Temporal Changes in Swimming Direction during the Phototactic Orientation of Individual Cells in Cryptomonas sp.

The response of individual Cryptomonas cells to continuous lightwas recorded using infrared video-micrography. Swimming directionsand temporal shifts in swimming direction of each cell weremeasured. White light of 0.1–1 W m^–2 elicited apositive phototactic orientation, but did not induce any photophobicresponse. Light of 100 W m^–2 induced a photophobic responseat the onset of actinic irradiation, but did not induce positivephototactic orientation. No correlation between positive phototacticorientation and photophobic response was found in this species.The direction toward the light source was defined as 0°,and the direction away from the source as 180°. Within 2s after the onset of lateral monochromatic light of 570 nm at0.1 W m^–2, cells which were swimming in a direction ofless than 120° predominantly shifted their course towardthe light source. Cells swimming in directions of larger than120° shifted their course as randomly as those in the dark.Thus, for phototactic orientation, the cells must perceive thelight from their anterior side. (Received July 29, 1985; Accepted November 4, 1985)     

Inhibition of Phototaxis and Motility by UV-B Irradiation in Dictyostelium discoideum Slugs

The effect of monochromatic irradiation in and near the UV-Bregion (280–320 nm) on motility (speed of movement) andwhite light-induced phototactic orientation was studied in slugsof the cellular slime mold Dictyostelium discoideum (NC-4).Motility decreased to about 50% in UV-B fluences (about 100Jm^–2 at 280 nm) well below those required to inhibit slugand sorocarp development or to impair viability. At wavelengthsof 270–300 nm, the phototactic orientation was almosttotally eliminated by an exposure to U V (about 0.1 Wm^–2)for as short a time as half an hour when administered at thebeginning of the 24 h exposure to white light. (Received July 22, 1983; Accepted September 27, 1983)     

The effect of UV-B radiation and humic substances on growth and motility of the flagellate, Euglena gracilis

The effects of ultraviolet-B radiation (UV-B, 250–315nm) were determined on Euglena gracilis with respect to speed,phototactic orientation, specific growth rate, and in the presenceof humic substances. Humic substances had a protective effectwhen studying the speed and specific growth rate. However, thedegree of phototactic orientation decreased in UV-B radiationboth with and without humic substances. The inhibition of O₂evolution and speed was most pronounced when using cutoff filtersWG280 and WG295. The photosynthetic inhibitor DCMU (10^–6M) did not have any effect on the speed, but the 0₂ evolutiondecreased to zero. The effect of different wavelengths in theUV-B region on the speed of E.gracilis showed the maximum sensitivityat 280 and 290 nm.     

Photomovement and Photobleaching In Two Gyrodinium Species

The phototactic orientation in Gyrodinium dorsum and G. aureolumhas been analyzed. Both species show only positive phototaxiswith an optimum at about 300 W-m^–2. The mechanism of photoorientationdoes not seem to be based on a dichroic orientation of the photoreceptorpigments as in the flagellate Euglena gracilis. Photobleachingexperiments have shown a far higher resistance toward continuousirradiation at even high fluence rates than in other flagellateswhich is in good agreement with the exclusive behavior of positivephototaxis. (Received August 7, 1987; Accepted July 14, 1988)     

Auxin Stimulates Cl-Uptake by Oat Coleoptiles

The effects of auxin on net ion fluxes near parenchyma of oatcoleoptiles were studied using the non-invasive MIFE systemto measure specific ion fluxes using ion selective microelectrodes.Application of 10 µM1-naphthaleneacetic acid (NAA) for3 h caused doubling of coleoptile segment growth, without changingthe pH of the unbuffered bathing solution from pH 5.4 duringthat time. Short term experiments revealed that auxin led toan immediate three-fold increase of chloride influx to 1200nmol m^-2s^-1, maintained for at least 1 h. In the first minutesafter auxin application, proton fluxes were small (-25 nmolm^-2s^-1, an efflux) and tended to decrease, whereas potassiumand calcium fluxes changed little, fluctuating from -100 to0 nmol m^-2s^-1and from -15 to 0 nmol m^-2s^-1, respectively. Itis suggested that one target of auxin action in plant cellsis the plasma membrane chloride transport system mediating increasedchloride uptake.Copyright 1998 Annals of Botany Company Auxin, chloride transport, ion flux,Avena sativaL., oat.     

