Effect of lithocholic acid feeding on plasma lipoproteins and binding of radioiodinated human lipoproteins to hepatic membranes in rats

1. Male Sprague-Dawley rats fed diets containing 0.25% lithocholic acid for 6 weeks exhibited elevated serum cholesterol. 2. The rats were fed diets containing 5 or 20% fat with and without the lithocholate and/or oxytetracycline-HCl. 3. The cholesterol elevation was associated with high density lipoprotein (HDL) and not very low density lipoprotein (VLDL) or low density lipoprotein (LDL). 4. Specific binding of human [125I]HDL to hepatic membranes was lowered in lithocholate-fed rats, but binding of human [125I]LDL to these membranes was not affected.

Effects of omega 3 fatty acids and vitamin E on hormones involved in carbohydrate and lipid metabolism in men

Forty healthy men were fed diets providing 40% of energy from fat and a minimum of 25 mg vitamin E for 28 wk. During the first 10 wk diets were supplemented with placebo, 15 g mixed fat/d. During the second 10 wk placebo was replaced by 15 g fish-oil concentrate/d. During the last 8 wk 200 mg vitamin E/d was added to fish oil. Compared with placebo, fish-oil feeding significantly increased plasma glucose and decreased triacylglycerol, insulin, glucagon, growth hormone, and somatomedin C. The changes in plasma cholesterol, cortisol, and dehydroepiandrosterone sulphate (DHEA-S) were not significant. Fish oil plus vitamin E further decreased insulin, growth hormone, and DHEA-S and reversed the effect of fish-oil on somatomedin C. The changes in glucose, glucagon, growth hormone, and cortisol were not significant. Thus, changes in plasma glucose and lipids caused by dietary fish oil alone and with fish oil plus vitamin E appear to be due to alterations in hormones involved in carbohydrate and lipid metabolism.

Dietary fat and hormonal influences on lipoprotein fluidity and composition in premenopausal women

LDL and HDL became more fluid when health, free-living, premenopausal women were fed reduced fat diets with higher proportions of polyunsaturated fatty acids. Lipoproteins were isolated from plasma of 31 female subjects fed one of two sets of diets from typical U.S.A. foods with P/S ratios of 0.3 or 1.0. All subjects were fed high-fat diets (40% of energy) for the duration of four menstrual cycles followed by low-fat diets (20% of energy) for the next four cycles. Blood samples were collected during mid-follicular and mid-luteal phases of the fourth menstrual cycle of each diet period to assess interactive dietary and hormonal control of lipoprotein fluidity. LDL was significantly more fluid, as determined by DPH fluorescence, upon reducing fat consumption from 40 to 20% of energy for subjects eating foods with P/S = 1.0 or 0.3. Generally LDL was more fluid during the follicular phase than the luteal phase of the cycles, thus indicating hormonal influences on LDL fluidity. HDL results were similar but not as pronounced as with LDL. Lipoprotein phospholipid (PL) and cholesteryl ester (CE) fatty acyl compositions were also subject to dietary and hormonal influences. Effects were noted in several fatty acids depending upon diet and hormonal state; however, generally diet fat reduction resulted in reduced linoleate and increased oleate contents. Regression analyses showed that fluidity was more dependent upon the lipoprotein cholesterol content than upon fatty acyl composition.

Relative effects of feeding saturated fats and cholesterol on fluidity of rabbit lipoproteins

1. The effects of saturated fat and cholesterol on lipoprotein fluidity were tested in New Zealand white rabbits fed diets containing corn oil (CO) or cocoa butter (CB) with and without added 0.2% cholesterol. 2. Saturated fats had little effect on fluidity in any lipoprotein fraction. 3. Cholesterol feeding dramatically reduced fluidity in VLDL and LDL, but minimal change was noted in HDL. 4. Cholesterol-fed rabbits were hypercholesteroloemic throughout the 10-month study. 5. The rabbits became adapted to cholesterol feeding as VLDL became more fluid with time.

Dietary fat and cholesterol induced modification of minipig lipoprotein fluidity and composition

1. Miniature swine were fed a low (2.7%) fat control stock diet alone or supplemented with either 20% lard plus 1% cholesterol or 20% lard alone for periods of up to 6 months. 2. Cholesterol feeding reduced VLDL fluidity drastically and LDL fluidity minimally but had no effect on HDL fluidity. 3. Lard feeding had no effect on lipoprotein fluidity. 4. The rigid VLDL produced by cholesterol feeding was enriched in cholesterol and phospholipid contents, similar to beta-VLDL. 5. Plasma cholesterol concentrations were increased by 1.5 to 5-fold in pigs fed stock diets supplemented with 20% lard, with or without added cholesterol, but plasma triacylglycerol concentrations were not affected by either diet modification. 6. Diet effects were complete within 4 weeks with no further changes for periods up to 6 months. 7. Regression of the induced hypercholesterolemia was also accomplished within one month of removing cholesterol from the diet.

