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PubMed1975

Studies on the absorption and retention of calcium and phosphorus by young and mature Ca-deficient sheep.

1. A combination of a mineral balance and a radioactive technique has been used to study the effects of increased levels of dietary calcium and phosphorus on the Ca and P metabolism of young and mature wether sheep previously made Ca-deficient. 2. Ca absorption was directly related to Ca intake until an absorption maximum was reached. Young animals absorbed dietary Ca more efficiently than mature animals and had a much higher maximum rate of absorption. Apparent P absorption was directly related to P intake and maximum P absorption was also higher for the young animals. 3. Excretion of Ca in the urine and into the intestine was not altered by changes in Ca intake. Excretion of P in the urine, however, varied considerably and it is suggested that this process may be largely responsible for P homoeostasis. 4. Ca retention was directly related to Ca absorption. Maximum retention was considerably higher for the young animals than for the mature animals and occurred for each group of animals at exactly the same dietary intake as maximum Ca absorption. 5. Ca and P were retained in a constant ratio of 1-25:1 and the results suggested that P retention was controlled by the rate of Ca retention. 6. Bone turnover was higher for the young than for the mature animals. Animals mobilized skeletal Ca when receiving the Ca-deficient diet and replaced their Ca losses when the dietary Ca was plentiful by changing the relative rates of bone resorption and bone accretion. Although bone resorption was mainly responsible for Ca homoeostasis, bone accretion appeared to play some part in the young animals. 7. Maximum retention of Ca occurred just as the rate of bone resorption became zero and was equal to the rate of bone accretion. The lower maximum retention of Ca for mature animals was due to a lower rate of bone accretion. 8. Results suggested that when bone resorption ceased, Ca absorption was regulated by homoeostatic mechanisms at a level just sufficient to maintain maximum retention, and that the maximum rates of absorption measured may not have been true maximum rates. 9. The consequences of a relationship between bone accretion and maximum Ca retention are discussed with reference to older, pregnant and lactating animals.

Age Factors↗

PubMed2004

Influence of operating parameters on the retention of chromatographic particles by thermal field-flow fractionation.

We have investigated the retention behavior of chromatographic particles in thermal field-flow fractionation (FFF). Retention time is found to increase with increasing temperature drop across the channel thickness, as expected for species exhibiting a thermophoretic mobility. Experiments have been performed with a vertically oriented channel rather than by using the classical horizontal configuration as this leads to much more reproducible retention data. In acetonitrile, silica-based particles are more retained than octadecyl-bonded silica particles, which confirms our previous finding, by means of a different method, that the thermophoretic mobility of the latter is smaller than that of the former. Whatever the type of particles and the nature of the carrier liquid, the relative retention time is observed to decrease with increasing carrier flow rate. This indicates that a hydrodynamic lift force acts on particles to move them away from the accumulation wall, as is usually observed in all FFF experiments with micrometer-sized particles. However, upward and downward flow directions in the vertical channel lead to similar retention data, indicating that inertial lift forces have a minor influence on retention. In addition, the relative retention time steadily decreases with increasing sample concentration, suggesting that the hydrodynamic lift force increases significantly with sample concentration. Accordingly, we speculate that a new transport phenomenon, called shear-induced hydrodynamic diffusion, not previously accounted for in the modeling of retention in FFF, is controlling the migration of the particles in the FFF channel. Implications of the influence of this phenomenon in other FFF experiments are discussed.

Chemical Fractionation↗

PubMed2006

Generating multiple independent retention index data in dual-secondary column comprehensive two-dimensional gas chromatography.

