Cigarette smoke-induced bronchoconstriction in dogs: vagal and extravagal mechanisms

We reassessed the severity of cigarette smoke-induced bronchoconstriction and the mechanisms involved in anesthetized dogs. To evaluate the severity of smoke-induced bronchoconstriction, we measured airway pressure and airflow resistance (Rrs, forced oscillation method). We studied the mechanisms in other dogs by measuring airway pressure, central airway smooth muscle tone in tracheal segments in situ, and respiratory center drive by monitoring phrenic motor nerve output, including the role of vagal and extravagal nerves vs. the role of blood-borne materials during inhalation of cigarette smoke. Rrs increased more than fourfold with smoke from one cigarette delivered in two tidal volumes. About half the airway response was due to local effects of smoke in the lungs. The remainder was due to stimulation of the respiratory center, which activated vagal motor efferents to the airway smooth muscle. Of this central stimulation, about half was due to blood-borne materials and the rest to vagal pulmonary afferents from the lungs. We conclude that inhalation of cigarette smoke in dogs causes severe bronchoconstriction which is mediated mainly by extravagal mechanisms.     

Cigarette smoke-induced bronchoconstriction: causative agents and role of thromboxane receptors

Hong, Ju-Lun, and Lu-Yuan Lee. Cigarette smoke-inducedbronchoconstriction: causative agents and role of thromboxane receptors. J. Appl. Physiol. 81(5):2053-2059, 1996.Inhalation of cigarette smoke induces a biphasicbronchoconstriction in guinea pigs: the first phase is induced by acombination of cholinergic reflex and tachykinins, whereas the secondphase involves cyclooxygenase metabolites (J.-L. Hong, I. W. Rodger,and L.-Y. Lee. J. Appl. Physiol. 78:2260-2266, 1995). This study was carried out to further determinethe causative agents in the smoke and the types of prostanoid receptorsand endogenous prostanoids mediating the bronchoconstriction. Inhalation of 10 ml of high-nicotine cigarette smoke consistently elicited the biphasic bronchoconstriction in anesthetized and artificially ventilated guinea pigs. Pretreatment with hexamethonium (10 mg/kg iv) significantly reduced the first-phase bronchoconstriction but did not have any measurable effect on the second-phase response. Insharp contrast, gas-phase smoke did not elicit any bronchoconstrictive effect. Furthermore, when the animals were challenged with low-nicotine cigarette smoke, only a single second-phase response was evoked, accompanied by increases in thromboxane (Tx)B₂ (a stable metabolite ofTxA₂), prostaglandin (PG)D₂,PGF₂ in the bronchoalveolar lavage fluid. The bronchoconstrictive response induced by low-nicotine smoke was completely prevented by pretreatment with SQ-29548 (0.3 mg/kgiv), a TxA₂-receptor antagonist.These results indicate that 1)nicotine is the primary causative agent responsible for the first-phasebronchoconstriction and 2)nonnicotine smoke particulates evoke the release ofTxA₂,PGD₂, andPGF₂, which act onTxA₂ receptors on airway smoothmuscles and induce the second-phase response to cigarette smoke.

     

Two different mechanisms for modulation of bronchoconstriction in guinea-pigs by cyclooxygenase metabolites

Leukotriene D₄ (LTD₄)-induced bronchoconstriction in guinea-pig airways has a cyclooxygenase (COX)-dependent component. The main objective of this study was to establish if prostaglandin (PG) D₂-induced bronchoconstriction also was modulated by COX products. The effects of non-selective and selective COX-1 and COX-2 inhibitors on bronchoconstriction induced by LTD₄ and PGD₂ were investigated in the perfused and ventilated guinea-pig lung (IPL). Both LTD₄-induced bronchoconstriction and thromboxane (TX) A₂ release was suppressed by COX inhibitors or by TX synthesis inhibition. The release of additional COX products following CysLT₁ receptor activation by LTD₄ was established by measurements of immunoreactive 6-keto PGF_1α (a stable metabolite of PGI₂) and PGE₂. In contrast, TP receptor-mediated bronchoconstriction by PGD₂ was somewhat enhanced by COX inhibitors, and there was no measurable release of COX products after TP receptor activation with U-46619. PGE₂ was bronchoprotective in IPL as it inhibited the histamine-induced bronchoconstriction. In the isolated guinea-pig trachea, neither PGD₂ nor U-46619 actively released PGE₂, but continuous production of PGE₂ and PGI₂ was established, and the response to PGD₂ was enhanced also in the trachea by COX inhibition. The study documented that bronchoconstriction induced by LTD₄ and PGD₂ in IPL was modulated differently by COX products. Whereas bronchoconstriction induced by LTD₄ was amplified predominantly by secondarily released TXA₂, that induced by PGD₂ was attenuated by bronchoprotective PGE₂ and PGI₂, presumably tonically produced in the airways.     

