Amelioration of glycerol-induced acute renal failure in the rat with 8-cyclopentyl-1,3-dipropylxanthine

Protection against acute kidney injury via A(1) adenosine receptor-mediated Akt activation reduces liver injury after liver ischemia and reperfusion in mice

Hepatic ischemia reperfusion (IR) injury causes acute kidney injury (AKI). However, the contribution of AKI to the pathogenesis of liver IR injury is unclear. Furthermore, controversy still exists regarding the role of A(1) adenosine receptors (A(1)ARs) in AKI. In this study, we determined whether exogenous and endogenous A(1)AR activation protects against AKI with subsequent liver protection after hepatic IR in mice. We found that after hepatic IR A(1) knockout (KO) mice and A(1)AR antagonist-treated A(1) wild-type (WT) mice developed worse AKI and liver injury compared with vehicle-treated A(1)WT mice. Moreover, a selective A(1)AR agonist protected against hepatic IR-induced AKI and liver injury in A(1)WT mice. Renal A(1)AR-mediated kidney protection plays a crucial role in protecting the liver after IR because: 1) selective unilateral renal lentiviral overexpression of human A(1)ARs [enhanced green fluorescent protein (EGFP)-huA(1)AR] in A(1)KO mice protected against both kidney and liver injury sustained after liver IR, 2) removal of the EGFP-huA(1)AR lentivirus-injected kidney from A(1)KO mice abolished both renal and hepatic protection after liver IR, and 3) bilateral nephrectomy before hepatic ischemia abolished the protective effects of A(1)AR activation in A(1)WT mice. Finally, inhibition of Akt, but not extracellular signal-regulated kinase mitogen-activated protein kinase, prevented the kidney and liver protection afforded by A(1)AR agonist treatment. Taken together, we show that endogenous and exogenous activation of renal A(1)ARs protect against liver and kidney injury after liver IR in vivo via pathways involving Akt activation.

Adenosine receptors and the kidney

The autacoid, adenosine, is present in the normoxic kidney and generated in the cytosol as well as at extracellular sites. The rate of adenosine formation is enhanced when the rate of ATP hydrolysis prevails over the rate of ATP synthesis during increased tubular transport work or during oxygen deficiency. Extracellular adenosine acts on adenosine receptor subtypes (A(1), A(2A), A(2B), and A(3)) in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate by constricting afferent arterioles, especially in superficial nephrons, and thus lowers the salt load and transport work of the kidney consistent with the concept of metabolic control of organ function. In contrast, it leads to vasodilation in the deep cortex and the semihypoxic medulla, and exerts differential effects on NaCl transport along the tubular and collecting duct system. These vascular and tubular effects point to a prominent role of adenosine and its receptors in the intrarenal metabolic regulation of kidney function, and, together with its role in inflammatory processes, form the basis for potential therapeutic approaches in radiocontrast media-induced acute renal failure, ischemia reperfusion injury, and in patients with cardiorenal failure.

Regional haemodynamic responses to adenosine receptor activation vary across time following lipopolysaccharide treatment in conscious rats

BACKGROUND AND PURPOSE

Studies using adenosine receptor antagonists have shown that adenosine-mediated vasodilatations play an important role in the maintenance of regional perfusion during sepsis, but it is unclear whether vascular sensitivity to adenosine is affected. Here, we assessed regional haemodynamic responses to adenosine agonists and antagonists in normal and lipopolysaccharide (LPS)-treated rats to investigate a possible role for adenosine in the haemodynamic sequelae.

EXPERIMENTAL APPROACH

Male Sprague-Dawley rats were chronically instrumented with pulsed Doppler flow probes to measure regional haemodynamic responses to adenosine-receptor agonists (adenosine, 2-choloro-N6-cyclopentyladenosine (CCPA)) and antagonists (8-phenyltheophylline (8-PT), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX)), at selected time points in control and LPS-treated rats.

KEY RESULTS

The responses to 8-PT were consistent with endogenous adenosine causing bradycardia, and renal and hindquarters vasodilatation in control rats, whereas in LPS-treated rats, there was evidence for endogenous adenosine causing renal (at 1.5 h) and hindquarters (at 6 h) vasoconstriction. In control animals, exogenous adenosine caused hypotension, tachycardia and widespread vasodilatation, whereas in LPS-treated rats, the adenosine-induced renal (at 1.5 h) and hindquarters (at 6 h) vasodilatations were abolished. As enhanced A1 receptor-mediated vasoconstriction could explain the results in LPS-treated rats, vascular responsiveness to a selective A1-receptor agonist (CCPA) or antagonist (DPCPX) was assessed. There was no evidence for enhanced vasoconstrictor responsiveness to CCPA in LPS-treated rats, but DPCPX caused renal vasodilatation, consistent with endogenous adenosine mediating renal vasoconstriction under these conditions.

CONCLUSIONS AND IMPLICATIONS

The results show changes in adenosine receptor-mediated cardiovascular effects in endotoxaemia that may have implications for the use of adenosine-based therapies in sepsis.

Lack of effect of extracellular adenosine generation and signaling on renal erythropoietin secretion during hypoxia

Previous studies have yielded conflicting results as to whether extracellular adenosine generation and signaling contributes to hypoxia-induced increases in renal erythropoietin (EPO) secretion. In this study, we combined pharmacological and genetic approaches to elucidate a potential contribution of extracellular adenosine to renal EPO release in mice. To stimulate EPO secretion, we used murine carbon monoxide exposure (400 and 750 parts per million CO, 4 h), ambient hypoxia (8% oxygen, 4 h), or arterial hemodilution. Because the ecto-5-nucleotidase (CD73, conversion of AMP to adenosine) is considered the pacemaker of extracellular adenosine generation, we first tested the effect of blocking extracellular adenosine generation with the specific CD73-inhibitor adenosine 5'-(alpha,beta-methylene) diphosphate (APCP) or by gene-targeted deletion of cd73. These studies showed that neither APCP-treatment nor targeted deletion of cd73 resulted in changes of stimulated EPO mRNA or serum levels, although the increases of adenosine levels in the kidney following CO exposure were attenuated in mice with APCP treatment or in cd73(-/-) mice. Moreover, pharmacological studies using specific inhibitors of individual adenosine receptors (A1 AR, DPCPX; A 2A AR, DMPX; A 2B AR, PSB 1115; A3AR, MRS 1191) showed no effect on stimulated increases of EPO mRNA or serum levels. Finally, stimulated EPO secretion was not attenuated in gene-targeted mice lacking A1A(-/-, A2A AR-/-, A2BAR(-/-), or A3AR-/-. Together, these studies combine genetic and pharmacological in vivo evidence that increases of EPO secretion during limited oxygen availability are not affected by extracellular adenosine generation or signaling.

