Fractional Retention Research Articles

Myocardial kinetics of Tc-99m glucarate in low flow, hypoxia, and aglycemia

Background. Technetium-99m glucarate is a myocardial infarct–avid imaging agent. Recent conflicting and inconclusive reports have suggested that the agent may be taken up by ischemic but viable myocardium. The purposes of this study were (1) to determine conclusively whether there is Tc-99m glucarate uptake in ischemic viable myocardium and (2) to investigate the potential mechanisms for such uptake by studying components of ischemia, namely, low flow, hypoxia, and aglycemia. Methods and Results. Rat hearts were isolated and perfused in a modified Langendorff preparation with a crytalloid perfusate. Tc-99m glucarate was studied in control (n = 6), low-flow (n = 5), hypoxic (n = 5), and aglycemic (n = 5) conditions. The experimental protocol consisted of 20-minute baseline (12 mL/min flow) and 20-minute treatment (low flow at 1 mL/min, hypoxia, or aglycemia), followed by tracer uptake (20 minute) and washout (20 minutes). Activity was monitored with a sodium iodide detector. The tracer was delivered continuously over a 20-minute uptake period. The injected dose was 150 μCi (5.6 MBq). Hemodynamics were monitored throughout. Triphenyltetrazolium chloride staining was used to assess myocardial viability. There was no evidence of myocardial necrosis. Low flow tended to delay tracer uptake compared with control for the first 10 minutes, but this did not reach statistical significance. Low flow increased end fractional retention significantly compared with control (mean ± SEM, 59.0% ± 0.9% peak vs 41.2% ± 1.4%, respectively; P < .05). Hypoxia resulted in a trend toward increased uptake; however, this was significant only at one early time point during the uptake phase. Retention in the hypoxia group was similar to control. Tc-99m glucarate uptake was significantly increased in aglycemia from 16 minutes to peak compared with control (1.36% ± 0.71% injected dose per gram vs 0.91% ± 0.37% injected dose per gram, respectively; P < .05). Aglycemia produced significantly higher end fractional retention compared with control (51.6% ± 1.8% peak vs 41.2% ± 1.4%, respectively; P < .05). Conclusions. Tc-99m glucarate myocardial retention is increased in the setting of ischemia, even in the absence of necrosis. This increased retention is not due to hypoxia. Furthermore, the retention is only partially explained by tissue hypoglycemia. Thus low flow per se appears to have a role in this increased retention, probably as a result of delayed flow-dependent washout. (J Nucl Cardiol 2003;10:168-76)

Differential retention and utilization of dissolved organic carbon by bacteria in river sediments

The differential ability of natural sediment biofilms to store and metabolize specific dissolved organic carbon (DOC) fractions was studied with a set of perfusion experiments that combined a chromatographic method for the analysis of several DOC fractions with the measurement of bacterial abundance and activity. High proportions of low‐molecular‐weight substances and polysaccharides and low but significant proportions of humic substances were retained in sediment cores after perfusion. Bacterial abundance and production in these cores were high (1.7—3.0 × 109 cells cm−3 and 0.7—12.0 µg C cm−3 h−1, respectively). Bacterial abundance, production, and turnover times were correlated with the retention of various DOC fractions, especially polysaccharides, indicating a differential microbial utilization of DOC. Temperature as well as the quantity and quality of retained organic matter were the major factors that influenced bacterial production. However, particulate organic carbon represented the main energy source for sediment bacteria. It is concluded that certain fractions of DOC may be readily utilized for bacterial growth, irrespective of molecular size. Instead, the presence and abundance of specific chemical groups might be decisive. Hence, the processing of organic substrates by the microbial biofilm may largely influence the biogeochemistry of DOC in river waters.

Retention of β‐Carotene Encapsulated in a Trehalose‐based Matrix as Affected by Water Content and Sugar Crystallization

ABSTRACT: Retention of β‐carotene encapsulated in an amorphous trehalose‐gelatin matrix and surface color changes were studied during storage of samples at several water activities. The fractional retention could be described by a first order reaction kinetics. The higher retention of β‐carotene (about 80% in 6 mo) was obtained in the not crystallized samples, while for crystallized samples exposed to water activity 0.75 the loss was almost complete. Trehalose crystallization could not be delayed by the presence of MgCl2 as occurs in the pure sugar systems. Surface color change was not a sensitive indication of carotene retention, which was mainly related to excess water over that needed to form the dihydrate crystals.

