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The authors describe the different "retentive complexes" proposed by the Akers, Roach and Ney schools and analyse their biomechanical validity.
The authors analyse the biomechanical bases of the Kratochvil "retentive complex" and show how they have induced the nowadays north-american propositions (RPI & RPA). They compare them to the european ones (Nally-Martinet).
Platelets and white cells contribute to thrombus formation. In the filterometer, platelet retention in normal citrated blood increases from 47.5% at 0-5 s to 81.5% at 20-40 s. White cell retention is normally closely related to platelet retention at 0-5 s but less so at 20-40 s. However at 20-40 s the number of white blood cells (WBC) retained has decreased relative to 0-5 s. This apparent paradox is now further explored using antibodies to glycoprotein (GP) Ib, to von willebrand factor (vWf) and to GPIIb-IIIa together with observations on a number of clinical conditions with abnormal platelet retention. In citrated blood, platelet retention of 0-5 s was significantly decreased relative to normal in von Willebrand's disease, in aortic valve stenosis and with the addition of anti-vWf-GPIb and minimally with anti-GPIIb-IIIa antibodies. WBC retention of 0-5 s in these groups was always 41 2%. However when platelet retention 0-5 s was raised (pregnancy and venous occlusion), white cell retention was also raised. At 20-40 s in all the nine conditions studied, the white cell and platelet retention were closely related, but the percentage WBC retained decreased relative to the platelet retention. Granulocyte retention was higher than lymphocyte retention, but both cell types were similarly affected. We conclude that the platelets are retained as previously described. The white cells (all types) initially bind at least in part in an independent unexplained way (0-5 s). Thereafter the degree of platelet activity (retention %) largely determines the degree of WBC retention. This is probably due to a proportional amount of P-selectin liberated by the activated platelets.
Patients with semantic STM deficits have difficulty comprehending sentences that require the retention of several lexical-semantic representations prior to their integration into higher-level propositions (Martin, 1995: Martin & Romani. 1994). In Experiment 1, patients with a semantic retention deficit had difficulty with the same type of constructions in speech production, namely noun phrases with one or two prenominal adjectives. Their performance improved when they could produce the nouns and adjectives in sentence form, which placed smaller demands on lexical-semantic retention. In Experiment 2 these patients were better able to produce syntactically complex sentences than the prenominal adjective phrases having an equal number of content words, indicating that the findings in Experiment 1 could not be attributed to syntactic complexity. These patients produced more pauses in the sentence constructions in Experiments 1 and 2, suggesting that the timing of such productions is abnormal. In contrast, patient EA, with a phonological retention deficit, performed better than the patients with a semantic retention deficit on the AN phrases despite having a smaller STM span. She showed no significant benefit of producing sentence compared to phrase constructions, and also made fewer and shorter pauses than the other patients. These findings support the multiple capacities view of verbal working memory and suggest that the same semantic retention capacity used in language comprehension is used in speech production.
Prospective longitudinal pregnancy cohorts can answer questions about fetal and early life exposures and later health outcomes; however, there are challenges to retaining participants in longitudinal studies, particularly over life transitions like the birth of a child. Optimal methods for retaining participants in longitudinal research are unclear. A systematic mapping review was conducted to identify prospective cohort studies and randomized controlled trials that enrolled pregnant participants and their infants. Data on retention rates and 17 retention strategies was extracted. A random effects meta-analysis generated pooled annual retention rates inversely weighted to the number of baseline participants. Spearman rank coefficients were used to assess correlation between strategy use and retention. A random-effects meta-regression was used to determine if select retention strategies were associated with participant retention. We identified 130 studies, involving 472 022 pregnancies. A downward trend in pooled mean retention rates were observed. Studies utilized an average of 6.8 (SD 3.9) retention strategies. Statistically significant associations were not observed between strategy use and retention rates at follow-up (p > 0.05). Prospective studies of pregnant people and their infants used multiple retention strategies. Participant retention rates declined over time, suggesting that additional factors may influence study participation in the postpartum period.