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.

The role of adenosine in the early respiratory and cardiovascular changes evoked by chronic hypoxia in the rat

Experiments were performed on anaesthetized normoxic (N) rats and chronically hypoxic rats that had been exposed to 12% O2 for 1, 3 or 7 days (1, 3 or 7CH rats). The adenosine A1 receptor antagonist DPCPX did not affect the resting hyperventilation of 1-7CH rats breathing 12% O2 and increased resting heart rate (HR) in 1CH rats only. DPCPX partially restored the decreased baseline arterial pressure (ABP) and increased femoral vascular conductance (FVC) of 1 and 3CH rats, but had no effect in N or 7CH rats. DPCPX also attenuated the decrease in arterial blood pressure (ABP) and increase in FVC evoked by acute hypoxia in N and 1-7CH rats. The non-selective adenosine receptor antagonist 8-SPT had no further effect on baselines or cardiovascular responses to acute hypoxia, but attenuated the hypoxia-evoked increase in respiratory frequency in 1-7CH rats. In N, and 1 and 3CH rats, the inducible nitric oxide synthase (iNOS) inhibitor aminoguanidine had no effect on baselines or increases in FVC evoked by acetylcholine. We propose: (i) that tonically released adenosine acting on A1 receptors reduces HR in 1CH rats and stimulates endothelial NOS in 1 and 3CH rats to decrease ABP and increase FVC, the remaining NO-dependent tonic vasodilatation being independent of iNOS activity; (ii) that in 7CH rats, tonic adenosine release has waned; (iii) that in 1-7CH rats, adenosine released by acute hypoxia stimulates A1 but not A2 receptors to produce muscle vasodilatation, and stimulates carotid body A2 receptors to increase respiration.

The extracellular cyclic AMP-adenosine pathway in renal physiology

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

Chronic hypoxia up-regulates expression of adenosine A1 receptors in DDT1-MF2 cells

As the first step to understand how chronic hypoxia might regulate smooth muscle function in health and disease, we have employed an established immortalised cell model of smooth muscle, DDT1-MF2 cells, to address the hypothesis that adenosine A1 receptor density is modulated by O2 availability. Maximal specific binding (Bmax) of the selective adenosine A1 receptor antagonist, [3H]-DPCPX, to cell membranes increased 3.5-fold from 0.48 +/- 0.02 pmol/mg to 1.7 +/- 0.5 pmol/mg protein after 16 hr of hypoxia and this effect was not accompanied by any statistically significant changes in either binding affinity (0.84 +/- 0.2 nM vs. 1.2 +/- 0.3 nM) or Hill coefficient (1.1 +/- 0.1 vs. 0.99 +/- 0.03). Hypoxia-evoked increases in membrane receptor density were paralleled in intact DDT1-MF2 cells. In addition, the increase in [3H]-DPCPX binding to intact cells was inhibited by co-incubation during hypoxia with the translational inhibitor cycloheximide, the transcriptional blocker actinomycin D and the NFkappaB inhibitor sulphasalazine. Together, these data show that adenosine A1 receptor density is modulated, at least in part, by O2-dependent activation of the transcription factor NFkappaB and adds to the list of processes dynamically regulated by ambient oxygen availability. Since hypoxia is an initiating factor in acute renal failure, similar changes in transcription may account for up-regulation of adenosine A1 receptors noted previously in the renal vasculature of rats with acute renal failure.

A1 adenosine receptor knockout mice exhibit increased renal injury following ischemia and reperfusion

Controversy exists regarding the effect of A1 adenosine receptor (AR) activation in the kidney during ischemia and reperfusion (I/R) injury. We sought to further characterize the role of A1 ARs in modulating renal function after I/R renal injury using both pharmacological and gene deletion approaches in mice. A1 AR knockout mice (A1KO) or their wild-type littermate controls (A1WT) were subjected to 30 min of renal ischemia. Some A1WT mice were subjected to 30 min of renal ischemia with or without pretreatment with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) or 2-chrolo-cyclopentyladenosine (CCPA), selective A1 AR antagonist and agonist, respectively. Plasma creatinine and renal histology were compared 24 h after renal injury. A1KO mice exhibited significantly higher creatinines and worsened renal histology compared with A1WT controls following renal I/R injury. A1WT mice pretreated with the A1 AR antagonist or agonist demonstrated significantly worsened or improved renal function, respectively, after I/R injury. In addition, A1WT mice pretreated with DPCPX or CCPA showed significantly increased or reduced markers of renal inflammation, respectively (renal myeloperoxidase activity, renal tubular neutrophil infiltration, ICAM-1, TNF-alpha, and IL-1beta mRNA expression), while demonstrating no differences in indicators of apoptosis. In conclusion, we demonstrate that endogenous or exogenous preischemic activation of A1 ARs protects against renal I/R injury in vivo via mechanisms leading to decreased necrosis and inflammation.

Nephroprotective effects of pentoxifylline in experimental myoglobinuric acute renal failure

The nephroprotective effects of pentoxifylline, a methylxantine, were studied in glycerol-induced acute renal failure. Glycerol treated rats exhibited collecting duct and medullary ascending limb dilation and casts, with focal tubular damage, confined mainly to the superficial cortex. In the interstitium focal mononuclear infiltration was observed. In some glomeruli there was swelling of mesangial spaces and mesangial cells. Pentoxifylline injected to glycerol pretreated rats exerted a protective effect. Only few groups of proximal tubules in the subcapsulary region of renal cortex showed necrosis and tubulorhexis. There were not leukocyte infiltrations or vascular congestion. Morphometric analysis showed increased surface area fraction of tubular lumen in rats treated with glycerol (p < 0.01) compared to those in controls. Intratubular cast formations in rats treated with glycerol alone were significantly higher than in rats given pentoxifylline in addition to glycerol. Kidney cortex ectopeptidases (APA, APN and DPP IV) were not significantly changed after glycerol administration. Serum creatinine and blood urea were markedly increased in glycerol treated rats, however, pentoxifylline reduced significantly their levels. This study in glycerol-induced acute renal failure showed a marked renal morphologic and functional protection by pentoxifylline.

Role of the extracellular cAMP-adenosine pathway in renal physiology

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

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.

A3 receptors mediate rapid inflammatory cell influx into the lungs of sensitized guinea-pigs

BACKGROUND

Inhaled adenosine causes bronchoconstriction in asthmatics and may modulate inflammatory cell activity. Elevated adenosine levels occur in the lungs after antigen challenge of asthmatics.

