Release of nitric oxide from nitrergic nerves of the guinea-pig small intestine using a bioassay system

It has been suggested that the transmitter released from nitrergic nerves is not free nitric oxide (NO) because a number of agents that block responses to exogenous NO do not block responses to nerve stimulation (Rand & Li, 1995) Recently Boeckxstaens el al. (1991) using a bioassay system consisting of rat fundus as a donor and rabbit aortic rings as a detector showed that stimulation of nitrergic nerves released a transferable factor that behaves very similar to free NO The aim of this study was to investigate the effect of the NO scavenger carboxy-PTIO on the factor released in a bioassay system using guinea-pig small intestine as a donor and rabbit aortic strip as a detector tissues An approximately 30 cm length of the longitudinal layer of smooth muscle of guinea-pig small intestine was mounted in a perfusion chamber and perfused at 3 ml/min with modified physiological salt solution (PSS). The PSS contained atropine (1 μM), guanethedine (30 μM), L-arginine (10 μM) and Superoxide dismutase (5 U/ml). A rabbit aortic strip denuded of endothelium was placed directly below the perfusion chamber and was superfused with the effluent of the guinea-pig small intestine and contracted by an infusion of phenylephrine (1 μM). Stimulation of the small intestine (15 Hz, 1 ms, 10 V for 20 s) induced the release of a vasorelaxant factor as detected by the rabbit aorta. Resonses of this factor was inhibited by tetrodotoxin (1 μM), and by the NO-synthase inhibitor K'-nitro-L-arginine (100 μM), to 48 ± 4.5 % (n=3) and 41 ± 7.3 % (n=4) of control responses, respectively. Addition of carboxy-PTIO (300 μM) to the perfusate also inhibited responses to this factor Thus, the findings indicate that stimulation of the guinea-pig small intestine releases a transferable relaxant factor that closely resembles NO.