Background-Inflammation in adipose tissue has been implicated in vascular dysfunction, but the local mechanisms by which this occurs are unknown. Methods and Results-Small arteries with and without perivascular adipose tissue were taken from subcutaneous gluteal fat biopsy samples and studied with wire myography and immunohistochemistry. We established that healthy adipose tissue around human small arteries secretes factors that influence vasodilation by increasing nitric oxide bioavailability. However, in perivascular fat from obese subjects with metabolic syndrome (waist circumference 111Ϯ2.8 versus 91.1Ϯ3.5 cm in control subjects, PϽ0.001; insulin sensitivity 41Ϯ5.9% versus 121Ϯ18.6% in control subjects, PϽ0.001), the loss of this dilator effect was accompanied by an increase in adipocyte area (1786Ϯ346 versus 673Ϯ60 m 2 , PϽ0.01) and immunohistochemical evidence of inflammation (tumor necrosis factor receptor 1 12.4Ϯ1.1% versus 6.7Ϯ1%, PϽ0.001). Application of the cytokines tumor necrosis factor receptor-␣ and interleukin-6 to perivascular fat around healthy blood vessels reduced dilator activity, resulting in the obese phenotype. These effects could be reversed with free radical scavengers or cytokine antagonists. Similarly, induction of hypoxia stimulated inflammation and resulted in loss of anticontractile capacity, which could be rescued by catalase and superoxide dismutase or cytokine antagonists. Incubation with a soluble fragment of adiponectin type 1 receptor or inhibition of nitric oxide synthase blocked the vasodilator effect of healthy perivascular adipose tissue. Conclusions-We conclude that adipocytes secrete adiponectin and provide the first functional evidence that it is a physiological modulator of local vascular tone by increasing nitric oxide bioavailability. This capacity is lost in obesity by the development of adipocyte hypertrophy, leading to hypoxia, inflammation, and oxidative stress. Key Words: hypoxia Ⅲ inflammation Ⅲ obesity Ⅲ microcirculation Ⅲ nitric oxide synthase M etabolic syndrome is a precursor to type 2 diabetes mellitus and cardiovascular disease, with a prevalence of almost 40% in the adult population. 1 Central obesity is believed to be the main cause of metabolic syndrome, and this is reflected in newer definitions of the condition with large waist circumference as a prerequisite. 2 Although associations of obesity with hypertension, 3 insulin resistance, 4 and cardiovascular disease 5 are well described, the underlying mechanisms are poorly understood. Two areas of research that may provide insight into these are the vasoactive properties of perivascular adipose tissue (PVAT) and the inflammatory changes that occur in fat as obesity develops. Demonstrated in 1991, 6 it is now accepted that healthy PVAT has an anticontractile effect. [7][8][9][10] The mechanism appears to be both endothelium dependent via release of nitric oxide 8,10 and endothelium-independent via generation of hydrogen peroxide. 8 In obesity and metabolic syndrome, there is also a conformational chang...

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S. Roberts is supported by the NIHR Manchester Biomedical Research Centre.