Electric responses of bilayer-immobilized films as models of a chemoreceptive membrane

Abstract

The membrane potential and resistance of a bilayer-immobilized film changed on addition of bitter substances or odorants in aqueous solution; there was a good correlation between the minimum concentration of these additives for inducing the membrane potential and the bitter taste or olfactory threshold in man.

Mechanism of the electric response of lipid bilayers to bitter substances

“…It is interesting to note how the present model induces the change in the membrane resistance when the membrane is stimulated with bitter substances because it has been reported that the resistance is not changed noticeably by the stimulation in lipid multibilayers (Okahata and En-na, 1987) and in taste cells (Akaike et al, 1976;Kumazawa et al, 1985). In this section, we consider only negatively charged membranes, where the contributions from cations H+ and M+ are dominant in determining the resistance.…”

“…On the other hand, the stimulation induces a large change in the resistance if at least one of the diffusion constants of H+ or M+ is increased noticeably, or both of them are decreased noticeably. The membrane resistance of the synthetic lipid multibilayer is not changed noticeably by the bitter stimulation (Okahata and En-na, 1987). It seems that the bitter substances do not induce large changes in the diffusion constants.…”

“…The membrane potential of planar lipid bilayers and liposomes are depolarized by the application of various bitter substances (Kumazawa et al, 1988). It also has been shown that the potential across a synthetic lipid multibilayer is hyperpolarized by bitter substances (Okahata and En-na, 1987). There exists a good correlation between the minimum concentrations of various bitter substances required to depolarize the membrane potential of lipid bilayers and those concentrations required to elicit bitter taste in humans (Kumazawa et al, 1988).…”

“…In addition, the lipid bilayer has interesting properties by itself. It has been found experimentally that the responses of the lipid bilayers to the bitter stimuli have the following unique properties, which cannot be accounted for by the usual ion channel model for membrane potential: 1) the response of the membrane potential appears even under the condition that there is no ion gradient across the membrane (Kumazawa et al, 1988), 2) the response remains even when the salt in the stimulating solution is replaced with the salt made of an impermeable cation, and then the response becomes rather larger (Kumazawa et al, 1988), and 3) the direction of the polarization of the potential is not reversed, even when the ion gradient across the multibilayer is reversed (Okahata and En-na, 1987). The membrane potential usually is described by the ion permeation, as in the widely accepted Goldman-Hodgkin-Katz model (Goldman, 1943;Hodgkin and Katz, 1949), for the potential induced by ion channels.…”

“…Kumazawa et al (1988) proposed that the adsorption of bitter substances on the lipid membranes induced the conformation change in the membranes and, as a result, changed the phase boundary potential. Okahata and En-na (1987) suggested, based on the analysis of their experimental results, that the bitter substances were adsorbed near the surface of the membrane, rather than penetrating the membrane. However, the microscopic mechanism through which the adsorption of the substances induces a change in the membrane potential has not yet been clarified.…”