Michael Schwemmer

Postprandial hypertriglyceridemia impairs endothelial function by enhanced oxidant stress

AIMS it appears that hypertriglyceridemia (HTG) is a risk factor of atherosclerosis as demonstrated by recent studies. In this study, we analyzed the effects of acute HTG on endothelial function and oxidative stress, which are important mechanisms in the pathogenesis of atherosclerosis. METHODS AND RESULTS in a high fat meal group (n = 11), serum triglycerides and PMA-activated leukocyte O(2)(-)* production were significantly (P < 0.005) increased from 146 +/- 69 mg/dl and 4.09 +/- 0.93 nmol/10(6) cells/min preprandially to 198 +/- 88 mg/dl and 5.49 +/- 1.19 nmol/10(6) cells/min, respectively, 2 h after eating a high-fat meal. The flow-mediated endothelium-dependent brachial artery dilation (FMD; high-resolution ultrasound) was decreased from 13.7 +/- 3.3% preprandially to 8.2 +/- 3.7%, 2 h after eating a high-fat meal (P < 0.005). However, following a low-fat meal (n = 9), there were no significant changes in triglycerides, leukocyte O(2)(-)* production and FMD. Changes of serum triglycerides were correlated negatively (r = -0.650, P < 0.005) with changes of FMD, but were correlated positively (r = 0.798, P < 0.001) with changes of leukocyte O(2)(-)* production, which - in turn - were correlated negatively (r = -0.784, P < 0.001) with changes of FMD in all study subjects (mean age: 56 years, n = 20). CONCLUSIONS this study suggests that acute HTG causes endothelial dysfunction via enhanced oxidant stress and this may pave the way for the development of atherosclerosis under chronic conditions.

Postprandial hypertriglyceridemia-induced endothelial dysfunction in healthy subjects is independent of lipid oxidation

BACKGROUND To analyze the effects of postprandial hypertriglyceridemia with or without antioxidant supplementation-on endothelial function as related to lipid oxidation in healthy young subjects. METHODS AND RESULTS Ten healthy male subjects (mean age: 26 years) were examined three times in fasting state (10 hours) following a high-fat meal, a low-fat meal, or a high-fat meal with additional antioxidant vitamin E (800 IU), respectively. Serum triglycerides significantly increased 2 and 4 hours after eating the high-fat meal with or without additional vitamin E. Endothelium-dependent, flow-mediated brachial artery vasodilations (FMD; percentage change in diameter) changed from 13.3+/-1.1% to 6.6+/-1.1% (p<0.05), 7.1+/-0.6% (p<0.05), or 13.2+/-0.8% at 2, 4, or 6 hours after eating a high-fat meal. However, there were no changes of FMD observed following either a low-fat meal or a high-fat meal with additional vitamin E. The flow-dependent vasodilation inversely correlated to postprandial hypertriglyceridemia (r=-0.54, p<0.05). Serum malondialdehydes (MDA; lipid oxidation products) did not significantly change following ingestion of any of the 3 types of meal. CONCLUSIONS This study suggests that postprandial hypertriglyceridemia-induced endothelial dysfunction is not associated with lipid oxidation and that the protective effects of vitamin E on endothelial function may be due to some alternative, as of yet unknown, mechanism.

Impact of postprandial hypertriglyceridemia on vascular responses in patients with coronary artery disease: effects of ACE inhibitors and fibrates

We analyzed vascular responses (endothelial function, oxidant stress) to postprandial hypertriglyceridemia (PHTG) in patients with coronary artery disease (CAD) to reveal potential therapeutical effects of angiotensin converting enzyme inhibition (ACE-I) and of lipid lowering (fibrate). The study population (n=39, mean age: 60 years) consisted of four groups, all of which had angiographically documented CAD. A high fat group (n=9) consumed a high fat meal, a low fat group (n=9) a low fat meal, and ACE-I (n=10) or fibrate (n=11) groups consumed a high fat meal plus lisinopril or fenofibrate. Serum triglycerides (TG) increased significantly 2 h after eating a test meal in all groups with the exception of the low fat group. In the high and low fat groups changes of serum TG were positively correlated (r=0.664, P<0.005) with changes of phorbol ester-activated leukocyte superoxide anion radical (O(2-.)) formation and were negatively correlated (r=-0.488, P<0.05) with flow-mediated brachial artery dilation (FMD). There was a negative correlation (r=-0.419, P=0.094) between FMD and changes of O(2-.) formation in the high and low fat groups. In the ACE-I and fibrate groups, O(2-.) formation decreased 2 h after eating a test meal (from 5.34+/-1.01 to 3.81+/-1.15 nmol/10(6)cells per min, P<0.01, and from 4.66+/-0.91 to 4.26+/-0.97 nmol/10(6)cells per min, P=0.374, respectively). However, endothelial function did not show any significant changes 2 h after eating a test meal in all groups. PHTG increases oxidant stress and further deteriorates endothelial function, even in patients with CAD. Both ACE-I and fibrates have an antioxidant effect but no acute beneficial effects in terms of endothelial function under conditions of PHTG in CAD patients.