A method producing simultaneously three retention indexes for compounds has been developed for comprehensive two-dimensional gas chromatography by using a dual secondary column approach (GC x 2GC). For this purpose, the primary flow of the first dimension column was equally diverted into two secondary microbore columns of identical geometry by means of a three-way flow splitter positioned after the longitudinally modulated cryogenic system. This configuration produced a pair of comprehensive two-dimensional chromatograms and generated retention data on three different stationary phases in a single run. First dimension retention indexes were determined on a polar SolGel-Wax column under linear programmed-temperature conditions according to the van den Dool approach using primary alcohol homologues as the reference scale. Calculation of pseudoisothermal retention indexes in both second dimensions was performed on low-polarity 5% phenyl equivalent polysilphenylene/siloxane (BPX5) and 14% cyanopropylphenyl/86% dimethylpolysiloxane (BP10) columns. To construct a retention correlation map in the second dimension separation space upon which KovAts indexes can be derived, two methods exploiting "isovolatility" relationships of alkanes were developed. The first involved 15 sequential headspace samplings of selected n-alkanes by solid-phase microextraction (SPME), with each sampling followed by their injection into the GC at predetermined times during the chromatographic run. The second method extended the second dimension retention map and consisted of repetitive introduction of SPME-sampled alkane mixtures at various isothermal conditions incremented over the temperature program range. Calculated second dimension retention indexes were compared with experimental values obtained in conventional one-dimensional GC. A case study mixture including 24 suspected allergens (i.e., fragrance ingredients) was used to demonstrate the feasibility and potential of retention index information in comprehensive 2D-GC.

Journal Article↗

PubMed1998

Pressure-induced retention variations in reversed-phase alternate-pumping recycle chromatography.

The progressions of peak width and peak separation in reversed-phase alternate-pumping (AP) recycle chromatography are found to be inconsistent with conventional chromatographic theory. These discrepancies are explained by subtle pressure-induced variations of solute retention that become amplified by AP recycling. The presence of these retention variations is demonstrated by multiply injecting a single solute into an AP system at offset times. As the serially injected peaks are recycled, the separation time between the peaks is shown to vary significantly, indicating that the retention of the solute is dependent upon the position of the peak. A new model of chromatographic retention that appropriately accounts for this variable retention is presented. When this retention model is applied to an AP system for the binary separation of phenylalanine and a pentadeuterated phenylalanine, the model accurately describes the experimentally observed progressions of peak width and peak separation. Furthermore, the retention model predicts that the improvement of resolution in AP recycling closely matches the expectations of conventional theory, so the effectiveness of AP recycling is not significantly compromised by the variations in retention.

Chromatography, High Pressure Liquid↗

PubMedDate not supplied

Fluctuations in continuous mammalian cell bioreactors with retention.

Continuous flow bioreactors with cell retention have been increasingly used for the cultivation of mammalian cells. The potential advantages of such bioreactors are high cell concentrations and volumetric productivities. In many reported cases, these systems have shown fluctuations in cell concentrations of various frequency and magnitude. To analyze the dynamics of the fluctuations, a model-based approach is followed. Simulations showed that large fluctuations in biomass resulted in response to fluctuations in the retention ratio when the system is operated at high dilution rate and high cell retention. The dependence of cell concentration fluctuations on variations in dilution rate and retention ratio was established by a cross-correlation statistical analysis on available experimental data. The slower dynamics and the fluctuation propensity of retention systems suggest that continuous culture without retention is more convenient for kinetic studies. In all likelihood, continuous culture with retention can be stabilized by controlling both the retention ratio and the dilution rate.

Animals↗

PubMedDate not supplied

Partial and total cell retention in a filtration-based homogeneous perfusion reactor.

Suspended mammalian cells can be cultivated in a variety of operational modes (pure chemostat, total cell retention, or partial cell retention) in a homogeneous perfusion bioreactor by varying the cell bleed rate. Hybridomas were grown in the reactor at a perfusion rate of 2.0 day-1 for over 10 weeks at different specific growth rates and viable cell densities achieved by varying the extent of cell retention. Cell metabolism in the reactor was found to vary with the extent of cell retention, which determined both cell density and specific growth rate. With partial cell retention, the nutrient consumption and metabolite production rates decreased with both increasing growth rate and increasing cell density. The specific and volumetric antibody production rates, however, increased dramatically with cell density (and to a lesser extent with decreasing growth rate). The specific MAb production rate was lower with total cell retention than with partial retention at the same growth rate. Since the reactor can be operated over a range of perfusion rates and extents of cell retention, the system can be used to culture cell lines with widely different productivity patterns.

Animals↗

PubMed2005

Pharmaceutical retention mechanisms by nanofiltration membranes.

