Food size selectivity of Artemia franciscana at three developmental stages

Food size selectivity was examined in Artemia franciscana metanaupliiat three different developmental stages. Clearance rates weredetermined in short-term experiments either by measuring thedecrease in concentration of live particles and plastic beads,or by measuring the radioactivity accumulated in animals thatgrazed ¹⁴C-labelled live particles. The maximum clearance rateofA.franciscana metanauplii increased during development andwas measured at 50–63 µl ind.^–1 h^–1,254 µl ind.^–1 h^–1 and 1.48–2.10 ml ind.^–1h^–1 in 2-, 4- and 7-day-old metanauplii, respectively.A preference for particles with a diameter of 4–8 µmwas observed at all three developmental stages. The abilityof A.franciscana metanauplii to graze bacterial particles wasalso demonstrated, although the efficiency in grazing such smallparticles was low compared to microalgae (28, 20 and 9% of themaximum clearance rate in 2-, 4- and 7-day-old metanauplii,respectively). Electron microscopy showed that the inter-setulardistance in antennae and thoracopods was 0.20 ± 0.07,0.16 ± 0.05 and 0.18 ± 0.04 µm in 2-, 4-and 7-day-old metanauplii, respectively, and accordingly independentof stage.     

Effects of sodium, potassium and chloride ions on the membrane potential of Valonia aegagropila

Changes of the resting potential of Valonia cell in sea wateragainst a 10-fold increase of the external concentrations ofK^$, Na^$ and Cl^– were 1±1, 6.2±0.1 and 38.9±4mV, respectively. The potassium conductance was smaller than7 µ/cm², while the Na and Cl conductances were 45 and281 µ/cm², respectively, in normal sea water. The positivevacuolar potential could be explained by these ionic conductances.On the other hand, the membrane became more sensitive to K^$,if the cell was incubated for about 30 min in K-rich (100 mM)sea water. It is worth noting, however, that the membrane conductancewas lower in the K-rich sea water than in the normal sea water. (Received October 7, 1974; )     

N-Ethylmaleimide Blocks the H+ Pump in the Plasma Membrane of Chara corallina Internodal Cells

The sulfhydryl (SH) modifying reagent N-ethylmaleimide (NEM)was applied to the internodal cells of Chara corallina to studythe role of SH residues in the activity of the plasma membraneH⁺ pump. NEM (1 µM) caused a marked depolarizing shiftof the resting potential by 6410mV (n=7) together with depressionof the conductance peak at around —200 mV, indicatinga marked depression of the H⁺ pump activity. This effect ofNEM was partly reversible, the membrane repolarized and theconductance peak was restored after extracellular washing. TheH⁺ pump inhibitor, dicyclohexylcarbodiimide (DCCD), caused noadditive membrane depolarization and/or depression of the H⁺pump conductance, in the presence of NEM. This suggests thatNEM blocks the H⁺ pump and that SH residues play a pivotal rolein maintaining the H⁺ pump activity in Chara corallina. (Received April 10, 1993; Accepted July 29, 1993)     

P2 purinoceptors regulate calcium-activated chloride and fluid transport in 31EG4 mammary epithelia

It has been reported thatsecretory mammary epithelial cells (MEC) release ATP, UTP, and UDP uponmechanical stimulation. Here we examined the physiological changescaused by ATP/UTP in nontransformed, clonal mouse mammary epithelia(31EG4 cells). In control conditions, transepithelial potential (apicalside negative) and resistance were 4.4 ± 1.3 mV (mean ± SD, n = 12) and 517.7 ± 39.4  · cm², respectively. The apicalmembrane potential was 43.9 ± 1.7 mV, and the ratio of apicalto basolateral membrane resistance (R_A/R_B) was 3.5 ± 0.2. Addition of ATP or UTP to the apical or basolateral membranescaused large voltage and resistance changes with an EC₅₀ of~24 µM (apical) and ~30 µM (basal). Apical ATP/UTP (100 µM)depolarized apical membrane potential by 17.6 ± 0.8 mV (n = 7) and decreasedR_A/R_B by a factor of3. The addition of adenosine to either side (100 µM) hadno effect on any of these parameters. The ATP/UTP responses werepartially inhibited by DIDS and suramin and mediated by a transientincrease in free intracellular Ca²⁺ concentration (427 ± 206 nM; 15-25 µM ATP, apical; n = 6). This Ca²⁺ increase was blocked by cyclopiazonic acid, by BAPTA,or by xestospongin C. 31EG4 MEC monolayers also secreted or absorbedfluid in the resting state, and ATP or UTP increased fluid secretion by5.6 ± 3 µl · cm² · h¹(n = 10). Pharmacology experiments indicate that 31EG4epithelia contain P2Y₂ purinoceptors on the apical andbasolateral membranes, which upon activation stimulate apicalCa²⁺-dependent Cl channels and cause fluid secretion acrossthe monolayer. This suggests that extracellular nucleotides could playa fundamental role in mammary gland paracrine signaling and theregulation of milk composition in vivo.

