These findings are comparable with other studies performed in the tracheal rings of a guinea pig model of asthma and human ASM using single or multiple frequencies of EFS [3,27], where the inhibitory effect of NOS inhibitor, Nω-nitro-L-arginine, on NO-mediated iNANC relaxation was restored by L-citrulline as well as L-arginine [33]. Here, NOS1 is linked to asthma.