Plant Physiol. (1992) 100, 637-6430032-0889/92/1 00/0637/07/$01 .00/0

Received for publication December 18, 1991Accepted April 25, 1992

Regulation of Calcium Influx in Chara'

Effects of K+, pH, Metabolic Inhibition, and Calcium Channel Blockers

Robert J. Reid* and F. Andrew SmithDepartment of Botany, University of Adelaide, Box 498, G.P.O. Adelaide, S.A. 5001, Australia

ABSTRACT

Measurements were made of 45Ca influx into isolated internodalcells of Chara corallina and also into internodal cells of intactplants. 45Ca influx was closely related to growth. In rapidly ex-panding internodal cells, the influx was approximately 1.4 nmolm-2 s-' compared to the influx in mature cells from slow-growingcultures of 0.2 nmol m-2 s-1. Isolated internodal cells had influxesin the range 0.2 to 0.7 nmol m-2 s-i, but this increased to approx-imately 2 nmol m-2 s-1 in high calcium solutions and to 4 nmol m-2s-' in high potassium solutions. No significant effects on calciuminflux were observed for changes in external pH or for treatmentsthat changed internal pH, except that NH4 was slightly inhibitory.Severe metabolic inhibition by carbonylcyanide-m-chlorophenyl-hydrazone stimulated influx, whereas dicyclohexylcarbodiimidehad no effect and darkness inhibited influx. La3" also inhibitedinflux, but the organic channel blockers nifedipine and bepridilstimulated influx. Verapamil had no effect. The results are generallyconsistent with voltage regulation of calcium channels as in animalcells.

MATERIALS AND METHODS

Plant Material

The giant alga Chara corallina was grown in the laboratoryin large plastic tanks on a substrate of garden soil and riversand that was supplemented with an APW2 containing 1 mmNaCl, 0.1 mm K2SO4 and 0.5 mm CaSO4. The cultures wereilluminated on a 16 h/8 h light/dark cycle at an intensity ofapproximately 50 ,umol m-2 s-' at the surface of the solution.Before experiments, individual internodal cells (40-90 mmlong and approximately 1 mm in diameter) were isolatedfrom the plant and stored overnight in APW buffered at theexperimental pH with 5 mm Mes (pH 5), Mops (pH 7), N-hydroxyethylpiperazine propane sulfonic acid (pH 8), n-cyclohexoaminoethane sulfonic acid (pH 9.2) or 3-cyclohex-ylaminopropane sulfonic acid (pH 10.4) and adjusted to therequired pH with NaOH. When intact plants were used, theywere taken from the culture tank and pretreated in APW atpH 7 for only 1 h before use. Unless stated otherwise,experiments were done at pH 7.

Flux Measurements

Fluctuations in the level of free calcium in the cytoplasmhave been implicated in the control of a range of intracellularand membrane-related processes in plants (see reviews byHepler and Wayne [12] and Kauss [14]). The relative impor-tance of membrane transport compared to cytoplasmic buff-ering of calcium in determining the level of free calciumunder a given set of conditions is at present impossible toassess, partly because so little is known about the magnitudeof calcium fluxes in plants, and in particular about the controlof calcium fluxes at the plasma membrane. This has beenlargely due to the considerable difficulties, common with alldivalent cations, in distinguishing between extracellular bind-ing in the cell wall and actual uptake to the cell. Recently,we reported the results of a detailed investigation of Ca2"exchange in the cell wall of Chara corallina (22), from whichmethods were developed for the measurement of both long-and short-term fluxes of Ca2". These methods have now beenrefined, and in this article, we present the first survey of theeffects of a range of treatments on 45Ca influx into a plantcell in an attempt to reveal the factors that control thepermeability of calcium channels in the plasma membrane.

'This work was supported by the Australian Research Council.

We used three methods to estimate 45Ca fluxes into thecell, cytoplasm and vacuole, and in each method, it wasnecessary to separate the cell wall from the cell contents in amanner that allowed minimal contamination of extracellular45Ca with intracellular calcium pools. A description of themethods used to measure fluxes of 45Ca in Chara and anoutline of the general problems associated with accuratelydetermining calcium influx into turgid plant cells is given inref. 22. The usual tracer method of loading whole cells andthen rinsing off most of the extracellular radioactivity wasused for long influx times with long rinse times to estimateflux to the vacuole and slowly exchanging cytoplasmic com-partments. The rinse solution was APW + 2 mm LaCl3. Thepurpose of the La3+ was to displace 45Ca from the cell walland to block calcium channels to prevent any further accu-mulation of 45Ca. Following the rinse period, the cell wasblotted and allowed to wilt slightly. The ends of the cell wereremoved and a hypodermic syringe was inserted into oneend and clamped in position with forceps. The vacuole wasdisplaced by injecting an air bubble through the cell and thecytoplasm was then flushed out by rapid injection of 1 mL

'Abbreviation: APW, artificial pond water.

