Lipid Metabolism in the Brown Marine Algae Fucus vesiculosus and Ascophyllum nodosum

Lipid metabolism and environmental effects on this process havebeen studied in the marine brown algae Fucus vesiculosus andAscophyllum nodosum. These algae showed very similar patternsof lipid metabolism during 24 h incubations. Labelling from[1-¹⁴C]acetate showed the major labelled lipids to be the ß-alanineether lipid and the neutral lipid fraction in both algae. Ofthe glycolipids, only sulphoquinovosyldiacylglycerol was welllabelled and the phosphoglycerides were all poorly labelled.The major labelled fatty acids were palmitate and oleate, againin both algae, although Fucus vesiculosus also showed significantlabelling of stearate and behenate. Although the amount of fattyacid labelling increased with time, the proportion of labelin palmitate and oleate remained approximately constant. Verylong chain fatty acids (arachidic, behenic) were increasinglylabelled with time. Lowered incubation temperatures decreased labelling of the saturatedfatty acids. Cu²⁺ increased the proportion of oleate labelledin both algae, and of linoleate in Fucus vesiculosus. This cationdecreased the percentage labelling of stearate and myristatein Ascophyllum nodosum. Lipid metabolism in Ascophyllum nodosumwas more sensitive to raised Cu²⁺ levels than in Fucus vesiculosus Key words: Acyl lipid metabolism, Fucus vesiculosus, temperature effects, Ascophyllum nodosum, copper pollution     

The concentration of selected cations in normal and crown-gall tumors obtained from potato discs

Crown-gall tumor tissue obtained from potato discs inoculatedwith virulent strains of Agrobacterium tumefadens containedhigher concentrations of K⁺, Mg²⁺ and Ca²⁺ than the correspondingnormal tissue. These tumors also contained higher concentrationsof these cations than normal tissue inoculated with an avirulentstrain of Agrobacterium tumefadens, or than normal tissue adjacentto the crown-gall tumors on the same potato disc. The concentrationof these cations remained significantly higher than controltissue regardless of the tumor age. 2,4-Dinitrophenol and myo-inositol,while affecting the concentration of Ca²⁺ in these tissues,had no effect on the Mg²⁺ and K⁺ concentrations. These resultssuggest increased concentrations of certain cations may be aspecific property of crown-gall tumors. (Received August 16, 1978; )     

Effects of cations on the sugar receptor-site of the blowfly, Phormia

1) Ca^{+ +} (1 to 10 mM) lowered the binding affinity of sugarreceptor-site for sucrose in the labellar sugar receptor ofthe blowfly, Phormia regina, without changing the maximum-responseamplitude. It also elevated the values of the Hill coefficient(n_H) in some degrees. 2) Other divalent cations such as Mg^{+ +}, Ba^{+ +} or Cd^{+ +} alsoshowed almost the same property as above. The sequence of theeffect is as follows: Ba^{+ +}, Mg^{+ +} x Ca^{+ +} x Cd^{+ +}. Trivalentcation, La^{+ + +} (1 mM), changed the value of n_H from 1 (La^{++ +}-free) to 2. 3) On the contrary, the action of monovalent cations such asK⁺ or Na⁺, of which ionic strength was made the same as thatof the divalents hardly suppressed the response. 4) The results obtained do not support the hypothesis, at leaston the sugar receptor of the fly, that the receptor potentialis attributable to a change of the surface potential (zeta potential)as is proposed for the frog sugar receptor.     

