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GASTROINTESTINAL, HEPATIC, PULMONARY, AND RENAL

The Extracellular cAMP-Adenosine Pathway Significantly Contributes to the in Vivo Production of Adenosine

Center for Clinical Pharmacology, Departments of Pharmacology (E.K.J.) and Medicine (Z.M., R.K.D.), University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania

Received August 18, 2006; accepted October 5, 2006.

	Abstract

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The extracellular cAMP-adenosine pathway is the cellular egress of cAMP followed by extracellular conversion of cAMP to adenosine by the sequential actions of ecto-phosphodiesterase and ecto-5'-nucleotidase. Although detailed studies in isolated organs, tissues, and cells provide evidence for an extracellular cAMP-adenosine pathway, whether this mechanism contributes significantly to adenosine production in vivo is unclear. 1,3-Dipropyl-8-p-sulfophenylxanthine is restricted to the extracellular compartment due to a negative charge at physiological pH and, at high concentrations (0.1 mM), blocks ecto-phosphodiesterase. Here, we show that administration of 1,3-dipropyl-8-p-sulfophenylxanthine at a dose that provided concentrations in plasma and urine of approximately 0.3 and 6 mM, respectively, inhibited urinary adenosine excretion. In Sprague-Dawley rats i.v., 1,3-dipropyl-8-p-sulfophenylxanthine (10 mg + 0.15 mg/min) significantly decreased by 48 and 39% the urinary excretion of adenosine (from 3.57 ± 0.38 to 1.87 ± 0.14 nmol/30 min; p = 0.0003) and the ratio of urinary adenosine to cAMP (from 0.93 ± 0.08 to 0.57 ± 0.06; p = 0.0044), respectively, without altering blood pressure, renal blood flow, or glomerular filtration rate. Although 1,3-dipropyl-8-p-sulfophenylxanthine transiently increased urine volume and sodium excretion, these effects subsided, yet adenosine excretion remained reduced. Thus, changes in systemic and renal hemodynamics and excretory function could not account for the effects of 1,3-dipropyl-8-p-sulfophenylxanthine on adenosine excretion. Additional experiments showed that 1,3-dipropyl-8-p-sulfophenylxanthine, as in Sprague-Dawley rats, significantly attenuated adenosine excretion and the ratio of urinary adenosine to cAMP in both Wistar-Kyoto rats and spontaneously hypertensive rats. We conclude that the extracellular cAMP-adenosine pathway significantly contributes to the in vivo production of adenosine.

The function of extracellular cAMP may be to provide adenosine for activation of adenosine cell surface receptors. In the interstitial compartment, extracellular AMP is rapidly dephosphorylated by ecto-5'-nucleotidase, an ubiquitous enzyme attached to the cell membrane by a lipid sugar linkage (Pearson et al., 1985; Misumi et al., 1990; Zimmermann, 1992). Therefore, if ecto-phosphodiesterase is expressed on the cell surface, hormonal activation of adenylyl cyclase would result in extracellular production of adenosine by metabolism of extracellular cAMP to extracellular AMP by ectophosphodiesterase, followed by metabolism of extracellular AMP to adenosine by ecto-5'-nucleotidase. This sequence of reactions is called the extracellular cAMP-adenosine pathway. Inasmuch as adenosine would be synthesized in the unstirred water layer by spatially linked, cell surface proteins, small increases in cAMP may produce biologically active concentrations of adenosine in the biophase of cell surface adenosine receptors, thus promoting autocrine and paracrine effects of adenosine.

Much in vitro data support the existence of the extracellular cAMP-adenosine pathway. In aortic vascular smooth muscle cells (Dubey et al., 1996) and cardiac fibroblasts (Dubey et al., 2000) in culture, exogenous cAMP is converted to AMP, adenosine, and inosine in a concentration- and time-dependent fashion. Significant increases in extracellular adenosine levels occur with concentrations of cAMP in the medium as low as 1 µM, and steady-state levels of adenosine are achieved in the culture medium within 5 min after adding exogenous cAMP. In addition to aortic vascular smooth muscle cells and cardiac fibroblasts, the extracellular cAMP-adenosine pathway is present in hepatocytes (Gorin and Brenner, 1976; Smoake et al., 1981), neurons (Rosenberg and Dichter, 1989; Rosenberg et al., 1994; Rosenberg and Li, 1995, 1996; Brundege et al., 1997), adipocytes (Kather, 1990; Zacher and Carey, 1999), isolated perfused kidneys (Mi and Jackson, 1995, 1998), cerebral blood vessels (Hong et al., 1999), renal microvessels (Jackson and Mi, 2000), collecting ducts (Jackson et al., 2003), and proximal tubules (Jackson et al., 2006). However, whether the extracellular cAMP-adenosine pathway accounts for a significant portion of adenosine biosynthesis in vivo is unknown and is the focus of the present investigation.

