Dietary cholesterol absorption, and sterol and bile acid excretion in hypercholesterolemia-resistant white rabbits

The New Zealand white (NZW) rabbit fed a 0.1% cholesterol-enriched diet (CD) typically responds (normoresponsive, NR) by quickly developing hypercholesterolemia. To study the underlying mechanisms responsible for the widespread phenomenon of inter-individual variability of response to dietary cholesterol, a unique hypercholesterolemia-resistant (RT) rabbit model was developed. These animals were utilized to investigate selected potential mechanisms that might enable the RT animal to compensate for dietary cholesterol overload. When rabbits were fed the low-cholesterol stock diet, there was no significant difference in the plasma cholesterol concentrations of the NR and the RT animals. However, a significant rise was observed in the NR rabbits within 1 month of their being placed on the cholesterol-enriched diet; the plasma cholesterol concentration of the RT animals was not affected. During consumption of the cholesterol diet the cholesterol absorption rate was somewhat greater in the NR rabbits (P less than 0.05), whereas intestinal transit times and the fecal excretion of neutral steroids were substantially the same in both groups. In contrast, the fecal bile acid excretion of the RT animals was more than twice as great (P less than 0.0001) as that of the NR animals. We conclude that the response to dietary cholesterol is a heritable trait in these rabbits and that, although less dietary cholesterol was absorbed by the RT animals, it appears that a major mechanism controlling plasma cholesterol levels involves the rate of conversion of cholesterol to bile acids and their subsequent excretion.

Cyclopentanoid analogs of dipalmitoyl phosphatidic acid: effect of backbone geometry on thermotropic properties

Seven geometrical or positional isomers of dipalmitoyl cyclopentanophosphoric acid (DPCPA) have been synthesized and studied: 1,3/2-1P (I); 1,2/3-1P (II); 1,2/3-3P (III), 1,2,3/0-1P (IV); 1,2,3/0-2P (V); 1,3/2-2P (VI); 1,2/3-2P (VII). When dispersed in 0.1 M Tris-HCl at pH 7.4, I-VII gave thermal transitions (Tc) of 60.0 degrees, 59.0 degrees, 56.8 degrees, 55.3 degrees, 38.3 degrees, 36.8 degrees and 34.0 degrees C, respectively, as measured by differential scanning calorimetry (DSC). When the lipids were dispersed at pH 9.5 in 0.1 M borate, Tc of I-IV decreased, whereas Tc of V-VII increased. In contrast, at pH 1.5 in 0.1 M HCl/KCl, Tc of I-IV decreased slightly, but Tc of V-VII rose markedly. To determine the effect of head group geometry and substitution pattern on acyl chain motion, EPR spectra of 1-palmitoyl, 2-[16-doxylstearoyl]-glycero-3-phosphoric acid in bilayers of DPCPA isomers were acquired. Abrupt spectral changes occurred at temperatures closely correlating with transition temperatures observed by DSC. These results have led to the conclusions that: (i) isomers I-IV containing vicinal acyl chains form bilayers that exhibit structural transitions at temperatures higher than those at which transitions are exhibited by isomers V-VII which have a polar phosphate group interposed between the two chains; (ii) the effects of differences in backbone structure are transmitted down the entire length of the acyl chains; (iii) the orientation of the cyclopentane ring in the isomers I-IV is significantly different from that in isomers V-VII at pH values where the phosphate group is doubly negatively charged.

Effect of particle size and temperature on the conformation and physiological behavior of apolipoprotein E bound to model lipoprotein particles

The effect of particle size and structural order/disorder of the lipid domain on the conformation and physiological behavior of lipid-associated apolipoprotein E (apoE) was evaluated. Circular dichroic (CD) spectra of apoE bound to large (LME) and small (SME) microemulsion particles, composed of dimyristoylphosphatidylcholine (DMPC) and cholesteryl oleate (CO), and to DMPC disks revealed that at 4 degrees C, where all of the lipid constituents were in an ordered state, apoE bound to LME displayed approximately 60% alpha-helicity, while apoE bound to SME and DMPC disks displayed 73% and 95% helicity, respectively. Over the temperature range 4-50 degrees C, encompassing the lipid thermal transitions, only apoE bound to LME demonstrated an abrupt change in its CD spectrum (decrease in alpha-helicity) in response to temperature. To determine the source of the abrupt CD change, the constants for dissociation (Kd) of apoE from the surface of the large and small microemulsion particles were determined at 4, 25, and 37 degrees C. These results demonstrated that at 4 degrees C, the KdS for binding of apoE to the LME and SME were approximately equal; however, between 4 and 25 degrees C, there was a 5-fold increase in the Kd for binding of apoE to the LME, whereas the Kd for binding to the SME remained constant. The physiological effects of these differences in apoE secondary structure and equilibrium binding were examined by measuring the capacity of each apoE-containing particle to complete with LDL for binding to human fibroblasts, and by measuring the capacity of the apoE-microemulsion particles to suppress HMG-CoA reductase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol metabolism in fibroblasts from rabbits resistant to diet-induced hypercholesterolemia

