Pharmacological activation of dimethylarginine dimethylaminohydrolase (DDAH) activity by inorganic nitrate and DDAH inhibition by N^G-hydroxy-L-arginine, N^ω,N^ω-dimethyl-L-citrulline and N^ω,N^ω-dimethyl-N^δ-hydroxy-L-citrulline: results and overview.