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Plant Physiol. Vol. 100, 1992

of distilled water through the lumen, leaving a clear sleeveof cell wall. The 45Ca activity in the vacuolar and cytoplasmicfractions was determined by liquid scintillation counting.

For shorter influxes, the technique that will be referred toas 'segment loading' was used. A single Chara internodal cellwas mounted in a three-chambered perspex block in whichthe ends of the cell were isolated from the central portion bymeans of grease barriers. 45Ca in APW was added to the cellsegment in the center for the influx period of 30 min, afterwhich it was rinsed away by 6 to 10 changes of APW + 2mM LaCl3 over 3 to 5 min. The cell was then removed fromthe chamber and fractionated as described above.These two methods measure different fluxes, and it is

important to understand what is actually being measured ineach case. 'Whole cell loading' is used to measure 45Ca influxfrom the extemal solution to the vacuole and slowly exchang-ing cytoplasmic compartments. For long influx times, thespecific activity of 45Ca in the cytoplasm will be similar tothat in the external solution, so that the vacuolar 45Ca activitywill reflect the Ca2" flux from the cytoplasm to the vacuole.Segment loading, i.e. short influx and short rinse times, isused to obtain the unidirectional influx at the plasmalemma.At high external pH (>8), problems were encountered with

both segment loading and whole cell loading because of theprecipitation of insoluble La` salts during the rinsing period.To overcome this problem, an alternative method referred toas 'half cell loading' was employed. For this method, aninternodal cell was mounted in a two-chambered perspexblock with the two halves isolated by a grease barrier. 45Cawas applied to one half only for 2 to 3 h, after which thesolution in both ends was sucked out and the cell removedfrom the chamber and blotted. The half cell from the non-radioactive end was excised and flushed as for segmentloading. The validity of this technique relies on efficientdistribution of 45Ca from the uptake end to the whole cell,and it is only useful over relatively long uptake periods andfor cells with normal protoplasmic streaming (22).The fluxes are expressed as mean ± SE of 7 to 10 cells. The

significance of the difference between means was tested usingthe Wilcoxon Rank Sum test as described in Sokal and Rohlf(25) for a probability of P < 0.1.

RESULTS

Measurement of Influx

Figure 1 shows a time course of uptake of 45Ca from theexternal solution to the cytoplasm and to the vacuole meas-ured by the segment loading method with a 4-min rinse inAPW containing La3+. For influx times less than 30 min, 45Calabeling of the cell was dominated by 45Ca in the cytoplasm.Tracer influx to the cytoplasm showed two distinct phases:an initial influx lasting approximately 15 min, followed by aslower influx that was linear up to at least 100 min. Influx tothe vacuole showed a lag over the first 5 to 15 min, but after100 min, the 45Ca activity in the vacuole and cytoplasm weresimilar. The variability of influx (as a percent of the mean)was much higher at short influx times. After 5 min, the 45Caactivity in the cell was small in comparison to the residual45Ca activity in the cell wall after the short rinse, so that even

1.2N

.9-l

co4n

Le)

1.0

0.8

0.6

0.4

0.2

0 M'.I

0 20 40 60 80 100

Influx time (min)

Figure 1. Time course of 45Ca influx into internodal cells of Charameasured using the "segment loading" method. Cells were rinsedfor 4 min in APW + 2 mm LaCI3; n = 14 cells/point.

a small degree of contamination during cell fractionationwould significantly increase the measured flux.The vacuolar flux was more accurately determined by

loading whole cells for much longer periods, so that thevacuolar activity would be high enough not to be significantlyaffected by contamination during sampling. The slow ex-change at the tonoplast and the large pool size of calcium inthe vacuole (22) mean that long rinse periods will not resultin significant loss of vacuolar 45Ca. Figure 2 shows an exper-iment in which cells were incubated in 45Ca for 3 h and thenrinsed in La3` solution. The 45Ca activity in the vacuole wasconstant for rinse times greater than about 0.5 h and inde-pendent of the "5Ca in the cell wall, which continued to fallover several hours. After 4 h, rinsing the 45Ca in the cell wallwas exchanging only slowly and the total activity in the wallwas similar to that in the combined intracellular fractions. Itis also worth noting that for long rinse times, a significantproportion of the cellular 45Ca remained in the cytoplasm(Fig. 2).

