References

280 K.Trebacz,H.Dziubinska,E.Krol
The picture is not so clear when calcium channels are considered. Thiel et
al. (1993) found 4 pS Ca 2+ -permeable channels in the Chara plasmalemma.
It is postulated that they serve in Ca 2+ accumulation in the vicinity of the
plasmalemma. The channels do not show clear voltage dependence.
There is a general problem with application of the patch-clamp technique
tostudythechannelsoperatingduringtheAP.Inmostcasesprotoplastsare
unexcitable. Thus, voltage-clamp experiments on intact turgid cells seem
more reliable. The early studies by Lunevsky et al. (1983) and subsequent
studies by Tsutsui and Ohkawa (1993) allowed the Ca 2+ currents to be
characterized in more detail. The pharmacology of channels carrying Ca 2+
resembles that of L-type calcium channels in animal cells.
Characean cells offer technical advantages over small cells of higher
plants, but because of their enormous dimensions they are highly specialized.
Thus, their usefulness as model cells in explaining excitation in plants
is often questionable. Conocephalum conicum –aliverwort–seemsabetter
model system for studying APs in terrestrial higher plants. C. conicum
belongs to the evolutionarily oldest land plants. The thallus has a simple
structure with relatively large cells connected symplasmically. It has no
conducting bundles. All cells including rhizoids are excitable. APs can be
evoked, among others, by electrical stimulation, illumination and cooling.
Wounding often leads to the generation of long-lasting trains of APs
(Paszewski et al. 1982). APs fulfill all basic electrophysiological principles.
They are generated according to the all-or-none law and they propagate
throughout the thallus with a velocity of 2.5−9 cm min −1 . Immersion, depending
on the resistance of the solution, causes acceleration of propagation
to more than 30 cm min −1 (Zawadzki and Trebacz 1985). Excitation is always
followed by refractory periods: absolute 2−4 min and relative 6−8 min
(Dziubinska et al. 1983). Spatial and temporal summation of subthreshold
stimuli takes place (Trebacz and Zawadzki 1985). C. conicum can be excited
by light stimuli. When shaded, its thallus cells hyperpolarize and depolarize
upon reillumination in a dose-dependent manner. Light-induced generator
potentials,whenstrongenough,leadtoexceedingofathresholdandAP
generation (Trebacz and Zawadzki 1985).
Intracellular microelectrodes combined with application of ion channel
and proton pump inhibitors allowed qualitative determination of ions participating
in resting potentials and APs. In C. conicum the resting potential
consists of passive and active components. The first is connected with the
plasma membrane permeability to potassium and the other with operating
of the electrogenic proton pump. Abolishing the active component by
proton pump inhibitors causes depolarization accompanied by a decaying
series of APs after which the cells became unexcitable. Treatment of
C. conicum cells with TEA, which blocks K + channels, makes them nonresponsive
to electrical stimulation. AP reduction or complete blockage was