Bioavailability of selenium from raw or cured selenomethionine-enriched fillets of Atlantic salmon (Salmo salar) assessed in selenium-deficient rats.

The bioavailability of Se from raw and cured selenomethionine-enriched (Se-enriched) salmon fillets was assessed in Se-deficient male albino rats (Mol: Wist). A low-Se Torula yeast feed was supplemented with 0, 50, 100, 150 or 200 microg Se/kg as sodium selenite or as Se from raw or cured Se-enriched salmon. The diets were fed to weanling rats for 10 and 30 d. Bioavailability of Se was assessed by metabolic balance, Se accumulation in femur, muscle, liver and plasma, and induction of Se-dependent glutathione peroxidase (EC 1.11.1.9; GSHpx) in plasma as response parameters. Except for the metabolic balance results, the slope-ratio method was used when calculating Se bioavailability from raw or cured Se-enriched fish fillets (test food) relative to sodium selenite (standard). The data for fractional apparent absorption and fractional retention showed differences (P<0.05) among all three Se sources in the order raw salmon > cured salmon > selenite. At 10 d, Se from raw and cured Se-enriched fish fillets tended to be more bioavailable than selenite. This was supported by the observations for Se accumulation in femur and muscle and induction of GSHpx activity. At 30 d, all response parameters showed a higher bioavailability of Se from raw and cured Se-enriched fish fillets compared with selenite. Differences (P<0.05) in Se accumulation in muscle at 10 and 30d, and differences (P<0.05) in fractional apparent absorption and fractional retention suggested that curing salmon altered the utilisation of Se. The experimental results showed that enrichment of fish fillets with selenomethionine yields fillets with high Se bioavailability.

Evaluation of Cadmium Toxicity on Survival, Accumulation and Depuration in an Intertidal Gastropod, Turbo Intercostalis

The effect of cadmium on tolerance, accumulation and depurationwas studied in an intertidal gastropod, Turbo intercostalis. Initially, the LC50 value for 96 hr wasdetermined by exposing the snails to different concentrations ofcadmium (2.5 to 12.5 ppm) and it was observed to be 6.18 mg L-1. Further, the accumulation and depuration of cadmium was investigated in different tissues namely GDG (Gonad-digestive gland) complex, mantle and foot of T. intercostalis on exposure to a sublethal concentration (1.24 mgL-1) for a period of 20 days. The accumulation of cadmium was found to be more in mantle and GDG complex and less in footduring this exposure and the tissue accumulation was in the order of GDG complex > mantle > foot. The GDG complex was largely responsible for most of the body burden of the metal. The percent body burden of the cadmium exhibited an increasingtrend with exposure time in GDG complex while in mantle therewas a decreasing trend. The foot tissue was observed to have aless percentage of body burden of cadmium. The fractional retention coefficients were also calculated and they showed a gradual increase with exposure time indicating more retention of metal in the tissues. However, depuration of the metal followed a similar trend of increase in all the tissues. The data suggestthat T. intercostalis have a good capacity ofaccumulation and depuration of heavy metal and it may be considered as a suitable species for biological monitoring of the coastal environment.

Effect of Different Operational Parameters in the Enhanced Biological Phosphorus Removal Process. Experimental Design and Results

The uncontrolled dumping of phosphorus into a water environment creates serious problems of eutrophication, affecting water quality and causing grave problems in the aquatic ecosystem. European legislation demands drastic reduction of phosphorus dissolved in wastewater. Enhanced Biological Phosphorus Removal (EBPR) is the current tendency in wastewater treatment. This biological process depends on a multiplicity of variables, but its three main factors are: influent COD/P ratio, anaerobic fraction and sludge retention time (SRT). The aim of this work is to statistically determine the effect of these three parameters in EBPR through a response surface methodology. The objective function that has been chosen is phosphorus removed per unit biomass formed. This function provides ample information on BPR, since the quantity of phosphorus removed depends on the accumulative capacity of the microorganisms present. Two levels were chosen for the SRT (5 and 10 days), two for the anaerobic fraction (0.1 and 0.2), and six for levels of the influent COD/P ratio (between 16 and 87). The experiments were undertaken at pilot scale (100 litres) with an A2/O configuration, with simultaneous nitrogen and phosphorous removaL The wastewater used is a synthetic mixture of complex sources of carbon and nitrogenwithout volatile fatty acids. The empirical model obtained indicates that the factor most influencing the EBPR process is the influent COD/P ratio, whilst the anaerobic fraction is that which has least influence. Additionally, there is an optimum in the influent COD/P ratio that is to be found between 41 and 48.

