Bioscience Reports 2, 77-90 (1982)
Printed in Great Britain
77
C e l l - m e m b r a n e receptors for purines
Review
T. W. STONE
Department of Physiology, St. George's Hospital Medical
School, University of London, London 574"17 ORE, U.K.
Purines are involved in many aspects of cell c h e m i s t r y - intermediary metabolism, nucleic acid synthesis, and the supply of highenergy phosphates to various active transport Systems.
In addition,
however, there appear to be specific receptor molecules located within
the plasma membrane of some cell% which mediate changes of cell
function in response to purines present in the extracellular fluid. It is
t h e p u r p o s e of this r e v i e w to s u m m a r i z e the kind of functions
subserved by those receptors as well as the basic structural requirements for their activation.
P h a r m a c o l o g i c a l Actions and the P l / P 2 Concept
It has been recognized for several decades that the extracellular
administration of some purines can have marked effects on the heart
or visceral smooth muscle. These actions were discussed by Burnstock
(1972) and f o r m a l i z e d into a concept of 'purinergic' neuronesneurones releasing a purine, probably adenosine triphosphate (ATP), as
a neurotransmitter.
This hypothesis arose partly to explain the
responses of tissue to nerve stimulation which could not be accounted
f o r by t h e i r c h o l i n e r g i c or adrenergic innervation.
More recent
reviews of peripheral purine pharmacology have appeared (Burnstock,
19gl; Baer & Drummond~ 1979)~ while Stone (1981a) has discussed in
d e t a i l th e a c t i o n s and functions of purines with emphasis on the
central nervous system.
The f i r s t a t t e m p t to classify purine receptors was made by
5urnstock (1978) on the basis of inverted potency series and susceptibility to antagonism. Thus the order of potency adenosine > AMP
> ATP had been demonstrated for several, mainly inhibitory, effects on
smooth muscl% including the relaxation of vascular tissues, and these
actions could be blocked by methylxanthines (caffei ne, theophylline,
e t c . , ) in significantly lower concentrations than were needed to inhibit
cyclic nucleotide phosphodiesterase.
The recept or for these e f f e c t s
was referred to as Pt (Burnstock, 1978).
On the other hand, several excitatory effects of purines, notably on
trachea, bladder, and parts of the intestine of some species are
produced by ATP > ADP > AMP > adenosine and are not blocked by
methylxanthines.
This ATP receptor was dubbed P2"
Although
uinidine and 2-pyridylisatogen have been claimed by some authors
Spedding et a l . 1975; Hunt et a l . 197g) to show antagonistic activity
~1982
The Biochemical Society
78
STONE
towards ATP, this view has not met an enthusiastic reception as both
drugs are rather nonspecific in their effect s.
(It is possible that the
apparent nonspecificity in the action of quinidine could actually r e f l e c t
blockade of engodenous ATP [see Stone, 19gla], but no hard evidence
for this is yet available.)
The PI/P2 classification is now frequently quoted in the pharmacol o g i c a l l i t e r a t u r e and used to draw a precise-sounding conclusion.
Critics of the Pl / p2 concept assert that some numerical est i m at e of
ranges of relative potency within the respective potency series should
be a criterion of the classification. Such sceptics point to the case of
several smooth muscles, where purine actions do not fit the above
scheme (e.g. Clark et al., 1980; Baer & Frew, 1979)7 and indeed
question the value of any specific nomenclature replacing the descriptive terms 'adenosine receptor' and 'ATP r e c e p t o r ' . As will be noted
b elo w, this particular classification is of dubious value when considering some of the more subtle interactions of purines with other
compounds.
It is perhaps appropriate here to reinforce the idea that these
r ecep to r s are accessible only to extracellularly located purines.
The
adenosine receptor, for example, can be activated by simple analogues
which do not cross cell membranes, such as 2-chloroadenosine, as well
as v e r y l a r g e m o l e c u l e s m a de by linking adenosine to a highmolecular-weight polymer (Olsson et al., 1976, 1977; Schrader et al.,
1977).
