Dipyridamole-induced capillary endothelial cell proliferation in the rat heart--a morphometric investigation.

20 male Wistar rats were randomly divided into two equal groups. Each experimental rat was injected with 4 mg . kg-1 dipyridamole ip twice daily for 4 weeks, the control rats received sham injections of an equal volume of saline. The rats were fixed by retrograde vascular perfusion. Left ventricular papillary muscles were sampled for light and electron microscopic morphometry. By means of a combination of serial section reconstruction and morphometry we could demonstrate that the number of capillary endothelial cell nuclei per tissue volume had increased by 37% (P less than 0.01), whereas the average nuclear volume remained constant. All other morphometric parameters (volume densities of myocardial cells, capillaries, cellular and non-cellular interstitial tissue, and myocardial cell nuclei; surface area, diameter, and density of capillaries; volume densities of myofibrils, mitochondria, and sarcoplasmic matrix; surface areas of the inner and outer mitochondrial membranes; dimensions of average mitochondrial section profiles) were unchanged. We conclude that chronic dipyridamole application induces proliferation of capillary endothelial cells in the rat myocardium, possibly by an increase of myocardial blood flow.