Substance P antagonists and the role of tachykinins in non-cholinergic bronchoconstriction

Electrical field stimulation of guinea-pig isolated hilus bronchi induced tetrodotoxin-sensitive contractions of which only a minor part could be inhibited by atropine. The remaining non-cholinergic bronchonstriction was antagonized by a heptapeptide and an undecapeptide substance P (SP) analogue (Arg⁵, D-Trp^{7, 9}) SP_5–11, IC50 = 24.0 μM and (D-Pro², D-Trp^{7, 9})SP, IC50 = 10.0 μM. Of the exogenously added tachykinins, both eledoisin (8 times) and physalaemin (3 times) were more potent bronchoconstrictors than SP. Pretreatment with the SP-analogues shifted concentration-response curves to the tachykinins to the right, eledoisin being most readily antagonized. (Arg⁵, D-Trp^{7, 9})SP_5–11 also antagonized the neural response more readily than that of SP. In addition, in the frog isolated sciatic nerve preparation the two SP-analogues were found to possess potent lidocaine-like neurodepressant actions which further complicated the interpretation of the neural inhibitory effects of these compounds. It is concluded that if a tachykinin contributes to non-cholinergic bronchoconstriction, an eledoisin-like peptide is a more likely candidate than SP itself. Since SP-antagonists may have local anaesthetic properties their value as tools in neurophysiology seems limited. Inferentially, the non-cholinergic bronchoconstrictive neurotransmitter remains to be identified.     

Central and local cholinergic components of histamine-induced bronchoconstriction in dogs

To determine the importance of central and local reflexes in the bronchoconstriction produced by inhaled aerosolized histamine, chloralose-urethan-anesthetized dogs were intubated with a double-lumen catheter, ventilated with a dual cylinder respirator, and instrumented for the measurements of pulmonary conductance (GL) and dynamic compliance (Cdyn) in each lung. In each dog, dose-response curves to inhaled aerosolized histamine were obtained in both lungs separately but synchronously. Four series of experiments were performed. In the first series (n = 10) the responses of the right and left lungs were compared and found to be approximately equal, indicating that one lung could be used as a control for the other. In the second and third series the dose-response curve of one lung that had either been treated with inhaled atropine sulfate (n = 6) (4 mg/ml) or vagotomized (n = 4) was compared with the contralateral control lung. At low concentrations of histamine, GL and Cdyn decreased more in the control lungs than in their atropine-treated or vagotomized counterparts, and approximately 40% of the bronchoconstriction induced was reflex in origin. At higher concentrations of histamine the responses of the control and atropine-treated or vagotomized lungs were not significantly different. In the fourth series of experiments (n = 6) histamine dose-response curves were obtained following combined bilateral vagotomy and unilateral delivery of inhaled aerosolized atropine. In these dogs GL, but not Cdyn, fell to a greater extent in the control than in the atropine-treated lung.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cigarette smoke-induced DNA-damage: role of hydroquinone and catechol in the formation of the oxidative DNA-adduct, 8-hydroxydeoxyguanosine

This study demonstrates the ability of cigarette smoke condensate to generate hydrogen peroxide and to hydroxylate deoxyguanosine (dG) residues in isolated DNA to 8-hydroxydeoxyguanosine (8-OHdG). Both the formation of hydrogen peroxide and that of 8-OHdG in DNA was significantly decreased when catalase or tyrosinase was added to the smoke condensates, and this also occurred when pure hydroquinone or catechol, two major constitutes in cigarette smoke, was used instead of smoke condensate. Moreover, pure hydroquinone and catechol both caused dose-dependent formation of hydrogen peroxide and 8-OHdG, and there was good correlation between the amounts of hydrogen peroxide and 8-OHdG formed. These findings suggest that (i) hydroquinone and catechol may be responsible for the ability of cigarette smoke to cause 8-OHdG formation in DNA, (ii) this oxidative DNA-damage is due to the action of hydroxyl radicals formed during dissociation of hydrogen peroxide and (iii) the hydrogen peroxide in cigarette smoke is generated via autooxidation of hydroquinone and catechol.     

