Effect of carbon monoxide exposure on aortic and coronary intimal morphology in the rabbit. A revaluation

Cigarette smoking increases the development of initial hyperplasia after vascular injury

PURPOSE

Our purpose was to determine whether exposure to cigarette smoke increases the development of intimal hyperplasia (IH) after vascular injury.

METHODS

Sixteen adult male Sprague-Dawley rats underwent standardized balloon catheter injury of the left common carotid artery. For 4 weeks before and 4 weeks after injury, animals in the experimental group (n=8) were exposed to cigarette smoke with an automated vacuum pump device. Animals in the control group (n=8) were restrained in the smoking device for an identical amount of time and underwent arterial injury at 4 vivo, prepared as histologic cross sections, and stained for elastin. IH was measured by planimetry and is reported both as the absolute area of IH and as the ratio (IH/IEL) of the absolute area of IH to the normalized area enclosed by the internal elastic lamina (expressed as a percent).

RESULTS

The absolute area of IH was 2.09 +/- 0.34 for the experimental group compared with 0.94 +/- 0.25 for the control group; mean IH/IEL was 43% +/- 7.1% for the experimental group versus 17.7% +/- 4.7% for the control group (p < 0.05, two tailed unpaired t test.

CONCLUSIONS

Inhalation of cigarette smoke increases the development of intimal hyperplasia in a rat model of a balloon catheter arterial injury.

Coronary vessel alterations following chronic carbon monoxide exposure in the adult rat

Adult male rats were exposed to 500 ppm CO continuously for 30 days, while litter-mate controls remained in room air (AIR). Heart weight-to-body weight ratio and hematocrit were increased significantly. Right ventricle (RV) free wall thickness was increased significantly as was right to left heart diameter and average heart diameter. Cross-sectional areas of the left ventricle (LV) free wall, interventricular septum (S) and RV midway between the apex and base were increased significantly. Morphometric analysis of the CO-exposed and AIR hearts revealed no significant differences in the number of small (27-114 microns) or larger (> 114 microns) blood vessels in any region; however, there was a trend towards an increased number of the smaller vessels, both arterioles and venules, in the CO-exposed rats. The larger arteries in the S and RV were significantly larger in the CO-exposed rats. There was a significant overall effect of CO on larger artery diameter across all heart regions, consistent with the appearance of heart radiographs taken. There were no differences in the diameter of the small vessels in any region of the heart between the CO-exposed and AIR rats. The vessel cross-sectional area of the larger vessels tended to be increased in all regions of the heart. The cross-sectional area of the large arteries in the LV was increased significantly. Arterial wall thickness was decreased significantly in RV and there was a significant overall effect of CO in decreasing wall thickness and the ratio of wall thickness-to-vessel luminal diameter in these vessels. No vascular pathology was observed. The results of this study suggest changes in coronary vessel architecture during chronic CO-induced cardiac hypertrophy and are consistent with earlier hemodynamic and morphometric studies of CO-exposed hearts.

Chronic carbon monoxide exposure in young rats alters coronary vessel growth

The goal of this study was to determine whether chronic monoxide exposure in the developing heart produces long-lasting coronary vasculature alterations. One-day-old male rat pups were exposed to 500 ppm CO continuously for 30 d, while littermate controls remained in room air (AIR). At 61 and 110 d of age hearts were removed, perfusion fixed, x-rayed, and processed for analysis of coronary vessel architecture. Body weight (BW) and heart weight (HW) increased with age; the ratio of HW/BW decreased. There were no differences in HW and ventricular dimensions at either age due to prior CO exposure. Morphometric analysis of the fixed hearts from CO-exposed and AIR rats revealed no significant individual group differences in the number of small (27-114 microns) or larger (> 114 microns) vessels in any heart region. The septum (S) in CO rats was an exception: There were more small veins at 61 d of age and more larger veins at 110 d of age. There was a significant increase in the number of small arteries at both ages in the CO rats across all heart regions, and in the smaller veins at 61 d of age. The large vessels in the S at 61 d of age had a significantly greater diameter in CO compared to AIR rats. This was also true for the large arteries in the S and right ventricle (RV) of the 110-d-old rats. Taking all heart regions together, the large arteries in CO rats were larger than in AIR rats. Previous CO exposure significantly increased large artery and total cross-sectional area in the S and RV at 61 d of age, and in RV at 110 d of age. Total cross-sectional area of veins in the S was also increased. Taking all heart regions together, CO significantly increased small artery cross-sectional area at 61 d of age, and small, large, and total artery cross-sectional area at 110 d of age. With one exception (small veins, 110 d of age), there was no effect of CO on vein cross-sectional area. These changes resulted in the percentage of total cross-sectional area contributed by the larger vessels being increased. Pathological examination showed nothing abnormal. The results suggest profound and persistent changes in coronary vessel architecture following chronic neonatal CO exposure.

