Hemodynamic sequelae of regression of experimental atherosclerosis.

Regression of experimental atherosclerosis is characterized by decreased intimal thickness and luminal enlargement, but intimal fibrosis becomes more dense. We tested the hypothesis that fibrosis of arteries during regression might limit vasodilator capacity and restrict hemodynamic improvement despite luminal improvement. We studied limb, coronary, and cerebral hemodynamics in 11 normal cynomolgus monkeys, 10 monkeys given an atherogenic diet for 20 mo and 8 monkeys given a regression diet for an additional 18 mo. The atherogenic diet induced lesions of moderate severity (50-60% stenosis); owing to characteristic vessel growth during the atherogenic period, luminal size did not decrease correspondingly. Regression monkeys showed typical changes of regression with luminal enlargement but increased fibrosis. The iliac artery was perfused at constant blood flow and maximal vasodilatation was produced with papaverine. Blood flow was measured with microspheres during maximal vasodilatation in the coronary bed (adenosine) and cerebral bed (hypercapnia). In normal monkeys, minimal vascular resistances were 1.95 +/- 0.19 mm Hg/ml/min X 100 g (mean +/- SE) (limb), 0.13 +/- 0.01 (coronary), and 0.44 +/- 0.02 (cerebral). In atherosclerotic monkeys minimal resistance increased (P less than 0.05) 108, 62, and 166% in the limb, coronary, and cerebral beds, respectively. In regression monkeys, minimal resistance increased from values found in atherosclerotic animals in the limb (+22%), decreased inconsistently in the coronary bed (-19%), and decreased significantly in the cerebral bed (-44%, P less than 0.05). Thus morphologic regression was accompanied by significant hemodynamic improvement during maximal dilatation only in cerebral vessels. We conclude that increases in luminal size during regression of atherosclerotic lesions may not be associated with increases in vasodilator capacity, as intimal fibrosis may limit physiologically important hemodynamic improvement.

Vascular dysfunction in monkeys with diet-induced hyperhomocyst(e)inemia.

Elevated plasma homocyst(e)ine may predispose to complications of vascular disease. Homocysteine alters vasomotor regulatory and anticoagulant properties of cultured vascular endothelial cells, but little is known about effects of hyperhomocyst(e)inemia on vascular function in vivo. We tested the hypothesis that diet-induced moderate hyperhomocyst(e)inemia is associated with vascular dysfunction in cynomolgus monkeys. Plasma homocyst(e)ine increased from 4.O +/- O.2 microM when monkeys were fed normal diet to 10.6 +/- 2.6 microM when they were fed modified diet (mean +/- SE; P = 0.02). Vasomotor responses were assessed in vivo by quantitative angiography and Doppler measurement of blood flow velocity. In response to activation of platelets by intraarterial infusion of collagen, blood flow to the leg decreased by 42 +/- 9% in monkeys fed modified diet, compared with 14 +/- 11% in monkeys fed normal diet (P = 0.008), Responses of resistance vessels to the endothelium-dependent vasodilators acetylcholine and ADP were markedly impaired in hyperhomocyst(e)inemic monkeys, which suggests that increased vasoconstriction in response to collagen may be caused by decreased vasodilator responsiveness to platelet-generated ADP. Relaxation to acetylcholine and, to a lesser extent, nitroprusside, was impaired ex vivo in carotid arteries from monkeys fed modified diet. Thrombomodulin anticoagulant activity in aorta decreased by 34 +/- 15% in hyperhomocyst(e)inemic monkeys (P = 0.03). We conclude that diet-induced moderate hyperhomocyst(e)inemia is associated with altered vascular function.

Supplementation of atherogenic diet with B vitamins does not prevent atherosclerosis or vascular dysfunction in monkeys.

