Blockage of the adenosine A2B receptor prevents cardiac fibroblasts overgrowth in rats with pulmonary arterial hypertension

Sustained pressure overload and fibrosis of the right ventricle (RV) are the leading causes of mortality in pulmonary arterial hypertension (PAH). Although the role of adenosine in PAH has been attributed to the control of pulmonary vascular tone, cardiac reserve, and inflammatory processes, the involvement of the nucleoside in RV remodelling remains poorly understood. Conflicting results exist on targeting the low-affinity adenosine A2B receptor (A2BAR) for the treatment of PAH mostly because it displays dual roles in acute vs. chronic lung diseases. Herein, we investigated the role of the A2BAR in the viability/proliferation and collagen production by cardiac fibroblasts (CFs) isolated from RVs of rats with monocrotaline (MCT)-induced PAH. CFs from MCT-treated rats display higher cell viability/proliferation capacity and overexpress A2BAR compared to the cells from healthy littermates. The enzymatically stable adenosine analogue, 5'-N-ethylcarboxamidoadenosine (NECA, 1-30 μM), concentration-dependently increased growth, and type I collagen production by CFs originated from control and PAH rats, but its effects were more prominent in cells from rats with PAH. Blockage of the A2BAR with PSB603 (100 nM), but not of the A2AAR with SCH442416 (100 nM), attenuated the proliferative effect of NECA in CFs from PAH rats. The A2AAR agonist, CGS21680 (3 and 10 nM), was virtually devoid of effect. Overall, data suggest that adenosine signalling via A2BAR may contribute to RV overgrowth secondary to PAH. Therefore, blockage of the A2AAR may be a valuable therapeutic alternative to mitigate cardiac remodelling and prevent right heart failure in PAH patients.

2,4,5-Triaminopyrimidines as blue fluorescent probes for cell viability monitoring: synthesis, photophysical properties, and microscopy applications

Monitoring cell viability is critical in cell biology, pathology, and drug discovery. Most cell viability assays are cell-destructive, time-consuming, expensive, and/or hazardous. Herein, we present a series of newly synthesized 2,4,5-triaminopyrimidine derivatives able to discriminate between live and dead cells. To our knowledge, these compounds are the first fluorescent nucleobase analogues (FNAs) with cell viability monitoring potential. These new fluorescent molecules are synthesized using highly efficient and cost-effective methods and feature unprecedented photophysical properties (longer absorption and emission wavelengths, environment-sensitive emission, and unprecedented brightness within FNAs). Using a live-dead Saccharomyces cerevisiae cell and theoretical assays, the fluorescent 2,4,5-triaminopyrimidine derivatives were found to specifically accumulate inside dead cells by interacting with dsDNA grooves, thus paving the way for the emergence of novel and safe fluorescent cell viability markers emitting in the blue region. As the majority of commercially available viability dyes emit in the green to red region of the visible spectrum, these novel markers might be useful to meet the needs of blue markers for co-staining combinations.

Molecular pathways underlying sympathetic autonomic overshooting leading to fear and traumatic memories: looking for alternative therapeutic options for post-traumatic stress disorder

The sympathoadrenal medullary system and the hypothalamic-pituitary-adrenal axis are both activated upon stressful events. The release of catecholamines, such as dopamine, norepinephrine (NE), and epinephrine (EPI), from sympathetic autonomic nerves participate in the adaptive responses to acute stress. Most theories suggest that activation of peripheral β-adrenoceptors (β-ARs) mediates catecholamines-induced memory enhancement. These include direct activation of β-ARs in the vagus nerve, as well as indirect responses to catecholamine-induced glucose changes in the brain. Excessive sympathetic activity is deeply associated with memories experienced during strong emotional stressful conditions, with catecholamines playing relevant roles in fear and traumatic memories consolidation. Recent findings suggest that EPI is implicated in fear and traumatic contextual memories associated with post-traumatic stress disorder (PTSD) by increasing hippocampal gene transcription (e.g., Nr4a) downstream to cAMP response-element protein activation (CREB). Herein, we reviewed the literature focusing on the molecular mechanisms underlying the pathophysiology of memories associated with fear and traumatic experiences to pave new avenues for the treatment of stress and anxiety conditions, such as PTSD.

