THE DISCHARGE OF IN SITU NEMATOCYSTS OF THE ACONTIA OF AIPTASIA MUTABILIS IS A Ca-INDUCED RESPONSE

The ionic events leading to discharge of in situ nematocysts were investigated in acontia excised from Aiptasia mutabilis Gravenhorst. The effect on discharge of various ionic solutions and ion channel blockers was tested. In the absence of Ca in the medium no discharge was elicited, whatever the composition of the medium. In the presence of lOmmolI" Ca total discharge was induced by NaSCN, Nal, choline iodide and KI, whereas KC1 induced both discharge and extrusion of undischarged nematocysts. The latter effect was prevented by La but not by 4-aminopyridine (4-AP) and tetraethylammonium (TEA). Aldan Blue induced total discharge. NaCl and choline chloride were ineffective. The discharge induced by lyotropic anions depended on Ca concentration and was prevented by the Ca channel blockers La, Co and Cd, but not by verapamil. It is proposed that the discharge of in situ nematocysts is caused by Ca conductance through the cell membrane of either the nematocyte or the supporting cell. Furthermore, cyclic AMP and cyclic GMP do not seem to be involved as second messengers in the discharge process. The combined effects of the metabolic poisons dinitrophenol (DNP) and monoiodoacetic acid did not affect the discharge process.