Adenosine modulates vasomotor tone in outer medullary descending vasa recta of the rat

Renal Autocrine and Paracrine Signaling: A Story of Self-protection

Autocrine and paracrine signaling in the kidney adds an extra level of diversity and complexity to renal physiology. The extensive scientific production on the topic precludes easy understanding of the fundamental purpose of the vast number of molecules and systems that influence the renal function. This systematic review provides the broader pen strokes for a collected image of renal paracrine signaling. First, we recapitulate the essence of each paracrine system one by one. Thereafter the single components are merged into an overarching physiological concept. The presented survey shows that despite the diversity in the web of paracrine factors, the collected effect on renal function may not be complicated after all. In essence, paracrine activation provides an intelligent system that perceives minor perturbations and reacts with a coordinated and integrated tissue response that relieves the work load from the renal epithelia and favors diuresis and natriuresis. We suggest that the overall function of paracrine signaling is reno-protection and argue that renal paracrine signaling and self-regulation are two sides of the same coin. Thus local paracrine signaling is an intrinsic function of the kidney, and the overall renal effect of changes in blood pressure, volume load, and systemic hormones will always be tinted by its paracrine status.

Short-term hypoxia and vasa recta function in kidney slices

BACKGROUND

Descending vasa recta (DVR) supply the inner part of outer renal medulla an area at risk for hypoxic damages.

OBJECTIVE

We hypothesize increased vasoreactivity after hypoxia/re-oxygenation (H/R) in DVR, which might contribute to the reduced medullary perfusion after an ischemic event.

METHODS

Live kidney slices (200μm) from SD rats were used for functional experiments. TUNEL assay and H&E staining were used to estimate slice viability. Kidney slices were treated with carbogen or hypoxia (1% O2) for 60 or 90 min and vasoreactivity to Ang II (10-7 M) was recorded by DIC microscopy after re-oxygenation with carbogen. Expression of NOS and NADPH enzymes mRNA were determined in iron-perfusion isolated VR.

RESULTS

Percentage of apoptotic cells increased in control and H/R after 90 min in the medulla. Ang II- induced constriction of DVR was reduced after 90 min in control (compared to 60 min), but not after H/R. NOS enzymes mRNA expression levels decreased over 90 min hypoxia.

CONCLUSIONS

Increased reactivity of DVR to Ang II after H/R compared to control (90 min) suggest a role of DVR in renal ischemia/reperfusion injury.

Reactive oxygen species as important determinants of medullary flow, sodium excretion, and hypertension

The physiological evidence linking the production of superoxide, hydrogen peroxide, and nitric oxide in the renal medullary thick ascending limb of Henle (mTAL) to regulation of medullary blood flow, sodium homeostasis, and long-term control of blood pressure is summarized in this review. Data obtained largely from rats indicate that experimentally induced elevations of either superoxide or hydrogen peroxide in the renal medulla result in reduction of medullary blood flow, enhanced Na(+) reabsorption, and hypertension. A shift in the redox balance between nitric oxide and reactive oxygen species (ROS) is found to occur naturally in the Dahl salt-sensitive (SS) rat model, where selective reduction of ROS production in the renal medulla reduces salt-induced hypertension. Excess medullary production of ROS in SS rats emanates from the medullary thick ascending limbs of Henle [from both the mitochondria and membrane NAD(P)H oxidases] in response to increased delivery and reabsorption of excess sodium and water. There is evidence that ROS and perhaps other mediators such as ATP diffuse from the mTAL to surrounding vasa recta capillaries, resulting in medullary ischemia, which thereby contributes to hypertension.

Renal pericytes: regulators of medullary blood flow

Regulation of medullary blood flow (MBF) is essential in maintaining normal kidney function. Blood flow to the medulla is supplied by the descending vasa recta (DVR), which arise from the efferent arterioles of juxtamedullary glomeruli. DVR are composed of a continuous endothelium, intercalated with smooth muscle-like cells called pericytes. Pericytes have been shown to alter the diameter of isolated and in situ DVR in response to vasoactive stimuli that are transmitted via a network of autocrine and paracrine signalling pathways. Vasoactive stimuli can be released by neighbouring tubular epithelial, endothelial, red blood cells and neuronal cells in response to changes in NaCl transport and oxygen tension. The experimentally described sensitivity of pericytes to these stimuli strongly suggests their leading role in the phenomenon of MBF autoregulation. Because the debate on autoregulation of MBF fervently continues, we discuss the evidence favouring a physiological role for pericytes in the regulation of MBF and describe their potential role in tubulo-vascular cross-talk in this region of the kidney. Our review also considers current methods used to explore pericyte activity and function in the renal medulla.

Medullary thick ascending limb buffer vasoconstriction of renal outer-medullary vasa recta in salt-resistant but not salt-sensitive rats

We have demonstrated previously that paracrine signaling occurs between medullary thick ascending limb (mTAL) and the contractile pericytes of outer-medullary vasa recta (VR), termed "tubular-vascular cross-talk." The aim of the current study was to determine whether tubular-vascular cross-talk has a functional effect on vasoconstrictor responses to angiotensin II and to determine whether this is altered in the Dahl salt-sensitive (SS) rat. Studies were performed on salt-resistant consomic SS.13 Brown Norway (BN) and SS rats using a novel outer medullary tissue strip preparation in which freshly isolated VRs within VR bundles were perfused either alone or in combination with nearby mTAL. In VRs from SS.13BN rats, angiotensin II (1 µmol/L) increased VR bundle intracellular Ca2+ concentration 19±9 nmol/L (n=8) and reduced focal diameter in perfused VRs by -20±7% (n=5). In the presence of nearby mTAL, however, VR bundle intracellular Ca2+ concentration (-9±8 nmol/L; n=8) and VR diameter (-1±4%, n=7) in SS.13BN rats were unchanged by angiotensin II. In contrast, in Dahl SS rats, angiotensin II resulted in rapid and sustained increase in VR bundle intracellular Ca2+ concentration (89±48 nmol/L, n=7; 50±24%, n=8) and a reduction in VR diameter of (-17±7%, n=7; -11±4%, n=5) in both isolated VRs and VRs with nearby mTAL, respectively. In VRs with mTAL from SS13BN rats, inhibiton of purinergic receptors resulted in an increase in VR bundle intracellular Ca2+ concentration, indicating that purinergic signaling buffers vasoconstriction. Importantly, our in vitro data were able to predict medullary blood flow responses to angiotensin II in SS and SS.13BN rats in vivo. We conclude that paracrine signaling from mTAL buffers angiotensin II vasoconstriction in Dahl salt-resistant SS.13BN rats but not SS rats.

