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K out type allowing K + efflux. Chloride and potassium fluxes in Chara dur-
ing one AP are much higher than the theoretical value of 1−2 pmol cm −2
and reach up to 10,000 pmol cm −2 (Oda 1976).
Recently, the question arose of what is the source of Ca 2+ ions entering
the cytosol (Tazawa and Kikuyama 2003). There is strong evidence that
Ca 2+ influx originates from the apoplast. Internal stores, mainly endoplas-
mic reticulum (ER), are indicated as an alternative source of Ca 2+ ions.
The former conclusion is supported, among others, by experiments with
45 Ca 2+ and near-Nernstian dependence of the AP amplitude on external
Ca 2+ concentration (Hayama et al. 1979). The latter statement is based
mostly on evidence that showed (1) a reduction of Ca 2+ currents after ap-
plication of IP 3 modulating factors (Biskup et al. 1999) and (2) quenching
of fura fluorescence in cells preincubated with Mn 2+ which coaccumulates
with Ca 2+ in the ER (Plieth et al. 1998). Combining these two sources
of Ca 2+ is also possible in a process of calcium-induced calcium release.
Accordingtoit,asmallportionofCa 2+ crossing the plasma membrane
from the apoplast would lead to Ca 2+ release from the ER and possibly
other internal stores, including the vacuole. An increase in cytoplasmic
Ca 2+ concentration ([Ca 2+ ] cyt )ispostulatedtomediatebetweentheAPon
the plasmalemma and that on the tonoplast. The regulation of transient
[Ca 2+ ] cyt increase during the AP is very precise and occurs according to the
all-or-none rule (Wacke and Thiel 2001). Involvement of voltage-dependent
IP 3 production in the mechanism of the AP is postulated (Wacke et al.
2003).
There were numerous attempts to characterize ion channels carrying
individual ion fluxes coupled with the AP, using the patch-clamp technique.
At least three calcium-dependent anion channels have been characterized in
the Chara plasmalemma. Channels of unitary conductance of 9 pS (Okihara
et al. 1991), 17 and 38 pS (Homann and Thiel 1994) have been examined.
The 9-pS channel has a narrow [Ca 2+ ] cyt dependence with a maximum
at approximately 1 µM. Addition of calmodulin transiently activates the
channel, while calmodulin inhibitors W-7 and chlorpromazine cause its
blockage (Okihara et al. 1993). The other two channel types show bursting
behavior in a cell-attached mode. Quantal release of Ca 2+ from stores in
the vicinity of the plasmalemma was postulated to activate these channels
(Thiel et al. 1993; Thiel and Dityatev 1998).
Apossibleroleinconductinganoutwardcurrentintherepolarization
phase of the AP was attributed to the 40 pS potassium channel in Chara
(Homann and Thiel 1994). In Nitellopsis a25−50pSK + channel was found
(Katsuhara et al. 1990). The channel is regulated by external Ca 2+ concen-
tration and by ATP. Other nucleotides like AMP and the nonhydrolyzable
ATPanalogueareequallyactiveindecreasingthechannelopenprobability
(Katsuhara and Tazawa 1992).
 
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