Hyperglycaemia and inflammatory can induce apoptosis in vascular endothelial cells, which contributes to the development of vascular complications in diabetes. Endothelial cells depend on glycolysis for their energy metabolism, and monocarboxylate transporters (MCTs) regulate intracellular pH by mediating the influx and efflux of lactate. Here, we evaluate the role of MCT4 in high glucose (HG) and interleukin 1 beta (IL-1 beta)-induced apoptosis in human umbilical vein endothelial cells (HUVECs). We demonstrate that aortic endothelium damage is severe in db/db mice by using scanning ion conductance microscopy (SICM). HG and IL-1 beta decrease MCT4 and its location on plasma membrane, as well as increase lactic acid accumulation and apoptosis in HUVECs. Knockdown of MCT4 blocks lactate efflux to result in lactic acid accumulation and pH dropping, which is involved in triggering apoptosis in HUVECs. (C) 2015 Elsevier Inc. All rights reserved.