- Sympathomimetic drugs are substances that mimic the effects of the sympathetic nervous system, such as catecholamines, epinephrine (adrenaline), norepinephrine (noradrenaline), dopamine, etc. Such drugs are used to treat cardiac arrest and low blood pressure, or even delay premature labor, among other things.
- These drugs act at the postganglionic sympathetic terminal, by directly activating postsynaptic receptors, blocking breakdown and reuptake, or stimulating production and release of catecholamines.
There are two types of adrenergic drugs.
- Direct Acting Drugs
- Indirect Acting Drugs
Direct Acting Drugs
These are of two types
Natural compounds (catecholamines)
- If these catecholamines are released from nerve fibers, these are called neurotransmitters.
- If released from glands, these are called as hormones.
- If released from the outside, these are called as drugs.
These are of further two types;
These stimulate α receptors. These include;
- Phenylephrine, Methoxyline (stimulate α-1 receptors)
- Clonidine, Detomedine, Xylezene (stimulate α-2 receptors)
- Isoprotalenol (stimulate β-1 & β-2 receptors)
- Dobutamine (stimulate β-1 receptors)
- Terbutalin, Sulbutamol, Clenbutarol (stimulate β-2 receptors)
Indirect Acting Drugs
- These increase level of nor-epinephrine on receptors. These stop removal of nor-Epinephrine by stopping re uptake or by increasing the release of nor-epinephrine from the storage vesicles. These are divided into 3 categories;
- These enhance release of Nor-epinephrine from the endings. These include Tyramine, Amphetamine and Ephedrine (mixed).
- These drugs block and reverse the activity of nor-epinephrine transporter (NET). NET is a transport protein present on surface of some cells that clears adrenaline and nor-adrenaline from the extracellular space and into the cells, thus terminating the signaling effect.
- These drugs include cocaine, Triphenylamine.
MAO Inhibitors (MAOI)
- Adrenaline and nor-adrenaline are metabolized by the enzyme monoamine oxidase. These drugs inhibit the activity of this enzyme, thus producing sympathomimetic effects.
- These drugs include Isocarboxazid, Phenelzine, Iproclozide, Procarbazine, Hydralazine and Phenoxypropazine.
Prototype drug in adrenergic drugs is;
- Adrenaline b/c it stimulates all four types of receptors.
- Nor-adrenaline cannot stimulate β-2 receptors.
- Others stimulate only 1 or 2 receptors.
- Asthma can be treated by β-2 agonists.
- Cardiac arrest can be treated by β-1 agonists.
- Retention of Urine
- To decrease the intestinal motility
- Vasoconstriction to increase blood flow and treat nasal congestion
- Spray of α-1 agonist during surgery to decrease bleeding from small blood vessels
- To enhance the duration of action of local anesthetics, adrenaline is used which decrease the absorption of anesthetic by causing vasoconstriction.
- Cocaine is used as local anesthetic but vasoconstrictor is not used along with it b/c it is itself a vasoconstrictor.
- Stimulate CNS E.g. Cocaine
- Some used in allergic reactions
- Some used to delay the labor condition of uterus near parturition to manage the delivery at proper time.
- Glycogenolysis and gluconeogenesis are increased by stimulation of β-receptors in liver, muscles and adipose tissue.