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OTHER PERIPHERAL MEDIATORS:
PURINES
26th Jan, 2017
Purine
• A double ringed, crystalline organic base,
C5H4N4, from which is derived the nitrogen
bases adenine and guanine, as well as uric acid
as a metabolic end product.
• Purines are found in all of the body’s cells, and
in virtually all foods.
• The high purine foods are also high-protein
foods and they include organ meats like kidney,
fish like mackerel, herring, sardines and also
yeast
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Overview
In this session we will describe:
• the role of purine nucleosides and
nucleotides as chemical mediators subserving a wide range of functions
• The mechanisms responsible for their
synthesis and release
• The various receptors on which they act
• Drugs that affect purinergic signalling
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Overview…
• There is an increasing interest in purine
pharmacology and the potential role of
purinergic agents in the treatment of pain
and a variety of disorders, particularly of
thrombotic and respiratory origin
• The full complexity of purinergic control
systems, and their importance in many
pathophysiological mechanisms, is only
now emerging.
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Overview…
• In comparison:
• 5-HT has a longer pharmacological history
than purines, and numerous drugs in
current use act wholly or partly on 5-HT
receptors, of which no fewer than 14
subtypes have been identified
• Purine pharmacology is much sparser
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Overview…
• In both cases, the physiological
significance; and hence therapeutic
relevance of the various receptor subtypes
is still being unravelled
• (NB: to unravel=to explain something that is difficult to undestand)
• Focus will be on the more secure
hypotheses, recognizing that the picture is
far from complete
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Learning Objectives
•
1.
2.
3.
4.
To understand and be able to describe:Purine receptors
Adenosine as a mediator
ADP as a mediator
ATP as a mediator
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Introduction
• Nucleosides, especially adenosine, and
nucleotides, especially ADP (adenosine
diphosphate) and ATP ( adenosine
triphosphate), will be familiar to you because of
their crucial role in DNA/RNA synthesis and
energy metabolism,
• but it may come as a surprise to learn that they
also produce a wide range of pharmacological
effects that are unrelated to their role in energy
metabolism.
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• Nucleotide=nitrogen base (purine or pyrimidine)
+ phosphate group + pentose sugar (ribose or
deoxyribose). They are units of DNA
• Nucleoside=nitrogen base (purine or
pyrimidine) +pentose sugar (ribose or
deoxyribose). They are units of RNA
• Nucleoside does not contain phosphate groups
• Nucleotidases break down nucleotides (such
as thymine nucleotide) into nucleosides (such as
thymidine) and phosphate groups.
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1. Purine Receptors
•
•
There are three main families of purine
receptors each with several subtypes.
The subtypes in each family may be
distinguished on the basis of their molecular
structure as well as their agonist and
antagonist selectivity
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1. Purine Receptors…
•
The three main types of purine receptor
are:1. Adenosine receptors
2. P2Y metabotropic receptors
3. P2X ionotropic receptors
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1. Purine Receptors…
1. Adenosine receptors (subtypes A1, A2A, A2B
and A3), formerly known as P1-receptors;
 These respond to adenosine, and are Gprotein-coupled receptors (GPCRs) that
regulate cAMP
 Linked to stimulation or inhibition of adenylate
cyclase
 They are present in many different tissues
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1. Purine Receptors…
2. P2Y metabotropic receptors (P2Y1-14),
 which are G-protein-coupled receptors
that utilize either cAMP or phospholipase
C activation as their signalling system
(refer to How Drugs Act: Molecular Aspects)
 they respond to various adenine
nucleotides, generally preferring ATP
over ADP or AMP
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1. Purine Receptors…
3. P2X ionotropic receptors (P2X1-7)
 are multimeric ATP-gated cation
channels

Refer table 16.1 pg 206 of Pharmacology by Rang and Dale
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Drugs Acting on Purine
Receptors
• Methylxanthines, especially analogues of
theophylline, are A1/A2-receptor
antagonists: however they also increase
the cAMP (cyclic 3’5’-adenosine
monophosphate) by inhibiting
phosphodiestrase, which contributes to
their pharmacological actions
independently of adenosine receptor
antagonism.