Effect of Different Operational Parameters in the Enhanced Biological Phosphorus Removal Process. Experimental Design and Results

The uncontrolled dumping of phosphorus into a water environment creates serious problems of eutrophication, affecting water quality and causing grave problems in the aquatic ecosystem. European legislation demands drastic reduction of phosphorus dissolved in wastewater. Enhanced Biological Phosphorus Removal (EBPR) is the current tendency in wastewater treatment. This biological process depends on a multiplicity of variables, but its three main factors are: influent COD/P ratio, anaerobic fraction and sludge retention time (SRT). The aim of this work is to statistically determine the effect of these three parameters in EBPR through a response surface methodology. The objective function that has been chosen is phosphorus removed per unit biomass formed. This function provides ample information on BPR, since the quantity of phosphorus removed depends on the accumulative capacity of the microorganisms present. Two levels were chosen for the SRT (5 and 10 days), two for the anaerobic fraction (0.1 and 0.2), and six for levels of the influent COD/P ratio (between 16 and 87. The experiments were undertaken at pilot scale (100 litres) with an A2/O configuration, with simultaneous nitrogen and phosphorous removal. The wastewater used is a synthetic mixture of complex sources of carbon and nitrogen without volatile fatty acids. The empirical model obtained indicates that the factor most influencing the EBPR process is the influent COD/P ratio, whilst the anaerobic fraction is that which has least influence. Additionally, there is an optimum in the influent COD/P ratio that is to be found between 41 and 48.

Bioavailability of cadmium from infant diets in newborn rats.

Infants are exposed to higher levels of cadmium (Cd) from infant and follow-on formulas than from breast milk. We studied the bioavailability of 109CdCl2 from cows' milk formula, soy formula, wheat/oat/milk formula, wholemeal/milk formula and water in 11-day-old rat pups. The pups received a single oral dose of one diet labelled with 109Cd, 0.1 or 0.3 mg Cd/kg body weight. After 2 or 24 h or 4, 9 or 12 days the fractional retention of 109Cd in the whole body, in segments of rinsed small intestine and in tissue was measured in a gamma counter. Pups receiving 109Cd in water or cows' milk formula had the highest mean whole-body retention. It ranged from 67% of the dose in the water group to 52% in the wholemeal/milk formula group 4 days after dosing. The retention of 109Cd in the rinsed small intestine was significantly higher in the water group and the cows' milk formula group than in the cereal-based formula groups at 24 h and 4 days after dosing. It was still high in all groups on day 9, ranging from 26 to 11%. Initially most of the 109Cd was retained in the duodenum but by day 4 it had moved further down into the jejunum. In the liver, the highest and lowest retention on day 4 was 16%, and 3 per thousand of the dose in the water group and wholemeal/milk formula group, respectively. In the kidney, 109Cd was still increasing 12 days after exposure in all groups. Whole-body retention and tissue levels were higher than previously reported in adult animals. The lower bioavailability of 109Cd from the cereal-based formulas compared to water and cows milk formula on the longer survival times is most likely explained by Cd binding to dietary fibre and phytic acid in the cereal-based formulas reducing the intestinal binding and decreasing the bioavailability of Cd. The high retention of 109Cd in the small intestine, leading to a prolonged absorption period, emphasizes the importance of extending studies on neonatal Cd absorption over a long time period in order to detect for example, endpoints, accumulation of Cd in the kidney.

A typical path model of tracheobronchial clearance of inhaled particles in rats.

