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Biomedical subjects

T P Stewart

Publications and source records attributed to T P Stewart.

At least 19 recordsLinked to original sources
PubMed1990

Pressure relief characteristics of an alternating pressure system.

The pressure relieving characteristics of an alternating pressure mattress are compared to the characteristics of a traditional hospital mattress. The trochanteric pressures of 20 healthy volunteers had a mean of 95 (SD = 49) on a hospital mattress while the trochanteric pressures on the alternating pressure mattress had a mean of 67 (SD = 9) for the inflated cells and a mean of 23 (SD = 7) for the deflated cells. Statistical analysis revealed significantly lower interface pressure for the alternating pressure mattress.

Adult↗
PubMed1988

Kinetic measurements of fusion of phosphatidylserine-containing vesicles by electron microscopy and fluorometry.

Large unilamellar vesicles (REV) containing phosphatidylserine and phosphatidylethanolamine at a ratio of 1:3 were induced to fuse by adding calcium (4 mM). The kinetics of fusion was monitored by fluorometry using terbium or dipicolinic acid-containing vesicles. The morphology and the states of vesicle aggregation and fusion were examined at approx. 2, 30, 60, 150 and 900 s after calcium addition, by rapid quenching and freeze-fracture electron microscopy. The size and the state of aggregation of vesicles are quantitated from 4000 randomly selected vesicles. The aggregation and fusion kinetics as assayed by fluorescence volume mixing is very well simulated and predicted by the mass action model. The model essentially predicts the time course of the distribution of the aggregates and the increase in size of fused particles as measured by electron microscopy, although in some cases the predicted fusion rate exceeds that by morphometric measurement. No morphological features can be defined as fusion intermediates, although bead-like and rim-like materials may be attributed to the remnants of broken diaphragms between fusion partners.

Calcium↗
PubMed1984

Alterations in phospholipid polymorphism by polyethylene glycol.

The fusogen polyethylene glycol is shown to alter the polymorphism of dimyristoyl phosphatidylcholine, soybean phosphatidylethanolamine, bovine phosphatidylserine, egg phosphatidylcholine/cholesterol mixture, dilinoleoylphosphatidylethanolamine/palmitoyl-oleoylphosphatidy lcholine mixture, and egg lysolecithin. Suspension of these lipids in 50% polyethylene glycol (mol wt = 6000) reduces both the lamellar and the hexagonal II repeat spacings as measured by X-ray diffraction. An increase in the gel to liquid crystalline and bilayer to hexagonal transition temperatures are observed by freeze-fracture, X-ray diffraction, differential scanning calorimetry and 31P NMR. Freeze-fracture electron micrographs revealed different bilayer defects depending on the physical states of the lipid. Lipidic particles in mixtures containing unsaturated phosphatidylethanolamine is eliminated. Some of the influences of polyethylene glycol on lipids may be explained by its dehydrating effect. However, other nonfusogenic dehydrating agents failed to produce similar results. These findings are consistent with the proposal that close bilayer contact and the formation of bilayer defects are associated with the fusogenic properties of polyethylene glycol.

Animals↗
PubMed1984

Thermal support for the very-low-birth-weight infant: role of supplemental conductive heat.

Heated water pads were placed underneath very-low-birth-weight preterm infants receiving care under overhead radiant warmers with plastic heat shields to evaluate the effects on their thermal environments. The electrical input (watts) to the overhead warmer was monitored and temperatures at various locations in the environment and on the infants were recorded. The electrical input to the radiant warmers decreased significantly (P = 0.0015) with the introduction of the heated water pads. Several of the temperatures recorded during the study also showed significant changes, including increased abdominal skin temperatures in these prone infants (P = 0.007) and decreased back-to-abdomen skin temperature gradients (P = 0.007). Less marked changes included higher mattress or pad temperatures (P = 0.019), lower ambient air temperatures within the plastic heat shields (P = 0.022), and increased infant foot temperatures (P = 0.036). We believe that conduction has a significant influence on the thermal environment of the VLBW infant cared for under an overhead radiant warmer and that the use of a heated water pad can markedly alter heat transfer. The impact of conduction appears to have been previously underestimated.

Abdomen↗
PubMed1983

The nature of lipidic particles and their roles in polymorphic transitions.

