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 auth ors have suggested that the contraction may be controlled by the cytos olic level of Ca2+, since glycerol-extracted Vorticella contracts when Ca2+ is added to the external solution. However, no direct evidence f or the increase in cytosolic [Ca2+] has yet been obtained in living Vo rticella. In the present study, by injecting a fluorescent Ca2+ indica tor into living Vorticella and monitoring the cytosolic [Ca2+] with a confocal microscope, we have demonstrated that a mechanical stimulus e voked an all-or-nothing rise in cytosolic [Ca2+] (Ca2+ 'spike'). The o nset of the Ca2+ spike was similar in its time course to that of cellu lar contraction. Since the Ca2+ spike was recorded in a Ca2+-deprived solution containing 1 mmoll(-1) EGTA, we concluded that release of Ca2 + from intracellular Ca2+ storage site(s) is responsible for the Ca2spike.