BACKGROUND: Hyperhomocysteinemia is associated with increased risk of atherosclerotic and thrombotic vascular disease. In many patients, hyperhomocysteinemia can be treated or prevented by dietary supplementation with B vitamins, but the clinical benefit of B vitamins for the prevention of vascular disease has not been proven. METHODS AND RESULTS: Using an atherogenic diet that produces both hyperhomocysteinemia and hypercholesterolemia, we tested the hypothesis that dietary supplementation with B vitamins (folic acid, vitamin B(12), and vitamin B(6)) would prevent hyperhomocysteinemia, vascular dysfunction, and atherosclerotic lesions in monkeys. After 17 months, plasma total homocysteine increased from 3.6+/-0.3 to 11.8+/-1.7 micromol/L in monkeys fed an unsupplemented atherogenic diet (P<0.01) but did not increase in monkeys fed an atherogenic diet supplemented with B vitamins (3.8+/-0.3 micromol/L). Serum cholesterol increased from 122+/-7 to 550+/-59 mg/dL in the unsupplemented group (P<0.001) and from 118+/-5 to 492+/-55 mg/dL in the supplemented group (P<0.001). Responses to endothelium-dependent vasodilators, both in resistance vessels in vivo and in the carotid artery ex vivo, were impaired to a similar extent in groups that did and did not receive vitamin supplements. Anticoagulant responses to the infusion of thrombin were also impaired to a similar extent in both groups. Vitamin supplementation failed to prevent intimal thickening in the carotid or iliac arteries. CONCLUSIONS: These findings demonstrate that supplementation with B vitamins prevents hyperhomocysteinemia but is not sufficient to prevent the development of vascular dysfunction or atherosclerotic lesions in monkeys with marked hypercholesterolemia, even in the absence of preexisting atherosclerosis.

Response of the mesenteric circulation to serotonin in normal and atherosclerotic monkeys: implications for the pathogenesis of non-occlusive intestinal ischaemia.

We tested the hypothesis that atherosclerosis potentiates vasoconstrictor responses in the mesenteric circulation to stimulation of serotonergic and alpha-adrenergic receptors. In normal monkeys, infusion of serotonin had little effect on blood flow to the stomach, duodenum, and colon. In contrast, in atherosclerotic monkeys, serotonin produced a modest decrease in blood flow to the stomach and duodenum, and virtually abolished blood flow to the colon. Vasoconstrictor responses to phenylephrine in the stomach and duodenum were not altered by atherosclerosis, but responses were potentiated in the colon of atherosclerotic monkeys. In summary, atherosclerosis greatly potentiates vasoconstrictor responses to serotonin in the mesenteric circulation, particularly in vessels to the colon. We speculate that release of serotonin during adherence and aggregation of platelets at atherosclerotic lesions, coupled with augmented vasoconstrictor responses to serotonin, may play a role in the pathogenesis of non-occlusive mesenteric ischaemia.

Effects of sympathetic nerves on collateral vessels in the limb of atherosclerotic primates.

This study was performed to examine effects of sympathetic nerves on collateral vessels in the limb. We studied normal (N) and atherosclerotic (AS) cynomolgus monkeys that were fed atherogenic diet for 21 months. A common iliac artery was ligated 13 months before hemodynamic measurements. Using histofluorescence microscopy, a plexus of noradrenergic nerves was identified in the adventitia of collateral vessels. We measured blood flow to the limb with microspheres, and the pressure gradient from aorta to the iliac artery beyond the occlusion. The lumbar sympathetic chain was stimulated electrically at 3 Hz (SNS-3) and 15 Hz (SNS-15). In normal monkeys, conductance of collateral vessels (in ml/min per 100 g per 100 mm Hg) was 19 +/- 3.6 (mean +/- SE) during control, 14 +/- 1.6 during SNS-3, and 9.8 +/- 0.9 during SNS-15 (P less than 0.05 vs control). In AS monkeys, collateral conductance was 12 +/- 2.9 during control, 7.5 +/- 1.7 during SNS-3 and 3.9 +/- 1.8 during SNS-15 (P less than 0.05). In summary, collateral vessels in the limb are innervated and sympathetic stimulation produces pronounced constriction of collateral vessels in both normal and atherosclerotic monkeys. Thus, the effectiveness of collateral vessels in maintaining blood flow to the limb may be compromised by increased activity of sympathetic nerves.

Activation of leukocytes with complement C5a is associated with prostanoid-dependent constriction of large arteries in atherosclerotic monkeys in vivo.

Activated leukocytes release a variety of substances which have been shown in vitro to modulate vascular tone. The chemotactic peptide complement C5a is a physiological activator of leukocytes. We injected human recombinant complement C5a (10 and 100 micrograms) into the blood-perfused hind limb of normal and atherosclerotic cynomolgus monkeys and examined vascular responses. In both normal and atherosclerotic monkeys, the high dose of C5a produced about 65% decrease in leukocyte cell count in venous blood drainage from the hind limb. Injection of C5a produced a pronounced increase in resistance of large arteries (segment from iliac artery to dorsal pedal artery) in atherosclerotic, but not in normal monkeys. The constrictor effect of C5a in atherosclerotic monkeys was abolished by the thromboxane A2 receptor antagonist SQ 29,548 (2 mg/kg i.v.). The platelet-activating factor antagonist WEB 2086 (5 mg/kg, i.v.) did not alter vascular responses to C5a. We conclude that activation of leukocytes produces constriction of large arteries in atherosclerotic, but not normal, monkeys in vivo. This response may be mediated in part by release of thromboxane A2.

