Contractions of the vagally innervated guinea pig tracheal tube preparation were induced by 1) stimulation of the preganglionic cervical vagus nerve (PGS), 2) activation of postganglionic intrinsic nerves by use of transmural stimulation (TMS) in the presence of hexamethonium, and 3) exogenous application of spasmogens, acetylcholine (ACh) and histamine. Contractions were recorded as increases in intraluminal pressure of the sealed Krebs-filled tube preparation. In the absence of basal tone, contractions induced by both PGS and TMS were monophasic. When the tone was raised with histamine (1 microM), the rapid contractions were followed by a slow relaxation during TMS but not during PGS. There was no evidence for any involvement of the putative inhibitory nonadrenergic noncholinergic neurotransmitters vasoactive intestinal peptide (VIP) and nitric oxide (NO) in the response to PGS, because neither the peptidase alpha-chymotrypsin nor the NO synthase inhibitor NG-nitro-L-arginine affected PGS-induced contractions. However, both alpha-chymotrypsin and NG-nitro-L-arginine facilitated contractions induced by TMS, suggesting that both VIP and NO are involved in responses to TMS. The facilitation of TMS-induced contractions by NG-nitro-L-arginine was unaffected by epithelium removal. Therefore, neither VIP nor NO appears to be released during PGS, but both are released during TMS, and the generation of NO during TMS is independent of the epithelium.
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