Dipyridamole decreases glomerular filtration in the sodium-depleted dog. Evidence for mediation by intrarenal adenosine

Regionally selective cardiovascular responses to adenosine A2A and A2B receptor activation

Adenosine is a local mediator that regulates changes in the cardiovascular system via activation of four G protein-coupled receptors (A₁ , A_2A , A_2B , A₃ ). Here, we have investigated the effect of A_2A and A_2B -selective agonists on vasodilatation in three distinct vascular beds of the rat cardiovascular system. NanoBRET ligand binding studies were used to confirm receptor selectivity. The regional hemodynamic effects of adenosine A_2A and A_2B selective agonists were investigated in conscious rats. Male Sprague-Dawley rats (350-450 g) were chronically implanted with pulsed Doppler flow probes on the renal artery, mesenteric artery, and the descending abdominal aorta. Cardiovascular responses were measured following intravenous infusion (3 min for each dose) of the A_2A -selective agonist CGS 21680 (0.1, 0.3, 1 µg kg^-1 min^-1 ) or the A_2B -selective agonist BAY 60-6583 (4,13.3, 40 µg kg^-1 min^-1 ) following predosing with the A_2A -selective antagonist SCH 58261 (0.1 or 1 mg kg^-1 min^-1 ), the A_2B /A_2A antagonist PSB 1115 (10 mg kg^-1 min^-1 ) or vehicle. The A_2A -selective agonist CGS 21680 produced a striking increase in heart rate (HR) and hindquarters vascular conductance (VC) that was accompanied by a significant decrease in mean arterial pressure (MAP) in conscious rats. In marked contrast, the A_2B -selective agonist BAY 60-6583 significantly increased HR and VC in the renal and mesenteric vascular beds, but not in the hindquarters. Taken together, these data indicate that A_2A and A_2B receptors are regionally selective in their regulation of vascular tone. These results suggest that the development of A_2B receptor agonists to induce vasodilatation in the kidney may provide a good therapeutic approach for the treatment of acute kidney injury.

Radiographic contrast-media-induced acute kidney injury: pathophysiology and prophylactic strategies

Contrast-induced acute kidney injury (CI-AKI) is one of the most widely discussed and debated topics in cardiovascular medicine. With increasing number of contrast-media- (CM-) enhanced imaging studies being performed and growing octogenarian population with significant comorbidities, incidence of CI-AKI remains high. In this review, pathophysiology of CI-AKI, its relationship with different types of CM, role of serum and urinary biomarkers for diagnosing CI-AKI, and various prophylactic strategies used for nephroprotection against CI-AKI are discussed in detail.

Ischaemic preconditioning of the graft in adult living related right lobe liver transplantation: impact on ischaemia-reperfusion injury and clinical relevance

BACKGROUND

Ischaemic preconditioning (IPC) of the right liver graft in the donor has not been studied in adult-to-adult living related liver transplantation (LRLT).

OBJECTIVE

To assess the IPC effect of the graft on ischaemia reperfusion injury in the recipient and compare recipient and donor outcomes with and without preconditioned grafts.

PATIENTS AND METHODS

Alternate patients were transplanted with right lobe grafts that were (n = 22; Group (Precond)) or were not (n = 22; Group (Control)) subjected to IPC in the living donor. Liver ischaemia-reperfusion injury, liver/kidney function, morbidity/mortality rates and outcomes were compared. Univariate and multivariate analyses were performed to identify factors predictive of the aspartate aminotransferase (AST) peak and minimum prothrombin time.

RESULTS

Both groups had similar length of hospital stay, morbidity/mortality, primary non-function and acute rejection rates. Post-operative AST (P = 0.8) and alanine aminotransferase (ALT) peaks (P = 0.6) were similar in both groups (307 +/- 189 and 437 +/- 302 vs. 290 +/- 146 and 496 +/- 343, respectively). In univariate analysis, only pre-operative AST and warm ischemia time (WIT) were significantly associated with post-operative AST peak (in recipients). In multivariate analysis, the graft/recipient weight ratio (P = 0.003) and pre-operative bilirubin concentration (P = 0.004) were significantly predictive of minimum prothrombin time post-transplantation.

CONCLUSIONS

Graft IPC in the living related donor is not associated with any benefit for the recipient or the donor and its clinical value remains uncertain.

Adenosine and kidney function: potential implications in patients with heart failure

Therapy of heart failure is more difficult when renal function is impaired. Here, we outline the effects on kidney function of the autacoid, adenosine, which forms the basis for adenosine A(1) receptor (A(1)R) antagonists as treatment for decompensated heart failure. A(1)R antagonists induce a eukaliuretic natriuresis and diuresis by blocking A(1)R-mediated NaCl reabsorption in the proximal tubule and the collecting duct. Normally, suppressing proximal reabsorption will lower glomerular filtration rate (GFR) through the tubuloglomerular feedback mechanism (TGF). But the TGF response, itself, is mediated by A(1)R in the preglomerular arteriole, so blocking A(1)R allows natriuresis to proceed while GFR remains constant or increases. The influence of A(1)R over vascular resistance in the kidney is augmented by angiotensin II while A(1)R activation directly suppresses renin secretion. These interactions could modulate the overall impact of A(1)R blockade on kidney function in patients taking angiotensin II blockers. A(1)R blockers may increase the energy utilized for transport in the semi-hypoxic medullary thick ascending limb, an effect that could be prevented with loop diuretics. Finally, while the vasodilatory effect of A(1)R blockade could protect against renal ischaemia, A(1)R blockade may act on non-resident cells to exacerbate reperfusion injury, where ischaemia to occur. Despite these uncertainties, the available data on A(1)R antagonist therapy in patients with decompensated heart failure are promising and warrant confirmation in further studies.