Colocalization of glycolytic enzyme activity and KATP channels in basolateral membrane of Necturus enterocytes

⁸⁶Rb fluxes throughATP-regulated K⁺(K_ATP) channels in membranevesicles derived from basolateral membranes ofNecturus small intestinal epithelialcells as well as the activity of single K_ATP channels reconstituted intoplanar phospholipid bilayers are inhibited by the presence of ADPplus phosphoenolpyruvate in the solution bathingthe inner surface of these channels. This inhibition can be preventedby pretreatment of the membranes with 2,3-butanedione, an irreversibleinhibitor of pyruvate kinase (PK) and reversed by the addition of2-deoxyglucose plus hexokinase. The results of additional studiesindicate that PK activity appears to be tightly associated with thismembrane fraction. These results, together with considerations of thepossible ratio ofNa⁺-K⁺pumps to K_ATP channels in thebasolateral membrane, raise the possibility that "cross talk"between those channels and pumps (i.e., the "pump-leakparallelism") may be mediated by local, functionallycompartmentalized ATP-to-ADP ratios that differ from those in the bulk cytoplasm.

     

Physiological and Biochemical Mechanisms Involved in the Response to Abscisic Acid in Maize Coleoptiles

The role of abscisic acid (ABA) in controlling growth and developmenthas been studied in maize (Zea mays L.) coleoptile segments.Application of ABA reduces the elongation rate by about 50%and affects ion fluxes. In particular, proton extrusion is decreasedwhile potassium efflux is greatly enhanced. Apparently, ABAdoes not: seem to influence calcium influx from the apoplastinto the cytosol, but more likely it influences its efflux.Alteration of cytosolic calcium concentration may also be obtainedby increasing its release from internal stores. This possibilitymight be sustained by the increased hydrolysis of phosphatidylinositolupon ABA application. Change in the balance of ion fluxes shouldresult from regulation of transport mechanisms at the membranelevel and should produce changes in the transmembrane electricalpotential. The H⁺- ATPase and the ATP-dependent calcium transportactivities are both influenced by the treatment with ABA, –55%and –40%, respectively. Under these conditions [Ca²⁺]_cytand pH_cyt can be modified and, as a consequence of their regulation,they may play an important role in mediating the physiologicaland biochemical effects of ABA, acting as second intracellularmessengers. ¹Research supported by National Research Council of Italy, SpecialProject RAISA, Sub-Project N. 2, Paper n. 2782.     

Arginine transport through system y(+)L in cultured human fibroblasts: normal phenotype of cells from LPI subjects

Inlysinuric protein intolerance (LPI), impaired transport of cationicamino acids in kidney and intestine is due to mutations of theSLC7A7 gene. To assess the functional consequences of the LPI defect in nonepithelial cells, we have characterized cationic aminoacid (CAA) transport in human fibroblasts obtained from LPI patientsand a normal subject. In both cell types the bidirectional fluxes ofarginine are due to the additive contributions of two Na⁺-independent, transstimulated transport systems. One ofthese mechanisms, inhibited by N-ethylmaleimide (NEM) andsensitive to the membrane potential, is identifiable with systemy⁺. The NEM- and potential-insensitive component,suppressed by L-leucine only in the presence ofNa⁺, is mostly due to the activity of systemy⁺L. The inward and outward activities of the two systemsare comparable in control and LPI fibroblasts. Both cell types expressSLC7A1 (CAT1) and SLC7A2 (CAT2B and CAT2A) aswell as SLC7A6 (y+LAT2) and SLC7A7 (y+LAT1). Weconclude that LPI fibroblasts exhibit normal CAA transport throughsystem y⁺L, probably referable to the activity ofSLC7A6/y+LAT2.

