Effects of equinatoxin II on isolated guinea pig taenia caeci muscle contractility and intracellular Ca2+.
Equinatoxin II (EqT II) is a approximately 20kDa 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 Ca(2+) indicator fura-2/AM, revealed that the toxin causes a marked increase in intracellular calcium, provided Ca(2+) is present in the external medium. The increase in intracellular Ca(2+) by EqT II was not blocked or diminished by the calcium channel blocker verapamil. Furthermore, pre-treatment of smooth muscle cells with Ca(2+)-ATPase inhibitor thapsigargin, or exposure of the cells to a high K(+) (75 mM) medium did not prevent EqT II-induced intracellular Ca(2+) increases. Replacement of external sodium by sucrose markedly modified the time course of Ca(2+) signals suggesting the involvement of the Na(+)/Ca(2+) exchanger in EqT II action. Our results strongly suggest that EqT II-induced increase in intracellular Ca(2+) and muscle tension are both dependent on the ability of EqT II to insert into the membrane and form pores allowing Ca(2+) 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 Ca(2+) in smooth muscle cells.
Frangez
Robert
R
Institute of Physiology, Pharmacology and Toxicology, Veterinary Faculty, University of Ljubljana, Ljubljana, Slovenia. robert.frangez@vf.uni-lj.si
Suput
Dusan
D
Molgó
Jordi
J
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003064:Cnidarian Venoms / Q000302:isolation & purification / Q000494:pharmacology*; D003603:Cytotoxins / Q000302:isolation & purification / Q000494:pharmacology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D005456:Fluorescent Dyes / Q000032:analysis; D016257:Fura-2 / Q000032:analysis; D006168:Guinea Pigs; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D012615:Sea Anemones / Q000737:chemistry; D019284:Thapsigargin / Q000494:pharmacology
D003064:Cnidarian Venoms; D003603:Cytotoxins; D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; C012307:equinatoxin; D019284:Thapsigargin; D002118:Calcium; D016257:Fura-2