An intact kidney slice model to investigate vasa recta properties and function in situ

Background

Medullary blood flow is via vasa recta capillaries, which possess contractile pericytes. In vitro studies using isolated descending vasa recta show that pericytes can constrict/dilate descending vasa recta when vasoactive substances are present. We describe a live kidney slice model in which pericyte-mediated vasa recta constriction/dilation can be visualized in situ.

Methods

Confocal microscopy was used to image calcein, propidium iodide and Hoechst labelling in ‘live’ kidney slices, to determine tubular and vascular cell viability and morphology. DIC video-imaging of live kidney slices was employed to investigate pericyte-mediated real-time changes in vasa recta diameter.

Results

Pericytes were identified on vasa recta and their morphology and density were characterized in the medulla. Pericyte-mediated changes in vasa recta diameter (10–30%) were evoked in response to bath application of vasoactive agents (norepinephrine, endothelin-1, angiotensin-II and prostaglandin E₂) or by manipulating endogenous vasoactive signalling pathways (using tyramine, L-NAME, a cyclo-oxygenase (COX-1) inhibitor indomethacin, and ATP release).

Conclusions

The live kidney slice model is a valid complementary technique for investigating vasa recta function in situ and the role of pericytes as regulators of vasa recta diameter. This technique may also be useful in exploring the role of tubulovascular crosstalk in regulation of medullary blood flow.

Extracellular nucleotides affect pericyte-mediated regulation of rat in situ vasa recta diameter

AIM

We hypothesized that extracellular nucleotides, established as being released from renal tubular epithelial cells, act at pericytes to regulate vasa recta capillary diameter.

METHODS

A rat live kidney slice model and video imaging techniques were used to investigate the effects of extracellular nucleotides on in situ (subsurface) vasa recta diameter at pericyte and non-pericyte sites. In addition, RT-qPCR was used to quantify P2 receptor mRNA expression in isolated vasa recta.

RESULTS

Extracellular ATP, UTP, benzylbenzyl ATP (BzATP) or 2-methylthioATP (2meSATP) evoked a significantly greater vasoconstriction of subsurface vasa recta at pericytes than at non-pericyte sites. The rank order of agonist potency was BzATP = 2meSATP > ATP = UTP. The vasoconstriction evoked at pericyte sites by ATP was significantly attenuated by the P2 receptor antagonists suramin, pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS) or Reactive Blue-2 (RB-2). UTP-evoked vasoconstriction at pericytes was attenuated by suramin or RB-2 but not PPADS. Interestingly, suramin or PPADS, when applied in the absence of a P2 receptor agonist, evoked a weak but significant vasoconstriction of vasa recta at pericyte sites, suggesting tonic vasodilation by nucleotides. Significant levels of P2X(1, 3 and 7) and P2Y(4 and 6) receptor mRNA were detected in vasa recta.

CONCLUSION

Extracellular nucleotides act at pericytes to cause vasoconstriction of in situ vasa recta. Pharmacological characterization, supported by RT-qPCR data, suggests that P2X(1 and 7) and P2Y(4) receptors mediate nucleotide-evoked vasoconstriction of vasa recta by pericytes. We propose that nucleotides released from renal tubular epithelial cells, in close proximity to vasa recta capillaries, are key in regulating renal medullary blood flow.

Pleiotropic effects of thyroid hormones: learning from hypothyroidism

Hypothyroidism induces several metabolic changes that allow understanding some physiopathological mechanisms. Under experimental hypothyroid conditions in rats, heart and kidney are protected against oxidative damage induced by ischemia reperfusion. An increased resistance to opening of the permeability transition pore seems to be at the basis of such protection. Moreover, glomerular filtration rate of hypothyroid kidney is low as a result of adenosine receptors-induced renal vasoconstriction. The vascular tone of aorta is also regulated by adenosine in hypothyroid conditions. In other context, thyroid hormones regulate lipoprotein metabolism. High plasma level of LDL cholesterol is a common feature in hypothyroidism, due to a low expression of the hepatic LDL receptor. In contrast, HDL-cholesterol plasma levels are variable in hypothyroidism; several proteins involved in HDL metabolism and structure are expressed at lower levels in experimental hypothyroidism. Based on the positive influence of thyroid hormones on lipoprotein metabolism, thyromimetic drugs are promising for the treatment of dyslipidemias. In summary, hypothyroid status has been useful to understand molecular mechanisms involved in ischemia reperfusion, regulation of vascular function and intravascular metabolism of lipoproteins.

Contrast-induced nephropathy

Interventional radiological procedures involving anaesthesia are generally increasing. Contrast-induced nephropathy (CIN), usually defined as an increase in serum creatinine of 44 micromol litre(-1) (0.5 mg dl(-1)) or a 25% increase from the baseline value 48 h after intravascular injection of contrast media, is a common and potentially serious complication of the use of iodinated contrast media in patients at risk of acute renal injury. It is an important cause of hospital-acquired renal failure, may be a difficult differential diagnosis and the incidence does not appear to have changed over the last few decades. In the general population, the incidence of CIN is estimated to be 1-2%. However, the risk for developing CIN may be as high as 50% in some patient subgroups, such as those with diabetes mellitus and pre-existing renal impairment. The impact of CIN on clinical outcomes has been evaluated most extensively in patients undergoing percutaneous coronary intervention where it is associated with increased mortality both in hospital and at 1 yr. As treatment is limited to supportive measures while awaiting the resolution of the renal impairment, emphasis needs to be directed at prevention.

Adenosine and Kidney Function

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 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.

Survival in Hostile Environments: Strategies of Renal Medullary Cells

Cells in the renal medulla exist in a hostile milieu characterized by wide variations in extracellular solute concentrations, low oxygen tensions, and abundant reactive oxygen species. This article reviews the strategies adopted by these cells to allow them to survive and fulfill their functions under these extreme conditions.

