Activation of A(2) adenosine receptors dilates cortical efferent arterioles in mouse

Adenosine reuptake inhibition reduces diabetes-induced glomerular hyperfiltration via the adenosine A2a receptor

Diabetes-induced glomerular hyperfiltration is an early alteration in kidney function in diabetes. Previous studies have shown that reduced adenosine A2a receptor signaling contributes to diabetes-induced glomerular hyperfiltration. The present study investigated the effects of enhanced interstitial adenosine concentration by inhibition of cellular adenosine reuptake, thereby promoting endogenous adenosine signaling. Insulinopenic diabetes was induced by streptozotocin in adult male Sprague-Dawley rats. Two weeks after diabetes induction, kidney function in terms of glomerular filtration rate, and total, cortical, and medullary renal blood flows were evaluated under thiobutabarbital anesthesia during baseline and after renal artery infusion of two doses of the adenosine reuptake inhibitor dilazep. Dilazep did not affect mean arterial pressure indicating that the effects of the interventions were intrarenal. Diabetics had increased glomerular filtration rate compared with controls and dilazep dose-dependently decreased glomerular filtration rate in diabetics, whereas it had no significant effect in controls. Dilazep increased cortical renal blood flows in controls, whereas medullary blood flow was not significantly changed. Dilazep did not affect total renal blood flow in any of the groups but decreased cortical blood flow in diabetics, resulting in decreased filtration fraction by dilazep in diabetics. Pretreatment with the adenosine A2a antagonist ZM241385 prevented intrarenal dilazep-mediated effects on glomerular filtration rate and filtration fraction in diabetics. In conclusion, enhancing intrarenal adenosine signaling by dilazep normalizes diabetes-induced glomerular hyperfiltration at least in part by activation of adenosine A2a receptors.

Adenosine receptors as emerging therapeutic targets for diabetic kidney disease

Diabetic kidney disease (DKD) is now a pandemic worldwide, and novel therapeutic options are urgently required. Adenosine, an adenosine triphosphate metabolite, plays a role in kidney homeostasis through interacting with four types of adenosine receptors (ARs): A1AR, A2AAR, A2BAR, and A3AR. Increasing evidence highlights the role of adenosine and ARs in the development and progression of DKD: 1) increased adenosine in the plasma and urine of diabetics with kidney injury, 2) increased expression of each of the ARs in diabetic kidneys, 3) the protective effect of coffee, a commonly ingested nonselective AR antagonist, on DKD, and 4) the protective effect of AR modulators in experimental DKD models. We propose AR modulators as a new therapeutic option to treat DKD. Detailed mechanistic studies on the pharmacology of AR modulators will help us to develop effective first-in-class AR modulators against DKD.

Time to Recover From Daily Caffeine Intake

Caffeine elicits widespread effects in the central nervous system and is the most frequently consumed psychostimulant worldwide. First evidence indicates that, during daily intake, the elimination of caffeine may slow down, and the primary metabolite, paraxanthine, may accumulate. The neural impact of such adaptions is virtually unexplored. In this report, we leveraged the data of a laboratory study with N = 20 participants and three within-subject conditions: caffeine (150 mg caffeine × 3/day × 10 days), placebo (150 mg mannitol × 3/day × 10 days), and acute caffeine deprivation (caffeine × 9 days, afterward placebo × 1 day). On day 10, we determined the course of salivary caffeine and paraxanthine using liquid chromatography-mass spectrometry coupled with tandem mass spectrometry. We assessed gray matter (GM) intensity and cerebral blood flow (CBF) after acute caffeine deprivation as compared to changes in the caffeine condition from our previous report. The results indicated that levels of paraxanthine and caffeine remained high and were carried overnight during daily intake, and that the levels of paraxanthine remained elevated after 24 h of caffeine deprivation compared to placebo. After 36 h of caffeine deprivation, the previously reported caffeine-induced GM reduction was partially mitigated, while CBF was elevated compared to placebo. Our findings unveil that conventional daily caffeine intake does not provide sufficient time to clear up psychoactive compounds and restore cerebral responses, even after 36 h of abstinence. They also suggest investigating the consequences of a paraxanthine accumulation during daily caffeine intake.

Effects of SGLT2 inhibitors on eGFR in type 2 diabetic patients-the role of antidiabetic and antihypertensive medications

Recent randomized trials demonstrating the beneficial effects of sodium-glucose cotransporter 2 inhibitors (SGLT2is) in type 2 diabetes suggest that early reductions in eGFR upon initiation of SGLT2i therapy are associated with improved renal outcomes. Multiple concomitant medications, including antidiabetic and antihypertensive agents, are commonly used, however, which may modify the renal hemodynamic action of SGLT2is. Here we found that background treatment with metformin diminished the SGLT2i-induced reductions in eGFR after 3 months of SGLT2i therapy in patients with type 2 diabetes and hypertension (-2.29 ± 0.90 vs -5.85 ± 1.27 mL/min/1.73 m² for metformin users (n = 126) and nonusers (n = 97), respectively). Other antidiabetic agents (DPP4 inhibitors, sulfonylureas and insulin) had no effect on the eGFR response to SGLT2is. Antihypertensive drugs, including calcium channel blockers (CCBs) and β blockers, did not affect the SGLT2i-induced changes in eGFR, whereas renin-angiotensin system inhibitors (RASis) tended to enhance this response (p = 0.059). Next, we evaluated the interaction between metformin and RASis in the eGFR responses to SGLT2is. Under no background treatment with RASis, metformin abrogated the eGFR response to SGLT2is, but this response was preserved when RASis had been given along with metformin (decreases of 0.75 ± 1.28 vs. 4.60 ± 1.15 mL/min/1.73 m² in eGFR, p = 0.028). No interaction between metformin and insulin or between metformin and DPP4 inhibitors was observed. In conclusion, metformin blunts the SGLT2i-induced decrease in eGFR, but coadministration of RASis ameliorates this response. Furthermore, the inability of CCBs to modify the SGLT2i-induced reduction in eGFR suggests that the SGLT2i-induced renal microvascular action is mediated predominantly by postglomerular vasodilation rather than preglomerular vasoconstriction.

