Hexamethonium-induced augmentation of the electrical twitch response in the guinea-pig ileum longitudinal muscle-myenteric plexus strip.

Longitudinal muscle-myenteric plexus strips of the guinea-pig ileum were used to investigate the nature of the hexamethonium-induced augmentation of the twitch response. All preparations were set up in Tyrode solution and intermittent longitudinal twitch contractions were evoked by single pulse electrical field stimulation. Hexamethonium, a blocker of nicotinic ganglionic transmission, at 300 μmol/l and 1 mmol/l augmented the twitch contractions by 21% and 35%, respectively. First we tested for a possible nicotinic drive onto an inhibitory neuronal component to the longitudinal smooth muscle cells. However, guanethidine (5 μmol/l), naloxone (1 μmol/l), or l-NAME (300 μmol/l) were without effect on the hexamethonium-induced augmentation. The P2 purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2'-4'-disulphonic acid (PPADS), 25-100 μmol/l, without altering the control twitch responses, dose-dependently reduced the hexamethonium-induced augmentation; at 100 μmol/l a statistically significantly inhibition was observed. Based on these functional experiments we found no evidence that blocking nicotinic transmission removed a tonic adrenergic, opioidergic or nitrergic inhibitory input to the longitudinal muscle. However, we provide evidence for a hexamethonium-induced augmentation of the P2 purinergic input to cholinergic motoneurons of the guinea-pig ileum longitudinal muscle. The P2-nicotinic receptor interaction presents a novel modulatory mechanism to cholinergic myenteric motor neurons.