Adenosine and hemodialysis in humans

Effect of hyperoxic and hyperbaric conditions on the adenosinergic pathway and CD26 expression in rat

The nucleoside adenosine acts on the nervous and cardiovascular systems via the A2A receptor (A2AR). In response to oxygen level in tissues, adenosine plasma concentration is regulated in particular via its synthesis by CD73 and via its degradation by adenosine deaminase (ADA). The cell-surface endopeptidase CD26 controls the concentration of vasoactive and antioxidant peptides and hence regulates the oxygen supply to tissues and oxidative stress response. Although overexpression of adenosine, CD73, ADA, A2AR, and CD26 in response to hypoxia is well documented, the effects of hyperoxic and hyperbaric conditions on these elements deserve further consideration. Rats and a murine Chem-3 cell line that expresses A2AR were exposed to 0.21 bar O2, 0.79 bar N2 (terrestrial conditions; normoxia); 1 bar O2 (hyperoxia); 2 bar O2 (hyperbaric hyperoxia); 0.21 bar O2, 1.79 bar N2 (hyperbaria). Adenosine plasma concentration, CD73, ADA, A2AR expression, and CD26 activity were addressed in vivo, and cAMP production was addressed in cellulo. For in vivo conditions, 1) hyperoxia decreased adenosine plasma level and T cell surface CD26 activity, whereas it increased CD73 expression and ADA level; 2) hyperbaric hyperoxia tended to amplify the trend; and 3) hyperbaria alone lacked significant influence on these parameters. In the brain and in cellulo, 1) hyperoxia decreased A2AR expression; 2) hyperbaric hyperoxia amplified the trend; and 3) hyperbaria alone exhibited the strongest effect. We found a similar pattern regarding both A2AR mRNA synthesis in the brain and cAMP production in Chem-3 cells. Thus a high oxygen level tended to downregulate the adenosinergic pathway and CD26 activity. Hyperbaria alone affected only A2AR expression and cAMP production. We discuss how such mechanisms triggered by hyperoxygenation can limit, through vasoconstriction, the oxygen supply to tissues and the production of reactive oxygen species.

Autotaxin/lysopholipase D and lysophosphatidic acid regulate murine hemostasis and thrombosis

The lipid mediator lysophosphatidic acid (LPA) is a potent regulator of vascular cell function in vitro, but its physiologic role in the cardiovasculature is largely unexplored. To address the role of LPA in regulating platelet function and thrombosis, we investigated the effects of LPA on isolated murine platelets. Although LPA activates platelets from the majority of human donors, we found that treatment of isolated murine platelets with physiologic concentrations of LPA attenuated agonist-induced aggregation. Transgenic overexpression of autotaxin/lysophospholipase D (Enpp2), the enzyme necessary for production of the bulk of biologically active LPA in plasma, elevated circulating LPA levels and induced a bleeding diathesis and attenuation of thrombosis in mice. Intravascular administration of exogenous LPA recapitulated the prolonged bleeding time observed in Enpp2-Tg mice. Enpp2+/- mice, which have approximately 50% normal plasma LPA levels, were more prone to thrombosis. Plasma autotaxin associated with platelets during aggregation and concentrated in arterial thrombus, and activated but not resting platelets bound recombinant autotaxin/lysoPLD in an integrin-dependent manner. These results identify a novel pathway in which LPA production by autotaxin/lysoPLD regulates murine hemostasis and thrombosis and suggest that binding of autotaxin/lysoPLD to activated platelets may provide a mechanism to localize LPA production.

Inconsistency of reported uremic toxin concentrations

Discrepancies in reported uremic toxin concentrations were evaluated for 78 retention solutes. For this analysis, 378 publications were screened. Up to eight publications per toxin were retained. The highest and the lowest reported concentrations, as well as the median reported concentration were registered. The ratio between the highest and the lowest (H/L) concentrations and, for some solutes, also the ratio between the highest and the median (H/M) concentrations were calculated. The compounds were arbitrarily subdivided into three groups based on their H/L ratio: group A, H/L < 3 (n = 33); group B, 3 < H/L < 8.5 (n = 20); and group C, H/L > 8.5 (n = 25). Solutes of groups A and B showed a low to intermediate scatter, suggesting a homogeneity of reported data. Group C showed a more substantial scatter. For at least 10 compounds of group C, extremely divergent concentrations were registered (H/M > 5.5) using scatter plot analysis. For all solutes of groups A and B, the highest reported concentration could be used as a reference. For some solutes of group C and for the compounds showing a divergent scatter analysis, however, more refined directives should be followed.