Effect of Divalent Cations on Na+ Permeability of Chara corallina and Freshwater Grown Chara buckellii

The effect of elevated Na⁺ concentration on Na⁺ permeability(P_Na) and Na⁺ influx in the presence of two levels of externaldivalent cations was determined in Chara corallina and freshwater-culturedChara buckellii. When Na⁺ in the medium was increased from 1.0to 70 mol m^–3, Na⁺ influx increased in both species ifCa²⁺ was low (0.1 mol m–3). If Ca²⁺ was increased to 7.0mol m^–3 when Na⁺ was increased, Na⁺ influx remained atthe low control level in C. corallina, and showed only a temporaryincrease in C. buckellii. Mg²⁺ was a better substitute for Ca²⁺in C. buckellii than in C. corallina. Na⁺ permeability data suggest that when the external Ca²⁺ concentrationis low, P_Na does not increase in the presence of elevated NaCl;the increase in Na⁺ influx appears to be due to the increasein external Na⁺ concentration alone. Ca^{2 +} supplementation appearsto decrease P_Na whereas supplemental Mg²⁺ has no effect. Na⁺ effluxes were computed from previously determined net fluxesand the influxes. It was found that for both species, fluxesin both directions were stimulated in response to all experimentaltreatments, but Na⁺ influx always exceeded efflux. This resultedin net Na⁺ accumulation in the vacuoles of both species. The results are discussed with reference to net flux and electrophysiologicaldata obtained previously under identical conditions, as wellas the comparative salinity tolerance of both species and theNa⁺/divalent cation ratio. Key words: Na⁺ influx, Na⁺ tolerance, membrane potential, permeability, Chara     

Effect of Substituted Pyridazinones on Chloroplast Structure and Lipid Metabolism in Greening Barley Leaves

The effect of the substituted pyridazinone herbicides, Sandoz9785 and Sandoz 6706, on lipid metabolism was studied in greeningbarley leaves. The herbicides had no effect on chlorophyll formationbut caused an altered chloroplast morphology during greening.In leaves supplied with {¹⁴C} acetate, Sandoz 9785 decreasedincorporation of radioactivity into linolenate while Sandoz6706 decreased incorporation into both linolenate and trans-3-hexadecenoate.Decreased linolenate labelling was accompanied by an accumulationof {¹⁴C}linolenate in diacylgalactosylglycerol. {¹⁴C}Palmitateaccumulated in phosphatidylglycerol when synthesis of trans-3-hexadecenoatewas inhibited. The results are discussed in relation to thefunction of acyl lipids in fatty acid desaturation and the roleof lipids in chloroplast morphology. Key words: Chloroplast structure, Lipid synthesis, Substituted pyridazinones, Fatty acid desaturation     

Changes in Endogenous Fatty Acids and Lipid Synthesis Associated with Copper Pollution in Fucus spp

Fatty acid synthesis from [¹⁴C]acetate was studied in the brownalgae, Fucus serratus and Fucus vesiculosus, which had beencollected from sites polluted by run-off from old copper mines.Algae collected from areas exposed to copper in situ showedchanges in the pattern of fatty acids labelled in vitro withan increase in oleate labelling and a decrease in palmitatelabelling in both species. The endogenous fatty acid patternsfor algae from polluted sites also differed from those fromunpolluted sites. Algae from sites polluted by copper containedmore oleate than samples from non-polluted sites. In addition,there were decreases in the polyunsaturated fatty acids -linolenate,octadecatetraenoate and docosapentaenoate for copper-exposedalgae. These differences are discussed with regard to the possiblemetabolic site of copper's action and to the relative toleranceof Fucus spp. to high environmental dissolved copper levels. Key words: Fucus spp., Copper pollution, Fatty acid synthesis     

Temperature dependence of chlorophyll {alpha} fluorescence in lettuce and spinach chloroplasts at sub-zero temperatures