Dietary trans fatty acids modulate erythrocyte membrane fatty acyl composition and insulin binding in monkeys

The substitution of trans- for half of the cis-monounsaturated fatty acids in the diet of Macaca fasicularis monkeys resulted in alterations in erythrocyte fatty acid composition and insulin receptor properties but not in membrane fluidity. Both cis and trans diets contained 10% fat and similar fatty acid compositions, except that approximately 50% of the cis-octadecenoate (c-18:1) in the cis diet was replaced with trans-octadecenoate isomers (t-18:1) in the trans diet. Compared with the cis diet, the trans diet resulted in the incorporation of approximately 11% t-18:1, an approximately 50% decrease in c-18:1, an approximately 16% decrease in total saturated fatty acids, and an approximately 20% increase in 18:2(n-6) in erythrocyte membrane lipids. The increase in 18:2(n-6) may reflect on homeostatic mechanisms designed to maintain overall membrane fluidity, as no diet-related changes in fluidity were observed with diphenylhexatriene steady state fluorescence polarization. Values observed for insulin binding and insulin receptor number were higher and binding affinity was lower in monkeys fed the cis diet. In the absence of an effect on overall membrane fluidity, altered receptor activity suggests that insulin receptor activity is dynamic, requiring specific fluid membrane subdomains or highly specific fatty acid-protein interactions.

Effect of feed deprivation on hepatic membrane and lipoprotein fluidity and binding of lipoproteins to hepatic membranes in the chick (Gallus domesticus)

1. Male chicks were deprived of feed for 48 hr to study the effect of metabolic stress on hepatic membrane and lipoprotein fluidity and binding of radioiodinated lipoproteins to hepatic membranes. 2. Plasma levels of low density lipoprotein (LDL) and high density lipoprotein (HDL) were markedly and slightly elevated, respectively. 3. There was a reduction in lipoprotein and hepatic membrane fluidity. 4. Binding of [125I]LDL, but not [125I]HDL, to hepatic membranes was decreased. 5. It is suggested that a reduction in the fluidity of LDL and/or hepatic membranes impedes LDL catabolism in vivo.

Characterization of two unique cholesterol-rich lipid particles isolated from human atherosclerotic lesions

The authors' laboratory, using histochemical methods, previously identified two types of cholesterol-containing lipid particles in the extracellular spaces of human atherosclerotic lesions, one particle enriched in esterified cholesterol and the other particle enriched in unesterified cholesterol. The authors isolated and characterized these lipid particles. The esterified cholesterol-rich lipid particle was a small lipid droplet and differed from intracellular lipid droplets found in foam cells with respect to size and chemical composition. It had an esterified cholesterol core surrounded by a phospholipid-unesterified cholesterol monolayer. Some aqueous spaces were seen within the particle core. Unesterified cholesterol-rich lipid particles were multilamellated, solid structures and vesicles comprised of single or multiple lamellas. The esterified cholesterol-rich particle had a density less than 1.01 g/ml, whereas the unesterified cholesterol-rich particle had a density between 1.03 and 1.05 g/ml. Both particles were similar in size (90% of both particles ranged in size between 40 to 200 nm in diameter) and had an unesterified cholesterol-to-phospholipid molar ratio of 2.5:1. The predominant phospholipid in both particles was sphingomyelin. The fatty acyl compositions of cholesteryl ester and phospholipid also were similar in both particles. Palmitate, oleate, and linoleate were the major fatty acids in the cholesteryl ester fraction, whereas palmitate, stearate, oleate, and linoleate were predominant in the phospholipid fraction. The origins and the role of these two unusual lipid particles in vessel wall cholesterol metabolism remain to be determined.

Inhibition of mitochondrial palmitate oxidation by calmodulin antagonists

1. The effect of calmodulin antagonists on the rate of palmitate oxidation by isolated rat liver mitochondria was studied. 2. In the presence of 100 microM amitriptyline, chlorpromazine, prenylamine, N-(4-aminobutyl)-5-chloro-2-naphthalenesulfonamide or N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide, palmitate oxidation was inhibited by 17, 34, 49, 31 and 37%, respectively. 3. The degree of inhibition of palmitate oxidation exerted by these chemical compounds did not appear to correlate appreciably with changes in mitochondrial membrane fluidity.

