Alpha Receptors Overview
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Questions and Answers

What cardiovascular effect does acetylcholine NOT cause?

  • Increase in heart rate (correct)
  • Vasodilation
  • Decrease in force of cardiac contraction
  • Decrease in conduction velocity

    • Which subtype of muscarinic receptors is primarily responsible for vasodilation when acetylcholine is administered?

  • M2
  • M1
  • M4
  • M3 (correct)

    • What does atropine primarily inhibit at high doses?

  • α-adrenergic receptors in blood vessels
  • M2 receptors on the heart (correct)
  • M1 receptors in the brain
  • M3 receptors in vascular smooth muscle

    • Which receptor is primarily responsible for causing vasodilation in the aorta and coronary artery?

    <p>Beta-adrenergic receptors</p> Signup and view all the answers

    Which effect of acetylcholine on the heart is primarily significant in the atria and less so in the ventricles?

    <p>Negative inotropic effect</p> Signup and view all the answers

    Which of the following actions results from stimulation of alpha-2a receptors?

    <p>Inhibition of sympathetic neurons</p> Signup and view all the answers

    What compensatory mechanism often obscures the effects of acetylcholine on heart rate and blood pressure?

    <p>Baroreceptor reflexes</p> Signup and view all the answers

    What system primarily utilizes acetylcholine to affect cardiac function?

    <p>Parasympathetic nervous system</p> Signup and view all the answers

    What is a significant effect of stimulating beta-1 adrenergic receptors?

    <p>Increase in cAMP</p> Signup and view all the answers

    What is the primary effect of stimulation of α1a-adrenoceptors?

    <p>Increase in PLC and Ca2+ channel activity</p> Signup and view all the answers

    Alpha-2b receptors are primarily associated with which location?

    <p>Liver</p> Signup and view all the answers

    What is the result of a small intravenous dose of acetylcholine?

    <p>Transient fall in blood pressure</p> Signup and view all the answers

    Which adrenergic receptor primarily mediates vasoconstriction in small procapillary vessels of skeletal muscle?

    <p>Alpha-2a receptors</p> Signup and view all the answers

    What cardiovascular effect does scopolamine primarily target?

    <p>Decreased salivation</p> Signup and view all the answers

    Which α1-adrenoceptor subtype is the most abundant in the heart?

    <p>α1B</p> Signup and view all the answers

    Which physiological factors can dynamically influence cardiac function?

    <p>Age and congestive heart failure</p> Signup and view all the answers

    Which mechanism describes how alpha-1d receptors contribute to erectile dysfunction?

    <p>Causing vasoconstriction in the prostate</p> Signup and view all the answers

    What common feature do alpha-2c receptors share with other alpha-2 receptor subtypes?

    <p>Decrease in adenylate cyclase activity</p> Signup and view all the answers

    What is a characteristic effect of α1B receptor stimulation?

    <p>Increase in renin release</p> Signup and view all the answers

    What is the role of beta-adrenergic receptors in the cardiovascular system?

    <p>Increase force of heart contractions</p> Signup and view all the answers

    Which system is primarily responsible for the regulation of cardiac function?

    <p>Autonomic nervous system</p> Signup and view all the answers

    What role do α1A receptors play in physiological responses?

    <p>Promoting vasoconstriction primarily</p> Signup and view all the answers

    Which statement accurately describes α1-adrenoceptors?

    <p>They primarily mediate sympathetic responses.</p> Signup and view all the answers

    How do physiological factors like aging affect the autonomic control of cardiac function?

    <p>They can alter receptor sensitivity and responsiveness.</p> Signup and view all the answers

    What are the primary mechanisms through which cardiac function is regulated?

    <p>Through the autonomic nervous system and physiological factors</p> Signup and view all the answers

    Which subtype of α1-adrenoceptors is most commonly found in the heart?

