The effect of exercise on myocardial interstitium. An ultrastructural morphometric study

Ultrastructural morphometric examinations show that the normal rat myocardium of the left ventricle contains about 15% interstitial connective tissue, half of which (53%) is composed of extracellular space, ground-substance and collagen, 41% of blood capillaries and 6% of interstitial cells. These percentages vary in the course of swimming training for periods of 45, 180 and 360 hours in dependence on the duration of training and load. The volume ratio of the interstitium increases after 45 hours of exercise from 15 to 17 %, after 180 hours of training to 19% and after 360 hours of training to 22%. Relative changes in morphometric parameters (volume and surface density, number per unit area) of the interstitial components are observed in relation to untrained animals. Owing to a shift in the volume ratio of interstitium to muscle cells in favour of the interstitium, the real volume ratio changes only in the case of extracellular space and capillaries.

After 45 hours of training the volume ratio of extracellular space to interstitium decreases to 50%, that of the capillaries increases to 44%. After 180 hours of training the volume ratio of blood capillaries diminishes to 32%, while that of extracellular space increases (62.5%). This ratio remains constant in the course of further training even if the load is increased. Exercise has virtually no bearing on the volume ratio of connective tissue cells.

There is an increase in the capillary density of the myocardium and a reduction in the extracellular space after 45 hours of training. After 180 hours of maintenance training the ratio is inversed again and after that is no longer influenced by exercise.

Unlike the number per unit area, the surface density of the capillaries increases with increasing intensity of the swimming load throughout the training period. The volume ratio of capillary to muscle cells changes in favour of the capillaries in the course of training. Specific surface density of the capillaries increases continuously up to a training period of 180 hours, and there is no further increase even if training is continued after that. The morphometrical changes in the capillaries suggest that a short, but intensive period of training brings about an increase in the capillary surface, which can be maintained but not increased through further training.