Читать книгу Fundamentals of Pharmacology for Paramedics - Группа авторов - Страница 32

Some drugs are very selective in their binding sites, and can bind to a very limited number of sites, or only one site, but most drugs will be able to bind to more than one site. For example, a bronchodilator medication that acts as an agonist at adrenergic beta receptors may bind at only beta‐2 receptor subtypes, in which case it would be a selective beta‐2 agonist, but is more likely to bind to both beta‐1 and beta‐2 receptors, because of the degree of chemical similarity between the two receptor subtypes. The more selective a drug is for a single receptor, the fewer effects it is likely to bring about, so a more selective drug is likely to be one with fewer side‐effects. On the other hand, a less selective drug which activates two or three related receptors may have more therapeutic uses, but it will also have more side‐effects. The selectivity of a drug will tend to decrease as the dose increases, because binding to other receptor types will become more likely as the concentration of the drug increases. This helps to explain the dose dependency of many side‐effects of medications.