Effect of dietary cholesterol on the lipoprotein profile and binding of radioiodinated lipoproteins to hepatic membranes in the cockerel (Gallus domesticus)

1. Cockerels fed a cholesterol-supplemented diet experienced a marked elevation of lipoprotein particles of density less than or equal to 1.006 g/ml (VLDL) and a diminution of lipoprotein particles of density 1.02-1.05 g/ml (LDL). 2. Unlike VLDL of some cholesterol-fed animals, cholesterol-fed cockerel VLDL did not display beta-mobility on agarose gel electrophoresis. 3. [125I]LDL and [125I]HDL binding to cockerel liver membranes was not affected by cholesterol feeding. 4. Different lipoprotein types appear to bind to a common site on cockerel liver membranes. 5. The results suggest that liver cells of cockerels may not possess LDL binding sites that are analogous to those of mammalian species.

Dietary fat and menstrual-cycle effects on the erythrocyte ghost insulin receptor in premenopausal women

The effect of high- and low-fat diets with different levels of fatty acid unsaturation on insulin receptors of erythrocyte ghosts was studied during different phases of the menstrual cycle in 31 healthy premenopausal women. Subjects were divided into two groups and consumed controlled diets containing 39% fat with a ratio of polyunsaturated to saturated fatty acids (P:S) of either 0.30 or 1.00 for four menstrual cycles. They were switched to 19% fat at the same P:S for another four cycles. Fasting blood samples were collected during the follicular and luteal phases. Insulin receptors were measured from right-side-out ghosts. Insulin binding was significantly lower due to fewer receptors when subjects were fed the low-fat, high-carbohydrate diet compared with the high-fat, low-carbohydrate diet. There was no significant effect of level of unsaturation or time of menstrual cycle on insulin binding. Thus, insulin receptors on erythrocytes respond to dietary lipids.

Dietary fat and hormonal effects on erythrocyte membrane fluidity and lipid composition in adult women

Erythrocyte ghost membrane fluidity and phospholipid linoleate were significantly increased when higher levels of polyunsaturated fats were fed to healthy, free living, premenopausal women. Fluidity was assessed by diphenylhexatriene (DPH) fluorescence polarization measurements with hypotonically lysed red blood cells from 31 female subjects fed one of two sets of diets, which were formulated from typical US foods to contain polyunsaturate to saturate ratios (P/S) of 1.0 or 0.3. Both groups of women were fed diets with 40% of energy as fat for four menstrual cycles followed by low-fat diets having 20% of energy as fat for the next four menstrual cycles. Blood was sampled during the fourth cycle of each dietary period at times estimated to correspond to maximum secretions of estrogen and progesterone to assess interactive hormonal and dietary effects on membrane composition and fluidity. Red blood cell membranes were most fluid following higher levels of linoleate intake, either by higher (40%) total fat or higher P/S levels. Membrane fluidity was directly related to the phospholipid oleate and linoleate contents and inversely related to the molar cholesterol/phospholipid ratio. Hormonal status effects on the membranes were not extensive. Membrane fluidity in cells from women fed P/S = 0.3 diets was higher at 40% than at 20% fat during the luteal phase of the fourth cycle. In contrast, women fed the P/S = 1.0 diets had more fluid red cells at 40% fat during the follicular phase of the cycle. Regression analysis showed a direct linear correlation between membrane fluidity and red cell membrane insulin binding demonstrating a relation between receptor binding and cell membrane fluidity in the human female.

Blood risk factor metabolites associated with heart disease and myocardial fatty acids in copper-deficient male and female rats

Intact and castrated males and intact and ovariectomized female rats were fed a copper-deficient diet in order to establish whether the protection provided in females against cardiovascular pathology and mortality is due to endogenous sex hormones, and different levels of blood lipids and/or myocardial fatty acids. Seventy-three male and female rats were assigned to a copper-deficient diet (0.6 micrograms of copper/g diet) containing 62% fructose for 8 weeks. Twelve of the male rats underwent castration and 12 of the females were ovariectomized. All animals exhibited high levels of plasma cholesterol, triglycerides, and uric acid, which were neither affected by the sex of the rat nor by the surgical treatment. The composition of fatty acids of the myocardium was similar in males and females. Except for those animals that were sacrificed by us, all other male rats died of heart pathology. In contrast, none of the female rats exhibited heart pathology and none died of the deficiency. It is suggested that heart pathology and mortality in copper deficiency are sex related and not due to high levels of plasma cholesterol, triglycerides, and uric acid or to differences in myocardial fatty acid composition.