The atherogenic potential of carbon monoxide

This paper reviews the available evidence concerning the atherogenic potential of carbon monoxide. The evidence comes from two different types of studies--epidemiology studies on populations of humans chronically exposed to carbon monoxide, and animal studies conducted under conditions of controlled exposure to carbon monoxide. Data from both epidemiology and animal studies suggest that carbon monoxide is not atherogenic. Therefore, the increased levels of atherosclerosis associated with smoking as reported in epidemiology studies of human smokers probably cannot be attributed to CO exposure.

Carbon monoxide, carbon disulfide, lead and cadmium--four examples of occupational toxic agents linked to cardiovascular disease

A variety of industrial agents to which large segments of the population are exposed have been linked to the development of cardiovascular disease. These toxic agents, which include carbon monoxide, carbon disulphide, lead, and cadmium, are produced by a wide variety of industrial processes and so are ubiquitous in the modern industrial environment. Although the mechanisms by which such toxins may contribute to cardiovascular disease are not well defined, there are at least four possible theories that have received clinical and experimental support. The postulated mechanisms by which such toxins may play a role in vascular disease are: (1) an increase in blood pressure; (2) an increase in the levels of blood cholesterol and/or an induction of lipid accumulation in vessel walls; (3) an induction of a lipid peroxidation process thereby increasing the tendency of blood clotting; and (4) a promotion of a mutation in the arterial cell wall.

Blood carbonmonoxyhaemoglobin levels are chronically elevated in alcoholics treated for detoxification

Self-reported daily consumption of cigarettes and ethanol was recorded from a group of 59 ethanol-dependent persons admitted to a detoxification clinic at 2000-2400 h and averaged 40 cigarettes per day and 320 g ethanol per day, respectively. In consequence, concentrations of blood COHb and ethanol were high. Mean values for COHb were 8.5 and 9.9% in men and women, respectively, while the corresponding levels for ethanol were 220 and 280 mg/100 ml. Blood acetate concentrations were also elevated and correlated negatively with mixed venous oxygenhaemoglobin concentrations. The mean carbonmonoxyhaemoglobin concentration at 0700 h was found to be approximately 65% of that on admission. Biochemical and haematological analysis, past medical history and current physical examination revealed a high degree of ethanol and/or cigarette-related pathology. Studies on lipid and lipoprotein metabolism in ethanol-dependent persons should take into account their simultaneous high cigarette abuse.

The effect of nicotine on aortic endothelial cell turnover. An autoradiographic study

Endothelial injury and increased mitotic activity are early features in the pathogenesis of intimal thickening in arteries. This study examines the effect of systemic nicotine on mitotic activity in endothelial cells. Nine adult mice were given nicotine in their drinking water for 5 weeks. The dose (5 mg/kg body wt/day) was equivalent to a human smoking 50-100 cigarettes/day. A group of 8 similar mice, not exposed to nicotine, was the control. At the end of the exposure period all mice were injected with [3H]thymidine (1 microCi/g body wt) and were killed 24 h later. After perfusion fixation, en-face preparations of aortic endothelium were processed for autoradiography. In nicotine-affected endothelium 0.46 +/- 0.11% (SEM) of cells were labelled, which was significantly higher (P less than 0.01) than in controls (0.14 +/- 0.06%). However, there was no difference in cell density between the groups. On this evidence it was concluded that the rate of cell loss, or cell turnover, was greater in nicotine-affected endothelium. Because other studies have shown that increased mitotic activity and cell loss are established features of endothelial injury, the present findings provide evidence in support of the hypothesis that nicotine contributes to the pathogenesis of arterial disease in smokers.

Cigarette smoke extracts, but not nicotine, inhibit prostacyclin (PGI2) synthesis in human, rabbit and rat vascular tissue

Ethanolic and aqueous cigarette smoke extracts were tested for their influence on prostacyclin (PGI2) synthesis in four vascular models: human umbilical artery, rabbit aorta, rat aorta and rat lung. Nicotine was also studied. In each tissue, a dose-dependent inhibition of the release of PGI2 (assessed by measurement of immunoreactive 6-oxo-PGF1 alpha, the stable hydrolysis product of PGI2), was seen on incubation with aqueous or ethanolic cigarette smoke extracts; nicotine, at concentrations of up to 1 g/1, was without effect. In vitro conversion of [14C]-arachidonic acid to [14C]-6-oxo-PGF1 alpha by human umbilical artery was also inhibited by both ethanolic and aqueous cigarette smoke extracts, whereas nicotine was again without effect. We conclude that cigarette smoke inhibits PGI2 synthesis at the level of cyclooxygenase or beyond it, and that components other than nicotine are responsible for this effect. In blood vessels, the inhibition of PGI2 synthesis may contribute to the pathogenesis of the vascular complications of smoking.