The temperature dependence of chlorophyll fluorescence wasmeasured in spinach and lettuce chloroplasts at sub-zero temperaturesin the presence of 50% ethylene glycol. In the presence of 5mM Mg²⁺, a fluorescence maximum appeared at –31?C in boththe spinach and lettuce chloroplasts, while in the presenceof only 5 mM Na⁺ as cations the maximum shifted to –20?Cin the spinach chloroplasts and to –11?C in the lettucechloroplasts. Since the occurrence of a maximum in the temperatureversus fluorescence curve is an indication for the transitionof the physical phase of thylakoid membrane lipids between theliquid crystalline and the phase-separation state (16, 18),these findings suggest that the (major) phase transition ofmembrane lipids occurs at these low temperatures in chloroplastsof higher plants and also that the phase transition temperatureis markedly lowered by the presence of divalent cations. Ethylene glycol at a concentration of 50% had almost no effecton the temperature dependence of chlorophyll fluorescence ina lamellar membrane preparation of Anabaena variabilis. In awater suspension of dimyristoylphosphatidylcholine, the additionof ethylene glycol to 50% did not alter the characteristic featureof the temperature dependence of fluorescence of 1-anilinonaphthalene-8-sulfonate.These findings suggest that 50% ethylene glycol does not affectthe temperature of the transition of the physical phase of membranelipids. ¹ C.I.W.-D.P.B. Publication No. 592. ² Present Address: Department of Biophysics and Biochemistry,Faculty of Science, University of Tokyo, Hongo 113, Tokyo, Japan. (Received June 22, 1977; )     

Opposite effects of Ni2+ on Xenopus and rat ENaCs expressed in Xenopus oocytes

Opposite effects of Ni²⁺ on Xenopus and rat ENaCs expressed in Xenopus oocytes. Am J Physiol Cell Physiol 289: C946–C958, 2005. First published June 8, 2005; .—The epithelial Na⁺ channel (ENaC) is modulated by various extracellular factors, including Na⁺, organic or inorganic cations, and serine proteases. To identify the effect of the divalent Ni²⁺ cation on ENaCs, we compared the Na⁺ permeability and amiloride kinetics of Xenopus ENaCs (xENaCs) and rat ENaCs (rENaCs) heterologously expressed in Xenopus oocytes. We found that the channel cloned from the kidney of the clawed toad Xenopus laevis [wild-type (WT) xENaC] was stimulated by external Ni²⁺, whereas the divalent cation inhibited the channel cloned from the rat colon (WT rENaC). The kinetics of amiloride binding were determined using noise analysis of blocker-induced fluctuation in current adapted for the transoocyte voltage-clamp method, and Na⁺ conductance was assessed using the dual electrode voltage-clamp (TEVC) technique. The inhibitory effect of Ni²⁺ on amiloride binding is not species dependent, because Ni²⁺ decreased the affinity (mainly reducing the association rate constant) of the blocker in both species in competition with Na⁺. Importantly, using the TEVC method, we found a prominent difference in channel conductance at hyperpolarizing voltage pulses. In WT xENaCs, the initial ohmic current response was stimulated by Ni²⁺, whereas the secondary voltage-activated current component remained unaffected. In WT rENaCs, only a voltage-dependent block by Ni²⁺ was obtained. To further study the origin of the xENaC stimulation by Ni²⁺, and based on the rationale of the well-known high affinity of Ni²⁺ for histidine residues, we designed -subunit mutants of xENaCs by substituting histidines that were expressed in oocytes, together with WT - and -subunits. Changing His²¹⁵ to Asp in one putative amiloride-binding domain (WYRFHY) in the extracellular loop between Na⁺ channel membrane segments M1 and M2 had no influence on the stimulatory effect of Ni²⁺, and neither did complete deletion of this segment. Next, we mutated His⁴¹⁶ flanked by His⁴¹¹ and Cys⁴¹⁷, a unique site for possible heavy metal ion chelation, and, with this quality, most proximal (100 amino acids upstream of the second putative amiloride binding site at the pore entrance), was found localized at M2. Replacing His⁴¹⁶ with arginine, aspartate, tyrosine, and alanine clearly affected amiloride binding in all cases, as well as Na⁺ conductance, as expressed in the xENaC current-voltage relationship, especially with regard to aspartate and tyrosine. However, similarly to those obtained with the WYRFHY stretch, none of these mutations could either abolish the stimulating effect of Ni²⁺ or reverse it to an inhibitory type. epithelia; divalent cations; amiloride; Na⁺; voltage clamp     