OBJECTIVES

The aim of this study was to investigate whether the bronchoconstrictor effects of the adenosine derivative, 5'-AMP, were associated with altered migration of inflammatory cells into the airways using a sensitized atopic guinea-pig model previously shown to display a bronchoconstrictor response. Comparisons were made with the effects of inhaled antigen.

METHODS

Airway responses of conscious sensitized guinea-pigs to inhalation exposures of 5'-AMP were determined by whole body plethysmography as the change in specific airway conductance (sGaw). Influx of leucocytes into the airways was determined by bronchoalveolar lavage (BAL).

RESULTS

5'-AMP caused bronchoconstrictor airway responses in sensitized animals. Dose-dependent infiltration of inflammatory cells into the lungs occurred 1 h after 5'-AMP exposure. No bronchoconstriction or cell influx was seen in unsensitized guinea-pigs. Exposure to ovalbumin (OA) also caused influx of inflammatory cells. Twenty-four hours after an OA exposure, 5'-AMP produced no bronchoconstriction. The P1-receptor antagonists, 8-PT and 8-SPT, inhibited the 5'-AMP-induced bronchoconstriction, indicating that the bronchoconstriction seen in sensitized animals is mediated by A1 or A2 receptors. They had no effect on the cell influx, whereas the A3 antagonist, MRS-1220, significantly inhibited cellular infiltration, suggesting mediation through A3 receptors. At 24 h after an OA challenge and accompanying the cellular influx, there was airway hyper-responsiveness to the bronchoconstriction by histamine. In contrast, no hyper-responsiveness to histamine was seen 1 h after 3 mM or 24 h after 300 mM 5'-AMP.

CONCLUSIONS

5'-AMP caused a rapid migration of eosinophils and macrophages into the airways only in sensitized guinea-pigs, and this was blocked by the A3 antagonist MRS-1220. This was not associated with bronchial hyper-reactivity to histamine.

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

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

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

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

Differential expression of renal adenosine A(1) receptors induced by acute renal failure

The distribution of renal adenosine A(1) receptors was investigated in rats with glycerol- or mercuric chloride (HgCl(2))-induced acute renal failure. Receptors were localised by autoradiography using [(3)H]8-cyclopentyl-1,3-dipropylxanthine ([(3)H]DPCPX), a selective A(1) adenosine receptor antagonist. In saline-injected control animals, significant labelling with [(3)H]DPCPX was detected in glomeruli, the inner stripe of outer medulla, and the inner medulla. Sixteen hours following induction of glycerol-induced acute renal failure (ARF), a 34% increase in labelling in glomeruli was noted compared to saline-injected controls, and by 48 hr, glomerular labelling had increased by 200%. In addition, 48 hr following glycerol injection, significant labelling was now detected in the cortical labyrinth and medullary rays whilst, in the inner medulla, labelling had decreased by 34%. By contrast to glycerol-induced ARF, the only significant change noted 48 hr following induction of HgCl(2)-induced ARF was a 39% decrease in labelling in the inner medulla. It is concluded that glycerol-induced ARF results in differential expression of renal adenosine A(1) receptors with increased expression in the cortex and reduced expression in the inner medulla. Increased density of A(1) receptors in glomeruli may account, at least in part, for the increased renal vasoconstrictor response to adenosine and depressed glomerular filtration rate noted previously in this type of acute renal failure.

No participation of adenosine A1 receptor in acute nephrotoxicity by 4-pentenoic acid administration in dogs

Intrarenal infusion of 4-pentenoic acid is known to lower renal cortical ATP content and cause a reduction in glomerular filtration rate (GFR). The alteration in nucleotide metabolism might augment the production of adenosine, thereby eliciting the fall in GFR. This study was conducted to examine whether 4-pentenoic acid stimulates renal production of adenosine, and if so, to examine the role of adenosine A1 receptor in the reduction of GFR by 4-pentenoic acid. With infusion of 4-pentenoic acid (1 micromol x kg(-1) x min(-1)) into the renal artery of anesthetized dogs, GFR gradually decreased and reached minimum at 60 min with values ranging from 33.9+/-2.2 to 20.2+/-2.8 ml/min. Neither renal blood flow nor mean arterial pressure was affected, but tubular reabsorption of water and sodium was significantly attenuated. Renal venous plasma concentration and urinary excretion of adenosine rose markedly (20-fold) without any change in arterial concentration, suggesting that renal adenosine production was augmented by 4-pentenoic acid. However, KW-3902 (8-(noradamantan-3-yl)-1,3-dipropylxanthine), a selective antagonist of the adenosine A1 receptor, did not affect the action of 4-pentenoic acid on GFR or renal handling of water and sodium. It is concluded that 4-pentenoic acid markedly increases renal adenosine production, but adenosine A1 receptor is not involved in the 4-pentenoic acid-induced nephrotoxicity.

Adenosine-induced renal vasoconstriction in diabetes mellitus rats: role of nitric oxide

In rats with streptozotocin (STZ)-induced diabetes, the renal vasoconstrictor effect of adenosine is enhanced. We investigated the role of nitric oxide (NO) in the renal vascular response to exogenous and endogenous adenosine in control and STZ diabetic rats. Exogenous adenosine (0.01-100 nmol) injected into the abdominal aorta decreased renal blood flow (RBF) in a dose-dependent manner to a much greater extent in STZ rats than in control rats (P < 0.001). Inhibition of NO synthesis with Nomega-nitro-L-arginine (L-NNA, 30 micromol/kg iv) and with renal perfusion pressure controlled potentiated the adenosine-induced renal vasoconstriction to a significantly greater extent in control rats than in STZ rats. In control rats, L-NNA shifted the dose-response curve of exogenous adenosine-induced RBF reductions to the left by a factor of 32 [half-maximal effective dose (ED50), from 5.5 to 0.17 nmol adenosine, n = 6] and in STZ rats only by a factor of 4.6 (ED50, from 0.32 to 0.07 nmol adenosine, n = 6). The renal response to endogenous adenosine was assessed by the magnitude of the postocclusive reduction of RBF (POR) after a 30-s renal artery occlusion. POR was markedly enhanced in STZ rats (-67.8 +/- 3.8%, P < 0.001) compared with control rats (-38.8 +/- 4.3%). L-NNA markedly enhanced POR in control rats but did not increase POR in STZ rats. These findings demonstrate a greater potentiation of the adenosine-induced renal vasoconstriction in the presence of L-NNA infusion in control rats compared with STZ rats. We conclude that the increased vasoconstrictor sensitivity of the diabetic renal vasculature to adenosine is caused by a defective NO-dependent renal vasodilation of the afferent arteriole in diabetic rats.

