Local Mitochondrial Enzyme Activity Correlates with Myocardial Blood Flow at Basal Workloads

To study whether heterogeneous myocardial blood flow relates to the local oxidative capacity of cardiac muscle, local blood flow at resting cardiac workloads and the activity of the mitochondrial enzyme succinate dehydrogenase (SDH) were determined in small regions of the left ventricle of seven anaesthetized, mechanically ventilated, open-chest pigs (25-35 kg). Following injection of radioactive microspheres (15 μm Ø) into the left atrium, the heart was rapidly excised and cut into five transverse slices, which were simultaneously freeze-clamped between two aluminum blocks precooled at -80°C. The left ventricle was then subdivided into 84 samples of about 0.9 g. Myocardial blood flow was 0.88 ± 0.34 ml/min/g wet weight (ww), and SDH activity 1.46 ± 0.33 μmol/min/g ww (mean ± S.D., n = 7). Local data were normalized to their respective mean values in each pig, and then pooled. Local blood flow ranged from 0.32 to 1.63 of the mean, and blood flow heterogeneity characterized by the coefficient of variation (CV=S.D./mean) was 18.4%. Normalized local SDH activity ranged from 0.16 to 1.94, with a CV of 21.8%, significantly exceeding measurement error (CV=4.5%). Local blood flows and SDH activities did not vary among transmural sublayers of the left ventricle, but variation within each sublayer was considerable. In six of the seven pigs, local blood flow correlated (P < 0.05) with SDH activity, with correlation coefficient (r) ranging from 0.26 to 0.54 (for pooled data: r=0.27, P < 0.0001). When expressed per gram dry weight, heterogeneity of SDH activity increased (P < 0.05), and here also local blood flow correlated with SDH activity in all pigs (for pooled data: r=0.45, P < 0.001). Hence, heterogeneity of mitochondrial capacity within cardiac muscle partly explains the heterogeneity of myocardial blood flow, even though myocardial perfusion at rest was studied in relation with a maximal enzyme rate. The low correlation coefficient clearly indicates that at resting workloads other factors also play a role.