| Abstract: | Ultraviolet radiation induces two photochemical alterations relevant to excitability in the nodal membranes: A selective blocking of the sodium permeability and a potential translation of the voltage dependent kinetic parameters of sodium inactivation and activation along the potential axis in the negative direction. The underlying processes are two different photoreactions, since 1) the action spectrum of the blocking effect shows a marked peak near 280 nm and rapidly decreasing sensitivity towards higher and lower wavelengths, while the action spectrum of the potential shift increase with lower wavelengths; 2) the blocking effect is enhanced by a more positive holding potential, while the potential shift is decreased; 3) the potential shift can be prevented intraaxonal application of l-cysteine or 2-mercaptoethanol, but the blocking effect is not affected. |