Matrix free Mg2+ and the regulation of mitochondrial volume

Mitochondria must maintain volume homeostasis inorder to carry out oxidative phosphorylation. It has been postulatedthat the concentration of freeMg²⁺([Mg²⁺]) serves as thesensor of matrix volume and regulates aK⁺-extrudingK⁺/H⁺antiport (K. D. Garlid. J. Biol. Chem.255: 11273-11279, 1980). To test this hypothesis, the fluorescentprobe furaptra was used to monitor[Mg²⁺] and freeCa²⁺ concentration ([Ca²⁺]) in the matrix ofisolated beef heart mitochondria, andK⁺/H⁺antiport activity was measured by passive swelling in potassium acetate. Concentrations that result in 50% inhibition of maximum activity of 92 µM matrix [Mg²⁺] and 2.2 µM[Ca²⁺] were determined for theK⁺/H⁺ antiport. Untreated mitochondria average670 µM matrix [Mg²⁺], a value that would permit <1%of maximumK⁺/H⁺antiport activity. Hypotonic swelling results in large decreases inmatrix [Mg²⁺], butswelling due to accumulation of acetate salts does not alter[Mg²⁺]. Swelling inphosphate salts decreases matrix[Mg²⁺], but not tolevels that permit appreciable antiport activity. We conclude that1) it is unlikely that matrix[Mg²⁺] serves as themitochondrial volume sensor, 2) ifK⁺/H⁺antiport functions as a volume control transporter, it is probably regulated by factors other than[Mg²⁺], and3) alternative mechanisms formitochondrial volume control should be considered.

     

Effects of Far-red Light on the Labelling of Acyl Lipids during the Greening of Barley (Hordeum vulgare)

Etiolated Hordeum vulgare (barley) Plants were greened underwhite light or far-red (> 700 nm) light. Exposure to far-redlight inhibited chlorophyll synthesis (especially chlorophyllb) and the development of photosystem II which were seen whengreening took place under white light. Primary leaves were detachedand the labelling of their acyl lipids from [¹⁴C]acetate wasstudied under white light or far-red light illumination. Greeningwith far-red light caused a reduction in the radiolabellingof polyunsaturated fatty acids and diacylgalactosylglycerol.Total fatty acid labelling rates were unaffected. Phosphatidylethanolamine,which was normally poorly labelled, accounted for up to 15 percent of the total radioactivity in acyl moieties of lipids inleaves greened with far-red light. The results are discussedin connection with the role that acyl lipids may play in normalthylakoid structure and function. Hordeum vulgare, barley, acyl lipids, white light, far-red light, chloroplasts, thylakoids     

Permeability of Lipophilic Cations

The permeability (P) of a lipophilic cation, triphenylmethylphosphonium(TPMP⁺) which is frequently used as a membrane potential probe,has been measured in Chara australis (Charophyceae). P_TPMP+across biological membranes is usually thought to be very highbut this is not the case across the plasmalemma of Chara. Thepermeability of TPMP⁺ across the plasmalemma was found to betypical of inorganic cations, about 1.0 nm s^–1. Estimateswere made of the permeability of lipophilic cations across someother cell membranes, based on previously published work. Thepermeability of TPMP⁺ across the plasma membranes of the redalga, Griffithsia monilis and the blue-green alga, Anabaenavariabilis was about 2–5 nm s^–1. The permeabilityof TPMP⁺ across the plasma membranes of eukaryotes and prokaryotesappears to be similar. The permeability of lipophilic cationsacross the cristae of isolated mitochondria are exceptionallyhigh, about 170 nm s^–1. TPMP⁺ did not behave as a thiamineanalogue in Chara, unlike in the case of yeast. The means ofentry of TPMP⁺ into the Chara cell, driven by the electrochemicalgradient across the plasmalemma, has not been identified. Thepresence of a second lipophilic cation probe, DDA⁺ (dibenzyldimethylammonium),caused a decrease in the uptake flux of TPMP⁺; this suggeststhat the two lipophilic cations compete for the same site atthe surface of the plasmalemma. Key words: Chara australis, TPMP⁺, Permeability, Lipophilic cation     

Apoplastic Solute Concentrations of Organic Acids and Mineral Nutrients in the Leaves of Several Fagaceae

