Pharmacological characteristics of the postsynaptically mediated contractile responses of guinea-pig ileum to long-lasting electrical field stimulation.

The contractile responses of the longitudinal smooth muscle layer of the isolated guinea-pig ileum which were elicited by long-lasting (40 sec train duration) electrical field stimulation (0.8 msec, 20 V), applied at a frequency of either 5 or 30 Hz, comprised an initial phasic component followed by a secondary tonic contraction, were studied against the background of postsynaptically-acting receptor antagonists. Atropine and apamin reduced both components of the responses while prazosin produced no change. Propranolol induced a slight reduction, decreasing mainly the responses evoked by stimulation at a frequency of 5 Hz. The simultaneous application of cholinoceptor and adrenoceptor blockers revealed a non-cholinergic, non-adrenergic component of the responses. The putative prostaglandin antagonist, SC 19220 was found to reduce mainly the tonic component of the non-cholinergic, non-adrenergic contractions evoked by stimulation at both frequencies used. These results demonstrated that the two-component contractile responses of the guinea pig ileum to long-lasting electrical stimulation were due to the release of more than one neurotransmitter and/or spasmogenic substance. It is concluded that the initial phasic component was evoked by acetylcholine and by a non-cholinergic, non-adrenergic neurotransmitter, while the tonic component was maintained predominantly by prostaglandins released during stimulation.