     

K+ current inhibition by amphiphilic fatty acid metabolites in rat ventricular myocytes

Fatty acid metabolites accumulate in the heart underpathophysiological conditions that affect -oxidation and can elicit marked electrophysiological changes that are arrhythmogenic. The purpose of the present study was to determine the impact of amphiphilic fatty acid metabolites on K⁺currents that control cardiac refractoriness and excitability. Transient outward(I_to) andinward rectifier(I_K1)K⁺ currents were recorded by thewhole cell voltage-clamp technique in rat ventricular myocytes, and theeffects of two major fatty acid metabolites were examined:palmitoylcarnitine and palmitoyl-coenzyme A (palmitoyl-CoA).Palmitoylcarnitine (0.5-10 µM) caused a concentration-dependent decrease in I_todensity in myocytes internally dialyzed with the amphiphile; 10 µMreduced mean I_todensity at +60 mV by 62% compared with control(P < 0.05). In contrast, externalpalmitoylcarnitine at the same concentrations had no effect, nor didinternal dialysis significantly alterI_K1. Dialysiswith palmitoyl-CoA (1-10 µM) produced a smaller decrease inI_to densitycompared with that produced by palmitoylcarnitine; 10 µM reduced meanI_to density at+60 mV by 37% compared with control(P < 0.05). Both metabolites delayedrecovery of I_tofrom inactivation but did not affect voltage-dependent properties.Moreover, the effects of palmitoylcarnitine were relatively specific,as neither palmitate (10 µM) nor carnitine (10 µM) alone significantly influencedI_to when added tothe pipette solution. These data therefore suggest that amphiphilicfatty acid metabolites downregulateI_to channels by amechanism confined to the cytoplasmic side of the membrane. Thisdecrease in cardiac K⁺ channelactivity may delay repolarization under pathophysiological conditionsin which amphiphile accumulation is postulated to occur, such asdiabetes mellitus or myocardial infarction.

     

Pacemaker activity in urethral interstitial cells is not dependent on capacitative calcium entry

The aim of the present study was to investigate the properties and role of capacitative Ca²⁺ entry (CCE) in interstitial cells (IC) isolated from the rabbit urethra. Ca²⁺ entry in IC was larger in cells with depleted intracellular Ca²⁺ stores compared with controls, consistent with influx via a CCE pathway. The nonselective Ca²⁺ entry blockers Gd³⁺ (10 µM), La³⁺ (10 µM), and Ni²⁺ (100 µM) reduced CCE by 67% (n = 14), 65% (n = 11), and 55% (n = 9), respectively. These agents did not inhibit Ca²⁺ entry when stores were not depleted. Conversely, CCE in IC was resistant to SKF-96365 (10 µM), wortmannin (10 µM), and nifedipine (1 µM). Spontaneous transient inward currents were recorded from IC voltage-clamped at –60 mV. These events were not significantly affected by Gd³⁺ (10 µM) or La³⁺ (10 µM) and were only slightly decreased in amplitude by 100 µM Ni²⁺. The results from this study demonstrate that freshly dispersed IC from the rabbit urethra possess a CCE pathway. However, influx via this pathway does not appear to contribute to spontaneous activity in these cells. smooth muscle; patch clamp; spontaneous transient inward currents     

A P2X7 receptor stimulates plasma membrane trafficking in the FRTL rat thyrocyte cell line

