BACKGROUND: It is well known that hyperglycemia activates protein kinase C (PKC) in vascular endothelial cells. However, the effect of hyperglycemia on internalization and recycling of insulin receptors by insulin in endothelial cells has not been examined thus far. METHODS: Human umbilical vein endothelial cells (HUVECs) were isolated from healthy, pregnant women. Confluent HUVECs were incubated in a culture media containing either 5 (NG group) or 25 mM glucose (HG group) for 4 days. Then, we measured the insulin binding, internalization and recycling of the insulin receptor and release of internalized insulin into the media. RESULTS: There was no difference in binding of 0.17 nM 125I-insulin between the two groups. However, the amount of internalized 125I-insulin, determined by the aeid washing method, was significantly greater in the HG group compared to the NG group. The addition of 10 pM 1-(5-isoquino-linesulfonyl)-2-methyl-piperazine (H7), a PKC inhibitor, to the HG group prevented the increase of internalization in 125I-insulin. In addition, preincubation with unlabeled insulin resulted in a decrease of 125I-insulin binding to a greater extent in the HG group compared with the NG group, indicating that high glucose levels increased internalizntion of insulin receptors. The high glucose-induced increase in internalization of insulin receptors was prevented by an addition of H7. Recycling of insulin receptors to the cell surface was not affected by high glucose. Internalized 125I-insulin released into media with time. The released amount of I-insulin in the HC group tended to be greater compared to the NG group. CONCLUSION: These results suggest that hyperglycemia may increase internalization of the insulin-receptor complexes in vascular endothelial cells through PKC activation.