FAST AND SLOW INACTIVATION OF SODIUM-CHANNELS - EFFECTS OF PHOTODYNAMIC MODIFICATION BY METHYLENE-BLUE

Illumination of crayfish giant axons, during internal perfusion with 0 .5 mM methylene blue (MB), produces photodynamic effects that include (i) reduction in total sodium conductance, (ii) shifting of the steady -state inactivation curve to the right along the voltage axis, (iii) r eduction in the effective valence of steady-state inactivation and, (i v), potentially complete removal of fast inactivation. Additionally, t he two kinetic components of fast inactivation in crayfish axons are d ifferentially affected by MB + light. The intercept of the faster comp onent (tau(h1)) is selectively reduced at shorter MB + light exposure times. Neither tau(h1) nor the slower (tau(h2)) process was protected from MB + light by prior steady-state inactivation of sodium channels. However, carotenoids provide differing degrees of protection against each of the photodynamic actions listed above, suggesting that the fou r major effects of MB + light are mediated by changes occurring within different regions of the sodium channel molecule.