| Abstract: | Ultrastructural myocardial cell changes were determined in eight miniswine after the development of pressure-overload hypertrophy induced by supra-valvular aortic constriction. Four miniswine served as control animals. Regional myocardial blood flows were measured at rest and during exercise stress with radioactive microspheres after two days and one month of aortic constriction. Exercise stress, causing the heart rate to increase to 85 percent of its maximum, was imposed twice weekly for 7 minutes on four pressure-overloaded animals and the four control animals to elicit differences between the control and experimental groups that might not occur at rest. After one month of pressure overload the swine were killed and myocardial samples were processed for electron microscopy. Ultrastructural changes similar to those in hypertrophied hearts were present throughout the left ventricular walls of the pressure-overloaded animals. Other changes consistent with ischemic injury were present in the subendocardial regions of pressure-overloaded animals subjected to exercise stress. These changes included disorganization of myofibrils, disintegration and broadening of Z-bands, swelling and aggregation of mitochondria, electron-dense deposits in mitochondria, decreased cristal density and vacuolization of mitochondria, intracellular edema, margination and clumping of nuclear chromatin, and a decrease of glycogen granules. Regional ischemia in the subendocardium of these animals was confirmed by functional studies which showed decreased regional myocardial blood flow to the subendocardium during exercise and S-T segment elevation for the first 2-10 days after inducing pressure overload. The ischemia, as shown by flow studies, during exercise stress persisted in the compensatory stage of hypertrophy although S-T segments returned to normal. Thus, the combined effect of pressure overload and exercise stress can produce focal subendocardial ischemia in the compensated, hypertrophied heart. |
