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Trichocyst discharge in Paramecium spp. is known to be mediated by rapid exocytosis. Applied stimuli induce fusion of the trichocyst membrane and plasma membrane within 30 ms. Both Ca2+ release from intracellular store(s) and Ca2+ influx from extracellular region have been suggested to be related to the trichocyst discharge. We constructed a new system in which to record intracellular levels of Ca2+ ([Ca2+]i) and microscopic images simultaneously without changing the optical path. With this system, we recorded [Ca2+]i at 2 ms intervals and microscopic images of trichocyst discharge at video rate (33 ms intervals) simultaneously in Paramecium caudatum. Simultaneous application of Ca2+ chelator at 100 mM with secretagogue onto Paramecium cells resulted in only a slight increase in [Ca2+]i (Δ[Ca2+]i). Furthermore, no extrusion of trichocysts occurred. In contrast, application of secretagogue concomitant with Ca2+ chelator at 20 mM induced a Δ[Ca2+]i composed of two phases. In this case, extrusion of trichocysts occurred. These observations directly indicated that Ca2+ influx from the extracellular medium in addition to Ca2+ release from intracellular store(s) contributes to Δ[Ca2+]i during trichocyst discharge.

Contribution of Calcium Influx on Trichocyst Discharge in Paramecium caudatum