By Amy S. Gorin
Caffeine inhibits some physiological responses to nicotine, an MIT research team has discovered.
According to the team's report, which will be published in the September issue of Proceedings of the National Academy of Science, adenosine (a substance found in tissues and blood) enhances nicotine's effect as a hypertensive (blood-pressure raising) agent. Caffeine inhibits the action of adenosine.
Reid W. von Borstel '78, one of the members of the research team, warned this information should not provide "a false sense of security. Caffeine may protect against nicotine's blood pressure- raising effects, but it will certainly not have a protective effect against many other potentially harmful effects of cigarette smoking."
Andrew A. Renshaw '85 and Professor Richard. J. Wurtman are the other members of the research team, which is part of the Laboratory of Neuroendocrine Regulation in the Department of Nutrition and Food Science. The team discovered the caffeine-nicotine link accidentally, while studying the effects of adenosine on blood pressure regulation.
Injecting adenosine into rats causes their blood pressure and pulse rate to drop. The team hoped to determine adenosine 's effects on the rat circulatory system, the amount needed to evoke a response, the amount naturally present, and whether naturally present adenosine acts as a blood pressure regulator.
To examine adenosine's regulatory effect, the blood pressure of a rat was artificially raised by electrical stimulation, injections of noradrenaline (similar to adrenaline), or injections of nicotine. The rat was then injected with adenosine. The researchers discovered adenosine dramatically enhanced the effects of nicotine, rather than inhibiting its effects as expected.
The team hypothesized that naturally present adenosine might also enhance the effects of nicotine. If so, nicotine would have less of an effect on blood pressure in the presence of an adenosine inhibitor, such as caffeine. The researchers found this to be true.
While no member of the scien tific community yet understands why adenosine enhances the effects of nicotine, smokers may al ready unknowingly be using caffeine to block adenosine's effects. The adenosine molecule is formed from adenosine-triphosphate in a reaction that provides fuel for the cell. It is released by the cell if cellular metabolism is impaired.
Adenosine binds to receptor sites on the surface of certain cells and in the brain, inducing a sedative effect, and possibly providing the brain with information about the energy levels of the body. It is thought that caffeine also binds to these sites, blocking the binding of adenosine, and thereby producing a well known stimulating effect.
Nicotine-induced high blood pressure is caused by constriction of the blood vessels, which also causes a drop in the amount of oxygen available to the cell. Lack of oxygen stimulates the cell to release adenosine, which in turn enhances the effects of the nicotine, creating a positive feedback loop. The resulting downward spiral of energy levels can be broken by the ingestion of caffeine. Studies have shown that cigarette smokers do in fact drink more coffee than non-smokers.