     

Effect of EGTA on Nyctinastic Closure Mediated by Phytochrome in Albizzia lophantha

The role of extracellular calcium on nyctinastic closure ofAlbizzia lophantha leaflets has been studied by testing theeffect of ethyleneglycol-bis-(ß-aminoethylether)-N,N,N',N'-tetraaceticacid (EGTA) and its reversibility by calcium. EGTA (1 and 5mM) causes an inhibition of nyctinastic closure and at a concentrationof 1 mM EGTA it decreases the difference between the effectof red light (R) and far-red light (FR) irradiation on leafletclosure. A simultaneous or subsequent supply of CaCl₂ (5 or10 mM) reverses EGTA (5 mM) inhibition on closure as well ascausing an additional promotion of closure. We suggest that external calcium could play a dual role in nyctinasticclosure. Phytochrome control of leaflet closure probably needsexternal Ca²⁺ and, in addition, Ca²⁺ could regulate the closuremechanism by controlling ionic fluxes through the plasma membranein pulvinular motor cells. (Received June 9, 1989; Accepted November 27, 1989)     

ATP-Dependent Ca2+ Transport in Tonoplast Vesicles from Apple Fruit

Protoplasts and vacuoles were isolated from immature apple fruit(Malus pumila Mill. cv. Golden Delicious). ATP-stimulated Ca²⁺uptake was identified in both protoplast vesicles and tonoplastvesicles. The apparent K_m for Ca²⁺ of the tonoplast transportsystem was 43.4 µM. The pH optima were 7.2 and 6.7 forCa²⁺ transport by protoplast and tonoplast vesicles, respectively.Ca²⁺ transport in tonoplast vesicles was strongly inhibitedby the calmodulin antagonists fluphenazine and N-(6-aminohexyl)-5-chloro-l-naphthalensulfonamidehydrochloride (W-7), while N-aminohexyl)-l-naphthalensulfonamidehydrochloride (W-5) was relatively ineffective. Addition ofexogenous calmodulin stimulated transport by 35%. Ca²⁺ uptakewas inhibited by vanadate, but not by the ionophores carbonylcyanidem-chlorophenyl hydrazone (CCCP) or valinomycin. The resultsindicate that apple tonoplasts have a Ca²⁺ transport systemthat is driven by the direct hydrolysis of ATP, and may be calmodulindependent. ¹Present address: Morioka Branch, Fruit Tree Research Station,Ministry of Agriculture, Forestry and Fisheries, Shimokuriyagawa,Morioka 020-01, Japan. To whom reprint requests should be addressed. (Received October 18, 1985; Accepted January 29, 1986)     

Polar transport of 45Ca2+ across the elongation zone of gravistimulated roots

The movement of calcium across the elongation zone of gravistimulatedprimary roots of maize (Zea mays L.) was measured using ⁴⁵Ca^2$.Radioactive calcium was applied to one side of the elongationzone about 4 mm back from the root tip and the distributionof radioactivity across the root in the region of applicationwas determined using scintillation spectrometry. The movementof ⁴⁵Ca^2$ across the elongation zone was non-polar in verticallyoriented roots. In gravistimulated roots the movement of labelwas polarized with about twice as much label moving from topto bottom as from bottom to top. A variety of treatments whichinterfere with gravitropism was found to eliminate the polarmovement of ⁴⁵Ca^2$ across the elongation zone. In maize cultivarswhich require light for gravitropic competency, dark grown rootsexhibited neither gravitropism nor polar movement of ⁴⁵Ca^2$across the elongation zone. Upon illumination the roots developedboth gravitropic competency and gravity-induced polar movementof ⁴⁵Ca^2$ across the elongation zone. Similarly, roots of light-grownseedlings lost both gravitropic competency and ⁴⁵Ca^2$ transportpolarity upon transfer to the dark. The results indicate a closecorrelation between calcium movement and gravitropism in primaryroots of maize. (Received July 20, 1985; Accepted September 25, 1985)     

