Hyoscine-resistant peristalsis in guinea-pig ileum

The effect of hyoscine on the peristaltic activity of the proximal and distal ileum of the guinea-pig was studied. Hyoscine strongly impaired peristalsis as indicated by the elevation of the threshold pressure and by the increased number of incomplete peristalses and blockades. Functional activity of the circular musculature was more markedly impaired. However, particularly in the distal ileum, complete peristalses occurred even after 70 min exposure to hyoscine at a concentration of 10(-6) g/ml. A tenfold increase in hyoscine concentration failed to produce further impairment of peristaltic activity and of the oral reflex contraction. The activity which remained in the presence of hyoscine was blocked by methysergide and by d-tubocurarine. The hypothesis is advanced that once the muscarinic receptors have been blocked, increased radial stretch of the circular coat results in activation of a separate, tetrodotoxin sensitive, excitatory nervous pathway, which is sufficient to maintain a discrete degree of peristaltic activity.