Ischemic preconditioning in solid organ transplantation: from experimental to clinics

This study reviews the current understanding of ischemic preconditioning (IP) in experimental and clinical setting, and the mechanisms that mediate the complex processes involved as a tool to protect against ischemia and reperfusion (I/R) injury, but is not intended as a complete literature review of preconditioning. IP has been mainly elucidated in cardiac ischemia. Recent reports confirm the efficacy of pre- and postconditioning in cardiac surgery and percutaneous coronary interventions in humans. IP utilizes endogenous as well as distant mechanisms in skeletal muscle, liver, lung, kidney, intestine and brain in animal models to convey varying degrees of protection from I/R injury. Specifically, preconditioned tissues exhibit altered energy metabolism, better electrolyte homeostasis and genetic reorganization, as well as less oxygen-free radicals and activated neutrophils release, reduced apoptosis and better microcirculatory perfusion. To date, there are few human studies, but recent trials suggest that human liver, lung and skeletal muscle acquire protection after IP. Present data address the potential therapeutic application of IP in the prevention of I/R damage specially aimed at clinical transplantation. IP is ubiquitous but more research is required to fully translate these findings to the clinical arena.

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.

Ischemic preconditioning for major liver resection under vascular exclusion of the liver preserving the caval flow: a randomized prospective study

BACKGROUND

Two randomized prospective studies suggested that ischemic preconditioning (IP) protects the human liver against ischemia-reperfusion injury after hepatectomy performed under continuous clamping of the portal triad. The primary goal of this study was to determine whether IP protects the human liver against ischemia-reperfusion injury after hepatectomy under continuous vascular exclusion with preservation of the caval flow.

STUDY DESIGN

Sixty patients were randomly divided into two groups: with (n=30; preconditioning group) and without (n=30; control group) IP (10 minutes of portal triad clamping and 10 minutes of reperfusion) before major hepatectomy under vascular exclusion of the liver preserving the caval flow. Serum concentrations of aspartate transferase, alanine transferase, glutathione-S-transferase, and bilirubin and prothrombin time were regularly determined until discharge and at 1 month. Morbidity and mortality were determined in both groups.

RESULTS

Peak postoperative concentrations of aspartate transferase were similar in the groups with and without IP (851 +/- 1,733 IU/L and 427 +/- 166 IU/L respectively, p=0.2). A similar trend toward a higher peak concentration of alanine transferase and glutathione-S-transferase was indeed observed in the preconditioning group compared with the control group. Morbidity and mortality rates and lengths of ICU and hospitalization stays were similar in both groups.

CONCLUSIONS

IP does not improve liver tolerance to ischemia-reperfusion after hepatectomy under vascular exclusion of the liver with preservation of the caval flow. This maneuver does not improve postoperative liver function and does not affect morbidity or mortality rates. The clinical use of IP through 10 minutes of warm ischemia in this technique of hepatectomy is not currently recommended.

Effects of 10 minutes of ischemic preconditioning of the cadaveric liver on the graft's preservation and function: the ying and the yang

SUMMARY BACKGROUND DATA

Although extensively studied in animal models, ischemic preconditioning has not yet been studied in clinical transplantation.

OBJECTIVE

To compare the results of cadaveric liver transplantation with and without ischemic liver preconditioning in the donor.

PATIENTS AND METHODS

Alternate patients were transplanted with liver grafts that had (n = 46, GroupPrecond) or had not (n = 45, GroupControl) been subjected to ischemic preconditioning. Liver ischemia-reperfusion injury, liver and kidney function, morbidity, and in-hospital mortality rates were compared in the 2 groups. Initial poor function was defined as a minimal prothrombin time within 10 days of transplantation <30% of normal and/or bilirubin >200 micromol/L.

RESULTS

The postoperative peaks of ASAT (IU/L) and ALAT (IU/L) were significantly lower in GroupPrecond (556 +/- 968 and 461+/-495, respectively) than in the GroupControl (1073 +/- 1112 and 997+/-1071, respectively). The rate of technical morbidity and the incidence of acute rejection were similar in both groups. Initial poor function was significantly more frequent in the GroupPrecond (10 of 46 cases) than in the GroupControl (3 of 45 cases). Hospital mortality rates were similar in the 2 groups. In multivariate analysis, body mass index of the donor, graft steatosis, and ischemic preconditioning were significantly predictive of the posttransplant peak of ASAT. In univariate analysis, only preconditioning was significantly associated with initial poor function.

CONCLUSIONS

Compared with standard orthotopic liver transplant, ischemic preconditioning of the liver graft in the donor is associated with better tolerance to ischemia. However, this is at the price of decreased early function. Until further studies are available, the clinical value of preconditioning liver grafts remains uncertain.

Is ischemic preconditioning of the kidney clinically relevant?

BACKGROUND

Renal ischemic preconditioning (IPC) is a phenomenon whereby a brief period of ischemia and reperfusion (I/R) provides tolerance to subsequent periods of ischemia. IPC has been demonstrated to protect rodent kidneys during I/R. The applicability to large mammals, including human beings, is unclear. The objective of this study was to determine if renal IPC has a beneficial effect in a large animal model of warm I/R and hypothermic preservation injury, which occurs with renal allografting.

METHODS

Renal ischemia (45 minutes) and reperfusion was studied in untreated dogs and in dogs receiving IPC (10-minute/10-minute I/R). IPC was administered immediately before I/R (early IPC) or 24 hours before I/R (delayed IPC). In another group of dogs, pharmacologically induced IPC was attempted with local intra-arterial administration of dipyridamole (2.4 mg/kg/min) to increase local adenosine concentrations. Finally, IPC was induced in kidneys before harvest, cold stored for 24 hours in University of Wisconsin flush solution, and subsequently reperfused for 4 hours in allogeneic recipients. Renal functional parameters, including vascular resistance, glomerular filtration rate, urine production, oxygen consumption, and proximal tubular fluid reabsorption, were monitored during the reperfusion period and were compared with the control ischemic group.

