Effects of Cations on the Cytoplasmic pH of Chara corallina

Smith, F. A. and Gibson, J.–L. 1985. Effects of cationson the cytoplasmic pH of Chara corallina.—J.exp. Bot.36: 1331–1340 Removal of external Ca²⁺ from cells of Chara corallina lowersthe cytoplasmic pH, as determined by the intracellular distributionof the weak acid 5,5–dimethyloxazolidine2–,4–dione(DM0), when the external pH is below about 60. This effect isreversed, at least partially, by addition of the following cationsto Ca²⁺-free solutions: tetraethylammonium (TEA⁺) and Na⁺ at5 or 10 mol m^-3, Li⁺ and Cs⁺ (10 mol m^-3), or Mg²⁺, Mn²⁺ andLa³⁺ (02 or 05 mol m^-3). Under the same conditions, increasesin pH sometimes, but not always, occur in the presence of 10mol m^-3 K⁺ or Rb⁺ The results are discussed in relation to the major transportprocesses that determine pH and the electric potential differenceacross the plasma membrane, namely fluxes of H⁺ and of K⁺. Thesimplest explanation of the effects of the various cations testedin this study is that they primarily affect pHi_c via changesin influx of H⁺ but direct effects on the H⁺ pump or on K⁺ fluxesmay also be involved Key words: Chara corallina, cytoplasmic pH, cations, H⁺transport     

Salinity-induced Malate Accumulation in Chara

Ion absorption by Chara corallina from solutions containingpredominantly KC1 or RbCl at up to 100 mol m^–3 resultedin accumulation of salts and turgor regulation. Turgor regulationdid not occur in solutions containing Na⁺ or Li⁺salts. Duringion absorption from various salts of K⁺ and Rb⁺ vacuolar cationconcentration exceeded Cl^– concentration. This differencewas shown to be balanced by the synthesis and accumulation ofmalate. Vacuolar malate concentration reached 48 mol m₃^–,with accumulation occurring at rates of up to 0.45 mol m^–3h^–1. Malate accumulation was inhibited by low externalpH and was dependent upon external HCO₃^– concentration.The synthesis of malic acid and its subsequent dissociationimposed a severe acid load on the cell. Biophysical regulationof cellular pH was achieved by a H⁺efflux at a rate of about40 nmol m^–2 s^–1from the cell. The results presentedargue against cytoplasmic Cl^–, HCO₃^– or pH regulatingmalate accumulation in Chara and it is suggested that malatetransport across the tonoplast may regulate malate accumulation. Key words: Malate, Chara corallina, pH regulation, salinity     

Regulation of the Cytoplasmic pH of Chara corallina in the Absence of External Ca2+: Its Significance in Relation to the Activity and Control of the H+ Pump

Effects of removal of external Ca²⁺ on the cytoplasmic pH (pH_c)of Chara corallina have been measured with the weak acid 5,5-dimethyl-oxazolidine-2,4-dione(DMO) as a function of external pH (pH₀) and of the externalconcentration of K⁺. Removal of Ca²⁺ always decreased pH_c whenpH₀ was below about 6.0; the decrease was about 0.2–0.4units at pH₀ 5.0, increasing to about 0.5 units at pH₀ 4.3.When pH₀ was 6.0 or higher the removal of Ca²⁺ had little orno effect on pH_c. This situation was not altered by changingthe concentration of K⁺, though in some experiments at pH₀ 5.0–5.2there was a slight decrease in pH₀ (about 0.2 units) when K⁺was increased from 0.2 to 2.0 mol m^–3, an effect apparentlyreversed when K⁺ was higher (5.0 or 10.0 mol m^–3). Theresults suggest that H⁺ transport continues in the absence ofexternal Ca²⁺, despite previous suggestions to the contrary,and that the H⁺ pump does not necessarily run near thermodynamicequilibrium with its chemical driving reaction. They indicate,rather, that the H⁺ pump is under kinetic control and providefurther evidence for the inadequacy of present models for theoperation of the H⁺ pump in charophyte cells, especially inrelation to its proposed role in regulating pH_c. Key words: Chara corallina, Cytoplasmic pH, Calcium     

Uptake of Imidazole and its Effects on the Intracellular pH and Ionic Relations of Chara corallina

