Adrenergic Drugs and Autonomic Nervous System

Adrenergic Drugs

• Adrenergic drugs stimulate the sympathetic nervous system (SNS) and are also known as adrenergic agonists or sympathomimetics, mimicking the effects of SNS neurotransmitters.

Autonomic Nervous System

• The autonomic nervous system (ANS) is a branch of the peripheral nervous system that controls autonomic bodily functions, consisting of the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). • The ANS is further subdivided into parasympathetic (cholinergic) and sympathetic (adrenergic) systems, which provide checks and balances to maintain homeostasis.

Characteristics of Adrenergic Agonists

• Mimic the effects of SNS neurotransmitters: catecholamines (norepinephrine, epinephrine, dopamine) • Divided into alpha-adrenergic receptors (alpha1, alpha2) and beta-adrenergic receptors (beta1, beta2, beta3)

Alpha-Adrenergic Receptors

• Located throughout the body • Receptors for sympathetic neurotransmitters • Divided into alpha1 and alpha2 receptors, differentiated by their location on nerves • Alpha1 receptors: located on postsynaptic effector cells, control vasoconstriction • Alpha2 receptors: located on presynaptic nerve terminals, control the release of neurotransmitters

Beta-Adrenergic Receptors

• Located on postsynaptic effector cells • Beta1 receptors: located primarily in the heart, stimulate cardiac muscle and increase heart rate • Beta2 receptors: located in smooth muscle of bronchioles, arterioles, and visceral organs, stimulate relaxation and dilation

Adrenergic Agonist Responses

• Alpha-adrenergic agonist responses: vasoconstriction, mimic naturally occurring neurotransmitters • Beta-adrenergic agonist responses: bronchial, gastrointestinal, and uterine smooth muscle relaxation, glycogenolysis, cardiac stimulation

Responses to Stimulation

• Cardiovascular: blood vessels (alpha1 - vasoconstriction, beta2 - vasodilation), cardiac muscle (beta1 - increased contractility), atrioventricular node (beta1 - increased heart rate) • Endocrine: liver (alpha1, beta2 - glycogenolysis), kidney (beta1 - increased renin secretion) • Gastrointestinal: muscle (alpha1, beta2 - decreased motility), genitourinary: bladder sphincter (alpha1 - constriction), penis (alpha1 - ejaculation), uterus (alpha1 - contraction, beta2 - relaxation) • Respiratory: bronchial muscles (beta2 - dilation), ocular: pupillary muscles of the iris (alpha1 - mydriasis)

Direct-acting, Indirect-acting, and Mixed-acting Sympathomimetics

• Direct-acting sympathomimetics: bind directly to the receptor and cause a physiologic response • Indirect-acting sympathomimetics: cause release of catecholamine from storage sites in nerve endings, which then binds to receptors • Mixed-acting sympathomimetics: directly stimulate the receptor and indirectly stimulate the receptor by causing release of stored neurotransmitters

Drug Effects

• Stimulation of alpha-adrenergic receptors on smooth muscles: vasoconstriction, constriction of bladder sphincter, contraction of uterus, male ejaculation, contraction of pupillary muscles of the eye • Stimulation of beta1-adrenergic receptors on the myocardium, atrioventricular node, and sinoatrial node: cardiac stimulation (increased force of contraction, heart rate, and conduction) • Stimulation of beta2-adrenergic receptors on the airways: bronchodilation, relaxation of uterine, GI smooth muscles, glycogenolysis in the liver, and increased renin secretion in the kidneys

Indications

• Respiratory: bronchodilators (stimulate beta2-adrenergic receptors), treatment of nasal congestion (alpha1-adrenergic receptors) • Cardiovascular: vasoactive adrenergics (pressors, inotropes), treatment of open-angle glaucoma (alpha-adrenergic receptors)

Contraindications and Adverse Effects

• Known drug allergy, severe hypertension • Alpha-adrenergic: headache, restlessness, excitement, insomnia, euphoria, chest pain, vasoconstriction, reflexive bradycardia, palpitations, dysrhythmias • Beta-adrenergic drugs: mild tremors, headache, nervousness, dizziness, increased heart rate, palpitations, fluctuations of blood pressure, sweating, nausea, vomiting, muscle cramps

Learn about adrenergic drugs, their effects on the sympathetic nervous system, and the autonomic nervous system, including its subdivisions and functions.