Adenosinergic System and BDNF Signaling Changes as a Cross-Sectional Feature of RTT: Characterization of Mecp2 Heterozygous Mouse Females

Rett Syndrome is an X-linked neurodevelopmental disorder (RTT; OMIM#312750) associated to MECP2 mutations. MeCP2 dysfunction is seen as one cause for the deficiencies found in brain-derived neurotrophic factor (BDNF) signaling, since BDNF is one of the genes under MeCP2 jurisdiction. BDNF signaling is also dependent on the proper function of the adenosinergic system. Indeed, both BDNF signaling and the adenosinergic system are altered in Mecp2-null mice (Mecp2-/y), a representative model of severe manifestation of RTT. Considering that symptoms severity largely differs among RTT patients, we set out to investigate the BDNF and ADO signaling modifications in Mecp2 heterozygous female mice (Mecp2+/-) presenting a less severe phenotype. Symptomatic Mecp2+/- mice have lower BDNF levels in the cortex and hippocampus. This is accompanied by a loss of BDNF-induced facilitation of hippocampal long-term potentiation (LTP), which could be restored upon selective activation of adenosine A2A receptors (A2AR). While no differences were observed in the amount of adenosine in the cortex and hippocampus of Mecp2+/- mice compared with healthy littermates, the density of the A1R and A2AR subtype receptors was, respectively, upregulated and downregulated in the hippocampus. Data suggest that significant changes in BDNF and adenosine signaling pathways are present in an RTT model with a milder disease phenotype: Mecp2+/- female animals. These features strengthen the theory that boosting adenosinergic activity may be a valid therapeutic strategy for RTT patients, regardless of their genetic penetrance.

The Carcinogenic Potential of Bisphenol A in the Liver Based on Transcriptomic Studies

Bisphenol A (BPA) is an environmental toxin widely used in the production of polycarbonate plastics. A correlation exists between BPA tissue contamination and the occurrence of pathological conditions, including cancer. First-passage detoxification of high BPA amounts in the liver promotes hepatotoxicity and morphological alterations of this organ, but there is a lack of knowledge about the molecular mechanisms underlying these phenomena. This prompted us to investigate changes in the liver transcriptomics of 3-month-old female mice exposed to BPA (50 mg/kg) in drinking water for 3 months. Five female mice served as controls. The animals were euthanized, the livers were collected, and RNA was extracted to perform RNA-seq analysis. The multistep transcriptomic bioinformatics revealed 120 differentially expressed genes (DEGs) in the BPA-exposed samples. Gene Ontology (GO) annotations indicated that DEGs have been assigned to many biological processes, including "macromolecule modification" and "protein metabolic process". Several of the revealed DEGs have been linked to the pathogenesis of severe metabolic liver disorders and malignant tumors, in particular hepatocellular carcinoma. Data from this study suggest that BPA has a significant impact on gene expression in the liver, which is predictive of the carcinogenic potential of this compound in this organ.

Could hypoxia rehabilitate the osteochondral diseased interface? Lessons from the interplay of hypoxia and purinergic signals elsewhere

The osteochondral unit comprises the articular cartilage (90%), subchondral bone (5%) and calcified cartilage (5%). All cells present at the osteochondral unit that is ultimately responsible for matrix production and osteochondral homeostasis, such as chondrocytes, osteoblasts, osteoclasts and osteocytes, can release adenine and/or uracil nucleotides to the local microenvironment. Nucleotides are released by these cells either constitutively or upon plasma membrane damage, mechanical stress or hypoxia conditions. Once in the extracellular space, endogenously released nucleotides can activate membrane-bound purinoceptors. Activation of these receptors is fine-tuning regulated by nucleotides' breakdown by enzymes of the ecto-nucleotidase cascade. Depending on the pathophysiological conditions, both the avascular cartilage and the subchondral bone subsist to significant changes in oxygen tension, which has a tremendous impact on tissue homeostasis. Cell stress due to hypoxic conditions directly influences the expression and activity of several purinergic signalling players, namely nucleotide release channels (e.g. Cx43), NTPDase enzymes and purinoceptors. This review gathers experimental evidence concerning the interplay between hypoxia and the purinergic signalling cascade contributing to osteochondral unit homeostasis. Reporting deviations to this relationship resulting from pathological alterations of articular joints may ultimately unravel novel therapeutic targets for osteochondral rehabilitation. At this point, one can only hypothesize how hypoxia mimetic conditions can be beneficial to the ex vivo expansion and differentiation of osteo- and chondro-progenitors for auto-transplantation and tissue regenerative purposes.

Silencing NTPDase3 activity rehabilitates the osteogenic commitment of post-menopausal stem cell bone progenitors

BACKGROUND

Endogenously released adenine and uracil nucleotides favour the osteogenic commitment of bone marrow-derived mesenchymal stromal cells (BM-MSCs) through the activation of ATP-sensitive P2X7 and UDP-sensitive P2Y₆ receptors. Yet, these nucleotides have their osteogenic potential compromised in post-menopausal (Pm) women due to overexpression of nucleotide metabolizing enzymes, namely NTPDase3. This prompted us to investigate whether NTPDase3 gene silencing or inhibition of its enzymatic activity could rehabilitate the osteogenic potential of Pm BM-MSCs.

