Effect of acute exercise stress in cardiac hypertrophy. II. Quantitative ultrastructural changes in the myocardial cell.

Stereological techniques were used to determine quantitatively the changes in subcellular organelles in the hypertrophied swine heart. Hypertrophy was induced by aortic stenosis. In addition, the animals were stressed by exercising twice weekly on the treadmill for 30 days. The controls were normal animals which were neither exercised nor had aortic stenosis. Tissue samples from the left ventricle and interventricular septum were processed for electron microscopy. Relative volumes of myofibrils, mitochondria, transverse tubular system (T-system) sarcoplasmic reticulum (SR), and clear intracellular space (ICS) were measured. Significant differences in the volumes of all the components except the T-system were found between experimental and control animals in the epicardial and endocardial regions of the left ventricle and interventricular septum. Mitochondria and myofibrils were significantly decreased in the exercise stressed hypertrophied hearts as reported in ischemia, while SR and ICS were significantly increased. These findings suggest that ischemic injury occurs in all regions of the hypertrophied heart wall subjected to acute exercise stress, not solely the endocardial region as previous qualitative studies suggested.