Ammonia (pK_a 9.25) and methylamine (pK_a, 10.65) increase cytoplasmicpH and stimulate Cl^– influx in Chara corallina, theseeffects being associated with influx of the amine cations ona specific porter. The weak base imidazole (pK_a 6.96) has similareffects but diffuses passively into the cell both as an unionizedbase and as a cation. When the external pH is greater than 6.0influx of the unionized species predominates. Imidazole accumulates to high concentrations in the vacuole,where it is protonated. Cytoplasmic pH and vacuolar pH riseby only 0.2–0.3 units, suggesting a large balancing protoninflux across the plasma membrane. Balance of electric chargeis partially maintained by net efflux of K⁺ and net influx ofCl^–. Calculation of vacuolar concentrations of imidazole(from (¹⁴C] imidazole uptake, assuming that there is no metabolism)plus K⁺ and Na⁺ indicates an excess of cations over inorganicanions (Cl^–). However, although the osmotic potentialof the cells increases, also indicating increased solute concentrations,the increase is less than that predicted by the calculated ionicconcentrations. This discrepancy remains to be resolved. Becausethe osmotic potential also increases when imidazole is absorbedfrom Cl^–-free solutions it is likely that maintenanceof charge-balance can also involve synthesis and vacuolar storageof organic or amino acids. Key words: Imidazole, potassium, intracellular pH, membrane transport, Chara     

Na+ fluxes in Chara under salt stress

The influx and efflux of Na⁺ across the plasma membrane of Characorallina and Chara longifolia were examined under mild saltstress conditions. Na⁺ influx was found to be rapid in bothspecies with the freely exchangeable cytoplasmic Na⁺ cominginto isotopic equilibrium with external ²²Na⁺ within 1 h ofexposure to isotope. Cytoplasmlc Na⁺ concentration and Na⁺ influxwere greater in C. corallina than in C. longifolla under thesame conditions. Na⁺ influx across the tonoplast was much lowerthan the flux across the plasma membrane. Na⁺ efflux was stimulatedat pH 5 relative to pH 7 by 218% in C. coralllna and 320% inC. longifolia. In both species externally applied Li⁺ inhibitedNa⁺ efflux at pH 5 but not at pH 7. Na⁺ etflux was not significantlyinhibited by amiloride. Key words: Na⁺ influx, Na⁺ efflux, Na⁺/H⁺ antiport, Chara     

The Influence of Ca2+ and K+ on H14CO3-Influx in Internodal Cells of Chara corallina

The influences of Ca²⁺-free solutions and increasing K⁺ concentrationson the H¹⁴CO^–₃ influx capacity of Chara corallina wereinvestigated. It was found that contact with Ca^2–freesolutions resulted in a gradual reduction in the H¹⁴CO^–₃influx capacity of these cells. Recovery of this influx capacity,following the return of Ca²⁺ to the experimental solution, followeda ‘mirror-image’ of the time course of decay. Potassium concentrations above a certain critical value (2 mM)induced a rapid reduction in H¹⁴CO^–₃ influx capacity.Normal activity was recovered within 60–90 min followingthe return to 0.2 mMK⁺ solutions. It was also shown that 10mM K⁺ can be used to determine the relative contribution of¹⁴C supplied by diffusion of ¹⁴CO₂ and transport of H¹⁴CO^–₃.The Ca²⁺ and K⁺ results are discussed in relation to the effectsof these treatments on the electrical properties of the plasmalemma.     

Water channels in Chara corallina

Water relations parameters ofChara corallina inter-nodes weremeasured using the single cell pressure probe. The effect ofmercurials, which are recognized as non-specific water channelinhibitors, was examined. HgCl₂ concentrations greater than5 mmol m^–3 were found to inhibit hydraulic conductivity{Lp) close to 90%, whereas pCMPS was found to have no effecton Lp. The activation energy of water flow was increased significantlyfrom 21.0 kJ mol^–1 to 45.6 kJ mol^–1, following theapplication of HgCl₂. These results are in accordance with evidencefor Hg²⁺sensitive water channels in the plasma membrane of charophytes(Henzler and Steudle, 1995; Tazawa et al., 1996). The metaboliceffects must, however, be considered in view of the rapid inhibitionof respiration and the depolarization of the membrane potentialwith HgCl₂ concentrations lower than those found to affect Lp.It was possible to measure simultaneously water relations andmembrane PD, in order to examine the contribution of potassiumchannels to Lp. Cells were induced into a K⁺ permeable state.The K⁺ channels, assumed to be open, were subsequently blockedby various blockers. No significant difference in Lp was foundfor any of these treatments. Finally, the permeability of C.corallina membranes to ethanol was examined. HgCl₂ was foundto cause a decrease in reflection coefficient, coinciding witha decrease in Lp, but there was no change in the ethanol permeabilitycoefficient. This has been interpreted in terms of both thefrictional model and composite model of non-electrolyte membranetransport. Key words: Water channels, Chara, hydraulic, conductivity, membrane transport models, reflection coefficient     