METHODS

MSCs were harvested from the bone marrow of Pm women (69 ± 2 years old) and younger female controls (22 ± 4 years old). The cells were allowed to grow for 35 days in an osteogenic-inducing medium in either the absence or the presence of NTPDase3 inhibitors (PSB 06126 and hN3-B3_s antibody); pre-treatment with a lentiviral short hairpin RNA (Lenti-shRNA) was used to silence the NTPDase3 gene expression. Immunofluorescence confocal microscopy was used to monitor protein cell densities. The osteogenic commitment of BM-MSCs was assessed by increases in the alkaline phosphatase (ALP) activity. The amount of the osteogenic transcription factor Osterix and the alizarin red-stained bone nodule formation. ATP was measured with the luciferin-luciferase bioluminescence assay. The kinetics of the extracellular ATP (100 µM) and UDP (100 µM) catabolism was assessed by HPLC RESULTS: The extracellular catabolism of ATP and UDP was faster in BM-MSCs from Pm women compared to younger females. The immunoreactivity against NTPDase3 increased 5.6-fold in BM-MSCs from Pm women vs. younger females. Selective inhibition or transient NTPDase3 gene silencing increased the extracellular accumulation of adenine and uracil nucleotides in cultured Pm BM-MSCs. Downregulation of NTPDase3 expression or activity rehabilitated the osteogenic commitment of Pm BM-MSCs measured as increases in ALP activity, Osterix protein cellular content and bone nodule formation; blockage of P2X7 and P2Y₆ purinoceptors prevented this effect.

CONCLUSIONS

Data suggest that NTPDase3 overexpression in BM-MSCs may be a clinical surrogate of the osteogenic differentiation impairment in Pm women. Thus, besides P2X7 and P2Y₆ receptors activation, targeting NTPDase3 may represent a novel therapeutic strategy to increase bone mass and reduce the osteoporotic risk of fractures in Pm women.

Purinergic Tuning of the Tripartite Neuromuscular Synapse

The vertebrate neuromuscular junction (NMJ) is a specialised chemical synapse involved in the transmission of bioelectric signals between a motor neuron and a skeletal muscle fiber, leading to muscle contraction. Typically, the NMJ is a tripartite synapse comprising (a) a presynaptic region represented by the motor nerve ending, (b) a postsynaptic skeletal motor endplate area, and (c) perisynaptic Schwann cells (PSCs) that shield the motor nerve terminal. Increasing evidence points towards the role of PSCs in the maintenance and control of neuromuscular integrity, transmission, and plasticity. Acetylcholine (ACh) is the main neurotransmitter at the vertebrate skeletal NMJ, and its role is fine-tuned by co-released purinergic neuromodulators, like adenosine 5'-triphosphate (ATP) and its metabolite adenosine (ADO). Adenine nucleotides modulate transmitter release and expression of postsynaptic ACh receptors at motor synapses via the activation of P2Y and P2X receptors. Endogenously generated ADO modulates ACh release by acting via co-localised inhibitory A₁ and facilitatory A_2A receptors on motor nerve terminals, whose tonic activation depends on the neuronal firing pattern and their interplay with cholinergic receptors and neuropeptides. Thus, the concerted action of adenine nucleotides, ADO, and ACh/neuropeptide co-transmitters is paramount to adapting the neuromuscular transmission to the working load under pathological conditions, like Myasthenia gravis. Unravelling these functional complexities prompted us to review our knowledge about the way purines orchestrate neuromuscular transmission and plasticity in light of the tripartite synapse concept, emphasising the often-forgotten role of PSCs in this context.

RNA-seq reveals that anti-obesity irisin and triiodothyronine (T3) hormones differentially affect the purinergic signaling transcriptomics in differentiated human adipocytes

The anti-obesity thyroid hormone, triiodothyronine (T3), and irisin, an exercise- and/or cold-induced myokine, stimulate thermogenesis and energy consumption while decreasing lipid accumulation. The involvement of ATP signaling in adipocyte cell function and obesity has attracted increasing attention, but the crosstalk between the purinergic signaling cascade and anti-obesity hormones lacks experimental evidence. In this study, we investigated the effects of T3 and irisin in the transcriptomics of membrane-bound purinoceptors, ectonucleotidase enzymes and nucleoside transporters participating in the purinergic signaling in cultured human adipocytes. The RNA-seq analysis revealed that differentiated adipocytes express high amounts of ADORA1, P2RY11, P2RY12, and P2RX6 gene transcripts, along with abundant levels of transcriptional products encoding to purine metabolizing enzymes (ENPP2, ENPP1, NT5E, ADA and ADK) and transporters (SLC29A1, SCL29A2). The transcriptomics of purinergic signaling markers changed in parallel to the upsurge of "browning" adipocyte markers, like UCP1 and P2RX5, after treatment with T3 and irisin. Upregulation of ADORA1, ADORA2A and P2RX4 gene transcription was obtained with irisin, whereas T3 preferentially upregulated NT5E, SLC29A2 and P2RY11 genes. Irisin was more powerful than T3 towards inhibition of the leptin gene transcription, the SCL29A1 gene encoding for the ENT1 transporter, the E-NPP2 (autotaxin) gene, and genes that encode for two ADP-sensitive P2Y receptors, P2RY1 and P2RY12. These findings indicate that anti-obesity irisin and T3 hormones differentially affect the purinergic signaling transcriptomics, which might point towards new directions for the treatment of obesity and related metabolic disorders that are worth to be pursued in future functional studies.

