Low cost polymer intramedullary nails for fracture fixation: a biomechanical study in a porcine femur model.

INTRODUCTION: Whilst intramedullary nailing is a commonly accepted technique for lower limb fracture fixation, the cost of nails can be prohibitive in hospitals in developing nations. In these institutions bone cement has found many off label applications, that whilst are effective do not meet manufacturers guidelines. The aim of this study was to examine the biomechanics of one such application, fracture fixation using a bone cement intramedullary nail. MATERIALS AND METHODS: Five porcine femurs underwent a mid-shaft osteotomy and were fixed using a nail made from antibiotic simplex bone cement. The torsional and flexural stiffness and shear modulus of these constructs were compared to five intact porcine femurs. RESULTS: The bone cement intramedullary nail was able to achieve relative stability in both torsion, with a mean shear modulus of 0.17 GPa and in flexion with a mean flexural stiffness of 358 N/mm. This corresponds to 47 and 22% of the respective measurements in the intact femurs. The mean ultimate flexural strength of fracture/nail constructs was 936 +/- 350 N, which is 20% of the ultimate flexural strength of an intact porcine femur (4,820 +/- 698 N). CONCLUSION: Intramedullary nails made from bone cement were able to provide sufficient promise in this situation to warrant further investigation for their applicability as a low cost alternative for use in developing countries.

Total parenteral nutrition stimulates hepatic cholesterol synthesis in the rat.

Cholesterol synthesis was studied in parenterally fed rats, as compared to orally fed rats with or without saline infusion. Conditions of total parenteral nutrition (TPN) involved the intravenous infusion of a nutritive mixture containing 20% Intralipid as the lipid source (50% of non-protein energy) at the continuous rate of 2 ml per h, for five days. In rats maintained in isotopic steady state by daily injections of [3H]cholesterol, isotope dilution indicated that the endogenous plasma cholesterol input was significantly higher (+15%, P < 0.05) in TPN than in orally fed rats, which suggested a slight stimulation of whole body cholesterogenesis. Cholesterol synthesis was assessed in TPN and orally fed rats by the in vivo incorporation of [1,2-13C]- and [1-14C]acetate into hepatic and intestinal sterols, and by the activity of HMG-CoA reductase in microsomes isolated from liver and small intestine. Both methods demonstrated that TPN markedly stimulated the hepatic cholesterol synthesis, since the radioactivity of liver sterols was 6- to 10-fold higher, and the activity of HMG-CoA reductase 5-fold higher, in TPN than in orally fed rats. Despite the weight reduction of the small intestine, by about 20% after TPN, the incorporation of exogenous [14C]acetate into intestinal sterols was similar in TPN and orally fed rats. As the liver and intestine are the main organs responsible for the appearance of endogenous cholesterol in plasma, it may be concluded that the increased endogenous plasma cholesterol input was mainly due to a strong stimulation of hepatic cholesterol synthesis in TPN rats.

A new rat liver phospholipid exchange protein.

The soluble fraction from several mammalian tissue homogenates is known to stimulate phospholipid exchange between cell membrane fractions or artificial vesicles. All phospholipid exchange proteins purified to data exhibit an acidic isoelectric point. Using an assay that measures the transfer of [32P] phosphatidylcholine from liposomes to beef heart mitochondria, we report the presence of a new phospholipid exchange protein with a basic isoelectric point (8.4) in rat liver cytosol. A purification procedure, consisting of pH adjustment to 5.1, gel filtrations on Sephadex G 75 and DE 52 cellulose, isoelectric focusing between a pH of 5 and 10, and gel filtration on Sephadex G-50, yielded a fraction with high phosphatidylcholine exchange activity per mg of protein. This fraction exhibits a major band and two minor bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular weight of the major band (18 700) is close to that for basic exchange protein fraction obtained by gel filtration (17 000). The distribution of basic and acidic exchange proteins differs markedly in various tissues and animal species. About 50 and 35% of phosphatidylcholine exchange activity from rat liver and rat intestine respectively are due to basic phospholipid exchange proteins. In contrast, no basic exchange protein was found in beef heart and only a small amount in beef liver. In the latter organ, less than 10% of phosphatidylcholine exchange activity was due to a basic phospholipid exchange protein fraction.

