Another class of drugs called antagonists blocks the access or binding of agonists to their receptors. For example, the cholinergic receptor antagonist ipratropium blocks the bronchoconstrictor effect of acetylcholine, the natural transmitter of cholinergic nerve impulses. Agonists and antagonists are used as different but complementary approaches to the treatment of asthma. The adrenergic receptor agonist albuterol, which relaxes bronchiolar smooth muscle, may be used together with the cholinergic receptor antagonist ipratropium, which blocks the bronchoconstrictor effect of acetylcholine. A widely used group of antagonists is the beta-blockers such as propranolol.
Antagonists are most effective when the local concentration of an agonist is high.
They work in much the same way that a roadblock affects a major highway. Similarly, beta-blockers, in doses that have little effect on normal heart function, may protect the heart against sudden surges of stress hormones. In addition to cell receptors, other important targets for drug action are enzymes, which help transport vital chemicals, regulate the rate of chemical reactions, or serve other transport, regulatory, or structural functions. For example, the drug lovastatin, used to treat some people who have high blood cholesterol levels, inhibits the enzyme HMG-CoA reductase, which is critical in the body's production of cholesterol. Affinity and Intrinsic Activity Two drug properties important to a drug's action are affinity and intrinsic activity.