Thyroid cells express a variety of P2Y and P2X purinergic receptor subtypes. G protein-coupled P2Y receptors influence a wide variety of thyrocyte-specific functions; however, functional P2X receptor-gated channels have not been observed. In this study, we used whole cell patch-clamp recording and fluorescence imaging of the plasma membrane marker FM1-43 to examine the effects of extracellular ATP on membrane permeability and trafficking in the Fisher rat thyroid cell line FRTL. We found a cation-selective current that was gated by ATP and 2',3'-O-(4-benzoylbenzoyl)-ATP but not by UTP. The ATP-evoked currents were inhibited by pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid, adenosine 5'-triphosphate-2',3'-dialdehyde, 100 µM Zn²⁺, and 50 µM Cu²⁺. Fluorescence imaging revealed pronounced, temperature-sensitive stimulation of exocytosis and membrane internalization by ATP with the same pharmacological profile as observed for activation of current. The EC₅₀ for ATP stimulation of internalization was 440 µM in saline containing 2 mM Ca²⁺ and 2 mM Mg²⁺, and 33 µM in low-Mg²⁺, nominally Ca²⁺-free saline. Overall, the results are most consistent with activation of a P2X₇ receptor by ATP^4–. However, low permeability to N-methyl-D-glucamine⁺ and the propidium cation YO-PRO-1 indicates absence of the cytolytic pore that often accompanies P2X₇ receptor activation. ATP stimulation of internalization occurs in Na⁺-free, Ca²⁺-free, or low-Mg²⁺ saline and therefore does not depend on cation influx through the ATP-gated channel. We conclude that ATP activation of a P2X₇ receptor stimulates membrane internalization in FRTL cells via a transduction pathway that does not depend on cation influx. purinergic receptor; internalization; patch clamp     

ENaC- and CFTR-dependent ion and fluid transport in mammary epithelia

Mammary epithelial 31EG4 cells (MEC) were grown as monolayers onfilters to analyze the apical membrane mechanisms that help mediate ionand fluid transport across the epithelium. RT-PCR showed the presenceof cystic fibrosis transmembrane conductance regulator (CFTR) andepithelial Na⁺ channel (ENaC) message, and immunomicroscopyshowed apical membrane staining for both proteins. CFTR was alsolocalized to the apical membrane of native human mammary ductepithelium. In control conditions, mean values of transepithelialpotential (apical-side negative) and resistance(R_T) are 5.9 mV and 829  · cm², respectively. The apical membranepotential (V_A) is 40.7 mV, and the mean ratioof apical to basolateral membrane resistance (R_A/R_B) is 2.8. Apicalamiloride hyperpolarized V_A by 19.7 mV andtripled R_A/R_B. AcAMP-elevating cocktail depolarized V_A by 17.6 mV, decreased R_A/R_B by60%, increased short-circuit current by 6 µA/cm²,decreased R_T by 155  · cm², and largely eliminated responses toamiloride. Whole cell patch-clamp measurements demonstratedamiloride-inhibited Na⁺ currents [linear current-voltage(I-V) relation] and forskolin-stimulated Clcurrents (linear I-V relation). A capacitance probe methodshowed that in the control state, MEC monolayers either absorbed orsecreted fluid (2-4µl · cm² · h¹). Fluidsecretion was stimulated either by activating CFTR (cAMP) or blockingENaC (amiloride). These data plus equivalent circuit analysis showedthat 1) fluid absorption across MEC is mediated byNa⁺ transport via apical membrane ENaC, and fluid secretionis mediated, in part, by Cl transport via apicalCFTR; 2) in both cases, appropriate counterions move throughtight junctions to maintain electroneutrality; and 3)interactions among CFTR, ENaC, and tight junctions allow MEC to eitherabsorb or secrete fluid and, in situ, may help control luminal[Na⁺] and [Cl].