Adenosine receptor subtypes and vasodilatation in rat skeletal muscle during systemic hypoxia: a role for A1 receptors

1. In anaesthetized rats we tested responses evoked by systemic hypoxia (breathing 8% O2 for 5 min) and adenosine (i.a. infusion for 5 min) before and after administration of a selective adenosine A1 receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine), or a selective adenosine A2A receptor antagonist ZM 241385. Arterial blood pressure, (ABP), heart rate (HR), femoral blood flow (FBF) and femoral vascular conductance (FVC: FBF/ABP) were recorded together with the K+ concentration in arterial blood ([K+]a) and in venous blood of hindlimb muscle ([K+]v) before and at the 5th minute of hypoxia or agonist infusion. 2. In 12 rats, DPCPX reversed the fall in ABP and HR and the increase in FVC evoked by the selective A1 agonist CCPA (2-chloro-N6-cyclopentyladenosine; i.a. infusion for 5 min). DPCPX also reduced both the increase in FVC induced by hypoxia and that induced by adenosine; the control responses to these stimuli were comparable in magnitude and both were reduced by approximately 50%. 3. In 11 rats, ZM 241385 reversed the fall in ABP and increase in FVC evoked by the selective A2A agonist CGS 21680 (2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadeno sin e hydrochloride; i.a. infusion for 5 min). ZM 241385 also reduced the increase in FVC induced by adenosine by approximately 50 %, but had no effect on the increase in FVC induced by hypoxia. 4. In these same studies, before administration of DPCPX, or ZM 241385, hypoxia had no effect on the venous-arterial difference for K+ ([K+]v-a), whereas after administration of either antagonist, hypoxia significantly reduced [K+]v-a suggesting an increase in hypoxia-induced K+ uptake, or a reduction in K+ efflux. 5. These results indicate that both A1 and A2A receptors are present in hindlimb muscle and can mediate vasodilatation and that A1 and A2A receptors contribute equally to dilatation induced by infused adenosine. However, they suggest that endogenous adenosine released during systemic hypoxia induces dilatation only by acting on A1 receptors. Given previous evidence that adenosine can stimulate receptors on skeletal muscle fibres that are coupled to ATP-sensitive K+ (KATP) channels so promoting K+ efflux, our results allow the proposal that KATP channels may be coupled to both A1 and to A2A receptors and may be stimulated to open by adenosine released during hypoxia, but indicate that, during systemic hypoxia, K+ efflux caused by either receptor subtype makes a very minor contribution to the muscle vasodilatation.

Natriuretic effect of an adenosine-1 receptor antagonist in cirrhotic patients with ascites

BACKGROUND & AIMS

The sodium and water retention and renal vasoconstriction exhibited by patients with cirrhotic ascites are similar to the changes observed by stimulation of renal adenosine 1 receptors. The aim of this study was to investigate the effects of FK352 (an adenosine 1 antagonist) on renal and systemic hemodynamics and renal function in cirrhotic patients with ascites.

METHODS

p-Aminohippuric acid and inulin clearance, urine flow rate, sodium and potassium excretion, and free water clearance were measured for 2 hours before and after FK352 administration. Cardiac output, systemic vascular resistance, plasma angiotensin II level, plasma renin activity, and noradrenaline, adrenaline, and adenosine 3', 5'-cyclic monophosphate (cAMP) levels were also measured before and after FK352.

RESULTS

Urine sodium excretion and urine flow rate increased after FK352 by a mean of 199.9% +/- 43.0% (P < 0.001) and 51.2% +/- 17.5% (P < 0.02), respectively. Plasma cAMP and angiotensin II levels and plasma renin activity also increased by 10. 8% +/- 3.2% (P < 0.01), 36.9% +/- 11.3% (P < 0.01), and 247.9% +/- 82.6% (P < 0.02), respectively. No change was detected in any other parameter.

CONCLUSIONS

The isokaliuretic improvement in natriuresis and diuresis suggests a role for adenosine 1 antagonism in the treatment of the renal abnormalities found in advanced cirrhosis.

Adenosine receptor mRNA levels during postnatal renal maturation in the rat

Adenosine may affect the pattern of intrarenal blood flow during renal development. It provides an angiogenic stimulus for the growth of new blood vessels and may be involved in compensatory renal growth. It is therefore of interest to investigate the expression of adenosine receptor genes during postnatal renal development. In the present study this was carried out by measuring adenosine receptor mRNA levels in rats aged between 2 and 60 days. The order of abundance of adenosine receptor mRNA levels in 60-day-old rats was A2A > A2B > or = A1 > A3. A1 receptor mRNA levels showed only small changes with increasing age although, by contrast, A3 receptor mRNA increased markedly with age with levels at 60 days twenty-fold greater than at 2 days. A2A receptor mRNA levels declined during renal maturation with transcript numbers four- to fivefold that at 12-18 days compared with numbers at 60 days. By contrast to the A2A receptor, there were no significant changes in the renal levels of A2B receptor mRNA during kidney maturation. During postnatal renal maturation, the levels of mRNA for A2A and A3 adenosine receptor subtypes undergo marked changes which may be related to functional maturation, morphological development, or both.

Evidence against a major role of adenosine in oxygen-dependent regulation of erythropoietin in rats

This in vivo study investigated whether adenosine (ADO) plays a role in oxygen-dependent production of erythropoietin (EPO). Exposure of rats to 0.075% carbon monoxide (CO) for four hours was used as a stimulus for EPO production. To inhibit potential effects of ADO, rats were treated with the non-specific ADO antagonist theophylline, the selective ADO A1 receptor blockers DPCPX and KW-3902, the selective ADO A2 receptor blocker DMPX, and AOPCP, an inhibitor of 5'-ectonucleotidase, an ADO generating enzyme that is expressed on the surface of EPO producing cells. To stimulate ADO receptor activity, animals were treated with the selective ADO A1 and A2 receptor agonists CHA and CGS 21680, the ADO reuptake inhibitors dipyridamole and soluflazine and the ADO desaminase inhibitor EHNA. At doses known to interfere with ADO signal transmission in vivo, none of these substances either influenced EPO serum levels in normoxic rats or affected the approximately 30-fold rise in EPO serum levels and the increase in renal EPO mRNA after exposure to carbon monoxide. Continuous administration of theophylline to normoxic rats for seven days did not alter hematocrit, hemoglobin or EPO serum levels. Taken together, these experiments do not support the hypothesis that ADO plays an important role in the regulation of EPO production.