RESULTS

Renal function significantly declined during I/R, relative to the nonischemic contralateral kidney but was not different with any form of IPC, relative to the ischemic control group not treated with IPC. IPC pretreatment also did not affect the preservation injury observed in cold-stored kidneys reperfused after transplantation.

CONCLUSIONS

It is concluded that IPC has no significantly measurable effects in warm or hypothermic renal I/R injury in large animals. The clinical usefulness of IPC in human renal ischemic conditions remains uncertain.

Effects of a nucleoside transporter inhibitor, dilazep, on renal microcirculation in rats

Adenosine, one of the endogenous modulators in renal hemodynamics, has recently been shown to be a mediator of tubuloglomerular feedback (TGF). Dilazep augments endogenous adenosine actions by blocking its cellular uptake. Our purpose in the present study was to clarify the effects of dilazep on renal microcirculation and the TGF mechanism. Clearance and micropuncture experiments were performed in anesthetized rats. TGF responsiveness was assessed in superficial nephrons by measuring the changes of early proximal flow rate (EPFR) in response to loop perfusion at 0-40 nl/min with artificial tubular fluid (ATF). Under dilazep administration (0.3 mg/kg+0.3 mg/kg/h i.v.) systemic BP and GFR were decreased and renal plasma flow was unaltered; as a result, the filtration fraction tended to decrease (p=0.076). Renal vascular resistance was reduced, but not to a significant degree. The reduction in EPFR by loop perfusion was similar between controls (47 +/- 2%) and rats administered dilazep i.v. (44 +/- 5%). Intraluminal application of dilazep in ATF suppressed TGF-mediated EPFR reduction to by 46 +/- 4%, 43 +/- 7%, and 37 +/- 3% at dilazep concentrations of 10(-6), 10(-5), and 10(-4) mol/l, respectively. TGF suppression with 10(-4) mol/l dilazep was reversed by co-perfusion of 10(-5) mol/l DMPX, a selective adenosine A2 receptor antagonist. DMPX alone did not affect TGF response. In conclusion, these results indicate that systemic dilazep dilates postglomerular arterioles and does not affect TGF, and thus reduces GFR. A pharmacological concentration of dilazep applied to single nephrons clearly attenuates TGF, indicating afferent arteriolar vasodilatation. Extracellular adenosine augmented by dilazep dilates glomerular vessels at both afferent and efferent sites, probably via the activation of A2 receptors.

A pilot study to investigate the effects of an infusion of aminophylline on renal function following major abdominal surgery

Acute renal failure is a frequent complication of critical illness and optimal preventive therapy remains elusive. There is increasing evidence from animal models and some human studies that adenosine receptor antagonism by aminophylline may reduce the severity of renal impairment caused by a variety of aetiologies. We studied the renal effects of intravenous aminophylline in an unblinded, within-patient study of 20 patients admitted to a general intensive care unit following major surgery. We demonstrated that there were no adverse cardiovascular complications related to aminophylline therapy. Renal sodium and osmolar clearance increased with a non-significant trend towards increased diuresis during treatment. Creatinine clearance, however, was unchanged but the study was not designed and did not have the power to test whether aminophylline increased renal blood flow or glomerular filtration rate. We suggest the renal actions of aminophylline in critical illness merit further investigation.

Renal interstitial adenosine metabolism during ischemia in dogs

The present study was conducted to determine the metabolism of renal interstitial adenosine under resting conditions and during ischemia. By using a microdialysis method with HPLC-fluorometric analysis, renal interstitial concentrations of adenosine, inosine, and hypoxanthine were assessed in pentobarbital-anesthetized dogs. Average basal renal interstitial concentrations of adenosine, inosine, and hypoxanthine were 0.18 +/- 0.04, 0.31 +/- 0.05, and 0.35 +/- 0.05 micromol/l, respectively. Local inhibition of adenosine kinase with iodotubercidin (10 micromol/l in perfusate) or inhibition of adenosine deaminase with erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA; 100 micromol/l in perfusate) did not change adenosine concentrations in the nonischemic kidneys (0.18 +/- 0.04 and 0.24 +/- 0.05 micromol/l, respectively). On the other hand, treatment with iodotubercidin+EHNA significantly increased adenosine concentration (0.52 +/- 0.07 micromol/l) with significant decreases in inosine and hypoxanthine levels (0.13 +/- 0.03 and 0.19 +/- 0.04 micromol/l, respectively). During 30 min of ischemia, adenosine, inosine, and hypoxanthine were significantly increased to 0.76 +/- 0.29, 2.14 +/- 0.45, and 21.8 +/- 4.7 micromol/l, respectively. The treatment with iodotubercidin did not alter ischemia-induced increase in adenosine (0.84 +/- 0.18 micromol/l); however, EHNA alone markedly enhanced adenosine accumulation (13.54 +/- 2.16 micromol/l), the value of which was not augmented by an addition of iodotubercidin (15.80 +/- 1.24 micromol/l). In contrast, ischemia-induced increases in inosine and hypoxanthine were inversely diminished by the treatment with iodotubercidin+EHNA (0.90 +/- 0.20 and 9.86 +/- 1.96 micromol/l, respectively). These results suggest that both adenosine kinase and adenosine deaminase contribute to the metabolism of renal interstitial adenosine under resting conditions, whereas adenosine produced during ischemia is mainly metabolized by adenosine deaminase and the rephosphorylation of adenosine by adenosine kinase is small.sent

