Proliferation of arterial smooth muscle cells is regarded as an important event in atherogenesis, which according to in vitro culture studies is influenced by diabetes and insulin. To assess whether this holds true in vivo, we studied the cellular kinetics of thoracic aorta in normal and streptozocin-induced diabetic rats with and without insulin treatment. We measured the incorporation of [3H]thymidine into intima-media, as well as its DNA content, 2 and 14 days after endothelial denudation. We found that the mitotic response of an injured artery is not modified by diabetes but is depressed by insulin treatment in nondiabetic rats, probably due to hypoglycemia. Our data in insulin-treated diabetic rats support but do not definitely settle the view that insulin is mitogenic as long as the treatment does not cause sustained hypoglycemia.

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