    <p>α1B</p> Signup and view all the answers

    What physiological effect is primarily associated with stimulation of α1A receptors?

    <p>Increased PLC and vasoconstriction</p> Signup and view all the answers

    Which adrenergic receptor subtype's stimulation is associated with cardiac growth and structure?

    <p>α1B</p> Signup and view all the answers

    What is the role of physiological factors in cardiac function?

    <p>They regulate cardiac function during aging and disease states.</p> Signup and view all the answers

    Which receptor subtype has been identified to function primarily in vascular contraction?

    <p>α1A</p> Signup and view all the answers

    What are the identified subtypes of muscarinic receptors involved in cardiac function?

    <p>M2 and M3</p> Signup and view all the answers

    In which organ systems are α1B receptors predominantly located?

    <p>Kidney, spleen, and lungs</p> Signup and view all the answers

    Which receptor is primarily responsible for vasoconstriction in the aorta and coronary artery?

    <p>α1a receptors</p> Signup and view all the answers

    What effect does stimulation of α2a receptors have on adenylate cyclase activity?

    <p>Decreases adenylate cyclase activity</p> Signup and view all the answers

    Which of the following receptors is associated with vasodilation in blood vessels?

    <p>β2 receptors</p> Signup and view all the answers

    Which adrenergic receptor is known to modulate dopamine neurotransmission?

    <p>α2c receptors</p> Signup and view all the answers

    What is the primary result of stimulating β1 adrenergic receptors?

    <p>Increase in cAMP levels</p> Signup and view all the answers

    Where in the body are α2b receptors primarily located?

    <p>Liver and kidney</p> Signup and view all the answers

    Which receptor primarily inhibits sympathetic neurons?

    <p>α2a receptors</p> Signup and view all the answers

    What role does cAMP play in the context of α2 receptor stimulation?

    <p>cAMP levels decrease</p> Signup and view all the answers

    What primary cardiovascular effect does acetylcholine induce?

    <p>Vasodilation</p> Signup and view all the answers

    How does atropine primarily affect the heart at high doses?

    <p>It inhibits vagal tone producing tachycardia</p> Signup and view all the answers

    What is the result of a large intravenous dose of acetylcholine on the heart?

    <p>Elicits bradycardia and AV nodal conduction block</p> Signup and view all the answers

    What compensatory response often mitigates the effects of acetylcholine on blood pressure?

    <p>Baroreceptor reflexes</p> Signup and view all the answers

    Which type of receptors primarily mediate the generalized vasodilation observed with acetylcholine?

    <p>M3 muscarinic receptors</p> Signup and view all the answers

    What cardiovascular effect does scopolamine selectively target?

    <p>Decreased bronchial secretions</p> Signup and view all the answers

    What effect does acetylcholine typically have on heart rate and rhythm?

    <p>It decreases heart rate and conduction velocity</p> Signup and view all the answers

    Which cardiovascular action is least affected by acetylcholine's influence?

    <p>Force of cardiac contraction in ventricles</p> Signup and view all the answers

    What is the ratio of β1 to β2 receptors in the ventricles?

    <p>4:1</p> Signup and view all the answers

    Which receptor types are primarily associated with 'smooth muscle relaxation'?

    <p>β2 receptors</p> Signup and view all the answers

    Which of the following agonists is primarily a beta agonist?

    <p>Dobutamine</p> Signup and view all the answers

    Which statement correctly describes the effects of β1 receptors on the heart?

    <p>They increase heart rate and contractility.</p> Signup and view all the answers

    What is the main function of β3 receptors in the human body?

    <p>Metabolic effects.</p> Signup and view all the answers

    Which of the following receptors primarily acts through the Gq protein pathway?

    <p>α1 receptors</p> Signup and view all the answers

    Which receptor has an affinity state proposed but has not been cloned?

    <p>β4 receptors</p> Signup and view all the answers

    Which of the following exhibits a mixed alpha and beta agonistic effect?