Afferent arteriolar adenosine A2a receptors are coupled to KATP in in vitro perfused hydronephrotic rat kidney

Adenosine is known to exert dual actions on the afferent arteriole, eliciting vasoconstriction, by activating A1 receptors, and vasodilation at higher concentrations, by activating lower-affinity A2 receptors. We could demonstrate both of these known adenosine responses in the in vitro perfused hydronephrotic rat kidney. Thus, 1.0 microM adenosine elicited a transient vasoconstriction blocked by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and 10-30 microM adenosine reversed KCl-induced vasoconstriction. However, when we examined the effects of adenosine on pressure-induced afferent arteriolar vasoconstriction, we observed a third action. In this setting, a high-affinity adenosine vasodilatory response was observed at concentrations of 10-300 nM. This response was blocked by both 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3, 5]triazin-5-yl-amino]ethyl)phenol (ZM-241385) and glibenclamide and was mimicked by 2-phenylaminoadenosine (CV-1808) (IC50 of 100 nM), implicating adenosine A2a receptors coupled to ATP-sensitive K channels (KATP). Like adenosine, 5'-N-ethylcarboxamidoadenosine (NECA) elicited both glibenclamide-sensitive and glibenclamide-insensitive vasodilatory responses. The order of potency for the glibenclamide-sensitive component was NECA > adenosine = CV-1808. Our findings suggest that, in addition to the previously described adenosine A1 and low-affinity A2b receptors, the renal microvasculature is also capable of expressing high-affinity adenosine A2a receptors. This renal adenosine receptor elicits afferent arteriolar vasodilation at submicromolar adenosine levels by activating KATP.

Effects of fructose and xylitol on the urinary excretion of adenosine, uridine, and purine bases

To examine whether fructose and xylitol increase the plasma concentration and urinary excretion of adenosine, as well as uridine and purine bases (hypoxanthine, xanthine, and uric acid), we intravenously administered xylitol and, 2 weeks later, fructose, to five healthy subjects. Analyses of blood and urine samples obtained during these infusion studies demonstrated that fructose increased the urinary excretion of adenosine and uridine 11.9- and 105.5-fold, respectively, and caused only a small increase in the plasma concentrations of uridine and purine bases. It was further demonstrated that xylitol increased the urinary excretion of uridine 58.4-fold, with a marked increase in the plasma concentrations of purine bases and uridine but without an increase in the urinary excretion of adenosine. However, neither infusion increased the plasma concentration of adenosine. These results suggest that in addition to many organs, including the liver, fructose is significantly metabolized by an abrupt adenosine triphosphate (ATP) consumption in the kidney, leading to an increase in the urinary excretion of adenosine and uridine. They also suggest that xylitol is not significantly metabolized in the kidney.

Effects of long-term caffeine consumption on renal function in spontaneously hypertensive heart failure prone rats

Our previous studies supported the hypothesis that prolonged administration of caffeine to animals with high-renin hypertension causes progressive deterioration of renal function. However, thus far this hypothesis has been tested with only a few animal models of hypertension. The aim of this study was to test this hypothesis further by investigating the effects of long-term caffeine consumption on renal function in adult spontaneously hypertensive heart failure (SHHF/Mcc-fa(cp)) rats, another model of high-renin hypertension. Lean, male, 9-month-old SHHF/Mcc-fa(cp) rats were randomized to receive either normal drinking water (control group) or drinking water containing 0.1% caffeine (caffeine group) for 20 weeks. No changes in body weight, food and fluid intake, urine volume, and sodium and potassium excretion were found in conscious SHHF/Mcc-fa(cp) rats after 10 or 20 weeks of caffeine treatment. However, caffeine treatment accelerated the time-related decline in renal function and augmented urinary protein excretion. Ten weeks into the protocol, creatinine clearance was 3.6+/-0.4 and 5.7+/-0.9 L/kg/day in the caffeine group and control group, respectively (p<0.02), whereas 20 weeks into the study, creatinine clearance was similarly diminished in both groups. Proteinuria was greater in the caffeine group compared with the control group at both 10 (928+/-131 vs. 439+/-21 mg/kg/day, respectively; p<0.02) and 20 weeks (1,202+/-196 vs. 603+/-30 mg/kg/day, respectively; p<0.01) into the protocol. After 20 weeks, all animals were anesthetized and instrumented. Caffeine treatment for 20 weeks had no effects on blood pressure, heart rate, or vascular resistance in four examined vascular beds (abdominal aorta and renal, carotid, and mesenteric arteries). No changes in renal hemodynamics and electrolyte excretion were found, whereas significantly lower glomerular filtration rate (GFR; inulin clearance) and creatinine clearance (p<0.05) were observed in caffeine-treated animals. These data support our hypothesis that prolonged consumption of caffeine has adverse effects on renal function, in high-renin hypertension.

Determination of adenosine and deoxyadenosine in urine by high-performance liquid chromatography with column switching

The means of measurement of adenosine and deoxyadenosine in urine was developed by separating adenosine and deoxyadenosine from other compounds using high-performance liquid chromatography with column switchings. This method is simple and convenient since no pretreatment of the urine is needed. Using this method, it could be demonstrated that urinary adenosine was higher in an adenosine deaminase (ADA) deficient patient who had a bone marrow transplant treatment (1.97 micromol/mmol creatinine) and in a heterozygote who had a markedly low erythrocyte ADA activity (1% of control ADA activity) (1.33 micromol/mmol creatinine) as compared to normal subjects (0.22+/-0.09 micromol/mmol creatinine, n=11). It was also noted that urinary deoxyadenosine was below the detection limits in the ADA-deficient bone marrow transplant patient, but it was detected in the heterozygote (3.7 micromol/mmol creatinine). Furthermore, it was also demonstrated that a fructose infusion increased the urinary concentration of adenosine from 0.21+/-0.03 to 2.66+/-1.21 micromol/mmol creatinine in five normal subjects.

