The Role of Nitric Oxide in Mediating Nonadrenergic, Noncholinergic Relaxation in Rat Pulmonary Artery

Experiments were undertaken to investigate the existence of inhibitory nonadrenergic, noncholinergic (i-NANC) nerve activity by using in vitro functional and immunohistochemical techniques in rat main pulmonary arterial rings. Vessels precontracted with phenylephrine (3 μM) relaxed in response to electrical field stimulation (EFS) (50 V, 0.2 ms, 0.1–10 Hz for 5 s) in the presence of atropine (1 μM) and guanethidine (1 μM). Tetrodotoxin (0.3 μM) abolished this response, indicating that it is neuronal in origin. l-NAME (30 μM), methylene blue (10 μM), and removal of endothelium significantly reduced the EFS-induced relaxations. The inhibitory action of l-NAME was completely reversed by l-arginine (1 mM) but not by d-arginine (1 mM). Moreover l-arginine alone potentiated the magnitude of the relaxations elicited by EFS. On the other hand, immunohistochemical work clearly demonstrated the existence of neuronal nitric oxide synthase in the pulmonary artery vessel wall. All these results are consistent with the suggestion that nitric oxide is the likely mediator of this vasodilatation. However, the incomplete blockade of the responses by l-NAME gives evidence of an additional inhibitory NANC neurotransmitter(s) mediating the residual relaxation, which requires further experiments to clarify its nature.