Insect Control: Biological and Synthetic Agents - Index of

Figure 8 Dihydropyrazole block appears as a parallel shift of
the steady state slow inactivation curve in the direction of
hyperpolarization. Ionic current traces were scaled by a common
factor so that the peak at 120 mV matched the peak
before treatment with the dihydropyrazole. Peak INa was depressed
most at depolarized potentials, whereas outward current,
IK, was not affected by the treatment. The graph shows
plots of peak current normalized to the value at 120 mV. RH-
3421 (10 mM) appears to shift the steady state inactivation relation
to the left by 8.6 mV. (Reproduced with permission from
Salgado, V.L., 1992. Slow voltage-dependent block of Na þ
channels in crayfish nerve by dihydropyrazole insecticides.
Mol. Pharmacol. 41, 120–126; ß American Society for Pharmacology
and Experimental Therapeutics.)
within 0.5 ms, then reversed and became outward.
The inward (downward) peak is the Na þ current
and the outward (upward) steady state current is
the K þ current (Pichon and Ashcroft, 1985). Depolarization
of the axon to potentials more positive
than 90 mV in the control suppressed the Na þ
current by a process known as inactivation, which
in this case had a midpoint potential of 77 mV.
Na þ channels have two partially independent inactivation
processes, known as fast and slow inactivation.
Fast inactivation occurs on a millisecond
timescale and serves to terminate the action potential,
while slow inactivation occurs on a much slower
timescale, over hundreds of milliseconds, and performs
a slow, modulatory function. Slow inactivation
occurs at more negative potentials than fast inactivation,
and is responsible for the inactivation seen in
Figure 8. After equilibration of the axon with RH-
3421, there was no effect on either Na þ or K þ current
at 120 or at 100 mV (Figure 8). At more depolarized
potentials, however, the Na þ current was
specifically depressed by RH-3421, whereas the K þ
current was unaffected. When peak Na þ current is
plotted against membrane potential, it appears that
2: Indoxacarb and the Sodium Channel Blocker Insecticides 43
Figure 9 A model showing the resting (R), Open (O), and
inactivated (I) states of the Na þ channel, and specific interaction
of the SCBIs (D) with the inactivated state.
RH-3421 has shifted the slow inactivation curve by
9 mV to the left.
The next step in the analysis is to consider the
mechanism of this apparent shift of slow inactivation.
Whereas fast inactivation occurs with a time course of
hundreds of microseconds to a few milliseconds, and
slow inactivation on the order of tens to hundreds of
milliseconds, the changes in peak Na þ current in the
presence of SCBIs occur on a much slower timescale,
on the order of 15 min. This slow readjustment of
peak current in response to voltage change can therefore
be attributed to a new process: the binding and
unbinding of SCBIs to Na þ channels.
The Na þ channel undergoes transitions between
many different conformations or states, which can
be grouped into resting (R), open (O), and inactivated
(I) states, each of which may have several
substates. The transitions between these naturally
occurring states are shown on the left in Figure 9.
In this simplified model, S1 ¼ [R]/[R] þ [I], is the
steady-state slow inactivation parameter, which
depends on membrane potential according to
1
S1 ¼
½1Š
1 þ exp½ðV VSÞ=kŠ
where V is membrane potential, VS is the potential
at which S1 ¼ 0.5 (half of the channels are in the
inactivated state), and k is a constant that includes
the Boltzmann constant and describes the voltage
sensitivity of the inactivation process (Hodgkin and
Huxley, 1952).
This model ignores the fast-inactivated state, but
interaction of the SCBIs with the channels is so slow,
that it and the open state, O, can be ignored in the
analysis. The observed enhancement of slow inactivation
can be explained if we assume that the SCBI
drug (D) binds selectively to an inactivated state, as
also shown on the right-hand side in the model
(Figure 9). Drug binding would then effectively
remove channels from the I pool, which would be
compensated for by mass action rearrangements
among the other states, leading to net flux of more
receptors from the R pool into the I and D I pools.
KI ¼ [D] [I]/[D I] is defined as the equilibrium
dissociation constant of the D I complex.
In the presence of SCBI drug, two processes lead to
the decrease in current upon depolarization: slow