Kinetics of Chloride-Bicarbonate Exchange Across the Human Red Blood Cell Membrane |
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| Authors: | FA Greco AK Solomon |
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| Institution: | (1) Faculty of Biology, University of Konstanz, P.B. Box 5560, D-78434 Konstanz, Germany, DE |
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| Abstract: | 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.
Received: 27 September 1996/Revised: 11 February 1997 |
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| Keywords: | : Calcium — Endocytosis — Exocytosis — Membrane fusion — Secretion — Paramecium |
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