Renal adenosine A1 receptor binding characteristics and mRNA levels during the development of acute renal failure in the rat

1. The binding characteristics and mRNA levels for renal adenosine A1 receptors were investigated in normal rats and rats with acute renal failure (ARF) induced by either glycerol or HgCl2. 2. Saturation isotherms determined from the binding of [3H]-1,3-dipropyl-8-cyclopentylxanthine ([3H]-DPCPX), a selective adenosine A1 antagonist, to renal membranes of untreated rats gave values of 0.62 nM for the equilibrium dissociation constant (Kd) and 19.9 fmol mg-1 protein for the density of binding sites (Bmax). No saturable binding was observed with [3H]-2-(p-(carboxylethyl)-phenylethylamino)-5'-N-ethylcar box amido adenosine ([3H]-CGS 21680), a selective adenosine A2a agonist. 3. By contrast to time-matched controls, renal membranes obtained from rats 16 and 48 h following the induction of ARF with glycerol, showed statistically significant increases (2-4 fold) in both Bmax and Kd for the binding of [3H]-DPCPX. No significant changes in the binding characteristics of [3H]-DPCPX were noted with membranes from rats 48 h following the production of ARF with HgCl2. 4. Adenosine A1 receptor mRNA levels were significantly elevated 0.5, 16 and 48 h following induction of ARF with glycerol, whilst no change was noted in mRNA levels for beta-actin at the same time points. No statistically significant changes in adenosine A1 receptor or beta-actin mRNA levels were noted 48 h after the induction of ARF with HgCl2. 5. This study indicates that glycerol-induced ARF in the rat is associated with an increase in renal adenosine A1 receptor density which appears to result from increased transcription of the gene for this receptor. An increase in adenosine A1 receptor density in renal resistance vessels may explain, at least in part, the enhanced renal vasoconstrictor response to adenosine in glycerol-induced ARF that was noted in a previous study.

Functional characterization of the adenosine receptor mediating inhibition of intestinal secretion

1. Previous studies have shown that the mixed A1/A2 adenosine agonist 5'-N-ethylcarboxamido-adenosine (NECA) inhibits intestinal fluid secretion which is thought to contribute to its antidiarrhoeal effect in the rat. The aim of this study was to characterize the adenosine receptor mediating this antisecretory effect via functional studies using a range of selective agonists and antagonists and by applying the pharmacological criteria of relative agonist and antagonist potencies. 2. Adenosine agonists and antagonists were administered i.v. to anaesthetized rats. Intestinal secretion was then stimulated by i.a. infusion of vasoactive intestinal peptide (VIP, 0.8 microgram min-1) and the net fluid transport across the wall of the jejunum was measured by a recirculation technique. 3. The rank order of agonist potency to reduce the response to VIP was: NECA > N6-cyclopentyladenosine (CPA) > R-N6-(2-phenylisopropyladenosine) (R-PIA) > S-PIA > chloroadenosine (2-CADO) > 2-phenylaminoadenosine (CV-1808). This order best complies with the rank order of agonist potency that represents activation of the recently described A2B receptor: NECA > 2-CADO > R-PIA = CHA > S-PIA > = CV-1808 > = CGS-21680. The most potent agonists (NECA, CPA and RPIA) had ED50 values in the low microgram range. 4. The anitsecretory action of NECA (submaximal dose of 40 micrograms kg-1) was antagonized equally (approximately 50%) by the selective adenosine antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 0.1 mg kg-1) and 8-phenyltheophylline (8-PT, 0.1 mg kg-1). This equipotent activity indicates the presence of an A2 and not an A1 receptor. 5. It is suggested that adenosine A2B receptor agonists could be evaluated for potential use as antidiarrhoeal drugs.

Interdependence of respiratory and cardiovascular changes induced by systemic hypoxia in the rat: the roles of adenosine

1. In ten spontaneously breathing, Saffan-anaesthetized rats (group I), respiratory and cardiovascular responses evoked by 10 min periods of hypoxia (arterial partial pressure of O2, Pa,O2, 33 mmHg) were recorded before and after the administration of the adenosine receptor antagonist 8-phenyltheophylline (8-PT, 10 mg kg-1 i.v.). Similar experiments were performed on nine constantly ventilated rats (group II; Pa,O2, 29 mmHg) with arterial partial pressure of CO2 (Pa,CO2) held constant. 2. In group I, hypoxia induced an initial increase and a secondary fall in ventilation (VE) with an accompanying secondary fall in heart rate (HR), arterial pressure (ABP) fell and cerebral vascular conductance (CVC) increased progressively. Cerebral blood flow (CBF) tended to fall with time during hypoxia. 8-PT abolished the secondary falls in VE and HR and reduced the fall in ABP and increase in CVC, while CBF was better maintained. 3. In group II, hypoxia induced a similar cardiovascular response to that in group I, but at the 1st minute of hypoxia, the HR was lower and the increase in CVC was greater. 8-PT did not affect the hypoxia-induced changes in HR, ABP, CVC or CBF. 4. These results indicate specific ways in which the ventilatory and cardiovascular responses induced by hypoxia in the spontaneously breathing rat are interdependent. They also indicate that the influences of 8-PT on the cardiovascular changes induced by hypoxia during spontaneous ventilation are mainly a consequence of its ability to block the centrally mediated contribution of adenosine to the secondary fall in ventilation.

Adenosine antagonist theophylline prevents the reduction of glomerular filtration rate after contrast media application

Radiographic contrast media (CM) can induce renal failure and this may serve as an experimental model of acute renal failure (ARF). One vasoactive factor likely to be involved in ARF is adenosine. In a double-blind, placebo-controlled study we investigated the effect of theophylline (TP), an adenosine receptor antagonist, regarding changes in renal hemodynamics induced by CM. Thirty-nine patients who received 100 ml of a non-ionic low osmolar CM (iopromide) were studied for changes in GFR and RPF by continuous inulin and PAH clearance before and until four hours after CM application. Forty-five minutes before the application of CM, patients were randomized and received either theophylline (5 mg/kg body wt) or the vehicle and placebo (saline) intravenously in a blinded manner. We additionally measured the creatinine clearance on the day before and two days after CM application. Sodium excretion, N-acetyl-beta-glucosaminidase (NAG) excretion, plasma renin activity (PRA) and aldosterone levels were also measured before and after CM application. Theophylline levels were within the therapeutic range in patients of the theophylline group during and four hours after CM application (59.0 +/- 10.6 mumol/liter and 40.1 +/- 10.9 mumol/liter). GFR, measured by inulin clearance significantly declined under CM application in patients without TP application (N = 19; 88 +/- 40 to 75 +/- 32 ml/min/1.72 m2; P < 0.01). In the group of patients receiving theophylline (N = 18) the GFR remained constant (75 +/- 26 vs. 78 +/- 33 ml/min/1.72 m2).(ABSTRACT TRUNCATED AT 250 WORDS)

1,3-Dipropyl-8-cyclopentyl xanthine (DPCPX): a useful tool for pharmacologists and physiologists?