The structural transition between bilayer (L alpha), inverted hexagonal (HII) and inverted cubic (CII) phases in mixtures of unsaturated phosphatidylethanolamines (PE) and phosphatidylcholines (PC) were investigated. Freeze fracture electron micrographs of intermediate stages of phase transitions showed that CII was a stable intermediate form between the L alpha--HII transition. The electron microscopic observation was supported by X-ray diffraction and 31P-NMR results. Detailed morphology revealed that during the L alpha--CII transition, interlamellae attachment points (conical lipidic particles) connect adjacent bilayers to form arrays of entrapped water pockets (inverted micelles). These water-containing spherical units were packed in a cubic lattice. In the CII to HII transition, these spherical units were linearly connected to form tubes. During the L alpha--HII transition, a ripple pattern was observed across the otherwise smooth lamellar. The troughs of the ripples were transformed into linear connections between adjacent bilayers, thereby converting multilayer structures into parallel tubes. No lipidic particles were involved in this type of transition. We show that there are different mechanisms involved in the L alpha, HII, CII polymorphic transitions, and that different types of 'lipidic particles' representing different molecular organizations may be involved in each case. Models of these transitions are proposed.

Lipid Bilayers↗
PubMed1982

Phase behavior of the major lipids of tetrahymena ciliary membranes.

The major lipids of Tetrahymena membranes have been purified by thin-layer and high pressure liquid chromatography and the phosphatidylethanolamine and aminoethylphosphonate lipids were examined in detail. 31P-NMR, X-ray diffraction and freeze-fracture electron microscopy were employed to describe the phase behavior of these lipids. The phosphatidylethanolamine was found to form a hexagonal phase above 10 degrees C. The aminoethylphosphonate formed a lamellar phase up to 20 degrees C, but converted to a hexagonal phase structure at 40 degrees C. Small amounts of phosphatidylcholine stabilized the lamellar phase for the aminoethylphosphonate. 31P-NMR spectra of the intact ciliary membranes were consistent with a phospholipid bilayer at 30 degrees C, suggesting that phosphatidylcholine in the membrane stabilized the lamellar form, even though most of the lipid of that membrane prefers a hexagonal phase in pure form at 30 degrees C. 31P-NMR spectra also showed a distinctive difference in the chemical shift tensor of the aminoethylphosphonolipid, when compared to that of phosphatidylethanolamine, due to the difference in chemical structure of the polar headgroups of the two lipids.

Animals↗
PubMed1981

The condensing effect of glucagon on phospholipid bilayers.

Glucagon forms discoidal particles with dimyristoylphosphatidylcholine at temperatures below the phase transition. Under these conditions and at a lipid to protein molar ratio of 20 : 1, glucagon is observed to induce a closer packing of the phospholipid bilayer. Similar effects are observed upon the interaction of glucagon with dilauroylphosphatidylcholine. In the region of the phase transition the discoidal particles are observed by freeze-fracture electron microscopy to undergo end-to-end association leading to the formation of multilamellar structures containing only a few layers and having a large internal volume. Above the phase transition temperature the properties of the lipid appear to be unperturbed by glucagon according to either freeze-fracture or densitometer studies. These results support the importance of phospholipid phase transitions in peptide-lipid interactions.

Densitometry↗
PubMed1981

Interactions of La2+ with phosphatidylserine vesicles: binding, phase transition, leakage, 31P-NMR and fusion.

The interaction of La2+ with phosphatidylserine vesicles is studied by differential scanning calorimetry, 140La binding, 31P-NMR chemical shifts and relaxation rates, carboxyfluorescein and [14C]sucrose release, X-ray diffraction and freeze-fracture electron microscopy. In the presence of La3+ concentrations above 1 mM and an incubation temperature of 38 degrees C, i.e., at the phase transition temperature of the complex La/phosphatidylserine, the binding ratio of La/lipid exceeds a 1/3 ratio, reaching saturation at a 1/2 ratio. Analysis, employing a modified Gouy-Chapman equation, indicates a significant increase in the intrinsic binding constant of La/phosphatidylserine when the La3+ concentrations exceeds the threshold concentration for leakage. The analysis illustrates that at the molecular level the binding of La3+ can be comparable to or even weaker than that of Ca2+, but that even when present at smaller concentrations La3+ competes with and partially displaces Ca2+ from membranes or other negatively charged surfaces. The results suggest that the sequence La3+ greater than Ca2+ greater than Mg2+ reflects both the binding strength of these cations to phosphatidylserine as well as their ability to induce leakage, enhancement of 31P spin-lattice relaxation rates, fusion and other structural changes. The leakage, fusion, and other structural changes are more pronounced at the phase transition temperature of the La/lipid complex.