Inhibition of 15-PGDH prevents ischemic renal injury by the PGE2/EP4 signaling pathway mediating vasodilation, increased renal blood flow, and increased adenosine/A2A receptors

In the present study, we demonstrated the marked activity of SW033291, an inhibitor of 15-hydoxyprostaglandin dehydrogenase (15-PGDH), in preventing acute kidney injury (AKI) in a murine model of ischemia-reperfusion injury. AKI due to ischemic injury represents a significant clinical problem. PGE₂ is vasodilatory in the kidney, but it is rapidly degraded in vivo due to catabolism by 15-PGDH. We investigated the potential of SW033291, a potent and specific 15-PGDH inhibitor, as prophylactic treatment for ischemic AKI. Prophylactic administration of SW033291 significantly increased renal tissue PGE₂ levels and increased post-AKI renal blood flow and renal arteriole area. In parallel, prophylactic SW033291 decreased post-AKI renal morphology injury scores and tubular apoptosis and markedly reduced biomarkers of renal injury that included blood urea nitrogen, creatinine, neutrophil gelatinase-associated lipocalin, and kidney injury molecule-1. Prophylactic SW033291 also reduced post-AKI induction of proinflammatory cytokines, high-mobility group box 1, and malondialdehyde. Protective effects of SW033291 were mediated by PGE₂ signaling, as they could be blocked by pharmacological inhibition of PGE₂ synthesis. Consistent with activation of PGE₂ signaling, SW033291 induced renal levels of both EP₄ receptors and cAMP, along with other vasodilatory effectors, including AMP, adenosine, and the adenosine A_2A receptor. The protective effects of SW0333291 could largely be achieved with a single prophylactic dose of the drug. Inhibition of 15-PGDH may thus represent a novel strategy for prophylaxis of ischemic AKI in multiple clinical settings, including renal transplantation and cardiovascular surgery.

Does ischemic preconditioning increase flap survival by ADORA2B receptor activation?

BACKGROUND

Ischemic preconditioning (IPC) is defined as raising tolerance to subsequent ischemic stress by exposing tissues to sub-lethal ischemia. Although many candidates have been suggested, recent studies have clearly demonstrated that adenosine-mediated ADORA2B receptor (ADORA2BR) activation is the main mechanism involved in IPC. While the tissue-protective role of this mechanism has been demonstrated in different ischemia/reperfusion (I/R) models, its role in flap surgery-derived I/R damage has not to date been investigated.

OBJECTIVE

To investigate the role of adenosine and ADORA2BR activation in IPC-mediated tissue protection in an epigastric flap model.

METHODS

Animals were divided into five main groups, all of which were then divided into two subgroups depending on whether or not they were exposed to IPC before the I/R procedure, which consisted of 6 hours of ischemia and 6 days of reperfusion. No drugs were administered in Group 1 (the control group). Animals in Group 2 were pretreated with CD73-inhibitor before IPC application or the ischemic period. Animals in Group 3 were pretreated with adenosine. Animals in Group 4 were pretreated with an ADORA2BR antagonist, and those in Group 5 with an ADORA2BR agonist. After 6 days of reperfusion, tissue survival was evaluated via histological and macroscopic analysis.

RESULTS

IPC application significantly enhanced CD73 expressions and adenosine concentrations (p < 0.01). Flap survivals were increased by IPC in Group 1 (p < 0.05). However, CD73 inhibition blocked this increase (Group 2). In Group 3, adenosine improved flap survival even in the absence of IPC (p < 0.01). While an ADORA2BR antagonist attenuated the tissue-protective effect of IPC (p < 0.01), the ADORA2BR agonist improved flap survival by mimicking IPC in groups 4 and 5.

CONCLUSION

These results provide pharmacological evidence for a contribution of CD73 enzyme-dependent adenosine generation and signaling through ADORA2BR to IPC-mediated tissue protection. They also suggest for the first time that ADORA2BR agonists may be used as a potential preventive therapy against I/R injury in flap surgeries.

The Role of P2X7 Purinergic Receptors in the Renal Inflammation Associated with Angiotensin II-induced Hypertension

Purinergic receptors play a central role in the renal pathophysiology of angiotensin II-induced hypertension, since elevated ATP chronically activates P2X7 receptors in this model. The changes induced by the P2X antagonist Brilliant blue G (BBG) in glomerular hemodynamics and in tubulointerstitial inflammation resulting from angiotensin II infusion were studied. Rats received angiotensin II (435 ng·kg^-1·min^-1, 2 weeks) alone or in combination with BBG (50 mg/kg/day intraperitoneally). BBG did not modify hypertension (214.5 ± 1.4 vs. 212.7 ± 0.5 mmHg), but restored to near normal values afferent (7.03 ± 1.00 to 2.97 ± 0.27 dyn.s.cm^-5) and efferent (2.62 ± 0.03 to 1.29 ± 0.09 dyn.s.cm^-5) arteriolar resistances, glomerular plasma flow (79.23 ± 3.15 to 134.30 ± 1.11 nl/min), ultrafiltration coefficient (0.020 ± 0.002 to 0.036 ± 0.003 nl/min/mmHg) and single nephron glomerular filtration rate (22.28 ± 2.04 to 34.46 ± 1.54 nl/min). Angiotensin II induced overexpression of P2X7 receptors in renal tubular cells and in infiltrating T and B lymphocytes and macrophages. All inflammatory cells were increased by angiotensin II infusion and reduced by 20% to 50% (p < 0.05) by BBG administration. Increased IL-2, IL-6, TNFα, IL-1β, IL-18 and overexpression of NLRP3 inflammasome were induced by angiotensin II and suppressed by BBG. These studies suggest that P2X7 receptor-mediated renal vasoconstriction, tubulointerstitial inflammation and activation of NLRP3 inflammasome are associated with angiotensin II-induced hypertension.