We have previously described a colony of New Zealand White rabbits that are resistant to hypercholesterolemia when fed a cholesterol-enriched diet. The present studies used skin fibroblasts obtained from normal and hypercholesterolemia-resistant rabbits to investigate cholesterol metabolism and lipid composition in vitro. The lipid compositions of the two cell lines after incubation in either fetal calf serum or lipoprotein-deficient serum were similar. The conversion of radiolabeled acetate into sterol and phospholipids was higher in resistant fibroblasts than in normal fibroblasts. In contrast, incorporation of radiolabeled oleic acid into cholesteryl ester was significantly lower in resistant fibroblasts than in normal cells. In parallel experiments, the 3-hydroxy-3-methylglutaryl coenzyme A reductase activity was higher and acyl-coenzyme A:cholesterol acyltransferase activity was lower in resistant cells compared to normal cells. Furthermore, binding, uptake, and degradation of normal rabbit 125I-labeled LDL (low density lipoproteins) were 30% higher in resistant than in normal fibroblasts. These observations are consistent with results from previous studies of cholesterol metabolism in the liver membranes of these rabbits. The results indicate that extrahepatic cells (such as fibroblasts) from the resistant rabbit exhibit the same altered cholesterol metabolism as that found in the hepatic tissues of these rabbits. These studies suggest that the resistant rabbit may provide an in vivo and in vitro system for studying the mechanisms by which some individuals of a species can minimize the effect of dietary cholesterol on the development of hypercholesterolemia and atherosclerosis.

Polymorphic forms of human apolipoprotein[a]: inheritance and relationship of their molecular weights to plasma levels of lipoprotein[a]

The plasma concentration of human lipoprotein[a], Lp[a], is highly correlated with coronary artery disease. The protein moiety of Lp[a], apoLp[a], consists of two apoproteins, apo[a] and apoB-100, linked by one or more disulfide bonds(s). Apo[a], the protein unique to Lp[a], exists in polymorphic forms that exhibit different apparent molecular weights (Mr). Polyacrylamide gel electrophoresis in sodium dodecyl sulfate followed by immunoblotting was used to separate and visualize these different forms and to determine the polymorphic pattern of apo[a] in the plasma samples of 692 individuals. A total of 11 different polymorph bands ranging in Mr from 419 kD to 838 kD could be resolved, but only 1 or 2 bands were present per individual. The polymorphic band pattern for an individual was assigned to 1 of the 66 different phenotype designations representing the total number of possible single- and double-band combinations of the 11 detectable bands. All 11 of the possible single-band phenotypes but only 32 of the 55 possible double-band phenotypes were represented. There were 412 plasma samples (59.5%) that contained a single band, 274 (39.6%) contained two bands, and only 6 (0.9%) had no detectable apo[a] band. A highly significant inverse correlation was found between the Mr of the band(s) present and the plasma apoLp[a] concentration (r = -0.461; rho = 0.0001). The correlation was better between apoLp[a] and single-band (r = -0.495; rho = 0.0001) than double-band (r = -0.382; rho = 0.0001) phenotypes. Of the 274 individuals exhibiting double-band phenotypes, the lower Mr band was more intense in 141 (51.4%), the two bands were equally intense in 85 (31.0%), while the higher Mr band was more intense in 48 (17.5%). Based upon the hypothesis that apo[a] polymorphism is controlled by different alleles at a single locus, the frequency of the 11 alleles determined from the observe phenotypes (low Mr----high Mr) was: band 1) 419 kD, 0.00875; band 2) 489 kD, 0.00510; band 3) 536 kD, 0.0555; band 4) 553 kD, 0.0758; band 5) 613 kD, 0.135; band 6) 680 kD, 0.0824; band 7) 705 kD, 0.104; band 8) 742 kD, 0.151; band 9) 760 kD, 0.246; band 10) 796 kD, 0.128; band 11) 838 kD, 0.00802. The observed distribution of phenotypes in the population was compared by chi-square analysis to that predicted on the basis of simple Mendelian inheritance, and the hypothesis was rejected (chi 2 = 921.7; rho less than 0.001). Significantly, the singleband phenotypes are over-represented in the population compared to that predicted.(ABSTRACT TRUNCATED AT 400 WORDS)