Calcium Influx and Growth

The relationship between influx and growth was examinedby incubating whole plants in 45Ca-APW and then rinsingand fractionating the cells as for individual isolated internodalcells. Figure 3 shows the 45Ca influx to the vacuole forinternodal cells of plants from two different cultures, one ofwhich was growing only slowly and the other was in a phaseof rapid growth. C. corallina normally grows by expansion ofthe uppermost internodes; increase in length is greatest inthe first internode from the terminal shoot, less in the secondinternode, and by the third internode, cells are virtuallymature. It can be seen from Figure 3 that there is a strongtendency for expanding cells to have higher calcium influxesthan mature internodes and for the faster growing plants tohave higher influxes than slower growing plants.

Because of the large variability in influx and the small size

638 REID AND SMITH

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CALCIUM INFLUX IN CHARA

0 1 2 3 4

Wall

L ,.1 -1

- 0.70

0.22 1.46 -

0.80

0.960.48

0.64

0.38-

Slow rowingcuture

Fast growingculture

Figure 3. 45Ca influx to the vacuole of internodal cells of intactplants of C. corallina from two different cultures. The numbers referto the influx of the adjacent internodal cell in nmol m-2 s-'. Plantswere incubated in 45Ca-APW for 5 h and then rinsed for 1 h in La3+-APW.

0 1 2 3 4

Rinse time (h)

Figure 2. The measured 45Ca influx to the vacuole, cytoplasm, andcell wall as a function of the rinse time in La3`-APW. Wholeinternodal cells were incubated in 45Ca-APW for 3 h.

of the first two internodes, experiments with isolated inter-nodes were normally conducted with larger mature cells frominternodes three to five from the top of the plant. For thesecells, the mean influxes measured by segment loading over

30 min lay in the range from 0.20 to 0.72 nmol m-2 s-. Inhindsight, the differences appear to be related to the rate ofgrowth of the culture, but there was also a tendency, more

marked in cells with higher influxes, for the influx to declinewith time after cutting.

Effects of K' and Ca21 on Calcium Influx

Figure 4 shows the response of 45Ca influx to increasingCa2+ concentration in the bathing medium. Influx doubledbetween 0.2 and 1 mm, was insensitive to Ca2' between 1and 4 mm, and then increased further by a factor of 3 between4 and 20 mM.

45Ca influx was insensitive to the extemal K+ concentrationbetween 0.1 and 5 mM (Fig. 5, data not shown for 0.1 mM),but was greatly stimulated at higher concentrations. Themaximum influx obtained at 20 mm was nearly 8-fold higherthan at 1 mm. Influx decreased between 20 and 100 mm K+.

2

-

0N

aC.

N1

1.6

1.2

0.8

0.4

O l l l l0.1 0.3 1 3 10 30

[Ca.] (mM)

Figure 4. Dependence of 45Ca influx on the calcium concentrationin the bathing solution. Influx measured by segment loading. Influxtime = 30 min; rinse time = 4 min.

0.50

0a)

0.40

0.30 r

0.20

0.10

Cell

Vacuole

0

2.0.-1

U)eq

0

0N

.E

-4

I*

1.6

1.2

0.8

0.4

0

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Plant Physiol. Vol. 100, 1992

4

1-1

W

F._

0

O

cout

3

2

1

0 1

1 10 100

IK+1 (mM)

Figure 5. Effect of K+ concentration on 45Ca influx to internodalcells of C. corallina measured by segment loading. Cells were

pretreated in solutions of different K+ concentrations for 1 h priorto influx. Influx time = 30 min; rinse time = 4 min.

Effects of pH and ATP Concentration

The effects on 45Ca influx of treatments that alter intra-cellular pH or the pH gradient across the plasma membraneare shown in Table I. There was no effect of changing externalpH or of application of butyric acid at concentrations thatwould be expected to acidify the cytoplasm by up to 1 pHunit (23). There was a significant inhibition of 45Ca influx byNH4, but not by methylamine. Because NH4 and methyla-mine normally have similar effects on cytoplasmic pH, itseems unlikely that the reduction in influx in the presence ofNH4 is solely a response to changing intracellular pH.

