Adenosine and Kidney Function

Physiol Rev 86: 901–940, 2006;
doi:10.1152/physrev.00031.2005.
Adenosine and Kidney Function
VOLKER VALLON, BERND MU
¨ HLBAUER, AND HARTMUT OSSWALD
Departments of Medicine and Pharmacology, University of California San Diego and Veterans Affairs
San Diego Health Care System, San Diego, California; and Institute of Pharmacology and Toxicology,
Medical Faculty, University of Tuebingen, Tuebingen, Germany
I. Introduction
902
II. Adenosine Generation in the Kidney
903
A. Renal tissue content of adenosine
903
B. Extracellular adenosine concentration
905
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C. Renal excretion of adenosine
907
D. Concluding remarks
907
III. Adenosine Receptors in the Kidney
907
A. Adenosine A receptors
907
1
B. Adenosine A
and A
receptors
908
2a
2b
C. Adenosine A receptors
909
3
D. Coupling of adenosine receptors
909
E. Concluding remarks
909
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IV. Vascular Actions of Adenosine in the Kidney
910
A. Exogenous adenosine
910
B. Endogenous adenosine
912
C. Mechanisms of adenosine-mediated vasoconstriction and vasodilation
913
D. Factors that modulate the vascular response to adenosine
913
E. Concluding remarks
915
V. Adenosine and Tubuloglomerular Feedback
915
A. Tubuloglomerular feedback
915
B. Altering TGF responses by manipulating adenosine receptor activation or adenosine formation
916
on May 4, 2011
C. Absence of TGF response in adenosine A receptor-deficient mice and consequences on the
1
single-nephron level
917
D. Adenosine is a mediator of TGF
918
E. Concluding remarks
920
VI. Adenosine and Renin Release
920
A. Effects of exogenous adenosine agonists on renin release
921
B. Role of endogenous adenosine in the control of renin release
921
C. Concluding remarks
923
VII. Role of Adenosine in Fluid and Electrolyte Transport in the Kidney
923
A. Proximal tubule
923
B. Medullary thick ascending limb
924
C. Distal convoluted tubule and cortical collecting duct
924
D. Inner medullary collecting duct
924
E. Concluding remarks
925
VIII. Adenosine and Metabolic Control of Organ Function: Does It Apply to the Kidney?
925
IX. Pathophysiological Aspects
928
A. Radiocontrast media-induced acute renal failure
928
B. Ischemia-reperfusion injury
929
X. Summary and Perspectives
930
Vallon, Volker, Bernd Mu
¨ hlbauer, and Hartmut Osswald. Adenosine and Kidney Function. Physiol Rev 86:
901–940, 2006; doi:10.1152/physrev.00031.2005.—In this review we outline the unique effects of the autacoid
adenosine in the kidney. Adenosine is present in the cytosol of renal cells and in the extracellular space of normoxic
kidneys. Extracellular adenosine can derive from cellular adenosine release or extracellular breakdown of ATP,
AMP, or cAMP. It is generated at enhanced rates when tubular NaCl reabsorption and thus transport work increase
or when hypoxia is induced. Extracellular adenosine acts on adenosine receptor subtypes in the cell membranes to
www.prv.org
0031-9333/06 $18.00 Copyright © 2006 the American Physiological Society
901

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902
VALLON, MU
¨ HLBAUER, AND OSSWALD
affect vascular and tubular functions. Adenosine lowers glomerular filtration rate (GFR) by constricting afferent
arterioles, especially in superficial nephrons, and acts as a mediator of the tubuloglomerular feedback, i.e., a
mechanism that coordinates GFR and tubular transport. In contrast, it leads to vasodilation in deep cortex and
medulla. Moreover, adenosine tonically inhibits the renal release of renin and stimulates NaCl transport in the
cortical proximal tubule but inhibits it in medullary segments including the medullary thick ascending limb. These
differential effects of adenosine are subsequently analyzed in a more integrative way in the context of intrarenal
metabolic regulation of kidney function, and potential pathophysiological consequences are outlined.
I. INTRODUCTION
systemic neurohumoral control, are the site of fine regu-
lation of NaCl balance.
The kidneys play a central role in body homeostasis
Because urinary fluid and NaCl excretion closely
by adapting the renal excretion of fluid and electrolytes to
match intake and because variations in fluid and NaCl
bodily needs under the control of a systemic neurohu-
intake are minor compared with the total amounts fil-
moral system. In addition to systemic control, primary
tered, it follows that GFR is the major determinant of
intrarenal local regulation mechanisms are important for
renal fluid and NaCl reabsorption. Reabsorption of NaCl
renal function and integrity. These local mechanisms are
requires energy, and the GFR is thus the major determi-
based on the way the mammalian kidney developed to
nant of renal O consumption. It follows that GFR is an
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2
important determinant of salt balance but also of meta-
fulfill its role in body fluid and NaCl homeostasis. More
bolic aspects of kidney function. Therefore, intrarenal
insights into intrarenal regulation of kidney function can
mechanisms that limit GFR when the ratio of renal O
provide a better understanding of pathophysiological pro-
2
supply to demand is significantly reduced could be bene-
cesses and eventually new therapeutic approaches. Vari-
ficial. Blood flow to the kidneys amounts to
20% of
ous autacoids are potential candidates to contribute to
cardiac output, and the cortical blood flow, which deter-
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the signaling cascades involved in local regulation mech-
mines GFR, is also rather high and thus the O supply of
anisms including molecules like nitric oxide, bradykinin,
2
the kidney cortex is generous. The situation is quite dif-
endothelin, angiotensin II, and prostanoids to name a few.
ferent, however, in the medulla, where blood flow derives
This review outlines the role of adenosine in the kidney.
from the postglomerular circulation of the deep cortex.
