Increases in pulse pressure impair acetylcholine-induced vascular relaxation.

Effects of pulse pressure on acetylcholine-induced endothelium-dependent relaxation were investigated using a cascade bioassay model. Intact carotid arteries from rabbits were perfused at constant flow, and activity of endothelium-derived relaxing factor (EDRF) was assayed by measuring changes in isometric tension in a detector ring without endothelium. When pulse pressure of the donor artery was raised from approximately 2 to 10 mmHg, relaxation to acetylcholine (10(-7) M) was reduced from 31 +/- 3 (means +/- SE) to 20 +/- 2% (expressed as percent relaxation of phenylephrine-induced tone). Responses of the detector ring to nitroprusside were unchanged. Superoxide dismutase (SOD) and indomethacin each prevented impairment of relaxation to acetylcholine at high pulse pressure. When the donor artery was perfused at a higher mean pressure, elevation of pulse pressure also impaired relaxation to acetylcholine, and this impairment was prevented by SOD. These findings suggest that elevation of pulse pressure inhibits acetylcholine-induced, endothelium-dependent relaxation, and this inhibitory effect is mediated by generation of oxygen radicals.

Interaction of human platelets and leukocytes in modulation of vascular tone.

We tested the hypothesis that the vasodilator response to human platelets is modulated by polymorphonuclear leukocytes (PMNs). Responses to platelets activated with thrombin, as well as PMNs activated with N-formylmethionyl-leucyl-phenylalanine (FMLP), were examined in perfused rabbit carotid arteries in vitro. Activation of platelets produced marked dilatation, and activation of PMNs produced modest constriction in arteries preconstricted with phenylephrine. Vasodilator responses to platelets were greatly impaired during infusion of activated PMNs. Pretreatment of PMNs with superoxide dismutase (SOD) partially restored dilator responses to platelets. Because SOD only partially restored vasodilator responses to platelets, we tested the possibility that adenosine-diphosphatase (ADPase) activity of PMNs may degrade ADP released by platelets and thus reduce vasodilator responses. After incubation with PMNs, dilator responses to ADP, but not acetylcholine, were significantly impaired. These findings indicate that vasodilatation produced by activated human platelets is profoundly impaired by activated leukocytes. We conclude that two mechanisms may account for this effect: 1) endothelium-derived relaxing factor, released in response to platelet-derived ADP, is inactivated by superoxide anion generated by activated PMNs and 2) ADP is degraded by ADPase activity of PMNs. We speculate that platelet-leukocyte interaction may have important effects on vasomotor tone.

Altered vascular responses to platelets from hypercholesterolemic humans.

Activated platelets release potent vasoactive factors. Previous studies have focused on mechanisms by which vascular abnormalities lead to altered responses of atherosclerotic arteries. We tested the hypothesis that the activation of platelets from hypercholesterolemic humans produces abnormal vascular responses. Responses to intraluminal and abluminal activation of platelets from normal subjects and type II hypercholesterolemic patients (total cholesterol, 274 +/- 16 [mean +/- SEM] mg/dl) were examined in carotid arteries from normal rabbits perfused in vitro. Intraluminal activation of normal platelets produced pronounced dilatation of arteries preconstricted with phenylephrine. Vasodilator responses to intraluminal activation of platelets from hypercholesterolemic patients were greatly impaired. Vasodilator responses to platelets from hypercholesterolemic patients were not restored to normal by LY53,857 (10(-5) M), a 5-hydroxytryptamine2-serotonergic antagonist, by SQ29,548 (10(-5) M), a thromboxane A2/prostaglandin H2 receptor antagonist, or by apyrase (1.5 units/ml), an enzyme with ADPase activity. Abluminal activation of normal platelets produced modest constriction in quiescent arteries, and abluminal activation of platelets from hypercholesterolemic patients produced augmented vasoconstrictor responses. The major finding is that vasodilator responses to platelets from hypercholesterolemic patients are profoundly impaired, and vasoconstrictor responses to platelets from hypercholesterolemic patients are augmented. Mechanisms in addition to increased release of serotonin, thromboxane, and ADP appear to contribute to impaired vasodilator responses to hypercholesterolemic platelets. Thus, alteration of platelets by hypercholesterolemia, as well as altered vascular reactivity, may contribute to abnormal vascular responses in atherosclerosis.

Asymmetry of vascular responses of perfused rabbit carotid artery to intraluminal and abluminal vasoactive stimuli.

