Post-tetanic potentiation at the nerve-muscle junction in the longitudinal muscle of the guinea-pig ileum. Possible role of substance P.

The effect of short tetanic stimulation (30 Hz for 25 s) on the following twitch responses of the myenteric plexus-longitudinal muscle preparation of guinea-pig ileum to electric stimulation (0.1 Hz) was investigated in the presence of naloxone and indomethacin. Post-tetanic potentiation (PTP) of the twitches observed in control experiments was abolished in preparations desensitized by substance P but it was not affected in preparations desensitized by serotonin or pretreated with methysergide. Immediately after 5 min tetanic stimulation a decreased sensitivity to substance P but unchanged sensitivity to serotonin were observed. Electromyogram (EMG) of the longitudinal muscle layer was picked up 4 and 10 mm aborally from the stimulation site in response to 1 to 16 impulse trains delivered at 100 Hz. In control conditions only the longer trains triggered this neurogenic response at the distal recording site. In the presence of substance P but not serotonin facilitation occurred so that the distal site was frequently recruited to respond with an EMG even to single impulses. A substance P-like compound rather than serotonin may be a candidate for the neuromodulator or neurotransmitter substance involved in PTP and changes in the response topography of muscarinic transmission.