    <p>Adrenaline</p> Signup and view all the answers

    Which area of the heart is primarily affected by β1 receptors in terms of automaticity and conduction velocity?

    <p>SA node</p> Signup and view all the answers

    What is the primary function of α1 receptors in the sympathetic nervous system?

    <p>Lead to smooth muscle contraction.</p> Signup and view all the answers

    Which statement about noradrenaline is incorrect?

    <p>It has high affinity for beta-2 receptors.</p> Signup and view all the answers

    What is the main effect of isoproterenol?

    <p>Increase in heart rate.</p> Signup and view all the answers

    At what dosage does dopamine begin to primarily affect DA receptors?

    <p>0.5-2 μg/kg/min.</p> Signup and view all the answers

    Which drug is considered the most potent vasoconstrictor?

    <p>Adrenaline.</p> Signup and view all the answers

    What is a characteristic effect of using high doses of dopamine (>10 μg/kg/min)?

    <p>Vasoconstriction via alpha receptors.</p> Signup and view all the answers

    Noradrenaline treatment leads to which change in blood pressure over time?

    <p>Rebound fall in blood pressure.</p> Signup and view all the answers

    Which of the following describes the main therapeutic use of isoproterenol?

    <p>Management of bradycardia.</p> Signup and view all the answers

    Which physiological effect does adrenaline primarily cause?

    <p>Produces positive inotropic effects.</p> Signup and view all the answers

    Study Notes

    Alpha 1 Receptors

    • Alpha 1 receptors are potent vasoconstrictors, causing narrowing of blood vessels.
    • Alpha 1 receptors are found in the prostate, aorta, and coronary artery.
    • Dysfunction of Alpha 1 receptors can cause erectile dysfunction.
    • Alpha 1 receptors utilize PLC, PLA2, Ca2+ channel activity, and Na+/H+ exchange as signaling pathways.
    • Alpha 1 receptors are found in platelets, prostate, aorta, coronary artery, cortex, and hippocampus.
    • Alpha 1a and Alpha 1d are predominantly found in the aorta and coronary artery, causing vasoconstriction.
    • Alpha 1b receptors are most abundant in the heart, promoting cardiac growth and structure.

    Alpha 2 Receptors

    • Alpha 2 receptors cause vasoconstriction indirectly by inhibiting sympathetic neurons.
    • Alpha 2a receptors decrease adenylate cyclase activity, reducing cAMP and PKA activity.
    • Alpha 2a receptors are located in platelets, sympathetic neurons, autonomic ganglia, pancreas, coronary/CNS vessels, Locus ceruleus, brainstem, and spinal cord.
    • Alpha 2a receptors are the predominant inhibitory receptors on sympathetic neurons.
    • Alpha 2a receptors cause vasoconstriction of small procapillary vessels in the skeletal muscle.
    • Alpha 2b receptors decrease adenylate cyclase activity, reducing cAMP and PKA activity.
    • Alpha 2b receptors are located in the liver, kidney, blood vessels, coronary/CNS vessels, diencephalon, and pancreas.
    • Alpha 2b receptors are the predominant receptors mediating vasoconstriction.
    • Alpha 2c receptors decrease adenylate cyclase activity, reducing cAMP and PKA activity.
    • Alpha 2c receptors are located in the basal ganglia, cortex, cerebellum, and hippocampus.
    • Alpha 2c receptors modulate dopamine neurotransmission and inhibit hormone release from the adrenal medulla.

    Beta Receptors

    • Beta 1 receptors increase adenylate cyclase activity, leading to increased cAMP and intracellular calcium.
    • Dopamine does not affect Beta 2 receptors.

