Effets des fluctuations rapides de la lumiere sur la photosynthese du phytoplancton

[¹⁴C]-assimilation rates were measured on cultures of two unicellulargreen algae (Chlamydomonas sp. and Oocystis sp.) as a functionof light intensities (saturation curves), under steady lightand also under rapidly alternating high and low light intensities.Assimilation rates vary according to the frequency of the intermittentlight regime and it falls under two categories: (1) at 0.1 and0.2 Hz, the assimilation rate is equal to the average of therates observed at high and low light intensities under steadylight, and (2) under 1.0, 1.6 and 10 Hz the assimilation rateis equal to the rate observed under a mean steady irradiance.Moreover, the range of assimilation rates at a given frequencydepends on the difference between the high and the low intensities.Batch cultures of Oocystis sp. have been grown under intermittentlight of 0.1, 1.0 and 10.0 Hz (same mean intensity). Growthrate under intermittent light of 0.1 Hz is –40% lowerthan the control under steady light. Photosynthetic potential(P^B_max)and efficiency () change with the growth stages of thecultures. At the end of the logarithmic growth phase, both photosyntheticparameters are maximum at 1.0 Hz and minimum at 0.1 Hz. Averagecell concentrations of chlorophyll a increase as the frequencyof the light regime decreases. During the log phase, concentrationof carotenoids relative to chlorophyll a increases at 1.0 Hz,decreases at 0.1 Hz, and remains constant at 10.0 Hz. Underclear sky conditions, wave-induced light fluctuations in thephotic layer may therefore enhance primary production, especially(1) in the lower part of the photic layer, where low frequencylight changes might cause cell chlorophyll a to increase, and(2) at a depth of 1–4 m, where the main frequencies (ofthe order of 1.0 Hz), might cause a significant increase ofboth the photosynthetic potential (P^B_max)and efficiency (). ¹Contribution au programme du GIROQ (Groupe interuniversitairede recherches océanographiques du Québec) ²Adresse actuelle: Centre de recherches en nutrition, UniversitéLaval, Québec, Qué. G1K 7P4, Canada     

Strategies d'adaptation photosynthetique chez des diatomees des I'Ocean Antarctique: variations du nombre et de la talile des unites photosynthetiques

Les diatomes Nitzschia turgiduloides et Chaetoceros deflandrei,rcoltes en fvrier–Mars 1980 dans le secteur indiende l'Ocan Antarctique, ont t maintenues en culture au laboratoire,sous des intensits himineuss variant de 140 2 µE m^–2s^–2. The production versus intensite lumineuse, tabliespar cellule et par unit de chlorophylle a, suggrent, selonla mthode de Richardson et al. (1983), une adaptation des espcesantarctiques la lumire par variation de la taille (estimepar la valeur I_k) et du nombre (volution de la pente initiale) des units photosynthtiques. Au plan ecologique, les valeursdes paramtres photosynthetiques en lumire Bmitante et en lumiresaturante sont faibles, caractristiques des algues ‘d'ombre’:concentration en chlorophylle a et productivit par celluleplus fortes quand les espces sont acclimates a des faiblesintensites lumineuses. Ce travail met ainsi en vidence I'importancedu facteur lumineux par rapport la basse temprature sur laproduction primaire de l'Ocan Antarctique.     

Observations sur la Presence de Certains Lamellibranches Lithophages Fossiles du Miocene dans le Sud-Est de l'Europe et dans la Mer Noire

Dans le Miocène moyen du Sud-Est de l'Europe on rencontretrès rarement des exemplaires fossiles de lamellibrancheslithophages, quelques-uns de ceux-ci appartenant au genre Lithophagaet d'autres au Pholas, Clavagella, Saxicava, Petricola, et Barnea.Certaines especes de Pholas et de Barnea sont rélativementsouvent rencontrees non seulement dans le Miocène moyen(etage Tortonien), mais aussi dans le Miocène supérieur(étage Sarmatien). En génèral ces espèces se rencontrent surun substrat calcaire. Å la fin du Miocène les espècesde lamellibranches fossiles lithophages disparaissent du basinde la Mer Noire, a cause que celui-ci est devenu un basin d'eaudouce. Mais, dans le temps actuel trois espèces de lamellibrancheslithophages vivent aussi dans la Mer Noire, venues dans le Quaternairede la Mer Mediterranee. C'est-à-dire: Pholas dactylusL., Barnea Candida var. pontica Mil., qui vivent sur un substratformé surtout de marnes argileuses ou de marnes calcaires.Sur un substrat de grès dures calcaire se rencontre presqueexclusivement Petricola lithophaga Retzius, sur la plateformerocheuse du sud de Constantza (Roumanie).     