• P2-receptors are blocked by Suramin 15
2. Adenosine as a Mediator
• The simplest of the purines, adenosine is
found in biological fluids throughout the body.
• Adenosine differs from ATP in that it is not
stored by and released from secretory
vesicles.
• Rather, it exists free in the cytosol of all cells
and is transported in and out of cells mainly
via a membrane transporter.
• Little is known about the way in which this is
controlled but the extracellular concentrations
are usually quite low compared with
intracellular levels
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2. Adenosine as a Mediator
• Adenosine in tissues comes partly from
this intracellular source and partly from
extracellular hydrolysis of released ATP or
ADP
• Virtually, all cells express one or more Areceptors and so adenosine produces
many pharmacological effects, both in the
periphery and in the CNS.
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2. Adenosine as a Mediator…
• Based on its ability to inhibit cell function
and thus minimize the metabolic requirements
of cells, one of its functions may be as an
‘acute’ protective agent that is released
immediately when tissue integrity is
threatened (e.g. by coronary or cerebral
ischaemia)
• Under less extreme conditions, variations in
adenosine release may play a role in
controlling blood flow and (through effects on
the carotid bodies) respiration, matching them
to the metabolic needs of the tissues
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2. Adenosine as a Mediator …
• Adenosine affects many cells and tissues,
including smooth muscle and nerve cells. It is
not a conventional transmitter but may be
important as local hormone and ‘homeostatic
modulator’.
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2. Adenosine as a Mediator …
• Important sites of action include the heart
and the lung
• Adenosine acts through A1-, A2- and A3G-protein receptors, coupled to inhibition
or stimulation of adenylate cyclase.
• A1- and A2-receptors are blocked by
xanthines, such as theophylline
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Functional Aspects-Adenosine
Receptors
• The main effects of adenosine are:
-hypotension (A2) and cardiac depression (A1)
-inhibition of atrioventricular conduction
(antidysrhythmic effect, A1)
-inhibition of platelet aggregation (A2)
-bonchoconstriction (probably secondary to mast
cell activation, A3)
-presynaptic inhibition in CNS (responsible for
neuroprotective effect, A1)
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• Adenosine is very short acting and
sometimes used for its antidysrhythmic
effect
• New adenosine agonists and antagonists
are in development, mainly for treatment
of ischaemic heart disease and stroke
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Uses of Adenosine
• Because of its inhibitory effects on cardiac
conduction, adenosine may be used as an
intravenous bolus injection to terminate
supraventricular tachycardia
• It is safer than Beta-adrenoceptor antagonists or
verapamil, because of its short duration of action
• Selective adenosine receptor antagonists could
also have advantages over theophylline in the
treatment of asthma
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Adenosine and the CVS
• Inhibits cardiac conduction and it is likely that all
four of the adenosine receptors are involved in
this effect
• Because of this, adenosine itself may be used
as a drug, being given as an intravenous bolus
injection to terminate supraventricular
tachycardia
• It is safer than Beta-adrenoceptor antagonists or
verapamil, because of its short duration of action
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Adenosine and the CVS…
• Longer lasting analogues have been
discovered that also show greater receptor
selectivity
• Adenosine uptake is blocked (thus its
action prolonged) by dipyridamole, a
vasodilator and antiplatelet drug
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Adenosine and Asthma
• Adenosine receptors are found on all the
cell types involved in asthma and the
overall pharmacology is complex
• However, by acting through its A1 receptor,
adenosine promotes mediator release
from mast cells, and causes enhanced
mucus secretion, bronchoconstriction and
leukocyte activation
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Adenosine and Asthma…
• Methylxanthines, especially analogues of
theophylline, are adenosine receptor
antagonists.
