Endocytosis by vascular smooth muscle cells in vivo and in vitro. Roles of vesicles and lysosomes

Overloading of lysosomes of smooth muscle cells with excess substrate may be a key event in the development of hypertensive and atherosclerotic vascular disease. Cellular uptake of materials and its relation to lysosomal function were studied by ultrastructural cytochemistry in aortic smooth muscle cells grown in vitro and in intact calves. Injection of horseradish peroxidase (HRP) into hypertensive rats resulted in rapid insudation of the material into the environs of medial smooth muscle cells, entrance into surface pinocytic vesicles, and transport via vesicles into the cell interior where material was seen to accumulate within lysosomes. In vitro exposure of calf aortic cells to HRP in the medium resulted in a similar sequence of events. Pinocytic vesicles, seen both in vitro and in vivo, ranged in diameter from 650-1000 Å. These dimensions are adequate to permit incorporation of intact lipoproteins of all classes, except the larger chylomicrons.