     

Rates of Photosynthesis in Characean Cells: I. PHOTOSYNTHETIC 14CO2 FIXATION BY NITELLA TRANSLUCENS

A study has been made of photosynthetic ¹⁴CO₂ fixation by isolated‘mature’ internodes of Nitella translucens. Experimentalconditions were similar to those used in studies of the ionicrelations of these cells. Maximum rates of photosynthesis were33–40µµmoles CO₂, fixed per cm² of surfacearea per second (equivalent to 12–15 /xmoles fixed permg chlorophyll per hour). ^l4CO₂ fixation was inhibited to thedark level by 3(3,4,dichlorophenyl)-1, 1-dimethylurea (at 0-6µM or 10µM) and by the uncoupler carbonyl cyanide-m-chlorophenylhydrazone(SµM). The presence of imidazole or ammonium sulphate(both of which uncouple ATP production in vitro) did not resultin an inhibition of 14CO2 fixation. These results are discussedin relation to published work on solute uptake by Nitella translucens.During photosynthesis there was rapid movement of ¹⁴C-labelledorganic compounds out of the chloroplasts. ¹⁴C-labelled sucrose,ammo-acids, and sugar phosphates were found in samples of vacuolarsap.     

Effect of IBMX and alkaline phosphatase inhibitors on Cl- secretion in G551D cystic fibrosis mutant mice

Some cysticfibrosis transmembrane conductance regulator (CFTR) mutations, such asG551D, result in a correctly localized Cl channel at the cellapical membrane, albeit with markedly reduced function. Patch-clampstudies have indicated that both phosphatase inhibitors and3-isobutyl-1-methylxanthine (IBMX) can induceCl secretion through theG551D mutant protein. We have now assessed whether these agents caninduce Cl secretion incftr^G551D mutantmice. No induction of Clsecretion was seen with the alkaline phosphatase inhibitorsbromotetramisole or levamisole in either the respiratory or intestinaltracts of wild-type orcftr^G551D mice.In contrast, in G551D intestinal tissues, IBMX was able to produce asmall CFTR-related secretory response [means ± SE: jejunum,1.8 ± 0.9 µA/cm²,n = 7; cecum, 3.7 ± 0.8 µA/cm²,n = 7; rectum (in vivo),1.9 ± 0.9 mV, n = 5]. Thiswas approximately one order of magnitude less than the wild-typeresponse to this agent and, in the cecum, was significantly greaterthan that seen in null mice(cftr^UNC). Inthe trachea, IBMX produced a transientCl secretory response (37.3 ± 14.7 µA/cm²,n = 6) of a magnitude similar to thatseen in wild-type mice (33.7 ± 4.7 µA/cm²,n = 9). This response was also presentin null mice and therefore is likely to be independent of CFTR. Noeffect of IBMX on Clsecretion was seen in the nasal epithelium ofcftr^G551D mice.We conclude that IBMX is able to induce detectable levels ofCFTR-related Cl secretionin the intestinal tract but not the respiratory tract through the G551Dmutant protein.

     

Photosynthesis of Stands of Tomato, Cucumber and Sweet Pepper Measured in Greenhouses under Various CO2-concentrations

The rate of net photosynthesis (P) of whole plant stands oftomato (Lycopersicon esculentum Mill.), cucumber (Cucumis sativusL.) and sweet pepper (Capsicum annuum L.) was measured in sixlong-term experiments in large greenhouses under normal operatingconditions and CO₂-concentrations between 200 and 1200 µmolmol^-1. The objective was to quantify the responses to lightand carbon dioxide and to obtain data sets for testing simulationmodels. The method of measuring canopy photosynthesis involvedan accurate estimation of the greenhouse CO₂ balance, usingnitrous oxide (N₂O) as tracer gas to determine, on-line, theexchange rate between greenhouse and outside air. The estimatedrelative error in the observed P was about ± 10%, exceptthat higher relative errors could occur under particular conditions. A regression equation relating P to the photosynthetically activeradiation, the CO₂ concentration and the leaf area index explained83-91% of the variance. The main canopy photosynthesis characteristicscalculated with the fitted regression equations were: canopyP_max 5-9 g m^-2 h^-1 CO₂ uptake; ratio P_max/LAI 1·5-3 gm^-2 h^-1; light compensation point 32-86 µmol s^-1 m^-2;light use efficiency (quantum yield) at low light 0·06-0·10µmol µmol^-1 and CO₂ compensation point 18-54 µmolmol^-1. The results were related to the prevailing conditions.Copyright1994, 1999 Academic Press Canopy photosynthesis, Capsicum annuum L., carbon dioxide, CO₂, CO₂ balance, CO₂ use efficiency, cucumber, _{Cucumis sativus} L., glasshouse, greenhouse, light use efficiency, Lycopersicon esculentum Mill., sweet pepper, tomato, tracer gas     