Evidence for the membrane location of the ATP r e c e p t o r is
m o r e c i r c u m s t a n t i a l , but i n c l u d e s t h e e x t r e m e rapidity of ATP
responses, as well as the high potencies of ATP analogues such as
B , y - m e t h y l e n e ATP and a recently described series of diadenosine
polyphosphates (Stone, 19glb, 1982; Stone & Perkins, 1981) which are
m e t a b o l i z e d r e l a t i v e l y slowly, and which p e n e t r a t e into cells very
poorly, if at all.
Mechanisms
of Action
Little is known about the mechanism of responses mediated by the
ATP receptor.
A limited amount of evidence from smooth-muscle
electrophysiology suggests activation of a potassium channel (T om i t a &
Watanabe, 1963; Burnstock, 1972; Hartzell, 1979), an action which also
seems to occur in isolated hepatocytes (Burgess et al., 1979) and even
HeLa ceils (Alton & Lamb, 1980). An increase of calcium influx has
also been noted in the presence of ATP (Goto et al., 1977, 1978;
Ribeiro, 1979), and such an e f f e c t might well underlie some cont r a c t i l e e f f e c t s of ATP.
Activation of the adenosine receptor has received more attention
than that of the ATP receptor, possibly because the form er entrains a
g r e a t e r v a r i e t y of c e l l u l a r responses and because it is of more
obviously direct practical value.
Thus, while the signilicance of the
ATP r e c e p t o r remains obscure, the adenosine recept or is now widely
a c c e p t e d as having a major physiological role, for example, in the
control of vascular tone (Berne et al., 1979).
The main consequence of adenosine recept or activation seems to be
a r e d u c t i o n in t h e a v a i l a b i l i t y of calcium ions.
This has been
demonstrated most clearly by direct studies of #5Ca2+ uptake i n t o
PURINE
RECEPTORS
79
cerebral synaptosomes (Ribeiro et al., 1979) as well as by electrophysiological studies of the cardiac action potential (Goto et al., 197g;
Schrader et al., 1975, 1979) and studies of the calcium dependence of
adenosine responses (Hayashi et al., 1981; Kuroda et al., I976b).
When acting on nerve terminals, this effect of adenosine results in a
marked inhibition of neurotransmitter release, a phenomenon which has
been observed on somatic (Ginsborg & Hirst, 1972; Ribeiro & Walker,
1975) and visceral autonomic nerves (Clanachan et al., 1977; Enero &
Saidman, 1977; Gustaffson et al., 1978; Hedqvist & Fredholm, 1979;
Ribeiro, ]979; Su, 1978; Verhaege et al., 1977; Vizi & Knoll, ]976), as
well as in the central nervous system (CNS) (Harms et al., ]978,
1979; Hollins & Stone, 1980).
The depressant effects of adenosine on neuronal firing rate in the
CNS are blocked by methylxanthines and may involve some reduction
of excitatory transmitter release (Phillis & Kostopoulos, 1975; Perkins
& Stone, ]980).
It is also clear, however, that as well as pre-synaptic inhibitory
a c t i o n s , adenosine has a c t i o n s at p o s t - j u n c t i o n a l sites.
Hence
adenosine causes a relaxation of vascular muscle even in the absence
oL a tonic c o n s t r i c t o r innervation (Mustafa, 19g0).
L u c h e l l i f o r t i s et
ai. ( [ 9 8 1 ) have also r e c e n t l y described a post-junctional inhibitory
a c t i o n on the cat n i c t i t a t i n g m e m b r a n e , a p r e p a r a t i o n which is one of
only a small number which seem to lack adenosine r e c e p t o r s on the
pre-synaptic
nerve terminals~
But adenosine does not have only
inhibitory e f f e c t s on smooth muscle.
On the guinea-pig t r a c h e a , for
e x a m p l e , adenosine causes a c o n t r a c t i o n of the muscle, and, unusually,
it is roughly e q u i p o t e n t with ATP in this r e s p e c t . As stable analogues
of ATP have l i t t l e e f f e c t on this tissue, Christie and S a t c h e l l ( ] 9 8 0 )
have proposed that both adenosine and ATP are acting on an adenosine
receptor.
One problem with this view is t h a t whereas an i n c r e a s e of
calcium influx, which could explain muscular c o n t r a c t i o n , has been
found in response to ATP in some tissues ( G o t o et al., 1977, 197g;
R i b e i r o e t al., 1 9 7 9 ) , t h i s e f f e c t does not seem to have been
d e m o n s t r a t e d in response to adenosine.