Cigarette smoke-induced pulmonary inflammation and emphysema are attenuated in CCR6-deficient mice

Chronic obstructive pulmonary disease (COPD) is mainly caused by cigarette smoking, and is characterized by an increase in inflammatory cells in the airways and pulmonary tissue. The chemokine receptor CCR6 and its ligand MIP-3alpha/CCL20 may be involved in the recruitment of these inflammatory cells. To investigate the role of CCR6 in the pathogenesis of COPD, we analyzed the inflammatory responses of CCR6 knockout (KO) and wild-type mice upon cigarette smoke (CS) exposure. Both subacute and chronic exposure to CS induced an increase in cells of the innate and adaptive immune system in the bronchoalveolar lavage, both in CCR6 KO and wild-type mice. However, the accumulation of dendritic cells, neutrophils, and T lymphocytes, which express CCR6, was significantly attenuated in the CCR6 KO mice, compared with their wild-type littermates. In the lung tissue of CCR6 KO mice, there was an impaired increase in dendritic cells, activated CD8(+) T lymphocytes, and granulocytes. Moreover, this attenuated inflammatory response in CCR6 KO mice offered a partial protection against pulmonary emphysema, which correlated with an impaired production of MMP-12. Importantly, protein levels of MIP-3alpha/CCL20, the only chemokine ligand of the CCR6 receptor, and MCP-1/CCL2 were significantly increased upon CS exposure in wild-type, but not in CCR6 KO mice. In contrast, CCR6 deficiency had no effect on the development of airway wall remodeling upon chronic CS exposure. These results indicate that the interaction of CCR6 with its ligand MIP-3alpha contributes to the pathogenesis of CS-induced pulmonary inflammation and emphysema in this murine model of COPD.     

Molecular and cellular mechanisms of cigarette smoke-induced myocardial injury: prevention by vitamin C

Background

Cardiovascular disease (CVD) remains one of the major killers in modern society. One strong risk factor of CVD is cigarette smoking that causes myocardial injury and leads to the genesis of pathological cardiovascular events. However, the exact toxic component(s) of cigarette smoke (CS) and its molecular and cellular mechanisms for causing myocardial injury leading to heart damage and its prevention are largely unknown.

Methodology/Principal Findings

Using a guinea pig model, here we show that chronic exposure to CS produces myocardial injury that is prevented by vitamin C. Male guinea pigs were fed either vitamin C-deficient (0.5 mg/day) or vitamin C-sufficient (15 mg/day) diet and subjected to CS exposure from 5 Kentucky Research cigarettes (3R4F)/day (6 days/week) in a smoke chamber up to 8 weeks. Pair-fed sham controls were subjected to air exposure instead of CS exposure under similar conditions. Myocardial injury was produced in CS-exposed marginal vitamin C-deficient guinea pigs as evidenced by release of cardiac Troponin-T and I in the serum, oxidative stress, inflammation, apoptosis, thrombosis and collagen deposition in the myocardium. Treatment of rat cardiomyocyte cells (H9c2) in vitro and guinea pigs in vivo with p-benzoquinone (p-BQ) in amounts derived from CS revealed that p-BQ was a major factor responsible for CS-induced myocardial damage. A moderately large dose of vitamin C (15 mg/day) prevented CS/p-BQ-induced myocardial injury. Population based studies indicated that plasma vitamin C levels of smokers without disease were significantly lower (p = 0,0000) than that of non-smokers. Vitamin C levels of CS-related cardiovascular patients were further lower (p = 0.0000) than that of smokers without disease.

Conclusions/Significance

The results indicate that dietary supplementation of vitamin C may be a novel and simple therapy for the prevention of pathological cardiovascular events in habitual smokers.     

Cigarette smoke-induced DNA damage in cultured human lung cells: role of hydroxyl radicals and endonuclease activation.