In 4 experiments with 3 X 3 male Wistar rats each, which received rations with different protein content on 3 levels (10-40%), a total of 240 measurings of the total metabolism with the purpose of ascertaining the energy requirement for protein retention were carried out under the conditions of ad libitum feeding in the growth range between approximately 65 and 200 g live weight at a temperature of the environment of 30 degrees C. With 25% crude protein (caseine supplemented with methionine) in the feed mixture, with an energy content of the ration between 18 and 20 kJ/kg dry matter, the N-retention capacity of the animals was largely exhausted. When 150 g live weight were exceeded, the N-retention capacity of the animals decreased considerably. Energy retention in % of gross energy depended on the protein level of the feed rations; in all 4 experiments the rats achieved the highest energy retention with 10% crude protein in the feed. The level of energy utilisation decreased with the increasing protein content of the feed mixture. Requirement values of metabolisable energy per 1 J protein energy retention of 1.69; 1.93; 1.81 and 1.74 J were derived by means of multiple regression analysis for all 4 experiments, assuming the exponent 0.75 for the variable metabolic body size. The application of multiple regression analysis to the values of energy retention measuring in all 4 experiments results in the mean energy requirement of 1.67 J/J protein retention, which completely coincides with previous investigations. When the exponent 0.67 was assumed for the variable metabolic body size, the energy requirement data for the individual experiments resp. after the joint evaluation of all 4 experiments were 1.56; 1.75; 1.68 and 1.56; 1.53 J/J resp. The significant influence of the live weight, the consumption quota of digested nitrogen (protein level of feeding) and the protein retention quota on the value of protein requirement for protein retention was proved. This evidence is discussed with respect to further research for the factorial assessment of the bioenergetic of protein retention.
The results of measurings of the total metabolism from experiments with 6 male broilers (origin: Tetra B) each, which mainly served the ascertainment of energy maintenance requirement (cf. 2) in the growth range between 100 and 2,000 g with alternating experiment phases on the maintenance and growth level, have also undergone interpretation for the further characterisation of the energy requirement for protein retention. Caused by the sequence of periods chosen, there were compensatory growth effects. Energy retention in % of gross energy varied between the experiments in the limits of 24 and 39%, and concerning energy retention per animal and day there were differences up to 100% between the experiments. The share of protein energy retention in energy retention on average amounted to 38% after a medium-high protein supply and 54% after a high protein supply. The utilisation of metabolisable energy for energy retention was significantly lower from rations of a high protein level (40% crude protein) than from those of a medium-high protein level (20% crude protein). Assuming an energy requirement for fat retention of 1.2 J metabolisable energy per J retention, a requirement of metabolisable energy for 1 J protein retention of 1.68; 1.66 and 1.86 was ascertained in the 3 experiments by means of regression analysis. An energy requirement for maintenance of 423 (30 degrees C), 511 (25 degrees C) and 432 kJ metabolisable energy/kg live weight 0 75 X d (30 degrees C) correlated with these requirement values. Energy requirement for protein retention increased significantly with N-intake and decreased significantly with the daily protein retention rate. There was no dependence on the live weight of the broilers.
A collaborative study among 10 laboratories has been undertaken to investigate the interlaboratory reproducibility of retention measurements in a high performance liquid chromatographic (HPLC) system for separating barbiturates. The system involved an ODS-Hypersil column with an eluent of methanol:phosphate buffer (40:60, v/v) at pH 8.5; all laboratories used the same batch of packing material. The conventional methods of recording retention properties (retention times and capacity factors) gave poor interlaboratory reproducibility. Better results were obtained by expressing the retentions relative to a standard barbiturate (quinalbarbitone); relative adjusted retention times proved to be more effective than straightforward relative retention times. Retention indices based on the alkylarylketone scale were not as reproducible as the methods based on a single closely related compound. The best reproducibility was obtained using corrected capacity factors based on the retention of four barbiturates in a standard mixture. The results of the study are discussed with a view to selecting the best methods of recording retention in HPLC when considering the establishment of databases for drug identification.