Impairment of endothelium-dependent arterial relaxation by lysolecithin in modified low-density lipoproteins

Atherosclerosis in animals and humans is associated with an unresponsiveness of arteries and arterioles to endothelium-dependent vasodilators--agents acting on smooth muscle indirectly by stimulating the release from endothelial cells of a vasodilator principle (endothelium-derived relaxing factor). Altered vasomotor regulation in atherosclerosis could partly reflect an injurious action of abnormal lipoproteins on endothelium. Recently, 'cell-modified' or 'oxidized' low-density lipoprotein (EC-LDL) has received increasing attention because of its potential cytotoxic and atherogenic properties. We report here that arteries exposed to EC-LDL in vitro show an endothelium-dependent vasoregulatory impairment closely resembling that of atherosclerotic arteries. Our results indicate that transfer of lysolecithin from EC-LDL to endothelial membranes produces a selective unresponsiveness to receptor-regulated endothelium-dependent vasodilators.

A technique for flow-enhanced magnetic resonance angiography of the lower extremities

A two-dimensional, flow-enhanced gradient echo pulse sequence for nuclear magnetic resonance angiography is described. It employs interleaved, presaturated slices to acquire data efficiently on imagers which favor interleaved acquisition over sequential acquisition for multislice imaging. It is useful on any imagers when the effective TR is extended to enhance the sensitivity to slow flow. The technique was applied to the region from aortic bifurcation to the iliac bifurcations of three normal volunteers. The right and left common iliac arteries and veins, the separation of the external and internal iliac arteries, and secondary branches were clearly depicted.

Quantitation and localization of apolipoproteins [a] and B in coronary artery bypass vein grafts resected at re-operation

Lp[a] is a lipoprotein whose plasma concentration is highly correlated with cardiovascular disease. Its protein moiety, apoLp[a], consists of two large polypeptides, apo[a] and apo B. The possible contribution of Lp[a] to atherosclerosis in saphenous vein aortocoronary bypass grafts was studied in a population of patients undergoing coronary re-bypass surgery. The vein graft tissue levels of apoLp[a] were compared with graft duration, gross and light microscopic pathology, as well as plasma levels of apoLp[a]. The localization pattern of apo[a] and apo B in vein graft tissue was determined. In addition, the plasma levels of cholesterol, triglycerides, apoproteins (apo) A-I, A-II, and E were measured. In a representative subpopulation of 17 patients with a mean age of 63 years from whom grafts with a mean duration of 112 months were resected, the mean total plasma cholesterol level was 221 mg/dl, the mean high density lipoprotein cholesterol level was 31 mg/dl, and the mean plasma triglyceride level was 228 mg/dl. In normal saphenous veins, the level of apoLp[a] was below measurable limits (less than 2 ng/mg), and the level of apo B was very low (3.3 ng/mg). In resected grafts, the mean tissue level of apoLp[a] was 32 ng/mg, and that of apo B was 70 ng/mg, demonstrating the net accumulation of these apoproteins in veins from the time of their grafting into the arterial bed. The apoLp[a]/apo B ratio was determined in 77 tissue segments from 59 grafts (28 patients) and was found to be 0.313. This tissue ratio was significantly higher (p = 0.02) than the plasma apoLp[a]/apo B ratio from these patients, which was 0.132. Immunostaining showed co-localization of apo[a] and apo B in the neointima of grafts. The most abundant pathologic features observed in resected grafts were proliferated intima (43/52), thrombus (28/52), and atherosclerotic core regions (21/52). The level of tissue apo B correlated well with the abundance of core regions (r = 0.501), whereas the level of tissue apoLp[a] did not correlate as well with this feature (r = 0.233). Although apo[a] and apo B are almost absent from normal saphenous vein, these apoproteins (and presumably the lipoproteins Lp[a] and low density lipoprotein) accumulate in bypass vein grafts. The data support the view that these lipoproteins play a significant role in vein graft atherosclerosis.

