Electrical characteristics in an excitable element of lipid membrane

Electrical characteristics in a membrane constructed from a porous filter adsorbed with a lipid analogue, dioleyl phosphate (DOPH), were investigated in a situation interposed between 100 mM NACl + 3 mM CaCl₂ and 100 mM KCl. Calcium ions affected significantly the membrane characteristics. The membrane potential was negative on the KCl side, which implies the higher permeability to K⁺ than Na⁺; this tendency was increased by a tiny amount of Ca²⁺. While the membrane showed a low electrical resistance of several kΩ. cm²under K⁺/Na⁺ gradient, it showed several MΩ · cm²⁺ by Ca²⁺. The surface structure of the membrane exhibited many voids in the low-resistance state, but the surface was covered by oil droplets in the high-resistance state. Oscillations of the membrane potential appeared spontaneously with application of the electrical current from the KCl side to the NaCl + CaCl₂ side. The frequency was increased with the electrical current. All these results were explained comprehensively using an electrochemical kinetic model taking account of the Ca²⁺ binding effect, where DOPH assemblies make a phase transition between oil droplets due to Ca²⁺ and multi-bilayers with excess K⁺. The oscillation arises from coupling of the phase transition to accumulation and release of K⁺ or Ca²⁺. This membrane can be used as an excitable element regulated by Ca²⁺ in neuro-computer devices.