Bruckdorfer Kr

Oxidative modification of low-density lipoproteins and the inhibition of relaxations mediated by endothelium-derived nitric oxide in rabbit aorta.

1 The mechanism by which Cu2+‐oxidized low‐density lipoproteins (oxLDL) inhibit acetylcholine (ACh)‐evoked relaxations mediated by endothelium‐derived nitric oxide (EDRF) in rabbit aortic rings was investigated. The proposed role of lysophosphatidylcholine (LPC) in the inhibition was also studied. 2 The kinetics of lipid peroxidation of native low‐density lipoproteins (LDL) from individual donors, as measured by changes in conjugated diene concentration, were related to the inhibitory effects of the resultant oxLDL. It was found that the more susceptible LDL was to oxidation, the greater the inhibition. 3 No correlation was found between the inhibitory effects of oxLDL and LPC content. 4 Synthetic 1‐palmitoyl LPC produced an inhibition of ACh‐induced relaxations and when added to precontracted rings evoked nitric oxide‐mediated endothelium‐dependent relaxation. This latter effect was not elicited by oxLDL. 5 Synthetic 1‐palmitoyl (10 μm) had no effect on relaxations evoked by glyceryl trinitrate in endothelium‐denuded aortic rings in contrast to the inhibition found previously for oxLDL. 6 Concentrations of oxLDL and phospholipase A2‐treated LDL which inhibited relaxation contained very different LPC concentrations. Unlike oxLDL, the inhibitory effects of phospholipase A2‐treated LDL preparations were independent of the donors and showed no lag period. 7 We sugggest that there are differences in the mechanisms by which oxLDL and 1‐palmitoyl LPC exert their inhibitory effects on relaxation. 8 The inhibition of relaxation by oxLDL (1–2 mg protein ml−1) was prevented by the presence of high‐density lipoproteins (HDL; 1–2 mg protein ml−1). 9 It is proposed that prevention of the inhibition of relaxation by HDL is consistent with the inhibitory factor(s) being lipophilic constituents of oxLDL. However, variations in the inhibitory effects of oxLDL preparations are not due to differences in their LPC content and factors other than LPC must contribute to the inhibition.

Low-density lipoproteins increase intracellular calcium in aequorin-loaded platelets

Low‐density lipoproteins activate isolated human platelets. The mechanism of this activation is unknown, but may involve increased phosphoinositide turnover. We have examined the effect of low‐density lipoproteins on intracellular calcium concentrations in platelets loaded with the photoprotein aequorin. The lipoproteins induced concentration‐dependent increases in intracellular calcium, associated with shape change and aggregation. These responses could be partially inhibited by the removal of extracellular calcium and by pre‐incubation with acetylsalicylic acid. They were also antagonised by agents which increase cellular concentrations of cyclic adenosine and guanosine monophosphates. It is not clear whether the platelet‐lipoprotein interaction involves a ‘classical’ lipoprotein receptor.

Differential effects of native and oxidatively modified low-density lipoproteins on platelet function

Low-density lipoproteins (LDL) have been various reported to induce platelet aggregation independently and/or sensitise platelets to other agonists. In these earlier studies the extent of oxidation of LDL was not always reported or addressed. We have now investigated the effects of native, minimally modified and fully oxidised LDL (0-1gapolipoproteinB(100)/l on platelet function using platelet aggregometry and fluorescence activated flow cytometry. Native LDL did not activate isolated platelets but inhibited ADP- and thrombin-induced aggregation of isolated platelets by 51 % in the presence or absence of added fibrinogen. Longer pre-incubations were required to produce a comparable inhibition by native LDL on platelets in plasma. Flow cytometric analysis showed that native LDL inhibited ADP-induced fibrinogen binding by up to 38%. In contrast, minimally modified LDL induced primary platelet aggregation and fibrinogen binding in the absence of other agonists, enhanced both submaximal (1-2mumol/l) ADP-induced aggregation, fibrinogen binding and degranulation (CD63 and P-selectin expression). Fully oxidised LDL, however, inhibited ADP-induced platelet aggregation and fibrinogen binding. The effects of minimally modified LDL on platelet aggregation could be reproduced partially by adding 15-hydroperoxy-eicosatetraenoic acid to native LDL. These data indicate that the extent of oxidation of LDL is critical in determining their effects on platelet function. Native LDL did not activate platelets, whilst minimally modified LDL exerted a pro-aggregatory effect, possibly due to the presence of lipid hydroperoxides near to the concentration range found in pathological states.

The Importance of Oxidation or Glycosylation of Low-density Lipoproteins in Relation to Platelet Activation.

Low-density lipoproteins (LDL) are thought to influence directly the sensitivity of platelets, but this may only be the case when the LDL are modified by oxidation. In diabetes, LDL are known to be modified by non-enzymatic glycosylation, especially when the blood glucose concentrations are poorly controlled: platelet activation is also concomitantly increased as is the concentration of plasma lipid peroxides. In this study we found that mild oxidation of LDL in vitro is more potent than strongly oxidised LDL in terms of the activation of platelets. Glycosylation of LDL per se has little effect on the aggregation of isolated platelets.

The influence of diet and diabetes on stearoyl Conenzyme A desaturase (EC 1.14.99.5) activity and fatty acid composition in rat tissues.