Functional expression of heterologous proteins in yeast: insights into Ca2+ signaling and Ca2+-transporting ATPases

The baker's yeast Saccharomyces cerevisiae is a well-developed, versatile, and widely used model organism. It offers a compact and fully sequenced genome, tractable genetics, simple and inexpensive culturing conditions, and, importantly, a conservation of basic cellular machinery and signal transducing pathways with higher eukaryotes. In this review, we describe recent technical advances in the heterologous expression of proteins in yeast and illustrate their application to the study of the Ca²⁺ homeostasis machinery, with particular emphasis on Ca²⁺-transporting ATPases. Putative Ca²⁺-ATPases in the newly sequenced genomes of organisms such as parasites, plants, and vertebrates have been investigated by functional complementation of an engineered yeast strain lacking endogenous Ca²⁺ pumps. High-throughput screens of mutant phenotypes to identify side chains critical for ion transport and selectivity have facilitated structure-function analysis, and genomewide approaches may be used to dissect cellular pathways involved in Ca²⁺ transport and trafficking. The utility of the yeast system is demonstrated by rapid advances in the study of the emerging family of Golgi/secretory pathway Ca²⁺,Mn²⁺-ATPases (SPCA). Functional expression of human SPCA1 in yeast has provided insight into the physiology, novel biochemical characteristics, and subcellular localization of this pump. Haploinsufficiency of SPCA1 leads to Hailey-Hailey disease (HDD), a debilitating blistering disorder of the skin. Missense mutations, identified in patients with HHD, may be conveniently assessed in yeast for loss-of-function phenotypes associated with the disease. Saccharomyces cerevisiae; calcium ion; transporters; functional complementation     

Regulation of Free Calcium Concentration in the Pitchers of the Carnivorous Plant Sarracenia purpurea: A Model for Calcium in the Higher Plant Apoplast?

The pitcher of the carnivorous plant Sarracenia purpurea L.contains an entrapped body of liquid within which its prey isdigested. Free calcium in the pitcher is derived from eitherthe pitcher walls or from prey falling into the pitcher; inthe absence of exogenous (prey-derived) calcium it will dependon the active and passive calcium regulatory properties of thepitcher walls and may to some extent therefore mimic calciumin the apoplast of plant cells. Using a calcium-specific electrode,the free calcium concentration of the pitchers of Sarraceniaplants was investigated and the effect of adding a variety ofconcentrations of calcium in water determined. The mean pitcherfree calcium concentration in vivo was 2.3 x 10^–5 M±2.5x 10^–5 M; when pitchers were washed and filled with watercontaining lower calcium concentrations, the concentration inthe pitcher water rose to 1–5 x 10^–5 M. When highercalcium concentrations (up to 1 x 10^–4 M) were added,the pitcher calcium concentration declined to 1–7 x 10^–5M. Concentrations of calcium above 1 x 10^–4 M were alsoreduced, but to a lesser extent. Metabolic inhibition of activeion transport, while inhibiting pitcher acidification, did notinhibit regulation of pitcher free calcium, suggested that itoccurs as a result of calcium exchange sites in the pitcherwalls. The data are discussed in relation to the physiologyof Sarracenia pitchers and to the usefulness of the pitcheras a model for free calcium in the higher plant apoplast. Sarracenia purpurea L., carnivorous plant, pitcher, free calcium, plant apoplast     

Effects of Some Amino Acid Analogues on the Uptake and Transport of K+ and Ca2+ in Wheat and Mung Bean Seedlings: I. CORTICAL CELL FLUXES AND TRANSPORT TO THE STELE IN EXCISED ROOT SEGMENTS, AS AFFECTED BY p-FLUOROPHENYLALANINE

