Biomedical subjects

F Svec

Publications and source records attributed to F Svec.

At least 55 records · Page 3Linked to original sources

PubMed1998

Molded rigid polymer monoliths as separation media for capillary electrochromatography. 2. Effect of chromatographic conditions on the separation.

The effect of chromatographic conditions on the performance of monolithic poly(butyl methacrylate-co-ethylene dimethacrylate-co-2-acrylamido-2-methyl-1-propanesulfonic acid) columns in capillary electrochromatography has been studied. The flow velocity was found to be proportional to the strength of the electric field and both the pH and the composition of the mobile phase. A column efficiency of 120,000 plates/m at the optimum flow velocity of 1.5 cm/min is achieved for all the monolithic capillary columns of identical composition and porosity, regardless of their length, which varied from 30 to 120 cm. The polymeric separation medium exhibits retention and selectivity properties similar to those of typical ODS packings for reversed-phase chromatography. In addition to the "classical" use of monolithic capillary columns for the electrochromatographic separation of small molecules in reversed-phase mode, larger styrene oligomers were also separated under isocratic elution conditions. In addition, the electroosmotically driven size exclusion chromatography of polystyrene standards with molecular weights up to 10(6) has been demonstrated for the first time.

Benzene Derivatives↗

PubMed1998

The design of chiral separation media using monodisperse functionalized macroporous beads: effects of polymer matrix, tether, and linkage chemistry.

Porous size monodisperse methacrylate beads containing amino and hydroxyl groups, which can be used as a platform for the production of chiral separation media, have been prepared using the staged templated suspension polymerization process. The monomers 2-hydroxyethyl methacrylate and several tert-butoxycarbonyl-protected aminoalkyl methacrylates were used for the preparation of both hydroxyl- and amino-functionalized beads. Attachment of a chiral selector based on L-valine-3,5-dimethylanilide through a carbamate and a urea linkage, respectively, provides chiral stationary phases with excellent enantioselectivities in the separation of the enantiomers of 3,5-dinitrobenzamido derivatives of alpha-amino acids. For comparison purposes, a separation medium was also prepared by the direct polymerization of a chiral monomer analogous to the hydroxyethyl methacrylate-based stationary phase. The chiral stationary phases prepared with the N-methyl-2-aminoethyl methacrylate platform exhibit the best selectivity, and separation factors as high as 15 were achieved under normal-phase HPLC conditions.

Carbamates↗

PubMed1998

Effect of porous structure of macroporous polymer supports on resolution in high-performance membrane chromatography of proteins.

The effect of porous structures of 2-mm thick diethylamine functionalized monolithic polymethacrylate discs on their chromatographic behavior in ion-exchange mode has been studied. Discs with small pores did not perform well because they exhibited high back pressure and substantial peak broadening. Discs characterized with pores larger than 1,000 nm did not provide good separations either because the time required for some protein molecules to traverse the length across the pore to reach the wall for adsorption/desorption process that is essential for the separation may be longer than their residence time within the matrix. Optimum pore size is centered at about 700 nm. Excellent separations have been achieved with these discs even at very steep gradients and high flow-rates which allow to shorten the separation times substantially.

Adsorption↗

PubMed1998

The actions of exogenous dehydroepiandrosterone in experimental animals and humans.

Dehydroepiandrosterone (DHEA) is the major adrenal steroid of young adults; however, its physiologic functions, if any, are not known. The purpose of this review is to evaluate the current literature in which DHEA was administered to either humans or experimental animals to discern what these functions might be. Reports are divided into five areas: neurologic, immunologic, cardiovascular, oncologic, and metabolic. Particular attention is paid to the dosage and route of administration. This type of analysis shows that at the lowest doses, DHEA has effects on neurologic and immunologic tissues, suggesting that these two sites may be physiologic targets. DHEA also affects cardiologic and metabolic functions as well as tumor growth, but such actions require higher doses and may reflect 'pharmacologic' activities. It is proposed that DHEA's pattern of activity represents a new class of steroid hormones, the "Regnantoids." Further progress in the endocrinology of this family of steroids may only come when synthetic, long-acting analogs of DHEA are available for in vitro studies to allow correlations between hormone action and receptor binding.