Role of interstitial ATP and adenosine in the regulation of renal hemodynamics and microvascular function

The role of adenosine in the regulation of renal hemodynamics and function has been studied extensively; however, another purine agent, ATP, is also gaining recognition for its paracrine role in the kidney. Adenosine and ATP bind to specific membrane-bound P1 and P2 purinoceptors, respectively, and initiate a variety of biological effects on renal microvascular tone, mesangial cell function, and renal epithelial transport. The purpose of this review is to summarize the potential roles of interstitial ATP and adenosine as regulators of renal hemodynamics and microcirculation. In vitro blood-perfused juxtamedullary nephron preparation was used to assess the roles of ATP and adenosine in the regulation of renal microvascular tone. This approach mimics the adventitial exposure of renal microvascular smooth muscle to ATP and adenosine synthesized locally and released into the interstitial fluid. ATP selectively vasoconstricts afferent but not efferent arterioles via P2X and P2Y receptors, whereas, adenosine vasoconstricts both vascular segments via activation of adenosine A(1) receptors. Furthermore, selective P2X and P2Y receptor stimulation increases intracellular calcium concentration in vascular smooth muscle cells that are freshly isolated from the preglomerular microvasculature. These data support the hypothesis that interstitial ATP plays a critical role in the control of renal microvascular function through mechanisms that are independent of adenosine receptors. We have recently developed a renal microdialysis method to determine the dynamics of ATP and adenosine levels in the renal cortical interstitium. In this review, we also summarize current knowledge pertaining to the alterations in renal interstitial ATP and adenosine in some pathophysiological conditions.

CELL SURVIVAL IN THE HOSTILE ENVIRONMENT OF THE RENAL MEDULLA

The countercurrent system in the medulla of the mammalian kidney provides the basis for the production of urine of widely varying osmolalities, but necessarily entails extreme conditions for medullary cells, i.e., high concentrations of solutes (mainly NaCl and urea) in antidiuresis, massive changes in extracellular solute concentrations during the transitions from antidiuresis to diuresis and vice versa, and low oxygen tension. The strategies used by medullary cells to survive in this hostile milieu include accumulation of organic osmolytes and heat shock proteins, the extensive use of the glycolysis for energy production, and a well-orchestrated network of signaling pathways coordinating medullary circulation and tubular work.

Experimental hypothyroidism modifies specific binding of A1 and A2A analogues to adenosine receptors in the rat kidney

1 Binding kinetic studies with the adenosine analogues [3H]CPA (0.250-50 nm) and [3H]CGS21680 (0.1-100 nm) were performed in renal tissue from control (NL) and thyroidectomised (HTX) rats. We propose that the low renal adenosine content reported in hypothyroid rats may induce changes in the density and/or affinity of adenosine receptor, distributed in the cortex (C), outer medulla (OM), and inner medulla (IM) of the kidney. 2 [3H]CPA and [3H]CGS21680 binding saturation isotherms were fitted by nonlinear regression analysis and evaluated by Furchgott's method. These results revealed high (KH) and low (KL) affinity (KD) sites for both compounds. As expected, a heterogeneous pattern was observed for Bmax and KD values. 3 Bound [3H]CPA and [3H]CGS21680 were displaced by increasing concentrations of nonlabelled DPCPX and NECA, respectively, indicating the presence of A1 and A2A adenosine receptors distributed in the renal segments studied. 4 The relative intrinsic efficacy (epsilon) for [3H]CPA and [3H]CGS21680 showed extreme values (far from 1.0), 0.5 in IM NL and 2.70 in IM HTX for [3H]CGS21680. 5 Our results indicate that A2A adenosine receptor is predominant in IM from HTX, but A1 receptors are expressed preferentially in C in NL. 6 We conclude that the changes observed in number, affinity, and epsilon for the A2A receptor in IM from HTX might be responsible from alterations in medullary function, that is, incapacity for urine concentration as observed in the hypothyroid kidney.

The extracellular cyclic AMP-adenosine pathway in renal physiology

Many cell types in the kidney express adenosine receptors, and adenosine has multiple effects on renal function. Although adenosine is produced within the kidney by several biochemical reactions, recent studies support a novel mechanism for renal adenosine production, the extracellular cAMP-adenosine pathway. This extracellular cAMP-adenosine pathway is initiated by efflux of cAMP from cells following activation of adenylyl cyclase. Extracellular cAMP is then converted to adenosine by the serial actions of ecto-phosphodiesterase and ecto-5'-nucleotidase. When extracellular cAMP is converted to adenosine near the biophase of cAMP production and efflux, this local extracellular cAMP-adenosine pathway permits tight coupling of the site of adenosine production to the site of adenosine receptors. cAMP in renal compartments may also be formed by tissues/organs remote from the kidney. For example, stimulation of hepatic adenylyl cyclase by the pancreatic hormone glucagon increases circulating cAMP, which is filtered at the glomerulus and concentrated in the tubular lumen as water is extracted from the ultrafiltrate. Conversion of hepatic-derived cAMP to adenosine in the kidney completes a pancreatohepatorenal cAMP-adenosine pathway that may serve as an endocrine link between the pancreas, liver, and kidney.

Gene expression of adenosine receptors along the nephron

BACKGROUND

In view of the multiple effects of adenosine on kidney function, this study aimed to determine the expression of adenosine receptors (AR) along the rat and mouse nephron.

METHODS

For this purpose, we semiquantified mRNA abundance for adenosine A1-, A2A-, A2B-, and A3 receptors by RNAse protection and by reverse transcription-polymerase chain reaction (RT-PCR) in the kidney zones and in the different nephron segments of mice and rats.

RESULTS

We found very similar expression patterns for rat and mice. For the kidney zones A1-AR mRNA and A2A-AR mRNA abundance displayed a marked difference, with an increase from cortex to the inner medulla. This was not seen for A2B receptors, which showed in general a rather weak expression. Along the nephron, A1-AR was strongly expressed in the thin limbs of Henle and in the collecting duct system and to a lesser extent in the medullary thick ascending limb. A2A-AR mRNA was clearly detected in glomeruli but not in other nephron segments. A2B-AR mRNA was strongly expressed in the cortical thick ascending limb of Henle and in the distal convoluted tubule. A3-AR mRNA was not found in any nephron segment.