New approaches to overcome tolerance to nitrates

In isolated cells (vascular smooth muscle, endothelium, platelets), perfused hearts, in vivo experiments, conscious instrumented animals, and in human subjects the induction of tachyphylaxis and tolerance to various exogenous NO-donors was analyzed. Various ways to circumvent tolerance were successfully tested.Different nitrovasodilators were associated with different rates and magnitudes of generation of tolerance and reactive oxygen radicals (ROS) in all models tested, beginning with PETN (pentaerithrityltetranitrate) (lowest rate) and concluding with GTN (highest rate). This pattern was found in all models tested (isolated cells, perfused organs, and in vivo experiments). The observed changes in ROS production in isolated cells were identical to changes in ROS production in vascular smooth muscle, endothelial cells, and platelets. Thus, blood cells such as washed platelets could be used as marker cells to identify induction of tolerance and rise in platelet activity, closely reflecting changes in the rate of tolerance generation to nitrates associated with enhanced oxidant stress (ROS generation).Generation of tachyphylaxis could be suppressed or even avoided by supplementation of appropriate antioxidants (SOD, vitamin C, DMSO, ß-blockers with antioxidant capacity, modulators of prostanoid metabolism such as ASS) in all models tested, including human subjects. Even fully developed tolerance (during non-intermittent GTN-administration) could be reversed by starting an appropriate antioxidant supplementation. This indicates that other potential factors involved in the generation of nitrovasodilator-associated tolerance (reducing the intended vasodilation and the concovactent decreases in blood pressure, namely augmented sympathetic and RAS-activity, changes in the activity of soluble guanylyl cyclase, protein kinase C, phosphodiesterase, etc.) are of minor importance. Thus, the treatment of tolerance under clinical conditions should closely target changes in redox potential and antioxidant capacity.

Tolerance to nitrates with enhanced radical formation suppressed by carvedilol.

Enhanced oxidant stress occurs under many pathophysiologic conditions (e.g., inflammation) and can be induced and mimicked by continuous nitrate therapy, eliciting increases in platelet activity, enhanced formation of reactive oxygen species (ROS), and impaired nitrate-induced vasorelaxation. Analysis was performed of effects of coinfusion of glycerol trinitrate (GTN) either with a carvedilol metabolite with antioxidant properties or with antioxidant vitamin C (Vit-C) on various hemodynamic parameters during enhanced oxidant stress associated with nitrate tolerance. Carvedilol metabolite (BM910228: 4.5 microg/kg/min) or Vit-C (55 microg/kg/min) was coadministered with GTN (1.5 microg/kg/min) for 5 days in chronically instrumented dogs. Changes in coronary diameters (CD) and other hemodynamic parameters were continuously monitored, as well as changes in platelet function. At the beginning of GTN treatment, CD increased by 9.8 +/- 0.4% and progressively declined to basal control values within 3 days. However, with additional antioxidant protection either with BM910228 or with Vit-C, the GTN-induced increase in CD was maintained (8.6 +/- 0.4% or 10.5 +/- 0.6%) and remained elevated for the entire infusion period. The thrombin-stimulated intracellular Ca2+ concentrations of platelets remained nearly unchanged during Vit-C or BM910228 in contrast to the increase with GTN. The basal cyclic guanosine monophosphate (cGMP) contents of platelets after GTN coadministered with BM910228 or with Vit-C increased on day 1 to 233 or to 250% versus control and remained at that level. Additional in vitro tests with xanthine oxidase-induced oxidant stress resulted in a more or less pronounced scavenging of O2- radicals by BM920228, Vit-C, or superoxide dismutase (SOD). Coadministration of carvedilol metabolite BM910228 or of Vit-C along with GTN suppressed noxious effects of GTN-induced oxidant stress such as increased platelet activity and impaired nitrate-induced vasorelaxation.