Before we focus on adenosine, we want to briefly intro-
To be able to concentrate the urine, the kidney uses a
duce some aspects of kidney function to illustrate the
mechanism that involves a low blood flow to the renal
need for intrarenal local regulation.
medulla and a counter-current system. As a consequence,
The way the mammalian kidney developed to fulfill
on May 4, 2011
the O supply to the renal medulla is low, although active
its role in body fluid and NaCl homeostasis includes a high
2
NaCl reabsorption in the medullary TAL is essential for
glomerular filtration rate (GFR,
180 l/day in humans).
the counter-current system. This situation asks for an
Subsequently, nearly all of the filtered fluid and NaCl is
intrarenal metabolic control to prevent hypoxic injury in
reabsorbed along the nephron such that only
1% of the
the medulla. Considering on the other hand that cortical
glomerular filtered amounts are excreted in the urine. As
blood flow is high but determines via GFR the tubular
a consequence, GFR and reabsorption have to be coordi-
NaCl load and thus transport work in cortex and medulla,
nated to prevent excessive renal losses of fluid and NaCl.
it follows that an intrarenal metabolic control of kidney
Intrarenal mechanisms that contribute to this coordina-
function requires differential effects on the vasculature
tion from minute to minute include glomerulotubular bal-
and transport systems of cortex and medulla.
ance and tubuloglomerular feedback (TGF). According to
Adenosine is a well-studied candidate that partici-
glomerulotubular balance, an increase in GFR and thus
pates in intraorgan control mechanisms including acute
the filtered amounts of NaCl causes a near-proportional
responses to increased work load (25, 26, 33, 234, 271,
increase in NaCl reabsorption in all segments of the tu-
299). The latter increases ATP hydrolysis and adenosine
bular and collecting duct system. In absolute amounts,
generation. Extracellular adenosine acts on specific G
this is particularly evident in the proximal tubule and the
protein-coupled receptors and, in organs like brain, heart,
medullary and cortical thick ascending limb of Henle
or skeletal muscle, induces vasodilation to match the
(TAL) where the bulk of the filtered NaCl is reabsorbed.
delivery of O and metabolic fuel to consumption. Addi-
2
The NaCl load at the end of the TAL is sensed, and the
tional potential defense mechanisms include a suppres-
TGF establishes an inverse relationship between this tu-
sion of the release of stimulatory neurotransmitters and a
bular NaCl load and the GFR of the same nephron. This
reduction of cell activity (see Fig. 1). The idea of adeno-
stabilizes and limits the NaCl load to the further distal
sine acting as a “retaliatory metabolite” or as a “homeo-
segments, which have a limited capacity to alter NaCl
static metabolite” has been reviewed (234, 299). Before
reabsorption from minute to minute, but which, under
this concept can be applied to the kidney and intrarenal
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ADENOSINE AND KIDNEY FUNCTION
903
ment in the TGF response of the nephron (see sect. V),
and that adenosine is involved in the regulation of renin
secretion (see sect. VI) and transport processes in the
tubular and collecting duct system (see sect. VII). Second,
we discuss the above findings in a more integrative way in
the context of an intrarenal metabolic control of kidney
function under physiological conditions (see sect. VIII).
Finally, we briefly extend this concept and the role of
adenosine in kidney function to aspects of renal patho-
physiology, namely, acute renal failure (see sect. IX). The
reader is also referred to three relatively recent reviews
on the role of adenosine in kidney function (48, 147, 315).
II. ADENOSINE GENERATION IN THE KIDNEY
FIG. 1. Schematic illustration of metabolic control of organ func-
tion. Mediators of metabolic control (M) are released from cells at a
A. Renal Tissue Content of Adenosine
rate that is determined by the phosphorylation potential (ATP/ADP · P ).
i
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The release increases when the phosphorylation potential falls (1).
The released mediators feed back on the cells to reduce cell activity
1. Normoxia and ischemia
(2), decrease stimulatory neurotransmitter release (3), and regulate
oxygen and substrate supply to the organ by vasodilation (4). NE,
When rat kidneys were snap frozen, an adenosine
norepinephrine.
tissue content of
5 nmol/g wet wt was found in nor-
moxic kidneys and the tissue content increased several-
control mechanisms, it is necessary to understand mech-
fold within a few minutes of renal ischemia (by occlusion
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anisms of adenosine formation in the kidney, the effects
of the renal artery) (253). These findings were confirmed
of adenosine on kidney function, and the determinants of
and extended to the kidneys of dogs and cats (215). The
kidney energy consumption and supply.
increase of adenosine tissue content during ischemia pre-
In this review we examine first the evidence that
ceded the increase of inosine and hypoxanthine, whereas
adenosine is present in the kidney and will be released at
ATP levels were rapidly reduced (245, 253) (see Fig. 2). In
an enhanced rate when ATP tissue levels fall (see sect. II),
vitro studies with suspensions of medullary TAL demon-
that the kidney expresses specific adenosine membrane
strated a hypoxia-stimulated adenosine release into the
on May 4, 2011
receptors (see sect. III), that adenosine mediates actions
medium. This increase was completely blocked by furo-
on renal vascular structures (see sect. IV) and is an ele-
semide or ouabain, indicating that adenosine release is
FIG. 2. Pathways and enzymes involved in adenosine
formation and metabolism. HCY, homocysteine; IMP, ino-
sine monophosphate; INO, inosine; SAH, S-adenosyl-ho-
mocysteine; SAM, S-adenosyl-methionine. 1: Adenosine ki-
nase; 2: adenylyl kinase; 3: ATPases; 4: 5 -nucleotidase; 5:
adenosine deaminase; 6: AMP deaminase; 7: inosine ki-
nase; 8: S-adenosyl-homocysteine hydrolase; 9: phospho-
diesterase; 10: adenylyl cyclase; 11: ecto-ATPases; 12:
ecto-phosphodiesterase; 13: ecto-5 -nucleotidase. NV, neu-
ronal varicosity.
