Posttetanic potentiation at the neuromuscular junction of the guinea-pig ileum longitudinal muscle strip: influence of calcium, temperature, ouabain and hexamethonium.

The effect of tetanic stimulation on the twitch responses of the longitudinal muscle-myenteric plexus preparation of the guinea-pig ileum was investigated. In the presence of naloxone plus indomethacin twitch contractions following tetanus were potentiated. This posttetanic potentiation (PTP) of twitches was absent after reducing calcium concentration (0.6 mmol/l) in the bath or lowering bath temperature (22 degrees C); doubling calcium concentration (5 mmol/l) enhanced PTP. The addition of ouabain (0.8-5 mumol/l) prevented PTP; following the washout of ouabain the time course of PTP was prolonged. Hexamethonium (0.5 mmol/l) did not prevent PTP. The results suggest that similarly to the situation at nicotinic synapses of the striated muscle the mechanism of PTP at this muscarinic synapse might be associated with changes in calcium distribution. Moreover, the effect of ouabain might suggest the importance of electrogenic sodium pumping from and hyperpolarization of cholinergic nerve terminals after tetanus.