Purinergic Signalling and Receptors

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Questions and Answers

What role does adenosine play in the body?

Inhibits blood flow regulation in the coronary system

Acts solely as a structural component of DNA

Functions exclusively as a neurotransmitter in the CNS

Serves as a signaling molecule in various physiological processes (correct)

Which signaling systems do purinergic receptors utilize?

Direct activation of gene transcription

Calcium ion signaling pathways exclusively

cAMP or phospholipase C activation (correct)

Only protein kinase A activation

Which receptor family does ATP primarily bind to?

P2Y receptors, which are membrane-bound enzymes

Purinergic A receptors, that exclusively activate cGMP pathways

A1 receptors, which act as transcription factors

P2X receptors, which are ion channels (correct)

What physiological process is NOT controlled by purinergic signaling?

Regulation of lipid metabolism Signup and view all the answers

Which subtype of adenosine receptors mediates the majority of adenosine's effects?

A1 receptors, which mainly influence cardiac function Signup and view all the answers

What happens to adenosine in the body after it is released?

It can be inactivated to inosine by adenosine deaminase Signup and view all the answers

Which of the following statements about purinergic receptors is true?

P2X receptors are ion channels and respond to ligands Signup and view all the answers

What is a key therapeutic implication of purinergic signaling?

Targeting neurotransmitter release manipulation Signup and view all the answers

What is the primary negative effect of adenosine on the heart's pacemaker activity?

Negative chronotropic effect Signup and view all the answers

Which of the following purine bases are categorized as purines?

Adenine and Guanine Signup and view all the answers

Which adenosine receptor subtype is primarily involved in promoting mediator release from mast cells?

A1 receptors Signup and view all the answers

What role do adenosine, ADP, and ATP play in the body?

They serve as extracellular signaling molecules. Signup and view all the answers

How does adenosine contribute to vasodilation in the cardiovascular system?

By acting on A2 receptors Signup and view all the answers

Which physiological process is primarily mediated by ATP?

Signal transduction in neurons Signup and view all the answers

What effect does adenosine have on platelet aggregation?

Inhibits platelet aggregation Signup and view all the answers

Which of the following clinical utilities is NOT associated with adenosine?

Paducah hypertension treatment Signup and view all the answers

Which statement best describes purinergic receptors?

They have various subtypes with different ligands. Signup and view all the answers

What is a potential therapeutic application of purinergic agents?

Management of thrombotic disorders Signup and view all the answers

What is the mechanism through which dipyridamole exhibits its antiplatelet effects?

Inhibition of adenosine cellular uptake Signup and view all the answers

Which purine nucleoside is mainly highlighted for its role as a mediator?

Adenosine Signup and view all the answers

Which physiological response is primarily mediated by A2A adenosine receptors in the respiratory system?

Anti-inflammatory response Signup and view all the answers

What distinguishes the physiological roles of ADP from ATP?

ADP primarily acts as a signaling molecule rather than an energy source. Signup and view all the answers

What should be considered when administering adenosine with dipyridamole?

Reduce the dose of adenosine Signup and view all the answers

Which dietary sources are notably high in purines?

Organ meats and fish Signup and view all the answers

Study Notes

Purine Nucleosides & Nucleotides as Mediators

Purine nucleosides and nucleotides play crucial roles in DNA/RNA synthesis and energy metabolism.
They function extracellularly as signalling molecules to produce diverse pharmacological effects outside of their function in metabolic processes.

Physiological Effects of Purinergic Signalling

Purinergic signalling regulates various physiological processes, which are crucial for maintaining homeostasis within the body. These processes include:
- Coronary blood flow regulation: This involves the modulation of blood supply to the heart muscle, ensuring adequate oxygen delivery and nutrient supply, which is vital for sustaining normal cardiac function.
- Myocardial function regulation: Purinergic signalling influences the heart's contraction strength and rate, contributing to overall cardiac efficiency and workload management.
- Platelet aggregation regulation: This process is essential for blood clotting and wound healing, helping to prevent excessive bleeding while ensuring that clots do not form unnecessarily.
- Immune response regulation: Purinergic signalling plays a role in modulating immune cell functions, influencing inflammation and the body's response to infections or injuries.
- Neurotransmitter activity in the CNS and PNS: Purinergic mechanisms regulate neurotransmission, affecting various aspects of neural communication, including pain sensation, mood regulation, and muscle control.

