Intracellular binding of cationized ferritin prolongs the time course of sodium channel inactivation in squid giant axons

Abstract

Cationized ferritin (CF) applied intracellularly in squid giant axons bound to the negatively charged sites on the cytoplasmic surface of the axolemma. Under the electron microscope, the distribution of CF was found to be dense and uniform over the axolemmal surface. However, the effect of CF on the membrane excitability was highly specific, the major effect being a prolonging of the inactivation time course of the sodium channel without altering the properties of the potassium channel. The binding of CF did not alter the surface potential related to the membrane excitability. When CF was present intracellularly, the time course of the inactivation was characterized by two time constants (slow and fast). The slow component increased with an increase in CF binding and its time constant had a unique value (26 msec) irrespective of the duration of perfusion and concentration of CF. The concentration of CF at which the half-maximum response occurred was about 150nm. Poly-l-glutamate, charged negatively at neutral pH, removed CF from the axolemma and counteracted the CF effect on the sodium channel, although this poly-acidper se did not affect the membrane excitability. Our results indicate that CF binds electrostatically to the inactivation site of the sodium channel but does not affect the voltage sensor, which is supposed to be located deep in the channel.