{Omega}-3 and {omega}-6 polyunsaturated fatty acids block HERG channels

Dietary polyunsaturated fatty acids (PUFAs) have been reported to exhibit antiarrhythmic properties, which have been attributed to their availability to modulate Na⁺, Ca²⁺, and several K⁺ channels. However, their effects on human ether-a-go-go-related gene (HERG) channels are unknown. In this study we have analyzed the effects of arachidonic acid (AA, -6) and docosahexaenoic acid (DHA, -3) on HERG channels stably expressed in Chinese hamster ovary cells by using the whole cell patch-clamp technique. At 10 µM, AA and DHA blocked HERG channels, at the end of 5-s pulses to –10 mV, to a similar extent (37.7 ± 2.4% vs. 50.2 ± 8.1%, n = 7–10, P > 0.05). 5,6,11,14-Eicosatetrayenoic acid, a nonmetabolizable AA analog, induced effects similar to those of AA on HERG current. Both PUFAs shifted the midpoint of activation curves of HERG channels by –5.1 ± 1.8 mV (n = 10, P < 0.05) and –11.2 ± 1.1 mV (n = 7, P < 0.01). Also, AA and DHA shifted the midpoint of inactivation curves by +12.0 ± 3.9 mV (n = 4; P < 0.05) and +15.8 ± 4.3 mV (n = 4; P < 0.05), respectively. DHA and AA accelerated the deactivation kinetics and slowed the inactivation kinetics at potentials positive to +40 mV. Block induced by DHA, but not that produced by AA, was higher when measured after applying a pulse to –120 mV (IO). Finally, both AA and DHA induced a use-dependent inhibition of HERG channels. In summary, block induced by AA and DHA was time, voltage, and use dependent. The results obtained suggest that both PUFAs bind preferentially to the open state of the channel, although an interaction with inactivated HERG channels cannot be ruled out for AA. K⁺ channel; membrane currents; ion channels; arrhythmia; antiarrhythmics     

Characterization of vectorial chloride transport pathways in the human pancreatic duct adenocarcinoma cell line HPAF

Pancreatic duct cells express a Ca²⁺-activated Cl^- conductance (CaCC), upregulation of which may be beneficial to patients with cystic fibrosis. Here, we report that HPAF, a human pancreatic ductal adenocarcinoma cell line that expresses CaCC, develops into a high-resistance, anion-secreting epithelium. Mucosal ATP (50 µM) caused a fourfold increase in short-circuit current (I_sc), a hyperpolarization of transepithelial potential difference (from -4.9 ± 0.73 to -8.5 ± 0.84 mV), and a fall in resistance to less than one-half of resting values. The effects of ATP were inhibited by mucosal niflumic acid (100 µM), implicating an apical CaCC in the response. RT-PCR indicated expression of hClC-2, hClC-3, and hClC-5, but surprisingly not hCLCA-1 or hCLCA-2. K⁺ channel activity was necessary to maintain the ATP-stimulated I_sc. Using a pharmacological approach, we found evidence for two types of K⁺ channels in the mucosal and serosal membranes of HPAF cells, one activated by chlorzoxazone (500 µM) and sensitive to clotrimazole (30 µM), as well as one blocked by clofilium (100 µM) but not chromanol 293B (5 µM). RT-PCR indicated expression of the Ca²⁺-activated K⁺ channel KCNN4, as well as the acid-sensitive, four transmembrane domain, two pore K⁺ channel, KCNK5 (hTASK-2). Western blot analysis verified the expression of CLC channels, as well as KCNK5. We conclude that HPAF will be a useful model system for studying channels pertinent to anion secretion in human pancreatic duct cells. Ussing chamber; short-circuit current; RT-PCR; immunoblot     