This study investigates the retention mechanisms of three pharmaceuticals-sulfamethoxazole, carbamazepine, and ibuprofen-by nanofiltration (NF) membranes. Laboratory-scale experiments were carried out with two well-characterized NF membranes, with the goal of relating pharmaceutical retention behavior to membrane characteristics, physicochemical properties of the pharmaceutical molecules, and solution chemistry. Results show that retention of pharmaceuticals by a tight NF membrane is dominated by steric (size) exclusion, whereas both electrostatic repulsion and steric exclusion govern the retention of ionizable pharmaceuticals by a loose NF membrane. In the latter case, speciation of pharmaceuticals may lead to a dramatic change in retention as a function of pH, with much greater retention observed for ionized, negatively charged pharmaceuticals. For uncharged pharmaceutical species, intrinsic physicochemical properties of the pharmaceutical molecules can substantially affect their retention. In its neutral form, ibuprofen adsorbs considerably to the membrane because of its relatively high hydrophobicity. Similarly, polarity (represented by the dipole moment) can influence the separation of molecules that are cylindrical in shape because they can be directed to approach the membrane pores head-on due to attractive interaction between the molecule polar centers and fixed charged groups on the membrane surface. This phenomenon is probably inherent for high dipole moment organic compounds, and the governing retention mechanism remains steric in nature.

Adsorption↗

PubMed2006

Chloride retention and release in a boreal forest soil: effects of soil water residence time and nitrogen and chloride loads.

The common assumption that chloride (Cl-) is conservative in soils and can be used as a groundwater tracer is currently being questioned, and an increasing number of studies indicate that Cl- can be retained in soils. We performed lysimeter experiments with soil from a coniferous forest in southeast Sweden to determine whether pore water residence time and nitrogen and Cl- loads affected Cl- retention. Over the first 42 days there was a net retention of Cl- with retention rates averaging 3.1 mg CI- m(-2) d(-1) (68% of the added Cl- retained over 42 days). Thereafter, a net release of Cl- at similar rates was observed for the remaining experimental period (85 d). Longer soil water residence time and higher Cl- load gave higher initial retention and subsequent release rates than shorter residence time and lower Cl- load did. Nitrogen load did not affect Cl transformation rates. This study indicates that simultaneous retention and release of Cl- can occur in soils, and that rates may be considerable relative to the load. The retention of Cl- observed was probably due to chlorination of soil organic matter or ion exchange. The cause of the shift between net retention and net release is unclear, but we hypothesize that the presence of O2 or the presence of microbially available organic matter regulates Cl- retention and release rates.

Adsorption↗

PubMed1999

Noise reduction of renograms: a new algorithm applied to simulated renograms for evaluation of the renal retention function.

Physiological information about an organ may be assessed from the retention function as derived by deconvolution analysis. However, noise in data may cause distortion of the retention curve and potentially induce methodological errors. To take full advantage of all parts of the retention function, i.e. even the vascular part, we have developed a non-linear noise reduction algorithm. The algorithm is an adaptive polynomial fit (APF) in a sliding segment over the renogram to be deconvoluted. Each segment is modelled by the lowest polynomial resulting in a root of mean square error lower than a pre-set value. APF was tested in comparison with conventional repetitive 1:2:1 smoothing and rectangular window smoothing, using a set of simulated retention functions and corresponding renograms with superimposed artificial noise. The outcome was evaluated by comparing the generated retention functions with the simulated ones. The conventional smoothing algorithms, as well as APF, induced some distortion of the retention function, but the deviation from the true retention function is essentially lower in the case of APF. In addition, APF seems to be more robust in cases of essentially reduced renal function and thereby relative high noise levels. APF reduces noise in data, leading to retention functions with reliable information in terms of a tracer's first passage, uptake and outflow.

Algorithms↗

PubMed2003

Effect of design of pre-fabricated posts and post heads on the retention of various cements and core materials.