Cigarette smoke can cause DNA single strand breaks in cultured human lung cells (T. Nakayama et al., Nature, 314 (1985) 462-464) but the mechanisms behind this DNA damage have not been clearly elucidated. In the present study we have investigated the possibility that one of the major constituents in cigarette smoke, hydroquinone, may be important for mediating smoke-induced DNA damage in the human epithelial lung cell line, A 549, and the mechanisms behind this damage. Cells were exposed to cigarette smoke, hydrogen peroxide, or hydroquinone, in the absence and presence of different inhibitors, and the resulting DNA damage was assessed either as DNA single strand break formation or formation of the oxidative DNA adduct, 8-hydroxydeoxyguanosine. It was found that (i) exposure to cigarette smoke, hydrogen peroxide or hydroquinone causes a rapid decrease in the intracellular thiol level and a considerable DNA single strand break formation, (ii) the formation of DNA single strand breaks in cells exposed to cigarette smoke is inhibited by catalase, dimethylthiourea, and o-phenantroline, suggesting that hydroxyl radicals generated from iron-catalyzed hydrogen peroxide dissociation are involved in the DNA damage, (iii) hydroquinone causes considerable DNA strand break formation that is blocked by aurintricarboxylic acid, an inhibitor of endonuclease activation, and by BAPTA, an intracellular calcium chelator, (iv) addition of hydroquinone to a smoke condensate greatly enhances its ability to cause DNA single strand breaks, and (v) smoke, but not hydroquinone, causes formation of 8-hydroxydeoxyguanosine, a DNA damage product induced by the action of hydroxyl radicals on the DNA base, deoxyguanosine. These findings suggest that the ability of cigarette smoke to cause DNA single strand breaks in cultured lung cells is due to mechanisms involving hydroxyl radical attack on DNA and endonuclease activation. They also suggest that hydroquinone is an important contributor to the DNA damaging effect of cigarette smoke on human lung cells.     

Megakaryocytopoiesis in culture: modulation by cholinergic mechanisms

Treatment of murine bone marrow cultures with the cholinergic agonist carbamylcholine enhanced megakaryocytic colony growth by as much as 65%. In contrast, adrenergic agonists had no such effect. Addition to cultures of dibutyryl cyclic GMP (db-cGMP) also enhanced megakaryocytic colonies up to 50%, whereas dibutyryl cyclic AMP (db-cAMP) had no effect. Sodium nitroprusside and sodium nitrite, putative guanyl cyclase activators, also enhanced colony numbers, as did imidazole, a postulated cGMP phosphodiesterase inhibitor. Preincubation of marrow for two hours with carbamylcholine resulted both an increase in colony numbers (58%) and percent of progenitors in DNA synthesis (48%, compared to 14% for controls) as determined by tritiated thymidine suicide studies. Treatment of mice with the acetylcholinesterase inhibitor neostigmine resulted in an increase in CFU-M/humerus (62%) and percent in DNA synthesis (45%). These data indicate that 1) cholinergic, but not adrenergic, agonists modulate megakaryocytopoiesis in culture; 2) this effect may be mediated by cyclic GMP; and 3) only a brief period of exposure of marrow cells to agonist results in enhancement of megakaryocytic colonies.     

Bronchial circulation and cyclooxygenase products in acute lung injury

The role of cyclooxygenase products in the response of the bronchial circulation to acute lung injury was examined in 30 dogs. By use of an open-chest preparation the left lower lobe (LLL) pulmonary circulation was isolated, continuously weighed, and perfused in situ. The anastomotic bronchial blood flow [Qbr(s-p)] was measured as the rate of increase in the volume of the LLL-perfusion circuit. Four groups of dogs were studied. In group A, six dogs received cyclooxygenase inhibition (COI) with either indomethacin (2 mg/kg) or ibuprofen (10 mg/kg). In group B (n = 10) lung injury caused by airway instillation of glucose (15 mg) with glucose oxidase (500 micrograms/kg) (G/GO) or LLL pulmonary arterial infusion of alpha-napthyl thiourea (ANTU, 2 mg/kg). Group C (n = 10) received COI, and 30 min later injury was induced as above with either ANTU or G/GO. Group D (n = 4) received COI immediately after anesthesia; then, 30 min after completion of the surgical preparation, injury was induced with ANTU or G/GO. After COI, Qbr(s-p) decreased to 35 +/- 9% of the basal values (P less than 0.05). After administration of ANTU or G/GO, Qbr(s-p) increased irrespective of whether COI was present. 6-Ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) and thromboxane B2 (TxB2) were measured by radioimmunoassay in the LLL pulmonary artery and systemic venous blood, demonstrating an increase in 6-keto-PGF1 alpha due to surgical preparation and confirming complete COI in those animals receiving COI immediately after anesthesia. These findings demonstrate that 1) the bronchial circulation is capable of a sevenfold increase in flow in response to acute lung injury.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cigarette smoke-induced lung emphysema in mice is associated with prolyl endopeptidase, an enzyme involved in collagen breakdown

