NDT Advance Access originally published online on March 7, 2006
Nephrology Dialysis Transplantation 2006 21(7):1961-1965; doi:10.1093/ndt/gfl082
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© The Author [2006]. Published by Oxford University Press on behalf of ERA-EDTA. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
Original Articles: Dialysis and Transplantation
Adenosine receptor antagonism in acute tacrolimus toxicity
Department of Pediatrics, Divisions of Critical Care Medicine University of Miami, Miami, FL, USA
Correspondence and offprint requests to: Gwenn McLaughlin, MD, PO Box 016960 (R-131), University of Miami Miller School of Medicine, Miami, FL 33130, USA. Email: gmclaugh{at}med.miami.edu
| Abstract |
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Background. Calcineurin inhibitors induce renal vasoconstriction and oliguria during acute toxicity. We previously demonstrated that the non-specific adenosine receptor antagonist theophylline improved glomerular filtration rate (GFR) and renal blood flow in the setting of acute tacrolimus (TAC) toxicity. This study was undertaken to determine which of the known adenosine receptor subtypes is responsible for the observed effect of theophylline.
Methods. The GFR was measured by clearance of 51Cr-EDTA in anaesthetized, instrumented SpragueDawley rats at three time points: at baseline, 60 min after intravenous administration of TAC (0.05 mg/kg) or vehicle (V) and at 100 min after TAC or V. Either DMSO (n = 5) or one of the three available specific adenosine receptor subtype antagonists 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 2 mg/kg, n = 5), a selective A1 receptor antagonist, 8-(3-chlorostyryl) caffeine (CSC, 2 mg/kg, n = 4), a selective A2a receptor antagonist and 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-dihydropyridine-3,5 dicarboxylate (MRS1191, 1 mg/kg, n = 5), a selective A3 receptor antagonist, was administered intra-peritoneally prior to the final GFR measurement. Repeated measures analysis of variance was used to detect differences between groups (P<0.05).
Results. Measured GFR declined by 30% from baseline 60 min after TAC. In DMSO treated animals, GFR decreased 51% from baseline at 100 min after TAC, but increased 45% from baseline at 100 min after TAC + MRS1191.
Conclusions. Only administration of the A3 adenosine antagonist increased GFR following TAC, suggesting that this receptor mediates the effect of theophylline on GFR.
Keywords: glomerular filtration rate; adenosine antagonist; tacrolimus; prograf; calcineurin inhibitor; adenosine; receptors; purinergic
Tacrolimus (TAC) is a potent immunosuppressant which has significantly advanced the success of transplantation through control of rejection episodes; however, both the calcineurin inhibitor and its predecessor cyclosporine A induce renal vasoconstriction [1]. Adenosine is one reputed mediator of this effect of calcineurin inhibition as it is elevated in the blood of patients receiving cyclosporine A [2]. In several types of cultured cells, incubation with calcineurin inhibitors blocks the uptake of adenosine through inhibition of adenylyl cyclase activity [3,4]. Increased adenosine has been implicated in various nephropathies associated with vasoconstriction including those induced by contrast media, amphotericin and cisplatin [2,5]. Adenosine receptors are found throughout the body and play important regulatory roles in the control of vasomotor tone and inflammatory processes [6]. In the kidney, adenosine acutely constricts pre-glomerular vessels via A1 receptor activation while dilating the efferent vessels by A2a receptor activation [6,7]. Both A1 and A2a receptors are G-coupled proteins that regulate adenylyl cyclase activity albeit in opposite directions [6]. Activation of A3 receptors, like the activation of A1 receptors increases cyclic AMP, but unlike A1 receptors increases intracellular calcium promoting vasoconstriction [8].
We previously demonstrated that the administration of theophylline, a non-specific adenosine receptor antagonist, ameliorated the TAC-induced decreases in renal blood flow (RBF) and glomerular filtration rate (GFR) seen in rats [9]. We also demonstrated that the addition of theophylline to standard loop diuretic therapy improves diuresis in children with acute TAC toxicity [10]. In the present study, available specific adenosine receptor antagonists were administered to rats after acute administration of TAC in order to identify which receptor(s) was (were) responsible for the vasoconstrictor effect of TAC and the vasodilator and diuretic effects of theophylline.
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Materials and reagents
The TAC was obtained in powdered form as a gift from Fujisawa Healthcare USA (Deerfield, IL). The powder was dissolved in a 1:3 solution of ethanol and Tween 80 to a concentration of 3 mg/ml. Just before administration the TAC solution was further diluted into 0.9% saline to a final concentration of 0.3 mg/ml. A similarly diluted 3:1 EthanolTween 80 solution was used as vehicle (V) for control studies. The three specific adenosine receptor subtypes were 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a selective A1 receptor antagonist, 8-(3-chlorostyryl) caffeine (CSC), a selective A2a receptor antagonist and 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-dihydropyridine-3,5 dicarboxylate (MRS1191), a selective A3 adenosine receptor antagonist. Each was obtained from Sigma-Aldrich chemical (St Louis, MO) in the powdered form and stored as per the manufacturer's recommendation. Prior to use each was dissolved in the vehicle DMSO.