Angiotensin Receptor Blocker Losartan Suppresses Platelet Activity by Interfering with Thromboxane Signaling

AbstractEnhanced platelet activity and platelet endothelial interaction are hallmarks of different vascular and metabolic diseases with subsequent thrombus formation. In atherosclerosis, coronary artery disease, congestive heart failure, nitrate tolerance, chronic inflammation, or diabetic states, platelet activation may in part be due to a stimulation of the renin-angiotensin-aldosteron system, which also contributes to enhanced oxidant stress in these conditions. Aims. We examined the putative role of the angiotensin receptor (AT1) and of phospholipase A2 (PLA2) in mediating platelet activation under defined in vitro conditions using the AT1 receptor antagonists losartan, EXP 3174, candesartan, and the PLA2 inhibitor arachidonyltrifluoromethyl ketone (AACOCF3), respectively. Results. In washed human or canine platelet suspensions, losartan (10−4–10−6 mol/L) dose-dependently suppressed thrombin-induced calcium transients as well as thromboxane (TxA2) release. In both species, aggregation of washed platelets in response to thrombin or ADP was substantially diminished by different doses of losartan. This inhibition of platelet aggregation was even maintained in ADP-stimulated platelet-rich plasma. While the PLA2 inhibitor AACOCF3 effectively inhibited thrombin-induced TxA2 release from washed human or canine platelets (similar to the effects observed with losartan), the AT1 agonist angiotensin II elicited platelet TxA2 release only at high supra-physiological doses (e.g., at 10−4 mol/L). The AT1 specific antagonist candesartan did not diminish stimulated platelet aggregation, TxA2 formation, or calcium transients. By contrast, the active losartan metabolite EXP 3174 dose-dependently inhibited stimulated platelet calcium transients as well as TxA2 release at 1–100 μmol/L. Conclusions. Losartan significantly counteracts ex vivo platelet activation, probably via the blockade of TxA2 receptor-dependent signaling (e.g. implying activation of phospholipase A2) rather than acting at the AT1 receptor itself. This implies that the TxA2 signaling pathway plays a significant role during platelet activation, which may be successfully antagonized in vivo under different pathological states with enhanced thrombus formation or platelet-endothelium interactions.

Effects of low-dose atorvastatin on vascular responses in patients undergoing percutaneous coronary intervention with stenting

Background: The primary endpoint of this study was to evaluate the effects of low-dose atorvastatin on carotid intima-media thickness (IMT) and endothelial function, and the secondary endpoint comprised restenosis and target lesion revascularization (TLR) in patients undergoing percutaneous coronary intervention (PCI) with stenting for the treatment of coronary artery disease. Methods: Two hundred five consecutive patients (mean age, 60 years) undergoing PCI were prospectively randomized to usual therapy (control group, n = 100) or to 10 mg of atorvastatin daily plus usual therapy (statin group, n = 105). Carotid IMT, endothelial function (flow-mediated dilatation [FMD] of the brachial artery), and coronary angiograms were taken before the study and 6 months after randomization. The 6-month follow-up measurements of the above factors were obtained in 83 patients (83%) of the control group and in 97 patients (92%) of the statin group. Results: No significant differences were noted in the baseline clinical and angiographic findings in either group. FMD was significantly improved during the 6 months in the statin group (4.38% ± 1.7% vs 4.85% ± 1.6%, P = .003), but did not change in the control group. Carotid IMT did not show any significant changes at 6 months in either group. There was a trend in favor of statin in terms of restenosis rate (26.8% vs 36.1%, P = .177) and TLR rate (18.6% vs 25.3%, P = .274). The changes of FMD were significantly correlated with the changes of total cholesterol and the changes of low-density lipoprotein, respectively (r= -0.336, P = .009, and r = -0.310, P = .046). Conclusion: Low-dose atorvastatin reduces endothelial dysfunction as measured by FMD, which coincides with the beneficial effects on lipid profiles, and can decrease restenosis and TLR rate in patients undergoing PCI with stenting.

Blockade of angiotensin signaling improves myocardial function in hypercholesterolemia independent of changes in eicosanoid release.

