Transient Membrane Potential in Response to a Bitter Substance in a Membrane Filter Impregnated with Phospholipid and 1-Octanol

Abstract

Abstract Transient and steady membrane potentials in response to a bitter substance have been measured in a membrane filter impregnated with phospholipid and 1-octanol. Quinine, which is one of the alkaloids, was used as the bitter substance. The internal and external solutions both contained 0.1 mol dm−3 KCl. A cation exchange liquid membrane was formed in a Teflon filter by impregnating it with a 1-octanol solution of dihexadecyl hydrogenphosphate (DCP). After the injection of the solution of quinine hydrochloride into the external solution, the membrane potential rapidly increased to reach a peak and gradually decreased to relax to a level of another steady potential. The time course curve of membrane potential was resolved in the early portion and the other portion after the peak to obtain the Donnan potential at the interface and the diffusion potential within the membrane at steady state, respectively. In comparison with the case of the response to HCl, even a small amount of aqueous quinine hydrochloride induced a great magnitude of change both in the Donnan potential and in the diffusion potential within the membrane. The amount of quinine transported across the membrane was measured by UV spectrophotometry. From the time dependence of the UV spectrum, we determined the time lag for the membrane diffusion to obtain the diffusion coefficient of quinine in the membrane. According to the theoretical expressions of the Donnan potential at the interface for the charged membrane and the intramembrane diffusion potential derived by solving the Nernst–Planck equations taking into account the Donnan equilibrium at the interface, these two constituents of the total membrane potential were estimated using the diffusion coefficient. By fitting the theoretical curves to the experimental results, large values of the partition coefficient and the equilibrium constant of the complex formation with DCP for monoprotonated quinine ion relative to potassium ion were obtained.