There is now ample evidence in the literature to demonstrate the selectivity of action of DPCPX for adenosine A1 vs other adenosine receptor types in tissues derived from a wide range of species. However, care has to be exercised to ensure that its physiochemical properties do not result in the production of quantitatively misleading data. In experiments using canine tissues the still limited data available in the literature clearly and consistently demonstrate that DPCPX has a lower affinity than expected in preparations which would be anticipated to contain A1 receptors. A range of in vitro experiments also demonstrate that DPCPX is not always a "neutral" or "silent" antagonist. The mechanism underlying these additional effects is unclear, but may result from an ability of the compound to disrupt the normal interaction of the A1 receptor with Gi, or may be indicative of a lack of specificity of action. The limited evidence available suggests that the compound retains its selectivity and specificity of action in vivo, and early work indicates that the compound is proving to be a useful tool with which to explore the potential of activation of adenosine A1 receptors as an important mechanism in physiological and pathophysiological processes.

Effects of repeated administration of KW-3902, a novel adenosine A1-receptor antagonist, on its pharmacological actions

Effects of repeated administration of KW-3902, a novel adenosine A1-receptor antagonist, on its pharmacological actions were studied with regards to: 1) in vivo adenosine A1-antagonism, 2) diuretic effects and 3) renal protective effects against glycerol-induced acute renal failure (ARF). After repeated oral administration of KW-3902 (0.1 mg/kg/day) for 24 days, neither enhancement of the sensitivity to 5'-N-ethylcarboxamidoadenosine (NECA) nor reduction of the inhibitory effect of KW-3902 on the NECA-induced bradycardic response were observed. After repeated oral administration of KW-3902 (0.01 and 0.1 mg/kg/day) for 20 days, the diuretic effects of KW-3902 did not change. Renal protective effects against glycerol-induced ARF were not reduced by repeated oral administration of KW-3902 (0.01 and 0.1 mg/kg/day) for 23 days. These results suggest that repeated oral administration of KW-3902 has no effect on its pharmacological actions. Additionally, changes in serum parameters, which occurred after repeated administration of furosemide or trichlormethiazide, were minimal after repeated oral administration of KW-3902 (0.001-1 mg/kg/day) for 27 days. From these results, KW-3902 proved to be a diuretic which has renal protective effects with less side effects.

Effect of the selective A1 adenosine antagonist 8-cyclopentyl-1,3-dipropylxanthine on acute renal dysfunction induced by Escherichia coli endotoxin in rats

The effect of the selective A1 adenosine antagonist, 8-cyclopentyl-1,3-dipropylxanthine (CPX), on Escherichia coli endotoxin-induced acute renal dysfunction was determined in anaesthetized rats. Bolus administration of endotoxin at doses of either 1 or 20 mg kg-1 evoked decreases in inulin clearance, renal blood flow, urine flow and excretion of sodium, potassium and chloride. The changes in renal function produced by 20 mg kg-1 endotoxin were more severe than those noted with 1 mg kg-1 toxin and, by contrast to this lower dose, renal function showed no signs of recovery. Intravenous administration of CPX (0.1 mg kg-1) elicited a statistically significant, although modest, attenuation of the decline in inulin clearance, renal blood flow, urine output and electrolyte excretion induced by 20 mg kg-1 endotoxin. By contrast, treatment with 0.1 mg kg-1 CPX resulted in statistically significant protection against the falls in excretory function evoked by 1 mg kg-1 endotoxin but not against the reductions in renal blood flow and inulin clearance produced by the lower dose of toxin. These results suggest that adenosine may play a role, albeit not a major one, in the pathophysiology of endotoxaemic acute renal failure.

Effects of adenosine A1-agonist and -antagonist on urinary volume and Na excretion in IAP-treated and non-treated rats

Effects of an adenosine A1-receptor agonist and antagonist were determined in pertussis toxin (IAP)-treated and non-treated rats. (-)-N6-(2-phenylisopropyl) adenosine, an adenosine A1-agonist, reduced the urine volume and sodium excretion without decreasing the glomerular filtration rate at 0.1 mg/kg (p.o.) in both IAP-treated and non-treated rats. Diuretic effects of KW-3902 (8-(noradamantan-3-yl)-1,3-dipropylxanthine) and 8-cyclopentyl-1,3-dipropylxanthine, adenosine A1-receptor antagonists, were not affected by pretreatment with IAP. These results suggest that endogenous adenosine may induce antidiuretic effects by accelerating the reabsorption of water and sodium at tubular sites via an IAP-insensitive mechanism, and that the diuretic effects of the adenosine A1-receptor antagonist may result from inhibiting this action of endogenous adenosine.

Adenosine receptor subtypes

The numerous and widespread effects of adenosine provide both an opportunity for the development of novel therapeutic agents acting via adenosine receptors and the challenge of achieving selectivity of action. The feasibility of achieving selectivity is enhanced if receptor subtypes can be identified. Biochemical, functional and receptor-cloning studies are beginning to provide convergent data supporting the existence of A1, A2A, A2B and A3 receptors. However, studies of the functional significance of these receptors in intact tissues both in vitro and in vivo have lagged behind the biochemical studies. In this article, Michael Collis and Susanna Hourani review the current status of adenosine receptor classification and propose that ligands with greater selectivity need to be evaluated in a wide range of functional preparations if the therapeutic potential of this area is to be realized.

The diuretic action of 8-cyclopentyl-1,3-dipropylxanthine, a selective A1 adenosine receptor antagonist

1. The diuretic effect of the selective A1 adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (CPX), was investigated in anaesthetized rats. 2. CPX (0.1 mg kg-1, i.v.) produced significant increases in urine flow, and the excretion rate and fractional excretion of both sodium and chloride. By contrast, CPX administration did not result in any significant change in the excretion of potassium. 3. The diuretic effect of CPX was accompanied by a transient increase in inulin clearance although p-amino-hippurate clearance was unaffected, indicating the CPX induced a temporary elevation of glomerular filtration rate but no change in renal blood flow. 4. The fractional excretion of lithium (a marker of delivery of fluid out of the proximal tubule) was also significantly increased by CPX. However, other measures of tubular function derived from lithium clearance indicated that there were no changes in the handling of sodium or water in the distal regions of the nephron. 5. CPX did not significantly alter the relationship between either free water reabsorption or free water clearance and the distal delivery of sodium, which suggests that CPX does not affect the renal concentration/dilution mechanism. 6. The results of this study show that the diuresis and increased excretion of sodium and chloride induced by CPX (0.1 mg kg-1) in the rat, occurs with only transient elevation in glomerular filtration rate and no change in renal blood flow. The primary reason for the diuresis appears to be inhibition of sodium reabsorption in the proximal tubule. Furthermore, the results provide evidence that production and release of endogenous adenosine modifies renal excretory function via stimulation of the A1 receptor subtype.

