The Possible Role of Dielectric Constant Variation and of Electro-osmosis in Excitable Natural and Artificial Membranes. An Extention of Teorell's “Membrane Oscillator”

Abstract

Artificial phosphatidic bilayers show ionic conductance and excitability only if they are in contact with cyclic ion-carriers and specific substances (Excitability inducing Materials). However many lipidic substances form ion-conducting and excitable membranes without any specific additives. They display a 10–20 fold transient increase of conductance under a weak field. Such “responses” repeat with a frequency which increases with the field. They are not a “noise” phenomenon and are similar to axon responses. The latter result, according to HODGKIN and HUXLEY, from the opening and closing of ionic pores. An alternative mechanism can be proposed. The lipidic membranes have a low dielectric constant. Therefore ions in membranes are bound as ion-pairs, formed by the cations and anions present in the membranes. The anions are always present in small amount because of a slight hydrolysis of the lipids. The dissociation constant of the ion-pairs (and thus the membrane conductance), as calculated by KRAUSS and FUOSS, is reciprocal to the interionic distance in the ion-pairs and to the dielectric constant. As the Na⁺ ions are smaller than the K⁺ ions the Na⁺ containing ion-pairs are less dissociated than the K⁺ containing pairs. Thus in the resting membrane the conductance due to Na⁺ can be calculated 20–30 times smaller than the conductance due to K⁺. Excitation could result from a slight increase of the dielectric constant of the membrane. A small afflux of water in the membrane, through the electroosmotic action of the stimulating current would suffice. Equations, similar to those proposed by TEORELL, describe the process. The notion of ion-pairs leads to other interesting phenomena. Ion pairs in a low dielectric constant medium, containing a coloured material can be photo-dissociated. The radiant energy absorbed by the coloured substance suffices to overcome the association energy and therefore increases the conductance.