Intermediate-conductance Ca2+-activated K+ channel is expressed in C2C12 myoblasts and is downregulated during myogenesis

We report here the expression in C₂C₁₂ myoblasts of the intermediate-conductance Ca²⁺-activated K⁺ (IK_Ca) channel. The IK_Ca current, recorded under perforated-patch configuration, had a transient time course when activated by ionomycin (0.5 µM; peak current density 26.2 ± 3.7 pA/pF; n = 10), but ionomycin (0.5 µM) + 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one (100 µM) evoked a stable outward current (28.4 ± 8.2 pA/pF; n = 11). The current was fully inhibited by charybdotoxin (200 nM), clotrimazole (2 µM), and 5-nitro-2-(3-phenylpropylamino)benzoic acid (300 µM), but not by tetraethylammonium (1 mM) or D-tubocurarine (300 µM). Congruent with the IK_Ca channel, elevation of intracellular Ca²⁺ in inside-out patches resulted in the activation of a voltage-insensitive K⁺ channel with weak inward rectification, a unitary conductance of 38 ± 6 pS (at negative voltages), and an IC₅₀ for Ca²⁺ of 530 nM. The IK_Ca channel was activated metabotropically by external application of ATP (100 µM), an intracellular Ca²⁺ mobilizer. Under current-clamp conditions, ATP application resulted in a membrane hyperpolarization of 35 mV. The IK_Ca current downregulated during myogenesis, ceasing to be detectable 4 days after the myoblasts were placed in differentiating medium. Downregulation was prevented by the myogenic suppressor agent basic FGF (bFGF). We also found that block of the IK_Ca channel by charybdotoxin did not inhibit bFGF-sustained myoblast proliferation. These observations show that in C₂C₁₂ myoblasts the IK_Ca channel expression correlates inversely with differentiation, yet it does not appear to have a role in myoblast proliferation. ATP; cell proliferation     

Inhibition of Cl- Channel Activation in Chara corallina Membrane by Lanthanum Ion

We examined a role of Ca²⁺ in the activation of the two majorion channels, i.e., Cl^– and K⁺ channels at the excitationof the characean plasmalemma. The current-voltage relation (I-Vcurve) of the Chara membrane was compared under the ramp voltageclamp condition before and after external application of 20µMof La³⁺ (a Ca²⁺ channel blocker). The transient inward currentcomponent, which is carried mainly by the efflux of Cl^–,disappeared almost completely in about 30 min with La³⁺ treatment.On the other hand, no effect was observed on the late largeoutward current, which is mainly carried by the efflux of K⁺in a large depolarization region (less negative than –50mV). These results suggest that the Cl^– channel in theChara plasmalemma is activated by Ca²⁺ influx, while the K⁺channel is simply activated by depolarization. (Received April 7, 1986; Accepted June 6, 1986)     

Micromolar Ca2+ from sparks activates Ca2+-sensitive K+ channels in rat cerebral artery smooth muscle

The goal of the present study was to testthe hypothesis that local Ca²⁺ release events(Ca²⁺ sparks) deliver high local Ca²⁺concentration to activate nearby Ca²⁺-sensitiveK⁺ (BK) channels in the cell membrane of arterial smoothmuscle cells. Ca²⁺ sparks and BK channels were examined inisolated myocytes from rat cerebral arteries with laser scanningconfocal microscopy and patch-clamp techniques. BK channels had anapparent dissociation constant for Ca²⁺ of 19 µM and aHill coefficient of 2.9 at 40 mV. At near-physiological intracellularCa²⁺ concentration ([Ca²⁺]_i; 100 nM) and membrane potential (40 mV), the open probability of a singleBK channel was low (1.2 × 10⁶). A Ca²⁺spark increased BK channel activity to 18. Assuming that 1-100% of the BK channels are activated by a single Ca²⁺ spark, BKchannel activity increases 6 × 10⁵-fold to 6 × 10³-fold, which corresponds to ~30 µM to 4 µM sparkCa²⁺ concentration.1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acidacetoxymethyl ester caused the disappearance of all Ca²⁺sparks while leaving the transient BK currents unchanged. Our resultssupport the idea that Ca²⁺ spark sites are in closeproximity to the BK channels and that local[Ca²⁺]_i reaches micromolar levels to activateBK channels.

