Impaired nitric oxide synthase pathway in diabetes mellitus: role of asymmetric dimethylarginine and dimethylarginine dimethylaminohydrolase.

BACKGROUND An endogenous inhibitor of nitric oxide synthase, asymmetric dimethylarginine (ADMA), is elevated in patients with type 2 diabetes mellitus (DM). This study explored the mechanisms by which ADMA becomes elevated in DM. METHODS AND RESULTS Male Sprague-Dawley rats were fed normal chow or high-fat diet (n=5 in each) with moderate streptozotocin injection to induce type 2 DM. Plasma ADMA was elevated in diabetic rats (1.33+/-0.31 versus 0.48+/-0.08 micromol/L; P<0.05). The activity, but not the expression, of dimethylarginine dimethylaminohydrolase (DDAH) was reduced in diabetic rats and negatively correlated with their plasma ADMA levels (P<0.05). DDAH activity was significantly reduced in vascular smooth muscle cells and human endothelial cells (HMEC-1) exposed to high glucose (25.5 mmol/L). The impairment of DDAH activity in vascular cells was associated with an accumulation of ADMA and a reduction in generation of cGMP. In human endothelial cells, coincubation with the antioxidant polyethylene glycol-conjugated superoxide dismutase (22 U/mL) reversed the effects of the high-glucose condition on DDAH activity, ADMA accumulation, and cGMP synthesis. CONCLUSIONS A glucose-induced impairment of DDAH causes ADMA accumulation and may contribute to endothelial vasodilator dysfunction in DM.