The effects of 4-aminopyridine on the isolated vas deferens and its effects on the inhibitory properties of adenosine, morphine, noradrenaline and gamma-aminobutyric acid.

1 Adenosine, adenosine 5'-triphosphate (ATP), morphine, noradrenaline, gamma-aminobutyric acid (GABA) phentolamine and amyl nitrite were used to inhibit electrically-evoked contractions of the isolated superfused vas deferens of the mouse. 2 The inhibitory effects of adenosine ATP, morphine, noradrenaline and GABA, which are thought to be due to presynaptic action, were reduced by perfusion with media containing 4-aminopyridine (4AP) or tetraethylammonium (TEA) ions. The inhibitory effects of phentolamine and amyl nitrite were unaffected by 4AP or TEA. 3 Quinidine, which like 4AP and TEA produced some increase to twitch height, did not reduce responses to the various agonists, indicating that an increased muscle contraction was not itself responsible for the reduced responses. 4 It is concluded that antagonism between 4AP and adenosine is not a specific interaction, as had been suggested, but probably reflects an interaction with Ca2+ requiring processes in the presynaptic terminal.