Adenosine A1 receptor antagonist improves intradialytic hypotension

Intradialytic hypotension is a most frequent complication of hemodialysis and may contribute to cardiovascular events and high mortality. There is a hypothesis that an increase in adenosine generation during hemodialysis may cause vasodilation and a decrease in cardiac output, which results in systemic hypotension. We studied whether this can be blocked by an adenosine A1 receptor antagonist. We investigated the effects of an A1 antagonist, FK352, injection in 30 chronic hemodialysis patients with frequent intradialytic hypotension by a prospective, multicenter, double-blind placebo-controlled study for 4 weeks after 4 weeks of the observation period. Intradialytic hypotension was defined as systolic blood pressure (SBP) less than 110 mmHg, with SBP drop of more than 30 mmHg from the predialysis level. The efficacy of FK352 was primarily assessed by the reduction rate of dialysis hypotension between the FK352 and placebo groups. Incidence of emergency treatments caused by hypotension was evaluated. FK352 (50 mg, intravenous) or an equivalent placebo was injected into the dialysis circuit 1 h after starting dialysis. Blood pressure and heart rate were monitored every 30 min during dialysis. FK352 significantly improved intradialytic hypotension (P=0.046), in that the reduction rates of intradialytic hypotension in the FK352 and placebo groups were -12.8% (Q1 (first quantile), Q3 (third quantile): -27.5, -1.7), and +8.3% (Q1, Q3: -16.6, +16.7), respectively. The frequency of discontinuation of dialysis was significantly reduced by FK352. No apparent side effects were observed from treatment with FK352. In conclusion, the A1 antagonist FK352 may offer a novel therapeutic option for chronic dialysis patients associated with intradialytic hypotension.

Influence of the Dialysis Membrane on Markers of Tissue Ischemia

BACKGROUND

Hemodialysis (HD) is often accompanied by adverse effects, such as tissue ischemia. We have already observed an increase in plasma adenosine (ADO) levels during HD sessions, which may be the result of tissue ischemia. Here we evaluate the influence of the dialysis membrane on two sensitive and early markers of ischemia: ADO and ischemia-modified albumin (IMA).

METHODS

We included in the study 50 patients with end-stage renal failure, 39 hemodialyzed (mean age 61+/-24 years; 24 male; membranes: 23 synthetic and 16 cellulose based) and 11 undialyzed (mean age 55+/-12 years; 6 male), and 10 healthy subjects (mean age 47+/-11 years; 4 male). We compensated for hemoconcentration during HD by measuring either the IMA to albumin (Alb) or the ADO to Alb ratio.

RESULTS

Under basal conditions, the IMA to Alb ratio was not significantly different in patients and controls and HD did not significantly modify this ratio. Conversely, the ADO to Alb ratio (mean+/-SD in micromol/g) was higher in patients before HD compared with either undialyzed patients or controls (before HD: 0.077+/-0.02; undialyzed patients: 0.026+/-0.11; controls: 0.022+/-0.01). During HD, there was a significant increase in the ADO to Alb ratio (before HD: 0.077+/-0.02; after HD: 0.09+/-0.029; p<.01). We found no significant difference in the IMA to Alb or ADO to Alb ratio using either synthetic or cellulose-based membranes.

CONCLUSIONS

We concluded that ADO is a more sensitive marker of ischemia than IMA and that, under our HD conditions, the ischemia caused by HD was very weak, independent of the dialysis membrane.

Adenosine and dialysis hypotension

In this issue, Imai et al. report the results of a double-blind placebo-controlled study on the effect of an adenosine A1 receptor antagonist, FK352, on the incidence of dialysis hypotension in hypotension-prone patients. This Commentary discusses the use of selective adenosine A1 receptor antagonists for the prevention of dialysis hypotension from the perspective of the potential role of adenosine in its pathogenesis.

