ADRENERGICS AND ADRENERGIC BLOCKERS
Andrenergics are medications that stimulate alpha 1 -receptors and beta 2 -adren-
ergic receptors. Alpha 1 -receptors are located in the smooth muscle of vascular
(vessels) tissues. Beta 2 -adrenergic receptors are in the smooth muscle of the
lungs, arterioles of skeletal muscles, and the uterine muscles. Adrenergics also
stimulate the dopaminergic receptor located in the renal, mesenteric, coronary,
and cerebral arteries to dilate and increase blood flow. Dopamine is the only
adrenergic that can activate this receptor.
Adrenergic blockers inactivate these receptors in three ways:1.They promote reuptake of the transmitter back into the neuron (nerve
cell terminal).
2.Transmitters are transformed or degraded by enzymes making them unable
to attach to a receptor. Two enzymes that inactive norepinephrine are
monoamine oxidase (MAO) and catechol-o-methyl-transferase (COMIT).
MAO is inside the neuron and COMIT is outside the neuron.
3.Transmitters are diffused away from receptors.Sympathomimetic drugs stimulate andrenergic receptors and are classified
into three categories according to its effect on organ cells. These categories are:1.Direct-acting sympathomimetics—directly stimulate receptors.
2.Indirect-acting sympathomimetics—stimulate the release of norepineph-
rine from terminal nerve endings.
3.Mixed-acting sympathomimetics—have the effect of both direct-acting
sympathomimetics and indirect-acting sympathomimetics. They simulate
the adrenergic receptor sites and stimulate the release of norepinephrine
from terminal nerve endings. Ephedrine is an example of a mixed-acting
sympathomimetic and is used to treat idiopathic orthostatic hypotension
and hypotension resulting from spinal anesthesia. Ephedrine also stimu-
lates beta 2 -receptors to dilate bronchial tubes and is used treat mild forms
of bronchial asthma.Many adrenergic medications stimulate more that one adrenergic receptor site.
For example, epinephrine (Adrenalin) acts on alpha 1 -, beta 1 -, beta 2 -receptor sites.
These receptor sites include an increase in blood pressure, pupil dilation,
increase in heart rate (tachycardia), and bronchodilation.
Epinephrine (Adrenalin) is used to treat cardiogenic and anaphylactic shock
because it increases blood pressure, heart rate, and airflow through the lungs(^284) CHAPTER 15 Nervous System Drugs