Modification of electroplax excitability by veratridine

Veratridine influences membrane-potential changes arising both from the action potential and from the application of external cholinergic agonists in the isolated monocellular electroplax preparation. The action potential shows a long depolarizing after-potential in the presence of veratridine. The effects of various pharmacological agents and of external ion changes on this after-potential are similar to those reported for other nerve and muscle fibers and are consistent with the view that veratridine acts chiefly to increase the Na⁺ conductance. Membrane depolarizations by cholinergic agonists are inhibited by veratridine at pH 7 but strikingly amplified at pH 9. The former effect appears to involve interaction with the cholinergic receptor at the surface of the membrane, while the latter potentiation parallels the increase in the spike after-potential at pH 9 and presumably arises from a Na⁺ conductance increase. Veratridine appears to interact with the component involved in the Na⁺ conductance in the interior membrane phase. The possible localization of this component in both the conducting and synaptic membrane is discussed.