Effect of triglyceride levels on methyl and methylene envelope line widths in proton nuclear magnetic resonance spectroscopy of human plasma

Recent studies have described a relation between the line widths of the methyl and methylene resonance envelopes in the proton nuclear magnetic resonance spectrum of human plasma and the occurrence of cancer. An average line width of less than 33 Hz has been reported to correlate with the presence of cancer, whereas greater line widths have not. In 26 normal volunteers, we found a significant inverse correlation between fasting triglyceride level and plasma spectral line width. We also observed that dietary lipids have measurable effects on spectral line widths. In another sample of seven normal persons (three of whom had elevated plasma triglyceride levels), the line widths of whole plasma varied widely (mean, 35.6 +/- 8.8 Hz); however, the mean line widths of the lipoprotein fractions isolated from those samples differed greatly, but the variance within each fraction was small (very-low-density lipoprotein, 22.0 +/- 1.9 Hz; low-density lipoprotein, 35.0 +/- 2.8; high-density lipoprotein, 28.8 +/- 1.9). The results of this study indicate that the plasma triglyceride level has a profound effect on the average spectral line width of plasma. This effect can be explained by the relative amounts of lipoprotein fractions in whole plasma. Plasma triglyceride concentrations of more than 1.24 mmol per liter (greater than 110 mg per deciliter), whatever the source, produce average plasma methyl and methylene line widths of less than 33 Hz.

A dual-precipitation method evaluated for measurement of cholesterol in high-density lipoprotein subfractions HDL2 and HDL3 in human plasma

The dual-precipitation method for measurement of cholesterol in high-density lipoprotein subfractions HDL2 and HDL3 (Warnick et al., Clin Chem 1982;28:1574) was compared with quantification of cholesterol in HDL2 and HDL3 by zonal ultracentrifugation (Patsch et al., J Lipid Res 1974;15:356-66). For 39 plasma specimens differing widely in their HDL subfraction cholesterol concentration, the coefficient of correlation between the two methods was 0.94 for HDL2-cholesterol, 0.82 for HDL3-cholesterol. Storage of plasma specimens at -70 degrees C decreased the apparent content of HDL3-cholesterol by 5%; no significant changes in HDL2-cholesterol were observed. In frozen plasma, interference by apoB-containing lipoproteins and by lipoprotein(a) was negligible. Mean weight ratios of apoA-I to cholesterol were twice as high for HDL3 as for HDL2, reflecting the increased cholesterol content of HDL2. The study suggests that quantification of HDL2 and HDL3 cholesterol by precipitation is appropriate for use in epidemiological studies.

A dual-precipitation method evaluated for measurement of cholesterol in high-density lipoprotein subfractions HDL2 and HDL3 in human plasma

The dual-precipitation method for measurement of cholesterol in high-density lipoprotein subfractions HDL2 and HDL3 (Warnick et al., Clin Chem 1982;28:1574) was compared with quantification of cholesterol in HDL2 and HDL3 by zonal ultracentrifugation (Patsch et al., J Lipid Res 1974;15:356-66). For 39 plasma specimens differing widely in their HDL subfraction cholesterol concentration, the coefficient of correlation between the two methods was 0.94 for HDL2-cholesterol, 0.82 for HDL3-cholesterol. Storage of plasma specimens at -70 degrees C decreased the apparent content of HDL3-cholesterol by 5%; no significant changes in HDL2-cholesterol were observed. In frozen plasma, interference by apoB-containing lipoproteins and by lipoprotein(a) was negligible. Mean weight ratios of apoA-I to cholesterol were twice as high for HDL3 as for HDL2, reflecting the increased cholesterol content of HDL2. The study suggests that quantification of HDL2 and HDL3 cholesterol by precipitation is appropriate for use in epidemiological studies.

