The role of vagal and intramural inhibitory reflexes in the regulation of intragastric pressure in the ferret

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Abstract
The step inflation technique combined with nerve section and pharmacological receptor blockade was used to determine the gastric distribution of the vagal inhibitory fibers and their role in the regulation of intragastric pressure in the anesthetized ferret. Under the conditions described, the predominant effect of the vagus was inhibitory. The dorsal abdominal vagus mediated more inhibition than the ventral vagal trunk. There was partial overlap in the inhibitory effects of the 2 trunks but, in contrast to excitatory effects, this overlap was less than complete. No evidence was found to indicate that the intramural inhibitory neurons (demonstrated pharmacologically) could be activated by a local distensive stimulus or that local inhibitory reflexes played an independent role in the regulation of intragastric pressure. The vago-vagal inhibitory reflexes play the major (possibly the only) part in the overall regulation of intragastric pressure. After a step gastric inflation (50 ml in < 3 s) the time taken for the intragastric pressure to fall to 1/e of the peak pressure was a function of the number of inflations; the time decreased with increased number of inflations. It was apparently unaffected by nerve lesions. The intragastric pressure in response to an inflation, besides being a function of the neural reflexes evoked, was also related to the gastric weight and hence the size of the animal. The sum of the prevailing intragastric pressure and the response to vagal stimulation (10 Hz, 10 s) were similar, irrespective of the intragastric fluid volume (10-80 ml). The volume of fluid in the corpus was a major determinant of the contraction produced in response to vagal stimulation: as the corpus volume increased the corpus contraction decreased, but the relaxation that follows the contraction increased. A similar effect was observed with intraarterial ACh [acetylcholine], which implies that the response to stimulation was modified directly by the degree of stretch of the smooth muscle cells. Inhibitory effects were not apparent unless the stomach was moderately inflated. The relative roles of the vagal reflexes, intramural reflexes and the smooth muscle in the regulation of intragastric pressure are discussed in the light of the above observations.