There is increasing evidence that the neutrophil chemoattractant proline-glycine-proline (PGP), derived from the breakdown of the extracellular matrix, plays an important role in neutrophil recruitment to the lung. PGP formation is a multistep process involving neutrophils, metalloproteinases (MMPs), and prolyl endopeptidase (PE). This cascade of events is now investigated in the development of lung emphysema. A/J mice were whole body exposed to cigarette smoke for 20 wk. After 20 wk or 8 wk after smoking cessation, animals were killed, and bronchoalveolar lavage fluid and lung tissue were collected to analyze the neutrophilic airway inflammation, the MMP-8 and MMP-9 levels, the PE activity, and the PGP levels. Lung tissue degradation was assessed by measuring the mean linear intercept. Additionally, we investigated the effect of the peptide L-arginine-threonine-arginine (RTR), which binds to PGP sequences, on the smoke-induced neutrophil influx in the lung after 5 days of smoke exposure. Neutrophilic airway inflammation was induced by cigarette smoke exposure. MMP-8 and MMP-9 levels, PE activity, and PGP levels were elevated in the lungs of cigarette smoke-exposed mice. PE was highly expressed in epithelial and inflammatory cells (macrophages and neutrophils) in lung tissue of cigarette smoke-exposed mice. After smoking cessation, the neutrophil influx, the MMP-8 and MMP-9 levels, the PE activity, and the PGP levels were decreased or reduced to normal levels. Moreover, RTR inhibited the smoke-induced neutrophil influx in the lung after 5 days' smoke exposure. In the present murine model of cigarette smoke-induced lung emphysema, it is demonstrated for the first time that all relevant components (neutrophils, MMP-8, MMP-9, PE) involved in PGP formation from collagen are upregulated in the airways. Together with MMPs, PE may play an important role in the formation of PGP and thus in the pathophysiology of lung emphysema.     

Cigarette smoke-induced proinflammatory alterations in the endothelial phenotype: role of NAD(P)H oxidase activation

Although the cardiovascular morbidity and mortality induced by cigarette smoking exceed those attributable to lung cancer, the molecular basis of smoking-induced vascular injury remains unclear. To test the link between cigarette smoke, oxidative stress, and vascular inflammation, rats were exposed to the smoke of five cigarettes per day (for 1 wk). Also, isolated arteries were exposed to cigarette smoke extract (CSE; 0 to 40 microg/ml, for 6 h) in organoid culture. We found that smoking impaired acetylcholine-induced relaxations of carotid arteries, which could be improved by the NAD(P)H oxidase inhibitor apocynin. Lucigenin chemiluminescence measurements showed that both smoking and in vitro CSE exposure significantly increased vascular O(2)(*-) production. Dihydroethidine staining showed that increased O(2)(*-) generation was present both in endothelial and smooth muscle cells. CSE also increased vascular H(2)O(2) production (dichlorofluorescein fluorescence). Vascular mRNA expression of the proinflammatory cytokines IL-1beta, IL-6, and TNF-alpha and that of inducible nitric oxide synthase was significantly increased by both smoking and CSE exposure, which could be prevented by inhibition of NAD(P)H oxidase (diphenyleneiodonium and apocynin) or scavenging of H(2)O(2). In cultured endothelial cells, CSE elicited NF-kappaB activation and increased monocyte adhesiveness, which were prevented by apocynin and catalase. Thus we propose that water-soluble components of cigarette smoke (which are likely to be present in the bloodstream in vivo in smokers) activate the vascular NAD(P)H oxidase. NAD(P)H oxidase-derived H(2)O(2) activates NF-kappaB, leading to proinflammatory alterations in vascular phenotype, which likely promotes development of atherosclerosis, especially if other risk factors are also present.     

Cigarette smoke-induced autophagy is regulated by SIRT1-PARP-1-dependent mechanism: Implication in pathogenesis of COPD

Autophagy is a fundamental cellular process that eliminates long-lived proteins and damaged organelles through lysosomal degradation pathway. Cigarette smoke (CS)-mediated oxidative stress induces cytotoxic responses in lung cells. However, the role of autophagy and its mechanism in CS-mediated cytotoxic responses is not known. We hypothesized that NAD⁺-dependent deacetylase, sirtuin 1 (SIRT1) plays an important role in regulating autophagy in response to CS. CS exposure resulted in induction of autophagy in lung epithelial cells, fibroblasts and macrophages. Pretreatment of cells with SIRT1 activator resveratrol attenuated CS-induced autophagy whereas SIRT1 inhibitor, sirtinol, augmented CS-induced autophagy. Elevated levels of autophagy were induced by CS in the lungs of SIRT1 deficient mice. Inhibition of poly(ADP-ribose)-polymerase-1 (PARP-1) attenuated CS-induced autophagy via SIRT1 activation. These data suggest that the SIRT1-PARP-1 axis plays a critical role in the regulation of CS-induced autophagy and have important implications in understanding the mechanisms of CS-induced cell death and senescence.     