Effluxes of K⁺ and Ca²⁺ from root segments of both wheat, Triticunaestivum L. cv. Capelle and mung bean, Vigna radiata (L.) Wilczek,were measured in the presence or absence of 20 mol m^–3para-fluorophenylalanine (p-FPA). The results were used to estimatethe compartment contents and transmembrane K⁺ and Ca²⁺ fluxesin root cortex cells. Using the Ussing-Teorell flux equationas the criterion, it was concluded that entry of K⁺ from theoutside solution to the cytoplasm, and from the cytoplasm tothe vacuole were active in both wheat and mung bean. Also, inboth species, Ca²⁺ entered the cytoplasm passively across theplasmalemma and was actively pumped back to the external solution.However, interpretation of the direction of active transportacross the tonoplast depends upon an assumption about Ca²⁺ activityin the cytoplasm. The only qualitative effect of p-FPA was to alter the drivingforce for K⁺ influx, across the plasmalemma in wheat, from anactive to a passive one. Quantitative effects of the analoguewere seen for K⁺ fluxes in both wheat and mung bean and forCa²⁺ fluxes in wheat. The p-FPA reduced transport of K⁺ in bothspecies, while transport of Ca²⁺ was unaffected. The implicationsof these results for the ‘two pump hypothesis’ arediscussed. Key words: Triticum aestivum, Vigna radiata, Two pump hypothesis     

Coupling of Proton Fluxes in the Polar Leaves of Potamogeton lucens L

An attempt has been made to quantify the light-induced H⁺ effluxand influx observed in polar leaves of Potamogeton lucens.Theseproton fluxes are spatially separated. The H⁺ efflux, mediatedby a plasmalemma bound H⁺ –ATPase, occurs across theplasmamembrane at the morphological lower epidermis and is accompaniedby an H⁺ influx (or OH^– efflux) at the upper side oftheleaf. As a result, these leaves exhibit a remarkable pH–polarityin the light. The pH near the lower epidermis may drop to avalueas low as 3.5, while a pH of about 10.5 can be observed at theupper epidermis. Obviously this phenomenon requires theco–ordinationof transport processes in the different cell layers of the leaftissue. These observations led to quantitative studies oftherelation between the H⁺ fluxes at either plasmalemma. Thesefluxes were calculated from the pH values recorded at twodistancesfrom the leaf surface. Although the H⁺ influx always exceededthe efflux, a coupling between the transport processesacrosseither plasma membrane became evident from the time–coursesof the two fluxes. Key words: Potamogeton lucens, proton flux, flux coupling, pH–;polarity     

Shoot-Dependent Regulation of Sodium and Potassium Fluxes in Roots of Whole Barley Seedlings

Unidirectional fluxes and the cytoplasmic and vacuolar contentsof potassium and sodium in root cells of intact barley seedlings(Hordeum vulgare L., cv. Villa) were determined by use of compartmentalanalysis. In addition, the net vacuolar accumulation J_cv andthe xylem transport ø_cx of K⁺ and Na⁺ were measured.Both of these data were needed for the evaluation of the effluxdata. Fluxes and compartmental contents of K⁺ and Na⁺ were comparableto data obtained with excised roots. The effect of the shoot-to-rootratio—as varied by partial excision of the seedlings seminalroots—on the fluxes and contents was investigated. Highershoot-to-root ratios induced an increase in xylem transport,in plasmalemma influx, and also in the cytoplasmic content ofK⁺ and Na⁺. With potassium the plasmalemma efflux was almostunaltered while the tonoplast fluxes and vacuolar content weredecreased (in presence of Na⁺). With sodium, on the other hand,the plasmalemma efflux and the tonoplast fluxes were also increasedin the plants having one root and a high shoot-to-root ratio.These changes occurred even under conditions of low humidity,when transpiration was low and guttation occurred. The latterwas also increased at the high shoot-to-root ratio. The observedchanges could be due to a relieved feedback control of ion fluxesby the shoot and mediated in part by a relatively higher supplyof photosynthates in the plants having one root In addition,hormonal signals were suggested to participate. In particulara possibly decreased level of cytokinins in the plants havingonly one root could contribute to the signal. The observed changesappear to be responses of the plant to an alteration that canoccur under natural conditions when the root system is damaged.     