Serotonin-induced constriction of ocular arteries in atherosclerotic monkeys. Implications for ischemic disorders of the retina and optic nerve head.

BACKGROUND: Ischemic disorders of the retina and optic nerve head, which constitute a common cause of visual loss, are usually seen in patients with atherosclerosis. OBJECTIVE: To test the hypothesis that serotonin, which is released when platelets aggregate, may produce vasospasm in atherosclerotic monkeys and, thus, may contribute to the ischemic disorders and that short-term dietary treatment of atherosclerosis causes the propensity to vasospasm to subside. METHODS: We studied the response of retinal and posterior ciliary circulation to serotonin in 18 atherosclerotic (25 eyes) and 5 normal (8 eyes) cynomolgus monkeys. The eyes were evaluated by color fundus photography and fluorescein fundus angiography. The eyes were examined under basal conditions and, at a different time, during the intravenous infusion of serotonin. In 6 of the 18 atherosclerotic animals, the evaluation was repeated 5 to 12 months after discontinuing the atherogenic diet (ie, the regression group). RESULTS: Serotonin had no effect in normal monkeys. In 18 atherosclerotic monkeys, serotonin produced transient occlusion or delayed filling of the central retinal artery and/or posterior ciliary artery (PCA) in 9 eyes of 9 animals, involving the central retinal artery in 5, lateral PCA in 8, and medial PCA in 5, in various combinations. In 6 animals (6 eyes) of the regression group, the vasoconstrictor effect of serotonin was abolished completely, except in the medial PCA in 1 eye. CONCLUSIONS: Serotonin, in the presence of atherosclerotic lesions, can cause transient, complete occlusion or impaired blood flow in the central retinal artery and/or PCA. We speculate that this mechanism may play a role in the development of ischemic disorders of the retina and optic nerve head. Discontinuing the atherogenic diet abolished or markedly improved the serotonin-induced vasoconstriction within a few months.

Potentiation of vasoconstrictor responses to serotonin in the limb of atherosclerotic monkeys.

Vasoconstrictor responses to serotonin and norepinephrine were examined in: normal cynomolgus monkeys, atherosclerotic monkeys that were fed atherogenic diet for 3-5 years and hypercholesterolaemic but non-atherosclerotic monkeys that were fed atherogenic diet for 4-5 months. Serotonin decreased total hindlimb resistance in normal and hypercholesterolaemic monkeys, but increased total resistance in atherosclerotic monkeys. There was a greater than 10-fold increase in constrictor responses of large arteries to serotonin in atherosclerotic compared with normal and hypercholesterolaemic monkeys. In contrast, responses to norepinephrine were not increased in atherosclerotic monkeys. Thus, atherosclerosis greatly potentiates constrictor responses to serotonin in large arteries of the limb. This potentiation appears to be somewhat selective for serotonin, as it is not observed with norepinephrine.

Effect of early and advanced atherosclerosis on vascular responses to serotonin, thromboxane A2, and ADP.

In monkeys with early and advanced atherosclerosis, we examined responses to the three major vasoactive agonists that are released when platelets aggregate. Measurements were obtained in normal cynomolgus monkeys and in monkeys fed an atherogenic diet for 4 +/- 1, 9 +/- 1, and 19 +/- 1 months (mean +/- SEM). Morphometry of femoral and iliac arteries indicated that 4 months of atherogenic diet produced only slight intimal thickening, 9 months produced early lesions, and 19 months produced approximately 5-10 fold greater intimal proliferation than did 9 months of atherogenic diet. Serotonin and adenosine 5'-diphosphate (ADP), which are endothelium-dependent agonists, and adenosine and phenylephrine, which are endothelium-independent agonists, were injected intra-arterially into the perfused hind limb. Thromboxane A2 analogue U46619 also was studied. Vasoconstrictor responses to serotonin were potentiated, and vasodilator responses to ADP were impaired by early and advanced atherosclerosis. In contrast, vasoconstrictor responses to phenylephrine and vasodilator responses to adenosine were similar in all groups. Vasoconstrictor responses to U46619 were potentiated by advanced atherosclerosis. Thus, vascular responses to serotonin, ADP, and thromboxane A2 are altered by atherosclerosis in a direction that would favor vasoconstriction when platelets aggregate. Furthermore, because responses to endothelium-dependent agonists are altered, these data suggest that endothelium is dysfunctional in early atherosclerosis. These findings may explain, in part, the propensity for exaggerated vasoconstriction even in arteries with minimal atherosclerotic lesions.