1. Responses of the carotid artery of rabbits to intraluminal and abluminal administration of purinergic agonists were examined. The carotid artery was perfused in vitro and changes in diameter were recorded. 2. Intraluminal acetylcholine, ADP, and ATP produced pronounced vasodilatation, whereas abluminal acetylcholine, but not ADP and ATP, produced dilatation of phenylephrine-preconstricted arteries. Intra- and abluminal adenosine and nitroprusside produced equipotent vasodilatation. 3. N omega-nitro-L-arginine abolished dilator responses to acetylcholine and adenine nucleotides, and unmasked vasoconstrictor responses to high concentrations of these agonists. Responses to adenosine and nitroprusside were not affected by nitro-L-arginine. 4. Intraluminal, but not abluminal, administration of nucleotidase-resistant adenine nucleotide analogues 2-methylthio-ATP and ADP beta S produced significant vasodilation in arteries preconstricted with phenylephrine. Intra- and abluminal administration of alpha,beta-methylene-ATP, a potent P2X-purinoceptor agonist, did not produce vasodilatation in preconstricted arteries. 5. Abluminal ADP failed to elicit dilatation of phenylephrine-preconstricted arteries even in the presence of the ADPase inhibitor beta,gamma-methylene-ATP (10(-5)M). When P2X-purinoceptors, which mediate adenine nucleotide-induced vasoconstriction, were stimulated with alpha,beta-methylene-ATP (10(-5)M), abluminal ADP produced vasodilatation, presumably because P2X-purinoceptors were occupied, thereby unmasking P2Y-purinoceptor-mediated dilatation. 6. These results suggest that asymmetric vascular responses of rabbit carotid arteries to adenine nucleotides may be due in part to preferential activation of P2Y-purinergic receptors on endothelium and P2X-purinergic receptors on vascular smooth muscle.

Effect of atherosclerosis on responses of the perfused rabbit carotid artery to human platelets.

Vascular responses to intraluminal and abluminal activation of human platelets were examined in carotid arteries from normal and atherosclerotic rabbits. The carotid artery was perfused in vitro, platelets were activated with thrombin (0.1 unit/ml), and changes in diameter were measured. In vessels from normal animals, intraluminal activation of platelets produced dilatation of preconstricted arteries. The dilator response was attenuated by N omega-nitro-L-arginine (10(-5) M), an inhibitor of synthesis of endothelium-derived relaxing factor-nitric oxide (EDRF-NO), and augmented by LY53,857 (10(-5) M), a 5-HT2-serotonergic antagonist. Abluminal activation of platelets produced modest constriction in quiescent arteries, which was inhibited by LY53,857. Intraluminal but not abluminal ADP produced pronounced dilatation of preconstricted arteries. In vessels from atherosclerotic animals, endothelium-dependent dilatation to intraluminal activation of platelets and to ADP was impaired and dilator responses to sodium nitroprusside were normal. These experiments indicate that 1) intraluminal activation of human platelets produces endothelium-dependent dilatation in perfused carotid arteries, whereas abluminal activation of human platelets produces vasoconstriction, which is mediated primarily by serotonin, and 2) atherosclerosis markedly impairs vasodilator responses to activation of human platelets, probably because vasodilatation to ADP released from platelets is impaired.

Mechanisms of contraction induced by human leukocytes in normal and atherosclerotic arteries.

Activation of leukocytes results in the release of a variety of vasoactive substances that may modulate vascular tone. We studied the effect of human polymorphonuclear (PMN) and mononuclear (MONO) leukocytes on quiescent femoral arteries in vitro. Arteries were obtained from normal and atherosclerotic cynomolgus monkeys. In normal arteries, stimulation of PMNs (3 and 5 x 10(6) cells/ml) with either thrombin (5 units/ml) or complement C5a (0.5 micrograms/ml) resulted in endothelium-independent contraction (approximately 25% of maximum contraction with 80 mM KCl). Vasocontraction was augmented in the presence of superoxide dismutase (150 units/ml) and was significantly impaired in the presence of the hydroxyl radical scavengers mannitol (20 mM) and deferoxamine (1 mM). Catalase (1,200 units/ml) or L-alanine (20 mM) did not modify this effect of PMNs. In contrast to PMNs, vasocontraction in response to MONOs was not altered by the addition of radical scavengers. Pretreatment of PMNs and MONOs with indomethacin (10 microM) or nordihydroguaiaretic acid (20 microM) did not influence vascular responses. Supernatant of thrombin-stimulated PMNs and MONOs also produced vasocontraction (approximately two thirds of the effect of intact cells). This vasocontractor factor (or factors) was heat stable (30 minutes, 95 degrees C) and had a molecular weight less than 1,000 as determined by ultrafiltration. Stimulation of MONOs or PMNs (3 and 5 x 10(6) cells/ml) produced a similar response in normal arteries. In contrast, the constrictor response in atherosclerotic arteries to MONOs (5 x 10(6) cells/ml) was significantly greater than to PMNs. We conclude that stimulated human PMNs and MONOs contract arteries in vitro by release of at least two factors. One factor appears to be heat stable, with a molecular weight less than 1,000. The vascular response to PMNs, but not to MONOs, appears to involve the generation of hydroxyl radicals. The response to MONOs is greater than the response to PMNs in atherosclerotic, but not in normal, arteries.