The proliferative activity of the myocardial tissue in various forms of experimental cardiac hypertrophy.

Various forms of experimental cardiac hypertrophy were investigated by light microscope autoradiography and measurements of tissue radioactivity after in vivo injections of 3H-thymidine in rats. In the autaradiograms an attempt was made to distinguish between labelled vessel wall cells, connective tissue cells and heart muscle cells. A highly significant increase in the nuclear incorporation of 3H-thymidine was recorded in both ventricle walls of rats with cardiac hypertrophy induced by swimming exercise. The increased incorporation was mainly confined to the nuclei of cells in the walls of the blood capillaries. A considerably smaller increase in the incorporation into capillary wall cells was recorded in the left ventricle wall of rats with cardiac hypertrophy secondary to prolonged arterial hypertension and aortic stenosis of 1–2 weeks1 duration. No increased incorporation of 3-thymidine into the myocardial tissue was found in rats if their swimming exercise was followed by a period of rest, in rats with short-term hypertension and in rats with aortic stenosis of 2 months' duration. The observations suggest a significant neoformation of myocardial blood vessels during swimming exercise, whereas any neoformation of myocardial blood vessels in arterial hypertension and aortic stenosis appears to be of minor degree.