Phorbol ester-induced phosphoinositide hydrolysis in rat aorta: role of cyclooxygenase products

This study investigates whether phorbol esters increase phosphoinositide hydrolysis in intact vascular smooth muscle, and the mechanism underlying the hydrolysis. Phorbol myristate acetate induced time- and concentration-dependent increases in phosphoinositide hydrolysis, as demonstrated by elevated inositol monophosphate levels, in deendothelialized rat aorta. The phorbol ester-elevated inositol monophosphate levels were abolished by indomethacin, a cyclooxygenase inhibitor, but were only partially decreased by SQ29548, a thromboxane A2/prostaglandin H2 receptor antagonist. SQ29548 also only partially decreased elevated inositol monophosphate levels due to prostaglandin E2, prostaglandin F2alpha, prostaglandin I2 and carbacyclin, a stable prostaglandin I2 analog. SQ29548 abolished elevated inositol monophosphate levels due to U46619, a stable thromboxane A2/prostaglandin H2 receptor agonist. These studies demonstrate that phorbol esters increase phosphoinositide hydrolysis in intact vascular smooth muscle, and that the increase is due, at lease in part, to endogenously released prostaglandins other than prostaglandin H2.     

Cigarette smoking: profiles of thromboxane- and prostacyclin-derived products in human urine

Thromboxane (TX) B2, 2,3-dinor-TXB2, 11-dehydro-TXB2, 6-oxoprostaglandin (PG)F1 alpha and 2,3-dinor-6-oxo-PGF1 alpha were measured in 24 h urine samples obtained from 30 apparently healthy chronic cigarette smokers and 37 closely matched non-smoking control subjects. Samples were analysed using a newly developed assay based on immunoaffinity chromatography and capillary column gas chromatography/electron capture negative ion chemical ionisation mass spectrometry. There were significant and comparable increases in the excretion rates of both 2,3-dinor-TXB2 and 11-dehydro-TXB2 in the smoking compared with the non-smoking group (2P less than 0.001). Excretion rates of 2,3-dinor-TXB2 were 418 +/- 35 and 265 +/- 26 pg/mg creatinine in the two groups, respectively. 11-Dehydro-TXB2 excretion rates were 440 +/- 54 and 221 +/- 18 pg/mg creatinine, respectively (mean +/- S.E.). There were significant (2P less than 0.05) positive correlations between average reported cigarette consumption and excretion of both thromboxane metabolites. There were small but significant (2P less than 0.02) increases in the excretion rates of both 6-oxo-PGF1 alpha and 2,3-dinor-6-oxo-PGF1 alpha in the smoking compared with the non-smoking group. There was no significant difference in the rates of excretion of TXB2 in the two groups. The effects of acute cigarette smoke exposure (five cigarettes in 2 h) was also studied in four normally non-smoking healthy volunteers. There was no significant change in the excretion rate of any of the eicosanoids measured during control and smoking periods (at least 2 weeks apart), indicating that increased TXA2 biosynthesis in chronic smokers is unlikely to be a consequence of acute platelet activation.     

Age-related endothelial dysfunction with respect to nitric oxide, endothelium-derived hyperpolarizing factor and cyclooxygenase products

Vascular aging is associated with both structural and functional changes that can take place at the level of the endothelium, vascular smooth muscle cells and the extracellular matrix of blood vessels. With regard to the endothelium, reduced vasodilatation in response to agonists occurs in large conduit arteries as well as in resistance arteries with aging. Reviews concerning the different hypotheses that may account for this endothelial dysfunction have pointed out alterations in the equilibrium between endothelium-derived relaxing and constricting factors. Thus, a decreased vasorelaxation due to nitric oxide and, in some arteries, endothelium-derived hyperpolarizing factor as well as an increased vasoconstriction mediated by cyclooxygenase products such as thromboxane A2 are likely to occur in age-induced impairment of endothelial vasodilatation. Furthermore, enhanced oxidative stress plays a critical role in the deleterious effect of aging on the endothelium by means of nitric oxide breakdown due to reactive oxygen species. The relative contribution of the above phenomenon in age-related endothelial dysfunction is highly dependent on the species and type of vascular bed.