Initial events in radiation-induced atheromatosis. II. Damage to intimal cells.

The carotid artery of the rabbit is a suitable blood vessel to study radiation induced atheromatosis in hypercholesteremic animals, because no plaque formation occurs within two months after the start of a 0.5% cholesterol diet. Cholesterol contents as high as 2% however, do give atheromatous plaques in the carotid artery without prior irradiation. As early as five hours after local irradiation of the carotid artery activation of the plasma membrane-bound enzyme alkaline phosphatase could be observed in some intimal cells. Two to three days after irradiation the activity disappeared. This phenomenon was observed in normo-and hypercholesteremic irradiated arteries. Depending on the lipid content of the blood, infiltration of lipids was observed at one day after the irradiation or later, accompanied by activation of beta-glucuronidase in the innermost layer of medial cells. Hereafter plaque formation started and cell proliferation could be found in the subendothelial space. It is assumed that because of the irradiation, the endothelial cells of the carotid artery are damaged in such a way that they do not function properly as a barrier against lipoprotein entrance from the blood into the arterial wall. The lipid infiltration caused lysosomal activation and probably cellular proliferation.