Development and partial metabolic characterization of a dietary cholesterol-resistant colony of rabbits

A colony of New Zealand white rabbits has been developed which, when fed a cholesterol-supplemented diet, exhibit unusual resistance to hypercholesterolemia and atherosclerosis, disorders usually observed in normal cholesterol-fed rabbits. When resistant rabbits (RT) were fed a normal low cholesterol diet (ND), their plasma lipoprotein patterns were significantly different from those of normal rabbits (NR) fed the same diet. The low density lipoprotein cholesterol (LDL-c)/high density lipoprotein cholesterol (HDL-c) ratio and LDL-c/very low density lipoprotein cholesterol (VLDL-c) ratio were lower in the resistant rabbits. The hydrated density of HDL of the normal-responsive rabbits was greater than that of the resistant rabbits. LDL from resistant rabbits contained a lower proportion of esterified cholesterol and protein than LDL from normal rabbits. Peripheral mononuclear cells from resistant rabbits bound about 30% more 125I-labeled rabbit LDL than mononuclear cells from normal rabbits. These results demonstrate that the plasma cholesterol levels of these animals is at least partly under genetic control and that compositional differences exist between the major plasma lipoprotein classes of normal and resistant rabbits even during the ingestion of low-cholesterol diet. The results indicate that at least a part of the difference in the cholesterolemic responses between the two rabbit groups is due to an enhanced LDL uptake by the mononuclear cells, and presumably by other somatic cells of the resistant group.

Lipolysis of phospholipids in model cholesteryl ester rich lipoproteins and related systems: effect of core and surface lipid phase state

Porcine pancreatic phospholipase A2 (PL A2) was used as a probe to study the structure of phospholipid domains of dimyristoylphosphatidylcholine (DMPC) vesicles +/- 2% cholesteryl oleate (CO), of discoidal structures formed by the interaction of apolipoprotein E (apoE) with these vesicles, and of large CO/DMPC microemulsion particles +/- apoE. Results of phospholipid hydrolysis over a range of temperatures were compared with the thermal transitions of the lipid components of the particles as measured by differential scanning calorimetry. These studies revealed that DMPC vesicles were most susceptible to digestion at or near the transition temperature. A similar result was obtained with DMPC/apoE disks; however, these particles were hydrolyzed over a broader temperature range than the vesicles. DMPC/CO vesicles were resistant to hydrolysis at every temperature tested; however, discoidal structures formed by interaction of apoE with these vesicles were hydrolyzed maximally above their thermal transition. Large microemulsion particles of CO and DMPC were poor substrates for the enzyme at every temperature; binding of apoE to these particles improved the ability of PL A2 to hydrolyze the phospholipid. These results suggest that the curvature of the surface, the presence of dissolved cholesteryl ester, and the binding of protein have profound effects on the temperature and breadth of the phospholipid phase transition and on the accessibility of the phospholipid to hydrolysis by PL A2.

Isolation and characterization of the two major apoproteins in human lipoprotein [a]

Human Lp[a] was isolated in preparative amounts from two donors; the native lipoprotein and its constituent apoproteins, apo[a] and apoB, were characterized extensively. Based on differences in apparent molecular weight, four different isoforms of apo[a], a1-a4, were observed between the two donors. The number and relative distribution of these isoforms varied between donors but were constant for each donor. Each apo[a] isoform was shown to be derived from a discrete apo[a]-B100 disulfide-linked complex present before reduction. Complete delipidation of Lp[a] was followed by solubilization, reduction, and carboxamidomethylation of the constituent apoproteins. These apoproteins were then separated by immunoaffinity chromatography using anti-apo[a]- or anti-apoB-Sepharose; their purity and structural integrity were demonstrated by Western blot analysis. ApoB isolated by this procedure was essentially identical to apoB from autologous LDL with respect to molecular weight, secondary structure, amino acid composition, and sialic acid content. However, apo[a] differed from apoB in that it exhibited: a much less alpha-helical, less beta, but much more disordered structure; a lower proportion of aspartate, isoleucine, leucine, phenylalanine, and lysine, but a higher proportion of proline, glycine, and threonine; and a much higher content of sialic acid. These results indicate that apo[a] is not a superglycosylated form of apoB but is distinctly different in its composition and structure.