Tubule-vascular feedback in renal autoregulation

Afferent arteriole (Af-Art) diameter regulates pressure and flow into the glomerulus, which are the main determinants of the glomerular filtration rate. Thus, Af-Art resistance is crucial for Na⁺ filtration. Af-Arts play a role as integrative centers, where systemic and local systems interact to determine the final degree of resistance. The tubule of a single nephron contacts an Af-Art of the same nephron at two locations: in the transition of the thick ascending limb to the distal tubule (macula densa) and again in the connecting tubule. These two sites are the anatomic basis of two intrinsic feedback mechanisms: tubule-glomerular feedback and connecting tubule-glomerular feedback. The cross communications between the tubules and Af-Arts integrate tubular Na⁺ and water processing with the hemodynamic conditions of the kidneys. Tubule-glomerular feedback provides negative feedback that tends to avoid salt loss, and connecting tubule-glomerular feedback provides positive feedback that favors salt excretion by modulating tubule-glomerular feedback (resetting it) and increasing glomerular filtration rate. These feedback mechanisms are also exposed to systemic modulators (hormones and the nervous system); however, they can work in isolated kidneys or nephrons. The exaggerated activation or absence of any of these mechanisms may lead to disequilibrium in salt and water homeostasis, especially in extreme conditions (e.g., high-salt diet/low-salt diet) and may be part of the pathogenesis of some diseases. In this review, we focus on molecular signaling, feedback interactions, and the physiological roles of these two feedback mechanisms.

Endothelial mineralocorticoid receptor ablation does not alter blood pressure, kidney function or renal vessel contractility

Aldosterone blockade confers substantial cardiovascular and renal protection. The effects of aldosterone on mineralocorticoid receptors (MR) expressed in endothelial cells (EC) within the renal vasculature have not been delineated. We hypothesized that lack of MR in EC may be protective in renal vasculature and examined this by ablating the Nr3c2 gene in endothelial cells (EC-MR) in mice. Blood pressure, heart rate and PAH clearance were measured using indwelling catheters in conscious mice. The role of the MR in EC on contraction and relaxation was investigated in the renal artery and in perfused afferent arterioles. Urinary sodium excretion was determined by use of metabolic cages. EC-MR transgenics had markedly decreased MR expression in isolated aortic endothelial cells as compared to littermates (WT). Blood pressure and effective renal plasma flow at baseline and following AngII infusion was similar between groups. No differences in contraction and relaxation were observed between WT and EC-MR KO in isolated renal arteries during baseline or following 2 or 4 weeks of AngII infusion. The constriction or dilatations of afferent arterioles between genotypes were not different. No changes were found between the groups with respect to urinary excretion of sodium after 4 weeks of AngII infusion, or in urinary albumin excretion and kidney morphology. In conclusion, deletion of the EC-MR does not confer protection towards the development of hypertension, endothelial dysfunction of renal arteries or renal function following prolonged AngII-infusion.

Physiopathological implications of P2X1and P2X7receptors in regulation of glomerular hemodynamics in angiotensin II-induced hypertension

Deleterious effects of purinergic P2X1and P2X7receptors (P2XRs) in ANG II-dependent hypertension include increased renal vascular resistance, and impaired autoregulation and pressure natriuresis. However, their specific effects on the determinants of glomerular hemodynamics remain incompletely delineated. To investigate the P2XR contributions to altered glomerular hemodynamics in hypertension, the effects of acute blockade of P2X1R, P2X7R, and P2X4R with NF449, A438079, and PSB12054, respectively, were evaluated in ANG II-infused rats (435 ng·kg−1·min−1). P2X1R or P2X7R blockade reduced afferent (6.85 ± 1.05 vs. 2.37 ± 0.20 dyn·s−1·cm−5) and efferent (2.85 ± 0.38 vs. 0.99 ± 0.07 dyn·s−1·cm−5) arteriolar resistances, leading to increases in glomerular plasma flow (75.82 ± 5.58 vs. 206.7 ± 16.38 nl/min), ultrafiltration coefficient (0.0198 ± 0.0024 vs. 0.0512 ± 0.0046 nl·min−1·mmHg−1), and single-nephron glomerular filtration rate (22.73 ± 2.02 vs. 51.56 ± 3.87 nl/min) to near normal values. Blockade of P2X4R did not elicit effects in hypertensive rats. In normotensive sham-operated rats, only the P2X1R antagonist caused an increase plasma flow and single-nephron glomerular filtration rate, whereas the P2X4R antagonist induced glomerular vasoconstriction that was consistent with evidence that P2X4R stimulation increases release of nitric oxide from endothelial cells. Mean arterial pressure remained unchanged in both hypertensive and normotensive groups. Western blot analysis showed overexpression of P2X1R, P2X7R, and P2X4R proteins in hypertensive rats. Whereas it has been generally assumed that the altered glomerular vascular resistances in ANG II hypertension are due to AT1receptor-mediated vasoconstriction, these data indicate a predominant P2X1R and P2X7R control of glomerular hemodynamics in ANG II hypertension.

Adenosine contribution to normal renal physiology and chronic kidney disease

Adenosine is a nucleoside that is particularly interesting to many scientific and clinical communities as it has important physiological and pathophysiological roles in the kidney. The distribution of adenosine receptors has only recently been elucidated; therefore it is likely that more biological roles of this nucleoside will be unveiled in the near future. Since the discovery of the involvement of adenosine in renal vasoconstriction and regulation of local renin production, further evidence has shown that adenosine signaling is also involved in the tubuloglomerular feedback mechanism, sodium reabsorption and the adaptive response to acute insults, such as ischemia. However, the most interesting finding was the increased adenosine levels in chronic kidney diseases such as diabetic nephropathy and also in non-diabetic animal models of renal fibrosis. When adenosine is chronically increased its signaling via the adenosine receptors may change, switching to a state that induces renal damage and produces phenotypic changes in resident cells. This review discusses the physiological and pathophysiological roles of adenosine and pays special attention to the mechanisms associated with switching homeostatic nucleoside levels to increased adenosine production in kidneys affected by CKD.

