cardiac mitochondrial oxygen consumption after a step in metabolic demand is constant in heart muscle, as has already been fou nd for skeletal muscle. The mean response time reflects the average de lay between the change in ATP hydrolysis due to a heart rate step and mitochondrial ATP production. Isolated rabbit hearts with a water-fill ed balloon in the left ventricle were perfused according to Langendorf f with a constant flow of Tyrode solution at 28-degrees-C. The mean re sponse time increased significantly from 7.6 s for a step in heart rat e from 60 to 70 min-1 to 12.1 s for a step from 60 to 120 min-1. The m ean response times for heart rate steps downward from 120 min-1 were a ll approximately 12 s, but for the step from 120 to 140 min-1 the resp onse time was 16.8 s. These results demonstrate that the mean response time of cardiac mitochondrial oxygen consumption in most cases increa ses with heart rate. These findings are in contrast to those obtained in skeletal muscle, where the response time at which ATP synthesis ada pts to a change in work load is constant.