Adenosine p h a r m a c o l o g y and r e c e p t o r classification b e c o m e s even
more confusing when i n t e r a c t i o n s with o t h e r compounds are considered.
The early biochemical observations of S a t t i n and Rail (1970) which
will be discussed below included the observation t h a t adenosine (>ATP)
and n o r a d r e n a l i n e exhibited a mutually p o t e n t i a t i v e i n t e r a c t i o n on the
a c t i v a t i o n of a d e n y l a t e cyclase in brain slices ( S a t t i n & Rail, 1970;
S a t t i n et al., 1975; Daly, 1976).
A p o t e n t i a t i o n b e t w e e n adenosine
and n o r a d r e n a l i n e could be d e m o n s t r a t e d e l e c t r o p h y s i o l o g i c a l l y but was
of much smaller m a g n i t u d e (Stone & Taylor, 1978).
Hedqvist and
F r e d h o l m ( ] 9 7 6 ) r e p o r t e d most c l e a r l y on the distinction b e t w e e n the
pre-synaptic
i n h i b i t o r y p r o p e r t i e s of adenosine and its ability to
e n h a n c e t h e e f f e c t s of n o r a d r e n a l i n e p o s t - s y n a p t i c a l l y .
The postsynaptic interaction
s e e m s to be r e s t r i c t e d to c~l_adrenoreceptor s
( H e d q v i s t & F r e d h o l m , 1976; Sattin & Rail, 1970; Jones, I981), the
a b s e n c e of a n y i n t e r a c t i o n in guinea-pig c e r e b e l l u m , for example,
p r e s u m a b l y r e f l e c t i n g the p r e s e n c e here only of B - a d r e n o r e c e p t o r s .
A similar potentiative
p h e n o m e n o n has been d e m o n s t r a t e d on
isolated vascular muscle and on the guinea-pig vas d e f e r e n s (Holck &
Marks, 1978).
This means, of course, t h a t on Some tissues, most
80
STONE
notably on vascular muscle, adenosine can have a direct action to
cause relaxation and an indirect action to reduce noradrenaline release,
as well as causing a potentiation of the post-junctional c o n t r a c t i l e
e f f e c t of noradrenaline. Clearly the system must normally function as
a d e l i c a t e l y b a l a n c e d e q u i l i b r i u m , and conceivably only a small
displacement of that equilibrium might be sufficient to trigger the
changes that lead to chronic hypertension (Kamikawa et al., 1980). It
is t h e r e f o r e especially exciting that one of the common antihypertensive drugs, hydrallazine, has recently been shown to act at a purine
r e c e p t o r site in so far as its muscle relaxant activity can be reversed
o r prevented by ATP (Chevillard et al., 1981).
Receptor Sensitivity
There are two further
interesXing f a c e t s of this p u r i n e noradrenaline interaction. One is that it may be partly related to the
functional s t at e of the relevant receptor.
Holck and Marks (1978),
for example, noted that not only did adenosine increase responses to
noradrenaline, but if the tissue was densensitized to the amine by
applying high concentrations into t h e b a t h , then adenosine hastened the
r e c o v e r y of responsiveness, i.e. resensitization.
The order of potency
of p u r i n e s was di f f er e nt for the two actions.
ATP for example
r e d u c e d r a t h e r than increased noradrenaline responses, and had no
e f f e c t on the rate of s - r e c e p t o r resensitization.
What makes this phenomenon particularly interesting is that similar
observations have been made on acetylcholine receptors. As long ago
as 19it4, Buchthal and Kahlson reported on the increased response to
acetylcholine produced by ATP at the neuromuscular junction.
As
these experiments were performed in vivo, it is possible that local
changes of blood flow were at least partly responsible for this e f f e c t .
H o w e v e r , Ewald ( 1 9 7 6 a , b ) , r e i n v e s t i g a t i n g t h e i n t e r a c t i o n with
intracellular electrophysiological methods, came to essentially similar
conclusions.
The similarity between the calcium dependence of the
potentiation and that of the normal acetylcholine recept or interaction
led Ewald ( 1 9 7 6 a , b ) to s u g g e s t t h a t ATP m i g h t f a c i l i t a t e the
transmitter-receptor
interaction.