Effects of intravenous infusions of commercial fat emulsions (Intralipid 10 or 20%) on rat plasma lipoproteins: phospholipids in excess are the main precursors of lipoprotein-X-like particles.

Like most commercial parenteral emulsions, Intralipid contains the same amount of phospholipids (12 mg/ml) to stabilize 100 or 200 mg of soybean oil (10 or 20% formula, respectively). By centrifugation, 10 or 20% Intralipid was separated into a supernatant, fat particles containing the bulk of triacylglycerols stabilized by a fraction of phospholipids and an infranatant--called mesophase--consisting mainly of phospholipids used in excess as emulsifier. We observed that the initial triacylglycerol/phospholipid ratio of the emulsion (100/12 and 200/12, respectively) determines the size of the triacylglycerol-rich particles (260 and 350 nm) as well as the phospholipid content of the mesophase (6.02 and 4.67 mg/ml). To understand the mechanism of the lipoprotein-X (LPX) accumulation generally reported after intravenous fat infusions, plasma lipid levels and lipoprotein profiles were first compared in the rats after infusion (at a constant rate of 0.5 or 1 ml/h for 43 h) of Intralipid 10 or 20%. For the same intravenous triacylglycerol load (100 mg/h), rats infused with Intralipid 10% at 1 ml/h displayed higher triacylglycerol levels than rats infused with the 20% emulsion at 0.5 ml/h, suggesting that the size of exogenous fat particles modulated the catabolic rate of their triacylglycerols. The plasma levels of LPX varied according to the infusion rate of phospholipids not associated with triacylglycerol-rich particles of the emulsion. Moreover, an apo E and apo B enrichment of plasma and an elevation of the apo B48/apo B100 ratio was always observed after Intralipid infusions. In order to confirm that phospholipids of the mesophase are the main LPX precursors, lipoprotein profiles were then compared in the rats after intravenous infusion, at a constant rate of 1 ml/h, of either the mesophase or a suspension of triacylglycerol-rich particles isolated from Intralipid 20%. As expected, significant LPX amounts were only detected in rats infused with the pure mesophase of the emulsion. It was concluded that products of the lipolysis of exogenous fat particles play only a minor role in the formation of LPX. In fact these abnormal lipoproteins are generated by phospholipids of the mesophase which, like infused liposomes, actively mobilize endogenous free cholesterol. Consequently, in order to be considered as true chylomicron models for safe fat delivery in parenteral nutrition and in order to prevent some detrimental effects on cholesterol metabolism, commercial emulsions should be cleared of phospholipid excess.

Total parenteral nutrition and plasma lipoproteins in the rat: evidence for accelerated clearance of apo-A-I-rich HDL.

The effect of total parenteral nutrition (TPN) containing fat on plasma lipoproteins and apo-A-I-rich HDL catabolism was studied in the rat. TPN rats were intravenously infused for 5 days with a nutritive mixture containing amino acids, lipids (Intralipid 20%) and glucose. In spite of similar plasma levels of total cholesterol in TPN and control orally fed rats, density gradient ultracentrifugation of plasma samples gave evidence of marked differences in the lipoprotein profiles. In the density range 1.010-1.040, were found elevated amounts of apo-B-100 and apo-B-48 containing lipoproteins, as well as an increase in free cholesterol and phospholipids, the latter indicating that the plasma of TPN rats contained abnormal lipoprotein-X-like particles. The level of apo-E-rich HDL (density: 1.040-1.063) was not markedly changed, whereas that of typical HDL (d > 1.063) was lowered, with less apo-A-I and apo-A-IV, and low amounts of cholesterol and phospholipids were found in the most dense HDL3 fractions (d > 1.090) containing the bulk of apo-A-I-rich particles. After intravenous infusion of homologous [14C]sucrose-labelled HDL3, the clearance of these particles was 2-fold faster in TPN than in control rats, with a tissue uptake increased in the liver (+40%) and decreased in the small and large intestines (-60%). Because the pool of apo-A-I-rich HDL was dramatically reduced after 5 days of artificial feeding, the absolute catabolic rate of these lipoproteins was similar in the two groups. These data suggest that, in TPN rats lacking of chylomicron coat components as a source for HDL material, the fall in plasma levels of apo-A-I-rich HDL resulted mainly from accelerated turnover of these particles, mediated by increased uptake by the liver. Conversely, mucosa atrophy was probably involved in the reduced uptake of apo-A-I-rich HDL by the gastrointestinal tract.

