Effect of 4-aminopyridine and verapamil on the inhibitory action of normorphine on the guinea-pig ileum.

The ability of normorphine to inhibit the muscular twitches elicited by electrical stimulation of the longitudinal muscle of the guinea-pig ileum was antagonized by 4-aminopyridine (4-AP). Incubation with 4-aminopyridine (4-AP) caused a marked increase of the normorphine IC50 and this effect was not modified by naloxone or by the Ca2+ channel blocker, verapamil. Although verapamil caused a significant decrease of the normorphine IC50 it also caused a potent reduction of the contractile effect of acetylcholine. The results suggest that 4-AP and verapamil act on different Ca2+ pools and that verapamil does not primarily affect the presynaptic Ca2+ pool involved in opiate action.