1. Rats were given low-fat diets for 3 d in which the carbohydrate source was starch. The livers of animals given the fructose or sucrose had increased hepatic activities of the fatty acid synthetase and stearoyl CoA desaturase (EC 1.14.99.5) enzyme complexes: in those given fructose there was a lower activity of the enzymes in adipose tissue. 2. Similar results were obtained in rats given fructose diets for 30 d, but in animals which had previously been made diabetic with streptozotocin, the activities were lower. The dietary treatment made little difference to the fatty acid profiles of the tissue lipids. The diabetic condition on the other hand produced considerable changes in fatty acid profile. 3. With diets containing approximately 200 g fat/kg in the form of butter or of polyunsaturated margarine, the tissue lipids from rats given sucrose had less linoleic acid than those from rats given starch. In addition, there was the expected difference between the rats given butter or margarine. The results are discussed in relation to the current literature.

The influence of dietary carbohydrate and fat on kidney calcification and the urinary excretion of N­acetyl­β­ glucosaminidase ( EC 3.2.1.30)

1. Male Sprague-Dawley rats were fed on diets containing either sucrose or starch as the carbohydrate component. In one experiment, the diets also contained 200 g either butter or polyunsaturated margarine/kg; in a second experiment, the diets contained less fat in the form of 20 g maize oil/kg. 2. Over a period of 11 months assays were made in the urine of several ions and of the activity of the enzyme N-acetyl-β-glucosaminidase (β-2-acetamido-2-deoxy-β-D glucoside acetamidodeoxygluco-hydrolase; EC 3.2.1.30); at 13 months, examination was made of some of the abdominal viscera, especially of the kidneys. 3. In rats fed on the higher amount of fat, dietary sucrose produced a higher activity of the enzyme than did dietary starch, and a greater excretion of inorganic phosphate. 4. With both the higher and lower amounts of dietary fat, sucrose led to an increase in the weight of the liver and of the kidneys, and an increase in the concentration of calcium and of phosphate in kidney tissue. With the higher amount of fat, sucrose also produced an increase in the concentration of magnesium in the kidney. There was no difference in the concentration of any of the ions assayed in the plasma or, apart from inorganic phosphate, in the urine. 5. The kidneys of the sucrose-fed rats showed nephrocalcinosis, mostly in the cortico-medullary region, and basophilic deposits in the tubules. Attention is drawn to this unusual occurrence of nephrocalcinosis in male rats.

Effects of high dietary sugar.

did not influence infection or inflammation, and 39 cannulae had been used once or not at all. No medication had been given through four of the 15 infected cannulae, including the septicaemic case. Significantly more of the infected than non-infected cannulae, however, had been used for blood transfusion (four out of 15 compared with four out of 87: p <0-002; y2 test with Yatess correction). We studied a second group of 20 patients to determine the risks of using the Venflon injection port for giving intravenous medication during operations. The cannulae were in situ for up to two hours and no cannula infection or local inflammation occurred.

Modification of tissue factor by peroxynitrite influences its procoagulant activity.

Peroxynitrite, a reactive oxidising species resulting from a reaction between nitric oxide and the superoxide anion, modifies proteins by nitration of certain amino acids such as tyrosine. Tissue factor (TF), a transmembrane protein, is expressed on cells under inflammatory conditions and initiates the coagulation cascade. The extracellular domain of TF is rich in tyrosine. Exposure of recombinant TF and cellular TF to peroxynitrite was associated with a reduction in procoagulant activity. This was accompanied by an elevated level of nitrotyrosine residues. Peroxynitrite may have a protective role by attenuation of the thrombotic properties of TF.

Dietary sucrose affects plasma HDL cholesterol concentration in young men.

Fourteen young male volunteers measured their habitual dietary intake for 2 weeks, then were told to increase their dietary sucrose while decreasing their other carbohydrates for 2 weeks and finally told to revert to their habitual diet while continuing to record their intake. Measurement of plasma constituents revealed a significant fall in HDL-cholesterol concentration after the period on the high sucrose diet, and a return to the higher concentrations after resumption of the habitual diet. Twenty-six young men whose habitual diet contained more than an average quantity of sucrose followed a similar regime, except that they were told to reduce their sucrose for 2 weeks and to compensate by increasing the intake of other carbohydrates. In the event, unlike the volunteers in the first experiment, they were found not to have made the compensatory change when lowering their sucrose intake. Measurement of their plasma constituents showed that the reduction in dietary sucrose resulted in a significant fall in the mean concentration of triglycerides. There was no significant change in the mean concentration of HDL cholesterol, although there was an increase in the concentration in 11 of the 26 subjects.

Some effects of different dietary carbohydrates on pregnancy and lactation in rats.

Female rats were fed from weaning on diets with sucrose, starch, glucose or fructose as the carbohydrate source. Animals were killed at various stages throughout pregnancy and early lactation. Maternal plasma triglycerides (TG), cholesterol, free fatty acids, glucose, insulin and corticosteroids were measured. Lipogenic activity was assayed in the livers, adipose tissue and mammary tissue, and the results compared with those from non-pregnant rats. Insulin, corticosteroids and hepatic lipogenesis were also assayed in the embryos and in newborn pups. Dietary sucrose and fructose produced a significantly higher concentration of plasma TG in the non-pregnant, pregnant and lactating rats than did starch and glucose. All the diets led to an increase in TG concentration at the 20th day of pregnancy, which returned to the original concentration 2 days post-partum. The hypertriglyceridaemia of late pregnancy was accentuated by the feeding of sucrose and fructose. Maternal concentrations of plasma glucose were significantly reduced towards the end of pregnancy in all dietary groups. The replacement of starch by sucrose, or of glucose by fructose, enhanced hepatic lipogenesis. Fructose but not sucrose depressed fat synthesis in the adipose tissue. Hepatic fatty acid synthetase activity was increased in late pregnancy on all diets except that with starch. Late pregnancy intensified hepatic lipogenesis in rats fed sucrose or fructose. The results are discussed in relation to the metabolic changes during pregnancy and to sucrose feeding.