C4荒漠植物猪毛菜与木本猪毛菜的叶片解剖结构及光合生理特征

 为了比较C₄荒漠植物猪毛菜(Salsola collina)和木本猪毛菜(S. arbuscula)的抗旱结构和适应环境的光合作用特征, 在二者混生的群落中, 选择代表性植株, 采集叶片进行叶片解剖结构分析, 在自然条件下测定了二者叶片的气体交换参数。研究结果表明：猪毛菜叶片具表皮毛, 具有更发达的薄壁贮水组织;木本猪毛菜叶片具有更厚的角质层, 表皮下有1层下皮细胞, 其栅栏组织细胞较长, 排列更紧密。猪毛菜的净光合速率明显高于木本猪毛菜, 日平均值分别为21.5和15.7 μmol CO₂·m^–2·s^–1。猪毛菜的蒸腾速率也明显高于木本猪毛菜, 日平均值分别为14.9和10.2 mmol·m^–2·s^–1。猪毛菜和木本猪毛菜的水分利用效率的日平均值分别为1.39和1.53 μmol CO₂·mmol^–1 H₂O, 特别是在14:00时分别为1.61和2.30 μmol CO₂·mmol^–1 H₂O, 木本猪毛菜高出猪毛菜约42%。猪毛菜的光补偿点低于木本猪毛菜, 而光饱和点和光量子效率较高, 具有更低的CO₂补偿点。这表明：二者的旱生结构不同, 木本猪毛菜具有更显著的荒漠植物特征;在适于二者混生的环境下, 猪毛菜比木本猪毛菜的光合能力更强, 而木本猪毛菜的水分利用效率更高。     

A Mutant of Arabidopsis thaliana That Defines a New Locus for Glycine Decarboxylation

A novel photorespiratory mutant of Arabidopsis thaliana, designatedgld2, was isolated based on a growth requirement for abnormallyhigh levels of atmospheric CO₂. Photosynthetic CO₂ fixationwas inhibited in the mutant following illumination in air butnot in atmosphere containing 2% O₂. Photosynthetic assimilationof ¹⁴CO₂ in an atmosphere containing 50% O₂ resulted in accumulationof 48% of the soluble label in glycine in the mutant comparedto 9% in the wild type. The rate of glycine decarboxylationby isolated mitochondria from the mutant was reduced to 6% ofthe wild type rate. In genetic crosses, the mutant complementedtwo previously described photorespiratory mutants of A. thalianathat accumulate glycine during photosynthesis in air due todefects in glycine decarboxylase (glyD, now designated gld1)and serine transhydroxymethylase (stm). Because glycine decarboxylaseis a complex of four enzymes, these results are consistent witha mutation in a glycine decarboxylase subunit other than thataffected in the gld1 mutant. The two gld loci were mapped tochromosomes 2 and 5, respectively. ³Present address: Department of Crop and Soil Sciences, MichiganState University, East Lansing, MI 48824, U.S.A. ⁴Present address: Department of Applied Bioscience, Facultyof Agriculture, Hokkaido University, Kita-Ku, Sapporo, 060 Japan ⁵Present address: Department of Biology, Carnegie Institutionof Washington, 290 Panama Street, Standford, CA 94305, U.S.A.     

THE RATIO OF CALCAREOUS AND ORGANIC SHELL COMPONENTS OF FRESHWATER SPHAERIID CLAMS IN RELATION TO WATER HARDNESS AND TROPHIC CONDITIONS

Shells from 14 populations of sphaeriid clams including Sphaeriumstriatinum, S. simile, Pisidium walkeri, Musculim partumeiumand M. iransversum were analyzed for organic carbon (µgCmg^–1 shell), nitrogen (µg,N mg^–1 shell) andCaCO_s (%CaCO₃ of total clam dry weight). Habitat waters wereassessed for total hardness (expressed as ppm CaCo₃), ppm Ca,ppm Mg, conductivity (µmho) and suspended organic Carbon(µgCl^–1). For all populations, shell organic C andN are positively correlated and there is an inverse relationshipbetween the amounts of shell CaCO₃ and shell organic carbon.Trophic considerations give the best correlation with shelltype at the generic level of consideration since species ofMusculium are found at the opposite end of the trophic scale(eutrophic) from all other populations studied. For S. striatinum,the most extensively studied species, the amount of shell CaCO₃is inversely related to water hardness. The selection of shellsin the Sphaeriidae is discussed in relation to structural-functionalneeds and habitat conditions ¹ Present Address: Department of Biology, Syracuse University,Syracuse, New York 13210, U.S.A. ² Present Address: Department of Zoology, Miami University,Oxford, Ohio 45056, U.S.A. (Received 5 December 1978;     