Plasma concentrations of adrenomedullin and ghrelin in hemodialysis patients with sustained and episodic hypotension

Sustained and/or episodic hypotension during hemodialysis (HD) is an important clinical issue. Plasma adrenomedullin (AM) is increased in HD patients with sustained hypotension, but little is known about its implications for episodic hypotension. Ghrelin may also contribute to the pathophysiology of hypotension in HD patients. We evaluated plasma levels of AM and total ghrelin in sustained hypotensive (SH; n = 23), episodic hypotensive (EH; n = 30) and normotensive (NT; n = 23) HD patients. In the EH group, the relationship between low blood pressure during HD and circulating levels of AM and ghrelin was also evaluated. Plasma levels of AM were significantly higher in SH (34.3 +/- 8.3 fmol/ml, p<0.01) than in NT patients (27.6 +/- 5.2 fmol/ml), but not in EH patients (30.8 +/- 6.1 fmol/ml). There was no significant difference of plasma total ghrelin in SH (548.1 +/- 426.5 fmol/ml) and in EH patients (544.6 +/- 174.3 fmol/ml), compared with NT patients (400.0 +/- 219.7 fmol/ml). On the other hand, in EH patients, the "suppressed blood pressure ratio" during HD significantly correlated with plasma AM (r = 0.77, p<0.001) and with total ghrelin levels (r = 0.44, p<0.05). Our results suggest that ghrelin, as well as AM, may play an important role as vasodilator local hormones and regulation of blood pressure during HD, especially the occurrence of EH. Further studies are necessary to clarify the implication of these hormones in the control of hypotension during HD.

Kinetic study of adenosine concentrations and the expression of adenosine deaminase in mononuclear cells during hemodialysis

BACKGROUND

We previously showed that high intralymphocytic adenosine (Ado) concentrations are found in hemodialyzed patients due to the reduced activity of mononuclear cell adenosine deaminase (MCADA). These abnormalities contribute to the immune defect observed in HD patients. The kinetics of these abnormalities and the causes of the low MCADA activity, however, have not been investigated. Here, we addressed this question. Since interferon gamma (IFNgamma) partially modulates MCADA, we also evaluated the effect of IFNgamma on MCADA activity in vitro.

METHODS AND RESULTS

The study included 12 patients (eight men and four women) who were observed from the first to the 36th hemodialysis (HD) session, and eight healthy subjects (controls). MCADA activity and Ado concentrations were normal before the first HD session. Ado concentrations progressively increased from the first (10.5 +/- 3.1 pmol/10(7) cells) to the fourth session (26.7 +/- 3 pmol/10(7) cells), before stabilizing at a high level. MCADA activity increased transiently until the second session (2.2 +/- 0.5 IU/10(7) cells before HD vs. 2.8 +/- 0.6 IU/10(7)cells), and then decreased and stabilized at a low level (1.0 +/- 0.5 IU/10(7)cells). The amount of MCADA mRNA transiently increased until the third session (mRNA MCADA/mRNA beta-actin: 0.6 +/- 0.2 vs. 0.8 +/- 0.2), and then decreased to 0.3 +/- 0.1 at the 36th session. MCADA activity underwent a dose-dependent increase after IFNgamma stimulation.

CONCLUSION

HD affects the transcription of the gene encoding MCADA after just three HD sessions, leading to decreased MCADA activity and increased plasma concentration of Ado.

Role of endogenous adenosine as a modulator of syncope induced during tilt testing

BACKGROUND

Previous reports that used head-up tilt testing and adenosine administration have suggested that adenosine may be an important endogenous mediator that may trigger a vasovagal response in susceptible patients. However, little is known regarding endogenous adenosine plasma levels (APLs) during vasovagal syncope provoked by tilt testing. The aim of this study was to determine whether APLs differ in patients with a positive head-up tilt test compared with those with a negative test and whether APLs are modified during tilt-induced vasovagal syncope.