A mathematical description of particle clearance from the ciliated conducting airways (tracheobronchial region) of the lungs in rats was developed, assuming that particles on the mucus blanket behave as a fluid and adhere to principles of fluid flow described by the continuity equation. Effective particle transport velocities for given generations of airways were estimated from reported tracheal mucus velocities. Using typical rat airway geometry and estimated particle transport velocities, solutions of sets of rate equations for transport from each generation of airways were summed to estimate total particle clearance from the tracheobronchial region of the lung as a function of time. Aerosol particle size distribution (MMAD ranging from 0.1 to 4.2 microm, and sigma(g) from 1 to 2.7) and concentration data from several investigators were used to predict short-term, tracheobronchial clearance (retention) in rats up to 24 h following exposure. Comparisons between predicted and observed retention showed an average difference between model predictions, and observed fractional retention of initial lung or body burden was 4.9%, with a tendency toward underprediction of clearance of particles >3.0 microm.

Behavior of tungsten exposed to high fluences of low energy hydrogen isotopes

Tungsten is a candidate plasma facing material under investigation in a Sandia National Laboratories project conducted at Los Alamos National Laboratory. Samples of 99.95% tungsten provided by Plansee Aktiengesellschaft were exposed to 100 eV deuterium and tritium ions at a range of fluxes from 2.3×10 17 to 1.3×10 18 ions/ cm 2 s for one hour at 623 K in the tritium plasma experiment. The samples were outgassed to determine the amount of retained hydrogen isotopes. The retention scaled at slightly greater than the square root of the fluence. The fractional retention was on the order of 10 −5. The data from these experiments were combined with previous results to construct a comprehensive model of the migration and retention behavior for hydrogen in tungsten. A second set of experiments involved exposing 99.95% tungsten foils provided by ÆSAR to 100 eV deuterons at a flux of 6×10 17 D/ cm 2 s for 30 min at 423 and 373 K. Scanning Electron Microscopy analysis was performed on the samples to determine the effects of the plasma exposure. Unannealed samples revealed extensive blistering with many blister caps removed. Samples annealed to 1473 K showed minor blistering, and samples annealed to 1273 K showed no blistering. The SEM analysis was used in conjunction with the retention results to understand the role of annealing and defects in trapping within the tungsten.

Changes in the regulation of calcium metabolism and bone calcium content during growth in the absence of endogenous prolactin and during hyperprolactinemia: A longitudinal study in male and female Wistar rats

Since endogenous prolactin has been shown to enhance food consumption, calcium absorption, and bone calcium turnover in the pregnant rat, the role of endogenous prolactin in the regulation of calcium metabolism was investigated in 3-day balance studies of female Wistar rats from the age of 3 to 11 weeks. The study was divided into two parts. In part I, calcium metabolism in males and females was compared. In part II, 3-week old female rats were divided into 5 groups: (i) control animals receiving 0.9% NaCl; (ii) animals receiving 6 mg bromocriptine/kg/day (- PRLendogroup); (iii) animals receiving 2.5 mg ovine prolactin/kg/day (+PRLexo); (iv) sham-operated animals receiving 0.9% NaCl, and (v) animals with two extra pituitaries implanted under the renal capsule, receiving 0.9% NaCl (AP group). Results showed that rapid growth occurred between 3 and 6 weeks with maximum fractional calcium absorption and calcium retention at 5 weeks of age in both sexes. The data also showed a physiological significance of endogenous prolactin in enhancing calcium absorption and retention in 5 week old rats. In an absence of prolactin, peak calcium absorption was delayed in 7-week old animals, and vertebral calcium content of 11-week old animals was reduced by 18%. Hyperprolactinemia in the AP group was found to enhance fractional calcium absorption and calcium retention at 7, 9, and 11 weeks and increased the femoral calcium content by 16%. It could be concluded that a physiological role of prolactin is the stimulation of calcium absorption and maintainance of bone calcium content during growth and development.Key words: bone calcium content, calcium absorption, calcium balance, hyperprolactinemia, prolactin.

The fractional urinary fluoride excretion in young children under stable fluoride intake conditions.