    Acetylcholine (ACh)

    • Acetylcholine has vasodilatory effects.
    • Acetylcholine decreases heart rate (negative chronotropic effect).
    • Acetylcholine decreases conduction velocity in the AV node (negative dromotropic effect).
    • Acetylcholine decreases the force of cardiac contraction (negative inotropic effect).
    • The effects of acetylcholine on the heart can be influenced by baroreceptor reflexes.
    • Acetylcholine rarely given systemically, but its effects are important for understanding the effects of other drugs.
    • Intravenous injections of acetylcholine cause vasodilation due to stimulation of endothelial nitric oxide (NO) production.
    • Vasodilation caused by acetylcholine is primarily due to muscarinic M3 receptor stimulation.

    Antimuscarinics

    • Scopolamine is selective for sweat, salivary and bronchial glands, eye and iris, ciliary muscle.
    • Atropine is selective for heart, gastrointestinal system (GIS), and bronchial muscle.
    • At high doses, Atropine blocks M2 receptors, leading to increased heart rate (tachycardia).
    • At low doses, Atropine can cause bradycardia.
    • Atropine has positive inotropic and chronotropic effects on the heart, increasing force and rate of contraction.
    • Atropine does not directly affect blood vessels because there is no parasympathetic innervation of blood vessels.
    • Atropine can prevent the vasodilator effects of parasympathetic drugs.

    Cardiovascular Pharmacology

    • Cardiac function is regulated by the autonomic nervous system, specifically the sympathetic and parasympathetic nervous systems.
    • The autonomic nervous system acts through adrenoceptors and muscarinic acetylcholine receptors.
    • There are at least 9 adrenoceptor subtypes and 5 muscarinic receptor subtypes.
    • Autonomic control of the heart can be influenced by physiological factors like aging and disease states like congestive heart failure.

    Autonomic Nervous System and Cardiac Function

    • Cardiac function is regulated by the sympathetic and parasympathetic nervous systems, acting through adrenoceptors and muscarinic acetylcholine receptors respectively.
    • At least nine adrenoceptor subtypes and five muscarinic receptor subtypes exist.
    • Autonomic control of cardiac function is dynamically regulated by factors such as aging and diseases like congestive heart failure.

    α₁ Adrenoceptors

    • Three subtypes of α₁ adreoceptors exist: α₁A, α₁B, and α₁D.
    • α₁A receptors are primarily responsible for vasoconstriction and are found in the heart, liver, smooth muscles, blood vessels, lung, and various brain regions.
    • α₁B receptors are found in kidney, spleen, lung, blood vessels, and cortex.
    • α₁B receptors are the most abundant in the heart and promote cardiac growth and structure, making them relevant to anti-remodelling benefits.
    • α₁D receptors are found in platelets, prostate, aorta, coronary artery, cortex, and hippocampus. They play a significant role in vasoconstriction of the aorta and coronary artery.
    • Dysfunction of α₁A and α₁D receptors, which are potent vasoconstrictors in the prostate, can lead to erectile dysfunction.

    α₂ Adrenoceptors

    • α₂A receptors are found in platelets, sympathetic neurons, autonomic ganglia, pancreas, coronary/CNS vessels, and brain regions like the locus ceruleus, brainstem, and spinal cord.
    • α₂A receptors are primarily inhibitory receptors on sympathetic neurons.
    • While not directly causing vasoconstriction, they can cause it indirectly by inhabiting sympathetic neurons in blood vessels.
    • α₂A receptors mediate vasoconstriction of small procapillary vessels in the skeletal muscle.
    • α₂B receptors are found in liver, kidney, blood vessels, coronary/CNS vessels, diencephalon, and pancreas.
    • They are the predominant receptors mediating vasoconstriction.
    • α₂C receptors are found in basal ganglia, cortex, cerebellum, and hippocampus.
    • They modulate dopamine neurotransmission and inhibit hormone release from the adrenal medulla.