Electron Spin Resonance Spectroscopy↗
PubMed1981

Membrane fusion through point defects in bilayers.

Fusion between bilayers of mixed egg phosphatidylcholine and soybean phosphatidylethanolamine was induced by freezing and thawing. Contact points between bilayers were observed by freeze fracture electron microscopy, and isotropic molecular motional averaging was detected by phosphorus-31 nuclear magnetic resonance under fusion conditions. A molecular model of point defect structure is proposed as an intermediate stage of fusion.

Freeze Fracturing↗
PubMed1981

Lipid-polyethylene glycol interactions: I. Induction of fusion between liposomes.

Fusion between unilamellar vesicles of both egg phosphatidylcholine and bovine phosphatidylserine was induced by polyethylene glycol. Aggregation and fusion events were monitored by electron microscopy and turbidity measurements. The threshold concentration of polyethylene glycol for aggregation and fusion is found to be independent of lipid concentration. Typically, aggregation of phosphatidylcholine vesicles starts at 2.5% (wt/wt) polyethylene glycol, but fusion is not significant until the polyethylene glycol concentration reaches 35%. Multilamellar vesicles were formed as a result of fusion.

Animals↗
PubMed1981

Lipid-polyethylene glycol interactions: II. Formation of defects in bilayers.

The kinetic properties of L-leucine transport across the human red blood cell membrane was analyzed according to the simple pore and carrier theory of Lieb and Stein (Biochim. Biophys. Acta, 1974, 373: 165-177 and 178-196) at 25 degrees C, pH 7.4. Several methods were used in order to obtain a thorough kinetic description of L-leucine transport. A rejection of the simple pore model was suggested from the result of zero-trans influx and zero-trans and equilibrium-exchange efflux experiments. Several predictions from the simple carrier model, based on the requirement of consistency among different kinetic parameters, were tested in infinite experiments, i.e. experiments performed at a high concentration of substrate at one of the faces of the membrane. The simple pore model was rejected, but no crucial evidence against a simple carrier model, which displays symmetric properties at 25 degrees C, was found in the concentration range considered (0.002-68 mM). The relative magnitudes of the rate constants of the translocation process are discussed, and it is concluded (a) that both the dissociation and translocation of carrier-complex is faster than the translocation of the empty carrier, (b) that no translocation step is rate determining, and (c) that the carrier-complex is equally distributed across the membrane at equilibrium. The present work provides a unique example of a carrier-mediated transport mechanism which displays symmetric properties. L-leucine transport in red blood cells may be a convenient system for studying molecular mechanisms of facilitated transport.

Cell Fusion↗
PubMed1980

Temperature-dependent morphological and phase behavior of sphingomyelin.

Aqueous dispersions of bovine brain sphingomyelin were studied as a function of temperature. 31P-NMR, X-ray diffraction, and negative-stain and freeze-fracture electron microscopy were used to determine the morphology and phase structure at several temperatures. 31P-NMR indicated a change in phase structure with an increase in temperature. Evidence was found only for the lamellar phase at all temperatures studied with X-ray diffraction. Electron microscopy unexpectedly revealed the spontaneous development of small unilamellar vesicles at elevated temperatures, consistent with the 31P-NMR data, in the absence of any outside disturbances.

Freeze Fracturing↗
PubMed1980

Effects of cholesterol on lipid organization in human erythrocyte membrane.

The molar ratio of cholesterol to phospholipid (C/P) in human erythrocyte membrane is modified by incubating the cells with liposomes of various C/P ratios. The observed increase in cell surface area may be accounted for by the addition of cholesterol molecules. Fusion between liposomes and cells or attachment of liposomes to cells is not a significant factor in the alteration of C/P ratio. Onset temperatures for lipid phase separation in modified membranes are measured by electron diffraction. The onset temperature increases with decreasing C/P ration from 2 degrees C at C/P = 0.95 to 20 degrees C at C/P = 0.5. Redistribution of intramembrane particles is observed in membranes freeze-quenched from temperatures below the onset temperature. The heterogeneous distribution of intramembrane particles below the onset temperature suggests phase separation of lipid, with concomitant segregation of intramembrane protein into domains, even in the presence of an intact spectrin network.

Cholesterol↗