Kyoko Ichida, Masahiro Ikeda*, Kuniyuki Goto# and Katsuaki
Ito
Department of Veterinary Pharmacology, Faculty of Agriculture, Miyazaki
University, Miyazaki 889 - 2192, Japan
*To whom correspondence should be addressed.
# Kuniyuki Goto (1972 - 1996) tragically killed in an auto accident
on 18 December 1996 had greatly contributed to our work. He is recognized
to be the co-author by K.I., M.I. and K.I.
Abstract: We used the whole-cell clamp and fura-2 techniques
to study the membrane current and intracellular Ca2+ concentration
([Ca2+]i) changes of mouse megakaryocytes in response to
palytoxin (PTX), a highly potent marine toxin. At a holding potential of
-60 mV, PTX induced a sustained inward current in a dose-dependent manner.
The reversal potentials measured in the presence of various extracellular
major cations indicated that the PTX-induced channel had a non-selective
permeability to alkali metal ions. Although elimination of intracellular
Ca2+ had no effect on the PTX-induced current, removal
of external Ca2+ inhibited the current activation. During the
sustained phase of the PTX-induced current, treatment with ADP activated
an additional current. Pretreatment with ouabain, an inhibitor of Na+-K+-ATPase,
suppressed the PTX-induced current. During the stable phase of the PTX-induced
current, challenge with NiCl2 (5 mM) or 2,4-dichlorobenzamil (DCB,
25 ƒÊM), a non-selective cation channel blocker, partially reversed the
current. Simultaneous measurement of the membrane current and [Ca2+]i
showed that PTX induced the current response without increasing the [Ca2+]i.
Taken together, these results indicate that PTX induces a non-selective
cation channel in mouse megakaryocytes. This channel is distinct from the
ADP-operated channel and is sensitive to ouabain, NiCl2 and DCB.
Keywords: Palytoxin, Megakaryocyte, Non-selective cation channel