Conractile responses of guinea-pig ileum to high-frequency electrical field stimulation

The responses of longitudinal muscle-myenteric plexus strips prepared from guinea-pig ileum to high-frequency electrical field stimulation (10-50 Hz) were investigated. The primary high-frequency responses consisted of a cholinergic component that was less susceptible to inhibition by adenyl compounds than the 0.1 Hz twitch response, and a non-cholinergic component which included a considerable degree of direct muscle stimulation which was not affected by adenosine and adenine nucleotides at concentrations up to 100 micro M. High frequency stimulation in the presence of atropine, at concentrations that effectively blocked the contractions produced by high doses of exogenously applied acetylcholine, elicited delayed complex secondary contractions. These contractions were abolished by tetrodotoxin and were inhibited by morphine and adenyl compounds. The secondary contractions were also elicited in choline-free Krebs buffer following treatment with 100 micro M hemicholinium-3 for 2 h; and they were not enhanced by the addition of 1.5 micro M eserine sulphate. By these criteria, it is concluded that the secondary contractions were mediated by a neurogenic, non-cholinergic mechanism. The nature of the spasmogen, which was not identified, is discussed.