Purinergic Receptor Families

There are three main families of purinergic receptors
- Adenosine receptors (A1, A2A, A2B, A3)
- P2Y and subtypes
- P2X and subtypes

Adenosine Receptors

G-protein coupled receptors (GPCRs) are a large family of membrane proteins that play an essential role in cellular communication and signal transduction. They are characterized by their ability to activate intracellular signaling cascades upon binding with specific ligands.
The four subtypes of adenosine receptors—A1, A2A, A2B, and A3—each have distinct physiological roles and are categorized based on their structural differences and the types of endogenous molecules that bind to them.
These receptors can be primarily distinguished by their unique molecular structures, as well as their selective responsiveness to various agonists and antagonists, which can either stimulate or inhibit their activity.
Adenosine receptors respond to several adenine nucleosides and nucleotides, with a particular affinity for adenosine itself, along with adenosine diphosphate (ADP) and adenosine triphosphate (ATP), influencing a wide range of biological processes such as neurotransmission, cardiovascular function, and immune response.

P2Y Receptors

G-protein coupled receptors
Various subtypes
Respond to various adenine nucleoside and nucleotides, particularly adenosine, ADP or ATP

P2X Receptors

Ligand-gated, receptor gated cation selective ion channels.
Various subtypes
Respond to various adenine nucleoside and nucleotides, particularly adenosine, ADP or ATP

Adenosine as a Mediator

Adenosine, a simple purine, exists free in the cytosol of all cells.
Transported in and out of cells via membrane transporters.
Extracellular adenosine is derived from intracellular sources and extracellular hydrolysis of ATP or ADP.
Inactivated to inosine by adenosine deaminase.
Nearly all cells express one or more A-receptors, leading to various pharmacological effects in both the periphery and CNS.

Adenosine in the Cardiovascular System

Heart:
- Negative chronotropic effect (suppresses automaticity of cardiac pacemakers)
- Negative dromotropic effect (inhibits AV nodal conduction) - antidysarrhythmic effect
Vasculature:
- Vasodilation
- Hypotension (A2?)
- Cardiac depression (A1)
Platelets:
- Inhibition of platelet aggregation via A2A and A2B receptors

Adenosine in the Respiratory System

Adenosine receptors are found on all cell types involved in asthma, with complex pharmacological implications:
A1 Receptors:
- Promotes mediator release from mast cells
- Enhanced mucus secretion
- Bronchoconstriction
- Activation of leukotriens
A2A Receptors:
- Anti-inflammatory response

Potential clinical utility in various areas:
- Cardiovascular system
- Asthma
- Inflammation
- CNS (not covered in this text excerpt)

Clinical Uses of Adenosine and Drugs Affecting Adenosine in the Cardiovascular System

Adenosine:
- Intravenous bolus injection to stop supraventricular tachycardia and convert to sinus rhythm
- Safer than beta-blockers or verapamil due to its short duration of action
Dipyridamole:
- Blocks adenosine cellular uptake
- Used as an antiplatelet drug with vasodilatory effects
- Used with other drugs to reduce blood clot risk after heart valve replacement

Drug Interactions of Adenosine and Drugs Affecting Adenosine in the Cardiovascular System

Adenosine is reported to interact with dipyridamole, requiring dosage adjustment of adenosine during concurrent administration.

Clinical Uses of Adenosine and Drugs Affecting Adenosine in the Respiratory System

Methylxanthines:
- Examples include caffeine and theophylline.
- These drugs antagonize adenosine receptors, particularly A1 receptors, leading to bronchodilation.

Other Peripheral Mediators

Other purine nucleosides and nucleotides, in addition to Nitric Oxide (NO), may also act as mediators.

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Description

This quiz explores the roles of purine nucleosides and nucleotides in DNA/RNA synthesis and their significance as signaling molecules. Additionally, it covers the physiological effects of purinergic signaling and the different families of purinergic receptors, focusing on adenosine receptors and their subtypes.