	Materials and Methods

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Animals. Studies used adult (14–16 weeks-of-age) male Sprague-Dawley rats, spontaneously hypertensive rats (SHRs), and Wistar-Kyoto (WKY) rats obtained from Taconic Farms (Germantown, NY). The Institutional Animal Care and Use Committee approved all procedures.

Protocol 1. Sprague-Dawley rats (n = 16) were anesthetized with Inactin (100 mg/kg i.p.) and placed on a Deltaphase isothermal pad (Braintree Scientific, Braintree, MA). Body temperature was monitored with a rectal temperature probe (Physitemp Instruments, Clifton, NJ) and maintained at 37 ± 0.5°C by adjusting a heat lamp positioned above the rat. A short section of polyethylene (PE) tubing (PE-240) was placed in the trachea to facilitate respiration. Two PE-50 cannulas were inserted into the right jugular vein, and infusions of saline were initiated at 25 µl/min in each cannula. A PE-50 catheter was placed in the left carotid artery for blood sample collection and for measurement of mean arterial blood pressure (MABP) via a digital blood pressure analyzer (model BPA; Micro-Med Enterprises, Inc., Louisville, KY). A PE-10 catheter was inserted into the left ureter for urine collection, and a flow probe (model 1RB; Transonic Systems, Inc., Ithaca, NY) was placed on the left renal artery and connected to a transit time flowmeter (model T206; Transonic Systems, Inc.) for determination of renal blood flow (RBF). A bolus and infusion of inulin [carboxyl-¹⁴C] (0.5 µCi bolus followed by 0.035 µCi/min infusion) was administered via one jugular cannula. In addition, a bolus (10 mg) and infusion (0.15 mg/min) of 1,3-dipropyl-8-p-sulfophenylxanthine (DPSPX; Sigma-Aldrich, St. Louis, MO) was administered via a separate jugular cannula in half (n = 8) of the animals; the other half (n = 8) continued to receive saline only (random assignment). DPSPX was rendered highly soluble in saline by titrating with sodium carbonate to convert the free acid to the sodium salt of DPSPX. DPSPX is restricted to the extracellular compartment due to a negative charge at physiological pH (Tofovic et al., 1991) and at high concentrations (equal to or greater than 0.1 mM) blocks ecto-phosphodiesterase activity (Dubey et al., 1996, 2000). An i.v. bolus of erythro-9-(2-hydroxy-3-nonyl) adenine hydrochloride (3 mg; Sigma-Aldrich) was administered to all animals to inhibit adenosine deaminase so as to enhance the ability to detect changes in adenosine production. After a 90-min rest period, parameters were recorded in all animals during three 30-min renal clearance periods. A midpoint blood sample (0.3 ml) for measurement of radioactivity was collected. Plasma and urine [¹⁴C]inulin radioactivity were measured, and renal clearance of [¹⁴C]inulin was calculated for estimation of glomerular filtration rate (GFR). Urinary adenosine and cAMP were measured by high-pressure liquid chromatography (HPLC) with fluorescence detection as described previously (Jackson et al., 1996). In addition, plasma levels of DPSPX were measured in a subset of rats using HPLC with UV detection as described previously (Tofovic et al., 1991). Urinary sodium and potassium were measured by flame photometry (Instrumentation Laboratory, Lexington, MA).