Ion chromatographic methods determined organic acids and mainnutrient minerals in the apoplastic solution from leaves ofseveral Fagaceae (Quercus ilex L., Quercus cerris L., Quercusvirgiliana (Ten.) Ten, and Fagus sylvatica L.). The anions oforganic acids found in high amounts (250 to 650 µM) werequinate, malate, and oxalate. Lactate, pyruvate, formate andacetate were detected in relatively low amounts with concentrationsbetween 20 and 200 µM. The total concentration of organicacids in the apoplastic sap ranged between 1.5 and 2 mM. Thetotal concentration of inorganic cations (K⁺, Mg²⁺, NH₄⁺, Ca²⁺,Na⁺) and anions (C1^–, NO₃^–, SO^2–₄ and PO^3–₄)in the apoplastic sap varied between 5 and 10 mM, and 0.35 and1.8 mM, respectively. We conclude that the concentration oforganic acid ions in the leaf apoplast depends mainly on theexchange with the leaf cells and is influenced by the electrochemicalgradient between the symplast and the apoplast in relation tothe water potential of the leaf. The determination of formateand acetate in the apoplastic compartment of leaves lend weightto the argument that the production of these acids by treesis a important emission source to the atmosphere. (Received June 9, 1998; Accepted April 8, 1999)     

Photosynthesis in Cell Suspension Culture of a C4 Plant, Gisekia pharnaceoides L.

Photomixotrophic cell suspension culture was established fromthe leaf derived callus cells of Gisekia pharnaceoides L., aC₄ dicotyledonous weed. The late log phase cells possessed shade-typecharacters such as low chlorophyll a/b ratio, less pronouncedO₂ evolution and CO₂ fixation, saturation of photosyntheticCO₂ fixation at low intensity. The chloroplasts from these cellscontained granal stacking with high degree of a very few granawhich are characterized by their wide and high degree of stackings. The predominant labelling of 3-phosphoglyceric acid and sugarphosphates (40% of the total ¹⁴C incorporated) during 5 s exposureto ¹⁴CO₂ in light and subsequent decrease in percentage of ¹⁴Cin these compounds with increase in exposure time indicatedthe operation of the C₃ pathway in these cells. The simultaneoussynthesis of malate (23% of the total ¹⁴C incorporated) is relatedto the much pronounced glycolytic and tricarboxylic acid cycleactivities in these cells. The initial proliferation of callimainly from the zones of vascular supplies in the leaf, highstarch content of the cells, presence of large starch grainsin all the chloroplasts, activities of Calvin cycle enzymes,heavy labelling of C₃ type intermediates and less labellingof aspartate as early photosynthates and rapid accumulationof radioactivity into starch during ¹⁴CO₂ assimilation indicatedthat most of the cells in photomixotrophic culture were derivedfrom bundle sheath cells or the leaf cells of Gisekia changetheir function under culture conditions. ¹Present address: Tropical Botanic Garden and Research Institute,Navaranga Road, Trivandrum 695 011, India. (Received January 29, 1982; Accepted June 4, 1983)     

Divalent cation and anion currents are activated during the dark-induced transient hyperpolarization of the plasma membrane of the green alga Eremosphaera viridis

Darkening after illumination induces a transient hyperpolarizationof the plasma membrane of the unicellular green alga Eremosphaeraviridis de Bary. With electro-physiological methods, in particularthe two electrode voltage-clamp technique, we investigated theion fluxes involved in this transient potential change (TP).The question was whether other ion currents besides those carriedby the known Ca²⁺-dependent K⁺ channel take part in this actionpotential-like, but hyperpolarizing, response. At maximum hyperpolarizationvoltage-clamp measurements resulted in ‘N-shaped’I/V curves, known from other botanical systems. The differentinstantaneous current components of the N-shaped I/V curvesoccurred at different times during a single transient potentialchange (TP). Substitution of alkali metal cations in the bathingsolution by NMG/NO₃ showed that the inward currents in the I/Vcurves were not carried by an influx of K⁺ into the cytoplasm.The voltage amplitude of the TP not only depended on the externalK⁺ concentration, but also on the Mg²⁺ concentration in thebathing solution. Increasing Mg²⁺ concentrations shifted themembrane potential in the top of the TP in the direction ofthe Nernst potential of Mg²⁺ and resulted in an increased inwardcurrent component of the N-shaped I/V curves. Another currentcomponent was found to be carried not by cations but by an effluxof anions. It was a voltage-dependent component with a maximumcurrent amplitude at voltages of about –220 to –240mV, and was blocked by the anion channel inhibitors anthracen-9-carboxylicacid (A9C), (5-nitro-2-3-phenylpropylamino) benzoic acid (NPPB)and ZnCI2. Based on these data a model is proposed which explainsthe N-shape of the I/V curves observed during the transientpotential change of the alga E. viridis by the combination ofan inward cation current with an inward anion current and theoutward cation current carried by the Ca²⁺-dependent K⁺ channels. Key words: Anion current, cation current, Eremosphaera viridis, potassium channel, voltage-clamp     

