This chapter considers the drugs that mimic the effects of adrenergic nerve stimulation (or stimulation of the adrenal medulla). In other words, these compounds mimic the effects of norepinephrine or epinephrine. These drugs are sometimes referred to as adrenomimetics or sympathomimetics. Remember that the actions of the sympathetic nervous system are mediated through α and β receptors.
|Remember that: |
| α1 = most vascular smooth muscle; agonists contract |
| β1 = heart; agonists increase rate |
| β2 = respiratory and uterine smooth muscle; agonists relax |
There are other effects of sympathetic stimulation, but the three listed in the preceding box are the most important.
The adrenergic agonists are often divided into direct- and indirect-acting agonists. This is a useful distinction for a number of reasons. The indirect-acting drugs do not bind to specific receptors, but act by releasing stored norepinephrine. This means that their actions are nonspecific. The direct-acting drugs bind to the receptors, so specificity of action is a possibility.
The drugs are also sometimes divided into catecholamines and noncatecholamines. This is yet another division based on structure (and our focus here is not on structures). However, this distinction is useful for one concept. Do you remember from Chapter 6 that norepinephrine is metabolized by catechol-o-methyltransferase (COMT) and monoamine oxidase (MAO)? Well, the other catecholamines are also metabolized by these enzymes; however, the noncatecholamines are not.
The focus here is to learn the specificity of the drugs for their receptor targets. If you know the effect of stimulation of the target receptors, then you can deduce the drug actions and adverse effects.
Only EPINEPHRINE and NOREPINEPHRINE activate both α and β receptors.
Although this is an oversimplification, it provides a useful starting point. The rest of the direct-acting drugs act on either α or β receptors (Figure 9–1). Epinephrine has approximately equal effects at α and β receptors. In addition, it has approximately equal effects at β1 and β2 receptors.
A classification of adrenergic agonists is presented. Affinity for the α receptors is shown at the top of the diagram and affinity for the β receptors at the bottom. Epinephrine and norepinephrine have affinity for both α and β receptors and are, therefore, placed in the middle.
Epinephrine has a number of uses, including the treatment of allergic reactions and shock, the control of localized bleeding, and the prolongation of the action of local anesthetics.
NOREPINEPHRINE has a relatively low affinity for β2 receptors.
Norepinephrine activates both α and β receptors, but activates β1 receptors more than β2...