Isolation of the ALG6 locus of Saccharomyces cerevisiae required for glucosylation in the N-linked glycosylation pathway

N-Linked protein glycosylation in most eukaryotic cells initiateswith the transfer of the oligosaccharide Glc₃Man₉GlcNAc₂ fromthe lipid carrier dolichyl pyrophosphate to selected asparagineresidues. In the yeast Saccharomyces cerevisiae, alg mutationswhich affect the assembly of the lipid-linked oligosaccharideat the membrane of the endoplasmic reticulum result in the accumulationof lipid-linked oligosaccharide intermediates and a hypoglycosylationof proteins. Exploiting the synthetic growth defect of alg mutationsin combination with mutations affecting oligosaccharyl transferaseactivity (Stagljar et al., 1994), we have isolated the ALG6locus. alg6 mutants accumulate lipid-linked Man₉GlcNAc₂, suggestingthat this locus encodes an endoplasmic glucosyltransferase.Alg6p has sequence similarity to Alg8p, a protein required forglucosylation of Glc₁Man⁹GlcNAc². Saccharomyces cerevisiae endoplasmic reticulum glycosyltransferase dolichol     

Distinct K+ conductive pathways are required for Cl- and K+ secretion across distal colonic epithelium

Secretion of Cl^– and K⁺ in the colonic epithelium operates through a cellular mechanism requiring K⁺ channels in the basolateral and apical membranes. Transepithelial current [short-circuit current (I_sc)] and conductance (G_t) were measured for isolated distal colonic mucosa during secretory activation by epinephrine (Epi) or PGE₂ and synergistically by PGE₂ and carbachol (PGE₂ + CCh). TRAM-34 at 0.5 µM, an inhibitor of K_Ca3.1 (IK, Kcnn4) K⁺ channels (H. Wulff, M. J. Miller, W. Hänsel, S. Grissmer, M. D. Cahalan, and K. G. Chandy. Proc Natl Acad Sci USA 97: 8151–8156, 2000), did not alter secretory I_sc or G_t in guinea pig or rat colon. The presence of K_Ca3.1 in the mucosa was confirmed by immunoblot and immunofluorescence detection. At 100 µM, TRAM-34 inhibited I_sc and G_t activated by Epi (4%), PGE₂ (30%) and PGE₂ + CCh (60%). The IC₅₀ of 4.0 µM implicated involvement of K⁺ channels other than K_Ca3.1. The secretory responses augmented by the K⁺ channel opener 1-EBIO were inhibited only at a high concentration of TRAM-34, suggesting further that K_Ca3.1 was not involved. Sensitivity of the synergistic response (PGE₂ + CCh) to a high concentration TRAM-34 supported a requirement for multiple K⁺ conductive pathways in secretion. Clofilium (100 µM), a quaternary ammonium, inhibited Cl^– secretory I_sc and G_t activated by PGE₂ (20%) but not K⁺ secretion activated by Epi. Thus Cl^– secretion activated by physiological secretagogues occurred without apparent activity of K_Ca3.1 channels but was dependent on other types of K⁺ channels sensitive to high concentrations of TRAM-34 and/or clofilium. epinephrine; prostaglandin E₂; cholinergic; Kcnn4; TRAM-34; clofilium     

K+ channel KVLQT1 located in the basolateral membrane of distal colonic epithelium is not essential for activating Cl- secretion

