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Biomedical subjects

G A Schwartz

Publications and source records attributed to G A Schwartz.

11 recordsLinked to original sources
PubMed2006

Water dynamics in n-propylene glycol aqueous solutions.

The relaxation dynamics of dipropylene glycol and tripropylene glycol (nPG-n=2,3) water solutions on the nPG-rich side has been studied by broadband dielectric spectroscopy and differential scanning calorimetry in the temperature range of 130-280 K. Two relaxation processes are observed for all the hydration levels; the slower process (I) is related to the alpha relaxation of the solution whereas the faster one (II) is associated with the reorientation of water molecules in the mixture. Dielectric data for process (II) at temperatures between 150 and 200 K indicate the existence of a critical water concentration (x(c)) below which water mobility is highly restricted. Below x(c), nPG-water domains drive the dielectric signal whereas above x(c), water-water domains dominate the dielectric response at low temperatures. The results also show that process (II) at low temperatures is due to local motions of water molecules in the glassy frozen matrix. Additionally, we will show that the glass transition temperatures (T(g)) for aqueous PG, 2PG, and 3PG solutions do not extrapolate to approximately 136 K, regardless of the extrapolation method. Instead, we find that the extrapolated T(g) value for water from these solutions lies in the neighborhood of 165 K.

Journal Article↗
PubMed2005

Combining configurational entropy and self-concentration to describe the component dynamics in miscible polymer blends.

We provide a new approach to describe the component segmental dynamics of miscible polymer blends combining the concept of chain connectivity, expressed in terms of the self-concentration, and the Adam-Gibbs model. The results show an excellent agreement between the prediction of our approach and the experimental data. The self-concentrations obtained yield length scales between 1 and 3.2 nm depending on the temperature, the flexibility of the polymer, expressed in terms of the Kuhn segment, and its concentration in the blends, at temperatures above the glass transition range of the blend.

Journal Article↗
PubMed2003

Dielectric relaxation studies of poly(propylene glycol) confined in vermiculite clay.

The molecular dynamics of oligomeric poly(propylene glycol) (PPG) liquids (M(w)=1200, 2000 and 4000 g/mol) confined in a two-dimensional layer-structured Na-vermiculite clay has been studied by broadband dielectric spectroscopy. In addition to the alpha-relaxation, the normal mode relaxation process was studied for all samples both in bulk and confinement. For the normal mode process the relaxation rate in the clay is drastically shifted to lower frequencies compared to that of the bulk material in contrast to the alpha-process whose relaxation time is only slightly affected by the confinement. Also the temperature dependence of the relaxation time for the normal mode process is strongly affected by the confinement. Moreover, in the clay the intensity of the normal mode is stronger than that of the alpha-process, in contrast to the bulk samples where the opposite is observed.

Journal Article↗
PubMed2003

Dynamics of propylene glycol and its oligomers confined in clay.

The dynamics of propylene glycol (PG) and its oligomers 7-PG and PPG, with Mw = 4000 (about 70 monomers), confined in a Na-vermiculite clay have been investigated by quasi-elastic neutron scattering and dielectric spectroscopy. The liquids are confined to a single molecular layer between the clay platelets, thus giving a true 2D liquid. The results show that the average relaxation time [tau], deduced from neutron scattering at a momentum transfer Q of about 1 A(-1) is in perfect agreement with the dielectric alpha-relaxation time, although neutron scattering does not only probe the main (alpha-) relaxation, but all motions of hydrogens on the experimental time scale. At room temperature 1/[tau] is proportional to Q(2), indicating that the relaxations are mainly due to ordinary translational diffusion. The most unexpected finding is that [tau](or the dielectric alpha-relaxation time) is almost unaffected by the 2D confinement, in contrast to the dielectrically active normal mode of PPG which is substantially slower in the confinement. Only the 7-mer has a significantly slower segmental translational diffusion in the clay. The results suggest that the interactions to the clay surfaces are weak and that the present 2D confinement has a very small influence on the time scale of all our observed relaxation processes, except the normal-mode relaxation.

Adsorption↗
PubMed2001

Post-translational phosphorylation of the slow/beta myosin heavy chain isoform in adult rabbit masseter muscle.

Four different phenotypes of slow muscle fibers, characterized by differential epitope expression in the slow/beta myosin heavy chain (MyHC) isoform, have been identified in adult rabbit masseter muscle. We investigated the role of post-translational phosphorylation in the expression of these four phenotypes. Serial cryostat sections were treated either with alkaline phosphatase to dephosphorylate proteins in the tissue, or with a brain kinase solution and ATP to phosphorylate them, and then stained, using four antibodies that bind specifically to the slow/beta MyHC isoform. In sections pre-treated with phosphatase, immunoreactivity to antibody A4.840 was abolished, but it could be restored by subsequent kinase/ATP treatment or ATP alone, indicating that the expression of its epitope requires phosphorylation. Phosphatase treatment resulted in an exposure of the epitope for antibody A4.951 in cells that normally bind this antibody only weakly or not at all, but since heat treatment alone produced similar effects, the role of phosphorylation in this enhancement is less certain. Immunoreactivity to antibodies S58 and BA-D5 were not influenced by phosphatase pre-treatment. Kinase/ATP treatment was only effective in changing antibody binding when tissues already had been phosphatase treated. We interpret these results to mean that sites of potential phosphorylation may already be occupied by O-linked glycosylation.

