G. Thiel et al., ION-CHANNEL ACTIVITY DURING THE ACTION-POTENTIAL IN CHARA - NEW INSIGHTS WITH NEW TECHNIQUES, Journal of Experimental Botany, 48, 1997, pp. 609-622
The dynamics of macroscopic currents underlying the electrically trigg
ered action potential (AP) in the giant alga Chara corallina were dire
ctly recorded with an action potential clamp method. In this technique
an AP is recorded and repetitively replayed as the command voltage to
the same cell under voltage control. Upon adding the channel blockers
niflumic acid and/or Ba2+ to the bath, the excitation current, i.e. t
he current crossing the membrane during an AP, can be dissected into a
transient, fast-appearing Cl- inward current and a transient delayed
K+ outward current. The delayed onset of the K+ outward current demand
s the postulation of an additional outward current in order to balance
the excess Cl- inward current at the onset of the AP. The capacitive
current that alters the charge on the membrane during excitation is se
veral orders of magnitude too small to be relevant for charge balance.
Measurements of single channel activity in the plasma membrane of C.
corallina by the patch clamp method shows two types of Cl- channel (15
and 38 pS with 100 mM Cl- in the pipette) and one type of K+ channel
(about 40 pS with 100 mM K+ in the pipette) which become transiently a
ctive during an AP. Typically, variable numbers of Cl- channels activa
te in a random fashion for short periods of time when favoured by posi
tive voltages in combination with high concentrations of extracellular
Ca2+ (Ca-o(2+)) or during an AP of the whole cell. The peak values of
these Cl- channel currents measured in a patch are such that they can
account quantitatively for the peak of the whole cell Cl- excitation
current studied under comparable ionic conditions. Furthermore, the sh
ort duration of channel activity, as well as the fast rising and somew
hat slower trailing kinetics is similar in duration and dynamics to AP
-associated changes in membrane permeability of the whole Chara cell t
o Cl- (PCl-). Taken together, the data stress that the characteristic,
transient activation of random numbers of Cl- channels seen in membra
ne patches is the elementary unit of the Cl- excitation current. Howev
er, due to the random nature of this transient activity, gating of Cl-
channels can not be explained on the basis of previous models for exc
itation: gating can neither be due to intrinsic voltage sensitivity of
the Cl- channels, nor to a voltage-dependent influx of Ca2+ and subse
quent activation of Ca2+-sensitive Cl- channels. to account for the sh
ort life-time and for the randomness of Cl- channel activity, the puta
tive gating factors Ca2+ and voltage must be uncoupled in time. This c
ould be explained by a random release of Ca2+ from stores, the latter
being filled in a voltage-sensitive manner via non-specific cation cha
nnels from the outside. A 4 pS non-selective cation channel in the pla
sma membrane may serve this purpose. The 40 pS K+ channel, which becom
es transiently active in C. corallina during a cell AP, is an outward
rectifier. At negative resting voltages the channel has a low open pro
bability (< < 1%). At voltages reached during an AP the open probabili
ty rises significantly reaching half-maximal open probability at -25 m
V. The elevated activity of the 40 pS channel associated with membrane
excitation relaxes at the end of an AP with a time constant of about
2.5 s. A comparable time constant of 2 s can be obtained for the decay
of the transiently elevated permeability of the membrane to K+ (PK+),
stressing that the kinetic properties of the 40 pS K+ channel are res
ponsible for the course of whole cell PK+ changes. Voltage sensitivity
of the K+ channels suggests that they are activated during an AP by t
he drop in membrane voltage in order to aid repolarization. However, t
he rise and decay of PK+ during an AP also shares similarity with the
timecourse of transient changes in cytoplasmic concentration of free C
a2+, [Ca2+](cyt), the latter being measured in parallel experiments wi
th the Ca2+-sensitive fluorescent dye, Fura-P, in excited C. corallina
cells. This similarity could suggest that gating of the 40 pS K+ chan
nel is also sensitive to [Ca2+](cyt) and that the latter sensitivity i
s rate-limiting for activity during an AP.