Vasorelaxant and antiaggregatory properties of the endothelium: a comparative study in normocholesterolaemic and hereditary and dietary hypercholesterolaemic rabbits

The effect of reactive oxygen species on whole blood aggregation and the endothelial cell-platelet interaction in patients with coronary heart disease

Background

The effect of reactive oxygen species (ROS) on platelet function in coronary heart disease (CHD) is complex and poorly defined. Platelet aggregation studies in healthy volunteers have demonstrated contrasting results when platelets are exposed to ROS. We investigated the effect of ROS on whole blood aggregation (WBA) and the endothelial cell-platelet interaction in patients with CHD.

Methods and Results

ROS generated by xanthine and xanthine oxidase caused a concentration-dependent inhibition of WBA in blood from healthy donors and patients with CHD. In patients with CHD, 100 μM xanthine and 100 mU/ml xanthine oxidase inhibited WBA in response to 3 μg/ml collagen by 28.9% (95% CI 15.9%-41.8%, p < 0.001) and in response to 5 μM ADP by 36.0% (95% CI 9.6%-62.4%, p = 0.005). Using nitrotyrosine expression, platelets isolated from patients with CHD were found to be susceptible to peroxynitrite damage. The addition of 1 × 10⁵ cultured endothelial cells inhibited WBA in response to 3 μg/ml collagen by 31.2% (95% CI 12.2%-50.2%, p < 0.05) and in response to 5 μM ADP by 31.6% (95% CI 2.5-60.7%, p < 0.05). Addition of xanthine and xanthine oxidase did not alter this effect, however pre-treatment of endothelial cells with a nitric oxide synthase inhibitor (L-NAME) partly reversed the inhibition.

Conclusion

ROS inhibit WBA in blood from patients with CHD. Whilst endothelial cells also inhibit WBA, the effect is attenuated by L-NAME, suggesting that nitric oxide is likely to remain an important protective mechanism against thrombosis in CHD.

Can we extrapolate the outcomes of in vitro studies on murine endothelium to studies of human platelet-endothelium interactions? A technical note

INTRODUCTION

Interactions between vascular endothelium and blood platelets play a crucial role in cardiovascular diseases. Ex vitro models which use endothelial cells and platelets were the essential tools to investigate these interactions and their impact on haemostasis. The impaired interplay between vascular endothelium, blood platelets and leukocytes is believed to contribute to the development of cardiovascular disease. In this study we compared the ability of human (HUVECs) and murine (HECa10) endothelial cells to inhibit human platelet function and reactivity under in vitro conditions.

MATERIAL AND METHODS

The aliquots of platelet-rich plasma obtained from 20 healthy donors were incubated with murine endothelial cell line HECa10 or human umbilical vein endothelial cells (HUVECs) (10 min, 37°C) prior to agonizing platelets with 5 µM ADP and monitoring platelet reactivity for 10 min using optical aggregation.

RESULTS

Significant reduction in ADP-induced platelet aggregation in the presence of endothelial cell cultures remained independent of cell count. HUVECs appeared much more effective in the inhibition of platelet aggregation compared to HECa10 (35.2 ±2.3 AU vs. 43.7 ±2.0 AU, p= 0.025).

CONCLUSIONS

HECa10 cells have much lower potential to inhibit platelet aggregation than HUVECs. This implies that these two cell lines may not be freely used interchangeably in in vitro experiments. These findings clearly indicate that the outcomes of in vitro studies performed with murine EC lines cannot be unreservedly extrapolated to human platelet-endothelium interactions.

N,N-Diacetyl-l-cystine Improves Endothelial Function in Atherosclerotic Watanabe Heritable Hyperlipidaemic Rabbits

