Regression of cardiac hypertrophy: morphometric and biochemical studies in rat heart after swimming training

There is currently little information about the morphological changes of the myocardium accompanying the reversal of cardiac hypertrophy. In this study the hypothesis was tested that myocardial alterations induced by exercise will regress within a short interval after the end of training. Rat hearts were examined using morphometric and biochemical methods at the end of a 9-week period of endurance training and also 7, 10 and 14 days after its termination. At the end of the training period the heart weight had increased by 65% but the weight ratio of the right and left ventricular wall remained unchanged. A decline in the DNA content by 27% as well as a decrease in the volume density of the interstitial space by 14% and in the number of interstitial cell nuclei by 32% against controls, are explained by a 30% increase in the width of myofibres. The capillary density was reduced by 22% but the volume density of capillaries remained nearly constant as a result of widening of the capillary diameter by 27%. The surface density of capillaries was diminished by 10%. Ultrastructurally an increase in the ratio of mitochondrial to myofibrillar volume density was observed in the myocytes of hypertrophied hearts as compared to controls (0.54 and 0.63, respectively). Fourteen days after termination of training, 80% of the increment in heart weight had regressed. At this time the width of the myofibres and the volume density of the interstitial space had nearly normalized, while the capillary to fibre ratio had significantly increased. The ratio of mitochondrial and myofibrillar volume density became nearly normal, and a confluence of intermyofibrillar mitochondria resulted in significantly longer organelles. The increased DNA content 10 days after the training, as compared to controls, is attributable to the genesis of non-myocardial cells during the hypertrophic growth and their persistence during regression. The study has shown that cardiac hypertrophy induced by physical training nearly completely regresses within 14 days after termination of conditioning. The increased capillary to fibre ratio indicates neoformation of transversely oriented capillary branches in hypertrophy which particularly becomes apparent in two-dimensional estimation in the regression period. In comparison with myofibres, regression of capillaries seems to be delayed. The decline of heart weight and a significantly diminished RNA content during the regression of hypertrophy suggest that reduced synthesis is responsible for the decrease in heart weight.