CONCLUSION

Our data demonstrate a distinct mutual expression of the AR subtypes along the nephron. A1 receptors are expressed in medullary tubular structures, while A2B receptors are predominant in cortical tubular structures. A2A receptor expression in the kidney appears to be restricted to vascular cells.

Vasoconstrictor and vasodilator effects of adenosine in the kidney

Adenosine is an ATP breakdown product that in most vessels causes vasodilatation and that contributes to the metabolic control of organ perfusion, i.e., to the match between oxygen demand and oxygen delivery. In the renal vasculature, in contrast, adenosine can produce vasoconstriction, a response that has been suggested to be an organ-specific version of metabolic control designed to restrict organ perfusion when transport work increases. However, the vasoconstriction elicited by an intravenous infusion of adenosine is only short lasting, being replaced within 1-2 min by vasodilatation. It appears that the steady-state response to the increase of plasma adenosine levels above normal resulting from the infusion is global renal vasorelaxation that is the result of A2AR activation in most parts of the renal vasculature, including larger renal arteries, juxtamedullary afferent arterioles, efferent arterioles, and medullary vessels. A2AR-mediated vasorelaxation is probably facilitated by endothelial receptors that cause the release of nitric oxide and other endothelial relaxing factors. In contrast, isolated perfused afferent arterioles of superficial and midcortical nephrons of rabbit and mouse, especially in their most distal segment at the entrance to the glomerulus, respond to adenosine with persistent vasoconstriction, indicating predominant or exclusive expression of A1AR. A1AR in afferent arterioles are selectively activated from the interstitial aspect of the vessel. This property can dissociate A1AR activation from changes in vascular adenosine concentration, a characteristic that is ideally suited for the role of renal adenosine as a paracrine factor in the control of glomerular function.

Renal protection from ischemia mediated by A2A adenosine receptors on bone marrow-derived cells

Activation of A2A adenosine receptors (A2ARs) protects kidneys from ischemia-reperfusion injury (IRI). A2ARs are expressed on bone marrow-derived (BM-derived) cells and renal smooth muscle, epithelial, and endothelial cells. To measure the contribution of A2ARs on BM-derived cells in suppressing renal IRI, we examined the effects of a selective agonist of A2ARs, ATL146e, in chimeric mice in which BM was ablated by lethal radiation and reconstituted with donor BM cells derived from GFP, A2AR-KO, or WT mice to produce GFP-->WT, A2A-KO-->WT, or WT-->WT mouse chimera. We found little or no repopulation of renal vascular endothelial cells by donor BM with or without renal IRI. ATL146e had no effect on IRI in A2A-KO mice or A2A-KO-->WT chimera, but reduced the rise in plasma creatinine from IRI by 75% in WT mice and by 60% in WT-->WT chimera. ATL146e reduced the induction of IL-6, IL-1beta, IL-1ra, and TGF-alpha mRNA in WT-->WT mice but not in A2A-KO-->WT mice. Plasma creatinine was significantly greater in A2A-KO than in WT mice after IRI, suggesting some renal protection by endogenous adenosine. We conclude that protection from renal IRI by A2AR agonists or endogenous adenosine requires activation of receptors expressed on BM-derived cells.

Physiology of the renal medullary microcirculation

Perfusion of the renal medulla plays an important role in salt and water balance. Pericytes are smooth muscle-like cells that impart contractile function to descending vasa recta (DVR), the arteriolar segments that supply the medulla with blood flow. DVR contraction by ANG II is mediated by depolarization resulting from an increase in plasma membrane Cl(-) conductance that secondarily gates voltage-activated Ca(2+) entry. In this respect, DVR may differ from other parts of the efferent microcirculation of the kidney. Elevation of extracellular K(+) constricts DVR to a lesser degree than ANG II or endothelin-1, implying that other events, in addition to membrane depolarization, are needed to maximize vasoconstriction. DVR endothelial cytoplasmic Ca(2+) is increased by bradykinin, a response that is inhibited by ANG II. ANG II inhibition of endothelial Ca(2+) signaling might serve to regulate the site of origin of vasodilatory paracrine agents generated in the vicinity of outer medullary vascular bundles. In the hydropenic kidney, DVR plasma equilibrates with the interstitium both by diffusion and through water efflux across aquaporin-1. That process is predicted to optimize urinary concentration by lowering blood flow to the inner medulla. To optimize urea trapping, DVR endothelia express the UT-B facilitated urea transporter. These and other features show that vasa recta have physiological mechanisms specific to their role in the renal medulla.

Importance of the renal medullary circulation in the control of sodium excretion and blood pressure

The control of renal medullary perfusion and the impact of alterations in medullary blood flow on renal function have been topics of research interest for almost four decades. Many studies have examined the vascular architecture of the renal medulla, the factors that regulate renal medullary blood flow, and the influence of medullary perfusion on sodium and water excretion and arterial pressure. Despite these studies, there are still a number of important unanswered questions in regard to the control of medullary perfusion and the influence of medullary blood flow on renal excretory function and blood pressure. This review will first address the vascular architecture of the renal medulla and the potential mechanisms whereby medullary perfusion may be regulated. The known extrarenal and local systems that influence the medullary vasculature will then be summarized. Finally, this review will present an overview of the evidence supporting the concept that selective changes in medullary perfusion can have a potent influence on sodium and water excretion with a long-term influence on arterial blood pressure regulation.

A(2A) adenosine receptor: a novel therapeutic target in renal disease

Present strategies in the treatment of inflammatory renal injury have focused on developing agents that specifically target individual mechanisms thought to contribute toward the pathogenesis of the disease. Such an approach is hindered by redundancies in the inflammatory cascade, rendering intervention suboptimal. The A(2A) adenosine receptor (A(2A)-AR) is a member of the family of guanine nucleotide binding proteins and has become a focus of major interest primarily because of its ability to broadly inactivate the inflammatory cascade. This review summarizes our present knowledge regarding the molecular biology and pharmacology of A(2A)-ARs as well as the physiological effects of activation of A(2A)-ARs in the kidney. We also review our recent experience in targeting this receptor subtype in abrogating the inflammatory cascade in ischemia-reperfusion injury.