N,N-diacetyl-L-cystine (DiNAC), a novel immunomodulator, stimulates contact sensitivity/delayed type hypersensitivity reactions in mice induced by oxazolone and reduces atherosclerosis in Watanabe heritable hyperlipidaemic (WHHL) rabbits. Forty-week-old WHHL rabbits were given DiNAC (3 micromol/kg per day) for 8 weeks, and endothelium-mediated dilatation was investigated in vivo using pulse wave analysis. A significant improvement in endothelial function was found after 3 weeks of treatment, which was further improved after 8 weeks. For experiments on isolated blood vessels, 40-week-old rabbits were treated for 3 weeks. Treatment did not affect plasma lipid levels. At termination, aortic rings from the thoracic and abdominal aorta were contracted with phenylephrine in vitro. Concentration-effect curves to acetylcholine and the calcium ionophore A 23187 were used to measure endothelium-mediated vasodilatation, and nitroprusside to elicit endothelium-independent relaxations. Abdominal aorta relaxations were generally larger than in thoracic aorta. DiNAC improved endothelium-dependent relaxations in the abdominal but not in the thoracic aorta. This effect was independent of the degree of atherosclerosis. It is concluded that DiNAC improved endothelial function in atherosclerotic rabbit arteries in vivo and in vitro, and may represent a new treatment modality for atherosclerosis-related diseases.

The role of cholesterol on the pressure sensing ability of kidneys in rats

We studied the effect of hypercholesterolemia on the pressure-sensing and regulating ability of the kidneys, using an acute hemorrhage model to provide 40% and 60% reduction in the blood pressure of hypercholesterolemic and control rats. The control group (n = 22) was fed a normal rat pellet diet and tap water; the experimental group (n = 22) received a diet containing 2% cholesterol/0.2% thaurocholate. Half the animals were subjected to 6 mL/kg bw and the others to 12 mL/kg bw of bleeding for 1 min. Blood pressure recording and proper samplings were done before bleeding and during the 20 min post-hemorrhagic period for analysis. Despite a finding of hypercholesterolemia in the experimental group, kidney cholesterol content as well as its function remained unchanged. Following an initial 40% decrease in rats bled 6 mL/kg bw, 20 min later the mean blood pressure returned to 90% of its initial value in control rats and to 70% of its basal level in hypercholesterolemic rats. A similar delay in pressure normalization occurred in rats subjected to 12 mL/kg of bleeding. Plasma renin activity remained unaffected. We conclude that dietary hypercholesterolemia delays the normalization of blood pressure after hemorrhage without affecting the sensing ability of kidneys, and that the kidneys are less sensitive than other organs to plasma cholesterol levels.

Involvement of cholinoceptors in cadmium-induced endothelial dysfunction

Cadmium (Cd) toxicity was produced in male rats to study the role of cholinoceptors in Cd-induced endothelial dysfunction. The changes in the tension of the aortic rings to constrictor and dilator agonists were compared with those of controls. A Cd-induced significant increase in phenylephrine response was associated with a decrease in basal dilator prostanoid release. In Cd-exposed rings, despite an obvious depression in the acetylcholine (ACh) response, the receptor-independent dilation to the calcium ionophore A23187, which elicits a receptor-independent endothelial relaxation, was slightly elevated (p<0.01), but the smooth muscle cell response to the NO donor, sodium nitroprusside (SNP) remained unaltered. Cadmium decreased both the maximal response to ACh (10(-5) M) and its pirenzepine (Prz) sensitive component. The M1 type cholinoceptor-mediated response to ACh decreased in Cd-exposed rings to 10.30 +/- 5.00% from 38.40 +/- 6.90% (p<0.001). Cadmium also reduced the share of indomethacin 1.64% to 13.92 +/- 2.89% (p<0.01), which correlated well with the changes in the M1-mediated response (r=0.991, p<0.0001). Most of the deleterious effect of Cd appears to be restricted to the M1-dependent ACh response. These findings suggest that Cd produces an endothelial dysfunction by impairing the M1 type cholinoceptor mediated response, which seems to be involved in prostanoid release.

Lipids and endothelium-dependent vasodilation--a review

Studies using both in vitro and in vivo techniques have repeatedly shown that endothelium-dependent vasodilation (EDV) is impaired in different forms of experimental as well as human hypercholesterolemia. Clearly this impaired EDV can be reversed by lowering cholesterol levels by diet or medical therapy. Competitive blocking of L-arginine, changes in nitric oxide synthase activity, increased release of endothelin-1, and inactivation of nitric oxide due to superoxide ions all contribute to the impairment in EDV during dyslipidemia. The oxidation of low density lipoprotein, with its compound lysophosphatidylcholine, plays a critical role in these events. However, data on the role of triglycerides and fat-rich meals regarding EDV are not so consistent as data for cholesterol, although a view that the compositions of individual fatty acids and antioxidants are of major importance is emerging. Thus, this review shows that while impaired EDV is a general feature of hypercholesterolemia, the mechanisms involved and the therapeutic opportunities available still have to be investigated. Furthermore, discrepancies regarding the role of triglycerides and fat content in food may be explained by divergent effects of different fatty acids on the endothelium.

