Alpha and Beta Adrenergic Receptors

Alpha Adrenergic Receptors

• Alpha-1 receptors are linked to Gq proteins and primarily located in smooth muscles.
• Vasoconstriction occurs in blood vessels, leading to increased blood pressure.
• Smooth muscle relaxation in the gastrointestinal tract and urinary bladder wall, while sphincters contract.
• Active mydriasis results from the stimulation of dilator pupillae muscle in the eye.
• Alpha-2 receptors, linked to Gi proteins, are primarily found in neuronal sites and inhibit various functions.
• They decrease central sympathetic outflow, leading to reduced blood pressure and induce lipolysis.
• Alpha-2 receptors inhibit insulin secretion and renin release, while promoting platelet aggregation and vasoconstriction.

Beta Adrenergic Receptors

• Beta-1 receptors are linked to Gs proteins and are predominantly found in cardiac muscles.
• Cardiac stimulation results in increased heart rate (HR), contraction strength, and conduction velocity, enhancing cardiac output and blood pressure.
• Beta-1 receptors also influence lipolysis and stimulate renin secretion, which leads to angiotensin II production and vasoconstriction.
• Beta-2 receptors, also linked to Gs proteins, mediate smooth muscle relaxation, primarily causing vasodilation in skeletal and coronary blood vessels.
• Bronchodilation and mast cell stabilization are key effects of beta-2 activation, alongside weaker insulin release and glycogenolysis in the liver and muscles, raising blood glucose levels.
• They also allow uterine and intestinal relaxation.
• Beta-3 receptors are involved in lipolysis and relaxation of the detrusor muscle of the urinary bladder.

Dopaminergic Receptors

• D1 receptors cause vasodilation in renal, coronary, cerebral, and mesenteric blood vessels.
• D2 receptors, found presynaptically, decrease dopamine and norepinephrine release from nerve endings.

Sympathomimetic Drugs (Adrenergic Agonists)

• Direct Acting: Act directly on adrenergic receptors.
• Indirect Acting: Promote norepinephrine release or inhibit its reuptake.
• Dual (Mixed) Acting: Combine both mechanisms.

Direct Acting Adrenergic Agonists

• Epinephrine: A non-selective agonist affecting α1, α2, β1, and β2 receptors, increases force of contraction, heart rate, and conduction speed, and may cause arrhythmias at high doses.
• Isoprenaline: A synthetic catecholamine acting on β1 and β2; increases cardiac output and induces bronchodilation and vasodilation.

Selective Alpha-1 Agonists

• Phenylephrine: Long-acting and not inactivated by COMT; used for hypotensive states, mydriasis, decongestion, and hemorrhoid treatment.
• Midodrine: A prodrug for postural hypotension with potential adverse effects like hypertension and bradycardia.

Selective Beta-Agonists

• Dobutamine: β1 selective, enhancing heart performance without significant cardiac complications.
• β2 Agonists:
  • Used in bronchial asthma and preventing premature labor.
  • Side effects include anxiety, muscle tremors, tachycardia, and potential for tolerance.

Selective D1 Agonist

• Fenoldopam: Induces peripheral vasodilation, mainly used intravenously for severe hypertension.

Adrenergic Neurotransmitters

• Norepinephrine: Functions primarily in postganglionic sympathetic fibers and select CNS tracts.
• Epinephrine: Major hormone of the adrenal medulla.
• Dopamine: Functions as both a central and peripheral neurotransmitter.

Overview of Sympathetic Nervous System

• The sympathetic system is essential for regulating heart and peripheral vascular activities, especially during stress responses.
• Adrenergic receptors belong to the G-protein family, facilitating various physiological responses in the body.

This quiz covers the functions and properties of alpha and beta adrenergic receptors, including their role in the inhibition of neurotransmitter release and cardiac stimulation. Test your understanding of how these receptors affect heart rate, contraction, and blood pressure regulation.