Time-dependent autoregulation of renal blood flow in conscious rats

Response of renal vasculature to changes in renal perfusion pressure (RPP) involves mechanisms with different frequency characteristics. Autoregulation of renal blood flow is mediated by a rapid myogenic response and a slower tubuloglomerular feedback mechanism. In 25 male conscious rats, ramp-shaped changes in RPP were induced to quantify dynamic properties of autoregulation. Decremental RPP ramps immediately followed by incremental ramps were made for four different rates of change, ranging from 0.118 to 1.056 mmHg/s. Renal blood flow and cortical and medullary fluxes were assessed, and the corresponding relative conductance values were calculated continuously. During RPP decrements, conductance increased. With increasing rate of change of RPP decrements, maximum conductance increased from 10% to 80%, as compared with control. This response, which indicates the magnitude of autoregulation, was more pronounced in cortical versus medullary vasculature. Pressure at maximum conductance decreased with increasing rate of change of RPP decrements from 88 to 72 mmHg. During RPP increments, dependence of maximum conductance changes on the rate of change was enhanced (-20 to 110% of control). Thus, a hysteresis-like asymmetry between RPP decrements and increments, a resetting of autoregulation, was observed, which in direction and magnitude depended on the rate of change and duration of RPP changes. In conclusion, renal vascular responses to changes in RPP are highly dependent on the dynamics of the error signal. Furthermore, the method presented allows differentiated stimulation of various static and dynamic components of pressure-flow relationship and, thus, a direct assessment of the magnitudes and operating pressure range of active mechanisms of pressure-flow relationships.

Role of the extracellular cAMP-adenosine pathway in renal physiology

Adenosine exerts physiologically significant receptor-mediated effects on renal function. For example, adenosine participates in the regulation of preglomerular and postglomerular vascular resistances, glomerular filtration rate, renin release, epithelial transport, intrarenal inflammation, and growth of mesangial and vascular smooth muscle cells. It is important, therefore, to understand the mechanisms that generate extracellular adenosine within the kidney. In addition to three "classic" pathways of adenosine biosynthesis, contemporary studies are revealing a novel mechanism for renal adenosine production termed the "extracellular cAMP-adenosine pathway." The extracellular cAMP-adenosine pathway is defined as the egress of cAMP from cells during activation of adenylyl cyclase, followed by the extracellular conversion of cAMP to adenosine by the serial actions of ecto-phosphodiesterase and ecto-5'-nucleotidase. This mechanism of extracellular adenosine production may provide hormonal control of adenosine levels in the cell-surface biophase in which adenosine receptors reside. Tight coupling of the site of adenosine production to the site of adenosine receptors would permit a low-capacity mechanism of adenosine biosynthesis to have a large impact on adenosine receptor activation. The purposes of this review are to summarize the physiological roles of adenosine in the kidney; to describe the classic pathways of renal adenosine biosynthesis; to review the evidence for the existence of the extracellular cAMP-adenosine pathway; and to describe possible physiological roles of the extracellular cAMP-adenosine pathway, with particular emphasis on the kidney.

Enhanced protection from renal ischemia-reperfusion [correction of ischemia:reperfusion] injury with A(2A)-adenosine receptor activation and PDE 4 inhibition

BACKGROUND

We previously demonstrated in rats and mice that agonists of A(2A)-adenosine receptors (A(2A)-ARs) reduce renal injury following ischemia-reperfusion. We now extend these studies and examine the effects of ATL-146e (formerly DWH-146e), an A(2A)-AR agonist, and rolipram, a type IV phosphodiesterase (PDE 4) inhibitor, on murine renal injury following ischemia-reperfusion.

METHODS

C57BL/6 mice were treated with rolipram, ATL-146e, or both compounds combined and were subjected to renal ischemia for 32 minutes and reperfusion for 24 to 48 hours. In vitro studies were performed on suspended and adhering human neutrophils.

RESULTS

Continuous delivery of rolipram or ATL-146e during reperfusion reduced renal injury in a dose-dependent manner. Maximal protection was observed when ATL-146e was infused for six hours during reperfusion. Elevated plasma creatinine and myeloperoxidase activity produced by ischemia-reperfusion were reduced by rolipram (0.1 ng/kg/min) and ATL-146e (10 ng/kg/min) by up to approximately 60% and 70%, respectively. Co-infusion of both compounds produced a maximum reduction of plasma creatinine of approximately 90% and myeloperoxidase activity. In vitro studies on suspended and adhering human neutrophils demonstrated that selective stimulation of A(2A)-ARs by ATL-146e increased cAMP accumulation, reduced oxidative activity of activated neutrophils, and decreased activated neutrophil adherence. These responses were potentiated by rolipram.

CONCLUSIONS

We conclude that the combined infusion of ATL-146e and rolipram leads to enhanced renal tissue protection from ischemia-reperfusion by mechanisms that may include reduced neutrophil adherence/recruitment and release of reactive oxygen species.

Adenosine signaling in outer medullary descending vasa recta

We tested whether dilation of outer medullary descending vasa recta (OMDVR) is mediated by cAMP, nitric oxide (NO), and cyclooxygenase (COX). Adenosine (A; 10(-6) M)-induced vasodilation of ANG II (10(-9) M)-preconstricted OMDVR was mimicked by the cAMP analog 8-bromoadenosine 3',5'-cyclic monophosphate (10(-10) to 10(-4) M) and reversed by the adenylate cyclase inhibitor SQ-22536. Adenosine (10(-4) M) stimulated OMDVR cAMP production greater than threefold. NO synthase blockade with N(G)-nitro-L-arginine methyl ester and N(G)-monomethyl-L-arginine (10(-4) M) did not affect adenosine vasodilation. Adenosine induced endothelial cytoplasmic calcium transients that were small. Indomethacin (10(-6) M) reversed adenonsine-induced dilation of OMDVR preconstricted with ANG II, endothelin, 4-bromo-calcium ionophore A23187, or carbocyclic thromboxane A(2). In contrast, selective A(2)-receptor activation dilated endothelin-preconstricted OMDVR even in the presence of indomethacin. We conclude that OMDVR vasodilation by adenosine involves cAMP and COX but not NO. COX blockade does not fully inhibit selective A(2) receptor-mediated OMDVR dilation.

