Effects of 4-aminopyridine on mechanical activity and noradrenaline release in the rat portal vein in vitro.

4-Aminopyridine (4-AP) increased the spontaneous mechanical activity of the isolated rat portal vein. Since denervation and adrenergic receptor blockade failed to prevent this effect of 4-AP it is suggested that the drug enhances the electrical excitability of the muscle membrane. 4-AP significantly increased the response of the muscle to electrical nerve stimulation in most experiments but had little effect on the response to applied noradrenaline (NA). Both spontaneous and evoked release of 3H-activity, following preincubation in 3H-noradrenaline, were increased in the presence of 4-AP (10(-3) M). The present results with 4-AP can be explained by its known ability to block the transient potassium conductance which accompanies the action potential in excitable tissues.