The effects of L-arginine on neointimal formation and vascular function following balloon injury in heritable hyperlipidaemic rabbits

OBJECTIVE

The aims of this study were to determine the morphological and functional consequences of balloon angioplasty of the left subclavian artery of Froxfield heritable hyperlipidaemic (FHHL) rabbits and the influence of oral L-arginine therapy on these changes.

METHODS

Sixteen-week-old FHHL rabbits were subjected to balloon injury of the left subclavian artery under halothane anaesthesia. Control rabbits (n = 7) were given free access to food and normal tap water. L-Arginine-treated rabbits were given L-arginine (5 g.l-1 in the drinking water for 2 days prior to angioplasty and then for either 2 weeks (n = 7) or 4 weeks (n = 7) after surgery. All rabbits were euthanised 28-30 days after surgery and blood and tissue removed for quantification of neointimal size and determination of endothelial function using isolated vessel tension studies. The ability of the endothelium to prevent platelet aggregation was determined by challenging a vessel ring with carbachol when incorporated into a whole blood sample in which platelet aggregation was induced with collagen.

RESULTS

Balloon injury in non-treated rabbits resulted in the development of marked intimal hyperplasia (18.8[3.6]% of the area within the internal elastic lamina) while endothelial function remained intact. Maximum responses to carbachol and calcimycin were, respectively, a 66.6[14.7]% and 46.9[12.9]% relaxation of 5HT-induced tone, compared to 58.0[3.2]% and 39.8[9.4]% in non-injured vessels. Maximum contractile responses to 5HT and KCl were unaffected by injury. L-Arginine therapy alone had no effect on the vasodilator function of the endothelium, but reduced the endothelium-dependent inhibition of platelet aggregation (68.4[7.8] vs 109[10]% of the maximum extent of platelet aggregation in non-treated and 2-week L-arginine-treated non-injured vessels, respectively). L-Arginine significantly reduced the extent of neointimal formation (7.2[3.9]% of the area within the IEL; P < 0.05 vs. non-treated group). However, L-arginine significantly attenuated the relaxant responses to both carbachol (26.5[10.4]% and 31.4[9.4]% for 2- and 4-week L-arginine groups) and calcimycin (38.7[15.4]% and 16.4[10.7]%) in the injured artery (P < 0.05 compared to non-treated controls).

CONCLUSIONS

L-Arginine reduces neointimal formation following balloon catheter injury in heritable hypercholesterolaemic rabbits, which is consistent with previous findings in normocholesterolaemic models. However, in the presence of hypercholesterolaemia, L-arginine has a detrimental effect on endothelial function following injury. This may be a consequence of the presence of lipids in the vascular wall on nitric oxide synthase activity.

Efficacy of simvastatin and pumpkin-seed oil in the management of dietary-induced hypercholesterolemia

Pumpkin-seed oil (PSO), a natural supplement rich with antioxidants and polyunsaturated fatty acids (PUFAs), was given in combination with simvastatin, as antihypercholesterolemic drug, to high cholesterol-fed rabbits, for three weeks. In comparison with normal rabbits, a significant increase of the aortic contractile response to norepinephrine was observed which could be attributed to endothelium dysfunction. In addition, serum levels of total lipids, triacylglycerols, total cholesterol and low density lipoprotein-cholesterol (LDL-C) were increased while phospholipids and high density lipoprotein-cholesterol (HDL-C) were decreased in hypercholesterolemic rabbits. These changes could be related to the predominance of LDL and oxidized-LDL particles caused by high levels of reactive oxygen species during hypercholesterolemia (HC). Treatment with simvastatin modulated most of the altered parameters affected during HC that might be, in part, due to inhibition of cholesterol biosynthesis. While concomitant administration of simvastatin and PSO succeeded to cause marked reduction of the aortic contractile response to norepinephrine and to normalize the most adverse effects observed during HC. These effects were explained by the potentiating effects of simvastatin with antioxidants and essential fatty acids in PSO. On the contrary, serum activities of aminotransferases and creatine phosphokinase were increased with simvastatin treatment but not with the combination therapy in hypercholesterolemic rabbits.