Immunolocalisation of adenosine A(1) receptors in the rat kidney

The location of adenosine A(1) receptors in the rat kidney was investigated using immunolabelling with antibodies raised to a 15-amino-acid sequence near the C-terminus of the receptor (antibody I) and to a 14-amino-acid sequence in the second extracellular loop (antibody II). In the cortex, antibody I bound to adenosine A(1) receptors in mesangial cells and afferent arterioles, whilst antibody II bound to receptors in proximal convoluted tubules. In the medulla, both antibodies bound to receptors in collecting ducts and the papillary surface epithelium. These observations provide support for the diverse functional roles previously proposed for the adenosine A(1) receptor in the kidney. The labelling of distinct but different structures in the cortex by antibodies raised to different amino acid sequences on the A(1) receptor protein suggests that differing forms of the receptor are present in this region of the kidney.

Role of adenosine in contrast media-induced acute renal failure in diabetes mellitus

Increased release of renal adenosine and stimulation of renal adenosine receptors have been proposed to be major mechanisms in the development of contrast media-induced acute renal failure (CM-ARF). Patients with diabetes mellitus or preexisting renal disease who have reduced renal function have a markedly increased risk to develop CM-ARF. This increased risk to develop CM-ARF in patients with diabetes mellitus is linked to a higher sensitivity of the renal vasculature to adenosine, since experimental studies have shown increased adenosine-induced vasoconstriction in the kidneys of diabetic animals. Furthermore, recent evidence suggests that administration of adenosine receptor antagonists reduces the risk of development of CM-ARF in both diabetic and nondiabetic patients. The purpose of this review is to discuss the role of adenosine in the development of CM-ARF, particularly in the kidneys of diabetic patients, and to evaluate the therapeutic potential of adenosine receptor antagonists in the prevention of CM-ARF. Selective adenosine A1 receptor antagonists may provide a therapeutic tool to prevent CM-ARF in patients with diabetes mellitus and reduced renal function.

Optimal strategies for preventing progression of renal disease: should angiotensin converting enzyme inhibitors and angiotensin receptor blockers be used together?

Interruption of the renin-angiotensin system (RAS) with angiotensin converting enzyme (ACE) inhibitors or angiotensin AT(1) receptor blockers has been shown to delay progression in a variety of renal diseases, suggesting that the RAS, and its major effector molecule, angiotensin II, are important players in renal pathophysiology. Both antagonists combine inhibition of deleterious effects of angiotensin II with activation of potentially beneficial pathways mediated by nitric oxide and prostaglandins. Some concerns have been raised about the completeness of the RAS blockade achieved by these agents. ACE-independent pathways can generate angiotensin II, whereas increases in angiotensin II levels may compete with the AT(1) receptor blocker at the receptor site. It has been suggested that an ACE inhibitor/AT(1) receptor blocker combination offers a better therapeutic effect than treatment with either agent alone. In this review, we focus on mechanisms of actions of ACE inhibitors and AT(1) receptor blockers, implicate them in the rationale for the use of an ACE inhibitor/AT(1) receptor blocker combination, and discuss evidence evaluating the renal effects of the combination as compared to the effects of a single agent. There is a surprising lack of information about the nephroprotective potential of the combination, allowing no consistent conclusions about the superiority of the combination over the single agent. Several experimental and clinical reports suggest that in some conditions, the combination may be beneficial. Rather than providing unequivocal evidence for the use of combination treatment in the renal disease, these studies should be considered as stimuli for more detailed exploration of this issue.

Inner medullary lactate production and accumulation: a vasa recta model

Since anaerobic glycolysis yields two lactates for each glucose consumed and since it is reported to be a major source of ATP for inner medullary (IM) cell maintenance, it is a likely source of "external" IM osmoles. It has long been known that such an osmole source could theoretically contribute to the "single-effect" of the urine concentrating mechanism, but there was previously no suggestion of a plausible source. I used numerical simulation to estimate axial gradients of lactate and glucose that might be accumulated by countercurrent recycling in IM vasa recta (IMVR). Based on measurements in other tissues, anaerobic glycolysis (assumed to be independent of diuretic state) was estimated to consume approximately 20% of the glucose delivered to the IM. IM tissue mass and axial distribution of loops and vasa recta were according to reported values for rat and other rodents. Lactate (P(LAC)) and glucose (P(GLU)) permeabilities were varied over a range of plausible values. The model results suggest that P(LAC) of 100 x 10(-5) cm/s (similar to measured permeabilities for other small solutes) is sufficiently high to ensure efficient lactate recycling. By contrast, it was necessary in the model to reduce P(GLU) to a small fraction of this value (1/25th) to avoid papillary glucose depletion by countercurrent shunting. The results predict that IM lactate production could suffice to build a significant steady-state axial lactate gradient in the IM interstitium. Other modeling studies (Jen JF and Stephenson JL. Bull Math Biol 56: 491-514, 1994; and Thomas SR and Wexler AS. Am J Physiol Renal Fluid Electrolyte Physiol 269: F159-F171, 1995) have shown that 20-100 mosmol/kgH(2)O of unspecified external, interstitial, osmolytes could greatly improve IM concentrating ability. The present study gives several plausible scenarios consistent with accumulation of metabolically produced lactate osmoles, although only to the lower end of this range. For example, if 20% of entering glucose is consumed, the model predicts that papillary lactate would attain about 15 mM assuming vasa recta outflow is increased 30% by fluid absorbed from the nephrons and collecting ducts and that this lactate gradient would double if IM blood flow were reduced by one-half, as may occur in antidiuresis. Several experimental tests of the hypothesis are indicated.

