Adrenergic receptors

Adrenergic receptors (or adrenoceptors) are a class of G-protein-coupled receptors (GPCRs) that are targets for catecholamines like adrenaline (epinephrine) and noradrenaline (norepinephrine). These receptors are classified into two main types, alpha (α) and beta (β) receptors, each with further subtypes:

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1. Alpha-Adrenergic Receptors (α)

a. Alpha-1 (α1) Receptors:

• Subtypes: α1A, α1B, α1D
• Location: Vascular smooth muscle, the heart, liver, and prostate.
• Physiological Activity:
  • Vasoconstriction (narrowing of blood vessels).
  • Increased peripheral resistance (raising blood pressure).
  • Pupil dilation (mydriasis).
  • Contraction of the bladder sphincter and prostate.

b. Alpha-2 (α2) Receptors:

• Subtypes: α2A, α2B, α2C
• Location: Presynaptic neurons, central nervous system (CNS), and blood vessels.
• Physiological Activity:
  • Inhibition of norepinephrine release (negative feedback mechanism).
  • Inhibition of insulin release from the pancreas.
  • Central sedation and analgesia.
  • Vasoconstriction at high doses.

2. Beta-Adrenergic Receptors (β)

a. Beta-1 (β1) Receptors:

• Location: Primarily in the heart and kidneys.
• Physiological Activity:
  • Increased heart rate (chronotropy).
  • Increased force of heart contraction (inotropy).
  • Release of renin from the kidneys (increasing blood pressure).

b. Beta-2 (β2) Receptors:

• Location: Lungs, skeletal muscle blood vessels, liver, and the uterus.
• Physiological Activity:
  • Bronchodilation (relaxation of airway smooth muscles).
  • Vasodilation in skeletal muscles.
  • Relaxation of uterine smooth muscles.
  • Glycogenolysis in the liver (increasing blood sugar).

c. Beta-3 (β3) Receptors:

• Location: Adipose tissue and the bladder.
• Physiological Activity:
  • Lipolysis (breakdown of fat in adipose tissue).
  • Relaxation of the bladder detrusor muscle, reducing urinary urgency.

Summary of Physiological Activities:

• α1 receptors: Mediate vasoconstriction and smooth muscle contraction.
• α2 receptors: Inhibit norepinephrine release and promote vasoconstriction.
• β1 receptors: Increase heart rate and contractility.
• β2 receptors: Promote bronchodilation and vasodilation.
• β3 receptors: Involved in fat metabolism and bladder relaxation.

Each receptor subtype has specific agonists and antagonists that can be used to modify their physiological activities for therapeutic purposes.

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