Animal preparation
All methods were described previously [9]. Briefly the rats were allowed food and water ad libitum. They were anaesthetized with isoflorane 1.5% and artificial ventilatory via tracheostomy to normal blood gases. The rectal temperature was kept at
38°C with radiant heat. Catheters implanted in the carotid artery, internal jugular and femoral vein were used for infusions, arterial blood sampling and continuous monitoring of mean arterial pressure and heart rate. The left kidney was exposed through a midline incision extending to the flank, and the left ureter was cannulated with polyethylene 10 tubing for urine collection into pre-weighed vials. Continuous free flow of clear urine was directly visualized. Heat and fluid loss was reduced by plastic wrap covering the rats and isoflorane was decreased to 0.5%.
Experimental protocol
Animals were divided into four subgroups who received either the adenosine receptor antagonist vehicle DMSO, DPCPX (2 mg/kg), CSC (2 mg/kg) and MRS1191 (1 mg/kg). These doses were used in the previously published studies to reverse receptor subtype activity when given by the intraperitoneal route [1117]. These adenosine receptor antagonists do not produce haemodynamic effects [11]. Due to concerns about the shelf-life of each reagent these experiments were conducted in series. After assignment to a specific adenosine receptor antagonist, animals were randomly assigned to receive either TAC or V (identical volume). Single kidney GFR was measured as described by Leyssac and Christensen [18] at baseline, for 40 min starting 1 h after TAC administration, and after administration of one of the three adenosine receptor subtype antagonists or DMSO. After completion of all surgical procedures, the animals received a loading dose of 90 µCi 51Cr EDTA in 1.5 ml of saline followed by a continuous intravenous infusion of 250 µCi mixed in 20 ml of saline infused at the rate of 6 ml/h. After an equilibration period of 60 min, 0.5 ml of blood was collected from the left femoral artery at the start and completion of a 20 min-timed urine collections into pre-weighed plastic vials. Thereafter, 0.5 mg/kg intravenous TAC was given slowly over 5 min. After 60 min, two 20 min-timed urine collections were obtained followed immediately thereafter by intraperitoneal injection of DMSO or an adenosine receptor subtype antagonist followed by two additional 20 min-timed urine collections. Again blood was collected as described above. At the end of the experiment the kidney was removed, drained and weighed. Blood samples were centrifuged at 10,000 revs/min for 10 min and 100 µl of plasma was assayed for radioactivity. Urine vials were weighed at the end of the experiment and the difference from baseline vial weights recorded as urine volume. These were also assayed for radioactivity. Renal clearance of 51Cr EDTA was calculated according to the conventional expression: Cs = (U/P)s x (Vu/KW)(ml/min/g KW) in which (U/P)s denotes the urine-to-plasma concentration ratio of the substance (s), or 51Cr-EDTA and Vu/KW denotes rate of urine flow divided by the kidney weight (KW). The average radioactivity of the plasma sampled at the start and completion of each collection period was used as Ps. The average of the two 20 min collections is reported for post-TAC and post-adenosine receptor antagonist results. Urine flow was calculated as ml/min/100 g of body weight. Blood gas determinations were performed with the pH/blood gas analyser.
All results are expressed as mean±SEM. The data were analysed using one-way analysis of variance (ANOVA) for repeated measurements. Post-hoc testing was performed using Tukey's comparisons. A P-value <0.05 was considered statistically significant.
| Results |
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The three groups were similar for weight and all baseline data. The mean arterial pressure and blood gas data are shown in the Table 1. There were relatively minor but statistically significant differences between groups in PaO2, but no animal experienced hypoxia. The adenosine receptor antagonists did not produce haemodynamic changes. Haemoglobin did not differ between groups or over time (data not shown).
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The GFR data is displayed in Figure 1. In the control group (V + DMSO), GFR did not change over time. The GFR, over time, in TAC treated animals was significantly different from normal controls. The GFR declined by 30 and 51% at 60 and 100 min after TAC, respectively. TAC resulted in a significant drop in GFR from baseline at 1 h in all the TAC treated groups except in those animals subsequently treated with the A3 antagonist MRS1191 (P = 0.051). In animals treated with CSC and DPCPX, a further decline in GFR at 100 min was not observed. The GFR increased significantly following MRS1191 administration to 145% of the baseline value.
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The urine flow data is displayed in Figure 2. Urine flow exhibited greater variability in general, therefore no statistical differences were detected. Administration of DPCPX did produce a trend toward increased diuresis in TAC-treated animals (P = 0.09).