The cellular mechanism for Cl^– and K⁺ secretion in the colonic epithelium requires K⁺ channels in the basolateral and apical membranes. Colonic mucosa from guinea pig and rat were fixed, sectioned, and then probed with antibodies to the K⁺ channel proteins K_VLQT1 (Kcnq1) and minK-related peptide 2 (MiRP2, Kcne3). Immunofluorescence labeling for Kcnq1 was most prominent in the lateral membrane of crypt cells in rat colon. The guinea pig distal colon had distinct lateral membrane immunoreactivity for Kcnq1 in crypt and surface cells. In addition, Kcne3, an auxiliary subunit for Kcnq1, was detected in the lateral membrane of crypt and surface cells in guinea pig distal colon. Transepithelial short-circuit current (I_sc) and transepithelial conductance (G_t) were measured for colonic mucosa during secretory activation by epinephrine (EPI), prostaglandin E₂ (PGE₂), and carbachol (CCh). HMR1556 (10 µM), an inhibitor of Kcnq1 channels (Gerlach U, Brendel J, Lang HJ, Paulus EF, Weidmann K, Brüggemann A, Busch A, Suessbrich H, Bleich M, and Greger R. J Med Chem 44: 3831–3837, 2001), partially (50%) inhibited Cl^– secretory I_sc and G_t activated by PGE₂ and CCh in rat colon with an IC₅₀ of 55 nM, but in guinea pig distal colon Cl^– secretory I_sc and G_t were unaltered. EPI-activated K⁺-secretory I_sc and G_t also were essentially unaltered by HMR1556 in both rat and guinea pig colon. Although immunofluorescence labeling with a Kcnq1 antibody supported the basolateral membrane presence in colonic epithelium of the guinea pig as well as the rat, the Kcnq1 K⁺ channel is not an essential component for producing Cl^– secretion. Other K⁺ channels present in the basolateral membrane presumably must also contribute directly to the K⁺ conductance necessary for K⁺ exit during activation of Cl^– secretion in the colonic mucosa. HMR1556; K⁺ secretion; epinephrine; prostaglandin E₂; cholinergic     

Induction and Repression of CAM in Sedum telephium L. in Response to Photoperiod and Water Stress

Lee, H. S. J. and Griffiths, H. 1987. Induction and repressionof CAM in Sedurn relephluni L. in response to photopcnod andwater stress.—J. exp. Bot. 38: 834–841. The introduction and repression of CAM in Sedurn telephiunmL, a temperate succulent, was investigated in watered, progressivelydrouglited and rewatered plants in growth chambers. Measurementswere made of water vapour and CO₂ exchange, titratable acidity(TA) and xylem sap tension. Effects of photoperiod were alsostudied. CAM was induced by drought under long or short days,although when watered no CAM activity was expressed. C₃-CAM intermediate plants were used for the investigation ofwater supply. Those which had received water and those drought-stressedboth displayed a similar nocturnal increase in TA, with a day-nightmaximum (H+) of 69 µmol g^–1 fr. wt. The wateredplants took up CO₂ at a maximum rate of 2?2 µmol m^–2s^–1 only in the light period, while the droughted plantsshowed a maximum nocturnal CO₂ uptake rate of 0?69 µmolm^–2 s^–1. Subsequently, as CAM was repressed, thewatered S. telephiwn displayed little variation in TA, withconstant levels at 42 µmol g^–1 fr. wt. (day 10).After 10 d of drought stress, the CAM characteristics of S.telephiurn were aLso affected, with reduced net CO₂ uptake andH+. The transition between C₃ and CAM in S. telephium can be describedas a progression in terms of the proportion of respiratory CO₂which is recycled and refixed at night as malic acid, in comparisonwith net CO₂ uptake. Recycling increased from 20% (day 1) to44% (day 10) as a result of the drought stress and was highin both the CAM-C₃ stage (no net CO2 uptake at night) and alsoin the drought-stressed CAM stage (reduced net CO₂ uptake atnight). The complete C₃-CAM transition occurred in less than8 d, and the stages could be characterized by xylem sap tensionmeasurements: CAM = 0?50 MPa C₃-CAM = 0?36 MPa C₃ = 0?29 MPa. Key words: CAM, Sedum telephium L., recycling     

Participation of Inorganic Orthophosphate in Regulation of the Ribulose-1,5-bisphosphate Carboxylase Activity in Response to Changes in the Photosynthetic Source-Sink Balance