Metabolic inhibition by carbonylcyanide-m-chloropheny-lhydrazone resulted in a 214% increase in influx (Table I).This was accompanied by a slowing of the streaming ratefrom 76 ± 2 to 6 ± 6 jim s-', which is consistent with a fall incellular ATP content of approximately 90% (24). However,dicyclohexylcarbodiimide, which reduced the streaming rateto 34 ± 4 um s- , had no significant effect on influx. It shouldbe noted that metabolic inhibitors could increase net Ca2+uptake without inhibiting influx if the reduced ATP concen-

tration slowed the active efflux of Ca2' at the plasmamembrane.

Darkness, which reduces the ATP concentration in Charaby 10 to 20% (24) and slightly acidifies the cytoplasm (21),inhibited 45Ca influx by nearly 50% (Table I).

Channel Blockers

Although the uptake of calcium by mature isolated inter-nodal cells was generally quite low by comparison withactively expanding cells, there was still a considerable reduc-

Table I. Effects of Various Treatments on Calcium Influx into Internodal Cells of Chara corallina

Pretreatment 45Ca influxConditions Percenttime Control Treatment

min nmol m-2 s-

Metabolic inhibitorsCCCP' (0.01 mM), pHs 30 0.44 ± 0.10 0.94 ± 0.16 214DCCDb (0.05 mM) 120 0.52 ± 0.16 nsc

AminesMethylamine (0.2 mM) 240 0.54 ± 0.16 0.44 ± 0.10 nsNH4 (0.2 mM) 240 0.20 ± 0.04 37

Weak acidButyrate (0.5 mM), pHs 30 0.46 ± 0.08 0.38 ± 0.06 ns

(2 mM), pHs 30 0.36 ± 0.10 nsLight/dark

Light -- dark 60 0.44 ± 0.06 0.24 ± 0.04 55External pHpH 5 60 0.22 ± 0.04dpH 7 60 0.20 ± 0.04dpH 9.2 60 0.22 ± 0.04dpH 10.4 60 0.28 ± 0.06da CCCp, Carbonylcyanide-m-chlorophenylhydrazone. b DCCD, Dicyclohexylcarbodiimide.

c ns, Not significant. d These experiments were done using the half cell loading technique, whichdoes not depend on La3+ rinsing, because the latter is not possible at pH > 8 because of theinsolubility of La3+ salts. All other experiments were done by segment loading.

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CALCIUM INFLUX IN CHARA

tion in influx following addition of low concentrations ofLa3" (Table II). Higher concentrations of La3" did not reducethe flux further, but acted more quickly. The La3"-insensitiveflux measured by segment loading was around 0.14 nmolm-2 s-i compared to the lowest influxes in untreated cells of0.20 nmol m-2 s-1 and more than 4 nmol m-2 s-1 in cells inhigh K+ solution (Fig. 5).The organic channel blockers nifedipine, bepridil, and ver-

apamil did not inhibit 45Ca influx (Table II). Verapamil hadno effect, whereas bepridil and nifedipine actually increasedthe influx. Verapamil and bepridil caused cells to die ifapplied at 0.05 mm for more than 30 min.

DISCUSSION

Flux Methods

The methods normally used to measure tracer fluxes inturgid plant cells are not feasible for ions whose binding inthe cell wall represents a significant proportion of the cell-associated radioactivity. For Chara, 45Ca in the cell wallexceeds that in the cell for 4 h of rinsing, and it is likely thatduring this time, fast exchanging cytoplasmic compartmentsare lost from the cell, leading to an underestimate of theplasma membrane flux. It is possible with charophytes toseparate the cell wall from the contents, but at short rinsetimes, there appears to be significant contamination of thecytoplasm and vacuole from extracellular 45Ca. We haveshown previously (22) that most of this contamination occurs

via contact of cell sap and cell wall at the cut ends duringinsertion of the syringe needle, which was used to blow outthe vacuole and flush out the cytoplasm. However, withsegment loading, a syringe can be introduced into an uncon-

taminated end portion of the cell and the cell contents re-

moved quickly to minimize exchange with wall-bound cal-cium. We found this method to be unreliable for rinse timesof 1 min or less (22), but after 4 min of rinsing, contaminationappears to be reduced to an acceptable level.

