The effect of carbon monoxide on cholesterol in the aortic wall of rabbits

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.

Effect of verapamil on accumulation of free and esterified cholesterol in the thoracic aorta of cholesterol-fed rabbits

In order to investigate the effect of the calcium antagonist verapamil on atherogenesis in cholesterol-fed rabbits, 3 groups of 11 animals were fed a 2% cholesterol-enriched diet for 10 weeks. One group received verapamil in a daily dose of 16 mg/kg orally plus 2 mg/kg subcutaneously. This dosage resulted in plasma concentrations of verapamil in the same range as the usual therapeutic levels in humans. Another group received verapamil in a daily dose of 8 mg/kg orally and 0.5 mg/kg subcutaneously. The third group received placebo capsules orally and isotonic saline subcutaneously. Total cholesterol concentrations in plasma over the 10 weeks were 37 +/- 4, 42 +/- 4 and 45 +/- 3 mM (mean +/- SE) in the high verapamil-, in the low verapamil- and in the placebo group, respectively. These values were not significantly different. The distribution of cholesterol between HDL, LDL and VLDL in plasma was similar in the 3 groups. The high verapamil group had a significantly (P less than 0.05) lower concentration of cholesterol in the thoracic aorta than the placebo group (29 +/- 5 vs 43 +/- 9 mumol/g wet weight). The low verapamil group and the placebo group had the same aortic cholesterol concentrations. Neither dosage of verapamil affected the permeability of the aortic endothelium to plasma lipoproteins and albumin, as measured by use of radioactive tracers at the end of the experiment.

No effect of nifedipine on atherogenesis in cholesterol-fed rabbits

It was recently reported that the calcium antagonist nifedipine suppresses aortic cholesterol accumulation in cholesterol-fed rabbits without reducing hypercholesterolemia. We extended this study on plasma lipoprotein levels and aortic influx of cholesteryl ester. We gave 40 mg per day of nifedipine orally to 17 rabbits fed a 2% cholesterol diet for 8 weeks. For the same period of time 15 control rabbits received placebo capsules and the same diet. During the study, total cholesterol, HDL, LDL, and VLDL cholesterol concentrations in plasma were not significantly different in the experimental and control animals. At the end of the study we found no difference in the two groups in accumulation of cholesterol in the intima media of the proximal thoracic aorta, the distal thoracic aorta, and the corresponding media layers. Furthermore, aortic influx of free cholesterol, cholesteryl ester, and albumin from plasma measured by radioactive tracers was not significantly affected by nifedipine.

Plasma lipid and lipoprotein profiles of cigarette smokers from randomly selected families: enhancement of hyperlipidemia and depression of high-density lipoprotein

Cigarette smoking is an important risk factor for coronary heart disease. However, its mechanisms of action remain to be fully explored. The hypothesis of the present study is that 1 mechanism whereby cigarette smoking enhances coronary disease might result from its effects upon the plasma lipids. Accordingly, we measured the plasma lipids and lipoproteins in cigarette smokers, exsmokers and nonsmokers from 233 randomly selected American families. Cigarette smokers (male and female) had significantly lower high-density lipoprotein levels and higher very low density lipoprotein and plasma triglyceride levels than the exsmokers and nonsmokers. The plasma levels of lipids and lipoproteins were related to the number of cigarettes smoked per day. Heavier cigarette smokers (greater than 25 cigarettes/day) had significantly lower high-density lipoprotein levels and significantly higher very low density lipoprotein total cholesterol and plasma triglyceride levels than those who smoked less than 25 cigarettes/day, nonsmokers and exsmokers. The lipid and lipoprotein values of those who smoked less than 15 cigarettes/day were similar to those of exsmokers and nonsmokers. Inasmuch as exsmokers had levels of plasma lipids and lipoproteins similar to those of nonsmokers, these findings add another health-enhancing benefit to the cessation of smoking.

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.