P2X7 receptor antagonist improves gastrointestinal disorders in spontaneously hypertensive rats

The purinergic system participates in the control of blood pressure. Hypertension promotes the occurrence of gastrointestinal disorders such as intestinal inflammation and gastric emptying delay. This study aimed i) to investigate the participation of the P2X7 receptor blocker Brilliant Blue G (BBG) on gastric emptying of solids and changes in oxidative stress in the gastric fundus, duodenum, and colon of spontaneously hypertensive rats (SHR) and ii) to study the putative relationship of this effect with the renin-angiotensin system. Rats were divided into five groups: Control, SHR, SHR+BBG, SHR+BBG+ATP, and SHR+BBG+ANG II. In the gastrointestinal tract, we assessed gastric emptying (GE) and oxidative stress markers (NOx, MPO, GSH, SOD). We observed a decrease in the GE rate (P<0.05) in SHR vs control rats (21.8±2.0% vs 42.8±3.5%). The decrease in GE was returned (P<0.05) to control levels by BBG in SHR rats (21.8±2.0% vs 41.6±3.2%). Co-administration of ATP or ANG II together with BBG bypassed the effect of the P2X7 antagonist on GE in SHR (P<0.05) (21.9±5.0% vs 25.6±3.0% vs 41.6±3.2%). The MPO activity increased (P<0.05) in the gastric fundus of SHR compared to control rats (6.12±2.26 vs 0.077±0.02 UMPO/mg tissue); this effect was prevented (P<0.05) by BBG (0.55±0.15 vs 6.12±2.26 UMPO/mg tissue). Data demonstrated that blockage of P2X7 receptors with BBG can improve the GE delay and oxidative stress biomarkers in SHR animals. This preventive effect of BBG on GE delay was abrogated by ANG II and ATP, thus prompting crosstalk between renin-angiotensin and the purinergic signaling systems underlying this phenomenon.

Mesial Temporal Lobe Epilepsy (MTLE) Drug-Refractoriness Is Associated With P2X7 Receptors Overexpression in the Human Hippocampus and Temporal Neocortex and May Be Predicted by Low Circulating Levels of miR-22

Objective: ATP-gated ionotropic P2X7 receptors (P2X7R) actively participate in epilepsy and other neurological disorders. Neocortical nerve terminals of patients with Mesial Temporal Lobe Epilepsy with Hippocampal Sclerosis (MTLE-HS) express higher P2X7R amounts. Overexpression of P2X7R bolsters ATP signals during seizures resulting in glial cell activation, cytokines production, and GABAergic rundown with unrestrained glutamatergic excitation. In a mouse model of status epilepticus, increased expression of P2X7R has been associated with the down-modulation of the non-coding micro RNA, miR-22. MiR levels are stable in biological fluids and normally reflect remote tissue production making them ideal disease biomarkers. Here, we compared P2X7R and miR-22 expression in epileptic brains and in the serum of patients with MTLE-HS, respectively.

Methods: Quantitative RT-PCR was used to evaluate the expression of P2X7R in the hippocampus and anterior temporal lobe of 23 patients with MTLE-HS and 10 cadaveric controls. Confocal microscopy and Western blot analysis were performed to assess P2X7R protein amounts. MiR-22 expression was evaluated in cell-free sera of 40 MTLE-HS patients and 48 healthy controls.

Results: Nerve terminals of the hippocampus and neocortical temporal lobe of MTLE-HS patients overexpress (p &lt; 0.05) an 85 kDa P2X7R protein whereas the normally occurring 67 kDa receptor protein dominates in the brain of the cadaveric controls. Contrariwise, miR-22 serum levels are diminished (p &lt; 0.001) in MTLE-HS patients compared to age-matched control blood donors, a situation that is more evident in patients requiring multiple (&gt;3) anti-epileptic drug (AED) regimens.