Ion Composition of the Chara Internode

Ion compositions of the cytoplasm and the vacuole of Chara australiswere analyzed according to Kishimoto and Tazawa (1964) and Kiyosawa(1979a). The ions in the cytoplasm and the vacuole analyzedwere K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^–, NO₃^– and H₂PO₄^–.Assuming that the volume of the cytoplasm V_p is 10% of thatof the whole cell V, the concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺,Cl^–, NO₃^– and H₂PO₄^– in the cytoplasm averaged70, 15, 13, 4.6, 31, 2.2 and 16 mM, respectively. If the volumeof the cytoplasm was assumed to be 5% of that of the whole cell,their averaged concentrations were 139, 31, 25, 9.2, 62, 4.4and 33 mM, respectively. The averaged ion compositions of thecell sap were K⁺, 111; Na⁺, 47; Ca²⁺, 4.4; Mg²⁺, 8.9; Cl^–,91; NO^–₃, 3.3 and H₂PO₄^–, 6.0 mM. These values,taking the concentrations and the charges of the protein (Kiyosawa1979b) and amino acids (Sakano and Tazawa 1984) into accountand assuming the presence of some uni- or oligovalent anionsand/or small nonelectrolyte molecules, could explain fairlywell both the electroneutrality and the osmotic pressure ofthe cell, except when V_p/V = 5%. (Received May 18, 1987; Accepted September 29, 1987)     

Potassium Transport Across the Membranes of Chara: IV. INTERACTIONS WITH OTHER CATIONS

Smith, J. R. and Kerr, R. J. 1987. Potassium transport acrossthe membranes of Chara. IV. Interactions with other cations.—J.exp. Bot. 38: 788–799. The ⁴²K influx () and the membrane electrical conductance (G_m were measured simultaneously forintemodal cells of Chara australis bathed in solutions containingdifferent concentrations of various cationic species. It wasfound that the potassium permeability (P_k,) of the membranewas reduced significantly when the bathing [CaSO₄ exceeded 01mol m^–1. Concentrations of tetra-ethylammonium ions (TEA)exceeding 0?3 mol m^–3 were found to reduce significantlyboth and , but even high concentrations (10 mol m^–3)usually did not reduce the fluxes by more than a factor of 3.Na⁺ ions were found to be capable of reducing P_K by a factorof 5?6 to a value of 4 nm s^–1. This appeared to be aminimum value for P_K which was not reduced even if several inhibitorycations were present simultaneously. This suggests that possiblyonly one of two different modes of K⁺ transport can be inhibitedby cations. The possible geometry of the inhibitable K⁺ channelis briefly discussed and the implications of the presence ofNa⁺ and Ca²⁺ ions in many common bathing solutions are considered. Key words: Potassium, calcium, tetraethylammonium, inhibition     

Biophysical and Biochemical Regulation of Cytoplasmic pH in Chara corallina during Acid Loads

The contribution of membrane transport to regulation of cytoplasmicpH in Chara corallina has been measured during proton-loadingby uptake of butyric acid. In the short-term (i.e. up to 20min) uptake of butyric acid is not affected by removal of externalK⁺, Na⁺ or Cl^– but over longer periods uptake is decreased(by 20–50% in different experiments) in the absence ofexternal Na⁺ or, sometimes, K⁺. Influxes of both Na⁺ and K⁺increase temporarily after addition of butyrate, Na⁺ immediatelyand K⁺ after a lag. Effects on Cl^– influx are small butCl^– efflux increases enormously after a short lag. Anapproximate comparison of internal butyrate with changes inthe concentration of K⁺, Na⁺, and Cl^– suggests that initially(i.e. for a few min) cytoplasmic pH is determined by bufferingand possibly by some decarboxylation of organic acids (biochemicalpH regulation), and that biophysical pH regulation involvingefflux of H⁺ balanced by influxes of K⁺, Na⁺ and especiallyefflux of Cl^– progressively becomes dominant. When butyric acid is washed out of the cells, cytoplasmic pHis restored completely or partially (depending on the butyrateconcentration used) and this is independent of the presenceor absence of external Cl^–. Where Cl^– is present,its influx is relatively small. It is suggested that cytoplasmicpH is then controlled biochemically, involving the synthesisof an (unidentified) organic acid and the accumulation of acidicanions in place of butyurate lost from the cell. During thesecond application of butyrate, net Cl^– efflux is small:it is suggested that control of cytoplasmic pH then involvesdecarboxylation of the organic acid anions. The questions of the source of Cl^– lost from the cell(cytoplasm or vacuole) and of possible cytoplasmic swellingassociated with the accumulation of butyrate are discussed. Key words: Chara corallina, butyric acid, cytoplasmic pH, membrane transport     