Canonical Transient Receptor Potential 6 Channel: A New Target of Reactive Oxygen Species in Renal Physiology and Pathology

SIGNIFICANCE

Regulation of Ca2+ signaling cascade by reactive oxygen species (ROS) is becoming increasingly evident and this regulation represents a key mechanism for control of many fundamental cellular functions. Canonical transient receptor potential (TRPC) 6, a member of Ca2+-conductive channel in the TRPC family, is widely expressed in kidney cells, including glomerular mesangial cells, podocytes, tubular epithelial cells, and vascular myocytes in renal microvasculature. Both overproduction of ROS and dysfunction of TRPC6 channel are involved in renal injury in animal models and human subjects. Although regulation of TRPC channel function by ROS has been well described in other tissues and cell types, such as vascular smooth muscle, this important cell regulatory mechanism has not been fully reviewed in kidney cells. Recent Advances: Accumulating evidence has shown that TRPC6 is a redox-sensitive channel, and modulation of TRPC6 Ca2+ signaling by altering TRPC6 protein expression or TRPC6 channel activity in kidney cells is a downstream mechanism by which ROS induce renal damage.

CRITICAL ISSUES

This review highlights how recent studies analyzing function and expression of TRPC6 channels in the kidney and their response to ROS improve our mechanistic understanding of oxidative stress-related kidney diseases.

FUTURE DIRECTIONS

Although it is evident that ROS regulate TRPC6-mediated Ca2+ signaling in several types of kidney cells, further study is needed to identify the underlying molecular mechanism. We hope that the newly identified ROS/TRPC6 pathway will pave the way to new, promising therapeutic strategies to target kidney diseases such as diabetic nephropathy. Antioxid. Redox Signal. 25, 732-748.

Genetic Abrogation of Adenosine A3 Receptor Prevents Uninephrectomy and High Salt-Induced Hypertension

Background

Early‐life reduction in nephron number (uninephrectomy [UNX]) and chronic high salt (HS) intake increase the risk of hypertension and chronic kidney disease. Adenosine signaling via its different receptors has been implicated in modulating renal, cardiovascular, and metabolic functions as well as inflammatory processes; however, the specific role of the A₃ receptor in cardiovascular diseases is not clear. In this study, gene‐modified mice were used to investigate the hypothesis that lack of A₃ signaling prevents the development of hypertension and attenuates renal and cardiovascular injuries following UNX in combination with HS (UNX‐HS) in mice.

Methods and Results

Wild‐type (A₃^+/+) mice subjected to UNX‐HS developed hypertension compared with controls (mean arterial pressure 106±3 versus 82±3 mm Hg; P<0.05) and displayed an impaired metabolic phenotype (eg, increased adiposity, reduced glucose tolerance, hyperinsulinemia). These changes were associated with both cardiac hypertrophy and fibrosis together with renal injuries and proteinuria. All of these pathological hallmarks were significantly attenuated in the A₃^−/− mice. Mechanistically, absence of A₃ receptors protected from UNX‐HS–associated increase in renal NADPH oxidase activity and Nox2 expression. In addition, circulating cytokines including interleukins 1β, 6, 12, and 10 were increased in A₃^+/+ following UNX‐HS, but these cytokines were already elevated in naïve A₃^−/− mice and did not change following UNX‐HS.

Conclusions

Reduction in nephron number combined with chronic HS intake is associated with oxidative stress, chronic inflammation, and development of hypertension in mice. Absence of adenosine A₃ receptor signaling was strongly protective in this novel mouse model of renal and cardiovascular disease.

Identification and function of adenosine A3 receptor in afferent arterioles

Adenosine plays an important role in regulation of renal microcirculation. All receptors of adenosine, A1, A2A, A2B, and A3, have been found in the kidney. However, little is known about the location and function of the A3 receptor in the kidney. The present study determined the expression and role of A3 receptors in mediating the afferent arteriole (Af-Art) response and studied the interaction of A3 receptors with angiotensin II (ANG II), A1 and A2 receptors on the Af-Art. We found that the A3 receptor expressed in microdissected isolated Af-Art and the mRNA levels of A3 receptor were 59% of A1. In the isolated microperfused Af-Art, A3 receptor agonist IB-MECA did not have a constrictive effect. Activation of A3 receptor dilated the preconstricted Af-Art by norepinephrine and blunted the vasoconstrictive effect of both adenosine A1 receptor activation and ANG II on the Af-Art, respectively. Selective A2 receptor antagonist (both A2A and A2B) had no effect on A3 receptor agonist-induced vasodilation, indicating that the dilatory effect of A3 receptor activation is not mediated by activation of A2 receptor. We conclude that the A3 receptor is expressed in the Af-Art, and activation of the A3 receptor dilates the Af-Art.

Differential effect of T-type voltage-gated Ca2+ channel disruption on renal plasma flow and glomerular filtration rate in vivo

Voltage-gated Ca(2+) (Cav) channels play an essential role in the regulation of renal blood flow and glomerular filtration rate (GFR). Because T-type Cav channels are differentially expressed in pre- and postglomerular vessels, it was hypothesized that they impact renal blood flow and GFR differentially. The question was addressed with the use of two T-type Cav knockout (Cav3.1(-/-) and Cav3.2(-/-)) mouse strains. Continuous recordings of blood pressure and heart rate, para-aminohippurate clearance (renal plasma flow), and inulin clearance (GFR) were performed in conscious, chronically catheterized, wild-type (WT) and Cav3.1(-/-) and Cav3.2(-/-) mice. The contractility of afferent and efferent arterioles was determined in isolated perfused blood vessels. Efferent arterioles from Cav3.2(-/-) mice constricted significantly more in response to a depolarization compared with WT mice. GFR was increased in Cav3.2(-/-) mice with no significant changes in renal plasma flow, heart rate, and blood pressure. Cav3.1(-/-) mice had a higher renal plasma flow compared with WT mice, whereas GFR was indistinguishable from WT mice. No difference in the concentration response to K(+) was observed in isolated afferent and efferent arterioles from Cav3.1(-/-) mice compared with WT mice. Heart rate was significantly lower in Cav3.1(-/-) mice compared with WT mice with no difference in blood pressure. T-type antagonists significantly inhibited the constriction of human intrarenal arteries in response to a small depolarization. In conclusion, Cav3.2 channels support dilatation of efferent arterioles and affect GFR, whereas Cav3.1 channels in vivo contribute to renal vascular resistance. It is suggested that endothelial and nerve localization of Cav3.2 and Cav3.1, respectively, may account for the observed effects.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Histamine-dependent prolongation by aldosterone of vasoconstriction in isolated small mesenteric arteries of the mouse

