L-NMMA (Tilarginine) hydrochloride competitively antagonizes L-arginine, and thus inhibits production of nitric oxide (NO). L-NMMA hydrochloride is utilized as NO synthase antagonist in experimental settings[1].
[1] Frey C, et al., L-thiocitrulline. A stereospecific, heme-binding inhibitor of nitric-oxide synthases. J Biol Chem. 1994 Oct 21;269(42):26083-91. PMID:7523401