Ultraviolet-induced alterations of the sodium inactivation in myelinated nerve fibers

Summary

Ultraviolet radiation irreversibly reduces the sodium permeability in nerve membranes and, in addition, induces a change of the potential dependence of the kinetic parameters of sodium inactivation in the node of Ranvier. This second ultraviolet effect shifts the kinetic parameters of sodium inactivationh _∞(V),α _h andβ _h (V) to more negative potentials (no changes of the slopes of the curves). The amount of the displacement ‡V along the potential axis is equal for the three parameters and depends on the ultraviolet dose. It is about ‡V=−10 mV after an irradiation dose of 0.7 Ws/cm² at 280 nm. Both ultraviolet-induced effects depend on membrane potential and on the wavelength of the applied radiation. But while the potential shift is enhanced at more negative holding potentials, the ultraviolet blocking is diminished andvice versa. Further, the ultraviolet-induced potential shift is greater at 260 nm than at 280 nm, whereas a maximum sensitivity of ultraviolet blocking is found at 280 nm. Therefore, the two radiation effects are the result of two separate photoreactions. For explanation of the radiation-induced potential shift it is assumed that ultraviolet radiation decreases the density of negative charges at the inner surface of the nodal membrane. From this hypothesis a value for the inner surface potentialψ _i was derived. -19 mV≦ψ _i ≦-14 mV.