Evidence for different isotopic enrichments of acetyl-CoA used for cholesterol synthesis in the liver and intestine: a study in the rat by mass fragmentography after intravenous infusion of [13C]acetate.

Wistar rats were killed 4 h after an intravenous infusion of [1,2-13C]- and [1-14C]acetic acid sodium salt (39 mg, 12.5 microCi/ml, constant rate: 1.2 ml/h). At this time, labeled free cholesterol movements between the organs are still weak and cholesterol labeling in each tissue mainly originates from the in situ incorporation of the exogenous substrate. In male rats, the specific radioactivity of free cholesterol was found to be higher in the intestine (mucosa and wall) than in the liver and plasma. In female and in cholestyramine-fed male rats, cholesterol 14C labeling was close to that of male rats in the intestine, and was markedly higher in the liver. The same variations of 13C excess, calculated by mass fragmentography, indicated that there was no isotopic effect between 13C and 14C precursors. The advantage of this method consisted in obtaining the proportions of labeled molecules according to their molecular weight (M + 1-M + 11) for each sample. Then the distribution of 13C atoms in newly synthesized cholesterol was assessed in each sterogenesis site. In the intestine, about 3/4 of the 13C atoms were found in molecules of weight of at least M + 4 (after incorporation of at least two labeled acetate units). This proportion was only 1/3 in hepatic and plasma free cholesterol. These distinct 13C-labeling patterns clearly indicate that local variations occurred in the isotopic enrichment of acetyl-CoA used for cholesterol formation. Whatever the experimental conditions of this study, cholesterol was synthesized from an acetyl-CoA more 13C enriched in the intestine than in the liver. Such variations probably result from the different dilutions of exogenous acetyl-CoA by the endogenous pool in the liver and intestine. Consequently, the 14C or 13C incorporations measured in the liver and intestinal sterols do not account for absolute rates of cholesterol production by these organs. This study also indicated that after a few hours of infusion, free cholesterol labeling in the plasma originated mainly from cholesterol newly formed in the liver, even when acetate incorporation into cholesterol was higher in the intestine than in the liver.

Cholesterol metabolism in the genetically hypercholesterolemic (RICO) rat. II. A study of plasma lipoproteins and effect of dietary cholesterol.

The high plasma cholesterol concentration of the genetically hypercholesterolemic RICO rats fed a low cholesterol base diet (1.28 mg/ml) compared to that of SW rats (0.73 mg/ml) results from an increase in the cholesterol content of the d greater than or equal to 1.006 lipoproteins. Since the composition of each type of lipoprotein is similar in the two groups of rats, the RICO rat, therefore, is hyperlipoproteinemic with an increase in the number of lipoprotein particles, except VLDL and chylomicrons. Furthermore, the apolipoprotein E (apoE) content in the d less than or equal to 1.063 lipoproteins is higher in RICO than in SW rats, while that of apoA-I in HDL is lower. In rats fed 0.5% cholesterol base diet, cholesterolemia doubles in the two groups (SWCH, 1.32 +/- 0.10 mg/ml; RICOCH, 2.10 +/- 0.09 mg/ml). This hypercholesterolemia is due to an increased cholesterol content in VLDL and chylomicrons. These lipoproteins carry 60% (in SWCH) and 45% (in RICOCH) of the plasma cholesterol and are cholesterol-enriched compared with the lipoproteins observed in rats fed the base diet. In RICOCH, 24% of the plasma cholesterol is found in apoE-rich LDL2 (1.040 less than or equal to d less than or equal to 1.063), whereas in SWCH, this fraction contains only 11% of the plasma cholesterol. Finally, as before with the base diet, RICOCH shows an apoE enrichment of the d less than or equal to 1.063 lipoproteins and an apoA-I depletion of HDL compared to SWCH. These data suggest that hypercholesterolemia of the RICO rats results from a modification in the turnover of apoE-containing lipoproteins.