In arterioles, aldosterone counteracts the rapid dilatation (recovery) following depolarization-induced contraction. The hypothesis was tested that this effect of aldosterone depends on cyclooxygenase (COX)-derived products and/or nitric oxide (NO) synthase (NOS) inhibition. Recovery of the response to high K+ was observed in mesenteric arteries of wild-type and COX-2−/− mice but it was significantly diminished in preparations from endothelial NOS (eNOS)−/− mice. Aldosterone pretreatment inhibited recovery from wild-type and COX-2−/− mice. The NO donor sodium nitroprusside (SNP) restored recovery in arteries from eNOS−/− mice, and this was inhibited by aldosterone. Actinomycin-D abolished the effect of aldosterone, indicating a genomic effect. The effect was blocked by indomethacin and by the COX-1 inhibitor valeryl salicylate but not by NS-398 (10−6 mol/l) or the TP-receptor antagonist S18886 (10−7 mol/l). The effect of aldosterone on recovery in arteries from wild-type mice and the SNP-mediated dilatation in arteries from eNOS−/− mice was inhibited by the histamine H2 receptor antagonist cimetidine. RT-PCR showed expression of mast cell markers in mouse mesenteric arteries. The adventitia displayed granular cells positive for toluidine blue vital stain. Confocal microscopy of live mast cells showed loss of quinacrine fluorescence and swelling after aldosterone treatment, indicating degranulation. RT-PCR showed expression of mineralocorticoid receptors in mesenteric arteries and in isolated mast cells. These findings suggest that aldosterone inhibits recovery by stimulation of histamine release from mast cells along mesenteric arteries. The resulting activation of H2 receptors decreases the sensitivity to NO of vascular smooth muscle cells. Aldosterone may chronically affect vascular function through paracrine release of histamine.

Adenosine A2 receptor-mediated regulation of renal hemodynamics and glomerular filtration rate is abolished in diabetes

Alterations in glomerular filtration rate (GFR) are one of the earliest indications of altered kidney function in diabetes. Adenosine regulates GFR through tubuloglomerular feedback mechanism acting on adenosine A1 receptor. In addition, adenosine can directly regulate vascular tone by acting on A1 and A2 receptors expressed in afferent and efferent arterioles. Opposite to A1 receptors, A2 receptors mediate vasorelaxation. This study investigates the involvement of adenosine A2 receptors in regulation of renal blood flow (RBF) and GFR in control and diabetic kidneys. GFR was measured by inulin clearance and RBF by a transonic flow probe placed around the renal artery. Measurements were performed in isoflurane-anesthetized normoglycemic and alloxan-diabetic C57BL/6 mice during baseline and after acute administration of 3,7-dimethyl-1-propargylxanthine (DMPX), a selective A2 receptor antagonist. GFR and RBF were lower in diabetic mice compared to control (258 ± 61 vs. 443 ± 33 μl min(-1) and 1,083 ± 51 vs. 1,405 ± 78 μl min(-1)). In control animals, DMPX decreased RBF by -6%, whereas GFR increased +44%. DMPX had no effects on GFR and RBF in diabetic mice. Sodium excretion increased in diabetic mice after A2 receptor blockade (+78%). In conclusion, adenosine acting on A2 receptors mediates an efferent arteriolar dilatation which reduces filtration fraction (FF) and maintains GFR within normal range in normoglycemic mice. However, this regulation is absent in diabetic mice, which may contribute to reduced oxygen availability in the diabetic kidney.

Targeting adenosine receptors in the development of cardiovascular therapeutics

Adenosine receptor stimulation has negative inotropic and dromotropic actions, reduces cardiac ischemia-reperfusion injury and remodeling, and prevents cardiac arrhythmias. In the vasculature, adenosine modulates vascular tone, reduces infiltration of inflammatory cells and generation of foam cells, and may prevent the development of atherosclerosis as a result. Modulation of insulin sensitivity may further add to the anti-atherosclerotic properties of adenosine signaling. In the kidney, adenosine plays an important role in tubuloglomerular feedback and modulates tubular sodium reabsorption. The challenge is to take advantage of the beneficial actions of adenosine signaling while preventing its potential adverse effects, such as salt retention and sympathoexcitation. Drugs that interfere with adenosine formation and elimination or drugs that allosterically enhance specific adenosine receptors seem to be most promising to meet this challenge.

Equilibrative nucleoside transporter 1 (ENT1) regulates postischemic blood flow during acute kidney injury in mice

A complex biologic network regulates kidney perfusion under physiologic conditions. This system is profoundly perturbed following renal ischemia, a leading cause of acute kidney injury (AKI) - a life-threatening condition that frequently complicates the care of hospitalized patients. Therapeutic approaches to prevent and treat AKI are extremely limited. Better understanding of the molecular pathways promoting postischemic reflow could provide new candidate targets for AKI therapeutics. Due to its role in adapting tissues to hypoxia, we hypothesized that extracellular adenosine has a regulatory function in the postischemic control of renal perfusion. Consistent with the notion that equilibrative nucleoside transporters (ENTs) terminate adenosine signaling, we observed that pharmacologic ENT inhibition in mice elevated renal adenosine levels and dampened AKI. Deletion of the ENTs resulted in selective protection in Ent1-/- mice. Comprehensive examination of adenosine receptor-knockout mice exposed to AKI demonstrated that renal protection by ENT inhibitors involves the A2B adenosine receptor. Indeed, crosstalk between renal Ent1 and Adora2b expressed on vascular endothelia effectively prevented a postischemic no-reflow phenomenon. These studies identify ENT1 and adenosine receptors as key to the process of reestablishing renal perfusion following ischemic AKI. If translatable from mice to humans, these data have important therapeutic implications.