In hypercholesterolemia in the presence or absence of atherosclerosis, cardiovascular dysfunction and altered signaling of angiotensin, nitric oxide, or prostanoids are closely related to enhanced oxidant stress. We analyzed the potentially beneficial effects of the specific angiotensin-converting enzyme inhibitor enalapril and the specific angiotensin receptor blocker losartan on cardiac performance, eicosanoid metabolism, and parameters of oxidant stress in hypercholesterolemic animals. Guinea pigs were fed a 1% cholesterol diet for 8 weeks (Chol) with or without equieffective doses of either enalapril (1.5 mg/kg/d; Ena) or losartan (3 mg/kg/d; Los). Hemodynamics were analyzed in Langendorff hearts. Detection of eicosanoids was by enzyme immunoassay. Estimation of plasma xanthine oxidase (XO) activity was determined by spectrophotometry. In hypercholesterolemic guinea pigs, enhanced oxidant stress (e.g., increased plasma XO activities) was associated with profound myocardial and coronary (e.g., endothelial) dysfunction. Both enalapril and losartan lowered plasma cholesterol levels slightly, but only the angiotensin receptor antagonist effectively suppressed the increased plasma XO activities (from 11.4 ± 0.7 to 7.6 ± 2.2 U/L), and at the same time decreased the augmented coronary flow (from 26.0 ± 1.0 to 23.0 ± 1.0 mL/min/g tissue) observed in hypercholesterolemic animals. Assessment of left ventricular pressure and contractility (e.g., dp/dtmax) as well as the diastolic relaxation parameter (τ) revealed substantial myocardial dysfunction (systolic and diastolic) in Chol that was more substantially (and comparably) improved during administration of losartan (Los) than during enalapril (Ena). Surprisingly, angiotensin signaling blockade by either antagonist further suppressed the diminished coronary dilator responses to bradykinin (BK; not significant for enalapril) or adenosine (Ado) that was demonstrated in Chol Langendorff hearts [ΔCPPBK/Ado: from 5.0 ± 0.5/0.9 ± 0.1 to 4.4 ± 1.5/0.4 ± 0.1 (Ena) or to 1.9 ± 0.5/0.4 ± 0.1 (Los) cm2 (area under the curve), respectively]. Finally, as expected from control studies using heart preparations from normocholesterolemic guinea pigs, enhanced cardiac release of eicosanoids, prostacyclin, and thromboxane in Chol (0.48 ± 0.03 and 0.6 ± 0.1 ng/min/g) was augmented even further by treatment with enalapril (Ena: 1.6 ± 0.4 and 1.0 ± 0.1 ng/min/g), but was significantly reduced to or below control levels in losartan-treated animals (Los: 0.4 ± 0.1 and 0.2 ± 0.1 ng/min/g). Blockade of angiotensin signaling via angiotensin-converting enzyme inhibition or receptor antagonism—although differentially acting on enhanced cardiac prostanoid metabolism and oxidant stress—efficiently restored proper systolic and diastolic myocardial performance (losartan was more beneficial than enalapril), probably by counterbalancing altered angiotensin II → angiotensin receptor signaling in the cardiovascular system of hypercholesterolemic animals. Impaired coronary vasodilator capacity seems to be irreversible after 8 weeks of a high-cholesterol diet, as shown by the unexpected lack of a dilator effect with both enalapril and losartan.

Potentiation of sildenafil-induced hypotension is minimal with nitrates generating a radical intermediate.

&NA; Recently the new specific phosphodiesterase‐5 inhibitor sildenafil was introduced into therapy for erectile dysfunction. Because of the phosphodiesterase‐5 inhibitor‐induced increases of cyclic GMP in the vasculature, vasodilation in various vascular beds is induced, which in combination with various nitrovasodilators (e.g., when used simultaneously for the treatment of coronary artery disease), may lead to excessive hypotension. Thus nitrovasodilators are contraindicated when sildenafil may be used and reports of a number of accidents have recently been published. We therefore studied the acute interactions of glyceryl trinitrate (GTN), pentaerythritol tetranitrate (PETN), and isosorbide dinitrate (ISDN) with sildenafil in six chronically instrumented conscious dogs for each nitrate to assess the magnitude of blood pressure drops (and compensatory increases in heart rate) during a 24‐h nitrate administration (infusion into the pulmonary artery). Sildenafil (3 mg/kg) was given orally (after a 24‐h fast) 30 min after start of nitrate infusion. GTN, PETN, or ISDN (which follow different steps of metabolic conversion to nitric oxide) were applied at submaximal dosages leading to 90% of maximal coronary artery dilation at 1.5 &mgr;g/kg per min, 0.75 &mgr;g/kg per min, or 6 &mgr;g/kg per min, respectively. During GTN infusion sildenafil caused a maximum drop in mean blood pressure of 21 ± 3 mm Hg (rise in heart rate from 117.0 ± 7.2 to 126.0 ± 6.0/min) and during ISDN infusion of 18 ± 3 mm Hg (rise in heart rate from 115.0 ± 7.0 to 125 ± 6/min), which was significantly less (p < 0.01) during PETN (only 6 ± 1 mm Hg with a rise in heart rate from 107.0 ± 5.0 to 122.0 ± 7.0/min). When sildenafil is used during exposure to nitrates (e.g., in coronary artery disease), the PETN‐induced drop in blood pressure at equi‐effective dosages (with regard to coronary dilation) is substantially smaller compared with that of GTN or ISDN, which is probably because of lesser potentiation of phosphodiesterase‐5 inhibitor‐induced effects in the arteriolar bed, thus minimizing critical drops in blood pressure.