Animals↗

PubMed1997

Molded rigid polymer monoliths as separation media for capillary electrochromatography.

Rigid, monolithic capillary columns for reversed-phase electrochromatography have been prepared within the confines of untreated fused-silica capillaries in a single step by a simple copolymerization of ethylene dimethacrylate, butyl methacrylate, and 2-acrylamido-2-methyl-1-propanesulfonic acid in the presence of a porogenic solvent. The composition of the specifically designed ternary porogenic solvent allows fine control of the porous properties of the monolithic material over a broad range. While the electroosmotic flow through these capillary columns increases with both increasing pore size of the monolith and content of charged functionalities, better chromatographic properties have been observed for monoliths with larger surface area and hydrophobicity. Using this technique, monolithic capillary columns with an efficiency higher than 120000 plates/m have been easily obtained.

Chromatography, High Pressure Liquid↗

PubMed1997

Monodisperse hydrolyzed poly(glycidyl methacrylate-co-ethylene dimethacrylate) beads as a stationary phase for normal-phase HPLC.

The basic characteristics of a rugged, stable, and highly efficient polymeric stationary phase for normal-phase HPLC prepared by hydrolysis of porous monodisperse poly(glycidyl methacrylate-co-ethylene dimethacrylate) beads have been studied and compared with those of bare Nucleosil silica and Nucleosil silica-diol. As a result of their overall chemical composition and the more than 10-fold larger surface coverage with hydroxyl groups, the polymer beads provide much higher retention of model solute molecules. In contrast to silica hydroxyls, all of the polymer surface diol groups are chemically equal and homogeneously distributed over all of the surface. In addition, they are less acidic than typical silanol functionalities. The exceptional selectivity of the polymeric column can be controlled by the composition of the mobile phase, as demonstrated with a broad spectrum of separations involving positional isomers of benzene derivatives, nonpolar isobutylene copolymers with minute composition differences, and hydrophilic poly(ethylene oxides) differing only in their chain length. Unlike silica phases, the properties of the polymeric stationary phrase are not affected by the presence of water in the mobile phase. As a result, repetitive gradient separations in eluents ranging in polarity from hexane through tetrahydrofuran to water can be easily accomplished.

Chromatography, High Pressure Liquid↗

PubMed1997

Rigid porous polyacrylamide-based monolithic columns containing butyl methacrylate as a separation medium for the rapid hydrophobic interaction chromatography of proteins.

Macroporous poly(acrylamide-co-butyl methacrylate-co-N,N'-methylenebisacrylamide) monoliths containing up to 15% butyl methacrylate units have been prepared by direct polymerization within the confines of HPLC columns. The hydrodynamic and chromatographic properties of these 50 mm x 8 mm I.D. columns-such as back pressure at different flow-rates, effect of percentage of hydrophobic component in the polymerization mixture, effect of salt concentration on the retention of proteins, dynamic loading capacity, and recovery-were determined under conditions typical of hydrophobic interaction chromatography. Using the monolithic column, five proteins were easily separated within only 3 min.

Acrylic Resins↗

PubMed1997

Polymer- versus silica-based separation media: elimination of nonspecific interactions in the chiral recognition process through functional polymer design.

A novel chiral stationary phase that contains a new chiral selector based on L-valine-3,5-dimethylanilide attached to monodisperse poly(glycidyl methacrylate-co-ethylene dimethacrylate) beads has been prepared. The polymeric separation medium provides greatly enhanced enantioselectivities and reduced retention times when compared to the analogous silica-based chiral stationary phase in the separation of the enantiomers of 3,5-dinitrobenzamido derivatives of alpha-amino acids under normal-phase HPLC conditions. Separation factors (alpha) of up to 7 were achieved with the polymeric separation medium, which is also useful for the separation of large samples under overload conditions.

Chemistry Techniques, Analytical↗

PubMed1997

Cyclo(His-Pro) augments the insulin response to oral glucose in rats.