Protein and energy metabolism in boars of different breeds, 10 each of Hampshire, Duroc and Danish Landrace was measured in balance and respiration experiments by means of indirect calorimetry in an open-air circulation system. Measurements were performed in four periods (Period I-IV) covering the body weight range from 25 to 100 kg. In order to achieve maximum protein retention (RP) a daily intake of digestible protein > 12 g/kg0.75 and metabolisable energy > 1100 kJ/kg0.75 was assumed to be necessary. Protein retention of Danish Landrace boars was inferior to that of Hampshire and Duroc boars in Periods III and IV, and therefore, 55 measurements on Hampshire and Duroc boars fulfilling the chosen criteria for digested protein and ME intake were used for calculation of maximum protein retention, giving the following significant quadratic relationship: RP [g/d] = 11.43.W0.75-0.144.W1.50 (n = 55, RSD = 15.2, CV = 9.2%, R2 = 0.851) with a summit of 227 g/d at 135 kg BW. In Period I, when BW was below 30 kg, 12 measurements fulfilled the chosen criterion for digested protein but not for ME, and these data were used comparatively. Protein retention of boars with a low ME intake in Period I was significantly below that of boars with a high ME intake (93 g/d vs. 107 g/d; P = 0.02). In summary, the present data have shown that boars of high genetic potential have capacity for maximum protein retention of about 230 g/d, and that there was a significant quadratic relationship between protein retention and metabolic body weight, indicating that maximum protein retention was not reached until 135 kg BW. Differences in capacity for protein retention were recorded between boars of different breeds, with Duroc and Hampshire boars being superior to Danish Landrace boars. Additionally, the crucial importance of a sufficient ME supply early in the growth period was underscored by a lower protein accretion rate of boars given a daily ME supply below 1100 kJ ME/kg0.75 at an approximate BW of 25 kg.
The retention index increment for addition of a methylene group to an analyte molecule is shown for 1-halo-n-alkanes to be different from 100 i.u., a value that is customarily assigned according to the current convention in retention index prediction. In temperature-programmed gas chromatography using linearly interpolated retention index I, a linear regression equation, I=AZ+(GRF), with the number of atoms (Z) in the molecule as variable can describe the retention of 16 homologous series of organic compounds on non-polar and polar columns with characteristic A (linear regression coefficient) and (GRF) (group retention factor) values. A molecular model of retention on the basis of electron density and electron density distribution relative to that of n-alkane is proposed. This model brings out the inter- and intramolecular electronic effects in the analyte molecule and its dipole-dipole interaction with the stationary liquid phases, as variations in the A value. The (GRF) value varies with the connectivity ability of a functional group for extended conjugation, substitution, etc., but is most influenced by hydrogen bonding (H-bonding) with the stationary liquid phase. One can estimate the sequence of elution of a mixture of organic compounds from any two of the three parameters on the right-hand side of the above equation or retrieve the retention indexes of an entire homologous series from its A and (GRF) values. The fact that each analyte molecule has its own A value on different columns makes column difference (deltaI) compound-specific rather than column-specific, a departure from previous assumptions.
The retention behavior of methoxy-substituted p-benzoquinones and the corresponding hydroquinones in reversed-phase chromatography was examined on octylsilica and two octadecylsilica stationary phases and with five hydroorganic mobile phases containing acetonitrile, methanol or tetrahydrofuran and additionally in most cases (NH3OH)3PO4 used as a reducing and buffering agent. The retention order of benzoquinones and hydroquinones was the same on each stationary phase with either methanol or acetonitrile as the organic modifier. On the other hand, minor differences in the retention order were observed with the various stationary phases. In all cases, satisfactory quantitative structure-retention relationships (QSRRs) were found and the data suggest that the differences in the retention behaviour of octadecylsilicas used in this study are silanophilic interactions which, together with solvophobic interaction contribute to the retention of these eluites. Further analysis showed that QSRRs of sterically crowded molecules must take into account reduced surface area available for binding. The retention data obtained with use of aqueous tetrahydrofuran as mobile phase failed to give rise to satisfactory QSRRs. This was attributed to selective solvation of eluite by tetrahydrofuran and/or nearly equipotent binding of eluite and tetrahydrofuran to stationary phase.
Quantitative structure-retention relationships (QSRR) combined with the linear solvent strength (LSS) model are demonstrated to provide approximate predictions of gradient reversed-phase high-performance liquid chromatography (HPLC) retention time for any structurally defined analyte on a once characterized column. The approach requires at first the determination of retention times for a predesigned model series of 15 analytes in two gradient runs. Then by employing the LSS theory a given HPLC system of interest is quantitatively characterized. Structure of the model analytes is next described quantitatively by means of three structural descriptors from standard molecular modeling: total dipole moment, electron excess charge of the most negatively charged atom and water-accessible molecular surface area. With those data the general QSRR equations are derived which describe gradient retention times of the model analytes in the specific column/eluent system. Having now the structural descriptors for any analyte to be chromatographed in such a characterized HPLC system, one employs respective general QSRR equations to calculate its expected gradient retention time at given gradient conditions by means of appropriate LSS equations. Additionally, the chromatographic parameters log kw and S can be calculated and retention coefficients corresponding to chosen isocratic conditions evaluated. The approach provides retention predictions which can be treated as a first approximation of actual data. Predictions are not yet precise enough for practical separation purposes but can be of use in rational modification of analytical conditions aimed at optimization of separations.
