Electrical Responses of the Carnivorous Ciliate Didinium Nasutum in Relation to Discharge of the Extrusive Organelles

1. The carnivorous ciliate Didinium nasutum discharged its extrusive organelles when a strong inward current was injected into the cell in the presence of external Ca ions. 2. In the absence of external Ca ions, the strong inward current produced fusion of the apex membrane of the proboscis. 3. External application of Ca ions after the fusion of the apex membrane produced discharge of the organelles. 4. An increase in Ca"" concentration around the organelles seems to cause discharge of the organelles. 5. Ca concentration threshold for the discharge of the pexicysts seems to be lower than that for the toxicysts. 6. External Ca ions were not necessary for discharge of the organelles upon contact with Paramecium. 7. Chemical interaction of the apex membrane with Paramecium membrane may cause intracellular release of Ca ions from hypothetical Ca storage sites around the organelles. 8. A small hyperpolarizing response seen before the discharge upon contact with Paramecium seems to correlate with the chemical interaction, 9. The depolarizing spike response associated with discharge of the organelles is caused by the depolarizing mechanoreceptor potential evoked by mechanical stimulation of the proboscis membrane by the discharging organelles.