A mean distance of more than 100 A separates the surfaces of lipoproteins and rat erythrocytes

Direct determination of unbound lipophilic ligands in aqueous solutions

Due to their hydrophobic nature, lipophilic compounds are always bound to proteins when transported in the organism. The transfer of such compounds between their binding proteins and cells as well as intracellular trafficking is mediated by a very low water-phase concentration of monomers. The use of protein filled resealed red cell membranes (erythrocyte ghosts) as semipermeable bags enables us to determine directly such water-phase concentrations in a biological system where the lipophilic compound is in equilibrium with the compound bound to its binding protein. Equilibrium dissociation constants (K(d)'s) and number of binding sites are determined by regression analyses of data. We describe the method with the hydrophobic anion arachidonate and the neutral N-arachidonoylethanolamide as examples.

Effects of Intralipid infusion on hemorheology and peripheral resistance in neonates and children

Deleterious microcirculatory effects of Intralipid (IL) infusion may be caused by hemorheological or vascular effects. The aim of this investigation was to study vascular and hemorheological effects of IL in preterm and fullterm neonates and children. Ten preterm newborns, 10 fullterm neonates, and 10 children received an initial infusion of IL (0.6 g/kg) over 4 h. Calf blood flow (venous occlusion plethysmography), blood pressure (Dinamap), whole blood and plasma viscosity (capillary viscometer), red blood cell deformability (rheoscope), and erythrocyte aggregation (aggregometer) were measured before and after administration of IL. Plasma triglyceride levels showed the greatest increase in preterm infants. Whole blood viscosity decreased by about 10% in all three groups because of a similar reduction in hematocrit. Red blood cell aggregation decreased by about 20% after IL infusion. Blood pressure rose by 10%, and peripheral blood flow declined by about 10% in the three groups. Vascular hindrance, a calculation of blood pressure divided by blood flow and viscosity, was raised by about 20%, suggesting marked vasoconstriction of peripheral arteries. Vasoconstriction rather than hemorheological changes during infusion of IL may play a crucial role in the pathogenesis of circulatory alterations in parenterally-fed neonates.

Palmitate binding to and efflux kinetics from human erythrocyte ghost

At 0 degrees C, pH 7.3, palmitate (PA) binds to human erythrocyte ghosts suspended in 0.2% bovine serum albumin (BSA) solution with molar ratios of PA to BSA, v, between 0.2 and 1.3. The binding depends on the water phase PA concentration, measured in equilibrium experiments, using BSA-filled ghosts as semipermeable bags. The saturable binding has a capacity of 19.4 +/- 7.5 nmol g-1 packed ghosts (7.2 x 10(9) cells) and Kd = 13.5 +/- 5 nM. PA exchange efflux kinetics to 0.2% BSA is recorded from ghosts without and with 0.2% BSA with a resolution time of about 1 s. Data are analyzed in terms of compartmental models. Using BSA-free ghosts the kinetics is essentially monoexponential. The rate constant is 0.0287 +/- 0.0022 s-1. Using ghosts with BSA, the kinetics is biexponential with widely different rate constants. Extrapolated zero-time values reflect, according to additional investigations, 'instantaneous' release of PA from the outer surface of the ghosts. Analyses of the biexponential curve up to about 55% tracer efflux assign unequivocally values to three model parameters. (1) k1, the dissociation rate constant of the PA-BSA complex is (1.47 +/- 0.03) x 10(-3) s-1 and (2.56 +/- 0.08) x 10(-3) s-1 and (4.08 +/- 0.13) x 10(-3) s-1 at v = 0.2, 0.6 and 1.4, respectively. (2) k3*, the overall rate constant of PA transport from the inside of the ghost membrane to the medium is 0.0269 +/- 0.0020 s-1 independent of v. (3) Qkin, the ratio of PA on the inside of the membrane to PA on BSA within the ghosts is v dependent and smaller than a corresponding ratio Qeq measured in equilibrium by a value corresponding to PA on the outer surface. This fraction is released with a rate constant, k5, which is of the order of 1 s-1. The data suggest a maximum PA transport capacity, Jmax, of 2 pmol min-1 cm-2, 0 degrees C, pH 7.3.