View larger version (19K): [in this window] [in a new window]   Fig. 1. Urinary excretion rate of adenosine (top) and cAMP (middle) and the ratio of adenosine to cAMP excretion in the urine (bottom) in control Sprague-Dawley rats (n = 8; solid bars) and Sprague-Dawley rats pretreated with DPSPX (10 mg bolus plus 0.15 mg/min infusion; n = 8; hatched bars) during three back-to-back renal clearance periods. a, p < 0.05 versus corresponding period in control rats, Fisher's LSD. Data are shown as means ± S.E.M.

Statistics. Data were analyzed by repeated measures two-factor analysis of variance (ANOVA) followed by a Fisher's Least Significant Difference test (Fisher's LSD) if the interaction term in the analysis of variance was significant. The criterion of significance was p < 0.05. All values in text and figures are means ± S.E.M.

View larger version (19K): [in this window] [in a new window]   Fig. 2. MABP (top), RBF (middle), and GFR (bottom) in control Sprague-Dawley rats (n = 8; solid bars) and Sprague-Dawley rats pretreated with DPSPX (10 mg bolus plus 0.15 mg/min infusion; n = 8; hatched bars) during three back-to-back renal clearance periods. Data are shown as means ± S.E.M.

	Results

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View larger version (20K): [in this window] [in a new window]   Fig. 3. Urine volume (top), sodium excretion (middle), and potassium excretion (bottom) in control Sprague-Dawley rats (n = 8; solid bars) and Sprague-Dawley rats pretreated with DPSPX (10 mg bolus plus 0.15 mg/min infusion; n = 8; hatched bars) during three back-to-back renal clearance periods. a, p < 0.05 versus corresponding period in control rats, Fisher's LSD. Data are shown as means ± S.E.M.

View larger version (29K): [in this window] [in a new window]   Fig. 4. Urinary excretion of adenosine during two experimental periods in WKY rats (top) and SHRs (bottom). WKY-saline (n = 8) and SHR-saline (n = 8) groups received only saline between experimental periods 1 and 2. WKY-DPSPX (n = 8) and SHR-DPSPX (n = 9) groups received DPSPX (10 mg bolus plus 0.15 mg/min) between experimental periods 1 and 2. a, p < 0.05 versus period 1, Fisher's LSD. Data are shown as means ± S.E.M.

View larger version (24K): [in this window] [in a new window]   Fig. 5. Adenosine to cAMP urinary excretion ratios during two experimental periods in WKY rats (top) and SHRs (bottom). WKY-saline (n = 8) and SHR-saline (n = 8) groups received only saline between experimental periods 1 and 2. WKY-DPSPX (n = 8) and SHR-DPSPX (n = 9) groups received DPSPX (10 mg bolus plus 0.15 mg/min) between experimental periods 1 and 2. a, p < 0.05 versus period 1, Fisher's LSD. Data are shown as means ± S.E.M.

	Discussion

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The goal of the present study was to test the hypothesis that metabolism of extracellular, endogenous cAMP contributes to adenosine formation in vivo. To test this hypothesis, we measured urinary adenosine as an index of adenosine production and treated animals i.v. with DPSPX to block ecto-phosphodiesterase, the enzyme that converts extracellular cAMP to AMP (which in turn is metabolized to adenosine by ecto-5'-nucleotidase). We used urinary adenosine as a marker for total body adenosine production for two reasons. First, under normal physiological conditions, urinary adenosine is derived in part from filtered adenosine that escapes uptake and metabolism by tubular epithelial cells and in part from renal tubular production of adenosine (Thompson et al., 1985; Heyne et al., 2004). Second, plasma, but not urinary, adenosine measurements are fraught with technical problems that give rise to misleading results because of rapid uptake and metabolism by cells in whole blood and because of rapid synthesis of adenosine in whole blood. Because urine does not normally contain enzymes of adenosine production or metabolism, measurement of urinary adenosine should provide a more reliable index of adenosine production.