Animals↗
PubMed2000

A digital 3-dimensional method for computing great artery flows: in vitro validation studies.

BACKGROUND: Conventional 2-dimensional Doppler large vessels are prone to inaccuracy. Three-dimensional (3D) volume imaging provides the opportunity to make cross-sectional flow calculations through digital spatiotemporal integration of flow velocity, area, and profile. METHODS: A new digital 3D color Doppler reconstruction method was used to generate radially acquired flow data sets. Raw scanline data with digital velocity assignments, obtained by scanning parallel to flow, were transferred from a specially programmed but otherwise conventional ultrasonographic system, which controlled a multiplane transesophageal probe, to a computer workstation via an Ethernet link for assimilation into color 3D data sets. This configuration was used to study 20 pulsatile laminar flows (stroke volumes 30 to 70 mL and peak flow rates 65 to 205 mL/s) in a curved tube model with an oval cross-sectional geometry. After generation of the color 3D data set, flow velocity values from cross sections perpendicular to the tubes were analyzed to determine flow rate and stroke volume. RESULTS: The flows from 3D digital velocity profiles showed close correlation with peak instantaneous flow rates (r = 0.99, y = 1.01x-0.9, standard error of estimate 4.1 mL/s). When interpreted with pulsed wave Doppler data obtained through the cardiac cycle, they also allowed computation of stroke volume (r = 0.98, y = 1.44x-2.5, standard error of estimate 3.8 mL). CONCLUSION: The ability to compute laminar flows from 3D digital data sets obtained parallel to the direction of flow and without the need for geometric assumptions represents an important opportunity for and advantage of 3D color Doppler echocardiography.

Coronary Vessels↗
PubMed2000

Sexually dimorphic expression of myosin heavy chains in the adult mouse masseter.

Little is known regarding the role of androgenic hormones in the maintenance of myosin heavy chain (MHC) composition of rodent masticatory muscles. Because the masseter is the principal jaw closer in rodents, we felt it was important to characterize the influence of androgenic hormones on the MHC composition of the masseter. To determine the extent of sexual dimorphism in the phenotype of masseter muscle fibers of adult (10-mo-old) C57 mice, we stained tissue sections with antibodies specific to type IIa and IIb MHC isoforms. Females contain twice as many fibers containing the IIa MHC as males, and males contain twice as many fibers containing the IIb MHC as females. There is a modest amount of regionalization of MHC phenotypes in the mouse masseter. The rostral portions of the masseter are composed mostly of type IIa fibers, whereas the midsuperficial and caudal regions contain mostly type IIb fibers. Using immunoblots, we showed that castration results in an increase in the expression of type IIa MHC fibers in males. Ovariectomy has no effect on the fiber type composition in females. We conclude that testosterone plays a role in the maintenance of MHC expression in the adult male mouse masseter.

Age Factors↗
PubMed1991

200 kD neurofilament protein and synapse elimination in the rat soleus muscle.

We studied the distribution and appearance of the phosphorylated form of the 200 kD neurofilament protein in the rat soleus muscle during the period of postnatal synapse elimination. Unlike many muscles, the appearance of singly innervated muscle cells in soleus occurs well after myogenesis has been completed, so that synapses are eliminated from a stable population of muscle cells. Immunoreactivity to the 200 kD neurofilament protein is present in the terminals of neuromuscular synapses of animals at all postnatal ages from 0 to 21 days. Before postnatal day 10, when physiological studies indicate that all soleus muscle cells receive more than one synaptic input, as many as 30% of soleus muscle cells contain phosphorylated 200 kD neurofilament protein immunoreactivity in only one synaptic terminal. At older ages the number of polyneuronally innervated muscle cells observed using immunostaining is similar to that observed physiologically. These findings suggest that not all developing neuromuscular synapses contain phosphorylated 200 kD neurofilament protein, and that those terminals lacking it comprise most of those eliminated early in the postnatal period. We conclude that the presence of phosphorylated 200 kD neurofilament protein might be highly correlated with the survival of motor nerve terminals during postnatal neuromuscular synapse elimination.

Animals↗
PubMed1991

Tenotomy delays both synapse elimination and myogenesis in rat lateral gastrocnemius.

To investigate a possible relationship between synapse elimination and myogenesis, we examined both phenomena during the first 2 weeks of postnatal life in the rat lateral gastrocnemius muscle. Synapse elimination and myogenesis occur simultaneously. Sixty per cent of the number of fibers observed in adult muscles is generated during the first 10 days of postnatal life; during this time, the majority of muscle cells in lateral gastrocnemius also become singly innervated. We delayed synapse elimination by cutting the tendon of insertion of lateral gastrocnemius (tenotomy) on the day of birth. Both synapse elimination and postnatal myogenesis were slowed by tenotomy. Tenotomized muscles contained fewer detectable cells than unoperated contralateral control muscles. These results suggest that synapse elimination may be altered by altering postnatal muscle fiber addition.

Animals↗