Renal circulatory effects of adenosine in patients with chronic heart failure

OBJECTIVES

We sought to study the renal circulatory effects of adenosine in patients with chronic congestive heart failure (CHF).

BACKGROUND

Renal blood flow (RBF) is often reduced in patients with chronic CHF and may lead to decreased renal function. The cause of reduced RBF is multifactorial and involves systemic as well as local vasoregulatory mechanisms. Stimulation of renal adenosine A1 receptors in animal models has resulted in a significant vasoconstriction of afferent and efferent glomerular arterioles and deterioration of renal function. Although adenosine serum levels have been shown to be elevated in patients with CHF, their effect on the renal circulation in this patient population has not been studied.

METHODS

Nine patients with CHF from left ventricular systolic dysfunction were studied. The effects of adenosine at a dose of 10(-5) mol/liter infused directly into the main renal artery on heart rate, renal artery blood pressure, renal artery cross-sectional area (measured by intravascular ultrasound), renal Doppler blood flow velocity (measured by a Doppler flow wire in the renal artery), RBF and renal vascular resistance (RVR) were evaluated.

RESULTS

Infusion of adenosine resulted in no significant effect on heart rate or renal artery blood pressure but caused a substantial increase in RVR (11,204 +/- 1,469 to 31,494 +/- 3,911 dynes x s x cm(-5), p = 0.0005), which led to a marked fall in RBF in every patient (mean values 376 +/- 36 to 146 +/- 22 ml/m2, p = 0.0002). These changes in RVR and RBF were associated with no significant change in renal artery cross-sectional area (0.389 +/- 0.040 to 0.375 +/- 0.033 cm2, p = 0.3).

CONCLUSIONS

Stimulation of renal adenosine receptors in patients with CHF results in marked renal vasoconstriction that leads to an important reduction in RBF. Lack of change in renal artery cross-sectional area suggests that adenosine affects intrarenal resistance blood vessels rather than large conductance vessels. These results may indicate a rationale for investigation of renal adenosine receptor blockade for enhancement of RBF and improvement of renal function in patients with chronic CHF.

Renal haemodynamic responses to exogenous and endogenous adenosine in conscious dogs

1. Adenosine has been suggested to be the mediator of a metabolic feedback mechanism which transfers acute changes in the tubular load into opposite changes in renal blood flow (RBF). The goal of the present experiments was to assess the importance of endogenously formed adenosine as a 'homeostatic metabolite' during short-term changes in metabolic demand. 2. In nine chronically instrumented conscious foxhounds, both the direct effects of adenosine injections (10, 30 and 100 nmol) into the renal artery and the temporal changes of RBF after short renal artery occlusions (15, 30 and 60 s duration), the most widely used experimental model to study the metabolic feedback mechanism in vivo, were studied. 3. Intrarenal bolus injections of adenosine (10, 30 and 100 nmol) induced dose-dependent decreases of RBF (RBF: -34 +/- 5, -59 +/- 4 and -74 +/- 4 %, respectively). This vasoconstrictor effect of adenosine was significantly larger (RBF: -51 +/- 4, -68 +/- 4 and -83 +/- 3 %, respectively) when the dogs received a low salt diet. 4. The post-occlusive responses were characterized by a transient hyperaemia with no detectable drop of RBF below the preocclusion level. The post-occlusive responses were affected neither by changes in local angiotensin II levels, nor by intrarenal infusions of hypertonic NaCl or blockade of A1 adenosine receptors. 5. When intrarenal adenosine levels were elevated by infusion of the adenosine uptake inhibitor dipyridamole, a transient, although weak, post-occlusive vasoconstriction was detected. 6. In summary, the present data demonstrate that adenosine acts as a potent renal vasoconstrictor in the conscious dog. The endogenous production of adenosine during short-lasting occlusions of the renal artery, however, appears to be too small to induce a post-occlusive vasoconstrictor response of RBF. These results suggest that a metabolic feedback with adenosine as 'homeostatic metabolite' is of minor importance in the short-term regulation of RBF in the conscious, unstressed animal.

Dipyridamole prevents diabetes-induced alterations of kidney function in rats

One possible factor for glomerular hyperfiltration in early diabetes could be a deficiency of renal vasoconstrictive mechanisms. Dipyridamole (DIP) inhibits cellular uptake of adenosine thereby increasing interstitial adenosine concentration. The effect of DIP on tubuloglomerular feedback (TGF) and on urinary protein excretion (UPE), glomerular filtration rate (GFR), and kidney weight was studied in early diabetes in rats. One day after onset of streptozotocin (STZ)-induced insulin-dependent diabetes mellitus (IDDM) daily treatment with DIP (50 mg/100 g twice a day via a gastric tube) was started in one group (STZ-DIP) and with vehicle alone in another group (STZ). Rats were housed in metabolic cages for 24 h to measure UPE 7, 14, and 21 days after STZ-injection. Non-diabetic animals, also receiving vehicle, served as controls (CON). While 7, 14, and 21 days after STZ-injection UPE was enhanced by 88, 123, and 153% in the STZ-group (n = 5) as compared to the CON-group (n = 6), the increase in UPE in the STZ-DIP-group (n = 5) was reduced by 82, 66, and 60%, respectively. Subsequently these diabetic rats were prepared for clearance and micropuncture study. Weight-matched (wm) non-diabetic rats served as controls (CONwm). TGF activity was assessed as the difference between stop flow pressures (delta SFP) in the early proximal tubule at 0 and 50 nl/min perfusion rates of Henle's loop. delta SFP was 8.8 +/- 0.7 mmHg (mean +/- SEM) in the CONwm-group (n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of dipyridamole on kidney function in cirrhosis

