AJP - Cell Physiology, Vol 232, Issue 1 23-C29, Copyright © 1977 by American Physiological Society
How Gymnodinium breve red tide toxin(s) produces repetitive firing in squid axons
M. Westerfield, J. W. Moore, Y. S. Kim and G. M. Padilla
Partially purified toxin(s), GbTX, extracted from Gymnodinium breve red
tide organisms elicits a spontaneous train of action potentials in the
squid giant axon. The spikes have a shape similar to that in the normal
seawater control except for an increase in the rate of recovery from the
afterhyperpolarization. With this more rapid recovery, the membrane
potential overshoots the resting potential and threshold, triggers another
spike, and thus produces repetitive firing. Voltage-clamp studies revealed
that the toxin has no effect on the normal sodium or potassium conductance
changes produced by step depolarization. However, consistent with the
faster recovery after an action potential, GbTX speeds recovery of the
"shut-off" currents to their steady-state values after a depolarization.
The most likely mechanism by which the toxin accelerates recovery after an
action potential (leading to repetitive firing) is the induction of a small
additional inward current which was found to be reduced by
prehyperpolarization. This toxin-induced current which speeds recovery is
blocked by tetrodotoxin and hence presumably flows through the sodium
channel.