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VALLON, MU
¨ HLBAUER, AND OSSWALD
related to electrolyte transport (21). Thus the kidney
makes no exception with respect to enhanced adenosine
generation and cellular release following ATP breakdown
during oxygen deficiency or enhanced organ work. Fur-
ther evidence is provided by the renal response to maleic
acid and hypertonic saline as outlined in the following
section.
2. Maleic acid
ATP depletion without inducing ischemia can be
achieved in the kidney by the use of maleic acid. This
stereoisomer of fumarate forms a stable complex with
coenzyme A, resulting in a fall in ATP levels mainly in the
proximal tubules (190, 256, 284). When in rat kidneys ATP
levels were reduced by intravenous administration of mal-
eate, adenosine tissue content was increased threefold
(245, 247). In dogs, maleate increased adenosine release
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into renal venous blood and into urine severalfold while
leaving arterial adenosine unchanged (10). The changes of
kidney function after maleate administration are dis-
cussed in section IV.
FIG. 3. Inverse changes of kidney contents of ATP (connected with
3. Hypertonic saline
the solid line) and adenosine (dashed line) in response to increasing
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renal Na
reabsorption rate (T
) by infusion of hypertonic saline (HS)
Na
An experimental maneuver to induce a rapid increase
into the thoracic aorta in rats on normal- or low-Na diet. [Adapted from
Osswald et al. (251).]
in renal transport work that leads to a fall in ATP and an
increase of adenosine tissue content is the short time
infusion of hypertonic saline into the thoracic aorta (251).
adenosine concentration of 5
M appears to be unrealis-
This maneuver increases renal NaCl reabsorption be-
tically high. Therefore, one has to assume that
90% of
cause the amount of NaCl filtered significantly increases
intracellular adenosine is protein bound, which makes it
due to the rise in plasma NaCl concentrations and a
on May 4, 2011
unavailable for deamination by the intracellular enzyme
concomitant increase in GFR by
20%. A rise in GFR in
adenosine deaminase (ADA). One candidate protein for
response to short time exposure to hypertonic saline
binding adenosine intracellularly is the cytosolic enzyme
(within 10 min) is in accordance with findings of Young
S-adenosylhomocysteine (SAH) hydrolase (163, 341) (see
and Rostorfer (378). In contrast, prolonged infusion of
Fig. 2). This enzyme hydrolyzes reversibly SAH into aden-
hypertonic saline directly into the renal artery leads to
osine and homocysteine. Two binding sites for adenosine
sustained vasoconstriction that can be blocked by the-
of the SAH hydrolase could be identified, one with high
ophylline (94). The results of this experimental series are
affinity (k
9.2 nM) and one with low affinity (k
1.4
shown in Figure 3. The experiments demonstrated a re-
D1
D2
M) (166, 167). Both adenosine binding sites of SAH
ciprocal relationship between the fall in ATP and the
hydrolase are controlled by the ratio of NAD /NADH
increase in adenosine tissue levels depending on the ab-
(162, 163, 165, 166). Based on the intracellular concentra-
solute amount of Na
reabsorbed by the kidney. Intrave-
tion of SAH hydrolase in the kidney of 2.2
M and its
nous infusion of hypertonic saline also led to a fall in renal
binding capacity, one can calculate that
20% of intracel-
ATP tissue levels, which returned to control levels follow-
lular adenosine is bound to SAH hydrolase under nor-
ing furosemide administration (80, 250). The hemody-
moxic conditions (124, 125, 341). Thus other proteins that
namic response of the kidney to intra-arterial infusion of
can bind intracellular adenosine have to be identified. The
hypertonic saline is discussed in section IV.
formation of SAH in the isolated perfused guinea pig
hearts was used by Deussen et al. (63) to calculate that
4. S-adenosylhomocysteine hydrolase binds adenosine
free adenosine concentrations in the cytosol are
80 nM.
An unsolved issue is the free intracellular adenosine
Notably, these estimated free cytosolic adenosine concen-
concentration in the kidney. Considering a renal tissue
trations are remarkably similar to the basal adenosine
content of adenosine of
5 nmol/g wet wt (253) and an
concentrations measured in the kidney interstitium (see
extracellular adenosine concentration of
100 nM under
below). This may not be unexpected given the presence of
normoxic conditions (see sect. IIB), a calculated cytosolic
equilibrative nucleoside transporters particularly in the
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ADENOSINE AND KIDNEY FUNCTION
905
basolateral membranes of kidney tubules (see below)
oxia (238). In dogs, renal [ADO]
was
117 nM and did
ISF
across which adenosine should equilibrate.
not change during reduction of renal perfusion pressure
In addition to adenosine binding, SAH hydrolase may
within the autoregulatory range when GFR remained con-
serve as a target of intracellular adenosine actions (167).
stant (236). One study actually looked at the time course
Because adenosine inhibits in vitro SAH hydrolase activ-
of [ADO]
in the effluent of the microdialysis tubes in
ISF
ity in nanomolar concentrations, this action would in-
conscious rats: [ADO]
revealed immediately after im-
ISF
crease cytosolic SAH levels and thus diminish the meth-
plantation very high levels that fell subsequently to con-
ylation potential [the ratio of S-adenosylmethionine
centrations between 100 and 200 nM within 2– 6 days
(SAM) to SAH], which regulates transmethylation reac-
(237). Although the microdialysis technique may have
tions in the cell. Moreover, it was shown that renal tissue
some limitations, the data show clearly that adenosine is
content of SAH increases severalfold after ischemia (161).
present in the renal interstitium at concentrations suffi-
In this respect, it is of interest to note that in addition to
cient to activate G-coupled adenosine receptors (see sect.
being expressed in the cytosol of nearly all kidney cells, a
III), i.e., in the mid to high nanomolar range under normal-
prominent staining of SAH hydrolase can be seen in the
NaCl diet and normoxic conditions with concentrations
nuclei of podocytes (164) and that SAH hydrolase accu-
being about two- to fourfold greater in medulla than in
mulates in nuclei of transcriptional activated cells (275).
cortex.