Adenosine is a potent endogenous renal vasoconstrictor. To investigate the sensitivity of the renal circulation to adenosine in cirrhosis, we evaluated kidney function and vasoactive hormones in 20 patients with cirrhosis before and after administration of dipyridamole (0.4 mg/kg, intravenously), a drug that increases extracellular levels of adenosine. In patients with ascites and increased plasma renin activity (6.9 +/- 4.0 ng/ml.hr [mean +/- S.D.]) (n = 7), dipyridamole induced marked reductions in renal plasma flow (from 623 +/- 294 to 374 +/- 188 ml/min, p = 0.03), glomerular filtration rate (from 89 +/- 22 to 48 +/- 16 ml/min, p = 0.009), urine volume (from 7.1 +/- 2.1 to 1.5 +/- 1.1 ml/min, p = 0.0001), free water clearance (from 4.0 +/- 1.7 to 0.4 +/- 0.6 ml/min, p = 0.001) and sodium excretion (from 28 +/- 36 to 7 +/- 15 mu Eq/min, p = 0.05) in the absence of changes in arterial pressure, plasma renin activity and levels of aldosterone, norepinephrine and antidiuretic hormone. In patients without ascites (n = 5) and in patients with ascites and normal plasma renin activity (0.9 +/- 0.5 ng/ml.hr) (n = 8), renal plasma flow and glomerular filtration rate did not change significantly after dipyridamole administration, whereas excretion of sodium and free water was reduced. These results indicate that in cirrhotic patients with ascites and overactivity of the renin-angiotensin system, dipyridamole induces renal vasoconstriction in the absence of changes in systemic hemodynamics, suggesting that these patients are particularly sensitive to the renal vasoconstrictor effect of endogenous adenosine.

Adenosine, the heart, and coronary circulation

Adenosine is known to regulate myocardial and coronary circulatory functions. Adenosine not only dilates coronary vessels, but attenuates beta-adrenergic receptor-mediated increases in myocardial contractility and depresses both sinoatrial and atrioventricular node activities. The effects of adenosine are mediated by two distinct receptors (i.e., A1 and A2 receptors). A1 adenosine receptors, located in atrial and ventricular myocardium and sinoatrial/atrioventricular nodes, are responsible for inhibition of adenylyl cyclase activity. A2 adenosine receptors, located in coronary endothelial and smooth muscle cells, are responsible for stimulation of this enzyme activity. During increased myocardial oxygen demand due to rapid pacing and exercise, although both coronary blood flow and adenosine concentrations in the myocardium and coronary efflux increased, there is no clear consensus explaining its cause and effect relation at present. However, ischemia/reperfusion-induced coronary hyperemia is believed to be mostly attributed to released adenosine, and it has been proven that adenosine attenuates the severity of ischemia due to its coronary vasodilatory action. The beneficial effects of adenosine during ischemia/reperfusion processes do not seem simple. This is because myocardial ischemia and reperfusion injury is caused by 1) activated leukocytes and platelets, 2) ATP depletion and calcium overload of myocardium, and 3) catecholamine release from the presynaptic nerves as well as 4) the impaired coronary circulation. Intriguingly adenosine attenuates all of these deleterious actions and thereby attenuates ischemia/reperfusion injury. Indeed, adenosine attenuates the severity of contractile dysfunction (myocardial stunning) and limits the infarct size. Thus, administration of adenosine or potentiators of adenosine production in the ischemic myocardium may be beneficial for the attenuation of ischemic and reperfusion injuries, although further clinical investigations are necessary.

Renal blood flow control by tubuloglomerular feedback (TGF) in normal and spontaneously hypertensive rats--a role for dopamine and adenosine

Following the elementary laws of hemodynamics and the functional characteristics of the renal myogenic and macula densa-mediated (TGF) vascular resistance control mechanisms, TGF-mediated changes of renal vascular resistance are amplified by cooperative changes of the myogenic mechanism. Myogenically induced changes, on the other hand, would be antagonized by TGF. Resetting of renal vascular flow resistance by alterations to the TGF mechanisms might thus be more effective than alterations to the myogenic mechanism. Dopamine and adenosine, two autacoids occurring normally in the tubular fluid, may play a key role in operating such a resetting mechanism. Dopamine and adenosine were found in proximal tubular fluid at concentrations of 10(-8) and 0.5 10(-6) M respectively. Dopamine inhibits the tubuloglomerular feedback mechanism, this inhibition is antagonized concentration-dependently by adenosine. These effects most likely occur via D1 and A1 receptors and hence by regulation of the adenyl cyclase activity in the macula densa cells. The balance between adenosine and dopamine in tubular fluid appears to be under the control of extrarenal parameters. In normal rats, high dietary salt intake, by influencing the secretion of an unknown adrenal hormone, and inhibition of Na-K-ATPase might be of importance. In spontaneously hypertensive rats unknown genetic parameters may also play a role.

Adenosine receptors and signaling in the kidney

It is now generally accepted that adenosine is capable of regulating a wide range of physiological functions. Nowhere is the diversity of this action better illustrated than in the kidney. When adenosine binds to plasma membrane receptors on a variety of cell types in the kidney, it stimulates functional responses that span the entire spectrum of renal physiology, including alterations in hemodynamics, hormone and neurotransmitter release, and tubular reabsorption. These responses to adenosine appear to represent a means by which the organ and its constituent cell types can regulate their metabolic demand such that it is maintained at an appropriate level for the prevailing metabolic supply. Extracellular adenosine, produced from the hydrolysis of adenosine 5'-monophosphate and stimulated by increased substrate availability and enzyme induction, acts on at least two types of cell surface receptors to stimulate or inhibit the production of cyclic adenosine-3',5'-monophosphate and also acts in some renal cells to stimulate the production of inositol phosphates and elevation of cytosolic calcium concentration. To understand when and why this complicated system becomes activated, how it interacts with other known extracellular effector systems, and ultimately how to manipulate the system to therapeutic advantage by selective agonists or antagonists, requires a detailed knowledge of renal adenosine receptors and their signaling mechanisms. The following discussion attempts to highlight our knowledge in this area, to present a modified hypothesis for adenosine as a feedback regulator of renal function, and to identify some important questions regarding the specific cellular mechanisms of adenosine in renal cell types.