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| Discussion |
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In a previous study, we demonstrated that the methylxanthine theophylline, a non-specific adenosine receptor antagonist reversed parallel changes in RBF and GFR observed in response to intravenous TAC in rats. We initially hypothesized that the A1 receptor antagonist, DPCPX, would reduce vasoconstriction in response to TAC. This agent has been shown to reverse hypoxia-induced vasoconstriction in isolated perfused rabbit kidney [19]. However, DPCPX did not fully prevent the fall in GFR, whereas theophylline was fully protective. These findings are consistent with a similar study after cyclosporine A exposure demonstrating that DPCPX attenuated the fall in RBF measured by electromagnetic flow meter by 57%, but was less effective than theophylline [20]. This data suggests that A1 receptor activation alone does not explain the decrease in RBF observed after TAC administration.
In children with oliguria, due to elevated TAC concentrations, low dose aminophylline promoted a brisk diuresis [10]. Intact adenosine A1 receptors are required for the diuretic and natriuretic effects attributed to methylxanthines [21,22]. Administration of FK 838, another A1 adenosine antagonist in chronically instrumented awake rats promoted a brisk diuresis, but no significant change in GFR [23]. A trend toward an increased urine flow was observed after DPCPX in both TAC- and V-treated animals. This suggests that theophylline enhances diuresis in children with TAC toxicity through its A1 adenosine antagonist property.
Because A2a agonists reportedly increase GFR and RBF through enhanced medullary vasodilation [24,25], we anticipated that A2a receptor inhibition would potentiate the vasoconstriction observed in response to TAC. Furthermore, in an animal model of renal ischaemia-reperfusion, A2a receptor activation prevents renal impairment through modulation of inflammation [6,24,25]. However, we observed no significant response in either GFR or urine volume in response to A2a receptor blockade. This may be explained by the fact that A2a receptors are only expressed at very low levels in the pre-glomerulus where TAC-induced vasoconstriction occurs [26].
Whereas A1 and A2a receptors are probably activated under normal physiological conditions, A3 receptors require relatively higher levels of adenosine for activation and therefore may be more important in pathological conditions [8]. To date A3 receptors have not been as well-studied as the A1 and A2a subtypes. In cultured renal epithelial cells, A3 receptor blockade with MRS 1191, but not A1 or A2a receptor inhibition, prevented calcium influx [27]. Calcineurin inhibition results in intracellular calcium influx which may be further aggravated by A3 receptor activation or ameliorated by A3 receptor antagonism. Lee and colleagues [11,28] have published a body of work which suggests that A3 receptor antagonism is renal protective in rats and mice. Using both agonists and antagonists, they evaluated several adenosine subtype receptors and identified that A3 receptor activation decreased creatinine clearance, while A3 receptor inhibition improved creatinine clearance in renal ischaemia [11]. They also demonstrated that knockout mice lacking A3 receptors are protected against both ischaemia and myoglobin-induced renal failure [28]. Our finding that A3 receptor antagonism improves GFR following TAC-induced vasoconstriction is consistent with these investigations.
Chronic renal failure is a consequence of long-term exposure to calcineurin inhibition which in turn contributes to morbidity and mortality from cardiovascular disease [29]. It is plausible that prevention of renal ischaemia can delay renal failure; however, there is conflicting available data. Several authors demonstrated that while acute non-specific adenosine receptor antagonism with theophylline may prevent renal vasoconstriction, there is no significant benefit to creatinine clearance over several weeks [19,3032]. This may be due to the diuretic effect of theophylline mediated through A1 receptor inhibition. Sodium depletion aggravates calcineurin inhibitor-induced renal insufficiency and therefore, the sodium and water loss induced by A1 receptor antagonism is undesirable [33]. In fact, Lee et al. [34] demonstrated that ischaemic reperfusion injury worsened in A1 receptor knockout mice and DPCPX-treated wild-type mice as evidenced by higher serum creatinine and worse histology. In contrast, A3 receptor antagonism which improves GFR without promoting sodium excretion may be renoprotective in TAC-induced nephropathy.
Application of the results of this study to patient care should proceed cautiously for several reasons. First, unopposed high-dose A3 receptor antagonists induce apoptosis in cultured cell lines, but low doses appear cytoprotective [8]. Second, the appropriate dose of MRS1191 in humans is unknown at the present time. Third, the ubiquitous nature of adenosine receptors in vivo raises concerns about the effects of A3 receptor inhibition on non-renal organs [8]. Nevertheless, research to address these concerns should proceed more rapidly, particularly because A3 receptor inhibition has no apparent haemodynamic effect and can therefore be tolerated in states of hypoperfusion.
Conflict of interest statement. None declared.
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[Abstract/Free Full Text]
Accepted in revised form: 7. 2.06
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