Effects of chronic glucagon administration on cholesterol and bile acid metabolism.

Male adult Wistar rats received daily, at 9 a.m. and 5 p.m., 10 micrograms of Zn-protamine glucagon for 21 days by subcutaneous injections. The blood glucose level was not significantly modified. Cholesterol and triacylglycerol levels were decreased by 40 and 70% in plasma but not in the liver. The rates of cholesterol turnover processes were determined in vivo with an isotope balance method. Internal secretion of cholesterol (13.8 +/- 0.5 mg/day per rat in control rats and 22.4 +/- 0.9 mg/day per rat in glucagon-treated rats) and cholesterol transformation into bile acids were strikingly increased by chronic administration of glucagon. Biliary secretion rates of bile acids measured by a wash-out method were increased by 139%, while the intestinal bile acid pool was not changed. The enterohepatic cycle number was increased from five per day in control rats to nine per day in glucagon-treated rats. An increased turnover rate of the exchangeable cholesterol would explain the hypocholesterolemic effect of glucagon.

Effects of hyodeoxycholic acid and alpha-hyocholic acid, two 6 alpha-hydroxylated bile acids, on cholesterol and bile acid metabolism in the hamster.

The effects of hyodeoxycholic (HDCA) and alpha-hyocholic acids (alpha-HCA), on cholesterol, bile acid and lipoprotein metabolism, were studied in hamsters. The animals were fed a low cholesterol control diet supplemented with 0.1% HDCA or alpha-HCA for 3 weeks. In both treated groups, the LDL-cholesterol concentration was significantly lowered and was associated with a global hypocholesterolemic effect. Moreover, hepatic cholesterol ester storage was reduced and HMGCoA reductase activity was respectively enhanced 13.5-times and 7.7-times in HDCA and alpha-HCA groups compared to controls. In contrast, cholesterol 7 alpha-hydroxylase activity and LDL-receptor activity and mass were not modified. In bile, the cholesterol saturation index was increased 5-fold (HDCA group) and 2-fold (alpha-HCA group) as a consequence of an enlarged proportion of biliary cholesterol. The two 6-hydroxylated bile acids induced an enhanced fecal excretion of neutral sterols (HDCA group: 11.6-times, alpha-HCA group: 3.2-times versus controls) which was consistent with a 59% decrease in intestinal cholesterol absorption in the HDCA group. The major effects due to bile acid treatments were a decrease in LDL-cholesterol concentration, a strong stimulation of hepatic cholesterol biosynthesis and an excessive loss of cholesterol in feces. These perturbations might be the result of the enrichment of bile with hydrophilic bile acids, leading to a limited return of endogenous cholesterol from the intestine to the liver.

Assay of microsomal oxysterol 7alpha-hydroxylase activity in the hamster liver by a sensitive method: in vitro modulation by oxysterols.

A method of assaying hepatic cytochrome P-450, oxysterol 7alpha-hydroxylase (CYP7B), was developed by combining the use of 25-[26,27-(3)H]hydroxycholesterol as a substrate and hydroxypropyl-beta-cyclodextrin as a substrate vehicle. When these assay conditions were tested, an undesirable transformation was observed of the reaction product, 7alpha,25-dihydroxycholesterol, into 3-oxo-7alpha,25-dihydroxy-4-cholesten by the activity of 3beta-hydroxy-Delta(5)-C(27) steroid oxydoreductase, a microsomal NAD(+) and NADP(+) dependent enzyme of bile acid metabolism. A great improvement was reached by using a continuous NADPH generating system which constantly re-transforms NADP(+) into NADPH, thus inhibiting this activity. This improved CYP7B assay, comparable to our previously described assay for cholesterol 7alpha-hydroxylase (CYP7A), allowed a 3-fold increase of the apparent enzyme activity. The possibility to simultaneously measure CYP7A and CYP7B activities on the same microsomal preparation was investigated. A marked decrease (-33%) in the CYP7B activity was noticed, while that of CYP7A remained unchanged. The CYP7B activity was observed to be inhibited by cholesterol (-30%) and also by the oxysterols 7alpha-hydroxycholesterol (-21%), 7beta-hydroxycholesterol (-25%) and epicoprostanol (-20%), and by cyclosporin A (-26%). It can be concluded that this sensible and easy to perform CYP7B assay allows to observe, at least in vitro, a modulation of the enzyme activity by oxysterols.