Ca2' Fluxes Across the Plasma Membrane and Tonoplast

There has been some doubt expressed about the feasibilityof measuring unidirectional influxes of Ca2' across theplasma membrane (see, for example, Wrona et al. [28]) be-cause the small pool size of free Ca2+ in the cytoplasm might

equilibrate too rapidly with 45Ca in the external solution.Efflux of 45Ca would then cause the influx to be underesti-mated. The plasma membrane influx will only be significantlyunderestimated if (a) the cytoplasmic free Ca2" becomeslabeled rapidly and (b) the efflux from the cell is largecompared to fluxes to intracellular compartments. Althoughthere may be only a very small pool of free Ca2" in thecytoplasm (approximately 0.2 yM [17]), it is likely that thispool is buffered by a much larger amount of Ca2" bound inthe free cytoplasm, possibly as much as 60 /.M (1). In addition,there appears to be a larger amount of Ca2" in a more slowlyexchanging pool, possibly within cytoplasmic organelles. 4"Caaccumulation in the cytoplasm (Fig. 1) indicated a moderatelyfast phase with a content of less than 20 gM, and a slowerphase with at least another 40 glM (based on a surface area/volume ratio of 5 x 103 m2/m3 and assuming 5% cytoplasm).The specific activity of a small pool of Ca2" in the cytoplasmthat is exchanging with large pools of nonradioactive Ca2" inthe ground cytoplasm, organelles, and vacuole, as well as

with radioactive Ca2+ in the external solution, will be deter-mined by the ratios of the radioactive and nonradioactivefluxes/exchanges. Our data do not indicate rapid equilibra-tion of cytoplasmic Ca2+ with 45Ca in the external solution.Moreover, the lag in the development of the vacuolar influxis inconsistent with rapid equilibration.

In relation to the size of the efflux from the cell, we reportedpreviously that the efflux at the plasma membrane was of a

magnitude similar to the influx from the cytoplasm to thevacuole. There is no evidence from the current work tosupport a large efflux from the cell or rapid cycling of Ca2"across the plasma membrane. Efflux must necessarily proceedagainst a considerable electrochemical gradient across theplasma membrane. The gradient for transport across thetonoplast is considerably smaller because of the smaller elec-trical potential difference across this membrane.Having considered these arguments, we are reasonably

confident that the early part of the time course shown inFigure 1 reflects the unidirectional influx across the plasma-lemma. The influx measured in the first 15 min of 0.36 nmolm-2 s-' is slightly higher than was reported previously usingthe half cell loading method (22). After 30 min, the influxwas lower by 33% than that measured at 5 to 15 min. Someof the reduction may be due to the decreasing effect ofresidual cell wall contamination as the intracellular 45Ca

Table II. Effects of Channel Blockers on Calcium Influx into Internodal Cells of Chara corallinaAll fluxes measured over 30 min using segment loading with a 4- to 5-min rinse. Verapamil and

LaCI3 experiments were conducted in low light (5 Amol m-2 s-'). The light intensity in the bepridilexperiment was approximately 45,umol m-2 s-'. ns, Not significant.

Pretreatment 45Ca InfluxConditions PercentTime Control Treatment

min nmol m-2 S-1

Verapamil (0.05 mM) 20 0.24 ± 0.04 0.28 ± 0.10 nsLaCI3 (0.10 mM) 60 0.48 ± 0.14 0.14 ± 0.04 29Nifedipine (0.05 mM)Low light 20 0.36 ± 0.02 0.42 ± 0.12 nsDarkness 0 0.24 ± 0.04 0.62 ± 0.12 258

Bepridil (0.05 mM) 0 0.24 ± 0.03 0.88 ± 0.40 200

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Plant Physiol. Vol. 100, 1992

activity increased, whereas some efflux of label is alsopossible.The flux of 45Ca from the external solution to the vacuole

was approximately 0.1 to 0.2 nmol m-2 s-1 (Figs. 1 and 2).The actual Ca2" flux across the tonoplast will be higher thanthese values by the ratio of the specific activity of 45Ca in theexternal solution to the specific activity of 45Ca in the freecytoplasm; i.e. if the specific activity of Ca21 in the cytoplasmwas half that in the external solution, the tonoplast influxwould be double the tracer influx, or 0.2 to 0.4 nmol m-2 s-1,which is similar to the plasma membrane influx.