Preserving postischemic reperfusion in the kidney: a role for extracellular adenosine

Several adenosine receptor subtypes on endothelial, epithelial, mesangial, and inflammatory cells have been implicated in ischemic acute kidney injury, a life-threatening condition that frequently complicates the care of hospitalized patients. In this issue of the JCI, Grenz and coworkers provide novel insight into how preservation of postischemic renal perfusion by endothelial cell adenosine A2B receptors is antagonized by adenosine reuptake into proximal tubule cells by equilibrative nucleotide transporter 1, which can be inhibited by dipyridamole. The work suggests that adenosine A2B receptor agonists and inhibition of equilibrative nucleoside transporters by dipyridamole may have therapeutic potential in ischemic acute kidney injury, a condition for which there are currently no specific therapeutic interventions.

Pharmacological targets in the renal peritubular microenvironment: implications for therapy for sepsis-induced acute kidney injury

One of the most frequent and serious complications to develop in septic patients is acute kidney injury (AKI), a disorder characterized by a rapid failure of the kidneys to adequately filter the blood, regulate ion and water balance, and generate urine. AKI greatly worsens the already poor prognosis of sepsis and increases cost of care. To date, therapies have been mostly supportive; consequently there has been little change in the mortality rates over the last decade. This is due, at least in part, to the delay in establishing clinical evidence of an infection and the associated presence of the systemic inflammatory response syndrome and thus, a delay in initiating therapy. A second reason is a lack of understanding regarding the mechanisms leading to renal injury, which has hindered the development of more targeted therapies. In this review, we summarize recent studies, which have examined the development of renal injury during sepsis and propose how changes in the peritubular capillary microenvironment lead to and then perpetuate microcirculatory failure and tubular epithelial cell injury. We also discuss a number of potential therapeutic targets in the renal peritubular microenvironment, which may prevent or lessen injury and/or promote recovery.

Role of the adenosine(2A) receptor-epoxyeicosatrienoic acid pathway in the development of salt-sensitive hypertension

Activation of rat adenosine(2A) receptors (A(2A) R) dilates preglomerular microvessels, an effect mediated by epoxyeicosatrienoic acids (EETs). High salt (HS) intake increases epoxygenase activity and adenosine levels. A greater vasodilator response to a stable adenosine analog, 2-chloroadenosine (2-CA), was seen in kidneys obtained from HS-fed rats which was mediated by increased EET release. Because this pathway is antipressor, we examined the role of the A(2A) R-EET pathway in a genetic model of salt-sensitive hypertension, the Dahl salt-sensitive (SS) rats. Dahl salt resistant (SR) rats fed a HS diet demonstrated a greater renal vasodilator response to 2-CA. In contrast, Dahl SS rats did not exhibit a difference in the vasodilator response to 2-CA whether fed normal salt (NS) or HS diet. In Dahl SR but not Dahl SS rats, HS intake significantly increased purine flux, augmented the protein expression of A(2A) R and cytochrome P450 2C23 and 2C11 epoxygenases, and elevated the renal efflux of EETs. Thus the Dahl SR rat is able to respond to HS intake by recruiting EET formation, whereas the Dahl SS rat appears to have exhausted its ability to increase EET synthesis above the levels observed on NS intake. In vivo inhibition of the A(2A) R-EET pathway in Dahl SR rats fed a HS diet results in reduced renal EETs levels, diminished natriuretic capacity and hypertension, thus supporting a role for the A(2A) R-EET pathway in the adaptive natriuretic response to modulate blood pressure during salt loading. An inability of Dahl SS rats to upregulate the A(2A) R-EET pathway in response to salt loading may contribute to the development of salt-sensitive hypertension.

Adenosine elicits an eNOS-independent reduction in arterial blood pressure in conscious mice that involves adenosine A2A receptors

AIMS

Adenosine plays an important role in the regulation of heart rate (HR) and vascular reactivity. However, the mechanisms underlying the acute effect of adenosine on arterial blood pressure in conscious mice are unclear. Therefore, this study investigated the effect of the nucleoside on mean arterial blood pressure (MAP) and HR in conscious mice.

METHODS

Chronic indwelling catheters were placed in C57Bl/6J (WT) and endothelial nitric oxide synthase knockout (eNOS(-/-)) mice for continuous measurements of MAP and HR. Using PCR and myograph analysis, involvement of adenosine receptors was investigated in human and mouse renal blood vessels.

RESULTS

Bolus infusion of 0.5 mg kg(-1) adenosine elicited significant transient decreases in MAP (99.3 ± 2.3 to 70.4 ± 4.5 mmHg) and HR (603.2 ± 18.3 to 364.3 ± 49.2 min(-1)), which were inhibited by the A(2A) receptor antagonist ZM 241385. Activation of adenosine A(2A) receptors with CGS 21680 (0.02 mg kg(-1)) caused a significant reduction in MAP from 99.6 ± 1.2 to 73.1 ± 3.6 mmHg accompanied by tachycardia (610.5 ± 9.3 to 677.5 ± 9.5 min(-1)). The reduction in MAP observed after adenosine or CGS 21680 administrations was not significantly different in WT and eNOS(-/-) mice. In isolated human and mouse intrarenal arteries, adenosine caused a relaxation dependent on A(2A) adenosine receptor activation. A(2A) receptors were present in both human and mouse arteries whereas A(1) and A(2B) receptors were only present in mouse arteries.

CONCLUSION

In conclusion, acute adenosine administration and selective stimulation of adenosine A(2A) receptors results in an immediate, transient eNOS-independent reduction in MAP. A(2A) receptor activation causes relaxation of human and mouse arteries.

