Biological activation of N(G)-nitro-D-arginine by kidney homogenate

N(G)-nitro-L-arginine (L-NNA) and D-NNA have been shown to inhibit endothelium-dependent relaxation. This study examined if the inhibitory effect of L-NNA or D-NNA on relaxation is increased following incubation of the drug with the supernatant of tissue homogenates. Acetylcholine (ACh) caused concentration-dependent relaxation of pre-constricted rat aortic rings with maximum relaxation of 95%. Maximum relaxations to ACh were reduced to 71 and 37% in the presence of D-NNA (40 microM) and L-NNA (1 microM), respectively. Relaxation to ACh was further reduced to 18% in the presence of D-NNA that was incubated for 1 h with the supernatant of kidney homogenate, but unaffected by D-NNA incubated with the supernatant of trichloroacetic acid-denatured kidney homogenate. Incubation of L-NNA (1 microM) with either kidney supernatant or denatured kidney supernatant for 1 h did not affect its inhibitory effect on ACh-induced relaxation. Neither 1 h's incubation with plasma, or supernatants of liver, lungs or aorta homogenates affected the inhibitory action of D-NNA (40 or 120 microM) on ACh-induced relaxation. After D-NNA was incubated in kidney supernatant, its inhibitory effect on ACh-induced relaxation of the aorta was abolished by pretreatment of the aorta with L-arginine (L-Arg) but not D-Arg suggesting involvement of the L-Arg pathway. The results suggest that D-NNA is converted by the kidney to a compound that acts similar to L-NNA. There appears to be little conversion of L-NNA to D-NNA.