Interaction between stimuli and their antagonists on frog esophageal peptic glands

The peptic glands, which are located in the mucosa of the distal esophagus in the frog, respond to multiple stimuli. In vitro studies were performed, using mucosal sheets of the esophagus of Rana catesbeiana, during the summer months when the glands are fully responsive to determine whether the receptors for the stimuli [cholinergic, beta-adrenergic, and peptidergic (bombesin but not cholecystokinin)] are specific to the stimuli. Through the use of three classes of antagonist, we found that 1) atropine defined the muscarinic nature of the bethanechol stimulation, 2) propranolol defined the beta-adrenergic nature of stimulation by isoproterenol, and 3) the substance P analogue [D-Arg1,D-Pro2,D-Trp7,9,Leu11]substance P was specific for the peptide bombesin. No cross-inhibition was seen, and dibutyryl cGMP did not inhibit any of the three stimuli. Moreover, any two of the three stimuli in combination stimulated more pepsinogen than either alone but the same as the sum of the two individual responses. Both lines of evidence indicate that there are at least three independent receptor pathways for stimulation of pepsinogen secretion in these glands.