Modulation of gastric contractions in response to tachykinins and bethanechol by extrinsic nerves.

1. Extrinsic reflexes elicited by changes in gastric wall tension play an important role in regulating gastric tone. The present study investigated whether such reflexes modulate gastric contractions induced by close arterially administered neurokinin A (NKA), substance P (SP), SP-methylester and bethancehol in anaesthetized rats. 2. Reflex pathways were acutely interrupted by either subdiaphragmatic vagotomy or prevertebral ganglionectomy. C-fibre afferent nerve activity was abolished by pretreating rats with capsaicin 10 to 16 days before the experiments. 3. The order of potency in inducing gastric contractions was NKA greater than SP greater than bethanechol. SP-methylester was markedly less effective than SP and its effects did not fit sigmoid dose-response curves (DRCs). The maximal responses to NKA, SP, and bethanechol were similar, whilst the DRC for SP was significantly flatter than those for NKA or bethanechol. Pretreatment of the rats with the peptidase inhibitors phosphoramidon or captopril did not increase the contractile response to SP. 4. Prevertebral ganglionectomy had no significant effect on the DRCs for SP and NKA, whereas vagotomy shifted the DRCs for all three test substances to the left. 5. Capsaicin pretreatment did not change the DRC for NKA in rats with intact vagus but shifted that for bethanechol to the left. The leftward of the DRC for NKA caused by vagotomy was prevented in capsaicin-pretreated rats whereas the vagotomy-induced shift of the DRC for bethanechol remained unaltered. The shift of the DRC for SP seen in response to vagotomy was only slightly reduced by capsaicin pretreatment. 6. These data may be interpreted as demonstrating two neuronal mechanisms for modulating drug-induced gastric contractions. First, the contractions themselves activate a vago-vagal negative feedback involving capsaicin-sensitive afferents. Second, NKA, and to a lesser degree SP, seem to induce a nonvagal non-splanchnic mechanism which via capsaicin-sensitive afferent neurones reinforces tachykinininduced gastric contractions.