Microtubules inside the plasma membrane of squid giant axons and their possible physiological function

The effects of application of the microtubule-disassembling reagents to squid giant axons upon resting potential, the height of the propagated action potential, and the threshold to evoke action potential were studied using colchicine, podophyllotoxin, vinblastine, griseofulvin, sulfhydryl reagents including NEM, diamide, DTNB and PCMB, and Ca2+ ions. At the same time, the effects of concentrations of K halides and K glutamate on the above physiological properties were studied in comparison with in vitro characteristics of microtubule assembly from purified axoplasmic tubulin. It was found that there was good correlation between conditions supporting maintenance of membrane excitability and microtubule assembly. The experiments suggest that associated with the internal surface of the plasma membrane there are microtubules which regulate in part both the resting and action potentials.