Evidence for more than one type of 5-hydroxytryptamine receptor in the human colon

The actions of methysergide, a 5-hydroxytryptamine (5-HT) antagonist, have been examined on muscle strips taken from the circular and longitudinal layers of the human colon. Relaxations of the longitudinal muscle to 5-HT were antagonized by concentrations of methysergide known to be selective. Relaxations of the circular muscle to 5-HT were unaffected by similar concentrations of methysergide. Responses of both types of muscle to 5-HT were partially reduced by tetrodotoxin. Furthermore there was evidence for 5-HT receptors in circular colonic muscle which were unaffected either by selective concentrations of methysergide or tetrodotoxin.