ATP also reduced the rat e of
end-plate desensitization to acetylcholine.
Most recently, however,
Akasu et al. (1981), using the voltage-clamp technique, have shown
that ATP does not alter the affinity of acetylcholine for its receptor,
and have concluded that the enhancement of acetylcholine sensitivity
r e s u l t s f r o m e i t h e r an increase in the conductance of unit ionic
channels or of the number of available channels, i.e. at a site distal
to the r ecept or molecule.
S e v e r a l Russian authors have come to the conclusion that the
presence of ATP can regulate the sensitivity of cholinoreceptors in the
h e a r t ( S a k h a r o v & T u r p a e v , 1968; T u r p a e v & S a k h a r o v , 1973;
Nistratova, 1968).
These observations raise again the proposal of
Stone (1978) that one of the main physiological functions of endogenous purines may be the regulation of the activity of tissue by
changing the balance between, say, receptors for catecholamines and
a c e t y l c h o l i n e , or between muscarinic and nicotinic receptors.
An
increase in the local concentration of ATP, for example, would lead to
PURINE
RECEPTORS
81
an increased sensitivity to acetylcholine but a decreased sensitivity to
noradrenaline.
Adenosine on the other hand would tend to increase
a - a d r e n o r e c e p t o r sensitivity but does not seem to a f f e c t cholinoceptor
sensitivity (Akasu et al., 1981).
Relationship to Cyclic-AMP-Generating Systems
T h e s e c o n d reason ior the particular interest in the functional
post-junctional potentiation between adenosine and noradrenaline is that
j u s t - s u c h a potentiation was among the first observations made by
Sattin and Rail (1970) in the study which demonstrated for the first
t i m e t h a t a d e n o s i n e was able to a c t i v a t e cyclic-AMP-generating
systems in slices of guinea-pig cerebral cortex.
It was later found
th at the failure of noradrenaline to cause an increased concentration
oi cyclic AMP on second or subsequent application could be prevented
by the inclusion of adenosine (Schultz & Daly, 1973).
This sounds
highly reminiscent of the f a c i l i t a t e d resensitization of smooth muscle
discussed above.
This then raises the question of the relationship
between the functional e f f e c t s of purines and their interactions with
n eu r o tr an s m i t t er s , and the activation of adenylate cyclase.
E x p e r i m e n t s by K u r o d a et al. ( 1 9 7 6 a ) showing a correlation
between the ext ent and time course of changes of cyclic AMP levels
and the depression by adenosine of synaptically evoked potentials in
the guinea-pig ol f a c t or y slice are still to be found quoted as support
for a positive relationship between these parameters.
However, later
work by the same group, in which a b e t t e r time resolution of the
b i o c h e m i c a l results was achieved, clearly showed that the el ect rophysiological changes occUrred well b e f o r e the cyclic AMP changes
(Kuroda, 197g).
Other groups have ats0 concluded that adenylate cyclase activation
is not necessary for functional changes to occur (Smellie et al,, 1979;
Okada & Saito, 1979; Scholfield, 197g; Reddington & Schubert, 1979;
Dunwiddie & H ol l e r , 19g0) and it has been noted earlier that the
s mall d e g r e e of p o t e n t i a t i o n between adenosine and noradrenaline
o b s e r v e d e l e c t r o p h y s i o l o g i c a l l y ( S t o n e & Taylor, 197g) would be
difficult to reconcile with the large potentiation Seen on cyclic AMP
levels (Sattin et al., 1975).
It would seem, then, that the receptors
for adenosine's functional e f f e c t s , such as inhibition of t r a n s m i t t e r
release, and elevation of cyclic AMP levels must be distinct entities.
The two receptors are clearly very similar as they exhibit a similar
relative p r e f e r e n c e for a number of adenosine derivatives, and both
are blocked by similar concentrations of methylxanthines. Part of the
reason for the differences between the two sites could, of course, be
m o r p h o l o g i c a l , t h e c y c l i c - A M P - e l e v a t i n g s i t e s being in general
post-synaptic and those inhibiting t r a n s m i t t e r release being pre-synaptic
(E)eMey et al., 1979).