Hormones regulating lipid and carbohydrate metabolism in premenopausal women: modulation by dietary lipids

The effect of high- and low-fat diets with different levels of fatty acid unsaturation on plasma hormones involved in lipid metabolism was studied during different phases of the menstrual cycle in 31 premenopausal women. Subjects were divided into two groups and were fed controlled diets containing 39% fat with a ratio of polyunsaturated to saturated fatty acids (P:S) of either 0.3 or 1.0 for four menstrual cycles and then switched to a 19% fat diet with the same P:S for another four cycles. Blood samples were analyzed during both the follicular and luteal phases. A significant direct effect of level of dietary fat was observed on plasma cortisol and dehydroepiandrosterone-sulphate whereas an inverse relationship was seen for plasma insulin. Both plasma insulin and growth hormone levels were higher during the luteal compared with the follicular phase of the menstrual cycle. None of the hormones was affected by the level of unsaturation of dietary fats.

The long-term prognosis of childhood asthma in a predominantly rural Swedish county

In 1985 a survey questionnaire was sent to 119 individuals who had been hospitalized as children (aged 5-15) because of asthma over a ten-year period (1953-1962). Four patients out of 123 had died, three of them because of asthma. The response rate was 98% and the follow-up covered a time range from 23 to 31 years. The mean age at follow-up was 33 (range 26-43) and sixty-four individuals (55%) were free from symptoms. The severity of asthma was evaluated by means of a symptom-score divided into 5 degrees (no symptoms to continuous asthma). Symptom onset under the age of 4, the occurrence of atopic eczema and increased sensitivity to pollen, furry animals and respiratory tract infections correlated significantly (p less than 0.05, p less than 0.05 and p less than 0.01, respectively) with the occurrence of continuous adult asthma. A factor which appears to predict the development of adult asthma is a high level of medical care, expressed as the number of days of hospitalization during childhood.

Dietary linoleate increases fluidity and influences chemical composition of plasma low density lipoprotein in adult men

Dietary linoleate was effective to increase LDL fluidity in adult men but did not significantly influence VLDL or HDL fluidities. Lipoproteins were isolated ultracentrifugally from plasma of sixteen healthy, free living male volunteers consuming controlled diets formulated from typical U.S.A. foods to have 35 energy % fat with 10 g (diet L) or 30 g (diet H) linoleate per day, 30-50 g saturated fatty acids/day and the balance mainly monounsaturated fatty acids. Calculated cholesterol intakes were 500 mg/day at each calorie level. Changes in LDL fluidity were detected as differences in diphenylhexatriene (DPH) fluorescence polarization upon crossover between the two controlled diets. Thermotropic measurement of DPH fluorescence anisotropy and compositional analyses indicated that LDL and HDL fluidities were dependent upon phospholipid and triacylglycerol concentrations, respectively, and were modulated by the presence of cholesteryl esters. Fatty acid analyses of the major lipid classes of the isolated lipoproteins indicated that changes, upon diet crossover, in DPH fluorescence anisotropy, were a linear function of the incremental change in LDL phospholipid linoleate. The fluorescent probe described an environment corresponding to the fatty acyl moieties of the phospholipids on the LDL periphery, which composition is apparently under dietary control. It is suggested that the diet induced fluidity changes may affect the conformation of the apoprotein moiety on the LDL surface and thus the potential for LDL interaction with cellular LDL receptors.

Influence of saturated and unsaturated fats on platelet fatty acids in cholesterol-fed rabbits