Acetylcholine-induced vasodilatation in rabbit hindlimb in vivo is not inhibited by analogues of L-arginine.

Nitric oxide (NO) or related nitroso compounds are an endothelium-derived relaxing factor (EDRF), originating from metabolism of L-arginine, L-Arginine analogues with chemically altered guanidino moity are potent and specific inhibitors of EDRF(NO) release. We evaluated effects of two L-arginine analogues, NG-monomethyl-L-arginine (L-NMMA, 100 microM) and N omega-nitro-L-arginine (L-NARG, 30 microM), on acetylcholine-, substance P-, and nitroglycerin-induced relaxation in the blood-perfused rabbit hindlimb in vivo and femoral arteries in vitro. L-NMMA and L-NARG selectively inhibited the vasodilator response to acetylcholine in rabbit femoral arteries in vitro, whereas endothelium-independent response to nitroprusside increased. L-NMMA (1.6 mg/min ia) in the blood-perfused rabbit hindlimb in vivo increased vascular resistance in the hindlimb by 23 +/- 3% (means +/- SE; n = 10) but did not inhibit the vasodilator responses to acetylcholine or substance P. L-NARG (10 mg/kg iv) increased systemic blood pressure by 26 +/- 3% (n = 7) and vascular hindlimb resistance by 22 +/- 9% (n = 8), and blood flow to hindlimb musculature, measured with microspheres, decreased by 46 +/- 5% (n = 6). Pretreatment with L-NARG, however, did not impair vasodilator responses to acetylcholine and substance P. These findings are consistent with the view that basal tone in resistance vessels in the rabbit hindlimb may be mediated by nitroso compounds, whereas agonist-stimulated vasodilation may be mediated by other mechanisms that do not involve the NO-synthesizing enzyme.

Vascular responses to endothelin-1 in atherosclerotic primates.

Endothelin-1 (ET-1) is a vasoactive peptide that is released by endothelial cells. This study was performed to determine whether vascular responses to ET-1 are altered by atherosclerosis. ET-1 (1 or 10 nmol) was injected intra-arterially into the perfused hind limb of normal cynomolgus monkeys and monkeys fed an atherogenic diet for 19 months. We calculated the resistance of the total limb and large arteries and estimated the resistance of the small vessels. The major finding was that ET-1 had minimal effects on large arteries in normal monkeys but produced pronounced constriction of large arteries in atherosclerotic monkeys. In both groups, ET-1 produced dilatation of small vessels at 1 nmol and constriction at 10 nmol. Indomethacin (6 mg/kg intravenously) did not affect the responses to ET-1 in normal or atherosclerotic monkeys. In summary, the major finding is that the constrictor responses of large arteries to ET-1 are potentiated by atherosclerosis.

Effect of short-term regression of atherosclerosis on reactivity of carotid and retinal arteries.

BACKGROUND AND PURPOSE: This study tested the hypothesis that functional abnormalities of carotid and ocular arteries may improve after short-term regression of atherosclerosis, before regression of structural abnormalities. METHODS: We examined effects of short-term dietary treatment of atherosclerosis on carotid and ocular vascular responses to serotonin and to platelet activation by collagen in vivo. Three groups of monkeys were studied: normal cynomolgus monkeys, monkeys fed an atherogenic diet for 34 months, and atherosclerotic monkeys that were fed a regression diet for 8.6 +/- 1.1 months (mean +/- SE). We measured changes in carotid blood flow (using a Doppler probe), retinal blood flow (using microspheres), and diameter of the internal carotid artery (using quantitative angiography). Endothelium-dependent relaxation to acetylcholine was studied in rings of internal carotid artery in vitro. RESULTS: Carotid blood flow increased in response to both serotonin and collagen in normal monkeys, decreased in response to both agents in atherosclerotic monkeys, and was restored toward normal after regression. Serotonin had little effect on retinal blood flow in normal monkeys and produced a marked decrease in retinal blood flow in atherosclerotic monkeys; the vasoconstrictor response to serotonin was reduced after regression. Activation of platelets by collagen increased blood flow in normal monkeys, decreased blood flow in atherosclerotic monkeys, and had little effect after regression. Alterations in responses of the internal carotid artery were consistent with changes in carotid and ocular blood flow. Endothelium-dependent relaxation in vitro was impaired by atherosclerosis and was restored toward normal by regression. There was no reduction in intimal area of the atherosclerotic lesion in common carotid and ophthalmic arteries from regression monkeys, despite a marked reduction in cholesteryl ester. CONCLUSIONS: Within a few months of regression of atherosclerosis, endothelial function and hyperresponsiveness of carotid and ocular arteries to serotonin and platelet activation return toward normal. Functional improvement is associated with resorption of lipid from atherosclerotic lesions, but with little reduction in size of intimal lesions.