Cyclo(His-Pro) (CHP) is a gut-brain peptide found in rat and man. Since plasma levels of CHP are altered by oral glucose ingestion, we wondered whether exogenous CHP might alter the insulin response to oral glucose ingestion. To this end, rats were given 3g/kg oral glucose load with either saline or increasing doses of CHP and plasma levels of insulin, C-peptide and glucose were measured. We found mean insulin but not C-peptide excursions and area under the insulin but not C-peptide response curves (AUC) were significantly higher in the CHP groups than controls despite similar glucose responses. In summary, these data show that in rats receiving oral glucose, CHP causes higher insulin excursions without any change in C-peptide suggesting that CHP may decrease hepatic insulin clearance.

Administration, Oral↗

PubMed1997

Dietary and hypothalamic changes in delta 4-androstenedione-treated Zucker rats.

Dehydroepiandrosterone (DHEA) has been shown to alter hypothalamic monoamines and reduce energy intake (EI) in Zucker rats (ZRs). We hypothesized that a metabolite of DHEA, delta 4-Androstenedione (delta 4), may mediate these effects. Male lean and obese ZRs (LZR, OZR) were fed control chow (CC) for 7 days, during which basal EI was recorded, various concentrations of delta 4 for 7 days, during which 0.6 and 0.3% delta 4 reduced EI significantly, and CC for 7 days, which resulted in a return of EI to basal levels. After delta 4 administration, neurotransmitter contents of various hypothalamic areas were determined. Serotonin (5-HT) has been shown to be correlated with feeding inhibition, and we have shown DHEA to increase lateral hypothalamic 5-HT synthesis; however, after 1 day and 7 days of delta 4, the OZR exhibited an increased metabolism, not synthesis, of 5-HT in the lateral and paraventricular hypothalamus, respectively, delta 4 was compared to DHEA in a macronutrient self-selection study with female OZRs. One group was injected intraperitoneally (IP) with sesame oil (control), another with DHEA (100 mg/kg), and another with delta 4 (100 mg/kg). Previous studies have shown that DHEA decreases both EI and % calories from fat. In this study, delta 4 decreased % calories from fat, but did not decrease total EI. Contrary to DHEA's effect of reducing serum insulin through 28 days of treatment, delta 4 in chow reduced insulin only acutely (1 day). We conclude, based on these differences, that DHEA has unique effects not mediated by its metabolite, delta 4-Androstenedione.

Androstenedione↗

PubMed1997

The effect of dehydroepiandrosterone (DHEA) on Zucker rat food selection and hypothalamic neurotransmitters.

The effect of dehydroepiandrosterone (DHEA) administered as an acute intraperitoneal (IP) injection on food selection and levels of hypothalamic neurotransmitters of obese Zucker rats was investigated. Animals consumed a 'macronutrient selection' diet which consisted of three separate foodstuffs; they were nearly pure selections of fat, protein or carbohydrate. On the day before DHEA treatment, food bowls were removed at 1700 h and the animals fasted. The next morning some were treated with IP DHEA (100-200 mg/kg) while the controls received vehicle. Two hours later, their food intakes over 4 h were quantitated or, in other experiments, the animals were decapitated and hypothalamic neurotransmitter levels determined. Results showed that the administration of DHEA clearly diminished the amount of fat the animals consumed while their intakes of carbohydrate and protein were unchanged. Levels of neurotransmitters in the paraventricular nuclear region were altered. It is concluded that DHEA diminishes the fat food consumption of obese Zucker rats by altering the levels of neurotransmitters in select regions of the hypothalamus.

Animals↗

PubMed1997

The acute effect of a noontime meal on the serum levels of cortisol and DHEA in lean and obese women.

Lean [n = 11, body mass index (BMI) < 22.5] and obese (n = 13, BMI > 31.5) women consumed a noontime meal while serum levels of cortisol and dehydroepiandrosterone (DHEA) were measured. Before the meal the obese had lower levels of serum cortisol compared to the leans. Within 20-40 min of consuming the meal the levels of cortisol rose in both groups; those of the obese rose by a greater increment and in the first postprandial hour there were no differences between lean and obese. After the meal-induced peak, the obese again had lower cortisol values. DHEA levels rose with the meal in both groups but the difference over baseline was only significant for the obese. These results show that the lean and obese respond differently to the physiologic stimulus of a meal. These results are consistent with the hypothesis that obesity is influenced by differences in adrenal function that are demonstrable during stimulation with a meal.