Calcium Influx and Growth

Calcium influx to the vacuole in Chara appears to be closelyrelated to growth. Expanding cells have high influxes, but inrapidly growing plants, mature internodal cells further downthe plant also have higher influxes than in slow-growingplants. This implies that symplasmic transport of calcium tothe shoot may also be important. In Chara, there is relativelyfree access between adjoining cells for small solutes (3, 20,29), and long distance transport is aided by protoplasmicstreaming. Ding et al. (5) have recently demonstrated polartransport of photoassimilates in Chara. However, plasmodes-mata close almost immediately after internodal cells are iso-lated (20), and it is, therefore, not surprising that calciuminflux decreases with time after cutting.

Regulation of Calcium Influx

There is a large electrochemical gradient for calcium entryto the cell, and calcium influx as a passive uniport throughmembrane channels, as in animal cells, seems the most likelymechanism. The question of whether there are specific chan-nels for Ca2', and if so how much of the flux is through thesechannels, has not been resolved by the current study. If theflux is predominantly mediated by channels, then it is appar-ent that most of the channels that pass Ca2+ in C. corallinaare closed under normal conditions. We have identified highexternal concentrations of Ca2' and K+ as conditions thatincrease the permeability of the plasma membrane to calcium.The correspondence between the K+ concentration requiredto stimulate calcium influx and the concentration of K+ thatwould normally depolarize the plasma membrane (refs. 2,15; R. J. Reid, unpublished results) point to voltage controlof calcium influx, as in animal cells (17). However, there aresome inconsistencies in the hypothesis that voltage is thedominant factor regulating calcium influx through channelsin Chara. The small effects on calcium influx of carbonyl-cyanide-m-chlorophenylhydrazone, the lack of response todicyclohexylcarbodiimide, and the fall-off in influx at veryhigh K+ concentrations are all difficult to reconcile withvoltage control of calcium channels. A detailed investigationof membrane potential difference and conductance underconditions that favor calcium influx is clearly indicated.Of the other possible control factors, severe metabolic

inhibition increased the influx, whereas darkness decreasedthe influx. However, these changes were modest in compar-ison to those caused by K+ and Ca2" . In the light of a bodyof literature on plant cells in which changes in cytoplasmic

pH were correlated with changes in intracellular free calcium(7-11, 18), with a general tendency for pCa to increase aspHcy, increased and vice versa, the effect on 45Ca influx oftreatments known to substantially alter cytoplasmic pH inChara was examined. Application of weak acids and bases oraltering external pH (Table I) had very little influence oninflux, and it must therefore be concluded that changes infree calcium in the cytoplasm with changes in pHcyt areintracellular in origin, quite possibly the effect of competitivebinding of H+ to calcium buffers in the cytoplasm.The higher calcium2" influxes into expanding cells indicate

either that there is a higher density of channels or thatchannels are open more often, or a combination of the two,than in mature cells.

Channel Blockers

The organic channel blockers nifedipine, verapamil, andbepridil have been shown to be effective in blocking calciumchannels in animal cells (13, 27), and there have been anumber of reports of effects of nifedipine and verapamil onvarious aspects of the physiology of plant cells (e.g. refs. 3,4, 6, 19). MacRobbie and Banfield (16) obtained variableeffects of nifedipine on 45Ca influx in Chara, but we havepreviously shown (22) that their flux methods do not distin-guish between fluxes into cells and extracellular binding, andtheir calcium fluxes were an order of magnitude higher thanthose reported here, suggesting considerable extracellularcontamination. Tester and MacRobbie (26) noted inhibitionby nifedipine of the inward current during the action poten-tial in Chara, which was consistent with blockage of calciumchannels in the plasma membrane. They found no effect ofmethoxyverapamil and a stimulation of the inward currentby bepridil. We found that none of these agents reducedcalcium influx. Verapamil and bepridil were toxic to cells, butthis is unlikely to be due to reduced calcium influx becausecells remain viable for at least several weeks in solutionscontaining La3" at concentrations high enough to substan-tially reduce calcium influx. Bepridil and verapamil clearlyhave severe side effects in Chara that are unrelated to theirperceived mode of action, and caution should be exercisedwith other plant cells in interpreting responses to organicchannel blockers in terms of inhibition of calcium influx; onthe basis of the results shown above, a stimulation of influxis more likely.

ACKNOWLEDGMENTS

This work was supported by the Australian Research Council. Theauthors wish to thank Patrick Kee for his excellent technical assist-ance and M. Tester for useful discussions on the experiments andcomments on the manuscript.

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CALCIUM INFLUX IN CHARA

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