EXCITATORY AND INHIBITORY MOTOR REFLEXES IN THE ISOLATED GUINEA-PIG STOMACH

1. We have described and analysed the movements of the isolated stomac h during distension by correlating intragastric pressure with video re cordings, and investigated the presence of intrinsic inhibitory and ex citatory reflexes. 2. Isolated guinea-pig stomachs, placed in an organ bath, were slowly distended with Krebs solution using a syringe pump via a cannula through the pylorus. The changes in intragastric pressur e during cycles of distension were monitored by pressure transducers c onnected to both oesophageal and pyloric cannulae. The resistivity of the gastric wall (change in pressure with volume, Delta P/Delta V) and the amplitude and frequency of phasic pressure events were calculated from pressure recordings. 3. The movements of the stomach were also r ecorded onto videotape. The motion of the gastric wall during distensi on cycles was analysed to establish the patterns of contractions, thei r propagation and the distribution of fluid in the stomach. During fil ling, fluid was preferentially accommodated in the fundus. Propagating (peristaltic) contractions, often starting in the fundus, moved abora lly towards the pylorus. The peak of the phasic pressure event was obs erved when a contraction reached the orad antrum. As it reached the py lorus, intragastric pressure was at its minimum. 4. During the initial phase of distension, intragastric pressure increased steeply. Tetrodo toxin and hyoscine reduced both the resistivity and amplitude of phasi c pressure events. Hexamethonium had a similar effect. Thus distension appears to activate an excitatory reflex pathway, involving nicotinic ganglionic transmission. This reflex increases wall tension and enhan ces myogenic peristaltic contractions. 5. In control preparations, wit h larger distension volumes, the intragastric pressure decreased, desp ite the continued infusion of Krebs solution. L-NAME and apamin abolis hed this drop in pressure, indicating that gastric enteric inhibitory mechanisms prevail at larger distension volumes. After blockade of the excitatory reflex, hexamethonium antagonized the inhibitory response, indicating that activation of inhibitory mechanisms involves nicotini c transmission, probably on enteric inhibitory motoneurons. 6. Both th e excitatory and inhibitory reflexes in the isolated stomach operate w ithin a physiological range of gastric volumes. The excitatory reflex predominates at small distension volumes, leading to large phasic prop agated contractions that mix the contents and may lead to emptying of the stomach. The inhibitory reflex, described previously as adaptive r elaxation, can maximally relax the stomach and is activated preferenti ally at higher distension volumes to accommodate the contents. The int erplay of these reflex pathways in the isolated stomach produces a ric h repertoire of gastric movements. 7. The isolated stomach preparation , used with a combination of kinematic, kinetic and pharmacological me thods, provides a, highly suitable means of investigating the mechanis ms of gastric motility.