Adult mouse kidneys become permissive to acute polyomavirus infection and reactivate persistent infections in response to cellular damage and regeneration

Kidneys of newborn (but not adult) mice are normally high permissive for polyomavirus (Py) infection and readily establish persistent infections. We have proposed that ongoing cellular differentiation, which occurs in newborn mice, may be necessary for a high level of in vivo Py replication (R. Rochford, J. P. Moreno, M. L. Peake, and L. P. Villarreal, J. Virol. 66:3287-3297, 1992). This cellular differentiation requirement may also be necessary for the reactivation of a persistent Py kidney infection and could provide an alternative to the accepted view that reactivation results from immunosuppression. To examine this proposal, the ability of adult BALB/c mouse kidneys to support primary acute Py infection or to reactivate previously established persistent Py infections after kidney-specific damage was investigated. Kidney damage was induced by both chemical (glycerol, cisplatin, or methotrexate) and mechanical (through renal artery clamping to produce unilateral renal ischemia) treatments. We also examined the effects of epidermal growth factor (EGF), which enhances the rate of kidney regeneration, on Py replication. Using histopathologic techniques, in situ hybridization for Py DNA, and immunofluorescence for Py VP1 production, we established that both chemical damage and damage through renal artery clamping of adult kidneys promoted high levels of primary Py replication in these normally nonpermissive cells. This damage also promoted the efficient reactivation of Py replication from persistently infected kidneys, in the absence of immunosuppression. EGF treatment significantly increased acute Py replication and also reactivation in damaged kidneys. These results support the view that ongoing cellular division and differentiation may be needed both for high levels of acute Py replication and for reactivation of persistent infections in vivo.

Amelioration of glycerol-induced acute renal failure in rats by an adenosine A1 receptor antagonist (FR-113453)

A potent adenosine A1 receptor antagonist, FR-113453, was tested for its preventive effect on glycerol-induced acute renal failure in rats. First, the optimum timing of FR-113453 administration was studied. Oral FR-113453 (100 mg/kg) given 1 h before or 5-10 min after glycerol injection produced a significant reduction of the serum creatinine at 24 h (4.3 +/- 0.8 mg/dL [vehicle] vs. 1.4 +/- 0.4 mg/dL [FR-113453], and 4.7 +/- 1.1 mg/dL vs. 1.3 +/- 0.6 mg/dL, respectively, p < 0.001). However, when FR-113453 was given 2 h after glycerol injection, the serum creatinine did not improve. Creatinine clearance at 24 h after the induction of acute renal failure was significantly better in rats given FR-113453 (100 mg/kg) 1 h before glycerol than in rats given vehicle alone (0.08 +/- 0.08 mL/min vs. 0.01 +/- 0.02 mL/min), (p < 0.01). The kidney weight was lower and less severe histologic changes were observed at 24 h in the FR-113453-treated group. Renal blood flow (measured using 85Sr microspheres) did not change at 24 h after glycerol injection (3.0 +/- 0.9 mL/min/g [vehicle] vs. 3.6 +/- 0.9 mL/min/g [FR-113453]), but renal vascular resistance was significantly reduced by FR-113453 (47.9 +/- 37.9 vs. 26.4 +/- 5.2 mm Hg/mL/min/g, p < 0.05). Beta-ATP levels (measured by 31P-magnetic resonance spectroscopy) were reduced in glycerol-induced acute renal failure, with no difference between the vehicle and FR-113453-treated groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Further characterization of the protective effect of 8-cyclopentyl-1,3-dipropylxanthine on glycerol-induced acute renal failure in the rat

In the rat, treatment with the alkylxanthine 8-cyclopentyl-1,3-dipropylxanthine (CPX) at a dose of 0.1 mg kg-1 antagonizes adenosine-induced falls in renal blood flow and reduces the severity of glycerol-induced acute renal failure. Treatment of glycerol-injected rats with 0.03 mg kg-1 of CPX resulted in no significant improvements in a range of indices of renal function. However, treatment with 0.1 or 0.3 mg kg-1 doses of CPX did significantly ameliorate acute renal failure although there were no significant differences in the degree of protection of renal function afforded by these two doses. In glycerol-injected rats, 0.1 or 0.3 mg kg-1 CPX administered either as a single dose or repeated doses every 12 h for two days did not inhibit renal phosphodiesterase. Thus the beneficial effects of CPX can be produced by doses that have no effect on renal phosphodiesterase activity whereas 0.1 mg kg-1 of CPX has been shown previously to antagonize the actions of adenosine. The findings provide further evidence that the beneficial effect of CPX in glycerol-induced acute renal failure is a consequence of adenosine antagonism and not phosphodiesterase inhibition.

A1 adenosine-receptor antagonists activate chloride efflux from cystic fibrosis cells

A1 adenosine-receptor-antagonist drugs such as 8-cyclopentyl-1,3-dipropylxanthine (CPX) and xanthine amine congener (XAC) are found to activate the efflux of 36Cl- from CFPAC cells. These cells are a pancreatic adenocarcinoma cell line derived from a cystic fibrosis (CF) patient homozygous for the common mutation, deletion of Phe-508. The active concentrations for these compounds are in the low nanomolar range, consistent with action on A1 adenosine receptors. In addition, drug action can be blocked by exogenous agonists such as 2-chloroadenosine and also can be antagonized by removal of endogenous agonists by treatment with adenosine deaminase. Cells lacking the CF genotype and phenotype, such as HT-29 and T84 colon carcinoma cell lines, appear to be resistant to activation of chloride efflux by either drug. CFPAC cells transfected with the CF transmembrane regulator gene, CFTR, are also resistant to activation by CPX. We conclude that, since these antagonists are of relatively low toxicity and appear to act somewhat selectively, they might be considered as promising therapeutic candidates for CF.