Origin and fate of rat plasma cholesterol in vivo. Modelling of cholesterol movements between plasma and organs.

A cholesterol system model was developed in the rat following a single injection of red cells containing free (unesterified) [3H]cholesterol. The radioactivity of free and esterified cholesterol in the different parts of the system was measured during the 48 h following tracer introduction. The model consisted of seven compartments (red cell free cholesterol, plasma and liver free and esterified cholesterol, total cholesterol in the rapidly and slowly exchangeable carcass pools). The model was validated by the similarity between simulated and experimental values during the 48 h following tracer introduction. Both the fractional rate of cholesterol esterification in the plasma (0.44 h-1) and liver (0.01 h-1) and the fractional exchange rate of free cholesterol from the plasma towards the various organs (particularly 3 h-1 towards the liver for a total of 7 h-1) can be estimated with this model. The results show that cholesterol movements between the plasma and the different organs take place mainly through intense free cholesterol exchanges, resulting in a low net flux.

Cholesterol metabolism in the genetically hypercholesterolemic rat (RICO). I. Measurement of turnover processes.

The rates of mobile cholesterol turnover processes were measured by the isotopic equilibrium method in normocholesterolemic (SW) and hypercholesterolemic homozygote (RICO) rats fed a semi-synthetic base diet containing 0.05% cholesterol. When the absorption rate is similar in SW and RICO rats, the internal secretion rate is 60% higher in RICO (25.3 mg/day) than in SW (16.2 mg/day). This increase is compensated by an increase in fecal excretion (RICO: 5 mg/day; SW: 3.8 mg/day), urinary excretion (RICO: 1.7 mg/day; SW: 1.1 mg/day) and above all the transformation of cholesterol into bile acids (RICO: 24.2 mg/day; SW: 15.3 mg/day). The fact that 70 minutes after [14C]acetate administration, the only variations obtained in RICO compared to SW rats are a doubled sterol radioactivity in the small intestine and a tripled one in the liver suggests that the increase in internal secretion of the RICO rat has both an intestinal and hepatic origin. This cholesterogenic stimulation in RICO rats takes place in the jejunum as well as in the ileum and in the crypt cells as well as in the villosities. It is concomitant with a doubled cholesterolemia, a doubled intestinal, caecal and colon bile acid pool and a 20% increase in the enterocyte protein content.

Effect of beta-cyclodextrin dietary supplementation on biliary proteins and their resulting cholesterol nucleating activity in pigs.

We explored the possibility that the biliary protein fraction may support part of the variation in the nucleating activity previously measured in gallbladder biles of pigs. Eighteen gallbladder aspirates freshly obtained from three dietary groups (0, 5, or 10% beta-cyclodextrin) of six pigs were chromatographed to purify their total protein fraction. Proteins were quantified, and analysed through electrophoresis and immunoblotting or enzyme-linked immunosorbent assay for albumin, and five putative effectors of cholesterol crystallisation, mucins, immunoglobulin A, 130 kDa, apolipoprotein A-I, and anionic polypeptide fraction. Each total protein fraction was also assayed for its ability to influence cholesterol precipitation, when added to supersaturated model bile. The current data provided evidence that the cholesterol crystallisation-promoting activity of biliary proteins in model biles increased with the beta-cyclodextrin dietary content. This occurred without any significant change in the total biliary protein content, but was associated with a significant decrease in the concentration of albumin and apolipoprotein A-I, resulting in changes in the overall balance of proteins in bile. Comparison of these results with the crystallisation figures previously obtained from the corresponding native biles led us to conclude that biliary proteins might influence the outcome of the crystallisation process, namely the final crystal concentration at equilibrium, but would not systematically represent a major driving force for determining the velocity of crystal formation in native bile of pigs.

