The injection of heparin prolongs the plasma clearance of oxidized low density lipoprotein in the rat

Citrate anticoagulation abolishes degranulation of polymorphonuclear cells and platelets and reduces oxidative stress during haemodialysis

BACKGROUND

During haemodialysis (HD), polymorphonuclear cells (PMNs) and platelets are activated and release various granule products, including myeloperoxidase (MPO) and platelet factor 4 (PF4). MPO triggers the generation of reactive oxygen species, leading to irreversible protein, carbohydrate and lipid modification. PF4 probably also contributes to oxidative stress. As previously shown, HD-induced PMN degranulation is almost completely abolished during citrate anticoagulation, most probably due to its calcium chelation ability.

METHODS

In the present study, apart from HD-induced PMN and platelet degranulation, oxidative stress was analysed during three modes of anticoagulation. Heparin, dalteparin and citrate (HDhep, HDdal and HDcit) were compared in a randomized, crossover fashion in eight chronic HD patients. Multiple blood samples were taken during the third HD session of each modality, from both the afferent and efferent line. Besides the degranulation markers MPO and PF4, various markers of oxidative stress were measured, including oxidized low-density lipoprotein (ox-LDL), malondialdehyde (MDA) and carboxymethyllysine (CML).

RESULTS

During HDhep and HDdal, marked degranulation was observed shortly after the start of HD. In contrast, during HDcit, PF4 and MPO levels remained unaltered, suggesting no release at all. After 1 week of HDcit, ox-LDL levels were markedly reduced [median 26% (3-65%), P=0.01], if compared with HDhep and HDdal. As regards MDA and CML, no differences were found.

CONCLUSIONS

This study shows first, that HD-induced PMN and platelet degranulation are early, most probably calcium-dependent processes and, secondly, that the formation of ox-LDL is clearly dependent on the type of anticoagulant applied.

Role of macrophage glycosaminoglycans in the cellular catabolism of oxidized LDL by macrophages

Macrophage binding sites for oxidized LDL (Ox-LDL) include class A scavenger receptors (SR-As), the CD-36 molecule, and an additional but hitherto unidentified binding site. Because cell-surface glycosaminoglycans (GAGs) were previously shown to be involved in the cellular uptake of native LDL and lipoprotein(a), several strategies to assess the participation of heparan sulfate (HS) and chondroitin sulfate (CS) in macrophage catabolism of Ox-LDL were used. First, incubation of J-774 A.1 macrophage-like cells with either heparinase or chondroitinase, or with both enzymes together, reduced the binding, uptake, and degradation of 125I-Ox-LDL by 20% to 45%, in comparison with control nontreated cells, while catabolism of 125I-labeled acetylated LDL (Ac-LDL) and native LDL were unaffected. Second, the proteoglycan (PG) cellular content was increased by cell enrichment with exogenous GAGs or by using human monocyte-derived macrophages from two patients with Sanfilippo mucopolysaccharidosis, which are characterized by cellular HS accumulation. In these macrophages, cellular uptake of 125I-Ox-LDL increased, while catabolism of 125I-Ac-LDL and native LDL were unaffected. Experiments using conditioned media from control, heparinase-digested, or chondroitinase-digested macrophages indicated that neither secreted GAGs nor released digestion products played any role in Ox-LDL catabolism. To evaluate potential interactions between cell-surface GAGs and known receptors for Ox-LDL, we used excess unlabeled Ac-LDL to block SR-As or anti-CD-36 antibodies to block CD-36, and then examined the catabolism of 125I-Ox-LDL by GAG-enriched or -depleted macrophages. Both excess unlabeled Ac-LDL and anti-CD-36 antibodies reduced 125I-Ox-LDL catabolism, but only excess unlabeled Ac-LDL completely abolished the increase in 125I-Ox-LDL catabolism on GAG enrichment of the cells, indicating a cooperation between exogenous GAGs and cell-surface SR-As in the catabolism of OX-LDL. Moreover, the addition of GAGases to macrophages that were preincubated with anti-CD-36 antibodies and excess Ac-LDL further reduced macrophage degradation of Ox-LDL in comparison with cells that were pretreated only with anti-CD-36 antibodies and Ac-LDL, indicating a more complex role for endogenous GAGs. Overall, these studies demonstrate a substantial contribution of macrophage-associated GAGs in the catabolism of Ox-LDL, which is mediated in part by a cooperation between GAGs and cell-surface SR-As.

Residence time of low-density lipoprotein in the normal and atherosclerotic rabbit aorta

Previous results from this laboratory found that the arterial low-density lipoprotein (LDL) residence time in lesion-prone aortic sites was longer in hyperlipidemic rabbits before lesion formation than in the corresponding sites in normolipidemic rabbits. The calculation of residence time in the previous study assumed that the arterial wall was homogeneous; the present study reexamines the issue using a method that does not require such an assumption. The concentration of radiolabeled arterial LDL was measured in New Zealand White rabbits killed at several different times (0.5 to 72 hours) after injection of labeled LDL. Using a stochastic analysis, arterial LDL residence time was calculated from the pooled labeled arterial LDL measurements from these rabbits. In these studies, the arterial LDL residence times in normolipidemic and hyperlipidemic rabbits before lesion formation were similar in both the lesion-prone and -resistant sites. However, immediately upon development of early fatty streak lesions, the arterial LDL residence time increased dramatically. After only 16 days of cholesterol feeding, the residence time was 10 times longer in the lesioned aortic arch compared with similar tissue from normolipidemic rabbits (4 to 45 hours). After 21 days of cholesterol feeding, the residence time of LDL in the lesioned aortic arch increased to > 25-fold that of normolipidemic tissue. Similar results were observed in the lesioned tissue of the abdominal branchings. This early retention of LDL suggests that significant changes are taking place within the arterial wall during this critical stage of early lesion development.