Characteristics of palytoxin-induced cation currents and Ca2+ mobilization in smooth muscle cells of rabbit portal vein

Abstract

We examined the nature of the palytoxin (PTX)-induced channel and its relevance to the Ca2+ mobilizing effect of the toxin on smooth muscle cells isolated from rabbit portal vein using whole-cell voltage-clamp and microfluorimetric techniques. PTX (1 nM) induced a sustained, irreversible inward current at a holding potential of -40 mV. The PTX-induced current reversed at 0.5 +/- 0.6 mV, and the PTX-induced channel permitted the passage of Na+, K+, Cs+ and, to a lesser extent, Li+, but not choline+ or Ca2+. During the sustained phase of the current, superfusion of Ni2+ (5 mM), La3+ (0.5 mM) or 2,4-dichlorobenzamil (2,4-DCB, 25 microM) reduced the current amplitude and decreased the slope conductance without changing the reversal potential. In 5 of 7 experiments, ouabain transiently increased the PTX-induced inward current and shifted the reversal potential in a positive direction. Subsequently, ouabain inhibited the current in every cell. PTX (10 nM) induced a sustained rise in cytosolic Ca2+ ([Ca2+]i), which was resistant to verapamil but suppressed by omission of extracellular Ca2+. When external Na+ was replaced by choline+, PTX did not increase [Ca2+]i. Pretreatment with 2,4-DCB prevented the elevation of [Ca2+]i due to PTX. These results suggest that PTX does not directly stimulate Ca2+ entry but induces entry through Na(+)-Ca2+ exchange as a consequence of increased cytosolic Na+. Ni2+, La3+, 2,4-DCB and ouabain were shown to act as blockers of the PTX-induced channel. Ouabain may also inhibit Na+ pump current activated by cytosolic Na+.

The cytoskeleton, a structure that is susceptible to the toxic mechanism activated by palytoxins in human excitable cells

“…The results obtained in this study suggest that depolarization is due to an increase in cytosolic Na + as a primary event in palytoxin and ostreocin‐D action. In agreement with this, stimulation of Na + entry induced by palytoxin has been described in several cellular models [11,12,28,29]. We also found an increase in cytosolic Ca 2+ induced by both toxins.…”

“…It has been reported that the elevation of cytosolic Ca 2+ induced by palytoxin is dependent on extracellular Ca 2+ [12,17,18,[34][35][36]. Our assays using neuroblastoma cells also indicated that palytoxin, as well as ostreocin-D, evoked Ca 2+ entry from the external medium.…”

Modulation of calcium entry and glutamate release in cultured cerebellar granule cells by palytoxin

“…PTX has been reported to increase both cytoplasmatic Na + and Ca 2+ concentrations under physiological conditions. The PTX‐induced calcium increase has been attributed to the activation of voltage‐dependent Ca 2+ channels, reverse of the Na + /Ca 2+ exchanger, or Ca 2+ entry through PTX‐induced channels (Ozaki et al,1983; Monroe and Tashjian,1995; Ishii et al,1997a).…”

“…PTX‐induced rise in Ca 2+ has been shown to be partially inhibited by verapamil, supporting a role for voltage‐dependent calcium channels that seem to be activated by PTX‐induced depolarization of the plasma membrane, on PTX‐induced calcium mobilization. The reverse operation of the Na + ‐Ca 2+ exchanger has also been implicated in the PTX‐induced calcium increase (Ozaki et al,1983; Ishii et al,1997a,b). In spite of the fact that PTX is not specific for the sodium pump and blocks also the H + ‐K + ‐ATPase (Scheiner‐Bobis et al,2002), other factors that could contribute to calcium accumulation such as inhibition of the calcium‐ATPase (Garcia and Strehler,1999) have not yet been investigated.…”

Channel induction by palytoxin in yeast cells expressing Na+,K+‐ATPase or its chimera with sarco/endoplasmic reticulum Ca2+‐ATPase

“…Palytoxin (PTX), isolated from Palythoa spp., is the most potent marine toxin known. We have reported that PTX induces a cation channel, which is permeable to monovalent cations, in cardiac ventricular or vascular smooth muscle cells [1–3]. Its lethal action is ascribed to an increase in cytosolic Na + concentration and the subsequent increase in cytosolic Ca 2+ [4].…”