[CO-pollution, a health risk in Geneva?]

The results from CO measurements at different locations in Geneva show that the highest levels are found in streets in the center of the city. An evaluation of the medical risks is performed against the most sensitive criteria--effects on the central nervous system, the cardiac muscle and the fetus. It is concluded that symptoms of anoxia may develop among persons with existing cardiovascular diseases.

Atherogenesis in the White Carneau pigeon. Further studies of the role of carbon monoxide and dietary cholesterol

Two experiments are described. In the first, 3 pairs of groups of 20 female White Carneau pigeons were fed on diets containing 0.5%, 1% or 2% cholesterol. Birds in one group from each pair were exposed to 150 ppm carbon monoxide (CO) for 6 h on 5 days of each week for 52 weeks, sufficient to raise their carboxyhaemoglobin (COHb) levels to approximately 10%, while those in the other group were sham-exposed under similar conditions. In the second experiment, 3 groups of 40 female pigeons were each fed on a diet containing 1% cholesterol, one group being exposed to CO to give COHb levels of 20%, one to give COHb levels of 10% and one being sham exposed. In addition, 20 birds in the second experiment were fed on the 1% cholesterol diet but were neither exposed to CO nor sham-exposed. Cholesterol enriched diets caused mean plasma cholesterol values in each group to rise sharply within 4 weeks of starting them, but the levels reached were as high with diets containing 0.5% cholesterol as for diets containing 1% or 2% added cholesterol. Exposure to CO increased plasma and aortic cholesterol levels, though this increase was only statistically significant for aortic levels in the second experiment. In both experiments combined exposure to the 1% cholesterol diet and CO resulted in a significant decrease in aortic triglyceride content. The incidence and severity of coronary artery atherosclerosis was associated with increasing dietary cholesterol. It was also associated with exposure to CO in birds given 0.5% or 1%, but not 2%, dietary cholesterol; the increase in birds given 1% was related to the dose of CO. Possible mechanisms are discussed for this effect of CO, which is not found in normally fed birds.

Arterial entrance and metabolism of free and esterified plasma cholesterol measured in vivo in experimental animals by a dual isotope method

The arterial walls of 3 cholesterol-fed rabbits were exposed for 3--4 hours in vivo to homologous cholesterol-labelled plasma with a 20-fold higher [3H/14C] ratio in esterified cholesterol (EC) than in free cholesterol (FC). The [3H/14C] ratio in total cholesterol (TC) in the thoracic aorta was 0.6--0.9 times the ratio prevailing in TC in plasma. This corresponds to a transfer from plasma into the arterial wall of relatively more FC, with the low ratio, than EC with the higher ratio. The [3H/14C] ratio in FC in the arterial wall was 1.4--2.0 times the ratio in FC in plasma. This corresponds to hydrolysis of some of the entered EC, adding cholesterol with the high ratio to FC with the lower ratio. The [3H/14C] ratio in EC in the arterial wall was 0.1--1.1 times the ratio in EC in plasma. This corresponds to no esterification in the arterial wall of entered FC. The same method applied to normolipidemic cockerels gave similar results. In a kinetic model the arterial influx from plasma of FC and EC combined (lipoprotein influx) corresponded in both groups to 20--90 nl plasma per cm2 arterial surface per hour with subsequent hydrolysis of 10--50% of the entered EC. The arterial influx of FC without EC (influx by exchange) accounted for 10--25% of the total influx of cholesterol in the rabbits and 40--70% in the cockerels.

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

Non-cholesterol-fed rabbits were exposed to carbon monoxide at concentrations in air of either 200, 2000, or 4000 parts per million (=0.02, 0.2 or 0.4%, vol/vol). Using the same criteria for intimal damage as in earlier morphological studies, no histotoxic effect on intimal/subintimal morphology of coronary arteries or the aorta could be demonstrated, when light-microscopic evaluation was performed blindly.