A key feature of this experimental strategy was the use of DPSPX. DPSPX is a xanthine that is restricted to the extracellular compartment due to a negative charge at physiological pH (Tofovic et al., 1991). At low concentrations, DPSPX blocks adenosine receptors (Daly and Jacobson, 1995), but at high concentrations, DPSPX inhibits ecto-phosphodiesterase (Zacher and Carey, 1999). For example, DPSPX at concentrations greater than or equal to 0.1 mM inhibits the metabolism of extracellular cAMP to AMP in the isolated perfused kidney (Mi and Jackson, 1995), cultured aortic vascular smooth muscle cells (Dubey et al., 1996), cultured cardiac fibroblasts (Dubey et al., 2000), freshly isolated preglomerular vascular smooth muscle cells (Jackson and Mi, 2000), cultured oviduct cells (Cometti et al., 2003), freshly isolated collecting ducts and cultured collecting duct cells (Jackson et al., 2003), and freshly isolated proximal tubules and cultured collecting duct cells (Jackson et al., 2006).

In the present study, we administered a bolus of DPSPX of 10 mg followed by an infusion of 0.15 mg/min. This experimental paradigm provided plasma levels of DPSPX, as measured by HPLC with UV detection, of approximately 0.3 mM, a concentration well within the range to inhibit ecto-phosphodiesterase. It is noteworthy that we achieved urinary concentrations of DPSPX of approximately 6 mM, 60-fold above the concentration required to inhibit ecto-phosphodiesterase. The high urinary concentrations of DPSPX are probably due to the fact that DPSPX does not cross cell membranes and therefore is concentrated in the urinary compartment as water is reabsorbed from the tubules. Therefore, DPSPX should afford a high degree of inhibition of ecto-phosphodiesterase in the renal tubular system.

It is unlikely that the ability of DPSPX to reduce the urinary excretion of adenosine was secondary to changes in systemic hemodynamics, renal hemodynamics, glomerular filtration rate, or renal excretory function. In the present study, we did not detect an effect of DPSPX on arterial blood pressure, renal perfusion, or glomerular filtration. Because DPSPX is an adenosine receptor blocker (Daley and Jacobson, 1995) and because blockade of A₁ adenosine receptors reproducibly increases urine volume and sodium excretion (Kuan et al., 1993), it is not surprising that in the current study, DPSPX caused a large initial increase in urine volume and sodium excretion. However, as has been noted previously, diuretic breaking occurs quickly following A₁ receptor blockade (Bak and Thomsen, 2004); therefore, with time, we observed a gradual return of urine volume and sodium excretion to basal values. Despite the return of urine volume and sodium excretion to baseline levels the excretion of adenosine remained attenuated in DPSPX-treated animals. This observation would seem to rule out the possibility that the effects of DPSPX on adenosine excretion were mediated by the transient changes in urine volume and sodium excretion. In addition, the fact that DPSPX did not significantly affect cAMP excretion but did reduce the ratio of adenosine to cAMP excretion argues strongly that the effects of DPSPX were not merely secondary to changes in urine volume or renal excretory function. Consistent with this conclusion, Heyne et al. (2004) report that urine volume and sodium excretion have little if any effect on the urinary excretion of adenosine.

Currently, DPSPX is the only know inhibitor of ecto-phosphodiesterase that is restricted to the extracellular compartment and, therefore, was the preferred inhibitor for the present study. However, as mentioned, DPSPX is also an adenosine receptor antagonist; therefore, it is conceivable that the reduction in urinary adenosine excretion was related to blockade of adenosine receptors. However, this seems unlikely because blockade of A₁ receptors increases, rather than decreases, extracellular levels of adenosine (Andresen et al., 1999). Although DPSPX decreases urinary adenosine excretion, it does not increase urinary cAMP excretion. This could be due to the fact that by blocking A₂ receptors and decreasing adenosine levels, DPSPX inhibits A₂ receptor-mediated activation of adenylyl cyclase.

The results of the present study are in accord with our previously published work using 3-isobutyl-1-methylxanthine (IBMX) (Mi et al., 1994), an inhibitor of both intracellular and extracellular phosphodiesterase. When added to the perfusate of a microdialysis probe inserted into the renal cortex of anesthetized rats, IBMX reduced the recovery of adenosine and inosine (metabolite of adenosine) by approximately 40 to 50%, which is very similar to the reduction in urinary adenosine achieved in the present study. However, because IBMX enters cells and inhibits intracellular as well as extracellular phosphodiesterase, our previous study left open the possibility that metabolism of cAMP to adenosine intracellularly was responsible for the cAMP-adenosine pathway, rather than extracellular cAMP-adenosine pathway. Taken together, the two studies support the concept that the extracellular cAMP-adenosine pathway importantly contributes to adenosine production in vivo.

