Palytoxin-induced permeability changes in excitable membranes

Palytoxin causes nonoxidative necrotic damage to PC12 cells in culture

Palytoxin (PTX) is a potent marine toxin that causies serious damage to various tissues and organs. It has been reported to affect the transport of cations across the plasma membranes, which is commonly recognized as being the principal mechanism of its highly toxic action on mammals, including humans. However, although some marine toxins have been shown to cause toxic effects on the nervous system by interfering with the transmission of nerve impulses, the effect of PTX on neuronal cells has not yet been fully elucidated. Therefore, the toxic action of PTX on PC12 cells was examined as an in vitro model experiment to elucidate the neurotoxic properties of this toxin, and PTX was shown to reduce the viability of PC12 cells in a concentration-dependent manner. The cytotoxic action of PTX was not significantly altered by the presence of the antioxidant N-acetylcysteine and reduced-form glutathione in the cultures. Fluorescence staining of the cells and the electrophoretic analysis of genomic DNA showed that PTX failed to cause chromatin condensation and DNA fragmentation within the cells. On the other hand, the exposure to PTX caused positive staining of the cytoplasmic space of the cells with propidium iodide and the release of lactate dehydrogenase into the culture medium. Based on these observations, PTX is considered to cause cell death as a consequence of disrupting the plasma membranes, thus causing nonoxidative necrotic damage to PC12 cells.

Palytoxin: membrane mechanisms of action

Palytoxin is a marine toxin originally isolated from the zoantharians of the genus Palythoa, but now is found in marine organisms ranging from dinoflagellates to fishes. With a MW of 2680, it is one of the largest nonpolymeric natural products ever found. Its complex structure has been elucidated and total synthesis has been achieved. With an LD(50) of 25 ng/kg for rabbits (the most sensitive species), it is one of the most lethal marine toxins. It binds to the Na,K-ATPase specifically with a K(D) of 20 pM. It has a unique action on the Na,K-ATPase, converting the pump into an ion channel and resulting in K(+) efflux, Na(+) influx and membrane depolarization. As a result palytoxin causes a wide spectrum of secondary pharmacological actions. By acting like a key to unlock the internal structure of the Na,K-ATPase, palytoxin holds promise as a useful tool for investigation of the pump molecule.

Involvement of the Na,K-ATPase in the induction of ion channels by palytoxin

The effects of ouabain, ATP, and vanadate on palytoxin induction of ion channels were examined with the aim of elucidating the role of Na,K-ATPase in palytoxin action. Palytoxin-induced membrane depolarization of crayfish giant axons and single channel currents of frog erythrocytes and mouse neuroblastoma N1E-115 cells were examined using the intracellular microelectrode and patch-clamp techniques. External application of palytoxin in nanomolar concentrations induced depolarization in the crayfish giant axons, and the depolarization was inhibited by pretreatment of the axon with ouabain (10 microM). Internally perfused axons were less sensitive to palytoxin unless ATP (6 mM) was added internally. In patch-clamp experiments, picomolar palytoxin in the patch electrode induced single channels in both cell-attached and inside-out patches of erythrocytes and neuroblastoma cells. The induced channels had a conductance of about 10 pS, reversed near 0 mV in physiological saline solution, and was permeable to Na+, K+, Cs+, and NH4+, but not to choline. Single channel activities induced by palytoxin were inhibited by ouabain (10 microM) and vanadate (1 mM), but promoted by ATP (1 mM). The modulating effects of ouabain, vanadate, and ATP on palytoxin action suggest that the Na,K-ATPase is involved in the induction of single channels by palytoxin. Palytoxin-induced and ouabain-inhibitable single channels were observed in planar lipid bilayer incorporated with purified Na,K-ATPase. The results indicate that an interaction between palytoxin and Na,K-ATPase leads to opening of a 10-pS ion channel. They further raise the possibility that a channel structure may exist in the sodium pump which is uncovered by the action of palytoxin.

Characterization of palytoxin-induced channels in mouse neuroblastoma cells

Palytoxin-induced whole-cell and single channel currents were recorded in mouse neuroblastoma cells. Palytoxin-induced single channel currents had a slope conductance of 26 pS (20-22 degrees C). Palytoxin-induced channels were permeable to sodium and potassium and slightly permeable to calcium, choline and tetramethylammonium. They did not seem to be significantly permeable to chloride or protons. Both the steady-state and the rate of the dose-dependent effects of palytoxin could be accounted for if one assumed that a palytoxin-induced channel resulted from the binding of two palytoxin molecules to a membrane receptor with respective dissociation constants of 5 nM and 10 pM. In the continued presence of low palytoxin concentrations (less than 1 nM) the effect was maintained. Higher palytoxin concentrations induced a transient and irreproducible effect. The effect of palytoxin was decreased when either external sodium was replaced by potassium or in the absence of calcium in external and/or internal media. The results suggest that ionic currents result from the binding of palytoxin molecules to a membrane receptor and that receptor-toxin complexes can be internalized.

Palytoxin acts through Na+,K+-ATPase

Palytoxin is the most potent animal toxin, with a unique structure. The author's group has searched for its mode of action with the following results: 1. Palytoxin (1 pM and less) causes a fast K+ outflow from erythrocytes; 2. Extracellular Ca2+ and borate, and intracellular ATP enhance, but ouabain potently inhibits the palytoxin effects; 3. Palytoxin increases the permeability for Na+ and K+ but not for Ca2+; 4. Palytoxin in comparatively high concentrations (100 nM and above) inhibits Na+,K+-ATPase; 5. Palytoxin can be radiolabeled with 125I. Its receptor is very similar to, but not identical to that of ouabain. A reaction scheme has been delineated which allows an explanation to be obtained for all the known actions of palytoxin. It centers on the hypothesis that palytoxin binds to Na+,K+-ATPase and converts the enzyme or its close vicinity into an open channel with the permselectivity measured on erythrocytes. Patch clamp data from myocytes were obtained in other laboratories. They prove the presence of the predicted palytoxin channel.

Does palytoxin open a sodium-sensitive channel in cardiac muscle?

The effect of palytoxin (PTX) on transmembrane potentials and currents of frog atrial fibres was studied using the double sucrose gap technique. PTX irreversibly depolarized the membrane. This depolarization was reversed only when Na ions were removed from the Ringer solution and replaced by a non-permeant cation such as choline. The depolarization was tetrodotoxin (TTX) insensitive and a function of the external Na concentration. In voltage clamp experiments PTX induced the development of a large inward resting current which did not inactivate and was insensitive to ouabain and to a lowering of the temperature. PTX and ouabain did not share the same receptor. Dose-response curves indicated a stoichiometry of 2, which suggested the aggregation of 2 molecules of PTX to form a channel. The channel formed by PTX remained insensitive to TTX, 4 aminopyridine, tetramethyl-ammonium, Cs and Cd, the classical blockers of Na, K and Ca conductances. PTX reduced the Na current, but not the apparent reversal potential for Na ions. It was concluded that PTX might act on frog atrial fibres as a Na ionophore.