Conclusion: Data show that there is an inverse relationship between miR-22 serum levels and P2X7R expression in the hippocampus and neocortex of MTLE-HS patients, which implies that measuring serum miR-22 may be a clinical surrogate of P2X7R brain expression in the MTLE-HS. Moreover, the high area under the ROC curve (0.777; 95% CI 0.629–0.925; p = 0.001) suggests that low miR-22 serum levels may be a sensitive predictor of poor response to AEDs among MTLE-HS patients. Results also anticipate that targeting the miR-22/P2X7R axis may be a good strategy to develop newer AEDs.

Pharmacological Tuning of Adenosine Signal Nuances Underlying Heart Failure With Preserved Ejection Fraction

Heart failure with preserved ejection fraction (HFpEF) roughly represents half of the cardiac failure events in developed countries. The proposed 'systemic microvascular paradigm' has been used to explain HFpHF presentation heterogeneity. The lack of effective treatments with few evidence-based therapeutic recommendations makes HFpEF one of the greatest unmet clinical necessities worldwide. The endogenous levels of the purine nucleoside, adenosine, increase significantly following cardiovascular events. Adenosine exerts cardioprotective, neuromodulatory, and immunosuppressive effects by activating plasma membrane-bound P1 receptors that are widely expressed in the cardiovascular system. Its proven benefits have been demonstrated in preclinical animal tests. Here, we provide a comprehensive and up-to-date critical review about the main therapeutic advantages of tuning adenosine signalling pathways in HFpEF, without discounting their side effects and how these can be seized.

Pitfalls and challenges of the purinergic signaling cascade in obesity

Obesity is a worldwide health problem which have reached pandemic proportions, now also including low and middle-income countries. Excessive or abnormal fat deposition in the abdomen especially in the visceral compartment is tightly associated with a high metabolic risk for arterial hypertension, type II diabetes, cardiovascular diseases, musculoskeletal disorders (especially articular degeneration) and some cancers. Contrariwise, accumulation of fat in the subcutaneous compartment has been associated with a neutral metabolic impact, favoring a lower risk of insulin resistance. Obesity results more often from an avoidable imbalance between food consumption and energy expenditure. There are several recommended strategies for dealing with obesity, including pharmacological therapies, but their success remains incomplete and may not compensate the associated adverse effects. Purinergic signaling operated by ATP and its metabolite, adenosine, has attracted increasing attention in obesity. The extracellular levels of purines often reflect the energy status of a given cell population. Adenine nucleotides and nucleosides fine tuning control adipogenesis and mature adipocytes function via the activation of P2 and P1 purinoceptors, respectively. These features make the purinergic signaling cascade a putative target for therapeutic intervention in obesity and related metabolic syndromes. There are, however, gaps in our knowledge regarding the role of purines in adipocyte precursors differentiation and mature adipocytes functions, as well as their impact among distinct adipose tissue deposits (e.g. white vs. brown, visceral vs. subcutaneous), which warrants further investigations before translation to clinical trials can be made.

Impairment of adenosinergic system in Rett syndrome: Novel therapeutic target to boost BDNF signalling

Rett syndrome (RTT; OMIM#312750) is mainly caused by mutations in the X-linked MECP2 gene (methyl-CpG-binding protein 2 gene; OMIM*300005), which leads to impairments in the brain-derived neurotrophic factor (BDNF) signalling. The boost of BDNF mediated effects would be a significant breakthrough but it has been hampered by the difficulty to administer BDNF to the central nervous system. Adenosine, an endogenous neuromodulator, may accomplish that role since through A_2AR it potentiates BDNF synaptic actions in healthy animals. We thus characterized several hallmarks of the adenosinergic and BDNF signalling in RTT and explored whether A_2AR activation could boost BDNF actions. For this study, the RTT animal model, the Mecp2 knockout (Mecp2^-/y) (B6.129P2 (C)-Mecp2tm1.1Bird/J) mouse was used. Whenever possible, parallel data was also obtained from post-mortem brain samples from one RTT patient. Ex vivo extracellular recordings of field excitatory post-synaptic potentials in CA1 hippocampal area were performed to evaluate synaptic transmission and long-term potentiation (LTP). RT-PCR was used to assess mRNA levels and Western Blot or radioligand binding assays were performed to evaluate protein levels. Changes in cortical and hippocampal adenosine content were assessed by liquid chromatography with diode array detection (LC/DAD). Hippocampal ex vivo experiments revealed that the facilitatory actions of BDNF upon LTP is absent in Mecp2^-/y mice and that TrkB full-length (TrkB-FL) receptor levels are significantly decreased. Extracts of the hippocampus and cortex of Mecp2^-/y mice revealed less adenosine amount as well as less A_2AR protein levels when compared to WT littermates, which may partially explain the deficits in adenosinergic tonus in these animals. Remarkably, the lack of BDNF effect on hippocampal LTP in Mecp2^-/y mice was overcome by selective activation of A_2AR with CGS21680. Overall, in Mecp2^-/y mice there is an impairment on adenosinergic system and BDNF signalling. These findings set the stage for adenosine-based pharmacological therapeutic strategies for RTT, highlighting A_2AR as a therapeutic target in this devastating pathology.