Adolescent↗

PubMed1997

Ageing and adrenal cortical function.

Ageing is associated with changes in the secretion of adrenal cortical steroids. In the elderly, cortisol secretion increases after stimulation, while the secretion of dehydroepiandrosterone (DHEA) decreases. Each of these hormones influences the age-related processes of energy metabolism, fat depot distribution, immune function and neurodegeneration. In animals the effects of adrenal steroids are dramatic and easily measured. In humans the effects are more subtle. This review summarizes these actions and emphasizes the differences of dosages used in various experimental designs. It is concluded that adrenal hormones may play a significant role in human ageing, but research is hindered because the molecular pathways of DHEA action are not known.

Adrenal Cortex↗

PubMed1997

Leptin levels do not change acutely with food administration in normal or obese subjects, but are negatively correlated with pituitary-adrenal activity.

BACKGROUND: Leptin is a peptide secreted by white adipose tissue which has been shown to have a major influence on body weight regulation, while animal studies have revealed widespread interconnections between leptin and other endocrine systems, especially with insulin. However, its acute regulation has been little studied in the human. We have therefore investigated the effect of a 1000 kcal meal and fasting on the levels of leptin, insulin and cortisol, in both normal and obese subjects. SUBJECTS AND DESIGN: We have studied the effect of food and fasting on circulating leptin levels in 20 subjects of normal body mass index (BMI range 18-25) and in a group of 12 moderately-severely obese subjects (BMI range 34-61). We also studied the effect of food and fasting in a patient both before and after the successful removal of a pancreatic insulinoma as a model of excess insulin secretion. RESULTS: Mean leptin levels were significantly higher in the obese than in the lean group (42.7 +/- 3.41 vs 5.35 +/- 1.55 micrograms/l, mean +/- SEM; P < 0.001), and showed a positive correlation with body mass index (r = +0.71; P < 0.001). Frequent (every 20 minutes) sampling for 3 hours after food did not show any acute changes in circulating leptin levels. On the fasting day we observed a small but significant fall in circulating leptin levels in the last 4 hours of a 20-hour fast in our subjects as a group (92 +/- 0.03% of basal, P = 0.03); however, in the lean subjects the fall was greater (86 +/- 0.04% of basal, P = 0.02) than in the obese, where it did not reach statistical significance (96 +/- 0.05% of basal). Pre-meal and peak insulin levels showed a positive correlation with circulating mean leptin levels (r = +0.65; P < 0.001 and r = +0.78; P < 0.001, respectively) in all subjects, while pre-meal and peak serum cortisol levels showed an inverse relation with leptin levels (r = -0.53; P = 0.002 and r = -0.41; P = 0.02, respectively); this effect was independent of BMI in the obese subjects. In the patient with the insulinoma the markedly elevated insulin and leptin levels measured before the operation returned to normal after removal of the tumour, in accord with reports of experimental animal data that long-term insulin excess per se is associated with increased circulating leptin concentrations. CONCLUSION: Leptin is a robust indicator of BMI and insulin levels, both basal and stimulated, but does not change acutely following food. Fasting causes a proportionately greater decline in leptin levels in lean subjects than in obese subjects. Circulating leptin is inversely correlated with the activity of the hypothalamo-pituitary-adrenal axis: whether this is a direct influence of leptin on hypothalamo-pituitary-adrenal activity, or whether both are indirect indicators of body fat stores, requires further investigation.

Adult↗

PubMed1997

Thyroid function in children with different lipoprotein profiles: observations in a biracial (black/white) population--the Bogalusa Heart Study.

