Changes of ultraviolet sensitivity of voltage-clamped sodium channels during their potential-induced conductance cycle.

The effects of ultraviolet (UV) radiation at various intermediate states of excitation of the frog nerve membrane were measured under voltage-clamp conditions. By synchronizing a UV flash source (flash duration: 10 microseconds) with the stimulating voltage-clamp pulses it was possible to irradiate the membrane at any state of excitation. The resulting UV sensitivities gamma Na and gamma h of gNa and h infinity (V = 0) were compared with those found at the end of a hyperpolarizing prepulse (m = 0, h = 1). The irradiation always induced a decrease of both parameters, gNa and h infinity (V = 0), and the more "h-gates" there were in the closed position at the moment of irradiation, the more pronounced were the effects. The normalized sensitivity of both parameters can be described fairly well by linear relations of the form: gamma x(m,h)/ gamma x(0, 1) = 1 + Px(1 - h) + Qxm2h (x = Na or h). The coefficients P and Q depend on wavelength. The results suggest a conformational change of the nodal membrane during excitation and an interdependence of m and h to some extent.