Adrenergic Agonists

Adrenergic Agonists

  • Definition: Adrenergic drugs target adrenergic receptors stimulated by norepinephrine (noradrenaline) and epinephrine (adrenaline).

  • Receptor Types:

    • Adrenergic receptors (adrenoceptors): Types include α (alpha) and β (beta) receptors.

    • Sympathomimetics: Activate adrenergic receptors.

    • Sympatholytics: Block adrenergic receptor activation.

Types of Adrenergic Agonists

  • Direct-acting: Activate adrenergic receptors directly.

    • Examples: Epinephrine, norepinephrine, isoproterenol, phenylephrine.

  • Indirect-acting: Enhance norepinephrine release or block its reuptake.

    • Examples: Cocaine, amphetamines.

  • Mixed-action: Stimulate receptors and release norepinephrine.

    • Examples: Ephedrine, pseudoephedrine.

Major Catecholamines

  • Noradrenaline (Norepinephrine): Released by sympathetic nerve terminals.

  • Adrenaline (Epinephrine): Hormone secreted by the adrenal medulla.

  • Dopamine: Precursor to norepinephrine and epinephrine; functions in the CNS.

  • Isoprenaline (Isoproterenol): Synthetic derivative of norepinephrine.

Neurotransmission in Adrenergic Neurons

  1. Synthesis of Norepinephrine:

    • Tyrosine → DOPA via tyrosine hydroxylase.

    • DOPA → Dopamine via aromatic L-amino acid decarboxylase.

    • Dopamine → Norepinephrine via dopamine β-hydroxylase.

  2. Storage of Norepinephrine:

    • Stored in vesicles; blocked by reserpine.

  3. Release of Norepinephrine:

    • Triggered by action potentials and calcium influx; blocked by guanethidine.

  4. Binding to Receptors:

    • Norepinephrine binds to postsynaptic and presynaptic receptors, activating secondary messenger systems (cAMP, phosphatidylinositol).

  5. Removal of Norepinephrine:

    • Mechanisms include diffusion, metabolic breakdown by COMT, and reuptake via norepinephrine transporter (NET).

  6. Fates of Recaptured Norepinephrine:

    • Repackaging into vesicles or oxidation by monoamine oxidase (MAO).

Adrenergic Receptors

  • Types: α (alpha) and β (beta) adrenoceptors.

α-Adrenoceptors:

  • α1 Receptors: Postsynaptic; induce smooth muscle contraction and vasoconstriction.

  • α2 Receptors: Presynaptic; inhibit norepinephrine release causing feedback inhibition.

β-Adrenoceptors:

  • β1 Receptors: Cardiac stimulation; equal affinity for epinephrine and norepinephrine.

  • β2 Receptors: Primarily in the vascular smooth muscle; have greater affinity for epinephrine.

  • β3 Receptors: Involved in lipolysis and bladder control.

Responses to Adrenergic Stimulation

  • α1 Activation: Vasoconstriction, increased peripheral resistance.

  • β1 Activation: Increased heart rate, contractility.

  • β2 Activation: Vasodilation, smooth muscle relaxation.

Mechanism of Action of Adrenergic Agonists

  • Direct-acting agents: Bind and activate adrenergic receptors directly.

  • Indirect-acting agents: Increase or prolong the effects of norepinephrine through various mechanisms.

Clinical Applications

Epinephrine:

  • Uses: Bronchospasm, anaphylactic shock, cardiac arrest, local anesthetic solutions.

  • Effects: Increases heart rate, dilates bronchi, raises blood pressure through vasoconstriction.

Norepinephrine:

  • Uses: Typically in treating shock; vasoconstriction leads to increased blood pressure.

Isoproterenol:

  • Uses: Cardiac stimulant; dilates arterioles in skeletal muscles.

Dopamine:

  • Uses: Cardiogenic shock, increases renal blood flow, improves cardiac output.

Non-Catecholamines:**

  • Phenylephrine: Vasoconstrictor; treats hypotension and nasal congestion.

  • Clonidine: α2 agonist; treats hypertension and withdrawal symptoms.

  • Albuterol, terbutaline: Short-acting β2 agonists used for asthma.

  • Salmeterol, formoterol: Long-acting β2 agonists for chronic respiratory conditions.

  • Mirabegron: β3 agonist for overactive bladder treatment.

Adverse Effects of Adrenergic Agonists

  • General: Anxiety, jitters, arrhythmias, hypertension.

  • Specific to drug class and receptor subtype activated.