Adenosine A2A receptor activation attenuates tubuloglomerular feedback responses by stimulation of endothelial nitric oxide synthase

Adenosine A(2) receptors have been suggested to modulate tubuloglomerular feedback (TGF) responses by counteracting adenosine A(1) receptor-mediated vasoconstriction, but the mechanisms are unclear. We tested the hypothesis that A(2A) receptor activation blunts TGF by release of nitric oxide in the juxtaglomerular apparatus (JGA). Maximal TGF responses were measured in male Sprague-Dawley rats as changes in proximal stop-flow pressure (ΔP(SF)) in response to increased perfusion of the loop of Henle (0 to 40 nl/min) with artificial tubular fluid (ATF). The maximal TGF response was studied after 5 min intratubular perfusion (10 nl/min) with ATF or ATF + A(2A) receptor agonist (CGS-21680; 10(-7) mol/l). The interaction with nitric oxide synthase (NOS) isoforms was tested by perfusion with a nonselective NOS inhibitor [N(ω)-nitro-L-arginine methyl ester hydrochloride (L-NAME); 10(-3) mol/l] or a selective neuronal NOS (nNOS) inhibitor [N(ω)-propyl-L-arginine (L-NPA); 10(-6) mol/l] alone, and with the A(2A) agonist. Blood pressure, urine flow, and P(SF) at 0 nl/min were similar among the groups. The maximal TGF response (ΔP(SF)) with ATF alone (12.3 ± 0.6 mmHg) was attenuated by selective A(2A) stimulation (9.5 ± 0.4 mmHg). L-NAME enhanced maximal TGF responses (18.9 ± 0.4 mmHg) significantly more than L-NPA (15.2 ± 0.7 mmHg). Stimulation of A(2A) receptors did not influence maximal TGF response during nonselective NOS inhibition (19.0 ± 0.4) but attenuated responses during nNOS inhibition (10.3 ± 0.4 mmHg). In conclusion, adenosine A(2A) receptor activation attenuated TGF responses by stimulation of endothelial NOS (eNOS), presumably in the afferent arteriole. Moreover, NO derived from both eNOS and nNOS in the JGA may blunt TGF responses.

Adenosine A(2) receptors modulate tubuloglomerular feedback

Adenosine can mediate the tubuloglomerular (TGF) response via activation of A(1) receptors on the afferent arteriole, but both adenosine A(1) and A(2) receptors can regulate preglomerular resistance. We tested the hypothesis that adenosine A(2) receptors offset the effect of A(1) receptors and modulate the TGF. Maximal TGF responses were measured in male Sprague-Dawley rats as changes in proximal stop-flow pressure (DeltaP(SF)) in response to increased perfusion of the loop of Henle (0 to 40 nl/min) with artificial tubular fluid (ATF). The maximal TGF response was studied after 5 min of intratubular perfusion (10 nl/min) with ATF alone, or with ATF plus the A(2A) receptor antagonist (ZM-241385; 10(-7) or 10(-5) mol/l), A(1) receptor antagonist (PSB-36; 10(-8) mol/l), or with a combination of A(1) (PSB-36; 10(-8) mol/l) and A(2A) (ZM-241385; 10(-7) mol/l) antagonists. The maximal TGF response (DeltaP(SF)) with ATF alone was 11.7 +/- 1.0 mmHg. Specific A(2) inhibition (low dose) enhanced the maximal TGF response (15.7 +/- 0.8 mmHg; P < 0.01), whereas a high dose (unspecific inhibition) attenuated the response (5.0 +/- 0.4 mmHg; P < 0.001). A(1) inhibition alone led to a paradoxical TGF response, with an increase in P(SF) of 3.1 +/- 0.5 mmHg (P < 0.05). Simultaneous application of A(1) and A(2) antagonists abolished the TGF response (DeltaP(SF): 0.4 +/- 0.3 mmHg). In conclusion, adenosine A(2) receptors modulate the TGF response by counteracting the effects of adenosine A(1) receptors.

Afferent arteriolar vasodilator effect of adenosine predominantly involves adenosine A2B receptor activation

Adenosine is an important paracrine agent regulating renal vascular tone via adenosine A(1) and A(2) receptors. While A(2B) receptor message and protein have been localized to preglomerular vessels, functional evidence on the role of A(2B) receptors in mediating the vasodilator action of adenosine on afferent arterioles is not available. The present study determined the role of A(2B) receptors in mediating the afferent arteriolar dilation and compared the effects of A(2B) and A(2A) receptor blockade on afferent arterioles. We used the rat in vitro blood-perfused juxtamedullary nephron technique combined with videomicroscopy. Single afferent arterioles of Sprague-Dawley rats were visualized and superfused with solutions containing adenosine or adenosine A(2) receptor agonist (CV-1808) along with adenosine A(2B) and A(2A) receptor blockers. Adenosine (10 micromol/l) caused modest constriction and subsequent superfusion with SCH-58261 (SCH), an A(2A) receptor blocker, at concentrations up 10 micromol/l elicited only slight additional decreases in afferent arteriolar diameter with maximum effect at a concentration of 1 micromol/l (-11.0 +/- 2.5%, n = 6, P < 0.05). However, superfusion of adenosine-treated vessels with MRS-1754 (MRS), an A(2B) receptor blocker, elicited greater decreases in afferent arteriolar diameter (-26.0 +/- 4.7%, n = 5, P < 0.01). SCH did not significantly augment the adenosine-mediated afferent constriction elicited by MRS; however, adding MRS after SCH caused further significant vasoconstriction. Superfusion with CV-1808 dilated afferent arterioles (17.2 +/- 2.4%, n = 6, P < 0.01). This effect was markedly attenuated by MRS (-22.6 +/- 2.0%, n = 5, P < 0.01) but only slightly reduced by SCH (-9.0 +/- 1.1%, n = 5, P < 0.05) and completely prevented by adding MRS after SCH (-24.7 +/- 1.8%, n = 5, P < 0.01). These results indicate that, while both A(2A) and A(2B) receptors are functionally expressed in juxtamedullary afferent arterioles, the powerful vasodilating action of adenosine predominantly involves A(2B) receptor activation, which counteracts A(1) receptor-mediated vasoconstriction.

