T he purpose of this study was to determine the interactions between bombesin and substance P at the feline lower esophageal sphincter (LES). Intraluminal pressures were recorded using a fixed, perfused catheter assembly. Myoelectrical activity was recorded using bipolar Ag-AgCl serosal electrodes. Bombesin, i.v., gave a dose-dependent increase in LES pressure and electronically counted spike activity. The threshold dose was 10(-7) g/kg, while the maximal dose, 10(-5) g/kg, increased LES pressure by 65.5 +/- 14.8 mmHg. The neuroantagonist, tetrodotoxin, decreased the LES response to bombesin by 74.1% +/- 7.9% (P less than 0.05), but had no significant effect on the LES response to substance P. The sphincteric response to bombesin was not inhibited by bilateral cervical vagotomy, atropine, propranolol, or phentolamine (P less than 0.10). Bombesin tachyphylaxis abolished the LES response to bombesin but had no effect on the response to substance P. Conversely, substance P tachyphylaxis completely abolished the LES response to bombesin (P less than 0.001). The substance P antagonist [D-Pro2, D-Trp7,9]substance P also significantly inhibited the LES response to bombesin (P less than 0.05). Acidification of the distal esophagus with 2.0 ml of 0.1 N HCl increased LES pressure by 32.5 +/- 5.2 mmHg (P less than 0.02). The LES response to acid was inhibited by bombesin tachyphylaxis (maximal pressure response, 4.7 +/- 2.1 mmHg, P less than 0.01 compared with control acid response). The tachyphylaxis techniques were specific for the peptides giving no effect on the LES responses to phenylephrine, bethanechol, or pentagastrin. We drew the following conclusions: (a) bombesin increased feline LES pressure via nonvagal neural pathways that were insensitive to adrenergic or cholinergic antagonists; (b) bombesin may be involved in the enteric pathways that mediate the feline LES response to distal esophageal acidification; and (c) substance P mediates the effect of bombesin at the LES and is a neurotransmitter in the LES response to acidification.