However, some clear pharmacological diff e r e n c e s bet w e e n these sites do exist, such as the fact that the
L-isomer of phenylisopropyladenosine is 100-fold more potent than the
D-isomer in causing a reduction of post-synaptic potentials, whereas it
is only #- to 5-fold more potent in elevating cerebral cyclic AMP
levels (Smellie et al., 1979).
The two receptor sites involved, of
course, would both belong to the Pt category.
82
STONE
Receptors Modulating Adenylate Cyclase Activity
Londos and Wolff (1977) drew attention to the fact that in some
tissues adenosine increased cyclic AMP levels while in others there
was a decrease.
By examining a series of adenosine analogues these
authors were able to conclude that at least two separate purine
receptors existed exhibiting different structural spec}ficities. The 'R'
site required an intact ribose portion of the molecule and mediated an
increase of adenylate cyclase activity.
The 'P' site showed little
tolerance to modification of the purine ring but was relatively
unaffected by changes of the ribose portion, 2'5'-dideoxyadenosine
being a good agonist.
Activation induced decreases of cyclase
activity. A further important difference between these sites was the
conclusion that the 'P' site was only accessible to compounds after
they penetrated into the cytoplasm and t h i s property presumably
accounted in part for the resistance of t h i s site to blockade by
methylxanthines.
Shortly after these experiments, Van Calker et al. (1979) reported
on a subdivision of the 'R' site. As in the case of the original 'R'
site, the receptors described by Van Calker et al. (1979) were
considered to be extracellularly directed, as inhibition of adenosine
uptake did not diminish the responses. Their subdivision was made
partly on the basis of concentration-dependent responses to adenosine
and partly on the basis of inverted potency series. Thus their Al site
was s t i m u l a t e d p r e f e r e n t i a l l y b y N6-phenylisopropyladenosine, whereas
t h e A 2 site was a c t i v a t e d more by adenosine itself. The A I r e c e p t o r
inhibited the accumulation
of c y c l i c AMP w h e r e a s the A 2 site
provoked an increase of cyclic AMP levels.
Both sites, of course,
w e r e blocked by m e t h y l x a n t h i n e s .
The most r e c e n t c h a p t e r in this sequence is the r e p o r t by Londos
e t al. (1980) of a subdivision of the 'R' site into R a and R i sites.
H o w e v e r , these r e c e p t o r s show e x a c t l y the same p r o p e r t i e s as Van
C a l k e r ' s A 1 and A2, viz., r e q u i r e m e n t for an i n t a c t ribose moiety,
block by m e t h y l x a n t h i n e s , e x t e r n a l location, phenylisopropyladenosine >
adenosine causes inhibition of c y c l i c AMP a c c u m u l a t i o n at the R i site,
but adenosine > phenylisopropyladenosine a c t i v a t e s a d e n y l a t e c y c l a s e at
t h e R a site. An additional finding c o n t r i b u t e d by Londos et al. (1980)
was that 5'-/V-ethylcarboxamide adenosine (ECA) was more p o t e n t than
adenosine and phenylisopropyladenosine at R a whereas it was w e a k e r
than e i t h e r of these at R i.
T h e v a r i o u s s c h e m e s of r e c e p t o r classification have been summ a r i z e d in Table l, and Table 2 is an a t t e m p t to r a t i o n a l i z e these
w h e r e s u f f i c i e n t l y close r e c e p t o r similarities have been d e m o n s t r a t e d .
It should be s t a t e d again, however, t h a t a number of p r e p a r a t i o n s or
tissues show purine responses which do not p e r m i t easy classification
into any of the c a t e g o r i e s of T a b l e 1. A b e t t e r a p p r e c i a t i o n of the
similarities or v a r i e t y of r e c e p t o r s ideally needs the testing of a large
series of compounds, the investigations of Bruns (1980a,b,c) in which
up to 128 analogues were e x a m i n e d being e x e m p l a r y in this r e s p e c t .
As the number of p o s t u l a t e d r e c e p t o r sites increases it also has to be
c o n s i d e r e d t h a t some subtle d i f f e r e n c e s of a p p a r e n t r e c e p t o r v a r i e t i e s
m a y result m e r e l y from the d i f f e r e n t proteo-lipid e n v i r o n m e n t s existing
in the cell walls of d i f f e r e n t tissues (Stone, 197#).