METHODS AND RESULTS

APLs (mean+/-SEM) were measured during head-up tilt test in 26 patients who presented with unexplained syncope. In the 15 patients with a negative test, APLs were 0.39+/-0.03 micromol/L at baseline, 0.22+/-0.03 micromol/L immediately after tilting, and 0.44+/-0.03 micromol/L after 45 minutes. APLs were significantly higher in the 11 patients with a positive test (2.66+/-0.67 micromol/L at baseline and 3.22+/-0.85 micromol/L immediately after tilting) than in those with a negative test. During tilt testing-induced syncope, APLs increased to reach 4.03+/-0.66 micromol/L (ie, a 52% increase compared with baseline levels; P<0.02). Furthermore, we observed that the higher the APL during syncope, the shorter the time to appearance of symptoms.

CONCLUSIONS

This study showed that APLs were higher in patients with a positive tilt test than in patients with a negative test and that they increased during tilt testing-induced syncope. These observations suggest that adenosine release may be involved in the triggering mechanism of syncope induced during tilt testing.

Pharmacological and biochemical characterization of adenosine receptors in the human malignant melanoma A375 cell line

1. The present work characterizes, from a pharmacological and biochemical point of view, adenosine receptors in the human malignant melanoma A375 cell line. 2. Adenosine receptors were detected by RT - PCR experiments. A1 receptors were characterized using [3H]-DPCPX binding with a KD of 1.9+/-0.2 nM and Bmax of 23+/-7 fmol x mg(-1) of protein. A2A receptors were studied with [3H]-SCH 58261 binding and revealed a KD of 5.1+/-0.2 nM and a Bmax of 220+/-7 fmol x mg(-1) of protein. A3 receptors were studied with the new A3 adenosine receptor antagonist [3H]-MRE 3008F20, the only A3 selective radioligand currently available. Saturation experiments revealed a single high affinity binding site with KD of 3.3+/-0.7 nM and Bmax of 291+/-50 fmol x mg(-1) of protein. 3. The pharmacological profile of radioligand binding on A375 cells was established using typical adenosine ligands which displayed a rank order of potency typical of the different adenosine receptor subtype. 4. Thermodynamic data indicated that radioligand binding to adenosine receptor subtypes in A375 cells was entropy- and enthalpy-driven. 5. In functional assays the high affinity A2A agonists HE-NECA, CGS 21680 and A2A - A2B agonist NECA were able to increase cyclic AMP accumulation in A375 cells whereas A3 agonists Cl-IB-MECA, IB-MECA and NECA were able to stimulate Ca2+ mobilization. In conclusion, all these data indicate, for the first time, that adenosine receptors with a pharmacological and biochemical profile typical of the A1, A2A, A2B and A3 receptor subtype are present on A375 melanoma cell line.

High adenosine and deoxyadenosine concentrations in mononuclear cells of hemodialyzed patients

Infections are one of the most important complications of hemodialysis (HD). The high concentrations of adenosine (Ado) and of its metabolites during HD may contribute to the dialysis-induced immune deficiency through their known ability to alter lymphocyte function. The influence of HD on Ado metabolism was assessed in mononuclear cells through the measurement of (1) the concentrations of nucleosides in mononuclear cells and (2) the activities of mononuclear cell Ado deaminase (MCADA) and Ado kinase, two enzymes involved in Ado concentration regulation. Nine end-stage renal failure hemodialyzed patients (five men and four women; mean age, 69 +/- 10 yr) and eight healthy volunteers (four men and four women; mean age, 53 +/- 19 yr) were included in the study. Before HD, Ado, deoxyadenosine, and inosine concentrations were respectively 2.9-, 2.5-, and 2.5-fold higher in mononuclear cells of patients than in healthy volunteers. During HD, Ado concentration decreased by 34%, whereas inosine concentration increased by 27%. Before HD, MCADA activity level was 2.1-fold lower in patients than in control subjects. After HD, MCADA activity increased by nearly 50% but remained lower than in control subjects. Ado kinase activity level of patients did not differ from that of control subjects and was unchanged by HD. The influence of Ado on in vitro mononuclear cell proliferation and interferon-gamma production also was evaluated. Ado inhibited cell proliferation and interferon-gamma production in a dose-dependent manner, and these inhibitions were stronger for patients than for healthy volunteers. The high concentrations of Ado and deoxyadenosine in mononuclear cells and the low MCADA activity level likely are involved in the immune defect of patients who are undergoing HD.