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 μM) 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⁺.