Calcium-salinity interactions affect ion transport in Chara corallina

Detached internodes of Chara corallina survived in solutions containing 100 mol m^?3 NaCl when the external concentration of Ca²⁺ was greater than 1 mol m^?3. Na⁺ influx was roughly proportional to external Na⁺ up to 100 mol m^?3 NaCl. Na⁺ influx involved two components: a Ca²⁺-insensitive influx which allowed the passage of Na⁺ independently of external Ca²⁺; and a Ca²⁺-inhibitable mechanism where Na⁺ influx was inversely proportional to external Ca²⁺. The Ca²⁺-inhibitable Na⁺ influx was similar to the Ca²⁺-inhibitable K⁺ influx. Mg²⁺ and Ba²⁺ were able to substitute for Ca²⁺ in partially inhibiting Na⁺ influx in the absence of external Ca²⁺. The effect of Ca²⁺ appears specific to Na⁺ and K⁺ influx since the effects of a Ca²⁺-free solution on the influx of some other cations, anions and neutral compounds is small. It is suggested that Na⁺ influx via the Ca²⁺-inhibitable mechanism represents Na⁺ leakage through K⁺ channels and that cell death at high salinity occurs due to a cytotoxic Na⁺ influx via this mechanism.     

The effect of divalent cations on Na+ tolerance in Charophytes. II: Chara corallina

Abstract The freshwater Charophyte Chora corallina dies when subjected to 70 molm^?3 NaCl if the Ca²⁺ concentration is 0.1 mol m ^?3. This stress is accompanied by a depolarization of the cell to a membrane potential more positive than E_K, a net influx of Na⁺ into the vacuole, and a net loss of K⁺ from the vacuole. Raising the Ca²⁺ concentration to 7 mol m ^?3 in the presence of elevated Na⁺ restores the Na⁺ to Ca²⁺ ratio to 10: 1 as in the control solution, and results in enhanced survival even though turgor is not regulated. Mg²⁺ is not a good substitute for Ca²⁺. It is suggested that the main reason that C. corallina fails to occupy saline habitats is its failure to regulate turgor, not sensitivity to Na ⁺, since the latter is similar to that seen in C. buckellii, which is found in saline habitats.     

The Permeability of Ammonia, Methylamine and Ethylamine in the Charophyte Chara corallina (C. australis)

Ritchie, R. J. 1987. The permeability of ammonia, methylamineand ethylamine in the charophyte Chara corallina (C. australis).—J.exp. Bot. 38: 67–76 The permeabilities of the amines, ammonia (NH₃), methylamine(CH₃NH₂) and ethylamine (CH₃CH₂NH₂) in the giant-celled charophyteChara corallina (C. australis) R.Br. have been measured andcompared. The permeabilities were corrected for uptake fluxesof the amine cations. Based on net uptake rates, the permeabilityof ammonia was 6?4?0?93 µm s^–1 (n = 38). The permeabilitiesof methylamine and ethylamine were measured in net and exchangeflux experiments. The permeabilities of methylamine were notsignificantly different in net and exchange experiments, norto that of ammonia (P_methylamine = 6?0?0?49 µm s^–1(n = 44)). In net flux experiments the apparent permeabilityof ethylamine was slightly greater than that of ammonia andmethylamine (P_{ethylamine, net} = 8?4?1?2 µm s^–1 (n= 40)) but the permeability of ethylamine based on exchangeflux data was significantly higher (P_{ethylamine, exchange} =14?1?2 µm s^–1 (n = 20)). Methylamine can be validlyused as an ammonium analogue in permeability studies in Chara. The plasmalemma of Chara has acid and alkaline bands; littlediffusion of uncharged amines would occur across the acid bands.The actual permeability of amines across the alkaline bandsis probably about twice the values quoted above on a whole cellbasis i.e. the permeability of ammonia across the permeablepart of the plasmalemma is probably about 12 µm s^–1. Key words: Chara, permeability, ammonia, methylamine     