Abnormalities of thyroid function are associated with hyperlipidemia, a risk factor for coronary artery disease that starts in childhood. We investigated the age-, race-, and sex-related differences in thyroid function and its relation to serum lipoprotein levels in children (n = 363) aged 6 to 18 years from the biracial (black/white) community of Bogalusa, Louisiana, using an ultrasensitive thyroid-stimulating hormone (TSH) assay. Serum levels of lipoprotein cholesterol fractions, triglycerides, triiodothyronine (T3), thyroxine (T4), and the Tanner stage of sexual development were determined. Serum T3 (P < 0.0001), T4 (P < 0.0001), and TSH (P < 0.0020) levels decreased significantly with Tanner stage. Serum T4 levels were significantly higher (P < 0.0001) in both black and white females than their male counterparts. An unexpected finding was a significantly increased mean serum TSH in whites (2.09 + 0.91; mean + standard error of mean) when compared to blacks (1.74 + 0.10; P = 0.0185). Overall, no significant correlation was noted between serum lipoprotein variables and TSH. However, those with the highest low-density lipoprotein to very low-density lipoprotein cholesterol fractions had a higher T4 and a T4/TSH ratio than those with the lowest low-density lipoprotein to very low-density lipoprotein cholesterol fractions. In summary, it is concluded that there is no simple relationship between lipoproteins and TSH or thyroid hormone levels in children.

Adolescent↗

PubMed1997

Outcomes analysis of teaching on a medical service with trainees of diverse medical experience.

BACKGROUND: Rotations in medical subspecialties often involve trainees with diverse levels of medical knowledge and experience. We conducted a study to determine whether teaching on such a clinical service could be delivered effectively to all members of the group despite their disparate backgrounds. METHODS: The Section of Endocrinology offers a 4-week rotation that is taken jointly by third- and fourth-year medical students, interns, residents and endocrinology fellows. Over the past 4 years enrollees were required to complete both a pre and postcourse test of written, multiple choice examination in general endocrinology. The mean and SEM percentage of correct answers were compared. RESULTS: It was found that the scores on the prerotation test were correlated with the level of experience; students scored the lowest and fellows the highest. There was a significant overlap; however, between groups, students' results, for example, were not statistically different from those of the interns but were lower than those of the residents and very different from fellows (p value < 0.0001). CONCLUSIONS: Rotations of trainees with mixed ability clearly lead to objective improvements in all groups except fellows. Students and interns appear to gain the greatest increment. Residents also improve. It is concluded that all levels of trainees (below the level of fellow) can increase their fund of knowledge by taking a subspeciality rotation even though the group may be of mixed levels of ability.

Education, Medical↗

PubMed1996

Molded continuous poly(styrene-co-divinylbenzene) rod as a separation medium for the very fast separation of polymers. Comparison of the chromatographic properties of the monolithic rod with columns packed with porous and non-porous beads in high-performance liquid chromatography of polystyrenes.

Gradient elution separations of polystyrene standards in a monolithic molded 50 x 8 mm I.D. poly(styrene-co-divinylbenzene) rod column and in 50 x 8 mm I.D. and 30 x 4.1 mm I.D. columns packed with porous and non-porous poly(styrene-co-divinylbenzene) beads has been carried out. All of these separation media differ in shape and porosity. Excellent separations of eight polystyrene standards were achieved with both the molded monolithic rod and porous beads at moderate flow-rates. However, the monolithic medium proved to be superior for high-speed separations using very steep gradients at a flow-rate of 20 ml/min. Three polystyrene standards were separated in the rod column within 4 s. The separation in the column packed with non-porous beads was poor at flow-rates of 2-8 ml/min, while higher flow-rates led to an unacceptably high back pressure.

Chromatography, High Pressure Liquid↗

PubMed1996

New designs of macroporous polymers and supports: from separation to biocatalysis.

Reactive polymers play many roles, from supports for solid-phase synthesis or catalysis to media for separations. Although macroporous polymer beads that provide high reactive capacities and excellent solvent tolerance are well established, approaches to monosized beads with optimized pore structures or multiple chemistries segregated within pores of different sizes have expanded their realm of application. Polymer monoliths containing intricate pore networks can be obtained in any desired shape by a simple molding process and provide unique advantages such as fast kinetics, high reactivity, and high throughput. Applications ranging from immobilized enzyme reactors to fast media for the separation of synthetic or biopolymers are presented.

Catalysis↗