DPSPX reduces urinary adenosine excretion in Sprague-Dawley rats, SHRs, and WKY rats. This indicates that the cAMP-adenosine pathway contributes to the in vivo production of adenosine in several different strains of rats. An unexpected finding of the present study was the fact that basal urinary adenosine excretion in SHRs is approximately 50% of the basal urinary adenosine excretion in WKY rats or Sprague-Dawley rats. We do not know the mechanism of this potentially important difference between genetically hypertensive and normotensive rats. However, given the importance of adenosine in modulating the growth of cardiac fibroblasts (Dubey et al., 1997), vascular smooth muscle cells (Dubey et al., 1998), and mesangial cells (Dubey et al., 2005), it is conceivable that reduced levels of adenosine could participate in the pathophysiology of genetic hypertension or its sequelae.

Biochemical and pharmacological evidence strongly supports the existence of the cAMP-adenosine pathway in vitro, and the present study suggests that the cAMP-adenosine pathway also exits in vivo. In addition to biochemical and pharmacological evidence supporting the cAMP-adenosine pathway, molecular characterization of the components of the system is progressing. In this regard, the molecular details of one component of the pathway, i.e., the ecto-5'-nucleotidase mediating the metabolism of AMP to adenosine, are well described previously (Zimmermann, 1992), and molecular studies identify several cAMP transporters that may participate in the efflux of cAMP to the extracellular compartment (Sekine et al., 1997; van Aubel et al., 2002). However, the molecular identity of the ecto-phosphodiesterase that mediates the extracellular metabolism of cAMP to AMP remains unclear. With regard to classic phosphodiesterases, in mammals, there are at least 11 gene families, and each family is composed of one to four distinct genes. Moreover, many of these genes give rise to splice variants so that mammals produce more than 50 different phosphodiesterase proteins (Lugnier, 2006). In addition, mammalian cells express at least three different functional ecto-nucleotide pyrophosphatases, and these cell surface enzymes are capable of hydrolyzing cAMP to AMP (Goding et al., 2003). It is conceivable that ecto-phosphodiesterase is comprised of one or more of the aforementioned enzymes or is a yet-to-be-discovered enzyme distinct from classic phosphodiesterases or ecto-nucleotide pyrophosphatases. Further work is needed to identify the precise molecular identity of ecto-phosphodiesterase.

In summary, the present study demonstrates that systemic administration of DPSPX achieves high levels of DPSPX in plasma and more notably in urine, and this is associated with a substantial decrease in the urinary excretion of adenosine. We conclude that the extracellular cAMP-adenosine pathway importantly contributes to adenosine production in vivo.

	Footnotes

 
This work was supported by the National Institutes of Health (Grant HL 69846).

Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.

doi:10.1124/jpet.106.112748.

ABBREVIATIONS: SHR, spontaneously hypertensive rat; WKY, Wistar-Kyoto; PE, polyethylene; MABP, mean arterial blood pressure; RBF, renal blood flow; DPSPX, 1,3-dipropyl-8-p-sulfophenylxanthine; GFR, glomerular filtration rate; HPLC, high-pressure liquid chromatography; ANOVA, analysis of variance; LSD, least significant difference test; IBMX, 3-isobutyl-1-methylxanthine.

Address correspondence to: Dr. Edwin K. Jackson, Center for Clinical Pharmacology, University of Pittsburgh School of Medicine, 100 Technology Drive, Suite 450, Pittsburgh, PA 15219-3130. E-mail: edj{at}pitt.edu

	References

Top Abstract Materials and Methods Results Discussion References

 

Andresen BT, Gillespie DG, Mi Z, Dubey RK, and Jackson EK (1999) Role of adenosine A₁ receptors in modulating extracellular adenosine levels. J Pharmacol Exp Ther 291: 76–80.[Abstract/Free Full Text]

Bak M and Thomsen K (2004) Effects of the adenosine A₁ receptor inhibitor FK 838 on proximal tubular fluid output in rats. Nephrol Dial Transplant 19: 1077–1082.[Abstract/Free Full Text]

