Hyperleptinemia accompanying obesity affects endothelial nitric oxide (NO) and is a serious factor for vascular disorders. NO, superoxide (O(2)(-)), and peroxynitrite (ONOO(-)) nanosensors were placed near the surface (5+/-2 microm) of a single human umbilical vein endothelial cell (HUVEC) exposed to leptin or aortic endothelium of obese C57BL/6J mice, and concentrations of calcium ionophore (CaI)-stimulated NO, O(2)(-), ONOO(-) were recorded. Endothelial NO synthase (eNOS) expression and L-arginine concentrations in HUVEC and aortic endothelium were measured. Leptin did not directly stimulate NO, O(2)(-), or ONOO(-) release from HUVEC. However, a 12-h exposure of HUVEC to leptin increased eNOS expression and CaI-stimulated NO (625+/-30 vs. 500+/-24 nmol/l control) and dramatically increased cytotoxic O(2)(-) and ONOO(-) levels. The [NO]-to-[ONOO(-)] ratio ([NO]/[ONOO(-)]) decreased from 2.0+/-0.1 in normal to 1.30+/-0.1 in leptin-induced dysfunctional endothelium. In obese mice, a 2.5-fold increase in leptin concentration coincided with 100% increase in eNOS and about 30% decrease in intracellular L-arginine. The increased eNOS expression and a reduced l-arginine content led to eNOS uncoupling, a reduction in bioavailable NO (250+/-10 vs. 420+/-12 nmol/l control), and an elevated concentration of O(2)(-) (240%) and ONOO(-) (70%). L-Arginine and sepiapterin supplementation reversed eNOS uncoupling and partially restored [NO]/[ONOO(-)] balance in obese mice. In obesity, leptin increases eNOS expression and decreases intracellular l-arginine, resulting in eNOS an uncoupling and depletion of endothelial NO and an increase of cytotoxic ONOO(-). Hyperleptinemia triggers an endothelial NO/ONOO(-) imbalance characteristic of dysfunctional endothelium observed in other vascular disorders, i.e., atherosclerosis and diabetes.