    β Adrenergic Receptors

    • Three functional β adrenergic receptors exist: β₁, β₂, and β₃.
    • β₁ receptors are found predominantly in heart, kidney, adipocytes, skeletal muscle, and various brain regions.
    • β₁ receptors are the predominant receptors in the heart, responsible for positive inotropic and chronotropic effects.
    • β₂ receptors are found in heart, lung, blood vessels, bronchial and GI smooth muscle, kidney, skeletal muscles, and various brain regions.
    • They are the predominant receptors involved in smooth muscle relaxation and skeletal muscle hypertrophy.
    • β₃ receptors are found in adipocytes, GI tract, and heart.
    • They are the predominant receptors in metabolic effects.
    • A fourth type of β adrenoceptor, β₄, has been proposed but hasn't been cloned.

    Organ System Effects

    • Sympathetic Nervous System (Adrenergic Receptors):

      • SA Node: Increased heart rate (β₁)
      • Atria: Increased contractility and conduction velocity (β₁ > β₂)
      • AV Node: Increased automaticity, conduction velocity (β₁ > β₂)
      • His-Purkinje System: Increased automaticity, conduction velocity (β₁ > β₂)
      • Ventricles: Increase in automaticity, conduction velocity, contractility, and pacemaker activity (β₁ > β₂)

    • Parasympathetic Nervous System (Muscarinic Receptors):

      • SA Node: Decreased heart rate (M₂ >> M₃)
      • Atria: Decreased heart rate, action potential duration (M₂ >> M₃)
      • AV Node: Decreased conduction velocity, AV block (M₂ >> M₃)
      • His-Purkinje System: Little effect (M₂ >> M₃)
      • Ventricles: Slight decrease in contractility (M₂ >> M₃)

    Agonists and Their Effects

    • Noradrenalin:

      • Weakly produces arrhythmia compared to adrenaline and isoprenaline.
      • Increases heart rate but bradycardia can occur as a reflex effect. Atropine can reverse this.
      • Low affinity for β₂ receptors.
      • Contracts pregnant uterus.
      • Used in hypotension as a vasoconstrictor.
      • Produces a rebound fall in blood pressure following long-term use.

    • Isoprenaline:

      • Most potent β adrenoceptors drug.
      • Produces vasodilation and bronchodilation.
      • Vasodilator effect is predominant in skeletal and splanchnic vascular beds.
      • Used in bradycardia, heart block, shock, and intoxications of β-blockers.

    • Adrenaline:

      • Very potent vasoconstrictor and cardiac stimulant.
      • Produces positive inotropic and chronotropic effects leading to increased systolic blood pressure.
      • Decreases Total Peripheral Resistance (TPR).

    • Dopamine:

      • Simple catecholamine structure.
      • At low doses (0.5-2 µg/kg/min) it affects DA receptors, increasing renal blood flow and glomerular filtration.
      • At higher doses (2-10 µg/kg/min) it affects β receptors, increasing heart rate.
      • At very high doses (>10 µg/kg/min) it affects α receptors, producing vasoconstriction and decreasing blood flow.
      • Does not affect vasodilator β₂ receptors.

    Acetylcholine (ACh)

    • Has four primary effects on the cardiovascular system:
      • Vasodilation.
      • Decreased heart rate (negative chronotropic effect).
      • Decreased AV nodal conduction velocity (negative dromotropic effect).
      • Decreased force of cardiac contraction (negative inotropic effect) - more significant in atria than ventricles.

    Antimuscarinics

    • Scopolamine: Selective for sweat, salivary and bronchial glands, eye and iris, ciliary muscle.
    • Atropine: Selective for heart, GI and bronchial muscle.
      • At high doses, blocks M₂ receptors, inhibiting parasympathetic vagal tonus on the heart, producing tachycardia.
      • At low doses, bradycardia can occur due to inhibition of pre-synaptic M₁ receptors.
      • Has positive inotropic and chronotropic effects.
      • No direct effect on blood vessels due to the lack of parasympathetic innervation.
      • Can prevent vasodilator effects of parasympathetic drugs.

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