How Does Chronic Sildenafil Prevent Vascular Oxidative Stress in Insulin-Resistant Rats?

Abstract

Insulin resistance features both endothelial dysfunction and increased oxidative stress. Both disorders are targeted by a chronic treatment with sildenafil. However, the mechanism of action by which chronic sildenafil exerts its effects on reactive oxygen species sources is still largely unknown. We therefore investigated how chronic sildenafil administration could impact vascular endothelial NO and superoxide release in a rat model of insulin resistance induced by fructose overload. Adult male Wistar rats were fed a fructose-enriched diet (fructose-fed rats [FFR]) for 9 weeks. From weeks 6-8, sildenafil was administered subcutaneously twice daily (20 mg/kg), followed by a 1-week washout. Vascular endothelial NO and superoxide release were monitored in vitro in thoracic aortic segments using oxidative fluorescence. Specific inhibitors were used to distinguish the respective role of the main superoxide-producing systems within the vascular wall (i.e., mitochondrial respiratory chain and NADPH oxidases). The levels of expression of eNOS, Akt, and NADPH oxidase subunits were determined in the abdominal aorta. Chronic sildenafil administration corrected hyperglycemia, hyperinsulinemia, and hypertriglyceridemia in FFR. Moreover, after 9 weeks of diet, while global unstimulated aortic endothelial NO and superoxide release were unchanged in FFR, the relative contribution of the mitochondrial respiratory chain and NADPH oxidases was modified. Chronic sildenafil treatment, even after the 1-week washout period, was able to increase endothelial NO release independently of Akt-dependent phosphorylation by up-regulating eNOS expression, and restored the relative contribution of each superoxide-producing system examined, yielding endothelial superoxide release. Finally, in vitro incubation of aortic segments with sildenafil markedly decreased the endothelial aortic superoxide release. The present study showed that chronic sildenafil produced sustained vascular antioxidant effects in insulin-resistant rats by increasing NO release and regulating vascular superoxide release, supporting therefore further investigations using chronic sildenafil administration in preventing cardiovascular alterations associated with oxidative stress.