Dietary treatment of atherosclerosis abolishes hyperresponsiveness to serotonin: implications for vasospasm.

Diet-induced atherosclerosis in primates impairs vasodilator responses and greatly potentiates vasoconstrictor responses to serotonin. Serotonin may play an important role in the pathogenesis of vasospasm. In diet-induced regression of atherosclerosis, intimal lesions are reduced, but maximal vasodilator responses do not improve, perhaps because of vascular fibrosis. Our goal was to determine whether dietary treatment of atherosclerosis reverses the augmented vasoconstrictor responses to serotonin and thus might reduce susceptibility to vasospasm. Normal cynomolgus monkeys, atherosclerotic monkeys, and atherosclerotic monkeys that were given a normal (regression) diet for 18 months were studied. Morphometric studies indicated that the regression diet reduced lesions in the iliac and femoral artery since intimal area was reduced by about 50%. In the hind limb perfused at constant flow, residual resistance during maximal vasodilatation produced by infusion of adenosine tended to be greater in atherosclerotic monkeys than in normals and failed to improve in regression monkeys. In contrast, vasoconstrictor responses to serotonin were greatly potentiated in atherosclerotic monkeys and were restored to normal in regression monkeys. Serotonin (20 micrograms i.a.) decreased hind limb resistance (in mm Hg/ml/min) 0.34 +/- 0.06 (mean +/- SE) in normal monkeys, increased resistance 0.58 +/- 0.17 in atherosclerotic monkeys (p less than 0.05 vs. normal), and decreased resistance 0.70 +/- 0.15 in regression monkeys (p less than 0.05 vs. atherosclerotic). Thus, dietary treatment of atherosclerosis abolishes augmented vasoconstrictor responses to serotonin. It is proposed that treatment of atherosclerosis may be beneficial, even when vasodilator responses fail to improve, by reducing susceptibility to serotonin-induced vasospasm.

Augmented responses to vasoconstrictor stimuli in hypercholesterolemic and atherosclerotic monkeys.

We examined effects of hypercholesterolemia and atherosclerosis on vasoconstrictor responses to norepinephrine and serotonin. Responses were compared in normal, atherosclerotic, and hypercholesterolemic but non-atherosclerotic cynomolgus monkeys. The hindlimb was perfused at constant flow so that changes in perfusion pressure indicated changes in vascular resistance. We measured the pressure gradient from the iliac to the dorsal pedal artery so that responses of the large artery segment could be determined. Serotonin decreased total hindlimb resistance in normal and hypercholesterolemic monkeys, but increased total resistance in atherosclerotic monkeys. There was a greater than 10-fold increase in constrictor responses of large arteries to serotonin in atherosclerotic monkeys, compared with normal and hypercholesterolemic monkeys. In contrast, we found that vasoconstrictor responses to norepinephrine are normal in atherosclerotic monkeys and increased in hypercholesterolemic monkeys prior to development of atherosclerosis. Hypercholesterolemia augmented responses of small vessels to norepinephrine. We conclude that, during early stages of hypercholesterolemia in cynomolgus monkeys, vasoconstrictor responses to norepinephrine are increased in small vessels. At a later stage, as atherosclerosis develops, responses to norepinephrine return to normal, but vasoconstrictor effects of large arteries to serotonin are greatly potentiated.

Structural and hemodynamic response of peripheral arteries of macaque monkeys to atherogenic diet.