In summary, as in other organs, most of the adenosine
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content of the kidney is sequestered to intracellular aden-
2. Sources of extracellular adenosine
osine binding proteins including SAH hydrolase, and
much has to be learned about the functional conse-
To what extent interstitial adenosine is derived from
quences of the interactions between intracellular adeno-
intracellular or from extracellular sources is incompletely
sine and SAH hydrolase.
understood. It is established that AMP is a major precur-
sor for intracellular and extracellular adenosine forma-
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tion (see Fig. 2). However, it would be important to know
B. Extracellular Adenosine Concentration
the free concentration of cytosolic AMP and adenosine;
the potential efflux rates of ATP, AMP, and adenosine
1. Microdialysis technique
from the cytosol into the interstitium; and the contribu-
With the use of the microdialysis technique in rat
tion of ecto-5 -nucleotidases to the interstitial adenosine
kidneys, it was found that mean values of interstitial fluid
concentration. With the use of the technique of nuclear
adenosine concentrations ([ADO]
) are 55 nM in cortex
resonance spectroscopy for 31P phosphorus, it was found
ISF
and 212 nM in medulla (381). Infusion of ATP-MgCl
in cardiac tissues that free AMP amounts to
5% of total
2
on May 4, 2011
resulted in a roughly twofold elevation of adenosine, ino-
extracted AMP. The measured (96) and calculated (39,
sine, hypoxanthine, and uric acid, indicating the capacity
355) concentrations of free AMP in the cytosol of cardiac
of the kidney to metabolize exogenous ATP (381) (see
tissue under normoxic conditions are in the range of 200
Fig. 2). Employing microdialysis tubes inserted into both
nM. These low concentrations are close to the assumed
kidney cortex and medulla in rats on a normal-NaCl diet,
levels of free adenosine in the cytosol (39). Changes in the
Siragy and Linden (308) found [ADO]
in the dialysate
AMP-adenosine cycle via the activities of cytosolic 5 -
ISF
from the cortex of 63 nM and from the medulla of 157 nM.
nucleotidase and adenosine kinase (see Fig. 2) can lead to
Notably, rats consuming a high-NaCl diet had renal corti-
significant changes of intracellular adenosine formation
cal and medullary dialysate adenosine concentrations
(58, 96, 211). The cytosolic 5 -nucleotidase has recently
that were increased about sevenfold, whereas low-NaCl
been cloned (for review, see Ref. 138), which should be
diet lowered [ADO]
by 64% in both kidney regions
helpful to further delineate this issue.
ISF
compared with normal diet (308). The mechanisms in-
A) NUCLEOSIDE TRANSPORTERS. Cellular uptake and re-
volved are not absolutely clear but may relate to the fact
lease of adenosine is mediated by nucleoside transport-
that rats can respond to a high NaCl diet with an increase
ers. The nucleoside transporter proteins differentiated so
in GFR (346). As a consequence, absolute and fractional
far are divided into five distinct superfamilies that are
renal NaCl excretion are increased to match increased
functionally characterized and vary in substrate specific-
intake but at the same time the associated increase in
ity. The concentrative nucleoside transporters CNT1-
GFR enhances absolute renal NaCl reabsorption (primar-
CNT3 [solute carrier (SLC) 28A1–28A3], which mainly
ily in proximal tubule and thick ascending limb)(346), and
localize to the apical membrane of renal epithelium,
thus possibly adenosine formation. Further studies are
mediate the intracellular flux of nucleosides. The equili-
required to clarify this issue. In another study using the
brative nucleoside transporters ENT1–2 (SLC29A1-
microdialysis technique in rabbit kidneys, [ADO]
was
SLC29A2), on the other hand, primarily localize to baso-
ISF
found to be 293 nM in the cortex under basal conditions
lateral membranes and mediate bidirectional facilitated
and increased threefold after induction of systemic hyp-
diffusion of nucleosides and may thus contribute to cel-
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906
VALLON, MU
¨ HLBAUER, AND OSSWALD
lular adenosine release when cytosolic concentrations
centrations were 117 nM and remained unaltered by these
increase. However, the knowledge base is far from clear
changes of renal perfusion pressure, which did not affect
to define satisfactorily how these transporters work,
renal blood flow or GFR (236). A similarly low interstitial
alone or in concert, and under varying intra- and extra-
ATP/adenosine ratio was reported in the isolated per-
cellular conditions. Much has to be learned in this regard,
fused rat heart (203). The 20- to 40-fold higher concentra-
and the interested reader is referred to recent reviews on
tions of adenosine than those of ATP do not readily
the topic (17, 101, 331).
support the assumption that this ATP is the major precur-
B) ECTO-5 -NUCLEOTIDASE IN GLOMERULI AND LUMINAL MEM-
sor of adenosine in the bulk phase of the interstitial fluid.
