The Electrophysiologic Effect of Local Anesthetics
The resting membrane potential of a nerve is little affected by
local anesthetics.[24]
As the concentration of
local anesthetic applied to a nerve is raised, a decrease in the rate and degree
of impulse-associated depolarization is produced until the impulse is abolished.
It is not possible, however, to derive direct data on the binding of local anesthetics
to Na+
channels from measurement of the changes in nerve impulses.
By using a "voltage-clamp" procedure, Na+
currents
and their inhibition by local anesthetics can be directly assayed ( Fig.
14-8A
). When the membrane of isolated neurons is rapidly depolarized to
a constant value, the time course of ionic currents is observed. Sodium currents
during one initial depolarization are reduced by subclinical doses of local anesthetic
(e.g., 0.2 mM lidocaine) and totally abolished by clinical doses (e.g., 1% lidocaine,
which equals about 40 mM). If the test depolarization is repeatedly applied, for
example, at frequencies above 5 Hz (5 pulses per second), the partially depressed
(tonically inhibited) Na+
current is further reduced, incrementally for
each pulse, until a new steady-state level of inhibition is reached.[19]
[24]
This frequency-dependent inhibition, also
called
phasic inhibition, is reversed when stimulation is slowed or stopped and currents
return to the level of tonic inhibition observed in the resting nerve. Paralleling
the phasic inhibition of Na+
currents in voltage-clamped membranes is
a "use-dependent" blockade of action potentials during normal physiologic function
( Fig. 14-8B
). The ability
of local anesthetics to produce both tonic and phasic inhibition is similarly dependent
on their structure, hydrophobicity, and pKa
.
[24]
[25]
At its
simplest, there appears to be a single binding site for local anesthetics on the
Na+
channel, with
Figure 14-8
"Use-dependent" actions of local anesthetics on excitable
membrane properties. A, Ionic Na+
currents
measured by voltage clamp are transiently activated by brief steps of depolarization
applied infrequently ("tonic" test) or in a train at 10 times per second ("phasic"
test, see Em
pattern in parentheses). After equilibration with 0.2 mM
(0.005%) lidocaine, the currents measured tonically are reduced by about 30% from
control currents. Application of the "phasic" train of depolarizations results in
a dynamic reduction in currents after each depolarization, with a steady-state value
of phasic inhibition reached during the train in 75% of control currents. Recovery
of currents to the tonic value occurs within a few seconds when phasic testing stops
(not shown). B, Action potentials are also inhibited
in a phasic manner by local anesthetics. After equilibration with 0.8 mM lidocaine
(0.02%), the action potential is tonically reduced by about 20% from its amplitude
in drug-free solution (not shown). Stimulation by a train at 20 stimuli per second
induces phasic inhibition, which further reduces the amplitude to about 30% of the
control value. As with ionic currents (A), phasic
inhibition of the action potential recovers rapidly when high-frequency stimulation
stops.
a "tonic" affinity at rest and increased "phasic" affinity occurring as a result
of depolarization. The phasic blocking mode can thus be used to reveal the true
kinetics of local anesthetic binding to the functional receptor, the Na+
channel itself.
Phasic actions are a manifestation of the selective affinity of
local anesthetics for conformations of the Na+
channel that occur during
depolarization. Both "open" and "inactivated" states of the channel bind local anesthetics
more avidly than the resting state does. Repeated depolarizations thus increase
the fraction of drug-bound channels; dissociation of these bound drug molecules is
usually a slower process than the normal recovery from inactivation (see earlier),
which results in the use-dependent accumulation of channels in the blocked condition
and the phenomenon of phasic block.
By its selective binding to a channel state, the local anesthetic
stabilizes that state. During phasic block, therefore, more inactivated channels
become drug bound, and reciprocally, less activation can occur. This relationship
between state-dependent affinities and modification of transitions among states through
drug binding is known as the "modulated receptor" model. Thus, overall binding of
anesthetic is increased by membrane depolarization for two reasons: more binding
sites become accessible during activation (the "guarded receptor" model), and drug
dissociation from inactivated channels is slower than from resting channels (the
modulated receptor model).
The specific binding rates and affinities for the different conformations
of the sodium channel depend on the particular local anesthetic drug. When the details
of this dependence are correlated with the physicochemical properties of the drug
and with the experimental conditions, they provide insight into the molecular features
of the local anesthetic binding site.