The arteries of monkeys given atherogenic diets develop marked intimal thickening and medial thinning, but luminal size apparently changes minimally. The hemodynamic significance of the atherosclerotic changes is therefore uncertain. To evaluate vascular function in atherosclerotic arteries, we studied the hind-limb vessels of adult male rhesus and cynomolgus monkeys to assess the structural and hemodynamic responses to an atherogenic diet given for about 1.5 years or for much longer periods (6.5 years for rhesus and 4.3 years for cynomolgus monkeys). The intimal cross-sectional area greatly increased after the atherogenic diet, but there was no significant luminal narrowing after either the 1.5-year diet or the longer diet periods. The media of atherosclerotic arteries showed focal atrophy and focal thinning after pressure fixation, but the total medial mass was not decreased even after the long diet periods. Hemodynamic studies indicated mild functional impairment in the atherosclerotic vessels; resting resistance increased and vasodilator responses decreased, but adrenergic responses were preserved. Thus, the marked changes that occur in the arterial wall in experimental primate atherosclerosis include adaptations to lesion formation that permit a long prestenotic phase of atherosclerosis in which vascular dysfunction is minimal.

Enhanced coronary vasoconstrictive response to serotonin subsides after removal of dietary cholesterol in atherosclerotic monkeys.

Constriction in response to serotonin is enhanced in the coronary arteries of atherosclerotic monkeys. The main objective of the present study was to determine whether abnormal responses to serotonin in atherosclerosis are reversed following removal of dietary cholesterol. In addition, we examined the effect of an atherogenic diet and reduction in dietary cholesterol on vascular responses to activation of ATP-sensitive K+ channels with aprikalim. Diameters of small coronary arteries were measured on the epicardial surface of the left ventricle in vivo by using stroboscopic illumination synchronized to the heart cycle to visually freeze the motion of the heart. Diameters were measured with a microscope-video system during topical application of two vasoconstrictor agonists, serotonin and the thromboxane mimetic U46619, and the vasodilator agonists aprikalim and nitroprusside. Responses were compared in normal (n = 9), atherosclerotic (n = 14; high-cholesterol diet), and regression (n = 8; high-cholesterol diet followed by normal diet) monkeys. Constriction of coronary arteries in response to serotonin was enhanced in monkeys on an atherogenic diet and was normal in regression monkeys. Vasoconstriction in response to U46619 and vasodilation in response to nitroprusside and aprikalim were not altered by atherosclerosis. Thus, abnormal vascular responses to serotonin in small coronary arteries of atherosclerotic monkeys without morphological evidence of disease can be reversed to normal by reducing dietary cholesterol.

Vascular responses to platelet activation in normal and atherosclerotic primates in vivo.

Platelets release vasoactive substances that may contribute to augmented vasoconstriction. In this study, we examined vascular responses to activation of platelets in vivo by infusion of collagen. Purified bovine collagen was infused into the blood-perfused hind limb of normal and atherosclerotic cynomolgus monkeys. Resistance of the total limb and large arteries was measured at constant flow. In normal monkeys, collagen produced a decrease in total limb resistance, with a modest constrictor response of the large arteries. In atherosclerotic monkeys, collagen produced a transient, small decrease in total limb resistance, with pronounced constriction of large arteries. Indomethacin (5 mg/kg i.v.) and the thromboxane A2/prostaglandin H2 receptor antagonist SQ29,548 (2 mg/kg i.v.) virtually abolished the large-artery constrictor response to collagen in atherosclerotic monkeys. The 5-HT2-serotonergic receptor antagonist ketanserin (0.6 mg/kg i.v.) had no effect on the vasoconstrictor response. We conclude that 1) large arteries constrict and small vessels dilate in response to collagen-mediated activation of platelets in vivo in normal and atherosclerotic monkeys, 2) large-artery constriction in response to activation of platelets is augmented in atherosclerotic monkeys, and 3) the augmented large-artery constriction in atherosclerotic monkeys may be mediated primarily by thromboxane. The findings provide evidence that platelets may contribute to augmented constrictor responses of atherosclerotic arteries.

Effects of atherosclerosis and regression on vascular responses to products of activated platelets in primates.

