Contribution of calcium influx on trichocyst discharge in Paramecium caudatum

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) (Delta [Ca-2+](i)). Furthermore, no extrusion of trichocysts occurred. In contrast, application of secretagogue concomitant with Ca2+ chelator at 20 mM induced a Delta [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 Delta [Ca2+](i) during trichocyst discharge.