An All-or-Nothing Rise in Cytosolic [Ca²⁺] in Vorticella SP.

The peritrich ciliate Vorticella sp. exhibits cellular contraction of an all-or-nothing type in response to a mechanical stimulus. Many authors have suggested that the contraction may be controlled by the cytosolic level of Ca²⁺, since glycerol-extracted Vorticella contracts when Ca²⁺ is added to the external solution. However, no direct evidence for the increase in cytosolic [Ca²⁺] has yet been obtained in living Vorticella.

In the present study, by injecting a fluorescent Ca²⁺ indicator into living Vorticella and monitoring the cytosolic [Ca²⁺] with a confocal microscope, we have demonstrated that a mechanical stimulus evoked an all-or-nothing rise in cytosolic [Ca²⁺] (Ca²⁺ ‘spike’). The onset of the Ca²⁺ spike was similar in its time course to that of cellular contraction. Since the Ca²⁺ spike was recorded in a Ca²⁺-deprived solution containing 1 mmol l⁻¹ EGTA, we concluded that release of Ca²⁺ from intracellular Ca²⁺ storage site(s) is responsible for the Ca²⁺ spike.