The Partitioning of Photosynthetically Assimilated 14C into Amino Acids in Wheat: 2. Distribution in Amino Acids of Wheat Grain Fractions

Changes in the distribution of ¹⁴C between free and bound aminoacids in wheat grains (Triticum aestivum L. cv. Arkas) at 10and 20 d post-anthesis are described. After ¹⁴CO₂, labellingof the flag leaf, ¹⁴C was initially more rapidly transferredto the grains of 20 d post-anthesis plants than for 10 d post-anthesisplants. However, after a 460 min chase period in the light theamount of ¹⁴C in the grains of the younger and older plantswere similar. In the younger, more rapidly growing grains, agreater proportion of the ¹⁴C was incorporated into structuraltissue and starch. ¹⁴C accumulation in the grains continuedduring the dark in the younger grains but not in the older grains. Although the overall ¹⁴C distribution between the free aminoacid and protein pools of the grain was similar for both treatments,the distribution within the albumin, prolamin and globulin fractionsand between the individual non-bound amino acids differed. Ofthe protein fractions, the albumins were initially the mostheavily labelled but after 460 min chase the prolamins containedmore ¹⁴C. The majority of the ¹⁴C in the albumin and globulinfractions after 280 min chase was in hydrolysable, non-aminoacid compounds. In both tissues, the free amino acid pools lostradioactivity in the dark but the solid residues and proteinscontinued to function as ¹⁴C sinks. Daily fluctuations in the radioactivity in free and bound alanineare consistent with the role of free alanine as a diurnal metabolicnitrogen pool. Wheat, Triticum aestivum¹⁴CO₂, amino acids, proteins, carbon metabolism     

Non-specificity of Salt Effects on Mg2+dependent ATPase from Grass Roots

The effect of tris, choline, and ethanolamine chlorides on theactivity of Mg^2–dependent ATPase in membrane fractions(cell walls, mitochondria, and microsomes) of Zea mays L. (cv.Neve Yaar 22), Avena saliva L. (cv. Mulga), and Hordeum vulgareL. (cv. Omer) was compared with the effect of KC1 and NaCl.Considerable salt effects on apparent Mg²⁺ATPase activity werefound only at relatively high pH values (8.2) at which Mg²⁺.ATPaseactivity was low in the absence of monovalent cation salts.The Mg²⁺-dependent ATP hydrolysis by ATPases from all the membranefractions increased in the presence of at least one of the organiccations to the same extent as in the presence of KCI or NaCl.The monovalent organic cations are only very slowly absorbedby corn roots in comparison with K⁺ and Na⁺. It is concluded that monovalent salt effects on ATPase fromthese plant roots are not cation specific and not related tothe capability of root cells to absorb cations. Present evidencefor the existence of a cation-transport ATPase in plant tissueis critically reviewed.     