Potassium Channels at Chara Plasmalemma

Exposure to high K⁺ medium transforms Chara plasmalemma into[K⁺]_osensitive state (K⁺ state). The current-voltage (I/V)characteristicsunder such conditions display a negative conductance region.This feature results from the complex time and voltage dependenceof K⁺ channel opening At potentials more negative than a thresholdp.d. the channels are closed and the I/V characteristics becomelinear with a low slope conductance of 0.8 S m² and only a weakdependence on [K⁺]_o. Such behaviour is usually associated witha non-specific leak current The threshold level for K⁺ channelclosing depends on [K⁺]_o. In 2.0 mol m^–3 and 5.0 mol m^–3K⁺ medium the membrane resting p.d. follows E_K, but hyperpolarizesgradually if the [K⁺]_o is lowered. The proton pump thus appearsto be non-operative, while the cell is in the K⁺ state, andrecovers slowly as the cell is returned to a low K⁺ medium.Excitation currents decline if the cells are kept in K⁺ statefor some hours. Key words: K⁺ channels, Chara corallina, Proton pump, Current/, oltage characteristics, Conductance     

Cl- Fluxes during Osmoregulation in Chara

Measurements of Cl^– influx in cells of Chara corallinashow that control of this flux contributes to the ability ofthis cell to regulate its osmotic pressure. Transcellular osmosiswas used to generate cell fragments with abnormally high ₁,(H-cells), and with abnormally low ₁, (L-cells). Plasmalemmainflux (_oc) was very high in L-cells, and markedly reduced inH-cells. Influx was not affected by the presence of sucrosein the pond water and the consequent reduction in turgor. InH-cells the chloride flux from cytoplasm to vacuole (_cv) wasalso strongly inhibited. It is suggested that control of Cl^–fluxes at both plasmalemma and tonoplast is involved in osmoregulationin these cells. Key words: Chara corallina, osmoregulation, Cl^– flux     

Effect of Cytoplasmic Ca2+ on the Membrane Potential and Membrane Resistance of Chara Plasmalemma

Effects of cytoplasmic Ca²⁺ on the electrical properties ofthe plasma membrane were investigated in tonoplast-free cellsof Chara australis that had been internally perfused with media,containing either 1 mM ATP to fuel the electrogenic pump orhexokinase and glucose to deplete the ATP and stop the pump. In the presence of ATP, cytoplasmic Ca²⁺ up to 2.5?10^–5M did not affect the membrane potential (about -190 mV), butmembrane resistance decreased uniformly with increasing [Ca²⁺]_i.In the absence of ATP, the membrane potential, which was onlyabout -110 mV, was depolarized further by raising [Ca²⁺]_i from1.4?10^–6 to 2.5?10^–5 M. Membrane resistance, whichwas nearly the twofold that of ATP-provided cells, decreasedmarkedly with an increase in [Ca²⁺]_i from zero to 1.38?10^–6M, but showed no change for further increases. Internodal cellsof Nitellopsis obtusa were more sensitive to intracellular Ca²⁺with respect to membrane potential than were those of Charaaustralis, reconfirming the results obtained by Mimura and Tazawa(1983). The effect of cytoplasmic Ca²⁺ on the ATP-dependent H⁺ effluxwas measured. No marked difference in H⁺ effluxes was detectedbetween zero and 2.5?10^–5 M [Ca²⁺]_i; but, at 10^–4M the ATP-dependent H⁺ efflux was almost zero. Ca²⁺ efflux experimentswere done to investigate dependencies on [Ca²⁺]_i and [ATP]_i.The efflux was about 1 pmol cm^–2 s^–1 at all [Ca²⁺]_iconcentrations tested (1.38?10^–6, 2.5?10^–5, 10^–4M).This value is much higher than the influx reported by Hayamaet al. (1979), and this efflux was independent of [ATP]_i. Thepossibility of a Ca²⁺-extruding pump is discussed. ¹ Present address: Botanisches Institut der Universit?t Bonn,Venusbergweg 22, 5300 Bonn, F.R.G. (Received September 22, 1984; Accepted February 19, 1985)     