Motion and surface accessibility of spin-labeled lipids in a model lipoprotein containing cholesteryl oleate, dimyristoylphosphatidylcholine, and apolipoprotein E

A series of spin-labeled phosphatidylcholines (PCs) and cholesteryl esters (CEs) bearing the paramagnetic 2,2-dimethyloxazolidinyl-1-oxy (doxyl) group at fatty acyl carbon C5', C12', or C16' were used to study acyl chain motions in the polar surface shell and hydrophobic core domains of microemulsion (ME) particles containing cholesteryl oleate and dimyristoylphosphatidylcholine (DMPC), and of particles with apolipoprotein E (apoE) bound to their surfaces. Electron paramagnetic resonance data obtained with the doxyl-labeled PCs indicated a gradient of motion in the ME surface monolayer similar to that observed with the same probes in a bilayer. The 5- and 12-doxyl-CEs clearly demonstrated a higher degree of order for the cholesteryl ester rich core than the corresponding doxyl-PCs showed for the phospholipid-rich surface over the entire range 10-60 degrees C. The temperature dependencies of spectra of the 16-doxyl-CE in the core and PC in the surface of the ME were almost identical, suggesting that there was no sharp boundary between core and surface domains. None of the probes detected either the surface phospholipid transition (31 degrees C) or the cholesteryl ester core transition (46 degrees C) measured previously by differential scanning calorimetry and 13C nuclear magnetic resonance. Binding of apoE to spin-labeled DMPC vesicles increased the order of the 5'-position of the sn-2 acyl chain over the range 15-33 degrees C; the thermal transition was broadened and its midpoint elevated. The effect of protein binding was not as striking for the ME particles.(ABSTRACT TRUNCATED AT 250 WORDS)

Apolipoprotein C-III/sphingomyelin recombinants: formation, isolation, and characterization

The association of apolipoprotein C-III (apoC-III) from human very low density lipoprotein with sphingomyelin from egg yolk (EYSM) has been studied at the transition temperature (Tc) of the phospholipid. Upon incubation of aliquots of the apoprotein with increasing amounts of sphingomyelin, the alpha-helical content of the apoprotein increased from 20% in the absence of EYSM to a limiting value of 67% at a protein:lipid molar ratio of 1:200. The tryptophan fluorescence spectrum of the apoprotein exhibited a gradual blue shift from 356 nm in the absence of EYSM to 348 nm when the protein:lipid ratio in the complex had reached 1:50. Gel filtration chromatography of complexes formed by incubating the apoprotein and phospholipid at differing apoC-III:EYSM ratios demonstrated a disintegration of sphingomyelin vesicles into particles of decreasing size with increasing proportion of protein. This effect was confirmed by sedimentation velocity experiments in which the observed sedimentation coefficient of EYSM decreased from 14.0 S (for vesicles) to a limiting value of 7.0 S when the apoprotein:phospholipid ratio reached 1:50 in the complex. Electron micrographs of negatively stained EYSM vesicles showed spherical particles of 380-A diameter. Addition of apoC-III led to the formation of disk-shaped structures whose diameter decreased to a limiting value of 204 +/- 34 A at a protein:lipid ratio of 1:50. In contrast, the disk thickness was relatively constant at 51 +/- 2 A for all isolated complexes.(ABSTRACT TRUNCATED AT 250 WORDS)

Human lysosomal sphingomyelinase: substrate efficacy of apolipoprotein/sphingomyelin complexes

Human apolipoprotein stimulation of sphingomyelin (SM) hydrolysis by sphingomyelinase from human skin fibroblasts has been studied. Apolipoproteins A-I, A-II, B, C-I, and E do not enhance sphingomyelin hydrolysis above control levels. In contrast, apoC-II stimulates sphingomyelin hydrolysis by approximately 2.5-fold. ApoC-III, the most potent apoprotein activator, stimulates hydrolysis by 3-4-fold. ApoC-III stimulation is not significantly different for the three different isoforms which carry 0, 1, or 2 sialic acid residues. The amino-terminal half of this apoprotein, C-III(1-40), which does not bind to phospholipid surfaces, does not activate sphingomyelinase. In contrast, the carboxyl-terminal half, C-III(41-79), which strongly binds to phospholipid surfaces, stimulates sphingomyelin hydrolysis to the same level as that produced by the intact, full-length apoprotein. Incubation of sphingomyelin vesicles with increasing proportions of apoC-III results in the formation of complexes of increasing apoC-III:SM ratio and decreasing radius. The hydrolysis of sphingomyelin in the 1:50 (mol/mol) complex was more than 2-fold greater than that of the 1:200 (mol/mol) complex. The rate of hydrolysis of egg yolk sphingomyelin in the 1:50 complex was maximal [0.9 mumol h-1 (mg of protein)-1] at the gel----liquid-crystalline phase transition temperature (Tm) of the complex (40 degrees C). The rate of hydrolysis fell markedly at either higher or lower temperature. Determination of the apparent Km and Vmax values below, at, and above Tm indicated that the temperature dependence of sphingomyelin hydrolysis was attributable primarily to changes in Vmax.(ABSTRACT TRUNCATED AT 250 WORDS)