Mechanisms and consequences of cellular cholesterol exchange and transfer

It is apparent from consideration of the reactions involved in cellular cholesterol homeostasis that passive transfer of unesterified cholesterol molecules plays a role in cholesterol transport in vivo. Studies in model systems have established that free cholesterol molecules can transfer between membranes by diffusion through the intervening aqueous layer. Desorption of free cholesterol molecules from the donor lipid-water interface is rate-limiting for the overall transfer process and the rate of this step is influenced by interactions of free cholesterol molecules with neighboring phospholipid molecules. The influence of phospholipid unsaturation and sphingomyelin content on the rate of free cholesterol exchange are known in pure phospholipid bilayers and similar effects probably occur in cell membranes. The rate of free cholesterol clearance from cells is determined by the structure of the plasma membrane. It follows that the physical state of free cholesterol in the plasma membrane is important for the kinetics of cholesterol clearance and cell cholesterol homeostasis, as well as the structure of the plasma membrane. Bidirectional flux of free cholesterol between cells and lipoproteins occurs and rate constants characteristic of influx and efflux can be measured. The direction of any net transfer of free cholesterol is determined by the relative free cholesterol/phospholipid molar ratios of the donor and acceptor particles. Cholesterol diffuses down its gradient of chemical potential generally partitioning to the phospholipid-rich particle. Such a surface transfer process can lead to delivery of cholesterol to cells. This mechanism operates independently of any lipoprotein internalization by receptor-mediated endocytosis. The influence of enzymes such as lecithin-cholesterol acyltransferase and hepatic lipase on the direction of net transfer of free cholesterol between lipoproteins and cells can be understood in terms of their effects on the pool sizes and the rate constants for influx and efflux. Excess accumulation of free cholesterol in cells stimulates the rate of cholesteryl ester formation and induces deposition of cholesteryl ester inclusions in the cytoplasm similar to the situation in the 'foam' cells of atherosclerotic plaque. Clearance of cellular cholesteryl ester requires initial hydrolysis to free cholesterol followed by efflux of this free cholesterol. The rate of clearance of cholesteryl ester from cytoplasmic droplets is influenced by the physical state of the cholesteryl ester; liquid-crystalline cholesteryl ester is removed more slowly than cholesteryl ester in a liquid state.(ABSTRACT TRUNCATED AT 400 WORDS)

Passage of ions and dextran molecules across the rete mirabile of the eel. The effects of charge

The countercurrent-perfused rete mirabile of the eel is a preparation in which capillary permeability values can be determined accurately in broadscale fashion. To provide insight into charge effects on transendothelial passage, the permeability values of small inorganic cations (labeled sodium and rubidium, and stable potassium) and anions (labeled chloride, iodide, sulfate, and ferrocyanide) were compared to those expected for neutral solutes with approximately matched diffusion coefficients, and that of a neutral dextran fraction was compared to that of a negatively charged dextran sulfate with a similar diffusion coefficient. In the small ion experiments, the labeled iodide values were unexpectedly high, apparently due to the contamination of the labeled iodide solution with I-3. The permeabilities of the rest of the ions clustered at a level about 0.5 of the values which would have been expected for neutral solutes with similar diffusion coefficients. The decrement was interpreted to reflect the presence of both positive and negative charges along the transendothelial pathway, which effectively decrease the dimensions of the limiting part of the pathway for the charged microions relative to that accessible to comparable nonelectrolytes. The larger negatively charged dextran sulfate was also reduced in its passage, in comparison with its matching neutral dextran; this was taken to indicate the presence of a larger scale average net negative charge along its pathway. The data indicate the presence of a staggering of positive and negative charges along the transendothelial pathway accessible to the microions, and a net negative charge in the more restricted part of the pathway available to the dextrans.