Effects of equinatoxin II on isolated guinea pig taenia caeci muscle contractility and intracellular Ca2+

Equinatoxin II (EqT II) is a ∼20 kDa cytotoxic and cytolytic protein isolated from the sea anemone Actinia equina. When injected intravenously to rats the toxin has been reported to produce a rapid cardiorespiratory arrest. In the present study, we show that EqT II increases the tension of spontaneous contractions and induces long-lasting contracture of guinea pig taenia caeci muscle. In taenia caeci, dissociated smooth muscle cells, microspectrofluorometric measurements, using the Ca2+ indicator fura-2/AM, revealed that the toxin causes a marked increase in intracellular calcium, provided Ca2+ is present in the external medium. The increase in intracellular Ca2+ by EqT II was not blocked or diminished by the calcium channel blocker verapamil. Furthermore, pre-treatment of smooth muscle cells with Ca2+-ATPase inhibitor thapsigargin, or exposure of the cells to a high K+ (75 mM) medium did not prevent EqT II-induced intracellular Ca2+ increases. Replacement of external sodium by sucrose markedly modified the time course of Ca2+ signals suggesting the involvement of the Na+/Ca2+ exchanger in EqT II action. Our results strongly suggest that EqT II-induced increase in intracellular Ca2+ and muscle tension are both dependent on the ability of EqT II to insert into the membrane and form pores allowing Ca2+ influx into the cells. To our knowledge this is the first report showing that EqT II causes contraction and contracture of taenia caeci muscles and increases intracellular Ca2+ in smooth muscle cells.