Brundege JM, Diao L, Proctor WR, and Dunwiddie TV (1997) The role of cyclic AMP as a precursor of extracellular adenosine in the rat hippocampus. Neuropharmacology 36: 1201–1210.[CrossRef][Medline]

Cometti B, Dubey RK, Imthurn B, Jackson EK, and Rosselli M (2003) Oviduct cells express the cyclic AMP-adenosine pathway. Biol Reprod 69: 868–875.[Abstract/Free Full Text]

Daly JW and Jacobson KA (1995) Adenosine receptors: selective agonists and antagonists, in Adenosine and Adenine Nucleotides: From Molecular Biology to Integrative Physiology (Belardinelli L and Pelleg A, eds) pp 157–166, Kluwer Academic Publishers, Boston, MA.

Dubey RK, Gillespie DG, Mi Z, and Jackson EK (1997) Exogenous and endogenous adenosine inhibits fetal calf serum-induced fetal calf serum-induced growth of rat cardiac fibroblasts: Role of A_2B receptors. Circulation 96: 2656–2666.

Dubey RK, Gillespie DG, Mi Z, and Jackson EK (1998) Adenosine inhibits growth of human aortic smooth muscle cells via A_2B receptors. Hypertension 31: 516–521.[Abstract/Free Full Text]

Dubey RK, Gillespie DG, Mi Z, and Jackson EK (2000) Cardiac fibroblasts express the cyclic AMP-adenosine pathway. Hypertension 36: 337–342.[Abstract/Free Full Text]

Dubey RK, Gillespie DG, Mi Z, and Jackson EK (2005) Adenosine inhibits PDGF-induced growth of human mesangial cells via A_2B receptors. Hypertension 46: 628–634.[Abstract/Free Full Text]

Dubey RK, Mi Z, Gillespie DG, and Jackson EK (1996) Cyclic AMP-adenosine pathway inhibits vascular smooth muscle cell growth. Hypertension 28: 765–771.[Abstract/Free Full Text]

Goding JW, Grobben B, and Slegers H (2003) Physiological and pathophysiological functions of the ecto-nucleotide pyrophosphatase/phosphodiesterase family. Biochim Biophys Acta 1638: 1–19.[Medline]

Gorin E and Brenner T (1976) Extracellular metabolism of cyclic AMP. Biochim Biophys Acta 451: 20–28.[Medline]

Heyne N, Benohr P, Muhlbauer B, Delabar U, Risler T, and Osswald H (2004) Regulation of renal adenosine excretion in humans: role of sodium and fluid homeostasis. Nephrol Dial Transplant 19: 2737–2741.[Abstract/Free Full Text]

Hong KW, Shin HK, Kim HH, Choi JM, Rhim BY, and Lee WS (1999) Metabolism of cAMP to adenosine: role in vasodilation of rat pial artery in response to hypotension. Am J Physiol 276: H379–H382.

Jackson EK and Dubey RK (2001) Role of the extracellular cyclic AMP-adenosine pathway in renal physiology. Am J Physiol 281: F597–F612.

Jackson EK and Mi Z (2000) The preglomerular microcirculation expresses the cAMP-adenosine pathway. J Pharmacol Exp Ther 295: 23–28.[Abstract/Free Full Text]

Jackson EK, Mi Z, Koehler MT, Carcillo JA Jr, and Herzer WA (1996) Injured erythrocytes release adenosine deaminase into the circulation. J Pharmacol Exp Ther 279: 1250–1260.[Abstract/Free Full Text]

Jackson EK, Mi Z, Zhu C, and Dubey RK (2003) Adenosine biosynthesis in the collecting duct. J Pharmacol Exp Ther 307: 888–896.[Abstract/Free Full Text]

Jackson EK, Zacharia LC, Zhang M, Gillespie DG, Zhu C, and Dubey RK (2006) cAMP-adenosine pathway in the proximal tubule. J Pharmacol Exp Ther 317: 1219–1229.[Abstract/Free Full Text]

Kather H (1990) Beta-adrenergic stimulation of adenine nucleotide catabolism and purine release in human adipocytes. J Clin Investig 85: 106–114.[Medline]

Kuan CJ, Herzer WA, and Jackson EK (1993) Cardiovascular and renal effects of blocking A₁ adenosine receptors. J Cardiovasc Pharmacol 21: 822–828.[Medline]