It is extremely important to pay attention to the person's growth pattern, and a distinction must be made in the selection of retention devices on the basis of the nature and the extent of dentofacial dysplasia (growth pattern). The nature and duration of retention should depend on the maturation status of the patient and on anticipated future growth. Retention guidance is necessary for adjustment of the dentition to late growth changes and maturation of neuromuscular balance. "Active retention" is a concept we accept as readily as the orthopedic surgeon does for his scoliosis patients. There is some merit in the philosophy of those clinicians who advocate permanent retention guidance. Without always being aware of the biomechanics of growth change, they are in fact carrying the patient through the active stages of growth with their retention appliances. Finally, one may philosophize that nothing about the human morphology is stationary. Aging is a well-documented process of change. Lifetime dentitional adjustment and changing dental relationships are known to all, even in otherwise healthy persons. Then why do we expect long-term stability in every case? The answer to the question of long-term stability is long-term retention--dynamic, not static.
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Quantitative structure retention relationships (QSRR) were derived allowing prediction of reversed-phase high-performance liquid chromatography (HPLC) retention of peptides. To quantitatively characterize the structure of a peptide, and then to predict its gradient retention time under given HPLC conditions, the following descriptors are employed: logarithm of the sum of retention times of the amino acids composing the peptide, log Sum(AA), logarithm of Van der Waals volume of the peptide, log VDW(Vol), and logarithm of its calculated n-octanol-water partition coefficient, clog P. The first descriptor is based on a set of empirical data for 20 natural amino acids. The next two descriptors are easily calculated from a structural formula. The predicted gradient retention times are in excellent agreement with the experimental data, determined for a structurally diversified series of 101 peptides. The QSRR equation obtained predicts in a convenient and reliable manner the retention times for any peptide in a once characterized HPLC system.
Amino acid retention coefficients were derived from the experimental retention data of 118 overlapping peptide heptamers related to the primary amino acid sequence of myohemerythrin. Individual retention coefficient values for each amino acid were derived by a multiple linear regression matrix approach. Retention data were derived for five different experimental conditions including different organic modifiers (acetonitrile, methanol or 2-propanol), different mobile phase additives (trifluoroacetic acid or potassium phosphate) and different silica-based stationary phase ligands (octadecyl or phenyl groups). A high degree of correlation was observed between these experimentally derived amino acid coefficients (EXP) and the coefficients (LIT) which we recently reported derived from the retention data of over 2000 peptides [M. C. J. Wilce et al., J. Chromatogr., 536 (1991) 165 and 548 (1991) 105]. These results demonstrated that the LIT and EXP coefficients can be used for the prediction of the retention of any peptide set. The effect of peptide length was also further investigated and the correlation results demonstrated the importance of peptide flexibility on the final value of the amino acid coefficient.
Quantitative structure-retention relationships (QSRRs) were derived for logarithms of retention factors normalised to a hypothetical zero percent organic modifier eluent, log kw, determined on 18 reversed-phase high-performance liquid chromatography (RP-HPLC) columns for 25 carefully designed, structurally diverse test analytes. The study was aimed at elucidating molecular mechanism of retention and at finding an objective manner of quantitative comparison of retention properties and classification of modern stationary phases for RP-HPLC. Three QSRR approaches were employed: (i) relating log kw to logarithms of octanol-water partition coefficient (log P); (ii) describing log kw in terms of linear solvation-energy relationship-based parameters of Abraham; (iii) regressing log kw against simple structural descriptors acquired by calculation chemistry. All the approaches produced statistically significant and physically interpretable QSRRs. By means of QSRRs the stationary phase materials were classified according to the prevailing intermolecular interactions in the separation process. Hydrophobic properties of the columns tested were parametrized. Abilities of individual phases to provide contributions to the overall retention due to non-polar London-type intermolecular interactions were quantified. Measures of hydrogen-bond donor activity and dipolarity of stationary phases are proposed along with two other phase polarity parameters. The parameters proposed quantitatively characterize the RP-HPLC stationary phases and provide a rational explanation for the differences in retention patterns of individual columns observed when applying the conventional empirical testing methods.