In the late 1970s, rumors began to spread in bodybuilding circles about a drug that appeared to produce the long-sought Holy Grail of bodybuilders: it helped to promote the loss of excess body fat, while building muscle mass simultaneously. This redistribution of fat and muscle is known as a repartitioning effect. The only problem was that thus far, this effect had only been seen in horses. The horses were provided with a drug that had been introduced in 1977 called clenbuterol. Clenbuterol was given to horses to help them breathe easier. This was so because clenbuterol was a beta-2 agonist drug. What that meant was that clenbuterol selectively interacted with beta-2 adrenergic cell receptors in the lungs. Doing so would dilate the bronchial tubes and increase breathing.
There are three main types of beta-2 adrenergic cell receptors. The beta-1 exists in the heart and elsewhere, less so in the lungs. When beta-1 receptors are stimulated, the heart rate increases and the heart works harder. With asthma, the problem is constriction of the bronchial tubes. For years, the primary cause of asthma was thought to be oversensitivity of the bronchial tubes, or allergic reactions in the lungs that caused them to constrict. It's now known, however, that asthma has a largely inflammatory component that is the actual primary cause of asthmatic reactions. But it's also been known for a long time that when you stimulate the beta-2 receptors in the lungs, the bronchial tubes dilate, thus preventing and curtailing an asthma attack. That is the primary use of clenbuterol and other beta-2 adrenergic receptor drugs. But the unexpected muscle mass increase shown by horses that were provided with clenbuterol caught the attention of some people. Also, the fact that stimulating beta-2 receptors also promotes the release of stored fat in fat cells made clenbuterol an especially interesting drug.
When initially marketed in 1977,clenbuterol was aimed at human use. However, some drawbacks of the drug soon became evident. For one, it had a much longer half-life compared to other beta-2 agonist drugs. Half-life refers to how long it takes the body to break down half of the original dose. For clenbuterol or "clen" as it's frequently known in bodybuilding and fitness, the half-life is 25 to 40 hours. The problem with that is that the longer time that clen stays in the body, the greater the chance of side effects, especially if repeated doses are taken. Although drugs such as clen are called beta-2 selective drugs, meaning that the majority of their activity involves an interaction with beta-2 adrenergic receptors, there is still some overlap with beta-1 receptors, and that will stimulate the heart. Because of excessive heart stimulation, clenbuterol soon fell out of favor as a drug to treat asthma. Indeed, it was never approved for sale in . . .