Feeding natural fats varying in contents of palmitate (16:0), stearate (18:0), oleate (18:1), and linoleate (18:2) to rabbits resulted in modulation of platelet phospholipid fatty acyl composition. Rabbits were fed high fat semipurified diets containing 2% corn oil (CO) + 18% CO, cocoa butter (CB) or milkfat (M) for periods of up to 300 d. Platelet phospholipid linoleate contents corresponded to diet levels with 18:2 highest in CO-fed rabbits and following the sequence CO greater than CB greater than M. Stearate was highest in CB-fed rabbits, corresponding to high 18:0 levels in CB, but palmitate levels were not affected by diet. Both CB and M-fed rabbits were higher than CO-fed rabbits in oleate. Though CO is highest in 18:2, the accepted 20:4 precursor, arachidonate was highest in M-fed rabbits. Adding cholesterol (0.2%) to the diets did not affect platelet phospholipid fatty acyl composition except to elevate 20:4 in M-fed rabbits. CO-fed rabbits showed uniquely high levels of tetracosadienoate (24:2). Fatty acyl composition data were essentially constant between 200 and 300 d on diet. Phospholipid fatty acyl unsaturation was apparently homeostatically controlled as mole percent unsaturate to saturate ratios were independent of diet. The observed homeostasis resulted in minimal diet influences on platelet membrane fluidity and ADP or collagen stimulated platelet aggregation. Platelet fluidity, determined by fluorescence polarization, was a function of oleate and linoleate contents of the cells. Cholesterol feeding generally lowered platelet fluidity and altered the dependence of fluidity on fatty acyl composition.

Dihydroergotamine therapy in orthostatic hypotension due to psychotropic drugs

A double-blind, placebo controlled study with 10 mg per day dihydroergotamine in patients with orthostatic hypotension induced by treatment with psychotropic drugs showed a significant effect in preventing immediate drop in blood pressure after standing up. Preventing an abrupt drop in blood pressure with change of posture hinders symptoms of dizziness and faintness and helps activating patients who otherwise prefer to stay in bed.

Acyl chain interactions and the modulation of phase changes in glycerolipids

Fluorescence polarization measurements with 1,6-diphenyl-1,3,5-hexatriene and differential scanning calorimetry (DSC) were used to monitor phase transitions and order in the liquid state in sonicated dispersions of mono-, di- and triacylglycerols. Residual order in melted glycerolipids was indicated when the structural order parameter, S, assumed non-zero values at temperatures, t greater than or equal to tf, the DSC-determined fusion temperature. Residual order was observed with cis unsaturated di- and triacylglycerols but not with corresponding trans unsaturated or with saturated compounds. The reduced fluidity was attributed to adjacent binding of fatty acids to the glycerol molecule and the resulting interactions between fatty acyl moieties and packing effects. Lipids were considered as in an isotropic liquid or highly fluid state when diphenylhexatriene fluorescence anisotropy, rs, was equal to or less than 0.08, corresponding to S = 0. Temperatures, t0.08, for transition from the fluid state upon cooling were noted when rs = 0.08, and delta t = t0.08-tf was then taken as a measure of residual order. Tri-, 1,2-di and 1,3-dioleoylglycerol delta t values were 75, 60.9 and 13.6 degrees C, respectively. Tri-, 1,3-di- and monolinoleoylglycerol delta t values were 86, 30 and 41 degrees C, respectively. Restrictions in mobility when observed are attributable to interactions between adjacent acyl chains. Double bond location in the hydrocarbon chain affected ordering in the liquid state as simple triacylglycerol esters of cis 18:1 delta 6, trans 18:1 delta 6 and cis 24:1 delta 15 exhibited t = 37, 14 and 18 degrees C, respectively.

Influence of age and sex on composition and lipid fluidity in miniature swine plasma lipoproteins

Age- and sex-related differences were observed in the plasma cholesterol level, the plasma concentrations of certain lipoprotein components, and the HDL lipid phase fluidity in miniature swine from post-weaning (6 weeks) through puberty (6 months), maturity (2-6 years), and old age (10-12 years). Age effects were more dominant in the males, with VLDL protein; LDL protein, triacylglycerol, and phospholipid; and HDL triacylglycerol, phospholipid, cholesterol, and polyunsaturated fatty acids showing statistically significant negative correlations with age. These effects were not observed in females. HDL cholesterol was positively correlated with age in females. Total plasma cholesterol decreased with age in males only, but plasma triacylglycerol was not influenced by age in either sex. Higher concentrations of all lipoprotein lipids were observed in the female minipigs regardless of age. HDL lipids became less fluid with age in the males alone suggesting a physical chemical basis for the lower incidence of heart disease among females. The more fluid HDL circulating in the female may be more capable of mobilizing peripheral tissue cholesterol for catabolism thus protecting her from developing atherosclerotic lesions.

