Polyamine triggering of exocytosis in Paramecium involves an extracellular Ca2+/(polyvalent cation)-sensing receptor, subplasmalemmal Ca-store mobilization and store-operated Ca2+-influx via unspecific cation channels

The polyamine secretagogue, aminoethyldextran (AED), causes a cortical [Ca2+] transient in Paramecium cells, as analyzed by fluorochrome imaging. Our most essential findings are: (i) Cortical Ca2+ signals also occur when AED is applied in presence of the fast Ca2+ chelator, BAPTA. (ii) Extracellular La3+ application causes within seconds a rapid, reversible fluorescence signal whose reversibility can be attributed to a physiological [Ca2+], transient (while injected La3+ causes a sustained fluorescence signal). (iii) Simply increasing [Ca2+], causes a similar rapid, short-lived [Ca2+](i) transient. All these phenomena, (i-iii), are compatible with activation of an extracellular "Ca2+/(polyvalent cation)-sensing receptor" known from some higher eukaryotic systems, where this sensor (responding to Ca2+, La3+ and some multiply charged cations) is Linked to cortical calcium stores which, thus, are activated. In Paramecium, such subplasmalemmal stores ("alveolar sacs") are physically linked to the cell membrane and they can also be activated by the Ca2+ releasing agent, 4-chloro-m-cresol, just like in Sarcoplasmic Reticulum. Since this drug causes a cortical Ca2+ signal also in absence of Ca2+, we largely exclude a "Ca2+-induced Ca2+ release" (CICR) mechanism. Our finding of increased cortical Ca2+ signals after store depletion and re-addition of extracellular Ca2+ can be explained by a "store-operated Ca2+ influx" (SOC), i.e., a Ca2+ influx superimposing store activation. AED stimulation in presence of Mn-o(2+), causes fluorescence quenching in Fura-2 loaded cells, indicating involvement of unspecific cation channels. Such channels, known to occur in Paramecium, share some general characteristics of SOC-type Ca2+ influx channels. In conclusion, we assume the following sequence of events during AED stimulated exocytosis: (i) activation of an extracellular Ca2+/polyamine-sensing receptor, (ii) release of Ca2+ from subplasmalemmal stores, (iii) and Ca2+ influx via unspecific cation channels. All three steps are required to produce a steep cortical [Ca2+] signal increase to a level required for full exocytosis activation. In addition, we show formation of [Ca2+] microdomains (less than or equal to 0.5 mu m, less than or equal to 33 msec) upon stimulation.