The retention of new Para Post (PP) rounded undercut post head (PP-XT) was compared with the conventional flattened head (PP-P), using amalgam Dentacore and Synergy composite as core materials. The retention of the posts PP (passive serrated), PP-P (passive serrated with venting channels), PP-XP (passive, raised diamond-style pattern) and PP-XT (threaded on the coronal portion, and passive, raised diamond-shaped pattern on the apical portion), cemented with zinc phosphate cement (ZPC) or Duo Cement to extracted teeth, was evaluated. Tensile strength plots were recorded using an Instron instrument, while pulling the post and core in opposite directions. The following conclusions were drawn from this study: (i) The round undercut PP-XT head was more retentive than the PP-P head with both core materials. (ii) Amalgam was more retentive than Synergy with both head designs. (iii) PP-XT was the most retentive post to extracted tooth canals and PP-XP was the least retentive with either ZPC or Duo Cement. Post retention with (iv) ZPC in decreasing order of magnitude: PP-XT > PP-P > PP > PP-XP; and with Duo Cement: PP-XT > PP > PP-P > PP-XP - ZPC exhibited the highest retention of cemented posts.

Bisphenol A-Glycidyl Methacrylate↗

PubMed2005

Adhesion, spreading, and aggregation of platelets in flowing blood and the reliability of the retention test Homburg.

Adhesion and aggregation are important parameters characterizing the function of intact platelets in flowing blood and in contact with a more or less thrombogenic surface. In the retention test Homburg (RTH), platelets are exposed to a standardized textured surface (Sysmex retention tubes) under defined conditions of flow. Platelet counts are performed before and after the Sysmex retention tube passage. The difference between these values indicates the percentage of retained platelets (retention index). Decreased retention in the RTH indicates a loss of function or defective platelet function; increase is associated with an increased activation of platelets, for example, in patients with vascular diseases. For further evaluation of the retention phenomenon the filters were fixed after cell passage and examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM micrographs show activated platelets adhering and spreading on the filter surface, comparable with platelets on a disturbed endothelium. Also, we examined the influence of different G forces and centrifugation times on the retention behavior of the platelets in citrated platelet-rich plasma (PRP) and whole blood (WB). G forces influenced the retention index in PRP and WB significantly and in a different way. Finally, we used a platelet standard, as customary in the quality check, to determine the serial as well as the day-to-day precision.

Biocompatible Materials↗

PubMed1987

Comparison of fibre digestion and digesta retention time between rabbits, guinea-pigs, rats and hamsters.

1. Digestive efficiencies of fibre components and retention time of digesta in the whole gut and in the large intestine were measured in rabbits, guinea-pigs, hamsters and rats when given a lucerne (Medicago sativa)-containing diet. 2. Co-EDTA and chromium-mordanted cell-wall constituents of Italian ryegrass (Lolium multiflorum L.) were used as liquid- and solid-phase markers respectively. Both markers were mixed with the experimental diet and given after digestion trials. 3. Mean retention times of each marker were calculated from time-course changes in concentrations of the markers in faeces. The mean retention times of the markers in the large intestine were calculated from exponential slopes fitted to the time-course changes of faecal concentrations of the markers. 4. The digestibilities of crude fibre, neutral-detergent fibre and acid-detergent fibre were highest in the guinea-pigs, followed by the hamsters, and lowest in the rabbits and rats. 5. The mean retention times of Cr in the whole tract were longer in the larger animals and shortest in the hamsters. The mean retention times of Cr in the large intestine were longest in the guinea-pig followed by the hamsters and the rats. The rabbits had an extremely short retention time of Cr in the large intestine. 6. These results suggest that the retention time of solid digesta in the large intestine can explain the difference in the digestive efficiencies of fibre components amongst non-ruminant small herbivores whereas retention of digesta in the whole gut is not related to the digestibility of fibre components.

Animals↗

PubMedDate not supplied

Retention of patients who entered methadone maintenance via an interim methadone clinic.

Time in treatment is considered an important predictor of good outcomes for drug abuse treatment. Patient retention would be of particular concern for low-service types of treatment. Interim methadone maintenance was developed as an alternative to waiting lists and as a method of providing HIV risk-reduction services to heroin addicts waiting for treatment, and does not include the regular counseling required in comprehensive methadone treatment. This analysis compares the retention of patients first admitted to an interim methadone clinic versus that of patients admitted directly to a comprehensive methadone clinic. The two groups did not differ with regard to demographic characteristics and drug of abuse at the time of admission. The two groups did not differ with respect to demographics. The three-, six-, and 12-month retention rates of patients first admitted to the interim clinic were 78%, 69%, and 62%, respectively. The three-, six-, and 12-month retention rates for patients admitted directly to a traditional methadone clinic were 84%, 76%, and 68%, respectively. Life-table analysis revealed that the two groups did not differ significantly with regard to retention (p = 0.17). Interim treatment does not appear to adversely affect overall retention in treatment. Three- and six-month retention rates of interim clinic patients fall within the range of six-month nationwide retention rates reported by the GAO. Factors associated with discharge from treatment are examined for both groups.