Radiolabelling Studies on the Lipid Metabolism in the Marine Brown Alga Dictyopteris membranacea

The lipid metabolism of the marine brown alga D. membranaceawas investigated using [2–¹⁴C]acetate, [1–¹⁴C]myristate,[l–^I4C]oleate and [l–¹⁴C]arachidonate as precursors.On incubation with [2–¹⁴C]acetate, 18:1 and 16:0 werethe main products formed by de novo synthesis and incorporatedinto polar lipids. With all the exogenous substrates used, DGTAwas strongly labelled and the subsequent rapid turnover of radioactivitysuggested a key role for this lipid in the redistribution ofacyl chains and most likely also in the biosynthesis of theeukaryotic galacto-lipids produced in the absence of PC. Inthe glycolipids a continuous accumulation of radioactivity wasobserved with all the substrates used. The labelling kineticsof molecular species of MGDG suggested the desaturation of 18:1to 18:4 and of 20:4 (n-6) to 20:5 (n–3) acids on thislipid. Both PG and PE were primary acceptors of de novo synthesizedfatty acids and exogenous [l–¹⁴C]oleate, but no evidenceexists for a further processing of acyl chains on these lipids.TAG, although strongly labelled with all exogenous [l–¹⁴CJacids,was not labelled when [2–¹⁴C]acetate was used as a precursorindicating the flux of endogenous fatty acids to be differentof that of exogenously supplied fatty acids. (Received November 4, 1997; Accepted February 23, 1998)     

Permeation of Ca2+ and monovalent cations through an outwardly rectifying channel in maize root stelar cells

A depolarization-activated outwardly-rectifying channel (OR),most likely involved in the passive release of K⁺ from the rootsymplasm into the stelar apoplast (for subsequent transportto the shoot via the xylem vessels), has been characterizedin the plasma membrane of maize root stelar cells (Roberts andTester, 1995). In the present study, the selectivity of thischannel was further characterized using single channel current-voltagecurves generated using a voltage ramp protocol. This protocolpermitted the accurate and unambiguous measurement of the reversalpotentials of currents resulting from single channel openings.Using the voltage ramp protocol, it was shown that the OR allowsboth K⁺ efflux and Ca²⁺ influx at potentials positive of E_Kand negative of E_Ca. The OR had a P_Ca/P_K of 1.72–0.21decreasing as extracellular Ca²⁺ was increased. The permeabilityof the OR for monovalent cations other than K⁺ was also investigated.In biionic conditions, a relative permeability sequence of was determined (i.e. Eisenman sequenceIV). The physiological implications of the selectivity of theOR are discussed. Key words: Maize roots, K⁺ channel selectivity, Ca²⁺ permeation     

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     

Modulation of Ca2+-gated cardiac muscle Ca2+-release channel (ryanodine receptor) by mono- and divalent ions

The effects of mono- and divalent ions onCa²⁺-gated cardiac muscleCa²⁺-release channel (ryanodinereceptor) activity were examined in [³H]ryanodine-bindingmeasurements. Ca²⁺ bound with thehighest apparent affinity to Ca²⁺activation sites in choline chloride medium, followed by KCl, CsCl,NaCl, and LiCl media. The apparentCa²⁺ binding affinities ofCa²⁺ inactivation sites were lowerin choline chloride and CsCl media than in LiCl, NaCl, and KCl media.Sr²⁺ activated the ryanodinereceptor with a lower efficacy thanCa²⁺. Competition studiesindicated that Li⁺,K⁺,Mg²⁺, andBa²⁺ compete withCa²⁺ forCa²⁺ activation sites. In 0.125 MKCl medium, the Ca²⁺ dependence of[³H]ryanodine bindingwas modified by 5 mM Mg²⁺ and 5 mM,-methyleneadenosine 5'-triphosphate (a nonhydrolyzable ATPanalog). The addition of 5 mM glutathione was without appreciable effect. Substitution of Clby 2-(N-morpholino)ethanesulfonic acid ion caused anincrease in the apparent Ca²⁺affinity of the Ca²⁺ inactivationsites, whereas an increase in KCl concentration had the oppositeeffect. These results suggest that cardiac muscle ryanodine receptoractivity may be regulated by 1)competitive binding of mono- and divalent cations toCa²⁺ activation sites,2) binding of monovalent cations toCa²⁺ inactivation sites, and3) binding of anions to anionregulatory sites.