Characterization of adenosine action in isolated rat renal artery. Possible role of adenosine A(2A) receptors

Adenosine (0.1-300 microM) induced concentration- and endothelium-dependent relaxation of rat renal artery (RRA). N(G)-Nitro-L-arginine (L-NOARG, 10 microM) significantly reduced adenosine-elicited dilatation, but not the application of indomethacin (10 microM), ouabain (100 microM) or tetraethylammonium (TEA, 500 microM). In the presence of high concentration of K(+) (100 mM) or glibenclamide (1 microM), adenosine-evoked relaxation was almost abolished. 8-(3-Chlorostyril)caffeine (CSC, 0.3-3 microM), a selective A(2A)-antagonist, significantly reduced adenosine-evoked dilatation in a concentration-dependent manner (pA(2)=7.29). Conversely, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 nM), an A(1)-antagonist, did not alter adenosine-induced relaxation. These results indicate that adenosine produces endothelium-dependent relaxation of isolated RRA. Dilatation evoked by adenosine is mediated by predominant releasing of endothelium-derived hiperpolarizing factor (EDHF) and also in one part of nitric oxide (NO) from endothelial cells. The obtained results also suggest that RRA response to adenosine is most likely initiated by activation of endothelial adenosine A(2A) receptors.

Adenosine and nitric oxide regulate regional vascular resistance via interdependent and independent mechanisms during sepsis

OBJECTIVE

Adenosine receptor blockade increases regional resting vascular resistance during sepsis. In healthy subjects, part of adenosine's actions are mediated via stimulation of nitric oxide synthase. Because nitric oxide synthase activity is thought to be a major contributor to altered vascular tone in sepsis, we tested the hypothesis that some of the nitric oxide-mediated resting regional resistance during sepsis is secondary to endogenous adenosine stimulation of nitric oxide synthase.

DESIGN

Prospective, randomized, controlled experiment.

SETTING

Shock-trauma and basic science laboratory.

SUBJECTS

Male Sprague-Dawley rats.

INTERVENTIONS

Twenty-four hours after sepsis or sham induction, rats were separated into two groups (n = 6 to 10 in each group). Group 1 received a 10-min infusion of the adenosine antagonist 8-sulfophenyltheophylline (0.9 mg/kg x min) followed by a 10-min infusion of L-nitro-arginine-methyl ester (0.5 mg/kg x min). Group 2 similarly received L-nitro-arginine-methyl ester followed by 8-sulfophenyltheophylline in the presence of L-nitro-arginine-methyl ester.

MEASUREMENTS AND MAIN RESULTS

Hemodynamic and blood flow measurements (microspheres) were made before infusions, 10 mins after the administration of each single-agent infusion, and 10 mins after combined-agent infusions were administered. No significant resistance alterations were observed in nonseptic rats. In septic rats, adenosine receptor blockade alone increased hepatosplanchnic and skeletal muscle vascular resistance, but no further increases were seen when L-nitro-arginine-methyl ester was added. Nitric oxide synthase inhibition alone increased hepatosplanchnic and skeletal muscle vascular resistances. When 8-sulfophenyltheophylline was added to the infusion, skeletal muscle vascular resistance increased significantly more than with L-nitro-arginine-methyl ester alone, but there were no further increases in hepatosplanchnic resistance. Renal and adipose vascular resistances increased with L-nitro-arginine-methyl ester infusions, and 8-sulfophenyltheophylline produced no effect.

CONCLUSIONS

During sepsis, nitric oxide caused resting vasodilation independent of adenosine in the renal and adipose vasculature. In the hepatosplanchnic circulation, there is reciprocal adenosine-nitric oxide interaction in maintaining resting regional resistance. Skeletal muscle displayed a dual adenosine-mediated (nitric oxide-independent) and nitric oxide-mediated (adenosine receptors required) interaction to regulate resting resistance during sepsis. These data indicate that in the hepatosplanchnic and skeletal muscle vasculature, all of the resting nitric oxide-mediated vasodilation is secondary to endogenous adenosine action, but in adipose and renal vasculature, resting nitric oxide mediated vasodilation is independent of adenosine. Endogenous adenosine also appears to play a significant role in determining resting skeletal muscle resistance that is independent of nitric oxide synthase during sepsis.

In vivo expression profile of an endothelial nitric oxide synthase promoter-reporter transgene

Endothelium-derived nitric oxide (NO) is primarily attributable to constitutive expression of the endothelial nitric oxide synthase (eNOS) gene. Although a more comprehensive understanding of transcriptional regulation of eNOS is emerging with respect to in vitro regulatory pathways, their relevance in vivo warrants assessment. In this regard, promoter-reporter insertional transgenic murine lines were created containing 5,200 bp of the native murine eNOS promoter directing transcription of nuclear-localized beta-galactosidase. Examination of beta-galactosidase expression in heart, lung, kidney, liver, spleen, and brain of adult mice demonstrated robust signal in large and medium-sized blood vessels. Small arterioles, capillaries, and venules of the microvasculature were notably negative, with the exception of the vasa recta of the medullary circulation of the kidney, which was strongly positive. Only in the brain was the reporter expressed in non-endothelial cell types, such as the CA1 region of the hippocampus. Epithelial cells of the bronchi, bronchioles, and alveoli were scored as negative, as was renal tubular epithelium. Cardiac myocytes, skeletal muscle, and smooth muscle of both vascular and nonvascular sources failed to demonstrate beta-galactosidase staining. Expression was uniform across multiple founders and was not significantly affected by genomic integration site. These transgenic eNOS promoter-reporter lines will be a valuable resource for ongoing studies addressing the regulated expression of eNOS in vivo in both health and disease.

Selective A2A adenosine receptor activation reduces ischemia-reperfusion injury in rat kidney

A2A adenosine receptors (A2A-ARs) are known modulators of renal hemodynamics and potent inhibitors of inflammation. We sought to determine whether selective activation of A2A-ARs protects kidneys from ischemia-reperfusion injury. The ester derivative of DWH-146 (DWH-146e), a selective A2A agonist, was found to be more potent and selective for A2A-ARs than the prototype compound CGS-21680. Osmotic minipumps were implanted subcutaneously to infuse into rats either vehicle or DWH-146e (0.004 microg. kg(-1). min(-1)), during and after ischemia-reperfusion injury. Following 24 and 48 h of reperfusion, the rise in serum creatinine and blood urea nitrogen for vehicle-treated rats was substantially elevated compared with DWH-146e-treated rats. Histological examination revealed widespread tubular epithelial necrosis and vascular congestion in the outer medulla of vehicle-treated compared with DWH-146e-treated animals. ZM-241385, a selective A(2A) antagonist, blocked the protective effect of DWH-146e. Delaying administration of DWH-146e until the initiation of reperfusion also decreased serum creatinine. We conclude that 1) selective A2A-AR activation by DWH-146e reduces ischemia-reperfusion injury in rat kidneys, 2) the effect of DWH-146e is A2A receptor mediated, and 3) the protective effects are mediated by preventing injury during the reperfusion period.