Lugnier C (2006) Cyclic nucleotide phosphodiesterase (PDE) superfamily: a new target for the development of specific therapeutic agents. Pharmacol Ther 109: 366–398.[CrossRef][Medline]

Mi Z, Herzer WA, Zhang Y, and Jackson EK (1994) 3-Isobutyl-1-methylxanthine decreases renal cortical interstitial levels of adenosine and inosine. Life Sci 54: 277–282.[CrossRef][Medline]

Mi Z and Jackson EK (1995) Metabolism of exogenous cyclic AMP to adenosine in the rat kidney. J Pharmacol Exp Ther 273: 728–733.[Abstract/Free Full Text]

Mi Z and Jackson EK (1998) Evidence for an endogenous cAMP-adenosine pathway in the rat kidney. J Pharmacol Exp Ther 287: 926–930.[Abstract/Free Full Text]

Misumi Y, Ogata S, Hirose S, and Ikehara Y (1990) Primary structure of rat liver 5'-nucleotidase deduced from the cDNA. J Biol Chem 265: 2178–2183.[Abstract/Free Full Text]

Pearson JD, Coade SB, and Cusack NJ (1985) Characterization of ectonucleotidases on vascular smooth muscle cells. Biochem J 230: 503–507.[Medline]

Rosenberg PA and Dichter MA (1989) Extracellular cAMP accumulation and degradation in rat cerebral cortex in dissociated cell culture. J Neurosci 9: 2654–2663.[Abstract]

Rosenberg PA, Knowles R, Knowles KP, and Li Y (1994) -Adrenergic receptor-mediated regulation of extracellular adenosine in cerebral cortex in culture. J Neurosci 14: 2953–2965.[Abstract]

Rosenberg PA and Li Y (1995) Adenylyl cyclase activation underlies intracellular cyclic AMP accumulation, cyclic AMP transport, and extracellular adenosine accumulation evoked by -adrenergic receptor stimulation in mixed cultures of neurons and astrocytes derived from rat cerebral cortex. Brain Res 692: 227–232.[CrossRef][Medline]

Rosenberg PA and Li Y (1996) Forskolin evokes extracellular adenosine accumulation in rat cortical cultures. Neurosci Lett 211: 49–52.[CrossRef][Medline]

Sekine T, Watanabe N, Hosoyamada M, Kanai Y, and Endou H (1997) Expression cloning and characterization of a novel multispecific organic anion transporter. J Biol Chem 272: 18526–18529.[Abstract/Free Full Text]

Smoake JA, McMahon KL, Wright RK, and Solomon SS (1981) Hormonally sensitive cyclic AMP phosphodiesterase in liver cells: an ecto-enzyme. J Biol Chem 256: 8531–8535.[Abstract/Free Full Text]

Thompson CI, Sparks HV, and Spielman WS (1985) Renal handling and production of plasma and urinary adenosine. Am J Physiol 248: F545–F551.

Tofovic SP, Branch KR, Oliver RD, Magee WD, and Jackson EK (1991) Caffeine potentiates vasodilator-induced renin release. J Pharmacol Exp Ther 256: 850–860.[Abstract/Free Full Text]

van Aubel RAMH, Smeets PHE, Peters JGP, Bindels RJM, and Russel FGM (2002) The MRP4/ABCC4 gene encodes a novel apical organic anion transporter in human kidney proximal tubules: putative efflux pump for urinary cAMP and cGMP. J Am Soc Nephrol 13: 595–603.[Abstract/Free Full Text]

Zacher LA and Carey GB (1999) Cyclic AMP metabolism by swine adipocyte microsomal and plasma membranes. Comp Biochem Physiol B Biochem Mol Biol 124: 61–71.[CrossRef][Medline]

Zimmermann H (1992) 5'-Nucleotidase: molecular structure and functional aspects. Biochem J 285: 345–365.

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This Article Abstract Full Text (PDF) All Versions of this Article: jpet.106.112748v1 320/1/117    most recent Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Jackson, E. K. Articles by Dubey, R. K. Search for Related Content PubMed PubMed Citation Articles by Jackson, E. K. Articles by Dubey, R. K.