Contribution of nitric oxide and substance P to nonadrenergic, noncholinergic transmission in the guinea pig ileum.

1. The possible contribution of the nonadrenergic noncholinergic (NANC) transmitters nitric oxide (NO) and substance P (SP) to the contractility of guinea pig isolated ileum was studied by the responses of the longitudinal muscle to electrical field stimulation (0.8 msec, 40 V, 1-20 Hz, 20 sec) of the intrinsic nerves and by the presence and distribution of NADPH-diaphorase- and SP-positive nerve structures in the myenteric plexus. 2. The electrically elicited, tetrodotoxin (0.3 microM)-sensitive responses, in the presence of phentolamine (5 microM), propranol (5 microM), and atropine (3 microM) consisted of relaxation, followed by twitch and tonic contraction on which phasic contractions were superimposed. 3. NG-nitro-L-arginine (L-NNA; 0.1 mM or 0.5 mM), an inhibitor of NO synthesis abolished the relaxation. L-arginine (0.1 mM), a substrate for NO synthesis, but not D-arginine, restored it. L-NNA concentration dependently increased the twitch and tonic contractions. Sodium nitroprusside (1 microM or 10 (M), an exogenous donor of NO, was without effect on the electrically evoked responses. 4. AP 13.2 ACOH (AP; 0.1 microM or 10 microM), a blocker of SP receptors, frequency dependently inhibited or even prevented the twitch and tonic contractions. AP concentration-dependently increased the relaxation or reversed the responses to electrical stimulation into a deep relaxation. 5. The concentration-response curve for SP (1 nM-0.1 microM) was shifted to the right by AP, the EC50 values being 5.2 +/- 0.4 nM and 88.0 +/- 3.0 nM, respectively. The effects of SP were not altered by L-NNA (0.1 mM). 6. These findings, supported by morphological data about distribution of NADPH-diaphorase-positive nerve cell bodies and processes and SP-positive varicose fibers, suggest the contribution of NO and SP to NANC transmission. It appears that NO inhibits prejunctionally the SP transmission whereas SP counteracts the NO effect at the postjunctional level.