ACCOMMODATION MEDIATED BY ENTERIC INHIBITORY REFLEXES IN THE ISOLATEDGUINEA-PIG SMALL-INTESTINE

1. The aim of the present study was to investigate whether the guinea- pig small intestine shows accommodation to infused fluid, similarly to other regions of the gastrointestinal tract. Tetrodotoxin, papaverine and transmitter antagonists were used to establish the existence of r eflex pathways and the nature of the neurotransmitters involved. 2. Co mpliance, measured as the change in volume of infused fluid divided by the intraluminal pressure change, was reduced by tetrodotoxin (0.6 mu M), indicating that there is an overall neurally mediated relaxation of the circular muscle in response to low rates of distension. Papaver ine (10 mu M) did not have any significant effect on compliance at the low rates of distension, suggesting that the circular muscle is fully relaxed. 3. At each rate of distension, 400 mu M N omega-nitro-L-argi nine methyl eater (L-NAME, a nitric oxide synthase inhibitor) signific antly decreased the compliance of the intestinal wall, indicating that the circular muscle was relaxed by a nitric oxide-mediated mechanism. Apamin (0.5 mu M) which blocks a component of inhibitory transmission , did not have a significant effect. 4. In control preparations, the i ntestinal wall was less compliant when distended by fluid at a fast ra te, compared with the lower rates of distension. This was not due to c hanges in passive components of the intestinal wall or a myogenic resp onse to rapid stretch. 5. When the intestine was distended rapidly, 1 mu M hyoscine and 100 mu M hexamethonium increased intestinal complian ce. However, they had no detectable effect on compliance with low rate s of distension. Thus, excitatory cholinergic neuronal input to the ci rcular muscle was evoked at the highest distension rate in addition to the inhibitory input that predominated at lower rates of distension. The lack of effect of hexamethonium at low distension rates indicates that nicotinic neuro-neuronal transmission is not involved in the inhi bitory reflex. 6. These studies show that during the preparatory phase of peristalsis, a separate, sustained accommodation reflex occurs in the circular muscle of the guinea-pig small intestine.