Role of renal medullary adenosine in the control of blood flow and sodium excretion

This study determined the levels of adenosine in the renal medullary interstitium using microdialysis and fluorescence HPLC techniques and examined the role of endogenous adenosine in the control of medullary blood flow and sodium excretion by infusing the specific adenosine receptor antagonists or agonists into the renal medulla of anesthetized Sprague-Dawley rats. Renal cortical and medullary blood flows were measured using laser-Doppler flowmetry. Analysis of microdialyzed samples showed that the adenosine concentration in the renal medullary interstitial dialysate averaged 212 +/- 5.2 nM, which was significantly higher than 55.6 +/- 5.3 nM in the renal cortex (n = 9). Renal medullary interstitial infusion of a selective A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 300 pmol. kg-1. min-1, n = 8), did not alter renal blood flows, but increased urine flow by 37% and sodium excretion by 42%. In contrast, renal medullary infusion of the selective A2 receptor blocker 3, 7-dimethyl-1-propargylxanthine (DMPX; 150 pmol. kg-1. min-1, n = 9) decreased outer medullary blood flow (OMBF) by 28%, inner medullary blood flows (IMBF) by 21%, and sodium excretion by 35%. Renal medullary interstitial infusion of adenosine produced a dose-dependent increase in OMBF, IMBF, urine flow, and sodium excretion at doses from 3 to 300 pmol. kg-1. min-1 (n = 7). These effects of adenosine were markedly attenuated by the pretreatment of DMPX, but unaltered by DPCPX. Infusion of a selective A3 receptor agonist, N6-benzyl-5'-(N-ethylcarbonxamido)adenosine (300 pmol. kg-1. min-1, n = 6) into the renal medulla had no effect on medullary blood flows or renal function. Glomerular filtration rate and arterial pressure were not changed by medullary infusion of any drugs. Our results indicate that endogenous medullary adenosine at physiological concentrations serves to dilate medullary vessels via A2 receptors, resulting in a natriuretic response that overrides the tubular A1 receptor-mediated antinatriuretic effects.

Evidence for A1 and A2B adenosine receptors in baby hamster kidney (BHK) cells

Adenosine is known to produce biphasic effects in the renal tissues via adenosine receptors. However, the presence of more than one subtype of adenosine receptor on a type of kidney cell or tissue has not been conclusively demonstrated. To address this issue, we investigated the presence of A1 and A2 adenosine receptors in baby hamster kidney (BHK) cells by use of radioligand binding and the reverse transcription-polymerase chain reaction. Ligand binding studies with (3H)-DPCPX revealed a single class of binding site with a K(D) of 9.2 +/- 2.0 nM, a Bmax of 1.7 +/- 0.2 pmol/mg protein and a pharmacological profile characteristic of A1 adenosine receptor on the BHK cell membrane. As the presence of A2 adenosine receptors could not be conclusively determined by ligand binding studies, the more sensitive method of RT-PCR was employed. The presence of A1 and A2B adenosine receptors was detected by RT-PCR with specific primers and the subsequent sequencing of the resultant amplification product. The sequences obtained were 75-90% homologous to the respective adenosine receptor mRNA of rat, mouse and human.

Intrarenal blood flow: microvascular anatomy and the regulation of medullary perfusion

1. The microcirculation of the kidney is arranged in a manner that facilitates separation of blood flow to the cortex, outer medulla and inner medulla. 2. Resistance vessels in the renal vascular circuit include arcuate and interlobular arteries, glomerular afferent and efferent arterioles and descending vasa recta. 3. Vasoactive hormones that regulate smooth muscle cells of the renal circulation can originate outside the kidney (e.g. vasopressin), can be generated from nearby regions within the kidney (e.g. kinins, endothelins, adenosine) or they can be synthesized by adjacent endothelial cells (e.g. nitric oxide, prostacyclin, endothelins). 4. Vasoactive hormones released into the renal inner medullary microcirculation may be trapped by countercurrent exchange to act upon descending vasa recta within outer medullary vascular bundles. 5. Countercurrent blood flow within the renal medulla creates a hypoxic environment. Relative control of inner versus outer medullary blood flow may play a role to abrogate the hypoxia that arises from O2 consumption by the thick ascending limb of Henle. 6. Cortical blood flow is autoregulated. In contrast, the extent of autoregulation of medullary blood flow appears to be influenced by the volume status of the animal. Lack of medullary autoregulation during volume expansion may be part of fundamental processes that regulate salt and water excretion.

Evidence for the presence of smooth muscle alpha-actin within pericytes of the renal medulla

This study was designed to determine whether smooth muscle alpha-actin mRNA and smooth muscle alpha-actin contractile protein elements were present within the renal medullary pericytes. Extraction of total RNA from microdissected outer medullary descending vasa recta allowed for the detection of smooth muscle alpha-actin mRNA expression using reverse transcription-polymerase chain reaction (RT-PCR). Expression of smooth muscle alpha-actin was specific to the descending vasa recta and not a result of tubular contamination because RT-PCR amplification of the vasopressin V2 receptor, which is a specific tubular marker, did not occur. To determine the exact cell type(s) that translate the mRNA into protein, we performed immunohistochemistry on the renal outer and inner medulla using a monoclonal smooth muscle alpha-actin antibody, whose specificity was determined by immunoblot analysis. Smooth muscle alpha-actin protein was found selectively within the pericytes surrounding the descending vasa recta from the outer and inner medullary tissue sections. This study demonstrates that the pericytes alone that surround the descending vasa recta within the outer and inner medulla contain smooth muscle alpha-actin mRNA and protein and are therefore the site of the contractile elements that could play a vasomodulatory role in the control of renal medullary blood flow and its distribution within the renal medulla.