Lipid composition and structure of commercial parenteral emulsions.

In order to study the influence of the phospholipid/triacylglycerol (PL/TG) ratio of parenteral emulsions on the distribution and the physico-chemical properties of their fat particles, commercial 10, 20 or 30% fat formulas were fractionated by centrifugation into an upper lipid cake (resuspended in aqueous glycerol) and a subnatant or mesophase, from which a PL-rich subfraction (d = 1.010-1.030 g/l) was purified by density gradient ultracentrifugation. Chemical and 31P-NMR analyses of these fractions indicated that at least two types of fat particles coexist in parenteral emulsions: (i) TG-rich particles (mean diameter: 330, 400, 470 nm in the 10, 20, 30% emulsion) which contain practically all the TG and esterified phytosterols of native emulsions, but only a fraction of their PL, unesterified cholesterol and phytosterols, and other minor lipids; (ii) PL-bilayer particles or liposomes (mean diameter: 80-100 nm) which are constituted with the remaining PL and relatively very small amounts of TG and other lipids. The higher the oil content of the emulsion, the lower the amount of these PL-rich particles, which represent the major particle population of the mesophase. Indeed, minute amounts of TG-rich particles (probably the smallest ones) are also present in the mesophase, even in the PL-rich subfraction which contains the bulk of liposomal PL. Since the PL-rich particles of the infused emulsion generate lipoprotein X-like particles, only the large TG-rich particles can be considered as true chylomicron counterparts.

Radiation-associated recurrent parathyroid adenoma.

Recurrent parathyroid adenoma occurred in a patient who had received radiation treatments to the lateral aspect of the pharynx for lymphoid hyperplasia at the age of 10 years. The interval between the development of the first and second adenomas was 20 years. Although an association between head and neck irradiation and primary hyperparathyroidism is known, we believe this to be the first report of recurrent hyperparathyroidism in an irradiated patient.

Dietary cholesterol, membrane cholesterol and cholesterol synthesis.

After describing the main steps of cholesterol biosynthesis the author recalls that the cholesterogenesis rate is feedback-inhibited by dietary cholesterol and examines the various processes of modulation. Hydroxy-3-methylglutaryl (HMG) CoA reductase, the key rate-limiting enzyme, is a 97 kDa endoplasmic reticulum glycoprotein, anchored 7-fold in this membrane. The N-terminal membrane-bound domain plays a fundamental role in the modulation of reductase activity. This modulation is essentially mediated by decreased gene transcription and enhanced degradation of the protein. The possible modulation by a bicyclic cascade system involving phosphorylation (inactivation) and dephosphorylation (activation) of reductase does not seem to play an essential role in vivo. Finally, recent data show that the lipid composition (C/P molar ratio) of some reticular membranes (fibroblasts, for example) can strongly modulate the activity of this ubiquitous enzyme.

Metabolism of intestinal triglyceride-rich lipoproteins in the genetically hypercholesterolemic rat (RICO)

Experiments were carried out to compare the catabolism of intestinal lipoproteins between genetically hypercholesterolemic (RICO) and normocholesterolemic (SW) rats. Kinetics of plasma cholesteryl ester were studied after injection of cholesterol-labeled chylomicrons or VLDL. The chylomicron clearance is reduced in the RICO rat (rate constant, K = 7.2 +/- 0.1 h-1 vs. 10.7 +/- 0.1 h-1 in SW rat), while a much more minor alteration was observed in the catabolism of lymph VLDL (K = 4.3 +/- 0.6 h-1 in the RICO rat vs. 5.1 +/- 0.4 h-1). The injection of chylomicrons from SW rats to RICO rats and from RICO rats to SW rats showed that the fall in the rate of catabolism of chylomicrons in RICO rats was not secondary to an increase in the production rate, but was related to the lipoprotein particle itself without any alteration of the catabolic system. The reduction in the rate of catabolism of chylomicrons in the RICO rat could be related to a change in their apolipoprotein composition (increase in the proportion of apolipoprotein E = 12 +/- 2% vs. 3 +/- 1% in the SW rat).