Adolescent↗

PubMed2007

A new method for estimating the retention of selected smoke constituents in the respiratory tract of smokers during cigarette smoking.

This report describes a new method for estimating the retention of selected mainstream smoke constituents in the respiratory tract of adult smokers during cigarette smoking. Both particulate-phase (PP) constituents including nicotine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), and N'-nitrosonornicotine (NNN), two tobacco-specific nitrosamines (TSNA), and gas-vapor-phase (GVP) constituents including carbon monoxide (CO), isoprene (IP), acetaldehyde (AA), and ethylene, were studied. To estimate the amounts of smoke constituents delivered during smoking, we used predetermined linear relationships between the measured cigarette filter solanesol content and machine-generated mainstream deliveries of these selected compounds. To determine the amounts of smoke constituents exhaled, the expired breath was directed through a Cambridge filter pad (CFP) attached to an infrared spectrometer. PP compounds were trapped on the CFP for later analysis and GVP compounds were analyzed in near real time. The smokers' respiratory parameters during smoking, such as inhalation/exhalation volume and time, were monitored using LifeShirt(R), a respiratory inductive plethysmography (RIP) device. The retention of each smoke constituent, expressed as a percentage, was then calculated as the difference between the amount delivered (estimated) and the amount exhaled relative to the amount delivered. We studied 16 adult male smokers who smoked cigarettes according to 3 predefined smoking patterns: no inhalation (pattern A), normal inhalation (pattern B), and deep inhalation (pattern C). For the three PP constituents, the mean retentions for pattern A ranged between 10 and 20%; and while the mean retentions of the two TSNAs were significantly higher for pattern C (84% for NNK and 97% for NNN) than those for pattern B (63% for NNK and 84% for NNN), the mean retentions of nicotine were basically the same between patterns B and C, which were both greater than 98%. For the GVP constituents, the retentions were similar between pattern B and pattern C, although different constituents were retained to different degrees (average values of 33%, 52%, 79%, and 99% for ethylene, IP, CO, and AA, respectively). The differences in the retention between different constituents could be interpreted in terms of each constituent's physical properties such as volatility and solubility. In conclusion, the method described is suitable for studying the retention of selected mainstream smoke constituents in the respiratory tract of smokers.

Acetaldehyde↗

PubMed1976

Respiratory retention of inhaled toluene and benzene in the dog.

This study deals with the inhalation of toluene and benzene found in the vapor phase of cigarette smoke. Determined in this study were the uptake of each substance by the total respiratory tract and by the upper and lower portions under varying conditions of ventilatory rate, tidal volume, and concentration inhaled. Retention by the total tract of toluene fell within the range of 91-94% at all ventilatory rates seen, indicating no relationship between rate and retention. With benzene, total tract retention varied from 88 to 68% with an inverse relationship between retention and rate apparent. This relationship was seen in all types of experiments conducted with benzene. Upper tract retention revealed an uptake of about 89% for both one- and two-way experiments with toluene. In the benzene experiments retention varied from 80 to 61% and from 81 to 63% in the one- and two-way procedures, respectively. Mean lower tract retention was 90% with toluene at all ventilatory rates considered. Benzene retention varied from 74 to 61% as the rate increased from 6 to 18 inhalations/min.

Aerosols↗

PubMed2005

Racial differences in calcium retention in response to dietary salt in adolescent girls.