The purpose of this study was to determine the fraction of the total daily fluoride intake that is excreted through the urine (FUEF) of children aged 3-5 years under usual intake conditions. Participating children were residents of an area with a fluoride (F) concentration of 0.5-0.6 mg/L in their drinking water. Assessments were made on two successive 24-h periods on 20 children, measuring the total amount of fluoride ingested through liquid and food consumption, and from ingestion of fluoridated toothpaste (500 microg F/g), together with the determination of the amount of fluoride excreted through urine. Fluoride retention was also estimated assuming a constant average F fraction of 10% excreted through faeces. It was found that the average proportion of liquids, solid foods, and toothpaste to the daily fluoride intake (1.02-mg F/day on average) were 40.8, 34.6, and 24.5%, respectively. The average FUEF value was 35.5% (95% C.I.=31.7-39.3%), and the estimated fractional F retention was 54.5%. The present data suggest a slight relationship between FUEF values and the inverse of the daily fluoride dose (1/dose) (r=0.513; P=0.021). When the present results are combined with those from previous studies on F-retention and urinary excretion, the correlation between both FUEF and fractional retention and 1/dose are very strong and highly significant (r=0.98, P<0.0001, and r=-0.986, P<0.0001, respectively). A possible mechanism is suggested in order to explain this latter finding. The potential usefulness of the current FUEF value for the estimation of daily F intake (or dose) from urinary F excretion data is also discussed.

Tolerance, accumulation and depuration in an intertidal gastropod, Turbo intercostalis, exposed to cadmium

Laboratory experiments were carried out to study cadmium (Cd) toxicity in an intertidal gastropod, Turbo intercostalis. Tolerance studies revealed an LC 50 of 6.18 ppm for 96 h. Experiments were also conducted to determine oxygen consumption and bioaccumulation of the metal by exposing the snails to a sublethal concentration of Cd (1.24 ppm) for 20 days. A significant fall in oxygen consumption was observed from 48 h onwards with maximum decrease (65.5%) at 20 days. The accumulation of Cd was determined in gonad-digestive gland (GDG) complex, mantle and foot at different intervals. There was a gradual increase in accumulation with exposure time in all tissues. The accumulation of Cd was: GDG complex>mantle>foot. The percentage body burden of the metal showed a decreasing trend in the mantle and an increasing trend in GDG complex. The fractional retention coefficients increased with exposure time. The snails exhibited a tissue-specific depuration of the metal and the trend was similar to that of accumulation. The results suggest that T. intercostalis may be a useful species for biomonitoring studies.

Tritium retention in tungsten exposed to intense fluxes of 100 eV tritons

Tungsten is a candidate material for the International Thermonuclear Experimental Reactor (ITER) as well as other future magnetic fusion energy devices. Tungsten is well suited for certain fusion applications in that it has a high threshold for sputtering as well as a very high melting point. As with all materials to be used on the inside of a tokamak or similar device, there is a need to know the behavior of hydrogen isotopes embedded in the material. With this need in mind, the Tritium Plasma Experiment (TPE) has been used to examine the retention of tritium in tungsten exposed to very high fluxes of 100 eV tritons. Both tungsten and tungsten containing 1% lanthanum oxide were used in these experiments. Measurements were performed over the temperature range of 423–973 K. After exposure to the tritium plasma, the samples were transferred to an outgassing system containing an ionization chamber for detection of the released tritium. The samples were outgassed using linear ramps from room temperature up to 1473 K. Unlike most other materials exposed to energetic tritium, the tritium retention in tungsten reaches a maximum at intermediate temperatures with low retention at both high and low temperatures. For the very high triton fluences used (>10 25 T/m 2), the fractional retention of the tritium was below 0.02% of the incident particles. This report presents not only the results of the tritium retention, but also includes the modeling of the results and the implication for ITER and other future fusion devices where tungsten is used.

Sample Mass Effects on Thermal Field-Flow Fractionation Retention and Universal Calibration

The effect of sample mass on retention in thermal field-flow fractionation (FFF) can be large, becoming more important as the molecular weight increases. Greater retention with increased sample mass is observed beginning at the detection limit of 0.1 μg of polystyrene in tetrahydrofuran (THF) with higher molecular weight material. Empirical equations have been developed to correct for changes in calibration parameters for the thermal FFF universal calibration curves due to the sample mass effect on polymer retention for polystyrene, polymethylmethacrylate, and polyisoprene in THF.

99mTc-HL91: "hot spot" detection of ischemic myocardium in vivo by gamma camera imaging.