Hepatic apolipoprotein and LDL receptor gene expression in the genetically hypercholesterolemic (RICO) rat.

The present study was designed to examine apolipoprotein and LDL receptor gene expression in genetically hypercholesterolemic RICO rats. In the plasma of RICO rats as compared to SW (control) rats, the hypercholesterolemia (+41%) was associated with a significant increase in plasma apo B (+23%) and apo E (+68%) concentrations. Study of apolipoprotein synthesis in the liver has shown that this increase in plasma apo B and apo E concentrations was not associated with modification in their synthesis and mRNA levels. Study of apo E mRNA level in various tissues has shown only the modification in adrenals in RICO as compared to SW rats (2.7-fold increase). Study of LDL binding, LDL receptor mass and LDL receptor mRNA level in the liver of RICO and SW rats has shown no significant differences between these two strains. EDTA-resistant binding of rat LDL was lower in RICO than in SW rats suggesting that binding sites others than the LDL receptor are present in lesser amount in this hypercholesterolemic strain.

The effect of lanosterol on platelet aggregation in human platelets.

It has been reported that lanosterol can sensitize isolated rat platelets to agonists such as ADP and thrombin (4). The purpose of this paper was to determine whether lanosterol had similar effects on human platelets and whether this was achieved by changes in membrane fluidity. Lanosterol did increase the sensitivity of human platelets, particularly to adrenaline and ADP at concentrations as low as 5 mg.L-1 when added from solutions in ethanol. At similar concentrations cholesterol, 4-cholesten-3-one or ethynyloestradiol had either no effect or were inhibitory. Measurement of membrane fluidity with diphenylhexatriene indicated that lanosterol did not affect membrane fluidity. Incubation of platelets with [4C]-mevalonic acid gave rise to a very small incorporation into lanosterol, squalene and farnesol. Sudden activation of the platelets did not accelerate lanosterol synthesis during or after platelet aggregation. It was concluded that lanosterol could only influence platelet behaviour if it came from the plasma. However the concentration of the steroid in both platelets and plasma is ten fold less than that required to sensitise the platelets.

Effects of ionizing radiation (neutrons/gamma rays) on plasma lipids and lipoproteins in rats.

Male Wistar rats weighing 250 g were exposed to 4 Gy of neutrons/gamma radiation (3.33 Gy of neutrons and 0.66 Gy of gamma rays). After whole-body irradiation, plasma cholesterol and phospholipid levels increased up to 62 and 37%, respectively, at day 4 and then returned to control values 12 days after irradiation. Plasma triglyceride concentrations decreased concomitantly with decreased food intake after irradiation but remained higher than in pair-fed control rats. Plasma lipoproteins were separated by ultracentrifugation on a density gradient (1.006-1.210 g/ml). Four days after irradiation, most of the cholesterol (62% compared to 31% in controls, P < 0.001) is transported by apolipoprotein E-rich high-density lipoproteins. At the same time, plasma levels of apolipoproteins B and E were increased by 28 and 65%, respectively, while those of apolipoproteins AI and AIV were reduced by 21 and 59%, respectively. While in the liver of irradiated rats the apolipoprotein B/E receptor number was not modified, the hydroxymethylglutaryl coenzyme A reductase activity was fivefold higher than in control pair-fed rats. Four days after irradiation, the susceptibility of lipoproteins to peroxidation, as measured by the formation of conjugated dienes in the presence of Cu2+, was markedly increased while plasma vitamin E levels were decreased, demonstrating that irradiation reduces antioxidant stores markedly. These results suggest that such modified lipoproteins could be involved in radiation-induced vascular damage.