Specificity Of Ion Channels Underlying The Action Potential

Other types of experiments further confirm the Hodgkin-Huxley hypothesis for the initiation and repolarization phases of the action potential. Compounds have been discovered that can be used selectively to block or inhibit these voltage-dependent permeability changes. One of these substances is known as tetrodotoxin (TTX), and the other is known as tetraethylammonium (TEA). TTX, a toxin that is isolated from the ovaries of the Japanese puffer fish, blocks the voltage-dependent changes in Na+ permeability but has no effect on voltage-dependent changes in K+ permeability. In contrast, TEA has absolutely no effect on the voltage-dependent changes in Na+ permeability but completely abolishes the voltage-dependent changes in K+ permeability. Thus, one substance (TTX) is capable of blocking the voltage-dependent Na+ permeability, and another (TEA) is capable of blocking the voltage-dependent K+ permeability.

Given the effects of TEA and TTX on permeabilities, how would one expect these substances to affect the action potential? If the voltage-dependent change in Na+ permeability is blocked, one would expect that no action potential could be initiated or propagated. If the voltage-dependent change in K+ permeability is blocked, one would expect the action potential to be somewhat longer in duration and, in addition, it should not have a hyperpolarizing afterpotential. Figure 18 illustrates these results; Fig. 18A is a normal action potential, and Fig. 18B is an action potential recorded in TEA. In TEA, the initiation and rising phase of the

A Normal

A Normal

B +TEA

action potential as well as its peak value are unaffected, but there is a dramatic increase in the spike duration and an absence of the hyperpolarizing afterpotential. Thus, the use of TEA confirms the Hodgkin-Huxley theory that the delayed increase in K+ permeability contributes to the repolarization phase of the action potential and to the undershoot. In the presence of TEA, the process of Na+ inactivation accounts entirely for the repolariza-tion. When one perfuses the axon with TTX, one finds that no action potential can be elicited and no action potential can be propagated. Thus, the use of TTX confirms the Hodgkin-Huxley theory for the critical role that the increase in Na+ permeability plays in initiating the action potential in nerve axons.

These experiments with TTX and TEA are also interesting in another respect because they demonstrate that the voltage-dependent changes in Na+ permeability are mediated by completely different membrane channels from the voltage-dependent change in K+ permeability because it is possible to selectively block one but not the other.

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