Effect of Ca2+ on Tonoplast Potential of Permeabilized Characeae Cells

Using permeabilized characean cells in which the ionic conditionsat the cytoplasmic side of the tonoplast are easily controlled,effects of Ca²⁺ ion on tonoplast potential were examined. Whenthe cell was treated with 1 µM Ca²⁺, the tonoplast potential(E_M became positive in a complicated manner in Chara corallinawhile it simply became negative in Nitella axilliformis. Whenthe cell was treated with 9-antracenecarboxylic acid, a Cl^–-channelinhibitor, E_m became more negative and the response of E_m toCa²⁺ was significantly suppressed. It is suggested that Ca²⁺activates Cl^–-channel at a low concentration and inactivatesat a higher one in C. corallina while it simply inactivate Cl^–-channelin N. axilliformis. ¹Present address: Biological Laboratory, The University of theAir, Wakaba 2-11, Wakaba, 260 Japan. (Received August 22, 1988; Accepted December 26, 1988)     

Entry of Methylammonium and Ammonium Ions into Chara Internodal Cells

Low concentrations of ammonia and methylamine greatly affectmembrane transport by, and the electrical properties of, cellsof Chara corallina (=C. australis). In the presence of theseamines, influx of Cl^– and efflux of K⁺ increase and alarge depolarizing current flows through the cell membrane. Measurements with [¹⁴C]methylamine show that methylamine isabsorbed rapidly over a wide pH range, and that the absorptionisotherm is complex. Methylamine influx is not affected by presenceor absence of Cl^–, K⁺, or Na⁺, but is decreased by additionof . The depolarizing current is associated with a small increase in membrane conductance, except at highpH, and both these effects are reversible. The current showssaturation with increasing amine concentration; when methylamineis 10–12 times more concentrated than ammonia, it producesa current of the same magnitude. The results show that the amines enter the cells as cations( or CH₃) except where external pH is high, and that a specific, selective electrogenicporter is involved. There is no need to invoke active transport,as there is no evidence that internal amine concentrations exceedthe equilibrium (Nernst) concentrations.     

Nitrate Transport into Chara corallina Cells using 36ClO-3 as an Analogue for Nitrate: I. INTERACTION BETWEEN 36ClO-3 AND NO-3 AND CHARACTERIZATION OF 36CIO-3/NO-3 INFLUX

The use of chlorate as an analogue for NO^–₃ during nitrateuptake into Chara corallina cells has been investigated. NO^–₃inhibits ³⁶C1O^–₃ influx into Chara over the concentrationrange 0–1000 mmol m^–3. Lineweaver-Burke plots ofthe data are characteristic of competitive inhibition by NO^–3in the low concentration range (0–300 mmol m^–3 ClO^–₃)and apparent K_INO3 is 140 mmol m^–3 which is of a similarorder of magnitude as apparent K_mCIO3- 180 mmol m^–3. Athigher substrate concentrations the inhibition by NO^–₃was not characteristic of competitive or uncompetitive inhibition. ³⁶C1O^–₃/NO^–₃ influx was dependent on K⁺ and Ca²⁺in the external medium and inhibited by FCCP. NO^–₃ pretreatmentor N starvation increased subsequent ³⁶C1O^–₃/NO^–₃influx into Chara. A comparison between rates of net NO^–₃uptake and ³⁶C1O^–₃/NO^–₃ influx supported the previoushypothesis that NO^–₃ efflux is an important componentin the determination of overall uptake rates. Key words: Nitrate, Chara, ³⁶CIO^–₃     

Membrane Potentials in Excitable Cells of Aldrovanda vesiculosa Trap-Lobes

The resting membrane potential in excitable cells of Aldrovandatrap-lobes is composed of diffusion and electrogenic potentials.The diffusion potential, about –100 mV in artificial pondwater, was determined from the external K⁺ and Na⁺ concentrations.The permeability ratio, P_Na/P_K of the membrane was estimatedto be about 0.3. The electrogenic potential hyperpolarized themembrane to about –140 mV. The peak value of the actionpotential increased by +26 mV with a tenfold increase in theexternal Ca²⁺ concentration. The action potential was blockedby an application of the Ca²⁺ chelater or the Ca channel blocker,LaCl₃. Cells showed additional Ca²⁺ influx (7.8 pmole/cm² impulse)during membrane excitation. These facts suggest that the transientincrease in Ca²⁺ influx causes the action potential presentin cells of Aldrovanda trap-lobes. ¹ Present address: Jerry Lewis Neuromuscular Research Center,School of Medicine, University of California Los Angeles, LosAngeles, CA90024, U.S.A. ² Present address: Biological Laboratory, Kyoritsu Women's University,Hachioji 193, Japan. (Received September 21, 1983; Accepted September 7, 1984)