PURINE
RECEPTORS
83
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84
STONE
Mechanism of Cyclase Regulation
Following the work of Blume and Foster (1976) and Levinson and
Blume (1977), Fain and Malbon (1979) have proposed a cyclical model
for purine regulation of cyclase activity in which a form of the
enzyme bound with GTP (EGTP) is the active form, whereas following
h y d r o l y s i s of the guanine nucleotide the EGD P and E 0 (unbound)
forms of the enzyme are inactive or of low activity. Adenosine and
p r o s t a g l a n d i n s are e n v i s a g e d as inducing a shift in the enzyme
e q u i l i b r i u m from the EGD P state towards E0, in which form the
e n z y m e is able to readily bind GTP again to produce the active
enzyme. Testing of this model is clearly feasible by pharmacological
m a n i p u l a t i o n s and should give useful insights into the relationship
between the mechanism of purine activation of cyclase and activation
by other guanine-nucleotide-dependent agonists.
Purines in Neuropharmacology
A new significance has become attached to membrane receptors for
purines since the reports by Marangos et al. (1979a,b) that inosine and
hypoxanthine would displace benzodiazepines from their high-affinity
binding sites on rat brain membranes. The benzodiazepines are minor
t r a n q u i l l i z e r s and anticonvulsants and include the widely prescribed
drugs chlordiazepoxide (Librium|
and diazepam (Valium|
With the
d e m o n s t r a t i o n of specific high-affinity binding of diazepam a huge
effort had been expended in attempts to determine the existence o f
endogenous l i g a n d s - hence the excitement generated by the findings
of Marangos et al. (1979a,b).
The concentration of these compounds
giving 50% inhibition (ICs0) of diazepam binding is high, around 1
mM, but it has been argued that both inosine and hypoxanthine could
reach extracellular concentrations of this order in brain, particularly
under conditions of high neuronal activity, when purines are known to
be released from nervous tissue (McIlwain, 1972; Stone, 1981a).
As
diazepam is known to be a poweHul anticonvulsant drug, it is an
a t t r a c t i v e hypothesis that receptor interactions between endogenous
purines, benzodiazepines, and perhaps a third factor may be important
in the regulation or stabilization of neuronal activity in the brain
(Tallman et al., 1980).
More r e c e n t work has r e v e a l e d t h a t o t h e r purines such as
1-methylisoguanosine, found to occur in nature though not so far in
m a m m a l s , are far more e f f e c t i v e displacers of benzodiazepines.
i - M e t h y l i s o g u a n o s i n e , for example, has an ICs0 of 19 pM against
[3H]-labelled diazepam (Davies et al., 19g0). Conceivably, therefore,
this or o t h e r purines e x i s t i n g in normal brain or arising during
abnormal circumstances may also prove very relevant to understanding
benzodiazepines and the convulsant and anxiety syndromes which they
alleviate.
A novel view of p u r i n e - b e n z o d i a z e p i n e relationships has been
championed by Phillis and his colleagues (Phillis et a l , 19gl; Wu et
al., 19gl), who have demonstrated remarkable correlations between the
t h e r a p e u t i c efficacy of benzodiazepines and their ability to inhibit
adenosine uptake into synaptosomes. Although very unclear at present,
it will be very interesting to assess the connection, if any, between
the purine uptake sites, purine receptors, and benzodiazepine receptors.
PURINE
RECEPTORS
85
Cell Growth and D e v e l o p m e n t
There is a vast l i t e r a t u r e on the role of purines in the growth,
development, and differentiation ol cells of normal or carcinomatous
origin.
Even adenosine itself is a potent inhibitor of cell growth,
o f t e n at micromolar concentrations (Henderson, 1980; Henderson &
Scott, 1980) although it is still not clear whether the mechanism of
this a c t i o n involves pyrimidine starvation as a result of phosphoribosylpyrophosphate depletion, the production of excessive s-adenosylh o m o c y s t e i n e l e v e l s , or t h e e l e v a t i o n of intracellular deoxyribonucteoside concentrations.