Non-genomic Actions of Methylprednisolone Differentially Influence GABA and Glutamate Release From Isolated Nerve Terminals of the Rat Hippocampus

Corticosteroids exert a dual role in eukaryotic cells through their action via (1) intracellular receptors (slow genomic responses), or (2) membrane-bound receptors (fast non-genomic responses). Highly vulnerable regions of the brain, like the hippocampus, express high amounts of corticosteroid receptors, yet their actions on ionic currents and neurotransmitters release are still undefined. Here, we investigated the effect of methylprednisolone (MP) on GABA and glutamate (Glu) release from isolated nerve terminals of the rat hippocampus. MP favored both spontaneous and depolarization-evoked [¹⁴C]Glu release from rat hippocampal nerve terminals, without affecting [³H]GABA outflow. Facilitation of [¹⁴C]Glu release by MP is mediated by a Na⁺-dependent Ca²⁺-independent non-genomic mechanism relying on the activation of membrane-bound glucocorticoid (GR) and mineralocorticoid (MR) receptors sensitive to their antagonists mifepristone and spironolactone, respectively. The involvement of Na⁺-dependent high-affinity EAAT transport reversal was inferred by blockage of MP-induced [¹⁴C]Glu release by DL-TBOA. Depolarization-evoked [³H]GABA release in the presence of MP was partially attenuated by the selective P2X7 receptor antagonist A-438079, but this compound did not affect the release of [¹⁴C]Glu. Data indicate that MP differentially affects GABA and glutamate release from rat hippocampal nerve terminals via fast non-genomic mechanisms putatively involving the activation of membrane-bound corticosteroid receptors. Facilitation of Glu release strengthen previous assumptions that MP may act as a cognitive enhancer in rats, while crosstalk with ATP-sensitive P2X7 receptors may promote a therapeutically desirable GABAergic inhibitory control during paroxysmal epileptic crisis that might be particularly relevant when extracellular Ca²⁺ levels decrease below the threshold required for transmitter release.

Nicotinic α7 receptor-induced adenosine release from perisynaptic Schwann cells controls acetylcholine spillover from motor endplates

Acetylcholine (ACh) spillover from motor endplates occurs after neuronal firing bursts being potentiated by cholinesterase inhibitors (e.g., neostigmine). Nicotinic α7 receptors (α7nAChR) on perisynaptic Schwann cells (PSCs) can control ACh spillover by unknown mechanisms. We hypothesized that adenosine might be the gliotransmitter underlying PSCs-nerve terminal communication. Rat isolated hemidiaphragm preparations were used to measure (1) the outflow of [³ H]ACh, (2) real-time transmitter exocytosis by video-microscopy with the FM4-64 fluorescent dye, and (3) skeletal muscle contractions during high-frequency (50 Hz) nerve stimulation bursts in the presence of a selective α7nAChR agonist, PNU 282987, or upon inhibition of cholinesterase activity with neostigmine. To confirm our prediction that α7nAChR-mediated effects require direct activation of PSCs, we used fluorescence video-microscopy in the real-time mode to measure PNU 282987-induced [Ca²⁺ ]_i transients from Fluo-4 NW loaded PSCs in non-stimulated preparations. The α7nAChR agonist, PNU 282987, decreased nerve-evoked diaphragm tetanic contractions. PNU 282987-induced inhibition was mimicked by neostigmine and results from the reduction of ACh exocytosis measured as decreases in [³ H]ACh release and FM4-64 fluorescent dye unloading. Methyllycaconitine blockage of α7nAChR and the fluoroacetate gliotoxin both prevented inhibition of nerve-evoked ACh release and PSCs [Ca²⁺ ]_i transients triggered by PNU 282987 and neostigmine. Adenosine deamination, inhibition of the ENT1 nucleoside outflow, and blockage of A₁ receptors prevented PNU 282987-induced inhibition of transmitter release. Data suggest that α7nAChR controls tetanic-induced ACh spillover from the neuromuscular synapse by promoting adenosine outflow from PSCs via ENT1 transporters and retrograde activation of presynaptic A₁ inhibitory receptors.

β3 Adrenoceptor-induced cholinergic inhibition in human and rat urinary bladders involves the exchange protein directly activated by cyclic AMP 1 favoring adenosine release

BACKGROUND AND PURPOSE

The mechanism by which β₃ receptor agonists (e.g. mirabegron) control bladder overactivity may involve adenosine release from human and rat detrusor smooth muscle. Retrograde activation of adenosine A₁ receptors reduces ACh release from cholinergic bladder nerves. β₃ -Adrenoceptors usually couple to adenylyl cyclase. Here we investigated, which of the cAMP targets, protein kinase A or the exchange protein directly activated by cAMP (EPAC) could be involved in this cholinergic inhibition of the bladder.

