Noncatecholamines

--> Noncatecholamine adrenergic drugs have a variety of therapeutic uses because of the many effects they can have on the body, including:

• local or systemic constriction of blood vessels (phenylephrine)
• nasal and eye decongestion and dilation of the bronchioles (albuterol, bitolterol, ephedrine, formoterol, isoetharine hydrochloride, isoproterenol, levalbuterol, metaproterenol, pirbuterol, salmeterol, and terbutaline)
• smooth-muscle relaxation (terbutaline).

Pharmacokinetics

Although these drugs are all excreted in urine, they’re absorbed in different ways.

Absorption and distribution

Absorption of the noncatecholamines depends on the administration route:

• Inhaled drugs, such as albuterol, are absorbed gradually from the bronchi and result in lower drug levels in the body.
• Oral drugs are absorbed well from the GI tract and are distributed widely in body fluids and tissues.
• Some noncatecholamines, such as ephedrine, cross the blood-brain barrier and can be found in high concentrations in the brain and cerebrospinal fluid (fluid that moves through and protects the brain and spinal canal).

Metabolism

Noncatecholamines are metabolized and inactivated primarily in the liver but also in the lungs, GI tract, and other tissues.

Excretion

Noncatecholamines and their metabolites are excreted primarily in urine. Some, such as inhaled albuterol, are excreted within 24 hours; others, such as oral albuterol, within 3 days. Acidic urine increases excretion of many noncatecholamines; alkaline urine slows excretion.

Pharmacodynamics

Noncatecholamines can be direct-acting, indirect-acting, or dual-acting (unlike catecholamines, which are primarily direct-acting).

• Direct-acting noncatecholamines that stimulate alpha receptors include phenylephrine. Those that selectively stimulate beta₂ receptors include albuterol, isoetharine, metaproterenol, and terbutaline.
• Dual-acting noncatecholamines include ephedrine.

Pharmacotherapeutics

Noncatecholamines stimulate the sympathetic nervous system, producing a variety of effects in the body. Phenylephrine, for example, causes vasoconstriction and is used to treat hypotension in cases of severe shock. Terbutaline is used to stop preterm labor.

Drug interactions

Here are a few examples of drugs that interact with noncatecholamines:

• Anesthetics (general), cyclopropane, and halogenated hydrocarbons can cause arrhythmias. Hypotension can also occur if these drugs are taken with noncatecholamines that exert predominantly beta₂ activity, such as terbutaline.
• Monoamine oxidase inhibitors can cause severe hypertension and even death.
• Oxytocic drugs that stimulate the uterus to contract can be inhibited when taken with terbutaline. When taken with other noncatecholamines, oxytocic drugs can cause hypertensive crisis or a stroke.
• Tricyclic antidepressants can cause hypertension and arrhythmias.
• Urine alkalizers, such as acetazolamide and sodium bicarbonate, slow excretion of noncatecholamines, prolonging their duration of action