Further characterization of adenosine transport in renal brush-border membranes

Adenosine transport has been further characterized in rat renal brush-border membranes (BBM). The uptake shows two components, one sodium-independent and one sodium-dependent. Both components reflect, at least partly, translocation via a carrier mechanism, since the presence of adenosine inside the vesicles stimulates adenosine uptake in the presence as well as in the absence of sodium outside the vesicles. The sodium-dependent component is saturable (Km adenosine = 2.9 microM, Vmax = 142 pmol/min per mg protein) and is abolished at low temperatures. The sodium-independent uptake has apparently two components: one saturable (Km = 4-10 microM, Vmax = 174 pmol/min per mg protein) and one non-saturable (Vmax = 3.4 pmol/min per mg protein, Km greater than 2000 microM). Inosine, guanosine, 2-chloroadenosine and 2'-deoxyadenosine inhibit the sodium-dependent and -independent transport, as shown by trans-stimulation experiments, probably because of translocation via the respective transporter. Uridine and dipyridamole inhibited only the sodium-dependent uptake. Other analogs of adenosine showed no inhibition. The kinetic parameters of the inhibitors of the sodium-dependent component were further investigated. Inosine was the most potent inhibitor with a Ki (1.9 microM) less than the Km of adenosine. This suggests a physiological role for the BBM ecto-adenosine deaminase (enzyme which extracellularly converts adenosine to inosine), balancing the amount of nucleoside taken up as adenosine or inosine by the renal proximal tubule cell.

Adenosine metabolism in kidney slices under normoxic conditions

The effect of adenosine in rat kidney under normoxic conditions has been studied. It is demonstrated that adenosine modulates cell nucleotide levels. HPLC analysis of the purine compounds inside the cell indicates that adenosine improves the ATP/ADP ratio, whereas it diminishes the adenine content. This behaviour is not due to mediation by specific receptors, as agonists at P1 purinoceptors did not have any effect. Further evidence using inosine as well as dipyridamole and deoxycoformycin indicates that all effects are dependent on the previous uptake of adenosine. The origin of free adenine in the kidney has been investigated, and it appears to come from the phosphorolysis of 5'-methylthio-adenosine. This report is the first to describe the activity of methylthioadenosine phosphorylase (E.C. 2.4.2.28) in the kidney. It is concluded that 1) extracellular adenosine improves guinea pig renal function by increasing the ATP level and the ATP/ADP ratio; and 2) there exists a functional pathway in the kidney that produces adenine and AMP coming from methionine and ATP. This latter pathway probably produces spermine and spermidine, which are likely to be important for renal function.

Absence of effect of dipyridamole on renal and platelet function in diabetes mellitus

We evaluated the effect of dipyridamole (5 mg/kg/day) for 12 months on renal and platelet function in 53 children with insulin dependent diabetes mellitus (IDDM) in a prospective double blind placebo controlled trial. Urine albumin excretion (expressed as the geometric mean albumin to creatinine concentration ratio (UA/UC) was measured every three months throughout the study. At 12 months, the geometric mean UA/UC was no different in diabetic children receiving dipyridamole, 0.60 mg/mmol, when compared with those receiving placebo, 0.87 mg/mmol. Glomerular filtration rate, urinary excretion of retinol binding protein, and N-acetyl-beta-D-glucosaminidase (NAG), blood pressure, and spontaneous platelet aggregation in response to stirring whole blood did not differ between the two groups at 12 months. Subgroup analysis to include only those children with high UA/UC before entry into the study also failed to show an effect of the drug on UA/UC. Eleven children had either persistently high UA/UC (n = 8: four on dipyridamole, four on placebo) or progression to high UA/UC (n = 3: two on dipyridamole, one on placebo). These children had significantly higher urinary excretion of retinol binding protein and NAG, bigger kidneys, and higher diastolic blood pressure both before and after treatment than the remaining 42 children, whereas there was no difference in spontaneous platelet aggregation between the two groups. These observations on the associations between UA/UC and other parameters of renal function suggest that measurement of 'tubular' proteins and diastolic blood pressure as well as UA/UC may contribute to the identification of those at risk of developing nephropathy.

Platelet-inhibitor treatment of diabetic nephropathy: a 10-year prospective study

We prospectively evaluated a platelet-inhibitor regimen of dipyridamole and aspirin in 28 patients with insulin-dependent diabetes mellitus and well-established nephropathy. After a mean treatment period of 4.3 years, iothalamate clearance (Ciot) was reasonably well maintained and urinary protein excretion was reduced in 7 patients (25%), whereas 21 (75%) had progressive nephropathy. Analysis of outcome revealed that all 7 patients with stable nephropathy and 9 of the 21 with progressive disease had baseline Ciot values that exceeded 50 ml/min per 1.73 m2. Shortened platelet survival improved after 3 months of treatment, and the distribution between patients who had stable and those who had progressive disease was approximately equal. Mean changes in fasting plasma glucose level, glycosylated hemoglobin, and blood pressure did not differ between these two groups. In a short-term protocol, urinary protein and thromboxane B2 significantly declined, whereas variable urinary levels of prostaglandin E2, 6-ketoprostaglandin F1 alpha, and Ciot did not change after 3 months of treatment with dipyridamole and aspirin. These findings suggest that treatment with dipyridamole and aspirin may stabilize renal function by reducing platelet hypersensitivity and production of thromboxanes by platelet or renal tissue (or both). In turn, constrictor activity in the glomerular vessels, mesangial contractility, and glomerular membrane permeability are decreased. These data also add evidence in support of a role for thromboxane A2 in the pathogenesis of experimental and human glomerular disease.

