Characterization of a Palytoxin-Induced Non-selective Cation Channel in Mouse Megakaryocytes

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 μM), 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.