Tidal volume (TV), the volume of air a person breathes in or out during normal breathing, will often be decreased if the asthmatic hyperventilates. Treatment of an asthma attack takes advantage of knowledge of cellular and molecular biochemistry. Formation of mast cell and smooth muscle cell cyclic AMP (cAMP) inhibits mast cell release of histamine and SRS-A and triggers smooth muscle cell relaxation. The goal then is to increase the cAMP concentrations in mast cells and smooth muscle cells to prevent the release of histamine and SRS-A,and to prevent smooth muscle cell constriction of airways. This can be accomplished by several means. The cell enzyme that degrades cAMP, cAMP phosphodiesterase, can be inhibited by a molecule known as theophylline. Theophylline is a common aerosol inhalant taken by asthmatics; it is also available in pill form. Theophylline, found in many teas, is an example of a class of molecules known as methylxanthines. caffeine is another example of a commonly occurring methylxanthine. Stimulation of beta receptors on the surface of mast cells and smooth muscle cells leads to an increase in their cellular cAMP, thereby inhibiting release of histamine and SRS-A. Epinephrine (adrenalin) stimulates beta receptors and is thus hrlpful in treating an acute asthma attack. Unfortunately, beta receptors are also found on heart muscle and on other tissues as well. Selective stimulation of lung beta receptors would be optimal, and indeed desired, particularly in someone with heart disorders. Beta receptors differ (biochemically) between the lung and the heart—but only slightly. Lung beta receptors are termed beta-2 and heart beta receptors are termed beta-1. Because of the slight molecular differences between beta-1 and beta-2 receptors, chemists have designed drugs to bind to one stronger than the other. Selective beta-2 stimulants include metaproteranol, terbutaline, and salbutamol; such agents are useful in treating asthma and avoiding effects on the heart. Because the mast cells are a leukocyte, they can be inhibitea with corticosteroids, such as hydrocortisone and prednisone. Corticosteroids inhibit leukocyte function by multiple mechanisms, and also stimulate beta-2 receptor synthesis. Cromolyn sodium is a newer drug used to prevent asthma attacks. It works by preventing histamine and SRS-A release from mast cells.