Mechanism of lower esophageal sphincter stimulation by bombesin in the opossum.

Effect of intravenously administered bombesin has been studied on the in vivo and in vitro motor function of the lower esophageal sphincter (LES) of the opossum. Intraesophageal pressures were monitored by an assembly of polyvinyl catheters attached to pressure transducers and a recorder. The catheters were continuously perfused with bubble-free water. Intravenous administration of 5, 10, 30, 50, and 100 ng/kg bombesin stimulated the LES 4, 34, 106, 148, and 130%, respectively, above control values. Atropine and hexamethonium did not antagonize the response of the LES to bombesin. Tetrodotoxin, phentolamine, and reserpine all significantly antagonized the response of the LES to bombesin. The LES stimulatory effect of bombesin injected locally in the left gastric artery (LGA) was antagonized significantly by local administration of phentolamine in the LGA. Experiments performed in vitro (25 ml baths containing oxygenated Krebs solution) showed that bombesin (10(-9) M) increased the tone of LES strips, and the effect was antagonized by phentolamine (10(-6) M). Serum gastrin concentrations increased significantly only 15 min after bombesin administration. It is concluded that bombesin is a potent stimulant of the LES. The mechanism of stimulation involves a direct effect on the smooth muscle as well as an indirect one by an effect on the postganglionic adrenergic neurons releasing norepinephrine.