EXPERIMENTAL APPROACH

[³ H]ACh and adenosine release from urothelium-denuded detrusor strips of cadaveric human organ donors and rats were measured by liquid scintillation spectrometry and HPLC, respectively. In vivo cystometry was also performed in urethane-anaesthetized rats.

KEY RESULTS

The exchange protein directly activated by cAMP (EPAC) inhibitor, ESI-09, prevented mirabegron- and isoprenaline-induced adenosine release from human and rat detrusor strips respectively. ESI-09, but not the PKA inhibitor, H-89, attenuated inhibition of [³ H]ACh release from stimulated (10 Hz) detrusor strips caused by activating β₃ -adrenoceptors, AC (forskolin) and EPAC1 (8-CTP-2Me-cAMP). Isoprenaline-induced inhibition of [³ H]ACh release was also prevented by inhibitors of PKC (chelerythrine and Go6976) and of the equilibrative nucleoside transporter 1 (ENT1; dipyridamole and NBTI), but not by PLC inhibition with U73122. Pretreatment with ESI-09, but not with H-89, prevented the reduction of the voiding frequency caused by isoprenaline and forskolin in vivo.

CONCLUSION AND IMPLICATIONS

Data suggest that β₃ -adrenoceptor-induced inhibition of cholinergic neurotransmission in human and rat urinary bladders involves activation of an EPAC1/PKC pathway downstream cAMP production resulting in adenosine outflow via ENT1.

Adenosinergic signalling in chondrogenesis and cartilage homeostasis: Friend or foe?

Chondrocytes and their mesenchymal cell progenitors secrete a variety of bioactive molecules, including adenine nucleotides and nucleosides, but these molecules are not usually highlighted in review papers about the secretome of these cells. Ageing and inflammatory insults compromise chondrocytes ability to keep ATP/adenosine synthesis, release and turnover. Cartilage homeostasis depends on extracellular adenosine levels, which acting via four P1 purinoceptor subtypes modulates the release of pro-inflammatory mediators, including NO, PGE₂ and several cytokines. Native articular cartilage is challenged by synovial fluid flow during normal joint motion transiently increasing ATP release and adenosine formation in the joint microenvironment. Excessive joint motion and shockwave trauma are deleterious to cartilage homeostasis due to HIF-1α overexpression, resulting in disproportionate ecto-5'-nucleotidase/CD73 production, adenosine accumulation and superfluous A_2B receptors activation. Scarcity of data however exists on the putative interplay between coexistent high affinity (A_2A and A₃) and low affinity (A_2B) adenosine receptors activation affecting stem cells fate towards preferential chondrogenic or osteogenic lineages in the human cartilage. Hints gathered in this commentary result mainly from studies using human immortalized cell lines and animal (e.g. rodent, equine, bovine) tissue samples. The available data point towards adenosine A_2A and A₃ receptors having cartilage protective roles, while excessive adenosine accumulation may be detrimental via low affinity A_2B receptors activation, with little reference to the putative role of the adenosine forming enzyme ecto-5'-nucleotidase/CD73. Thus, emphasizing the multiple pathways responsible for controlling adenosine signalling in cartilage will certainly impact on the search for novel therapeutic targets for highly disabling articular disorders.

Endomyocardial Fibrosis: an Update After 70 Years

PURPOSE OF REVIEW

This review aims at highlighting the need to better understand the pathogenesis and natural history of endomyocardial fibrosis when set against its changing endemicity and disease burden, improvements in diagnosis, and new options for clinical management.

RECENT FINDINGS

Progress in imaging diagnostic techniques and availability of new targets for drug and surgical treatment of heart failure are contributing to earlier diagnosis and may lead to improvement in patient survival. Endomyocardial fibrosis was first described in Uganda by Davies more than 70 years ago (1948). Despite its poor prognosis, the etiology of this neglected tropical restrictive cardiomyopathy still remains enigmatic nowadays. Our review reflects on the journey of scientific discovery and construction of the current guiding concepts on this mysterious and fascinating condition, bringing to light the contemporary knowledge acquired over these years. Here we describe novel tools for diagnosis, give an overview of the improvement in clinical management, and finally, suggest research themes that can help improve patient outcomes focusing (whenever possible) on novel players coming into action.

