Oxidized lipoproteins, altered cell function and atherosclerosis

A correlation between atherogenesis and lipoprotein oxidation was first suggested by experiments showing increased uptake by macrophages of oxidized LDL and oxidized LDL injury to cultured cells. Recent data which demonstrate the existence of oxidized lipoproteins in vivo, combined with studies showing a 'protective' effect of antioxidants against atherosclerosis progression, have greatly increased the interest in theories posing that lipoprotein oxidation is causally related to arterial disease. The fact that dozens of new compounds are produced upon the oxidation of low density lipoprotein has led, perhaps not surprisingly, to numerous discoveries in vitro of altered cell function induced by exposure of cells to oxidized LDL that are distinct from those resulting from exposure to native LDL. This brief overview will describe selected altered cell functions of oxidized lipoproteins and how they may impact on atherosclerosis.

Relative significance of endothelium and internal elastic lamina in regulating the entry of macromolecules into arteries in vivo

A role for the internal elastic lamina (IEL), which separates the intima and media of an artery wall, as a restrictive barrier to macromolecular movement has been suggested in atherosclerotic lesion development or restenosis during angioplasty. The permeability coefficient of the IEL, however, has never been quantified in unperturbed vessels in vivo. Using a newly developed technique, we measured the concentration distributions in both intima and media of cationic (pI approximately 8.5) and anionic (pI approximately 6.3) isozymes of the 44-kD macromolecule horseradish peroxidase (HRP). Two mathematical models of arterial wall transport differing in their resolution of the intima were required to simulate the concentration distribution data and to estimate the parameters of interest. Optimal estimates of the permeability coefficients of the endothelium (PE) and IEL (PIEL) to HRP were determined by the best least-squares fit of the two models to experimental data. These estimates (anionic: PE = 0.050 +/- 0.021 microns/min, PIEL = 0.146 +/- 0.082 microns/min, n = 8; cationic: PE = 0.034 +/- 0.018 microns/min, PIEL = 0.110 +/- 0.047 microns/min, n = 8) indicate that the IEL is responsible for approximately 25% (anionic, 26 +/- 9%; cationic, 25 +/- 13%) of the resistance to HRP transport from the blood into the arterial media. Although both parameters were less for the cationic preparation, the differences were not significant, and the relative role of the IEL was similar for both molecules.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of hypertension and aging on coronary arteriolar density

Coronary reserve has been shown repeatedly to be depressed in hypertension and aging. The underlying mechanisms remain elusive, but structural alterations of the coronary vasculature have been implicated. In this study, we measured maximal coronary dilator capacity and structural characteristics relevant to coronary resistance in aging normotensive (Wistar-Kyoto, n = 22) and spontaneously hypertensive rat (SHR) strains (n = 25) at 1.5, 4, 11, 16, and 22 months of age. Coronary flow measurements, using radiolabeled microspheres, demonstrated a significant (p < 0.01) hypertension- and age-related decline in maximal coronary dilator capacity. After flow measurements, vascular dimensions and arteriolar density were obtained from 1-micron sections prepared from perfusion-fixed hearts. A total of 10,012 arterioles were analyzed, 4,820 in hypertensive and 5,192 in normotensive rats. There was an 18-28% reduction in arteriolar density in hypertensive rats that specifically affected the terminal arteriolar bed at 1.5-11 months. However, the decrement in arteriolar density stabilized at 10% and 6% in 16- and 22-month-old hypertensive rats, respectively. Arteriolar density was not affected by aging. In both strains, there was a significant (p < 0.01) age-related decrease in the ratio of lumen diameter to wall thickness in arterioles > 50 microns. In addition, there was an overall 30% decrease (p < 0.01) in the ratio of lumen diameter to wall thickness in hypertensive compared with normotensive rats. These data indicate that both hypertension and aging are accompanied by structural alterations of the coronary resistance vasculature. These structural alterations may contribute to the depression in coronary reserve that complicates hypertension and aging.