Preservation of endothelium-dependent vascular relaxation in cholesterol-fed rabbit by treatment with the calcium blocker PN 200110

We tested the effects of low doses of a dihydropyridine calcium antagonist, PN 200110, on endothelium-dependent vascular relaxation in rabbits fed a 1% cholesterol diet. The drug was given orally, 1 mg/day, and control rabbits received placebo. Plasma total cholesterol after 10 weeks, was similar in the placebo- and PN 200110-treated groups. The respective values averaged 2140 +/- 116 (n = 14; mean +/- SEM) and 2012 +/- 115 mg/dl (n = 13). In placebo-treated rabbits, sudanophilic aortic lesions covered 52 +/- 5% of the intimal surface, and the aortic cholesterol concentration was 72 +/- 6 mg/g protein. Corresponding values in aortas from PN 200110-treated rabbits were significantly lower [36 +/- 5% (P less than 0.03) and 52 +/- 3 mg/g protein (P less than 0.03)]. Maximal endothelium-dependent cholinergic relaxation of aortic strips in untreated (n = 14) and treated cholesterol-fed rabbits (n = 13) differed significantly (P less than 0.01) and averaged 31 +/- 4% and 61 +/- 7% of the value in normocholesterolemic controls (n = 13). We conclude that cholesterol feeding suppresses endothelium-dependent relaxation evoked by acetylcholine, and that PN 200110 reduces the severity of atherosclerosis and impairment of endothelium-dependent relaxation.

Microemulsions of cholesteryl oleate and dimyristoylphosphatidylcholine: a model for cholesteryl ester rich very low density lipoproteins

This study describes the preparation, purification, and characterization of a cholesteryl oleate/dimyristoylphosphatidylcholine microemulsion as a model for the interaction of lipid domains in cholesteryl ester rich very low density lipoproteins. These lipids were chosen specifically because their thermal transitions were distinct from each other, and their differences in chemical structure permitted the motion(s) of each lipid component to be monitored independently by 13C nuclear magnetic resonance (NMR). The model particles were formed by cosonication of cholesteryl oleate and dimyristoylphosphatidylcholine in a 4:1 molar ratio for 45 min at 55-60 degrees C (above both lipid phase transition temperatures). The crude microemulsion was fractionated by low-speed centrifugation and Sepharose CL-2B chromatography. Microemulsion particles which eluted from the column at a volume similar to that of cholesteryl ester rich very low density lipoproteins had high cholesteryl ester:phospholipid ratios (2.5:1----6:1). Electron micrographs of negatively stained particles showed them to be large spheres devoid of multilamellar or unilamellar vesicle structures. Particle size calculated from a simple compositional model correlated well with sizes determined by electron microscopy (500-1000 A) for various column fractions. Differential scanning calorimetry studies of the microemulsion revealed two thermal transitions for the model particles, at 31.0 and 46.6 degrees C, which were tentatively assigned to the surface phospholipid and core cholesteryl ester domains, respectively. These assignments were confirmed by 13C NMR which demonstrated that, at temperatures near the lower thermotropic transition, only resonances derived from carbon atoms of dimyristoylphosphatidylcholine (DMPC) were observable. As the temperature was raised to 38.6 degrees C, resonances from the olefinic carbons in the cholesteryl ester acyl chain appeared in the spectrum. At 46.6 degrees C, the center of the higher temperature endotherm, resonances from both the steroid ring and remaining acyl chain carbons of cholesteryl oleate became observable in the spectrum. Further increases in temperature did not result in the appearance of new resonances; however, those that were present narrowed and increased in intensity. The elevation in transition temperature for DMPC in these particles (31 degrees C) as compared to that for DMPC in small unilamellar (18 degrees C) and large multilamellar (23 degrees C) vesicles suggested a stabilization of the phospholipid monolayer, possibly by interaction with the nonpolar core lipids.(ABSTRACT TRUNCATED AT 400 WORDS)