     

Protease-activated receptor regulation of Cl- secretion in Calu-3 cells requires prostaglandin release and CFTR activation

Human lung epithelial (Calu-3) cells were used to investigate the effects of protease-activated receptor (PAR) stimulation on Cl^– secretion. Quantitative RT-PCR (QRT-PCR) showed that Calu-3 cells express PAR-1, -2, and -3 receptor mRNAs, with PAR-2 mRNA in greatest abundance. Addition of either thrombin or the PAR-2 agonist peptide SLIGRL to the basolateral solution of monolayers mounted in Ussing chambers produced a rapid increase in short-circuit current (I_sc: thrombin, 21 ± 2 µA; SLIGRL, 83 ± 22 µA), which returned to baseline within 5 min after stimulation. Pretreatment of monolayers with the cell-permeant Ca²⁺-chelating agent BAPTA-AM (50 µM) abolished the increase in I_sc produced by SLIGRL. When monolayers were treated with the cyclooxygenase inhibitor indomethacin (10 µM), nearly complete inhibition of both the thrombin- and SLIGRL-stimulated I_sc was observed. In addition, basolateral treatment with the PGE₂ receptor antagonist AH-6809 (25 µM) significantly inhibited the effects of SLIGRL on I_sc. QRT-PCR revealed that Calu-3 cells express mRNAs for CFTR, the Ca²⁺-activated KCNN4 K⁺ channel, and the KCNQ1 K⁺ channel subunit, which, in association with KCNE3, is known to be regulated by cAMP. Stimulation with SLIGRL produced an increase in apical Cl^– conductance that was blocked in cells expressing short hairpin RNAs designed to target CFTR. These results support the conclusion that PAR stimulation of Cl^– secretion occurs by an indirect mechanism involving the synthesis and release of prostaglandins. In addition, PAR-stimulated Cl^– secretion requires activation of CFTR and at least two distinct K⁺ channels located in the basolateral membrane. cystic fibrosis transmembrane conductance regulator; KCNQ1; calcium-activated potassium channels; KCNN4; cAMP     

Are Mistletoes Shade Plants? CO2Assimilation and Chlorophyll Fluorescence of Temperate Mistletoes and their Hosts

Mistletoes usually have slower rates of photosynthesis thantheir hosts. This study examines CO₂assimilation, chlorophyllfluorescence and the chlorophyll content of temperate host–parasitepairs (nine hosts parasitized by Ileostylus micranthus and Carpodetusserratus parasitized by Tupeia antarctica). The hosts of I.micranthus had higher mean annual CO₂assimilation (3.59 ±0.41 µmol m^-2 s^-1) than I. micranthus(2.42 ± 0.20µmol m^-2 s^-1), and C. serratus(2.41 ± 0.43 µmolm^-2 s^-1) showed higher CO₂assimilation than T. antarctica(0.67± 0.64 µmol m^-2 s^-1). Hosts saturated at significantlyhigher electron transport rates (ETR) and light levels thanmistletoes. The positive relationship between CO₂assimilationand electron transport suggests that the lower CO₂assimilationrates in mistletoes are a consequence of lower electron transportrates. When photosynthetic rates, ETR and chlorophyll a /b ratioswere adjusted for photosynthetically active radiation, hostsdid not have significantly higher CO₂assimilation (3.21 ±0.37 µmol m^-2 s^-1) than mistletoes (2.54 ± 0.41µmol m^-2 s^-1), but still had significantly higher ETRand chlorophyll a / b ratios. The electron transport rates,saturating light and chlorophyll a / b ratios of sun leavesfrom mistletoes were similar to host shade leaves. These responsesindicate that in comparison with their hosts, mistletoe leaveshave the photosynthetic characteristics of the leaves of shadeplants. Copyright 2000 Annals of Botany Company CO₂assimilation, photosynthetic active radiation (PAR), chlorophyll fluorescence, electron transport rate (ETR), photochemical quenching (q_p), non-photochemical quenching (q_n), sun and shade leaves, chlorophyll content, Ileostylus micranthus, Tupeia antarctica, New Zealand     