• Theophylline has been used for the
treatment of asthma
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Adenosine in the CNS
• Act through A1 and A2A receptors
• Has an inhibitory effect on many CNS
neurons and
• The stimulation experienced after
consumption of methylxanthines such as
caffeine occurs partly as a result of block
of these receptors
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ADP AS A MEDIATOR
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ADP as a Mediator
• ADP is usually stored in vesicles in cells.
• When released, it exerts its biological
effects predominantly through the P2Y
family of receptors.
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ADP and Platelets
• The secretory vesicles of blood platelets store
both ATP and ADP in high concentrations, and
release them when the platelets are activated.
• One of the many effects of ADP is to promote
platelet aggregation, so this system provides
positive feedback-an important mechanism for
controlling this process.
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ADP and Platelets…
• The receptor involved is P2Y12.
• Clopidogrel, prasugrel and the earlier
agent, ticlopidine, are P2Y12 antagonists
and exert their anti-aggregating effects
through this mechanism
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ADP and Platelets…
• ADP acts on platelets, causing
aggregation.
• This is important in thrombosis.
• It also acts on vascular and other types of
smooth muscle, as well as having effects
on CNS
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ATP AS A MEDIATOR
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ATP as a Mediator
• ATP exerts its action primarily through the P2X
receptors.
• The extracellular domain of these multimeric
receptors can bind three molecules of ATP.
• When activated, the receptor gates the cationselective ion channels that trigger ongoing
intracellular signalling
• The other actions of ATP in mammals are
mediated through the P2Y receptors.
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ATP as a Mediator…
• Suramin (a drug originally developed to
treat trypanosome infections) and an
experimental compound PPADS
antagonize ATP and have broad-spectrum
inhibitory activity at most P2X and P2Yreceptors
• Cytoplasmic ATP may be released,
independently of exocytosis, when the
cells are damaged (e.g. by ischaemia)
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ATP as a Mediator…
• ATP released from cells is rapidly
dephosphorylated by a range of tissue-specific
nucleotidases, producing ADP and adenosine,
both of which produce a wide variety of receptor
mediated effects.
• The role of intracellular ATP in controlling
membrane potassium channels, which is
important in the control of vascular smooth
muscle and of insulin secretion is quite distinct
from its transmitter function
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ATP as a Neurotransmitter
• ATP functions as a neurotransmitter (or cotransmitter) at peripheral neuroeffector junctions
and central synapses.
• ATP is stored in vesicles and released by
exocytosis.
• Cytoplasmic ATP may be released when cells
are damaged.
• It also functions as an intracellular mediator,
inhibiting the opening of membrane potassium
channels
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ATP as a Neurotransmitter…
• ATP acts on two types of purinoceptors
(P2), one of which is (P2x) is a ligandgated ion channel responsible for fast
synaptic responses.
• P2X2, P2X4, P2X6, are the predominant
receptor subtypes expressed in neurons
• P2X1 predominates in smooth muscles
• The other (P2Y) is coupled to various
second messengers.
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ATP as a Neurotransmitter…
• Suramin blocks the P2x-receptor.
• Released ATP is rapidly converted to ADP
and adenosine that may act on other
purinergic receptors
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ATP in Nociception
• ATP causes pain when injected, as a result of
activation of P2X2 and/or P2X3 receptors in
afferent neurons involved in the transduction of
norciception
• Oddly, perhaps, the same receptors seem to be
involved in taste perception on the tongue.
• Elsewhere in the CNS, P2X4 receptors on
microglia may be important in the development
of neuropathic pain
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ATP in Inflammation
• The P2X7 receptor is widely distributed on
cells of the immune system, and ATP,
apparently acting through this receptor,
causes the release from macrophages and
mast cells of cytokines and other
mediators of the inflammatory response.
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Future Prospects
• While it is true that few currently available
drugs act through purinergic receptors
when compared, e.g. with 5-HT receptors,
the area as a whole holds promise for
future therapeutic exploitation, particularly
in the treatment of asthma, pain and
gastrointestinal disorders, provided
compounds with sufficient receptor
selectivity can be found.
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Further Reading on Purines:
• Rang & Dale’s Pharmacology (7th
Edition-Chapter 16)
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