L-ARGININE EVOKES BOTH ENDOTHELIUM-DEPENDENT AND ENDOTHELIUM-INDEPENDENT RELAXATIONS IN L-ARGININE DEPLETED AORTAS OF THE RAT

This study was designed to investigate the effects of L-arginine (the substrate for the formation of endothelium-derived nitric oxide) in vascular tissues. Rat aortic rings, with or without endothelium, were suspended in organ chambers for the measurement of isometric tension; they were contracted with phenylephrine (10(-6) M). After a short incubation period (0.5 hour) in physiological salt solution, L-arginine induced minor changes in both types of rings. In contrast, when the incubation time was increased (2, 4, 6, and 8 hours), L-arginine evoked concentration- and time-dependent relaxations in aortic rings both with and without endothelium. The relaxations were larger in rings with endothelium. The presence of L-arginine (10(-3) M) in the incubation medium inhibited subsequent relaxations evoked by the amino acid. The concentration-relaxation curves associated with acetylcholine in rings with endothelium and the curves associated with Sin-1, a spontaneous donor of nitric oxide, in rings with or without endothelium were slightly but significantly shifted to the right after a 6-hour incubation. Nitro-L-arginine (3 x 10(-5) M) and methylene blue (3 x 10(-7) M) attenuated the relaxations evoked by L-arginine in rings both with and without endothelium. Other basic amino acids (D-arginine, L-homoarginine, L-citrulline, L-lysine, and L-ornithine; all tested at 10(-3) M) either had no effect or induced small relaxations and did not affect the response to L-arginine. These observations suggest that L-arginine specifically and stereoselectively relaxes aortic rings with and without endothelium, probably by restoring the endogenous pool of the amino acid, which is likely depleted by prolonged incubation. Since the relaxations in response to L-arginine are inhibited by nitro-L-arginine in rings both with and without endothelium, the present experiments demonstrate that both the endothelial cells and the vascular smooth muscle possess biochemical pathways converting L-arginine to nitric oxide.