Biochemical Limitations to Carbon Assimilation in C3 Plants--A Retrospective Analysis of the A/Ci Curves from 109 Species

Species-specific differences in the assimilation of atmosphericCO₂ depends upon differences in the capacities for the biochemicalreactions that regulate the gas-exchange process. Quantifyingthese differences for more than a few species, however, hasproven difficult. Therefore, to understand better how speciesdiffer in their capacity for CO₂ assimilation, a widely usedmodel, capable of partitioning limitations to the activity ofribulose-1,5-bisphosphate carboxylase-oxygenase, to the rateof ribulose 1,5-bisphosphate regeneration via electron transport,and to the rate of triose phosphate utilization was used toanalyse 164 previously published A/C_i, curves for 109 C₃ plantspecies. Based on this analysis, the maximum rate of carboxylation,Vc_max, ranged from 6µmol m^–2 s^–1 for the coniferousspecies Picea abies to 194µmol m^–2 s^–1 forthe agricultural species Beta vulgaris, and averaged 64µmolm^–2 s^–1 across all species. The maximum rate ofelectron transport, J_max, ranged from 17µmol m^–2s^–1 again for Picea abies to 372µmol m^–2 s^–1for the desert annual Malvastrum rotundifolium, and averaged134µmol m^–2 s^–1 across all species. A strongpositive correlation between Vc_max and J_max indicated that theassimilation of CO₂ was regulated in a co-ordinated manner bythese two component processes. Of the A/C_i curves analysed,23 showed either an insensitivity or reversed-sensitivity toincreasing CO₂ concentration, indicating that CO₂ assimilationwas limited by the utilization of triose phosphates. The rateof triose phosphate utilization ranged from 4·9 µmolm^–2 s^–1 for the tropical perennial Tabebuia roseato 20·1 µmol m^–2 s^–1 for the weedyannual Xanthium strumarium, and averaged 10·1 µmolm^–2 s^–1 across all species. Despite what at first glance would appear to be a wide rangeof estimates for the biochemical capacities that regulate CO₂assimilation, separating these species-specific results intothose of broad plant categories revealed that Vc_max and J_maxwere in general higher for herbaceous annuals than they werefor woody perennials. For annuals, Vc_max and J_max averaged 75and 154 µmol m^–2 s^–1, while for perennialsthese same two parameters averaged only 44 and 97 µmolm^–2 s^–1, respectively. Although these differencesbetween groups may be coincidental, such an observation pointsto differences between annuals and perennials in either theavailability or allocation of resources to the gas-exchangeprocess. Key words: A/C_i curve, CO₂ assimilation, internal CO₂ partial pressure, photosynthesis     

Effect of Abscisic Acid on the K+ Efflux and Membrane Potential of Nicotiana tabacum L. Leaf Cells

K⁺ efflux from tobacco (Nicotiana tabacum L, cv. Samsun NN)leaf discs into the external medium was increased and the membranepotential (Em) changed in the positive direction with a changein pH from 8.0 to 4.0. E_m was affected by the external concentrationof KCl, greatly decreasing with a change in concentration from1 mM to 100 mM. The equilibrium potential of the membrane forK⁺ (E_k) was decreased in a Nernst fashion with increasing externalconcentrations of KCl. E_k is more positive than E_m above ca.50 µM KCl. Most of the experiments were carried out underconditions in which the difference between the electrochemicalpotential for K⁺ on the inside to the outside of the cell (µ_kis positive. Thus, K⁺ may passively flow to the outside of thecells accompanied by the depolarization of the membrane. Abscisic acid (ABA) increased the K⁺ efflux under conditionsof passive transport. K⁺ efflux was accelerated with an increasingconcentration of ABA, being maximal at 10^–4 M–10^–3M. This acceleration was due to the enhancement of the potassiummotive force (µ_k/F) which is the force causing the netpassive transport of K⁺. The membrane potential was decreasedfrom –205 mV to –170 mV by 2 x 10^–4 M ABAwithin 10 min. The depolarization was not transient, being lostfor at least 3 hr. These results show that ABA accelerated passive K⁺ efflux, whichaccompanied depolarization of the membrane. (Received June 22, 1981; Accepted August 24, 1981)