We aggregated human platelets in vitro and examined vascular responses to injection of the supernatant in atherosclerotic primates. Platelets were washed, suspended, and aggregated with thrombin. Thrombin was then inactivated with D-Phe-Pro-ArgCH2Cl, and the suspension was centrifuged. The supernatant was injected intra-arterially into the perfused hindlimb within 30 s after aggregation of platelets. We studied normal cynomolgus monkeys, atherosclerotic monkeys that were fed atherogenic diet for 18 mo, and regression monkeys that were fed an atherogenic diet for 18 mo followed by a normal diet for 20 mo. Products of activated human platelets produced vasodilation in normal monkeys, as effects of platelet-derived vasodilators (presumably adenine nucleotides) may override platelet vasoconstrictor products. Vasodilator responses to platelet products were impaired in atherosclerotic monkeys, probably as a result of endothelial dysfunction. Regression of atherosclerosis restored vasodilator responses to platelet products toward normal. These data suggest that the predominant response to human platelet products is vasodilatation. Atherosclerosis impairs vasodilator responses to human platelet products, and regression of atherosclerosis restores responses toward normal.

Consequences of hyperhomocyst(e)inemia on vascular function in atherosclerotic monkeys.

Moderate elevation of plasma homocyst(e)ine is associated with increased risk for atherosclerotic vascular disease. In a previous study, we observed impaired vascular function in nonatherosclerotic monkeys with moderate hyperhomocyst(e)inemia. In this study, we tested the hypothesis that dietary intervention to lower plasma homocyst(e)ine corrects vascular dysfunction in atherosclerotic monkeys. Cynomolgus monkeys were fed an atherogenic diet that produces both hypercholesterolemia and moderate hyperhomocyst(e)inemia. After 17 months, the atherogenic diet was supplemented with B vitamins (5 mg folic acid, 400 micrograms vitamin B-12, and 20 mg vitamin B-6 daily) for 6 months. Total plasma homocyst(e)ine decreased from 12.8 +/- 2.8 to 3.5 +/- 0.3 mumol/L (n = 9; mean +/- SE; P < .01) after vitamins were added to the diet, but plasma cholesterol remained elevated (522 +/- 63 versus 514 +/- 41 mg/dL; P > .05). In response to intra-arterial infusion of collagen, blood flow to the leg decreased by 30 +/- 3% and 38 +/- 5%, respectively, before and after vitamin supplementation (P > .05). In vivo responses of resistance vessels to endothelium-dependent vasodilators (acetylcholine or ADP) were impaired at baseline and did not improve after vitamin supplementation. In carotid artery studied ex vivo, relaxation to low doses of acetylcholine improved after vitamin supplementation, but maximal relaxation remained impaired. Ex vivo thrombomodulin anticoagulant activity was threefold higher in monkeys fed the atherogenic diet (with or without B vitamins) than in normal monkeys (P < .05). We conclude that normalization of plasma homocyst(e)ine is insufficient to restore normal vascular function in atherosclerotic monkeys with persistent hypercholesterolemia and that atherosclerosis, with or without hyperhomocyst(e)inemia, is associated with elevated thrombomodulin activity.

Functional improvement precedes structural regression of atherosclerosis.

BACKGROUND: Vasoconstrictor responses to serotonin are augmented in monkeys with diet-induced atherosclerosis and improve after 18 months of normal diet. We tested the hypothesis that functional improvement may occur early during regression, before evidence of structural improvement. METHODS AND RESULTS: Responses of the iliac artery to serotonin were measured by quantitative angiography and a Doppler flow probe in several groups of monkeys: (1) normal monkeys, (2) monkeys fed an atherogenic diet for 2 years (atherosclerotic), and (3) monkeys fed an atherogenic diet for 2 years (preregression) followed by a normal diet for 4, 8, or 12 months (regression). In normal monkeys, serotonin produced minimal constriction of the iliac artery, and blood flow to the legs increased. In atherosclerotic monkeys, there was pronounced constriction of the iliac artery, and blood flow to the legs decreased markedly. After 4 months of regression diet, four of eight monkeys demonstrated marked reduction in hyperresponsiveness to serotonin angiographically, and by 8 months, six of eight monkeys had significant improvement. After regression, serotonin produced minimal changes in flow. There was no reduction in intimal area (ie, atherosclerotic lesion) in iliac arteries from regression monkeys compared with atherosclerotic monkeys, but there was a marked reduction in cholesteryl ester in arteries from regression monkeys. CONCLUSIONS: Abnormal vasoconstrictor responses to serotonin usually return to or toward normal within a few months during regression of atherosclerosis. Functional improvement occurs in conjunction with early resorption of lipid from the arterial wall and occurs before detectable changes in mass of the atherosclerotic lesion.