Sink-limited conditions, defined as treatment with continuousillumination, cause a reduction in the rate of photosyntheticfixation of CO₂ in single-rooted leaves of soybean (Glycinemax. Merr. cv. Turunoko). We suggested previously that thisreduction is due to a deactivation of ribulose-1,5-bisphosphatecarboxylase (RuBPcase, E.C. 4.1.1.39 [EC] ) that is caused by a decreasein the level of P_i in the leaves [Sawada et al. (1989) PlantCell Physiol. 30: 691, Sawada et al. (1990) Plant Cell Physiol.31: 697]. In the present study, the mechanism of regulationof RuBPcase activity by P_i was examined. The activity of RuBPcasein the sink-limited leaves, exposed for 6 or 7 d to continuousillumination to alter the source/sink balance, was enhancedwith increasing concentrations of P_i, in a CO₂-free preincubationmedium in the presence of 5 mM MgCl₂ The maximum value [6.3µmole CO₂ (mg Chl)^–1 min^–1] was obtained atapproximately 5 mM P_i after a 5 min incubation, being 3 timesof the activity without the preincubation. The activity of acrude preparation of RuBPcase that had been deactivated by removalof CO₂ and Mg²⁺ ions by the gel filtration was 5.2–9.3nmole CO₂ (mg protein)^–1 min^–1 and was also enhancedby P_i plus Mg²⁺ ions. The maximum value [147–151 nmoleCO₂ (mg protein)^–1 min^–1] was attained at 5 mM P_iafter a 5 min incubation. The cycle of activation and inactivationof deactivated crude RuBPcase was perfectly reversible by additionof P_i to the enzyme and removal of P_i from the enzyme. Levelsof free P_i and of esterified phosphate in the sink-limited leaveswere 69% and 31% of the total phosphate, respectively. By contrast,in the control leaves, these values were 87% and 13%, respectively.These results support our previously stated hypothesis and indicatean important role for free P_i in the regulation of RuBPcaseactivity, in particular in sink-limited plants. (Received February 21, 1992; Accepted July 23, 1992)     

Functional characterization of two S-nitroso-L-cysteine transporters, which mediate movement of NO equivalents into vascular cells

In myogenic C₂C₁₂ cells, 5 mM creatine increased the incorporation of labeled [³⁵S]methionine into sarcoplasmic (+20%, P < 0.05) and myofibrillar proteins (+50%, P < 0.01). Creatine also promoted the fusion of myoblasts assessed by an increased number of nuclei incorporated within myotubes (+40%, P < 0.001). Expression of myosin heavy chain type II (+1,300%, P < 0.001), troponin T (+65%, P < 0.01), and titin (+40%, P < 0.05) was enhanced by creatine. Mannitol, taurine, and -alanine did not mimic the effect of creatine, ruling out an osmolarity-dependent mechanism. The addition of rapamycin, the inhibitor of mammalian target of rapamycin/70-kDa ribosomal S6 protein kinase (mTOR/p70^s6k) pathway, and SB 202190, the inhibitor of p38, completely blocked differentiation in control cells, and creatine did not reverse this inhibition, suggesting that the mTOR/p70^s6k and p38 pathways could be potentially involved in the effect induced by creatine on differentiation. Creatine upregulated phosphorylation of protein kinase B (Akt/PKB; +60%, P < 0.001), glycogen synthase kinase-3 (+70%, P < 0.001), and p70^s6k (+50%, P < 0.001). Creatine also affected the phosphorylation state of p38 (–50% at 24 h and +70% at 96 h, P < 0.05) as well as the nuclear content of its downstream targets myocyte enhancer factor-2 (–55% at 48 h and +170% at 96 h, P < 0.05) and MyoD (+60%, P < 0.01). In conclusion, this study points out the involvement of the p38 and the Akt/PKB-p70^s6k pathways in the enhanced differentiation induced by creatine in C₂C₁₂ cells. protein synthesis; insulin-like growth factor; mitogen-activated protein kinase; extracellular signal-regulated kinase 1/2; 70-kDa ribosomal S6 protein kinase     

Green Light Drives CO2 Fixation Deep within Leaves

Maximal ^l4CO₂-fixation in spinach occurs in the middle of thepalisade mesophyll [Nishio et al. (1993) Plant Cell 5: 953],however, ninety percent of the blue and red light is attenuatedin the upper twenty percent of a spinach leaf [Cui et al. (1991)Plant Cell Environ. 14: 493]. In this report, we showed thatgreen light drives ¹⁴CO₂-fixation deep within spinach leavescompared to red and blue light. Blue light caused fixation mainlyin the palisade mesophyll of the leaf, whereas red light drovefixation slightly deeper into the leaf than did blue light.¹⁴CO₂-fixation measured under green light resulted in less fixationin the upper epidermal layer (guard cells) and upper most palisademesophyll compared to red and blue light, but led to more fixationdeeper in the leaf than that caused by either red or blue light.Saturating white, red, or green light resulted in similar maximal¹⁴CO₂-fixation rates, whereas under the highest irradiance ofblue light given, carbon fixation was not saturated, but itasymptotically approached the maximal ¹⁴CO₂-fixation rates attainedunder the other types of light. The importance of green lightin photosynthesis is discussed. ¹Supported in part by grants from Competitive Research GrantsOffice, U.S. Department of Agriculture (Nos. 91-37100-6672 and93-37100-8855).     

