Dr Dohi said: "Smokers are twice as likely to have a heart attack as people who have never smoked. Quitting smoking is the most important thing people can do to reduce their risk of cardiovascular disease. But until now, studies have not examined whether the increased risk caused by smoking is completely reversed after smoking cessation."
The current study investigated how the vascular system is altered by smoking and whether the changes can be normalised by smoking cessation. The researchers focused on the effects of smoking and smoking cessation on arterial endothelial function and circulating serotonin concentration.
Both endothelial dysfunction and serotonin contribute to the development of atherosclerosis. Serotonin released from platelets induces platelet aggregation, which initiates blood coagulation and contractions in arteries especially those with damaged endothelium.
Smoke from cigarettes contains toxic molecules including nicotine, carbon monoxide and hydrogen cyanide which may cause and promote atherosclerosis via endothelial dysfunction and increased activity of blood coagulation.
The study included 27 apparently healthy male smokers aged 40±8 years and 21 age-adjusted non-smokers (40±7 years). Endothelial function was assessed by flow mediated dilation and peripheral arterial tonometry (PAT). Both methods assess endothelial function as the ability to dilate arteries through the release of endothelium-derived relaxing factors.
Dr Dohi said: "As expected, smoking damaged arterial endothelial function and increased plasma serotonin levels."
Only 21 subjects agreed to stop smoking for 8 weeks. Smoking cessation was confirmed in 11 out of the 21 subjects by measuring serum levels of cotinine, the principal metabolite of nicotine. Smokers who completely attained smoking cessation had a significantly increased PAT ratio (P<0.05) but serotonin levels were not significantly changed (see figure). Dr Dohi said: "This indicates that endothelial function had improved after 8 weeks of smoking cessation but serotonin levels remained at dangerously high levels."
An analysis of all 21 subjects who tried to stop smoking showed that serotonin levels were positively correlated with serum levels of cotinine (152±140 ng/ml, r=0.45, P<0.05) and inversely with PAT ratio (endothelial function: 0.25±0.10, r=-0.45, P<0.05). Dr Dohi said: "These results show that smoking cessation may have a beneficial effect on serotonin levels, but this effect is too small to normalise serotonin levels within 8 weeks. It will take a long time to see a significant effect on serotonin levels after smoking cessation."
Dr Dohi concluded: "Short term smoking cessation only partially improved the undesirable alterations caused by smoking. After 8 weeks of no smoking endothelial function improved but plasma serotonin remained high, suggesting that patients are still at increased risk of cardiovascular disease. Further studies are needed to see if longer term smoking cessation can completely reverse the damage caused by smoking."