BRANES OF TUBULES. Ecto-5 -nucleotidase is expressed in
ATP can serve as a major precursor of adenosine, how-
glomeruli including mesangial cells (45, 134, 189), where
ever, if higher ATP concentrations exist in the unstirred
it may contribute to the generation of adenosine, which
layer at the surface of the plasma membranes, which are
mediates the TGF mechanism (see sect. V). In the first
equipped with ectoenzymes to metabolize adenine nucle-
loops of the proximal convoluted tubule, prominent stain-
otides (see Fig. 2) and/or the generation of adenosine
ing of ecto-5 -nucleotidase is visible in the luminal brush-
by these pathways is faster than the downstream
border membrane (56, 91, 112). In the cortical pars recta
adenosine metabolism or cell uptake. Extracellular gen-
of the proximal tubule, ecto-5 -nucleotidase staining is
eration of adenosine from ATP may contribute to the
low but increases slightly towards the medullary pars
signaling mechanisms of the tubuloglomerular feedback
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recta of the proximal tubule. Also in segments of the
(see sect. V).
distal tubule (intercalated cells), a luminal staining was
E) INTERSTITIAL cAMP AS A PRECURSOR OF ADENOSINE. Accord-
shown. The luminal localization of ecto-5 -nucleotidase is
ing to the scheme of Figure 2, extracellular adenosine can
likely to be involved in the purine salvage pathway of the
be generated from cAMP, which is released by cells of the
filtered or luminal released nucleotides that after dephos-
tubular or vascular system. Jackson and co-workers ex-
phorylation can affect luminal adenosine concentrations
amined this possibility in several experiments. The cAMP
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or be taken up by nucleoside carriers in the brush-border
added to the perfusate of isolated perfused kidneys can be
membrane (4, 306). It is, however, unlikely that these
converted to AMP by ecto-phosphodiesterases and subse-
luminal ecto-5 -nucleotidase activities produce changes in
quently to adenosine by ecto-5 -nucleotidases resulting in
adenosine concentrations at the basolateral sites of the
a release of AMP, adenosine, and inosine into the venous
tubular epithelium and in the interstitium (189, 283).
effluent (213). Addition of the phosphodiesterase inhibi-
C) ECTO-5 -NUCLEOTIDASE IN PERITUBULAR SITES. The peritu-
tor 3-isobutyl-1-methylxanthine (IBMX) to the perfusate
bular staining of ecto-5 -nucleotidase in the kidney has
resulted in an almost complete block of AMP, adenosine,
first been attributed to the cells of blood capillaries (112).
and inosine release. However, when the inhibitor of ecto-
on May 4, 2011
Le Hir and Kaissling (189), however, demonstrated that
5 -nucleotidases,
, -methylene-adenosine-5 -diphosphate
the ecto-5 -nucleotidase-positive perivascular cells were
(AMPCP), was infused together with cAMP, the release of
in fact fibroblasts. Endothelial cells of the capillaries were
AMP was unchanged but adenosine and inosine release
negative. The fibroblasts in the interstitium of the kidney
make contact with tubular cells and peritubular capillar-
were nearly completely inhibited (213). Treating the iso-
ies and form a sheath around afferent and efferent arte-
lated perfused kidneys with isoproterenol, the endog-
rioles of the glomerulus. Under normoxic conditions the
enously released cAMP is also extracellularly converted
ecto-5 -nucleotidase-positive cells are exclusively located
to AMP and adenosine yielding a threefold increase of
in the cortex and cannot be demonstrated in the medulla
adenosine,
inosine,
and
hypoxanthine
(214).
The
(56, 91). The intensity of fibroblast staining in the cortex
-blocker propranolol, IBMX, and AMPCP blocked the
changes in parallel to the increased production of eryth-
isoproterenol-induced increase of purines (214). Also in
ropoietin. Under normal conditions, the staining is lo-
the isolated guinea pig gallbladder, a model of a trans-
cated predominantly in the deep cortex. The staining
porting epithelium, substantial cAMP release into the ex-
increases, however, throughout the whole cortex under
tracellular space was found after stimulation with pros-
challenges of hypobaric oxygen breathing or anemia
taglandins (265). Thus the formation of adenosine from
(189). The exact role of ecto-5 -nucleotidase on the fibro-
extracellular cAMP suggests that adenosine by activation
blasts remains to be determined.
of adenosine A receptors, which can be coupled to an
1
D) INTERSTITIAL ATP AS A PRECURSOR OF ADENOSINE. Re-
inhibitory G protein (see sect.
i
IIID), can function as a
cently, renal cortical interstitial ATP concentrations as-
local feedback inhibitor of adenylyl cyclase. This pathway
sessed by the microdialysis technique in anesthetized
may play a role for effects of adenosine on proximal
dogs were found to be 6.5 nM at a renal artery pressure of
tubular reabsorption (see sect. VII) as well as adenosine-
131 mmHg. Stepwise reduction of renal perfusion pres-
mediated inhibition of both, renin release (see sect. VI)
sure to 105 and 80 mmHg lowered ATP concentrations to
and vasopressin-stimulated transport in the inner medul-
4.5 and 2.8 nM, respectively. Interstitial adenosine con-
lary collecting duct (see sect. VII).
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ADENOSINE AND KIDNEY FUNCTION
907
C. Renal Excretion of Adenosine
III. ADENOSINE RECEPTORS IN THE KIDNEY
Is the renal adenosine excretion of physiological or
In recent years, the databases on adenosine recep-
pathophysiological significance? Only few data in the lit-
tors, especially on those located in the central nervous
erature address this question. Thompson et al. (329) ana-
system, have intensively grown. The knowledge on the
lyzed in anesthetized dogs the renal arterial-venous dif-
distribution of adenosine receptors in the kidney is less
ference of adenosine and adenosine excretion kinetics
clear, which can be attributed, among other reasons, to
following single injections of radiolabeled adenosine into
low, or, at most, intermediate expression levels in this
the renal artery (329). Under basal conditions the concen-
organ. The concept that adenosine exerts its actions also
trations of endogenous adenosine in plasma of renal vein
in the kidney via specific receptors was originally based
and artery were 52– 60 nM and not statistically different.
on binding characteristics or functional experiments us-
Urinary adenosine concentration was 312 nM, and the
ing selective pharmacological ligands. Several compre-
excretion rate was 0.67 nmol/min. With the use of the
hensive reviews have been published in recent years that
single injection method, it was found that 12% of the in-
focused on adenosine receptors in general or on adeno-
jected adenosine was recovered in urine and 11% in the
sine receptor subtypes A , A , A , or A (78, 81, 84 – 86).
