Direct acting sympathomimetics

Direct-acting sympathomimetic drugs act by activating one or more different subtypes of adrenergic receptors. The typical representative of direct-acting sympathomimetic drugs is the physiological transmitter adrenaline. Adrenaline acts on both alpha-adrenoreceptors and beta-adrenoreceptors. This particular functionality has led to the discovery of the duality of adrenoreceptors (alpha and beta receptors).

There is a large number of drugs that selectively act on one or more subtypes of adrenoreceptors and can therefore induce different effects. They can be applied topically or systemically. With topical application locally high drug concentrations are achieved. Systemic side effects are possible.

Depending on the bound receptor, the following active ingredients are distinguished:

alpha + beta sympathomimetics:

• Adrenalin (epinephrine)
• Noradrenaline (Norepinephrine)
• Xylometazoline
• Phenylephrine
• Oxymetazoline
• Miodrin

alpha2 sympathomimetics (α 2>α1):

• Clonidine
• Moxonidine
• alpha-methyldopa
• Brimonidine (centrally acting)

beta-sympathomimetics

beta1+beta2 sympathomimetics

• Orciprenalin
• Theodrenalin/Cafedrine

beta2 sympathomimetics (beta2>beta1 and beta2>>beta1))

• Terbutaline
• Fenoterol
• Salbutamol
• Clenbuterol
• Reproterol
• Formoterol
• Salmetrol
• Tolbuterol
• Bambuterol

Topical indications:

• Addition to local anaesthetics (adrenaline)
• Swelling of the mucous membranes in rhinitis, sinusitis, allergic conjunctivitis (alpha1-receptor agonists)
• Bronchial asthma (beta2 agonists)

Systemic indications (sympathomimetics are generally used as antiarrhythmics in acute events):

• Cardiogenic shock: the drug of choice is dobutamine, which acts as a beta and alpha1 receptor agonist; it also has a positive inotropic effect, without significant tachycardia; dopamine is also used in this indication)
• Anaphylactic shock: The drug of choice is adrenaline (500ug i.m. or 200-400ug slow i.v. can be life-saving; for therapeutic benefit, the activated alpha receptors (vasoconstriction), the beta1 receptors (positive inotropy and chronotropy) and the beta2 receptors (bronchodilation) are effective. Adrenaline can be administered for cardiopulmonary resuscitation (1.0 mg i.v.).
• AV block and bradycardic arrhythmias: Orciprenalin 0,25.0,5mg i.v.
• Premature labour: inhibition of premature labour. Fenoterol 20-25ugi.v. in 2-3 minutes, then infusion of 1.0-4.0ug/min. The tocolytic effect is increasingly reduced (tolerance phenomenon)
• Essential hypertension: Inhibition of blood pressure in essential hypertension: alpha2-receptor agonists.

All alpha- and beta-receptor antagonists cause tachycardic rhythm disturbances, blood pressure increase, pectangionous complaints, headaches, insomnia, heavy sweating.

All alpha- and beta-receptor agonists are interfered with by a number of other drugs. Directly acting sympathomimetics are reinforced by tricyclic antidepressants, thyroid hormones, MAO inhibitors, etc. They are attenuated by alpha-receptor antagonists and even reduce the blood sugar-lowering effect of all antidiabetics.

Mechanical obstruction of ventricular filling or ventricular outflow. Narrow-angle glaucoma, severe ventricular arrhythmias

1. Graefe KH (2016) Sympathetic nervous system. In: Graefe KH et al (Ed.) Pharmacology and Toxicology. Georg Thieme Publisher Stuttgart S. 85-103
2. Rasmussen N et al (2016) History full circle: 'Novel' sympathomimetics in supplements. Drug Test Anal 8(3-4):283-286.