Ca2+ triggers toxicyst discharge in Didinium nasutum

A carnivorous ciliate, Didinium nasutum, captures a prey, Paramecium spp., by discharging extrusomes (i.e., toxicysts) from the proboscis. To directly examine the role of Ca2+ for the discharge, we injected Ca2+ into D. nasut um. Injection of Ca2+ evoked discharge of toxicysts, if the site of the inj ection was the periphery region of the proboscis. After the discharge, D. n asutum opened the proboscis and swallowed the discharged toxicysts. These o bservations demonstrate that a rise in cytoplasmic Ca2+ level is an actual cause of toxicyst discharge in D. nasutum.