Exacerbation of coronary artery disease by occupational carbon monoxide exposure: a report to two fatalities and a review of the literature

The recent deaths of two workers with coronary artery disease (CAD) following exposure to carbon monoxide (CO) at work reinforced our appreciation of the hazard of this exposure to individuals with preexisting heart disease. Carbon monoxide acts to precipitate ischemia by reducing oxygen delivery to the myocardium. Animal and in vitro experiments suggest that CO may accelerate the development of atherosclerosis, particularly if exposure is in association with other risk factors. Thus, persons with known CAD who are exposed to CO at work are at risk for both the acceleration of the course of the underlying disease and for precipitation of acute ischemia or infarction following excessive exposure. Particular attention should be given to control of CO exposures in light of this hazard. For various reasons, preplacement evaluations or other job selection procedures do not adequately address his hazard. In view of the high prevalence of CAD in the U.S. and the high frequency of workplace exposure to CO, particular attention should be given to control of CO exposure through industrial hygiene measures.

The influence of passive smoking on the cardiovascular system

Cardiovascular effects of tobacco smoke have been studied in passive smokers far less extensively than in active smokers. Under real-life conditions, passive smokers inhale approximately 0.02 to 0.01 of the amount of particulate matter taken up by active smokers. Their nicotine concentration in serum is within a range that is barely distinguishable from the background level. The increase in carboxyhemoglobin rarely exceeds 1%. In healthy subjects heavily exposed to tobacco smoke, no or only slightly acute effects on the cardiovascular system are found. Whether or not passive smoking is likely to aggravate symptoms in patients with advanced coronary heart disease has not yet been unequivocally established and requires further investigation. From a few studies on occupational groups exposed to carbon monoxide (CO) and from experiments with animals chronically treated with CO or nicotine, the conclusion can be drawn that neither CO nor nicotine is likely to play a role in the development and progression of coronary heart disease in those concentrations normally found in passive smokers.

Cardiovascular disease and work place exposures

The typical occupational cohort study includes all causes of mortality. However, emphasis is usually placed on the presence or absence of excess cancer mortality. A systematic review of completed occupational cohort studies to assess the findings and patterns of cardiovascular mortality would be useful. Although many of these studies will illustrate the "healthy worker effect" with deficits in mortality, particularly from cardiovascular causes, a thorough review should indicate certain exposures needing further research. A recently published study of heart disease mortality in the rubber industry illustrates the potential use of such a literature review with subsequent follow up. Production workers in the rubber industry have shown small excesses in CAHD mortality. A follow-up study at one plant confirmed the known association between carbon disulfide and atherosclerosis, as well as suggested two new causal associations between CAHD and the use of phenol and ethanol as solvents. What additional techniques can be used to generate hypotheses on heart disease and occupation? Some possibilities include: A recent article describes the use of the results of occupational disease surveillance systems for occupational cancer research. A review of such systems for heart disease would be equally useful. It would be useful to review the quality and quantity of occupational data that has been collected in prospective cohort studies, such as those in Framingham and Evans County. The importance of examining the association between occupational exposures and heart disease include: Assessing whether adequate protection is afforded by current limits on exposure to substances known to cause heart disease (carbon disulfide, nitrates, and carbon monoxide).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of carbon monoxide on atherogenesis in normal pigs and pigs with von Willebrand's disease

The extent of coronary and aortic atherosclerosis was examined in pigs following balloon-catheter injury of coronary arteries and subsequent feeding of an atherogenic diet for 4 months. The pigs were either exposed intermittently to 100 ppm carbon monoxide or to ambient air alone. Three types of pigs were used: normals, homozygotes for von Willebrand's disease (bleeders), and heterozygotes (carriers). The 3 types of pigs developed coronary artery intimal lesions of similar thickness. Aortic lesions, quantified as percent of aortic surface involved with sudanophilia and raised fibrous plaques, were slightly less extensive in bleeder pigs than in normals. Carbon monoxide exposure did not increase the thickness of coronary artery intimal lesions, nor did it increase the percent of aortic surface involved with sudanophilia or raised fibrous lesions. These results suggest that exposure to low levels of carbon monoxide does not perceptibly enhance atherogenesis induced by hypercholesterolemia. None of 14 bleeder pigs showed evidence of myocardial infarction, despite significant coronary artery narrowing. Of the 24 normal and carrier pigs, 5 showed myocardial infarction. Four of these 5 pigs were exposed to carbon monoxide, while 1 was not exposed. These findings suggest that exposure to low levels of carbon monoxide may increase the incidence of myocardial infarction and that the absence of von Willebrand factor may be protective.