Effect of Buckberg cardioplegia and peripheral cardiopulmonary bypass on infarct size in the closed chest dog

OBJECTIVES

To simulate a human catheterization laboratory setting of controlled reperfusion during myocardial infarction, regional infusion of commercially available Buckberg cardioplegic solution and peripheral vented bypass were administered in the closed chest dog.

BACKGROUND

Studies in open-chest dogs have demonstrated a significant reduction in infarct size and improvement in regional wall motion with a similar controlled reperfusion method using infusion of substrate-enriched (Buckberg) cardioplegic solution during cardiopulmonary bypass coupled with left ventricular venting.

METHODS

After 100 or 180 min of balloon occlusion of the proximal left anterior descending artery, controlled reperfusion was performed with cardioplegic infusion and vented bypass. Dogs matched for occlusion time underwent balloon deflation without bypass or cardioplegia (uncontrolled reperfusion groups). Microspheres were used to quantify coronary ischemia during balloon inflation. All four groups (n = 8 to 9 per group) were followed up at 1 week to determine regional wall motion and infarct size.

RESULTS

Qualitative echocardiographic analysis demonstrated no significant difference among groups in recovery of regional wall motion at 1 week; however, wall motion improved significantly in all groups between the ischemia and 1-week recovery periods. The histologic infarct size compared with the area at risk for dogs with uncontrolled versus controlled reperfusion, respectively, was 17.9 +/- 10.5% versus 31.9 +/- 8.3% (p < 0.05) for dogs with 100 min of occlusion and 40.1 +/- 11.7% versus 46.2 +/- 8.4% (p = NS) for dogs with 180 min of occlusion. A greater rate-pressure product in the dogs with controlled reperfusion after 100 min of occlusion (p < 0.05) may explain the larger infarct size observed for that group.

CONCLUSIONS

These results demonstrate that regional infusion of substrate-enriched cardioplegic solution in combination with peripheral vented bypass does not further reduce infarct size after prolonged ischemia in the closed chest dog (compared with uncontrolled reperfusion).

Lipoprotein oxidation and lipoprotein-induced cell injury in diabetes

There is ample evidence that oxidized lipoproteins exist in vivo, not only in atherosclerotic lesions, but also associated with some experimental models of diabetes. Whether the lipoprotein oxidation is an epiphenomenon of other atherogenic or diabetogenic agents or processes or whether it is causally related to lesion formation in atherosclerosis or other forms of tissue damage in people with diabetes is unresolved. Intense interest in testing these ideas derives from in vitro observations of the ways in which oxidized lipoproteins interact with cells that are unlike the interactions with native lipoproteins. Many of these altered interactions suggest known features of atherosclerotic lesions, and recent data show that antioxidant treatment reduces the progression of vascular lesions. There are reasons to believe that hyperglycemia may worsen lipid and lipoprotein oxidation. If this observation is the case in vivo, and if it is ultimately proved that lipoprotein oxidation facilitates lesion development, these events may help explain the accelerated atherosclerosis suffered by diabetic patients. The multiple pathways for which there is evidence that hyperglycemia may contribute to oxidative events--for example, by enhancing free radical production in stimulated inflammatory cells or by forming glycation products that can propagate free radical events--suggest avenues for further research and may ultimately indicate points for intervention in the various manifestations of the disease.

Vascular injury by endotoxin: changes in macromolecular transport parameters in rat aortas in vivo

Vascular injury can lead to enhanced macromolecular transport into the arterial wall. We previously demonstrated that lipopolysaccharide (LPS) -induced injury to rat aorta in vivo caused increases in intimal and medial horseradish peroxidase (HRP) accumulation. In the present study, we quantitatively interpret these LPS-induced changes in HRP transport parameters. The parameters of interest are the permeability (PL) of the luminal blood-tissue boundary (combination of endothelium and internal elastic lamina, IEL), the effective diffusivity (D), and the convective velocity (V) across the media. The parameter values that yield the best fit of the model to the data provide a basis for understanding the tissue changes. The time of peak transmural (medial) accumulation (24 h after LPS injection) correlated with increases in PL (peak, 12-48 h) and preceded the maximum increase in V (peak, 36 h). The monotonic increase in the intimal accumulation during the 5 days after the injury has a time course distinct from the transient increases in PL and from the changes in D, which implies that endothelial permeability has only limited influence on transport beyond the intima. These data implicate the IEL as a barrier to macromolecular transport in the normal aorta and demonstrate that the endothelium and IEL work in concert to determine intimal macromolecular accumulation.