1
2a
2b
3
renal vein. This low venous recovery was due to cellular
For nomenclature and classification of the adenosine re-
uptake by nucleoside transporters as evidenced by a
ceptor family, the reader is referred to a recent publica-
threefold increase of adenosine recovery in the renal vein
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tion of the International Union of Pharmacology (86).
by the nucleoside transport inhibitor dipyridamole (329).
Studies in mice with genetically modified adenosine re-
Heyne et al. (127) studied renal adenosine excretion in 12
ceptors support the concept that these specific receptors
healthy volunteers under basal conditions, after water
mediate effects of adenosine on the kidney. Currently, all
loading, and following low and high Na
intake. It was
receptor subtypes have been genetically deleted in mouse
found that adenosine excretion (3.2 nmol/min) was inde-
models except for the adenosine A
receptor, and some
2b
pendent of urinary flow rate (2–19 ml/min), indicating that
physrev.physiology.org
have been overexpressed in selective tissues of trans-
passive tubular back-diffusion does not significantly con-
genic mice. Studies involving these transgenic mice indi-
tribute to net adenosine excretion. Moreover, low- and
cated that receptor levels are rate limiting, as effects were
high-Na
diet did not change adenosine excretion per
amplified upon increases in receptor level (369). This
milliliter of GFR. These remarkably constant values under
underscores the value of studies on the expression level
normal conditions could provide a basis for the evalua-
of adenosine receptors. In this section we review the
tion of renal adenosine excretion as a marker of renal
current knowledge on the distribution and signaling path-
injury in various clinical settings. In fact, enhanced renal
ways of adenosine receptors in the kidney. The functional
on May 4, 2011
adenosine excretion rates were found during renal isch-
aspects of renal adenosine receptors, including observa-
emia (215), maleic acid (10), radiocontrast media appli-
tions from knockout-models, are addressed in sections
cation (154), and methotrexate (12).
IV–IX.
D. Concluding Remarks
A. Adenosine A Receptors
1
The available data have shown that adenosine is
present in the normoxic kidney. The tissue content,
5
With the use of the selective ligands cyclohexylad-
nmol/g wet wt, represents mainly the cytosolic fraction of
enosine (CHA) or N6-p-hydroxy-phenyl-isopropyl-adeno-
renal adenosine, whereas only 2–5% of this amount is
sine (PIA), adenosine A receptors were first identified by
1
present in the extracellular compartments, such as tubu-
autoradiography in sections of human and guinea pig
lar fluid/urine and interstitium. Since a calculated cytoso-
kidney (257, 358). Moreover, binding sites on glomerular
lic adenosine concentration of 5
M appears to be unre-
structures for adenosine A
receptor ligands were re-
1
alistic, most of the intracellular adenosine must be bound
ported in both species (55, 337). Specific binding was
to intracellular proteins including SAH hydrolase. Sources
more recently reported of 3H-labeled 1,3-dipropyl-8-cyclo-
of extracellular adenosine in the kidney include cellular
pentylxanthine (DPCPX), an adenosine A receptor antag-
1
adenosine release as well as extracellular adenosine for-
onist, to plasma membranes of immortalized cells derived
mation from ATP, AMP, and cAMP being released from
from normal adult human proximal tubule (328). Among
the cells. After ATP depletion by ischemia, maleic acid, or
the adenosine receptors, the A subtype was the first to be
1
hypertonic saline, extracellular adenosine concentrations
cloned (208). The respective gene, in humans, was allo-
increase indicating that in the kidney as in other organs
cated to chromosome 1q32.1 (336). Several studies using
adenosine generation is controlled by the phosphoryla-
molecular techniques showed the presence of the adeno-
tion potential of the cell. The functional implications are
sine A
receptor in the rodent kidney. As depicted in
1
discussed in section VIII.
Table 1, adenosine A receptors are present in afferent
1
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908
VALLON, MU
¨ HLBAUER, AND OSSWALD
TABLE 1.
Expression of adenosine receptors in the kidney
Receptor Subtype/Localization
Species
Method
References Nos.
Adenosine A receptors
1
Whole kidney
Rat
RT-PCR
69, 99, 105, 225, 260, 280
Northern
313
Whole kidney
Rabbit
Cloning
28
Cortical and medullary membranes
Rat
Western
260, 382
Glomeruli, proximal tubule, mTAL, cTAL, MCD
Rat
RT-PCR
373
Thin limbs of Henle, CD, mTAL
Rat, mouse
RT-PCR
356
Afferent arterioles, mesangial cells, proximal tubules, collecting ducts
Rat
Immunocytochemistry
311
Preglomerular microvessels
Rat
Western/Northern, RT-PCR
150
Glomerular epithelial cells and medullar tubules
Rat
Immunohistochemistry
260
IMCD, JGA slices
Rat
ISH
357
Outer medulla descending vasa recta
Rat
RT-PCR
176
Adenosine A
receptors
2a
Whole kidney
Rat
PCR
69, 105, 225, 260
Whole kidney
Guinea pig
Northern
212
Cortical and medullary membranes
Rat
Western
260, 382
Glomerular epithelial cells and capillaries
Rat
Immunohistochemistry
260
Glomeruli
Rat, mouse
RT-PCR
356
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Papilla
Rat
ISH
357
OMDVR
Rat
PCR
176
Adenosine A
receptors
2b
Whole kidney
Rat
RT-PCR
69, 225, 260
Cortical and medullary membranes
Rat
Western
260, 382
Preglomerular microvessels
Rat
Western/Northern, RT-PCR
150
OMDVR
Rat
RT-PCR
176
cTAL, DCT
Rat, mouse
RT-PCR
356
Baby hamster kidney cells
Hamster
RT-PCR
218
physrev.physiology.org
Adenosine A receptors
3
Whole kidney
Rat
Cloning/RT-PCR
379
RT-PCR
69, 200, 225, 260
Western
260
Whole kidney
Human
RT-PCR
200
Northern
288
Whole kidney
Sheep
Cloning/RT-PCR
201
Northern
288
Cortical and medullary membranes
Rat
Western
260, 382
on May 4, 2011
DCT, distal convoluted tubule; cTAL and mTAL, cortical and medullary thick ascending limb of Henle’s loop, respectively; IMCD, inner
medullary collecting duct; ISH, in situ hybridization; JGA, juxtaglomerular apparatus; MCD, medullary collecting duct; OMDVR, outer medullary
descending vasa recta; RT-PCR, reverse transcription-polymerase chain reaction.