99mTc-HL91 is a new hypoxia imaging agent that demonstrates increased uptake and retention in globally hypoxic myocardium in vitro. The purpose of this study was to determine whether 99mTc-HL91 could detect regional ischemia in vivo by gamma camera imaging. Eight open-chest dogs with left circumflex (LCx) stenoses were studied. Injection of 5 mCi of 99mTc-HL91 and microspheres was followed by imaging over 4 hours. Heart slices were imaged, then stained with triphenyltetrazolium chloride (TTC), and tissues were well-counted. TTC staining demonstrated no injury. Mean LCx blood flow was 0.32+/-0.04 mL x min(-1) x g(-1), and mean left anterior descending coronary artery (LAD) flow was 0.96+/-0.02 mL x min(-1) x g(-1) (ratio, 0.33). "Hot spots" were detected in 8 of 8 experiments in vivo within 60 minutes and improved over 4 hours. Region of interest analysis of LCx/LAD activity ratios demonstrated significant increases within 30 minutes (final ratio, 3.0; P<0.05). LCx and LAD washout curves demonstrated significant differences within 15 minutes. Washout curves were biexponential over 1 hour, followed by linear retention from 1 to 4 hours. Four-hour fractional retention was 0.12 for LAD and 0.44 for LCx (P<0.01). Myocardial flow versus tracer uptake demonstrated 2 phases: phase 1 (flow, 0.05 to 0.7 mL x min(-1) x g(-1)) had an inverse linear correlation (r= -0.80); phase 2, (flow, >0.7 mL x min(-1) x g(-1)) had no correlation. Ischemic heart/liver ratios remained near 1.0 for 4 hours. 99mTc-HL91 positively identifies regional myocardial ischemia in a canine model using 99mTc imaging. Quantitative techniques allowed identification of ischemic myocardium within 15 minutes of tracer administration.

Influence of Fluid Velocity and Cell Concentration on the Transport of Motile and Nonmotile Bacteria in Porous Media

The effect of fluid velocity on the transport of motile and nonmotile bacteria was studied in saturated soil columns using radiolabeled cells. According to colloid filtration theory, decreasing the bulk fluid velocity in a porous medium increases the number of collisions of passive colloids with particles and, therefore, should result in increased colloid retention in porous media. However, for motile cells, there was a variation in cell retention significantly different from that predicted by filtration theory at low fluid velocities, leading to the conclusion that filtration theory is not applicable for this motile bacterial strain at low fluid velocities. As the pore velocity was decreased from 120 to 0.56 m/day, the fractional retention of motile cells (Pseudomonas florescens P17) decreased by 65%, and the collision efficiency (α) defined as the ratio of particles that attach to soil grains to particles that collide with the soil (calculated using a filtration equation) decreased from 0.37 (120 m/day) to 0.003 (0.56 m/day). For passive colloids, the fractional retention (if α is a constant equal to 0.01) would increase by more than 800% over this same velocity range. To support our conclusion that cell motility was the factor producing this change from filtration theory, we rendered P17 cells nonmotile and tested this strain and a second nonmotile strain [Burkholderia (Pseudomonas) cepacia G4] under the same conditions. Collision efficiencies for both nonmotile suspensions were constant. For nonmotile P17, α was equal to 0.018 ± 0.003 (0.56−590 m/day). Over a wide velocity range for nonmotile G4, α was equal to 0.22 ± 0.067 (11−560 m/day). Swimming cells were presumably able to avoid sticking to soil grains at low fluid velocities, but at high fluid velocities, cell motility did not reduce attachment. Two additional factors known to affect cell transport (solution ionic strength and cell concentration) were also examined with these two strains in porous media. Decreasing the ionic strength from 4.14 to 0.0011 mM (at a constant pH) decreased cell retention for motile P17 by 39 ± 12%, but this is less of a reduction than is typically observed for nonmotile strains. Increasing the cell concentrations of motile P17 increased the overall retention of cells, suggesting that previously deposited cells provided a more favorable surface for adhesion than the native soil (ripening). In contrast, increasing the cell concentrations of G4 resulted in lower retention, suggesting that deposited cells provided a less favorable collector surface (blocking). These results need to be further investigated with other motile and nonmotile species. However, our results do suggest that wider dispersal of cells during bioaugmentation than previously thought possible may be achieved by using a combination of motile cells, low pumping velocities, and low ionic strength solutions. Optimal cell concentrations to use for in situ bioaugmentation of contaminated soil will depend on the adhesion of the bacterial strains for soil grains and with each other, but in general blocking-type cells are capable of greater dispersal at higher concentration than ripening-type cells.

