THE BLOG
05/29/2013 03:29 pm ET Updated Jul 29, 2013

Rushing to Conclusions About Adrenaline

In 'Mission Impossible III', a secret agent (Tom Cruise) injects adrenaline into his barely conscious protégé (Keri Russell); she immediately responds by fighting off dangerous adversaries. News reports attribute to shots of adrenalin the capacity to lift a car off a trapped child or kayak away faster than an onrushing shark. Also, adrenaline is described as the root of the emotional thrills experienced by BASE jumpers and other daring adventurers. How do these popular accounts about adrenaline stack up under scrutiny?

In 1894, George Oliver demonstrated that an unknown substance in extracts of adrenal glands powerfully raised blood pressure. Six years later, Jokichi Takamine, a Japanese scientist in New York City, succeeded in chemically isolating this substance which he named 'adrenaline'. During the early part of the 20th century, Walter Cannon developed a conceptual framework that made sense of adrenaline's many physiological effects. He proposed that the brain stimulated secretion of adrenaline from the adrenal glands; after entering the bloodstream, adrenaline stimulated major organs such as the heart, arteries, lungs, and muscles to deal with threats; he termed this the 'fight or flight' response.

We now know a great deal about how both adrenaline and its partner noradrenaline--the neurotransmitter in the sympathetic nervous system--stimulate responses in the heart and other organs. Adrenaline and noradrenaline fit into specific receptors; activation of these receptors triggers a sequence of intricate cellular responses. In the heart activation of beta receptors causes more rapid and stronger beating, pumping more blood to exercising muscles.

What do we really know about the assumption that adrenaline or noradrenaline could almost instantaneously transform a weakling into a heroic performer? People with diseases that limit the release of noradrenaline from the sympathetic nervous system have diminished responses to exercise. Beta blockers--drugs that antagonize the actions of adrenaline at beta receptors--also diminish some maximal physical responses. However, even at very large doses, the effects of adrenaline-like drugs on strength and endurance range from none at all to 5-10 % improvements at most. No scientific data even hints that adrenaline could induce dramatic and rapid increases in strength.

Many athletes believe that large doses of drugs that mimic adrenaline can improve performance in endurance sports; indeed, these drugs are banned from the Olympics and other major sporting events such as the Tour de France, except in limited doses to treat asthma. Clenbuterol, a drug of this type, increases muscle mass in cattle. The winner of the 2010 Tour was stripped of his title after testing positive for clenbuterol. It is actually uncertain whether clenbuterol improves athletic performance. Nonetheless, very small effects could be enormously meaningful to elite athletes. Some Paralympic athletes with spinal cord transection can vastly augment release of noradrenaline from the sympathetic nervous system--cut off from control by the brain--by clamping off urinary catheters. This practice, known as 'boosting', possibly augments exercise capacity but is banned in competition because of potentially life-threatening rises in blood pressure. Sadly, problems with adrenaline-related athletic abuse are not new. In 1946, Time Magazine reported that a racing turtle dropped dead after being injected with adrenaline; apparently, the rules did not prohibit pharmacological aids.

Quantitatively, the known effects of adrenaline and related drugs are far removed from the alleged many-fold increases in strength reported in emergencies. Consequently, even if everyday citizens can have remarkable increases in speed or strength when confronted with life-threatening situations, there is little indication that adrenaline is responsible. On the other hand, there is convincing evidence that blocking some of the effects of adrenaline can help win medals in Olympic sports that depend on fine motor control. Beta blockers, such as propranolol, are banned from events involving marksmanship because they decrease normal tremors in the arms, improving the accuracy of expert shooters. Armed snipers, however, have more freedom in preparing for difficult shots.

'Adrenaline junkies' seek out extreme adventures for highly pleasurably rushes of excitement, sometimes risking life and limb jumping from tall cliffs or skiing in avalanche country. However, does adrenaline actually enhance human emotional responses? Extensive research over the past 100 years has overwhelming demonstrated that exciting events lead to a surge of adrenaline in the blood. Cannon used cats exposed to barking dogs in many of his experiments. His work supported the common sense view that perception of emotional events by the brain led to the release of adrenaline from the adrenals. However, his animal experiments could not test the hypothesis that adrenaline might then modify emotional feelings in humans.

Earlier, William James--one of Cannon's professors at Harvard--had suggested a rather different general view about the development of emotions. In 1884, James published an influential paper proposing that emotions were caused by physical responses to stimulating events. Accordingly, bumping into a bear in a forest provided a cue to run away; James claimed that it was the act of running away that led to the emotion of fear. In other words, he believed that emotions were produced by signals from the body to the brain although he had no experimental evidence that confirmed this hypothesis. We now know that even if changes in the body modify human emotions, adrenaline in the blood is not directly responsible because many experiments have shown that adrenaline does not penetrate the blood-brain barrier. Furthermore, experiments infusing adrenaline into the blood of volunteers in calm laboratory settings does not stimulate intense emotions. Nonetheless, the possibility that adrenaline has indirect effects that influence the perception of emotions in conjunction with exciting events in the real world remains an open question.

While shots of adrenaline alone do not enormously boost physical or emotional responses, adrenaline and noradrenaline play major roles in helping us cope with what the world throws our way. Moreover, the depth and importance of scientific and medical discoveries associated with adrenaline over the past hundred years, while more subtle than the usual fare of adrenaline fanatics, continues to provide considerable excitement to those interested in the history of medicine and scientific discovery.

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