arterioles, glomeruli including mesangial cells, juxtaglo-
Both the mRNA and the protein of the adenosine A2a
merular cells, vasa recta, as well as in various segments of
receptor were demonstrated in whole kidney prepara-
the tubular and collecting duct system including proximal
tions. Furthermore, mRNA for the adenosine A
receptor
2a
tubule, thin limbs of Henle, TAL, and collecting ducts. In
was detected in the papilla of the rat kidney (357) and in
spite of the numerous renal effects of adenosine A re-
1
glomeruli of rat and mouse kidney (356). Finally, adeno-
ceptor activation in humans, data on the localization of
sine A
receptor mRNA was found in the outer medullary
2a
this receptor subtype in the human kidney on the molec-
descending vasa recta (176).
ular level have not been reported so far.
Adenosine A
receptors were first cloned from brain
2b
regions of human (272) and rat (282). The responding hu-
B. Adenosine A
and A
Receptors
man gene was localized to chromosome 17p12 (151). There
2a
2b
is only sparse information on the presence of adenosine A2b
The adenosine A
receptor family consists of two
adenosine receptors in the kidney (see Table 1). Adenosine
2
subtypes, the A
and the A
receptor, which possess a
A
receptor mRNA or protein was detected in whole kidney
2a
2b
2b
high and a low agonist affinity, respectively. The adeno-
preparations. In addition, mRNA was detected in the cortical
sine A receptor was first cloned from a canine thyroid
TAL and in the distal convoluted tubule (356) as well as in
2
cDNA library (196). The human adenosine A
receptor
the outer medullary descending vasa recta (176), and more
2a
gene was localized to chromosome 22q11.2 (181, 205).
recently, adenosine A
receptor mRNA was reported in
2b
Similar to the adenosine A receptor, renal A
receptors,
baby hamster kidney cells (218) and at the protein and
1
2a
so far, have been identified only in rodents (see Table 1).
mRNA level in rat preglomerular vessels (150).
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ADENOSINE AND KIDNEY FUNCTION
909
C. Adenosine A Receptors
bition and phospholipase C (PLC) stimulation, whereas
3
adenosine A
and A
receptor stimulation leads, via
2a
2b
The fourth distinct adenosine receptor is the A sub-
cholera toxin-sensitive stimulatory G proteins, to adenylyl
3
type. It was first cloned from the rat striatum (379). The
cyclase activation (for detailed information, see reviews
recombinant striatal adenosine A receptor differs com-
in Refs. 79, 85, 86, 88, 229). The adenosine A receptor
3
3
pletely compared with the other adenosine subtypes in
appears to couple in a similar fashion as A receptors, via
1
agonist or antagonist binding. The human adenosine A
inhibitory G
protein, to adenylyl cyclase and PLC (for
3
q/11
receptor gene was localized to chromosome 1p (221).
review, see Ref. 86). Thus adenosine receptor subtypes
Adenosine A receptors have been detected, both on the
appear to couple to more than one G protein and/or
3
mRNA and on the protein level, in whole kidney prepara-
effector system. As outlined in Table 2 for adenosine A1
tions of various species (see Table 1). In contrast, no
and A receptors, the coupling of adenosine receptors to
2
distinct intrarenal localization has been reported so far.
the various effector systems in the kidney is, in general, in
From radioligand binding studies, the presence of the
agreement with the above-mentioned concepts derived
adenosine A receptor in brush-border membranes iso-
from extrarenal cell types.
3
lated from pig kidney had been suggested (32). Interest-
ingly, adenosine A
receptor mRNA in the kidneys of
3
E. Concluding Remarks
young rats increased with age from the newborn state to
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early adolescence by more than one order of magnitude
(225).
Each of the family of adenosine receptors has been
demonstrated in virtually all organs. In the last years, the
knowledge on expression and signal transduction of
D. Coupling of Adenosine Receptors
adenosine receptors in the kidney has grown but is still
sparse. In particular, differentiated localization of the
The adenosine receptors belong to the superfamily of
adenosine receptors in this organ is incompletely defined.
physrev.physiology.org
G protein coupling receptors. According to a consensus
In general, the existing data, however, indicate that the
definition, adenosine A receptors induce, via pertussis
mechanisms of adenosine receptor coupling as obtained
1
toxin-sensitive G and G proteins, adenylyl cyclase inhi-
from other tissues also apply to the kidney.
i
o
TABLE 2.
Effector coupling of adenosine A and A receptors in the kidney
1
2
on May 4, 2011
Type
Effector
Site/Cell
Reference Nos.