Elevated resting [Ca(2+)](i) in myotubes expressing malignant hyperthermia RyR1 cDNAs is partially restored by modulation of passive calcium leak from the SR

Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle triggered in susceptible individuals by inhalation anesthetics and depolarizing skeletal muscle relaxants. This syndrome has been linked to a missense mutation in the type 1 ryanodine receptor (RyR1) in more than 50% of cases studied to date. Using double-barreled Ca²⁺ microelectrodes in myotubes expressing wild-type RyR1 (_WTRyR1) or RyR1 with one of four common MH mutations (_MHRyR1), we measured resting intracellular Ca²⁺ concentration ([Ca²⁺]_i). Changes in resting [Ca²⁺]_i produced by several drugs known to modulate the RyR1 channel complex were investigated. We found that myotubes expressing any of the _MHRyR1s had a 2.0- to 3.7-fold higher resting [Ca²⁺]_i than those expressing _WTRyR1. Exposure of myotubes expressing _MHRyR1s to ryanodine (500 µM) or (2,6-dichloro-4-aminophenyl)isopropylamine (FLA 365; 20 µM) had no effects on their resting [Ca²⁺]_i. However, when myotubes were exposed to bastadin 5 alone or to a combination of ryanodine and bastadin 5, the resting [Ca²⁺]_i was significantly reduced (P < 0.01). Interestingly, the percent decrease in resting [Ca²⁺]_i in myotubes expressing _MHRyR1s was significantly greater than that for _WTRyR1. From these data, we propose that the high resting myoplasmic [Ca²⁺]_i in _MHRyR1 expressing myotubes is due in part to a related structural conformation of _MHRyR1s that favors "passive" calcium leak from the sarcoplasmic reticulum. ryanodine; FLA 365; bastadin 5; resting intracellular calcium concentration; sarcoplasmic reticulum     

CORRIGENDUM

Vol. II, N. S. No. 5 (January 1938), article by A. H. K. PETRIEand j . G. WOOD. In Table VII, p. 53, all values of the coefficientsof A²_R and A²_T should be preceded by minus signs.     

Further Characteristics of Salt-Dependent Bicarbonate Use by the Seagrass Zostera muelleri

Millhouse, J. and Strother, S. 1987. Further characteristicsof salt-dependent bicarbonate use by the seagrass Zostera muelleri.—J.exp. Bot. 38: 1055–1068. The contribution of HCO^–₃to photosynthetic O₂ evolutionin the seagrass Zostera muelleri Irmisch ex Aschers. increasedwith increasing salinity of the bathing seawater when the inorganiccarbon concentration was kept constant. K_1/2 (seawater salts)for HCO^–₃ -dependent photosynthesis was 66% of seawatersalinity. Both short- and long-term pretreatment at low salinitiesstimulated photosynthesis in full strength seawater. Twentyfour hours pre-incubation of seagrass plants in 3·0 molm^–3 NaHCO₃ resulted in increased photosynthesis at allsalinities, apparently due to stimulation of HCO^–₃ use(K_1/2 (seawater salts) = 26%). V_max (HCO^–₃) was not affectedby low salinity pretreatment. The kinetics of HCO^–₃ stimulationby the major seawater cations was investigated. Ca²⁺ was themost effective cation with the highest V_max (HCO^–₃) andwith K_1/2(Ca²⁺) = 14 mol m^–3. Mg²⁺ was also very effectiveat less than 50 mol m^–3 but higher concentrations wereinhibitory. This inhibition cannot be accounted for solely byprecipitation of MgCO₃. Na⁺ and K⁺ were both capable of stimulatingHCO^–₃ use. Stimulation was in two distinct parts. Up to500 mol m^–3, both citrate and chloride salts gave similarresults (K_1/2(Na⁺) 81 mol m^–3, V_max(HCO^–₃) 0·26µmol O₂ mg^–1 chl min^–1), but use of citratesalts above 500 mol m^–2 caused a second stimulation ofHCO^–₃ use (K_1/2(Na⁺) 830 mol m^–3, V_max(HCO^–₃)0·68 µmol O₂ mg^–1 chl min^–1). V_max(HCO^–₃)for the second-phase Na⁺ or K⁺ stimulation was of the same orderas for Ca²⁺-stimulated HCO^–₃ use. To further characterizesalt-dependent HCO^–₃ use, the sensitivity of photosynthesisto Tris and TES buffers was investigated. The effects of Trisappear to be due to the action of Tris⁺ causing stimulationof HCO^–₃ -dependent photosynthesis in the absence of salt,but inhibition of HCO^–₃ use in saline media. TES has noeffect on photosynthesis. External carbonic anhydrase, althoughimplicated in salt-dependent HCO^–₃ use in Z. muelleri,could not be detected in whole leaves. Key words: Zostera muelleri, HCO^–₃ use, salinity     

