Facilitation of membrane fusion during exocytosis and exocytosis-eoupled endocytosis and acceleration of Ghost detachment in Paramecium by extracellular Calcium : a quenched-flow/freeze-fracture analysis

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Journal of Membrane Biology. 1997, 158(3), pp. 198-208. ISSN 0022-2631. eISSN 1432-1424. Available under: doi: 10.1007/s002329900257
Zusammenfassung

We had previously shown that an influx of extracellular Ca2+ (Ca2+e), though it occurs, is not strictly required for aminoethyldextran (AED)-triggered exocytotic membrane fusion in Paramecium. We now analyze, by quenched-flow/freeze-fracture, to what extent Ca2+e contributes to exocytotic and exocytosis-coupled endocytotic membrane fusion, as well as to detachment of ghosts a process difficult to analyze by any other method or in any other system. Maximal exocytotic membrane fusion (analyzed within 80 msec) occurs readily in the presence of [Ca2+]e >= 5 × 10 −6 M, while normally a [Ca2+e = 0.5 mM is in the medium. A new finding is that exocytosis and endocytosis is significantly stimulated by increasing [Ca2+]e even beyond levels usually available to cells. Quenching of [Ca2+]e by EGTA application to levels of resting [Ca2+]i or slightly below does reduce (by ~50%) but not block AED-triggered exocytosis (again tested with 80 msec AED application).
This effect can be overridden either by increasing stimulation time or by readdition of an excess of Ca2+e. Our data are compatible with the assumption that normally exocytotic membrane fusion will include a step of rapid Ca2+-mobilization from subplasmalemmal pools ( alveolar sacs ) and, as a superimposed step, a Ca2+-influx, since exocytotic membrane fusion can occur at [Ca2+]e even slightly below resting [Ca2+]i. The other important conclusion is that increasing [Ca2+]e facilitates exocytotic and endocytotic membrane fusion, i.e., membrane resealing. In addition, we show for the first time that increasing [Ca2+]e also drives detachment of ghosts a novel aspect not analyzed so far in any other system. According to our pilot calculations, a flush of Ca2+, orders of magnitude larger than stationary values assumed to drive membrane dynamics, from internal and external sources, drives the different steps of the exo-endocytosis cycle.

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570 Biowissenschaften, Biologie
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Calcium, Endocytosis, Exocytosis, Membrane fusion, Secretion, Paramecium
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ISO 690PLATTNER, Helmut, Claudia BRAUN, Joachim HENTSCHEL, 1997. Facilitation of membrane fusion during exocytosis and exocytosis-eoupled endocytosis and acceleration of Ghost detachment in Paramecium by extracellular Calcium : a quenched-flow/freeze-fracture analysis. In: Journal of Membrane Biology. 1997, 158(3), pp. 198-208. ISSN 0022-2631. eISSN 1432-1424. Available under: doi: 10.1007/s002329900257
BibTex
@article{Plattner1997Facil-8165,
  year={1997},
  doi={10.1007/s002329900257},
  title={Facilitation of membrane fusion during exocytosis and exocytosis-eoupled endocytosis and acceleration of   Ghost   detachment in Paramecium by extracellular Calcium : a quenched-flow/freeze-fracture analysis},
  number={3},
  volume={158},
  issn={0022-2631},
  journal={Journal of Membrane Biology},
  pages={198--208},
  author={Plattner, Helmut and Braun, Claudia and Hentschel, Joachim}
}
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    <dcterms:abstract xml:lang="deu">We had previously shown that an influx of extracellular Ca2+ (Ca2+e), though it occurs, is not strictly required for aminoethyldextran (AED)-triggered exocytotic membrane fusion in Paramecium. We now analyze, by quenched-flow/freeze-fracture, to what extent Ca2+e contributes to exocytotic and exocytosis-coupled endocytotic membrane fusion, as well as to detachment of   ghosts     a process difficult to analyze by any other method or in any other system. Maximal exocytotic membrane fusion (analyzed within 80 msec) occurs readily in the presence of [Ca2+]e &gt;= 5 × 10 −6 M, while normally a [Ca2+e = 0.5 mM is in the medium. A new finding is that exocytosis and endocytosis is significantly stimulated by increasing [Ca2+]e even beyond levels usually available to cells. Quenching of [Ca2+]e by EGTA application to levels of resting [Ca2+]i or slightly below does reduce (by ~50%) but not block AED-triggered exocytosis (again tested with 80 msec AED application).&lt;br /&gt;This effect can be overridden either by increasing stimulation time or by readdition of an excess of Ca2+e. Our data are compatible with the assumption that normally exocytotic membrane fusion will include a step of rapid Ca2+-mobilization from subplasmalemmal pools (  alveolar sacs  ) and, as a superimposed step, a Ca2+-influx, since exocytotic membrane fusion can occur at [Ca2+]e even slightly below resting [Ca2+]i. The other important conclusion is that increasing [Ca2+]e facilitates exocytotic and endocytotic membrane fusion, i.e., membrane resealing. In addition, we show for the first time that increasing [Ca2+]e also drives detachment of   ghosts    a novel aspect not analyzed so far in any other system. According to our pilot calculations, a flush of Ca2+, orders of magnitude larger than stationary values assumed to drive membrane dynamics, from internal and external sources, drives the different steps of the exo-endocytosis cycle.</dcterms:abstract>
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