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) reduction in the effective valence of steady-state inactivation and, (iv), potentially complete removal of fast inactivation. Additionally, the two kinetic components of fast inactivation in crayfish axons are differentially affected by MB + light. The intercept of the faster component (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 four major effects of MB + light are mediated by changes occurring within different regions of the sodium channel molecule.