Theophylline in rats during maintenance phase of post-ischemic acute renal failure

We have shown previously that theophylline increases both renal plasma flow (RPF) and glomerular filtration rate (GFR) during the initiation phase of post-ischemic acute renal failure (ARF) in rats. The purpose of the present experiments was to determine the effects of theophylline during the maintenance phase of ARF, five days after initiation. Clearance techniques were used to measure renal function in a control group of pentobarbital anesthetized rats (group C) and in three experimental groups, five days after subjecting the left kidney to a thirty-minute period of complete ischemia. Group SS received saline during both the ischemic episode and the clearance measurements; group ST received saline during ischemia and theophylline, acutely, during the clearance measurements; group TS received theophylline during ischemia and saline during the clearance measurements. In comparison with the values for the control group (group C), RPF and GFR of the post-ischemic left kidneys of group SS were approximately half normal. In groups ST and TS, RPF and GFR of the left kidneys were higher than in group SS. Collectively, these results demonstrate that pretreatment with theophylline during the initiation phase of ischemia-induced ARF leads to increased RPF and GFR during the maintenance phase, and that acute theophylline treatment during the maintenance phase acutely increases RPF and GFR. Since increases in GFR were associated with increases in RPF, and since theophylline is an adenosine receptor antagonist, these results are consistent with the hypothesis that adenosine-mediated hemodynamic changes play a pathogenic role in ischemia-induced ARF in rats.

Effects of dipyridamole on adenosine concentration, insulin sensitivity and glucose utilisation in soleus muscle of the rat

Adenosine has been shown to modulate the sensitivity of skeletal muscle to insulin (Budohoski et al. 1984). In an attempt to further characterize the modulatory action of adenosine on insulin sensitivity in skeletal muscle we have investigated the effect of the nucleoside transport inhibitor dipyridamole in isolated incubated soleus muscle strips. At a concentration of 50 microM, dipyridamole increased the concentration of adenosine in the soleus muscle by 36% and in the incubation medium by 32%. At this concentration of dipyridamole, the basal rates (in the presence of 1 microunit of insulin/ml) of lactate formation, 2-deoxy [2,6-3H]glucose phosphorylation and glucose oxidation were decreased by 48%, 43% and 47% respectively, whilst the rate of glycogen synthesis was unaffected. Insulin-stimulated rates (in the presence of 10,000 microunits of insulin/ml) of lactate formation, 2-deoxy [2,6-3H] glucose phosphorylation, glycogen synthesis and glucose oxidation were decreased by 70%, 30%, 26% and 20% respectively in the presence of 50 microM dipyridamole. Although 50 microM dipyridamole was required to exert a significant effect on medium and soleus muscle adenosine concentrations, statistically significant effects on glycolytic rate were observed at concentrations as low as 2 microM dipyridamole. It is concluded that the results are not consistent with dipyridamole exerting an effect on skeletal muscle carbohydrate metabolism solely through elevation of the intracellular or interstitial adenosine concentration, but strongly suggest that dipyridamole inhibits glucose transport and/or phosphorylation in skeletal muscle.

Elevation of intrarenal adenosine by maleic acid decreases GFR and renin release

Maleic acid administration produces a defect in tubular reabsorption resembling that seen in the Fanconi syndrome and also causes a decrease in glomerular filtration rate (GFR). The mechanism by which maleic acid alters renal function is uncertain, though the tubular defect is known to be associated with decreased ATP levels. Because of this alteration in nucleotide metabolism the present study was undertaken to determine the role of elevated endogenous adenosine in mediating the maleic acid-induced changes in renal function. Since the renal effects of exogenous adenosine are enhanced by sodium-depletion and attenuated by sodium-loading, the present study compared the time course of the effects of maleic acid on renal function in 10 dogs maintained on a low sodium diet, and 10 dogs maintained on a high sodium diet. In addition, we examined the effect of maleic acid on adenosine levels in renal venous plasma, on the urinary excretion of adenosine, and the effect of the adenosine antagonist, theophylline, on the maleic acid-induced changes in renal function. After 100 min of maleic acid, GFR was decreased significantly by 55 +/- 4% of control in the sodium-depleted dogs, and by 39 +/- 4% of control in the sodium-loaded dogs. In the sodium-depleted dogs, renin release was also significantly depressed (12 +/- 8% of control) during the infusion of maleic acid. The fractional excretion of sodium was significantly increased in both groups. The renal venous concentration of adenosine and the urinary excretion of adenosine were both significantly increased during maleic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

How does dipyridamole elevate extracellular adenosine concentration? Predictions from a three-compartment model of adenosine formation and inactivation

Steady-state mathematical models are developed according to which adenosine is formed in the cytoplasm of a group of cells, arises in the extracellular space via the symmetric nucleoside transporter and is inactivated in the adenosine forming cells and after rate-limiting transport into other cell-types. Dipyridamole increases the Km and Vmax. of the transporter symmetrically with respect to influx and efflux. Models incorporating differing degrees of compartmentation are used to predict intracellular and extracellular adenosine concentration as a function of dipyridamole concentration and adenosine formation rate. The vasodilator action of dipyridamole is explained since it is predicted to elevate interstitial fluid adenosine concentrations at all rates of adenosine formation provided that washout of the interstitial compartment is restricted.