The relevance of this topic to the present review is that many
purine e f f e c t s on cell-growth-related phenomena could be mediated by
externally directed cell-membrane receptors. To begin with, Henderson
and Scott (1980) have pointed out that ' t h e r e is strong evidence that,
at least in many biological systems, growth inhibition produced upon
t r e a t m e n t with cyclic AMP, adenylate, ADP, ATP and nicotinamide
adenine dinucleotide is mediated through adenosine'. Such a view does
not exclude the potential importance of, for example, cyclic nucleotides inside the cell, but may imply a role of e x e r a e e l i u l a r
adenosine
in modulating those levels.
Thus Schwartz et al. (1978) have shown
using adenosine and its analogues that an external adenosine recept or
can regulate intracellular cyclic AMP levels in lymphocytes and thus
produce inhibition of l y m p h o c y t e - m e d i a t e d cytolysis.
G u r o f f et al. (1981) have demonstrated that ECA can act to
p o t e n t i a t e neurite extension of PC 12 cells produced by nerve growth
f acto r . This action was also produced by phenylisopropyladenosine, but
at concentrations 10-fold greater.
As seen in Table i, this suggests
that the externally directed R a site may be particularly involved in
this phenomenon.
The potential role of cell-surface purine receptors in the control of
growth processes and their implication for abnormal growth is clearly
worthy of further investigation.
Binding Studies
Table 3 summarizes some of the binding studies which have been
carried out since suitable radiolabelled ligands became available for
the purpose.
It is to be hoped that appropriate studies of the
pharmacology ol the binding sites will reveal the relationship between
them and the functional receptors discussed above. To date, however,
some of the binding studies have led to rather odd results which do
not allow ready comparison with functional studies.
For example,
Malbon et al. ( 1 9 7 8 ) reported that labelled adenosine binding to
f a t - c e l l m e m b r a n e s could be displaced by theophylline but not by
3-isobutyl-l-methylxanthine, a compound with approximately the same
p o t e n c y as t h e o p h y l l i n e as an adenosine antagonist in functional
experiments.
S i m i l a r l y t h e binding could not be prevented by
phenylisopropyladenosine.
Part ol the explanation for these observations, which superficially
at least do not seem to agree with the functional data (Table 1), is
undoubtedly the f a c t that all potential adenosine binding sites were
86
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PURINE
RECEPTORS
87
labelled.
The more recent arrival of labelled adenosine analogues
which promise to be more specific for a particular receptor population
(such as N6-cyclohexyl-adenosine and 173-diethyl-g-phenylxanthine)
should help to clarify the properties o1: individual binding sites.
F u t u r e Trends
Probably the main goal of much purine r e s e a r c h within the next
f e w y e a r s will be to c o r r e l a t e
the r e c e p t o r types d e m o n s t r a t e d
b i o c h e m i c a l l y with those observed physiologically, and to c o r r e l a t e
e i t h e r with a p p r o p r i a t e binding studies.
A s t a r t in this direction has been evident from the publications of
Smellie et al. (1979) and of P a t o n (19gi)7 who have shown the much
greater
a c t i v i t y of L-phenylisopropyladenosine with r e s p e c t to the
D - i s o m e r when p r o d u c i n g inhibition of t r a n s m i t t e r r e l e a s e in the
c e n t r a l and peripheral nervous systems r e s p e c t i v e l y .
This p o t e n c y
d i f f e r e n c e .is c h a r a c t e r i s t i c of the A t or R i site ( T a b l e [) and implies
a physiological as well as a biochemical r e l e v a n c e o1: this r e c e p t o r .
In s u m m a r y , it m a y be e m p h a s i z e d again t h a t the actions of
purines on cells discussed in this review are m e d i a t e d by purines in
the e x t r a c e l l u l a r space acting on r e c e p t o r s in the cell m e m b r a n e .
Several means o1: classifying those r e c e p t o r s have been described
and are s u m m a r i z e d in Table 1. While it remains an open question as
to whether any of the proposed schemes have any real r e l e v a n c e or
relationship to the actions of endogenous purines, the e x i s t e n c e of
multiple r e c e p t o r s of one type or a n o t h e r must be considered established.
This in turn implies an i m p o r t a n t , well-developed 7 and highly
c o m p l e x web of functions for purines outside the cell wall c o m p l e menting and i n t e r a c t i n g with the v a r i e t y o1: functions r e c o g n i z e d inside
the cell.
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