(a) Sildenafil-induced concentration-dependent relaxation in the absence and presence of the NO synthase inhibitor L-NNA(100 nM). (b) In vitro effects of sildenafil (100 mM, 5 min) on acetylcholine-induced, endothelium-dependent and (c) endothelium-independent vasorelaxation by the NO donor DEA-NONOate in aorta from control or diabetic (STZ) rats. Data are means±s.e.mean from 10 to 15 experiments, **P<0.01 vs STZ. DEA-NONOate, 2-(N,N-diethylamino)-diazenolate-2-oxide; L-NNA, NG-nitro-L-arginine; STZ, streptozotocin.