Destruction of endogenous low density lipoprotein in incubated intima

Glycosaminoglycans and apolipoproteins B and A-I in human aortas. Chemical and immunological analysis of lesion-free aortas from children and adults

To study changes in the contents of plasma lipoproteins in human arteries with age and the relationship of lipoproteins with other arterial constituents, we analyzed the contents of apolipoproteins B (apo B) and A-I (apo A-I), free and esterified cholesterol, and glycosaminoglycans (GAG) in lesion-free aortic intimas of 30 children and adults. The content of apo B increased significantly with age, whereas that of apo A-I remained relatively constant. Apo B and apo A-I had significant positive correlations with the content of chondroitin sulphates A + C (CS A + C), which comprised 35% to 47% of the aortic GAG. The correlations remained significant after correction for the effect of age. Aortic apo B, but not apo A-I, also showed significant positive correlations with the contents of intimal free and esterified cholesterol. The results indicate that: considerable amounts of apo B and apo A-I can be found in lesion-free aortic intimas; there is an age-related rise in the content of apo B and a fall in the ratio of apo A-I to apo B, which are unfavorable developments in the light of current views on atherogenesis; the contents of the apolipoproteins are proportional to that of CS A + C, which might have a role in the retention of lipoproteins in the arteries.

Effect of post-mortem time on the biochemical composition of coronary arteries

The effects of post-mortem autolysis on the biochemical composition of coronary intima-medias were studied by keeping the coronary arteries of 25 slaughterhouse pigs at +4 degrees C for periods up to 7 days. The samples were analysed for DNA, total protein, collagen, glycosaminoglycans (GAGs), free and esterified cholesterol, triglycerides, phospholipid fractions, and fatty acid composition in various lipid classes. The content of sulphated GAGs decreased steadily, whereas that of hyaluronic acid remained unchanged. These alterations led to a significant decrease in the content of total GAGs and to an increase in the percentage of hyaluronic acid in total GAGs. The results indicate a continuous post-mortem degradation of arterial proteoglycans. After 7 days storage, there was a significant increase in the content of free fatty acids, and the composition of this fraction was changed by increases in the relative amounts of oleic and linoleic acids. These changes were accompanied by an increase in lysolecithin and a decrease in lecithin, which suggests a post-mortem action of arterial phospholipases.

Plasma protein concentrations in interstitial fluid from human aortas

The concentration of plasma proteins was examined in interstitial fluid collected from human aortas, obtained at autopsy, from patients from whom a blood sample had been taken for routine analysis shortly before death. The interstitial fluid was absorbed onto small, preweighed pieces of filter paper which were inserted into natural strip planes in the tunica intima and inner tunica media. After equilibration the papers were removed and weighed to measure the amount of interstitial fluid collected, then analysed by quantitative immunoelectrophoresis for three plasma proteins covering a range of molecular masses ( M r ): low density lipoprotein (LDL) with molecular mass 2.4 x 10 6 , α 2 -macroglobulin ( M r 720 000) and serum albumin ( M r 68 000). In interstitial fluid obtained from normal intima of 20 men and women, the mean levels of the proteins were LDL 215%, α 2 -macroglobulin 115% and albumin 54% of the concentration in the patient’s own plasma. Concentrations were not influenced by depth within the intima, or by distance down the aorta. The patients covered the age range 31-96 years; relative concentrations of LDL, α 2 -macroglobulin and albumin increased in parallel by about 10% per decade. Large samples (up to 7 μl) of interstitial fluid were collected from inner media but they contained no measurable LDL; the levels of α 2 -macroglobulin and albumin were respectively 11 and 18% of plasma concentration. Analysis of interstitial fluid from one sample of normal intima obtained at vascular surgery, and from two freshly killed pigs, suggested that results were not invalidated by the use of autopsy material. Direct comparison of whole intimal tissue and interstitial fluid provided no evidence of preferential binding of LDL in tissue. There was no evidence of preferential adsorbtion of LDL by the filter paper. Interstitial fluid of six samples of normal intima from four patients was compared with normal plasma by two-dimensional immunoelectrophoresis. The peaks produced in the antiserum to LDL were identical in mobility, shape and staining properties. Thus the concentration of LDL found in interstitial fluid from normal aortic intima was more than twice the plasma concentration, and the relation between concentration and molecular mass was the inverse of that reported for peripheral interstitial fluid and lymph. Endothelium appears to trap LDL in the intima instead of keeping it out, and this raises fundamental questions about the function of arterial endothelium.

Role of endothelium in sequestration of lipoprotein and firbrinogen in aortic lesions, thrombi, and graft pseudo-intimas

In the soft, gelatinous thickenings that appear to be the precursors of fibrous plaques, concentrations of plasma low-density lipoproteins (LP) and fibrinogen were three to four times higher than that in normal intima, but their concentrations of fibrin was not significantly higher. By contrast, in 60% of cholesterol-rich areas from more advanced plaques, fibrin (mean concentration 22.7 mg/100 mg lipid-extracted dry weight) was ten times higher than normal, and this was associated with a twelve-fold increase in a bound LP fraction that was released by incubation with fibrinolytic enzymes. The origin of this fibrin is uncertain, and to test similarity of fibrin in the lesion with that of mural thrombus free and bound LP, fibrinogen, and cholesterol were measured in thrombi and pseudo-intimas from prosthesis grafts. In the pseudo-intimas and the thrombi that were not covered with endothelium the concentrations of all these substances were very low, but in thrombi covered with endothelium the concentrations of free and bound LP were similar to those in intimal lesions. Thus, endothelialisation appears to promote accumulation of LP in mural thrombi.

Characterization of free and tightly bound lipoprotein in intima by thin layer isoelectric focusing

Thin layer isoelectric focusing followed by second dimension quantitative immunoelectrophoresis has been used to characterize the free and tightly bound lipoprotein (lp) fractions in human aortic intima. In 3.3% acrylamide gels plasma low density lipoprotein (LDL) focuses rapidly, but very low density lipoprotein (VLDL) fails to enter the gell and remains at the origin (point of application). Samples of normal intima and lesions were placed directly on the gel and focused for 10--12 h; in normal intima and gelatinous lesions which had not accumulated cholesterol 60--70% of the free LP focused with plasma LDL, but with increasing accumulation of cholesterol the proportion focusing with LDL decreased and the component remaining at the origin increased. In 9 of the 39 samples examined one or more components with pI acid to LDL were present, and in one sample the whole peak focused at a more acid pI. In 4 of 5 lipid-rich centres of gelatinous plaques all the free LP remained at the origin, and the bound LP released by incubation with plasmin remained at the origin in all samples in 3.3% gels although it would focus in 2.6% gels. With antiserum to apo-C this large LP did not behave like VLDL, and it appears to be an aggregated LDL. An increased proportion of aggregated LDL was associated with (a) accumulation of cholesterol, (b) topographical location towards the centre of plaques, and (c) partial destruction of endogenous LP during incubation of fresh samples of tissue.