Renal homeostasis and tubuloglomerular feedback

PURPOSE OF REVIEW

We review some basic homeostatic principles that are frequently disregarded to provide boundary conditions to test any new theory containing new details. Homeostasis as applied to total body salt is discussed with a linear model for salt homeostasis that is extraordinarily simple wherein total body salt drives the salt excretion. The basics of tubuloglomerular feedback (TGF) and its implications for salt homeostasis are then reviewed.

RECENT FINDINGS

Advances in the field discussed include new details on the apical and basolateral transport of sodium chloride (NaCl) in the macula densa cells during TGF response, direct evidence of contribution of TGF to renal autoregulation and the description of vasodilatory adenosine A2b receptors in the 'efferent' TGF response. Finally, recent information about the role of proximal tubular microvilli as mechanosensors in the flow-dependent tubular reabsorption as a mechanism to explain glomerulotubular balance is reviewed.

SUMMARY

Notwithstanding the complexity of salt balance at a molecular level, the overall salt homeostasis is simple. Various natritropic nerves and hormones stabilize any disturbance in salt balance. A change in glomerular filtration rate (GFR) brought about by these natritropes will be partially counteracted by the impact of TGF on nephron function. Thus, by stabilizing GFR, TGF reduces the usefulness of GFR as an instrument of salt balance, and lessens the efficiency of salt homeostasis.

Proinsulin C-peptide reduces diabetes-induced glomerular hyperfiltration via efferent arteriole dilation and inhibition of tubular sodium reabsorption

C-peptide reduces diabetes-induced glomerular hyperfiltration in diabetic patients and experimental animal models. However, the mechanisms mediating the beneficial effect of C-peptide remain unclear. We investigated whether altered renal afferent-efferent arteriole tonus or alterations in tubular Na+ transport (T(Na)) in response to C-peptide administration mediate the reduction of diabetes-induced glomerular hyperfiltration. Glomerular filtration rate, filtration fraction, total and cortical renal blood flow, total kidney O2 consumption (QO2), T(Na), fractional Na+ and Li+ excretions, and tubular free-flow and stop-flow pressures were measured in anesthetized adult male normoglycemic and streptozotocin-diabetic Sprague-Dawley rats. The specific effect of C-peptide on transport-dependent QO2 was investigated in vitro in freshly isolated proximal tubular cells. C-peptide reduced glomerular filtration rate (-24%), stop-flow pressure (-8%), and filtration fraction (-17%) exclusively in diabetic rats without altering renal blood flow. Diabetic rats had higher baseline T(Na) (+40%), which was reduced by C-peptide. Similarly, C-peptide increased fractional Na+ (+80%) and Li+ (+47%) excretions only in the diabetic rats. None of these parameters was affected by vehicle treatments in either group. Baseline QO2 was 37% higher in proximal tubular cells from diabetic rats than controls and was normalized by C-peptide. C-peptide had no effect on ouabain-pretreated diabetic cells from diabetic rats. C-peptide reduced diabetes-induced hyperfiltration via a net dilation of the efferent arteriole and inhibition of tubular Na+ reabsorption, both potent regulators of the glomerular net filtration pressure. These findings provide new mechanistic insight into the beneficial effects of C-peptide on diabetic kidney function.

Enhanced tubuloglomerular feedback in mice with vascular overexpression of A1 adenosine receptors

Adenosine 1 receptors (A1AR) in the kidney are expressed in the vasculature and the tubular system. Pharmacological inhibition or global genetic deletion of A1AR causes marked reductions or abolishment of tubuloglomerular feedback (TGF) responses. To assess the function of vascular A1AR in TGF, we generated transgenic mouse lines in which A1AR expression in smooth muscle was augmented by placing A1AR under the control of a 5.38-kb fragment of the rat smooth muscle alpha-actin promoter and first intron (12). Two founder lines with highest expression in the kidney [353 +/- 42 and 575 +/- 43% compared with the wild type (WT)] were used in the experiments. Enhanced expression of A1AR at the expected site in these lines was confirmed by augmented constrictor responses of isolated afferent arterioles to administration of the A1AR agonist N6-cyclohexyladenosine. Maximum TGF responses (0-30 nl/min flow step) were increased from 8.4 +/- 0.9 mmHg in WT (n = 21) to 14.2 +/- 0.7 mmHg in A1AR-transgene (tg) 4 (n = 22; P < 0.0001), and to 12.6 +/- 1.2 mmHg in A1AR-tg7 (n = 12; P < 0.02). Stepwise changes in perfusion flow caused greater numerical TGF responses in A1AR-tg than WT in all flow ranges with differences reaching levels of significance in the intermediate flow ranges of 7.5-10 and 10-15 nl/min. Proximal-distal single-nephron glomerular filtration rate (SNGFR) differences (free-flow micropuncture) were also increased in A1AR-tg, averaging 6.25 +/- 1.5 nl/min compared with 2.6 +/- 0.51 nl/min in WT (P = 0.034). Basal plasma renin concentrations as well as the suppression of renin secretion after volume expansion were similar in A1AR-tg and WT mice, suggesting lack of transgene expression in juxtaglomerular cells. These data indicate that A1AR expression in vascular smooth muscle cells is a critical component for TGF signaling and that changes in renal vascular A1AR expression may determine the magnitude of TGF responses.

Regulation of renal arteriolar tone by adenosine: novel role for type 2 receptors

Tubuloglomerular feedback regulation of glomerular filtration rate (GFR) is mediated by adenosine, which acts on type 1 receptors in the afferent arteriole to increase resistance. However, new findings in isolated mouse tissue suggest that adenosine also dilates the efferent arteriole, which would reinforce the ability of adenosine to reduce GFR. This new information extends the concept that adenosine acts as a paracrine agent on both afferent and efferent arterioles.