Failure of chronic cigarette smoke exposure to alter plasma lipoproteins of stumptailed macaques (Macaca arctoides)

Twenty-one 8-14 kg adult male stumptailed macaques, Macaca arctoides, were fed a standard laboratory diet and divided into 3 groups. The high-dose group and low-dose group were exposed to cigarette smoke at the human equivalent of 3 packs and 1 pack per day, respectively, 7 days per week, for 3-5 years. Eight animals served as cage an sham controls. Peak blood carboxyhemoglobin (COHb) levels measured immediately after smoking showed levels of 0.5+/- 0.1%, 3.6+/-1.0%, and 5.7+/-2.8% for sham controls, low, and high dose smokers, respectively. Hemoglobin and hematocrit values were 2-7% higher (N.S. to P less than 0.05) for smoking groups, presumably as a consequence of chronically elevated COHb levels. No significant differences were seen in total plasma cholesterol and lipoprotein cholesterol concentration measured at four intervals over period of one year. We conclude from these data that, while fed a low fat diet, chronic cigarette smoke inhalation fails to alter plasma lipoprotein levels in this animal model.

Myocardial morphology in rabbits exposed to various gas-phase constituents of tobacco smoke--an ultrastructural study

Rabbits were continuously exposed to 200 ppm carbon monoxide. Using the same criteria as applied by earlier investigators for morphological myocardial damage, no histotoxic effect on myocardial morphology could be demonstrated when electron-microscopic investigations were performed blindly. Similarly, exposure to 0.5 ppm hydrogen cyanide, 0.5 ppm hydrogen cyanide + 200 ppm carbon monoxide, 0.5 ppm hydrogen cyanide + 200 ppm carbon monoxide + 5 ppm nitric oxide and to 50 ppm carbonyl sulphide for 1-7 weeks had no significant effect on myocardial ultrastructure.

In vivo oxygen transport in the normal rabbit femoral arterial wall

In vivo measurements of tissue oxygen tension were made at 10-micrometer intervals through functioning in situ rabbit femoral arterial walls, using inhalation anesthesia and recessed microcathodes with approximately 4-micrometer external diameters. External environment was controlled with a superfusion well at 30 torr PO2, 35 torr PCO2. Blood pressure, gas tension levels, and blood pH were held within the normal range. Radial PO2 measurements closely fit a mathematical model for unidimensional diffusion into a thick-walled artery with uniform oxygen consumption, and the distances traversed fit measured dimensions of quick-frozen in vivo sections. Using standard values of diffusion and solubility coefficients, mean calculated medial oxygen consumption was 99 nl0/ml-s. Mural oxygen consumption appeared to be related linearly to mean tangential wall stress. Differences in experimental design and technique were compared with previous in vivo and in vitro measurements of wall oxygenation, and largely account for the varying results obtained. Control of environment external to the artery, and maintenance of normally flowing blood in the lumen in vivo appeared critical to an understanding of mural oxygenation in life. If the conditions of this experiment prevailed in arteries with thicker avascular layers, PO2 could have been 20 torr at approximately 156 micrometer and 10 torr at 168 micrometer from blood (average values).

The effect of carbon monoxide exposure on morphology of lungs and pulmonary arteries in rabbits. A light- and electron-microscopic study

Rabbits were continuously exposed either to 200 parts per million carbon monoxide for 3, 4, and 6 weeks or to 1,900 parts per million carbon monoxide for 5 h. Blind light and transmission electron microscopic investigations showed no morphological changes in the lung parenchyma, lung vasculature or in pulmonary arteries of the exposed animals when compared with controls. The discrepancies with findings previously reported by other authors are discussed.

Cardiovascular disease and environmental exposure

This paper reviews the possible association between cardiovascular disease and occupational and environmental agents. The effects of carbon monoxide, fibrogenic dusts, carbon disulphide, heavy metals, noise, radiation, heat, cold, solvents and fluorocarbons are discussed. New directions for investigation are suggested.