Role of microtubules and axolinin in membrane excitation of the squid giant axon.

The squid giant axon possesses numerous microtubules and a large pool of axolinin, both of which are preferentially localized at the peripheral region of the axoplasm just underlying the axolemma. A series of perfusion experiments presents evidence that microtubules localized in the clad are directly associated with the membrane excitability. The internal perfusons which cause microtubules to depolymerize reduce the excitability and those supporting microtubule assembly increase the peak sodium current. Direct evidence that microtubules are essential for membrane excitation is obtained by the restoration of diminished membrane excitability. The restoration system consists of squid brain or axoplasmic tubulin, axolinin, ATP, Mg2+, and cAMP or of porcine brain tubulin, tubulin-tyrosine ligase, axolinin, GTP, ATP, Mg2+, and cAMP. The former restoration system causes the diminished excitability to recover in such a way that the Na conductance, resting potential, and Na reversal potentials are much improved. One hypothesis is that tyrosinated tubulin is polymerized into microtubules underneath the axolemma. Axolinin binds to the axolemma at one end of the rod-like molecule and cross-links the assembled microtubules with the other end, thereby reconstructing cytoskeletons just underneath the axolemma which are necessary for membrane excitation.