Foliar retention of 15N tracers: implications for net canopy exchange in low- and high-elevation forest ecosystems

Nitrogen-15 ( 15N) labelled ammonium and nitrate were applied to understory saplings in a low-elevation deciduous forest and a high-elevation coniferous forest during the 1993 growing season to estimate the minimum retention of inorganic N from wet deposition. Tissue samples were collected at both sites before and after the tracer applications to determine the initial and the final 15N content. The fraction of the tracer application retained by deciduous leaves or by current-year leaves and stems on conifers was calculated from measurements of 15N and total N content. At both study sites, tracer applications significantly increased the 15N content of sapling tissues, although there was no significant difference between the fractional retention of ammonium and nitrate. The results support earlier studies indicating that the foliar retention of 15N applied in simulated wet deposition is small. Our estimates of annual N uptake from wet deposition by the forest canopy at the low- and high-elevation study sites were only 14 and 7%, respectively. This study, along with prior studies of the different chemical forms of N deposition, indicates that canopy uptake of dry N deposition is a major contributor to the net canopy exchange of N in both forests.

Benthic organic matter dynamics in an intermittent stream in South-East Spain

This study investigates (1) the standing stocks of benthic particulate organic matter (BPOM) and its fractions; (2) their temporal and spatial variability; (3) the environmental variables that explain their dynamics; and (4) the relationship between the standing stocks of the different fractions and total BPOM on the one hand and functional feeding group densities on the other. The study was conducted at four sties (two pools and two runs) located along a perennial middle section (354 m length) of Chicamo stream, a 4th-order saline and intermittent tributary of the Segura river basin in southeast Spain. Three benthic organic matter samples were collected monthly by corers in a stratified pattern in each site during 1994. Coarse (CPOM, >1 mm), fine (FPOM, 56 μm-1 mm) and ultrafine (UPOM, 0.7-56μm) size fractions were determined. Benthic POM at all sites was dominated by FPOM, followed by UPOM and CPOM fractions. There were no significant differences in the overall amount of BPOM or its fractions between dates. In contrast, spatial variation was considerable between sites, in accordance with the high spatial heterogeneity of the stream. Habitat-specific variables (type and size of habitat, substrate, current, depth and type of primary producers) determined the production, retention and storage of the different BPOM fractions in the streambed. Shredders were absent from the stream due to the scarcity of their food (CPOM). No significant correlations were found between the densities of the detritivorous feeding groups and their presumed food resources. This lack of correlation may reflect an over-abundance of detritus in the stream.

Designed Disulfide between N-terminal Domains of Lactose Repressor Disrupts Allosteric Linkage

Substitution of Cys for Val at position 52 of the lac repressor was designed to permit disulfide bond formation between the two N-terminal DNA binding domains that comprise an operator DNA binding site. This position marks the closest approach of these domains based on the x-ray crystallographic structures of the homologous purine holorepressor-operator complex and lac repressor-operator complex (Schumacher, M. A., Choi, K. Y., Zalkin, H., and Brennan, R. G. (1994) Science 266, 763-770; Lewis, M., Chang, G., Horton, N.C., Kercher, M. A., Pace, H. C., Schumacher, M. A., Brennan, R. G., and Lu, P. (1996) Science 271, 1247-1254). The V52C mutation was generated by site-specific methods, and the mutant protein was purified and characterized. In the reduced form, V52C bound operator DNA with slightly increased affinity. Exposure to oxidizing conditions resulted in disulfide bond formation, and the oxidized protein bound operator DNA with approximately 6-fold higher affinity than wild-type protein. Inducer binding for both oxidized and reduced forms of V52C was comparable to wild-type lac repressor. In the presence of inducer, the reduced protein exhibited wild-type, diminished DNA binding. In contrast, DNA binding for the oxidized form was unaffected by inducer, even at 1 mM. Thus, the formation of the designed disulfide between Cys52 side chains within each dimer renders the protein-operator complex unresponsive to sugar binding, presumably by disrupting the allosteric linkage between operator and inducer binding.