Relation between lipopolysaccharide-induced endothelial cell injury and entry of macromolecules into the rat aorta in vivo

Lipopolysaccharide (LPS) causes endothelial cell injury both in vitro and in vivo. It is widely believed that this injury in vivo enhances the transport of macromolecules from plasma into the interstitial space of the underlying artery wall. A new technique was used in rats to obtain high resolution transmural profiles of macromolecules in vivo. We compared the time course of the macromolecular transport into the aortic tissue in vivo after LPS injection to that of LPS-induced endothelial cell death and the proliferative response of the endothelium to LPS injury. At a dose of 1 mg LPS/kg body wt, endothelial cell death reached a maximum by 36 hours after LPS injection and remained elevated for 96 hours; the peak of the S phase of endothelial cell proliferation was observed 48 hours after injection. To examine the effect of LPS on macromolecular accumulation, we measured aortic intimal and medial transmural concentration profiles of horseradish peroxidase (HRP) after circulation of HRP for 15 minutes. The data revealed a transient increase in total aortic accumulation (reflecting predominantly the media), which was maximal between 12 and 48 hours after LPS injection. Although total medial accumulation was found to return to near control levels by 72 hours after LPS injection, intimal accumulation remained elevated above control levels for 120 hours. When HRP was added to the perfusate of an in situ aorta preparation at a near zero transmural pressure gradient, the resulting transmural concentration profiles across aortas from control rats and from rats given LPS 24 hours previously were indistinguishable, whereas a pressure gradient of 60 mm Hg revealed LPS-altered concentration profiles analogous to those in vivo. This suggests that the accumulation of HRP observed in vivo was driven by increased convective transport. These results reveal that LPS enhances entry of macromolecules into the aorta wall in vivo. The changes in macromolecular transport do not, however, correlate temporally with endothelial cell death or proliferation. The results are consistent with an LPS-induced decrease in the endothelial barrier function, which precedes, and may be independent of, cell death and a transient increase in convective transport across the media due to alterations in the barrier function of the internal elastic lamina.

Visualization and quantification of transmural concentration profiles of macromolecules across the arterial wall

Transport parameters that describe a macromolecule entering the arterial wall from plasma can be obtained from concentration profiles of the labeled macromolecule entering the tissue. A new technique has been developed for measuring such concentration profiles, which offers spatial resolution superior to methods that measure profiles of radiolabeled macromolecules by serially sectioning tissue in planes parallel to the endothelium. In addition, this new method preserves cellular organization and tissue structure and permits measurement of concentration profiles underlying focal endothelial injuries or vascular lesions. The technique quantifies the concentration of a protein by measuring associated peroxidase activity. Although the present study was performed using horseradish peroxidase (HRP), the same principles can be applied to other macromolecules linked to HRP or microperoxidase. The colored reaction product of HRP was detected in transverse aortic sections using an image processing system. In the present study, profiles obtained by this new method were validated by comparison with HRP concentration profiles in rat aortas obtained by a serial slicing technique using radiolabeled HRP. We used the technique to measure high-resolution HRP concentration profiles in the intima and media of normal animals. These concentration profiles suggest that the internal elastic lamina acts as a major barrier to transport of macromolecules across the wall of the normal rat aorta. The new method should allow concentration profiles for macromolecules to be quantified in tissue surrounding vessels in the microcirculation, within the thickened intima of large vessels, and across coronary artery walls.