Cardiovascular Dysfunction in Hypercholesterolemia Associated With Enhanced Formation of ATI-Receptor and of Eicosanoids

Background: In hypercholesterolemia with or without atherosclerosis cardiovascular dys function and altered signalling of angiotensin (Ang II), nitric oxide (NO), or prostanoids are intimately related to enhanced oxidant stress and concomitant changes in gene expression. We analyzed cardiac angiotensin receptor (AT1) expression and metabolism of Ang II, eicosanoids, and NO in hypercholesterolemic animals. Methods: Guinea pigs were fed a 1% cholesterol diet for 8 weeks (Chol). Hemodynamics were analyzed in Langendorff hearts. Spectrophotometric determination of plasma lipids and radioimmunological detection of eicosanoids/cyclic guanosine monophosphate (cGMP). Ac tivities of NO synthase III (NOS-III) or angiotensin converting enzyme (ACE) were deter mined by enzymatic assays. AT1 receptor density was assessed by radioligand binding assay. NOS-III mRNAs were quantitated by reverse transcription polymerase chain reaction. Results: Hypercholesterolemia was associated with fatty degeneration of the liver and pro found myocardial and coronary (e.g., endothelial) dysfunction. In Chol Langendorff hearts we observed significant increases in coronary flow (26.0 ± 1.0 vs. 17.5 ± 0.5 mL/min/g tissue) but diminished coronary responses to bradykinin (Bk, 250 ng bolus) or adenosine (Ado, 250 μg bolus) (ΔCPPBk/Ado: 5 ± 0.5 vs. 7.2 ± 1/0.9 ± 0. 1 vs. 1.9 ± 0.3 cm2 (area under the curve)). AT1 receptor expression was significantly increased in Chol hearts (72 ± 6.8 vs. 45 ± 5.6 fmol/mg protein), whereas marked suppression of cardiac activities of ACE (1.96 ± 0.34 vs. 4.90 ± 0.20 nmol/min/mg tissue) and of the entire cardiac nitric oxide-cGMP axis (e.g., NOS-III activity: 1.9 ± 0.4 vs. 3.1 ± 0.1 pmol/min/mg tissue; NOS-III mRNA: 0.82 ± 0.16 vs. 1.20 ± 0.12 arbitrary units; cGMP release: 0.41 ± 0.02 vs. 0.54 ± 0.04 pmol/min/g tissue) were shown in Chol. Finally, cardiac release of eicosanoids prostacyclin (PGI2) and throm boxane (TxA2) were significantly enhanced (0.48 ± 0.05 vs. 0.38 ± 0.05 and 0.60 ± 0.10 vs. 0.24 ± 0. 10 ng/min/g tissue, respectively). Enhanced cardiac PGI2 release and suppression of cGMP synthesis in Chol were even more pronounced on stimulation with Bk (38.2 ± 3.0 vs. 28.2 ± 2.0 ng/min/g tissue and 1.9 ± 0.3 vs. 3.0 ± 0.3 pmol/min/g tissue, respectively). Conclusions: Altered angiotensin-mediated signal transduction probably related to aug mented eicosanoid formation does not compensate for the limited endogenous NO production and for cardiovascular dysfunction in hypercholesterolemic guinea pigs. In this context, changes in redox-sensitive regulation of gene expression (ATI receptor, NOS-III—caused by enhanced oxidant stress—coutd play a pivotal role.