Proximal tubule (PT)
A
AC2, cAMP2, PKA2
Rabbit or rat PT, OKC, LLC-PK cell line, human PT epithelial cell line
41, 42, 53, 191, 326, 328
1
1
A
PKC1
OKC
53, 54
1
A
AC1
Isolated tubules of rabbit renal cortex
87
2
Thick ascending limb of Henle’s loop (TAL)
A
cAMP2
Cultured rabbit or rat mTAL, mouse mTAL
19, 40, 335
1
A
G , Ca2 1
Cultured rabbit mTAL cells
40
1
i
A
cAMP1
Cultured rabbit mTAL, mouse TAL
19,40
2
Cortical collecting duct (CCD)
A
cAMP2
Cultured rabbit or human CCD
6, 7, 9, 274, 317
1
A
Ca2 1, PLC1, PKC1
Cultured rabbit or human CCD
6, 7, 274, 304, 317
1
A
cAMP1
Cultured rabbit or human CCD
9, 274
2
Inner medullary collecting duct (IMCD)
A
cAMP2
Primary cell culture rat IMCD, perfused rat IMCD, mouse IMCD cell line
73, 226, 371
1
A
cAMP2
Primary cell culture rat IMCD
371
2
Renal vasculature
A
Ca2 1
Isolated, perfused rat kidney
286
1
A
G , PLC
Isolated, perfused afferent arterioles
111
1
i
A
K
opening
Afferent arteriole
327
2a
ATP
A
eNOS activation
Whole kidney
109
2a
Renin/juxtaglomerular cells
A
Ca2 1
Cortical slices
285
1
AC, adenylyl cyclase; eNOS, endothelial nitric oxide synthase; G , inhibitory G protein; mTAL, medullary TAL; OKC, opossum kidney cells;
i
PKA, protein kinase A; PKC, protein kinase C; PLC, phospholipase C.
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910
VALLON, MU
¨ HLBAUER, AND OSSWALD
IV. VASCULAR ACTIONS OF ADENOSINE
of renal blood flow (“overshoot”) can be observed (8, 243,
IN THE KIDNEY
246, 255, 325) (see Fig. 4). The factors that can modu-
late the renal response to adenosine are discussed in
We divide the actions of adenosine on renal vascula-
section IVD.
ture into two different sections, exogenous and endoge-
Although whole kidney renal blood flow can return to
nous adenosine, for the following reasons: 1) adenosine
preinfusion levels within 1–2 min, whole kidney GFR
injected or infused into the renal artery is reaching all
remains to be reduced during steady-state continuous
structures of the kidney not taking into account the het-
adenosine infusion (255) (see Fig. 4). When micropunc-
erogeneity of its local generation under physiological con-
ture experiments were performed in rats and dogs, con-
ditions, 2) high intra-arterial concentrations of exog-
tinuous adenosine infusion into the renal artery reduced
enously administered adenosine may activate a variety of
single-nephron GFR (SNGFR) (derived from superficial
endothelial responses like release of prostaglandins or
nephrons) to a larger extent than whole kidney GFR,
nitric oxide (NO) which may not be induced by increases
indicating that deep-cortical vasodilation counteracts su-
of endogenous adenosine released into the interstitium,
perficial vasoconstriction (103, 254, 255) (see Fig. 4). In
and 3) the actions of endogenous adenosine being re-
fact, adenosine, after an initial vasoconstriction in all
leased or formed at an enhanced rate under physiological
cortical zones, induced deep-cortical vasodilation while
and pathophysiological conditions, such as an increased
superficial cortical vasoconstriction persisted (206, 219).
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metabolic rate or hypoxia, may elicit quite different re-
The adenosine-induced fall in SNGFR at the kidney sur-
sponses at their receptors compared with the basal state.
face was the result of afferent arteriolar vasoconstriction
Renal vascular actions of adenosine have recently been
with a parallel fall of the hydrostatic pressure in glomer-
reviewed (110, 240).
ular capillaries and in postglomerular star vessels. Effer-
ent arteriolar dilation did not contribute to the fall in
SNGFR (at the applied doses of 0.5 nmol · min 1 · ml renal
A. Exogenous Adenosine
physrev.physiology.org
blood flow 1) (103, 255). Other investigators employing
intrarenal infusion of adenosine in the rat at four- to
1. Effects of intrarenal administration of adenosine
fivefold higher doses did not observe significant changes
on renal blood flow and GFR
in whole kidney GFR (83, 219). The reasons for these
Two reports in 1964 described adenosine-induced
discrepancies can be due to 1) differences in the routes
renal vasoconstriction in the anesthetized dog (333) and
and doses of infused adenosine which can lead to activa-
in the blood-perfused dog kidney (115), respectively.
tion of different adenosine receptors (see below) or 2)
These findings were confirmed by other investigators
differences in the Na
and volume status of the animals
on May 4, 2011
(116, 243, 325). Importantly, also conscious dogs respond
which is known to modulate the renal vascular response
to intrarenal adenosine injection with vasoconstriction
to adenosine (see sect. IVD1).
(27). In anesthetized dogs, rats, and cats, intra-arterial
A different approach of intrarenal adenosine admin-
single injections of adenosine elicited a renal vasocon-
istration was chosen by Pawlowska et al. (261) who in-
striction, which is rapid in onset and short in duration
fused adenosine into the renal interstitial space of rats via
(243, 252, 319). Continuous intra-arterial infusion of aden-
implanted capsules. The data clearly demonstrated that
osine leads to an initial fall in renal blood flow that lasts,
adenosine and two stable analogs, 2-chloro-adenosine and
however, for only 1–2 min and then whole renal blood
NECA, decreased GFR by 50 – 80% while leaving total
flow returns to or slightly above preinfusion levels. After
renal blood flow unchanged (261). To analyze the intrare-
cessation of adenosine infusion, a short-lasting increase
nal blood flow distribution during interstitial adenosine
FIG. 4. Renal blood flow (RBF) response to adenosine
infusion into the renal artery in anesthetized dogs. Original
tracing of RBF in a dog subjected to micropuncture. Mea-
surements of whole kidney and single-nephron glomerular
filtration rate (SNGFR) were performed during steady-
state of continuous adenosine infusion for 25 min. Whereas
RBF normalizes within minutes, kidney GFR and particu-
larly SNGFR in superficial nephrons remain reduced.
[Adapted from Osswald et al. (255).]
Physiol Rev • VOL 86 • JULY 2006 • www.prv.org

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