Photosynthesis in Panicum milioides, a Species with Reduced Photorespiration

The capacity for C₄ photosynthesis in Panicum milioides, a specieshaving reduced levels of photorespiration, was investigatedby examining the activity of certain key enzymes of the C₄ pathwayand by pulse-chase experiments with ¹⁴CO₂. The ATP$P₁ dependentactivity of pyruvate,P₁ dikinase in the species was extremelylow (0.14–0.18 µmol mg chlorophyll^–1 min^–1).Low activity of the enzyme was also found in Panicum decipiensand Panicum hians (related species with reduced photorespiration)and in Panicum laxum (a C₃ species). The antibody to pyruvate,P₁dikinase caused about 70% inhibition of the ATP$P₁ dependentactivity of the enzyme in P. milioides. The activity of NAD-malicenzyme and NADP-malic enzyme in ^P. ^milioides was equally low(approximately 0.1–0.2 µmol mg chlorophyll^–1min^–1) and similar to the activity in P. decipiens, P.hians and P. laxum. Photosynthetic pulse-chase experiments underatmospheric conditions showed a typical C₃-like pattern of carbonassimilation including the labelling of glycine and serine asexpected during photorespiration. During the pulse with ¹⁴CO₂only about 1% of the labelled products appeared in malate and2–3% in aspartate. During a chase in atmospheric levelsof CO₂ for up to 6 min there was a slight increase in labellingin the C₄ acids. The amount of label in carbon 4 of aspartatedid not change during the chase, indicating little or no turnoverof the C₄ acid via decarboxylation. The results indicate thatunder atmospheric conditions P. milioides assimilates carbondirectly through the C₃ pathway. Photorespiration as indicatedby the CO₂ compensation point may be repressed in the speciesby a more efficient recycling of photorespired CO₂. (Received June 8, 1982; Accepted July 22, 1982)     

DCEBIO stimulates Cl- secretion in the mouse jejunum

We investigated the effects of 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one(DCEBIO) on the Cl^– secretory response of the mouse jejunum using the Ussing short-circuit current (I_sc) technique. DCEBIO stimulated a concentration-dependent, sustained increase in I_sc (EC₅₀ 41 ± 1 µM). Pretreating tissues with 0.25 µM forskolin reduced the concentration-dependent increase in I_sc by DCEBIO and increased the EC₅₀ (53 ± 5 µM). Bumetanide blocked (82 ± 5%) the DCEBIO-stimulated I_sc consistent with Cl^– secretion. DCEBIO was a more potent stimulator of Cl^– secretion than its parent molecule, 1-ethyl-2-benzimidazolinone. Glibenclamide or NPPB reduced the DCEBIO-stimulated I_sc by >80% indicating the participation of CFTR in the DCEBIO-stimulated I_sc response. Clotrimazole reduced DCEBIO-stimulated I_sc by 67 ± 15%, suggesting the participation of the intermediate conductance Ca²⁺-activated K⁺ channel (IK_Ca) in the DCEBIO-activated I_sc response. In the presence of maximum forskolin (10 µM), the DCEBIO response was reduced and biphasic, reaching a peak response of the change in I_sc of 43 ± 5 µA/cm² and then falling to a steady-state response of 17 ± 10 µA/cm² compared with DCEBIO control tissues (61 ± 6 µA/cm²). The forskolin-stimulated I_sc in the presence of DCEBIO was reduced compared with forskolin control tissues. Similar results were observed with DCEBIO and 8-BrcAMP where adenylate cyclase was bypassed. H89, a PKA inhibitor, reduced the DCEBIO-activated I_sc, providing evidence that DCEBIO increased Cl^– secretion via a cAMP/PKA-dependent manner. These data suggest that DCEBIO stimulates Cl^– secretion of the mouse jejunum and that DCEBIO targets components of the Cl^– secretory mechanism. 1-ethyl-2-benzimidazolinone; forskolin; glibenclamide; clotrimazole; H89     