Fluorescence polarization order parameters and phase transitions in lipids and lipoproteins

Fluorescence polarization measurements with 1,6-diphenyl-1,3,5-hexatriene (DPH) were used to monitor phase changes in sonicated dispersions of triacylglycerols, cholesteryl esters and phospholipids. Lipid transitions to a fluid state were detected in a novel way by noting the temperature, t0.08, at which rs, the steady state anisotropy, was equal to 0.08. According to published equations (Van Blitterswijk , W.J., Van Hoeven , R.P. and Van der Meer , B.W. (1981) Biochim. Biophys. Acta 644, 323-332), this value for rs corresponds to a value of zero for S, a structural order parameter. Saturated and trans monounsaturated fatty triacylglycerols and distearoylphosphatidylcholine yielded t0.08 values in close agreement with transition temperatures found by differential scanning calorimetry (DSC), whereas cis unsaturated triacylglycerols displayed residual anisotropy, rs greater than 0.08, at temperatures above the DSC transition. The bent configuration of the cis double-bonded fatty acyl chains probably limits freedom of movement even in the liquid state when three such chains are bound to the glycerol molecule. Cholesteryl esters of 14:0, 18:0 and cis 18:1 fatty acids all showed rs greater than 0.08 above the DSC melting point. The difference in rotational freedom of DPH in triacylglycerol and cholesteryl esters even in the 'liquid' state explains the low t0.08 in the more fluid plasma VLDL and the contrastingly high t0.08 in plasma LDL, and HDL, which contain more cholesteryl ester an less triacylglycerol.

Heats of combustion of chemically different lipids

The caloric content of foods listed in food composition tables, such as the USDA Agriculture Handbook No. 8, is calculated by multiplying the gram amount of the protein, fat, and carbohydrate in the food by specific caloric factors derived around the turn of the century by Atwater and co-workers. To evaluate the accuracy of these specific caloric factors, we determined the heats of combustion of vegetable oils as purchased; of lipids extracted from red meats, chicken, fish, egg yolk, and cereal grains; and of the residue (protein) left after lipid extraction of the meats. These heats of combustion were converted to available energy values by the method of Atwater . The specific caloric factors used to calculate the caloric content of foods in Agriculture Handbook No. 8 and other tables of food composition may exaggerate the calories contributed by the lipids in some foods. When the food lipid was mainly triglyceride, the available energy values calculated in this study were within 2% of the specific factors used in Agriculture Handbook No. 8. However, when the food lipid had a high content of phospholipid, our calculated available energy values were lower than the specific factors used currently. The energy content of the fat-free residue (protein) of meat, poultry, and fish was also less than that currently used for those foods in food composition tables.

Platelet membrane fluidity and aggregation of rabbit platelets

Aggregation of rabbit platelets from citrated plasma in response to ADP was directly correlated with platelet plasma membrane fluidity as determined by fluorescence depolarization measurements with the probe diphenylhexatriene. Rabbits were maintained for periods of 200 and 400 days on potentially hyperlipidemic diets (20% fat by weight) with varying levels of saturated and polyunsaturated fatty acids. Dietary variations were effective in modulating the mole percentage distribution patterns of the platelet phospholipid fatty acids. The major chemical control of membrane fluidity was the actual mass of unsaturated lipid in the cells and not simply the relative percentage distributions of such unsaturated fatty acids. Substantially higher phospholipid/protein ratios were observed upon analysis of platelets and platelet membranes from rabbits after 200- than after 400-day diet periods. Accordingly lipid structures were significantly more fluid in either whole platelets or membrane isolates at the end of the shorter diet period. The observations pertaining to the extent of aggregation and membrane fluidity are in consonance with the general role of membrane fluidity in controlling biological activity and support the concept that platelet aggregation is a membrane-associated phenomenon.

Effects of fat level, feeding period, and source of fat on lipid fluidity and physical state of rabbit plasma lipoproteins

Elevating fat content from 5 to 20% of diet by weight or extending the feeding period from 6 months to more than 1 year did not substantially alter the fluidity of rabbit plasma lipoprotein lipid domains. Dietary fatty acid saturation was not adequate as a predictor of lipoprotein fluidity. Rabbits fed corn oil, high in polyunsaturated fatty acid content, did not have more fluid lipoproteins than rabbits fed cocoa butter which contains a high level of saturated long chain fatty acids. Order parameters calculated from fluorescence depolarization measurements with diphenylhexatriene (DPH) showed that very low density lipoprotein (VLDL) lipids were in highly fluid or 'liquid' states at or below body temperature. Order parameter data showed transitions from ordered phase to isotropic liquid in low density lipoproteins (LDL) that were heretofore unnoted with DPH fluorescence depolarization measurements. The transition temperature was inversely related to the LDL triglyceride content, indicating probe intercalation between the fatty acyl chains of the core triacylglycerols in VLDL and LDL.