Cyclic nucleotide-gated cation channels mediate sodium and calcium influx in rat colon

We found mRNA for the three isoforms ofthe cyclic nucleotide-gated nonselective cation channel expressed inthe mucosal layer of the rat intestine from the duodenum to the colonand in intestinal epithelial cell lines in culture. Because thesechannels are permeable to sodium and calcium and are stimulated by cGMPor cAMP, we measured 8-bromo-cGMP-stimulated sodium-mediatedshort-circuit current (I_sc) inproximal and distal colon and unidirectional⁴⁵Ca²⁺fluxes in proximal colon to determine whether these channels couldmediate transepithelial sodium and calcium absorption across the colon.Sodium-mediatedI_sc, stimulatedby 8-bromo-cGMP, were inhibited by dichlorobenzamil andl-cis-diltiazem, blockers of cyclicnucleotide-gated cation channels, suggesting that these ion channelscan mediate transepithelial sodium absorption. Sodium-mediated I_sc and nettransepithelial⁴⁵Ca²⁺absorption were stimulated by heat-stable toxin fromEscherichia coli that increases cGMP.Addition of l-cis-diltiazem inhibited the enhanced transepithelial absorption of both ions. These results suggest that cyclic nucleotide-gated cation channels simultaneously increase net sodium and calcium absorption in the colon of the rat.

     

Interaction of reactive oxygen species with ion transport mechanisms

The use ofelectrophysiological and molecular biology techniques has shed light onreactive oxygen species (ROS)-induced impairment of surface andinternal membranes that control cellular signaling. These deleteriouseffects of ROS are due to their interaction with various ion transportproteins underlying the transmembrane signal transduction, namely,1) ion channels, such asCa²⁺ channels (includingvoltage-sensitive L-type Ca²⁺currents, dihydropyridine receptor voltage sensors, ryanodine receptorCa²⁺-release channels, andD-myo-inositol1,4,5-trisphosphate receptor Ca²⁺-release channels),K⁺ channels (such asCa²⁺-activatedK⁺ channels, inward and outwardK⁺ currents, and ATP-sensitiveK⁺ channels),Na⁺ channels, andCl channels;2) ion pumps, such as sarcoplasmicreticulum and sarcolemmal Ca²⁺pumps,Na⁺-K⁺-ATPase(Na⁺ pump), andH⁺-ATPase(H⁺ pump);3) ion exchangers such as theNa⁺/Ca²⁺exchanger andNa⁺/H⁺exchanger; and 4) ion cotransporterssuch asK⁺-Cl,Na⁺-K⁺-Cl,andP_i-Na⁺cotransporters. The mechanism of ROS-induced modificationsin ion transport pathways involves1) oxidation of sulfhydryl groups located on the ion transport proteins,2) peroxidation of membrane phospholipids, and 3) inhibition ofmembrane-bound regulatory enzymes and modification of the oxidativephosphorylation and ATP levels. Alterations in the ion transportmechanisms lead to changes in a second messenger system, primarilyCa²⁺ homeostasis, which furtheraugment the abnormal electrical activity and distortion of signaltransduction, causing cell dysfunction, which underlies pathologicalconditions.

