an antagonist is determined by the inhibitory (I) action of the drug concentration
on the receptor site. An inhibitory action of 50 (I 50 ) indicates that the drug effec-
tively inhibits the receptor response in 50% of the population.
Agonists and antagonists lack specific and selective effects. They are called
nonspecific and have nonspecificity properties. Each receptor can produce a
variety of physiologic responses. Cholinergic receptors are located in the blad-
der, heart, blood vessels, lungs, and eyes. A cholinergic stimulator or blocker
will affect all of these sites. These drugs are called nonspecific or are said to
have nonspecificity properties. A drug that is given to stimulate the cholinergic
receptors will decrease the heart rate and blood pressure, increase gastric acid
secretion, constrict bronchioles, increase urinary bladder contraction, and con-
strict the pupils. The effects may be beneficial or harmful.
Categories of Drug Action
Drugs are categorized by the type of action it causes on the body. There are four
types of responses:
- Stimulation or Depression. These are drugs that either increase or depress
cellular activity. - Replacement. These are drugs that replace an essential body compound
such as insulin or estrogen. - Inhibition. These drugs interfere with bacterial cell and limit bacterial
growth or eliminate the bacteria, such as penicillin. - Irritation. These drugs irritate cells to cause a natural response that has a
therapeutic effect such as a laxative that irritates the colon wall to increase
movement of the colon resulting in defecation.
Therapeutic Index and Therapeutic Range
Drugs have a pharmaceutical response as long as the dose remains within the
drug’s margin of safety. Some drugs have a broad margin of safety. This means
that a patient can be given a wide range of dose levels without experiencing a
toxic effect. Other drugs have a narrow margin of safety where a slightest
change in the dose can result in an undesirable adverse side effect.
CHAPTER 2 Drug Action and Drug Interactions^35