BACKGROUND: Sodium is an important determinant of urinary calcium excretion, and race is an important determinant of calcium retention. The effect of dietary sodium on calcium retention and the influence of race have not been studied in adolescence, the life stage during which peak bone mass is accrued. OBJECTIVE: The study reported here was undertaken to compare racial differences in calcium retention as a function of dietary salt intake. DESIGN: A total of 35 adolescent girls (22 black and 13 white) participated in two 20-d metabolic summer camps, separated by 2 wk, that simulated a free-living environment. The effect of changes in dietary sodium on calcium retention was tested in a randomized-order, crossover design with 2 concentrations of sodium-1.30 g/d (57 mmol/d) and 3.86 g/d (168 mmol/d)-and a constant calcium intake of 815 mg/d (20 mmol/d). RESULTS: Both race and sodium intake significantly affected calcium retention (P < 0.01). Calcium retention was significantly greater in black girls than in white girls, regardless of dietary sodium intake (P < 0.001). The high-sodium diet significantly reduced calcium retention in both whites and blacks (P < 0.01), primarily through a decrease in net calcium absorption. Black girls excreted significantly less calcium in the urine than did white girls, regardless of diet (P < 0.05). CONCLUSIONS: Calcium retention is significantly greater in black girls than in white girls but is significantly reduced in girls of both races in response to salt loading.

Adolescent↗

PubMed1987

Influence of usual zinc intake and zinc in a meal on 65Zn retention and turnover in the rat.

The influences of zinc in a meal and usual zinc intake on zinc retention and turnover were investigated in 7-wk-old male rats fed diets containing 12-151 mg Zn/kg for 3 wk before and after consuming a 65Zn-labeled meal containing ZnCl2. Retention corrected to zero time and turnover rate were determined by whole-body counting. Percent zinc retention was inversely proportional to the natural logarithm of the meal zinc, between 0.09 and 26 mumol. In comparison to lower doses, higher doses resulted in lower percent retention but greater amounts of zinc retained. Although the latter relationship was slightly curvilinear, there was no indication of a limited capacity for zinc retention with high doses. However, doses above 4 mumol resulted in higher turnover rates in rats accustomed to lower zinc intakes. Percent retention and the reciprocal of the turnover rate were proportional to the reciprocal of the dietary zinc concentration. The greatest differences in retention and turnover occurred between 12 and 26 mg Zn/kg diet. The zinc dose in a meal and the usual dietary zinc separately influenced percent zinc retention. These factors also interacted, such that greater dose effects were observed at lower zinc intakes and greater dietary zinc effects were observed at lower doses.

Animals↗

PubMed2001

Astrocytes as a predominant cellular site of (99m)Tc-HMPAO retention.

Technetium-99m-d,l-hexamethylpropylene amine oxime ((99m)Tc-HMPAO) retention in the brain monitored by single photon emission computed tomography (SPECT) is currently used as a marker of cerebral blood flow. The purported mechanism by which (99m)Tc-HMPAO accumulates in the brain is through its intracellular conversion from a lipophilic form to more hydrophilic derivatives within the brain parenchyma. The issue of the contribution of different cell types on (99m)Tc-HMPAO retention was investigated in vitro by studying the accumulation of (99m)Tc-HMPAO in primary cultures of mouse cortical astrocytes and neurons. Results show that (99m)Tc-HMPAO retention predominates in astrocytes over neurons by a factor of approximately 2.5 (0.26 +/- 0.05 vs. 0.095 +/- 0.042 fmol/mg protein after 120 minutes, respectively). Diethyl maleate (60 micromol/L), ethacrynic acid (1 mmol/L) and buthionine sulfoximine (1 mmol/L), 3 agents which significantly reduced glutathione levels also decreased (99m)Tc-HMPAO retention in both astrocytes (29%, 3%, and 46% of control, respectively) and neurons (69%, 11% and 63% of control). Decrease did not always correlate with glutathione levels, however, which suggests that other factors could be involved. The possibility that cell energy status determines (99m)Tc-HMPAO retention was also assessed. Agents that activate (glutamate, azide) or inhibit (cytochalasin B) glucose utilization in astrocytes, as measured by the (3)H-2-deoxyglucose method, were without effect on (99m)Tc-HMPAO retention. In conclusion, the data presented indicate that astrocytes may constitute a prominent site of (99m)Tc-HMPAO retention and most likely contribute significantly to the SPECT signal. In addition, the data also suggest that specific alterations in glial cell metabolism could explain flow-independent changes in (99m)Tc-HMPAO retention in the brain as observed by SPECT in some pathologies.

Animals↗