Amelioration of cisplatin-induced acute renal failure with 8-cyclopentyl-1,3-dipropylxanthine

1. The effect of the selective adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (CPX), on the development of cisplatin-induced acute renal failure was investigated in the rat. 2. CPX at doses of 0.03, 0.1 and 0.3 mg kg-1, i.v. caused increasing degrees of antagonism of adenosine-induced bradycardia in anaesthetized rats. The magnitude of antagonism was not directly proportional to the increment in dose, but for each dose, it was similar in rats injected with either saline or cisplatin. CPX at a dose of 0.03 mg kg-1 significantly antagonized adenosine-induced bradycardia for up to 2.5 h, while doses of 0.1 and 0.3 mg kg-1 produced significant blockade for periods longer than 5 h. 3. Administration of cisplatin (6 mg kg-1, i.v.) caused acute renal failure characterized by decreased inulin and p-aminohippurate clearances, increased urine volume but decreased excretion of Na+, K+ and Cl- ions and by increased plasma levels of urea and creatinine. Kidney weight was increased in cisplatin-treated rats and renal tubule necrosis occurred. 4. Administration of CPX (0.03 mg kg-1, i.v.; twice daily for two days) to rats given cisplatin did not reduce the severity of the resultant renal failure. However, treatment with 0.1 mg kg-1 CPX attenuated the increases in plasma creatinine/urea levels observed in rats on days 3 and 7 after induction of renal failure. In addition, this dose significantly reduced renal tubule damage and increased inulin and p-aminohippurate clearances. A similar pattern of protection was noted with CPX at a dose of 0.3 mg kg-1 although the increase in inulin clearance was not statistically significant. However, this higher dose of CPX significantly increased Na+ and K+ excretion compared to vehicle-treated rats. 5 CPX at doses of 0.03, 0.1 and 0.3 mgkg- produced blockade of an A1-receptor mediated response i.e. adenosine-induced bradycardia, but only treatment with the higher doses of CPX (0.1 and 0.3mgkg-1) ameliorated nephrotoxicity produced by cisplatin. The lack of any protective effect afforded by the lowest dose of CPX could be a result of its shorter duration of action.6. This study indicates that adenosine plays a significant role in the pathophysiology of cisplatin-induced acute renal failure.

The effect of 8-cyclopentyl-1,3-dipropylxanthine on the development of cyclosporin-induced acute renal failure

The effect of the selective A1-adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (CPX, 0.1 mg kg-1 i.v.) administered twice daily to rats has been assessed on the development of renal dysfunction induced by four daily injections of cyclosporin (60 mg kg-1 i.p.). The series of cyclosporin injections resulted in a polyuria accompanied by a 64-70% increase in plasma urea and creatinine concentrations and a 50% reduction in inulin clearance. However, cyclosporin administration resulted in no change in p-aminohippurate clearance nor was there any evidence of tubular necrosis or vascular damage. CPX treatment did not improve any index of renal function perturbed by cyclosporin. The findings provide evidence that adenosine does not play a role in the pathophysiology of cyclosporin nephrotoxicity.

Diuretic and saliuretic effects of 1,3-dipropyl-8-cyclopentylxanthine, a selective A1-adenosine receptor antagonist

We have previously shown that 8-phenyltheophylline (8-PT), a non-selective antagonist at adenosine A1- and A2-receptors, has a diuretic effect. In this study, the diuretic and adenosine antagonist effects of the A1-receptor selective compound 1,3-dipropyl-8-cyclopentylxanthine (CPX) have been examined in the conscious rat. CPX (0.1 and 0.3 mg kg-1 i.v.) significantly attenuated bradycardic but not hypotensive responses evoked by adenosine. In contrast, 8-PT (3 mg kg-1 i.v.) significantly antagonized both adenosine-induced bradycardia and hypotension. CPX (0.1 and 0.3 mg kg-1 i.v.) evoked a dose-related diuretic and saliuretic response in the conscious rat. These results indicate that the diuretic effects of adenosine antagonists are associated with blockade of the A1-receptor sub-type.

The role of adenosine in the respiratory and cardiovascular response to systemic hypoxia in the rat

1. In rats anaesthetized with Saffan we have studied the effects of the adenosine receptor antagonists, theophylline and 8-phenyltheophylline, upon the respiratory and cardiovascular responses evoked by 5 min periods of systemic hypoxia. 2. In the group of animals that were to receive theophylline (15 mg kg-1 i.v.), arterial O2 pressure (Pa,O2) fell from 83 +/- 2 mmHg during air breathing to 38 +/- 3 or 34 +/- 3 mmHg during the 5th minute of two different control periods of hypoxia, while in the group that were to receive 8-phenyltheophylline (10 mg kg-1 i.v.), Pa,O2 fell from 83 +/- 1 to 53 +/- 2 mmHg. Neither drug significantly altered the levels of Pa,O2 reached during hypoxia. 3. During the control periods of hypoxia respiration increased, but the increase evoked at the 5th minute was significantly less than that evoked at the 2nd minute of hypoxia. This secondary waning of the hyperventilation was abolished by both drugs. 4. Similarly, both drugs attenuated the tendency for the hypoxia-induced tachycardia to wane between the 2nd and 5th minute. 5. Further, both drugs substantially reduced both the hypoxia-induced fall in arterial pressure and the increases in vascular conductance in hindlimb muscle, carotid vasculature and kidney. 6. Thus, we propose that in the rat the release of adenosine by hypoxic tissues makes a major contribution to the secondary decrease in respiration and heart rate that occurs during systemic hypoxia and to the accompanying vasodilatation in muscle and fall in arterial pressure. The effects of the adenosine antagonists on the carotid and renal vasculature are more equivocal and may be partly explained as a smaller autoregulatory dilatation to a smaller fall in systemic arterial pressure. 7. These results and proposals are discussed in relation to the conditions that are known to cause release of adenosine and in relation to its known effects upon the respiratory and cardiovascular systems.

Benefit of vascular decongestion in glycerol-induced acute renal failure

The post insult administration of vascular decongestants has resulted in attenuation of experimental acute renal failure (ARF) following the introduction of various nephrotoxins including drugs, heavy metals, and endotoxin. In the present study, the dose-dependent effects of a novel methylxanthine, HWA-138, were studied in the glycerol-induced murine model of ARF. Renal function, assessed by serial inulin clearances at 24 and 48 h after glycerol injection, urinary electrolyte excretion rates, and renal morphology, was compared between controls and those given glycerol and single i.v. doses of 0.1, 0.5, 1.0, 5.0, and 10.0 mg/kg of HWA-138, or physiologic saline. Whereas significant renal dysfunction was found in all animal groups given glycerol, the mean inulin clearance values of animals given HWA-138 1 mg/kg closely approximated values found in control rats. There were no changes in renal electrolyte excretion rates in animals given HWA-138 compared with relative natriuresis found in untreated glycerol ARF rat. Although a modest decrease in medullary congestion was associated with rats given 1 mg/kg of HWA-138, there was no obvious structural improvement found with HWA-138. The present data provide further evidence of the potential of methylxanthines in the glycerol-ARF murine model.