Functional characterization of voltage-gated K+ channels in mouse pulmonary artery smooth muscle cells

Mice are useful animal models to study pathogenic mechanisms involved in pulmonary vascular disease. Altered expression and function of voltage-gated K⁺ (K_V) channels in pulmonary artery smooth muscle cells (PASMCs) have been implicated in the development of pulmonary arterial hypertension. K_V currents (I_K(V)) in mouse PASMCs have not been comprehensively characterized. The main focus of this study was to determine the biophysical and pharmacological properties of I_K(V) in freshly dissociated mouse PASMCs with the patch-clamp technique. Three distinct whole cell I_K(V) were identified based on the kinetics of activation and inactivation: rapidly activating and noninactivating currents (in 58% of the cells tested), rapidly activating and slowly inactivating currents (23%), and slowly activating and noninactivating currents (17%). Of the cells that demonstrated the rapidly activating noninactivating current, 69% showed I_K(V) inhibition with 4-aminopyridine (4-AP), while 31% were unaffected. Whole cell I_K(V) were very sensitive to tetraethylammonium (TEA), as 1 mM TEA decreased the current amplitude by 32% while it took 10 mM 4-AP to decrease I_K(V) by a similar amount (37%). Contribution of Ca²⁺-activated K⁺ (K_Ca) channels to whole cell I_K(V) was minimal, as neither pharmacological inhibition with charybdotoxin or iberiotoxin nor perfusion with Ca²⁺-free solution had an effect on the whole cell I_K(V). Steady-state activation and inactivation curves revealed a window K⁺ current between –40 and –10 mV with a peak at –31.5 mV. Single-channel recordings revealed large-, intermediate-, and small-amplitude currents, with an averaged slope conductance of 119.4 ± 2.7, 79.8 ± 2.8, 46.0 ± 2.2, and 23.6 ± 0.6 pS, respectively. These studies provide detailed electrophysiological and pharmacological profiles of the native K_V currents in mouse PASMCs. K_V channels     

Cl- secretory effects of EBIO in the rabbit conjunctival epithelium

Experiments were conducted to determine whether the Cl^– secretagogue, 1-ethyl-2-benzimidazolinone (EBIO), stimulates Cl^– transport in the rabbit conjunctival epithelium. For this study, epithelia were isolated in an Ussing-type chamber under short-circuit conditions. The effects of EBIO on the short-circuit current (I_sc) and transepithelial resistance (R_t) were measured under physiological conditions, as well as in experiments with altered electrolyte concentrations. Addition of 0.5 mM EBIO to the apical bath stimulated the control I_sc by 64% and reduced R_t by 21% (P < 0.05; paired data). Under Cl^–-free conditions, I_sc stimulation using EBIO was markedly attenuated. In the presence of an apical-to-basolateral K⁺ gradient and permeabilization of the apical membrane, the majority of the I_sc reflected the transcellular movement of K⁺ via basolateral K⁺ channels. Under these conditions, EBIO in combination with A23187 elicited nearly instantaneous 60–90% increases in I_sc that were sensitive to the calmodulin antagonist calmidazolium and the K⁺ channel blocker tetraethyl ammonium. In the presence of an apical-to-basolateral Cl^– gradient and nystatin permeabilization of the basolateral aspect, EBIO increased the Cl^–-dependent I_sc, an effect prevented by the channel blocker glibenclamide (0.3 mM). The latter compound also was used to determine the proportion of EBIO-evoked unidirectional ³⁶Cl^– fluxes in the presence of the Cl^– gradient that traversed the epithelium transcellularly. Overall, EBIO activated apical Cl^– channels and basolateral K⁺ channels (presumably those that are Ca²⁺ dependent), thereby suggesting that this compound, or related derivatives, may be suitable as topical agents to stimulate fluid transport across the tissue in individuals with lacrimal gland deficiencies. Ussing chamber; short-circuit current; electrolyte transport; chloride secretagogue; potassium conductance; 1-ethyl-2-benzimidazolinone; 1,10-phenanthroline