     

Chloride in Soils and its Uptake and Movement within the Plant: A Review

Natural inputs of chlorine (Cl) to soils come mainly from rainwater,sea spray, dust and air pollution. In addition, human practices,such as irrigation and fertilization, contribute significantlyto Cl deposition. In the soil solution, Cl occurs predominantlyas the chloride anion (Cl^-). The Cl^-anion does not form complexesreadily, and shows little affinity (or specificity) in its adsorptionto soil components. Thus, Cl^-movement within the soil is largelydetermined by water flows. Chlorine is an essential micronutrientfor higher plants. It is present mainly as Cl^-. Chloride isa major osmotically active solute in the vacuole and is involvedin both turgor- and osmoregulation. In the cytoplasm it mayregulate the activities of key enzymes. In addition, Cl^-alsoacts as a counter anion, and Cl^-fluxes are implicated in thestabilization of membrane potential, regulation of intracellularpH gradients and electrical excitability. Chloride enters plantsthrough the roots, and there is some concern over the uptakeof the long-lived radionuclide³⁶Cl, which enters into the foodchain through plants. Chloride is thought to traverse the rootby a symplastic pathway, and Cl^-fluxes across the plasma membraneand tonoplast of root cells have been estimated. These fluxesare regulated by the Cl^-content of the root. Chloride is mobilewithin the plant. The Cl^-concentrations of xylem and phloemsaps have been determined and Cl^-fluxes through the xylem andphloem have been modelled. Measurements of transmembrane voltagesand Cl^-activities in cellular compartments suggest (1) thatactive Cl^-transport across the plasma membrane dominates Cl^-influxto root cells at low Cl^-concentrations in the soil solutionand that passive Cl^-influx to root cells occurs under more salineconditions, and (2) that both active and passive Cl^-transportoccurs at the tonoplast. Electrophysiological studies have demonstratedthe presence of an electrogenic Cl^-/2H⁺symporter in the plasmamembrane of root-hair cells and Cl^-channels mediating eitherCl^-influx or Cl^-efflux across the plasma membrane. Similarly,there is both biochemical and electrophysiological evidencethat Cl^-channels mediate Cl^-fluxes in either direction acrossthe tonoplast and that a Cl^-/nH⁺antiport mediates Cl^-influxto the vacuole. This article reviews the availability of Cl^-inthe soil, the roles and distribution of Cl^-within the plant,the magnitude of Cl^-fluxes across membranes and between tissues,the mechanisms of Cl^-transport across membranes and the electricalcharacteristics and molecular biology of Cl^-channels. Copyright2001 Annals of Botany Company Review, Arabidopsis thaliana, channel, chloride (Cl^-), influx, phloem, plasma membrane, radiochlorine (³⁶Cl), soil, tonoplast, transport, uptake, xylem     

Calcium-independent inhibition of glucose transport in PC-12 and L6 cells by calcium channel antagonists

The goal of these studies was todetermine whether different calcium channel antagonists affect glucosetransport in a neuronal cell line. Rat pheochromocytoma (PC-12) cellswere treated with L-, T-, and N-type calcium channel antagonists beforemeasurement of accumulation of 2-[³H]deoxyglucose(2-[³H]DG). The L-type channel antagonistsnimodipine, nifedipine, verapamil, and diltiazem all inhibited glucosetransport in a dose-dependent manner (2-150 µM) withnimodipine being the most potent and diltiazem only moderatelyinhibiting transport. T- and N-type channel antagonists had no effecton transport. The L-type channel agonist l-BAY K 8644 alsoinhibited uptake of 2-[³H]DG. The ability of these drugsto inhibit glucose transport was significantly diminished by thepresence of unlabeled 2-DG in the uptake medium. Some experiments wereperformed in the presence of EDTA (4 mM) or in uptake buffer withoutcalcium. The absence of calcium in the uptake medium had no effect oninhibition of glucose transport by nimodipine or verapamil. To examinethe effects of these drugs on a cell model of a peripheral tissue, westudied rat L6 muscle cells. The drugs inhibited glucose transport in L6 myoblasts in a dose-dependent manner that was independent of calciumin the uptake medium. These studies suggest that the calcium channelantagonists inhibit glucose transport in cells through mechanisms otherthan the antagonism of calcium channels, perhaps by acting directly onglucose transporters.