Activation of Prejunctional P2x2/3 Heterotrimers by ATP Enhances the Cholinergic Tone in Obstructed Human Urinary Bladders

The objective of this study was to investigate the role of ATP in cholinergic neurotransmission in the urinary bladder of control men and of patients obstructed as a result of benign prostatic hyperplasia (BPH). Human detrusor samples were collected from 41 patients who submitted to transvesical prostatectomy resulting from BPH and 26 male organ donors. The release of [³H]acetylcholine ([³H]ACh) was evoked by electrical field stimulation (10 Hz, 200 pulses) in urothelium-denuded detrusor strips. Myographic recordings were performed to test detrusor strip sensitivity to ACh and ATP. Nerve-evoked [³H]ACh release was 1.5-fold higher in detrusor strips from BPH patients compared with controls. This difference was abolished after desensitization of ionotropic P2X1-3 receptors with an ATP analog, α,β-methylene ATP (30 μM, applied for 15 minutes). TNP-ATP (10 nM, a preferential P2X2/3 antagonist) and A317491 (100 nM, a selective P2X3 antagonist) were about equipotent in decreasing nerve-evoked [³H]ACh release in control detrusor strips, but the selective P2X1 receptor antagonist NF023 (3 μM) was devoid of effect. The inhibitory effect of TNP-ATP (10 nM) increased from 27% ± 9% to 43% ± 6% in detrusor strips of BPH patients, but the effect of A317491 (100 nM) [³H]ACh release unaltered (20% ± 2% vs. 24% ± 4%). The amplitude of ACh (0.1-100 μM)-induced myographic recordings decreased, whereas sensitivity to ATP (0.01-3 mM) increased in detrusor strips from BPH patients. Besides the well characterized P2X1 receptor-mediated contractile activity of ATP in pathologic human bladders, we show here for the first time that cholinergic hyperactivity in the detrusor of BPH patients is facilitated by activation of ATP-sensitive P2X2/3 heterotrimers. SIGNIFICANCE STATEMENT: Bladder outlet obstruction often leads to detrusor overactivity and reduced bladder compliance in parallel to atropine-resistant increased purinergic tone. Our data show that P2X1 purinoceptors are overexpressed in the detrusor of patients with benign prostatic hyperplasia. Besides the P2X1 receptor-mediated detrusor contractions, ATP favors nerve-evoked acetylcholine release via the activation of prejunctional P2X2/3 excitatory receptors in these patients Thus, our hypothesis is that manipulation of the purinergic tone may be therapeutically useful to counteract cholinergic overstimulation in obstructed patients.

The Ionotropic P2X4 Receptor has Unique Properties in the Heart by Mediating the Negative Chronotropic Effect of ATP While Increasing the Ventricular Inotropy

Background: Mounting evidence indicate that reducing the sinoatrial node (SAN) activity may be a useful therapeutic strategy to control of heart failure. Purines, like ATP and its metabolite adenosine, consistently reduce the SAN spontaneous activity leading to negative cardiac chronotropy, with variable effects on the force of myocardial contraction (inotropy). Apart from adenosine A₁ receptors, the human SAN expresses high levels of ATP-sensitive ionotropic P2X4 receptors (P2X4R), yet their cardiac role is unexplored. Methods: Here, we investigated the activity of P2 purinoceptors on isolated spontaneously beating atria (chronotropy) and on 2 Hz-paced right ventricular (RV, inotropy) strips from Wistar rats. Results: ATP (pEC ₅₀ = 4.05) and its stable analogue ATPγS (pEC ₅₀ = 4.69) concentration-dependently reduced atrial chronotropy. Inhibition of ATP breakdown into adenosine by NTPDases with POM-1 failed to modify ATP-induced negative chronotropy. The effect of ATP on atrial rate was attenuated by a broad-spectrum P2 antagonist, PPADS, as well as by 5-BDBD, which selectively blocks the P2X4R subtype; however, no effect was observed upon blocking the A₁ receptor with DPCPX. The P2X4R positive allosteric modulator, ivermectin, increased the negative chronotropic response of ATP. Likewise, CTP, a P2X agonist that does not generate adenosine, replicated the P2X4R-mediated negative chronotropism of ATP. Inhibition of the Na⁺/Ca²⁺ exchanger (NCX) with KB-R7943 and ORM-10103, but not blockage of the HCN channel with ZD7288, mimicked the effect of the P2X4R blocker, 5-BDBD. In paced RV strips, ATP caused a mild negative inotropic effect, which magnitude was 2 to 3-fold increased by 5-BDBD and KB-R7943. Immunofluorescence confocal microscopy studies confirm that cardiomyocytes of the rat SAN and RV co-express P2X4R and NCX1 proteins. Conclusions: Data suggest that activation of ATP-sensitive P2X4R slows down heart rate by reducing the SAN activity while increasing the magnitude of ventricular contractions. The mechanism underlying the dual effect of ATP in the heart may involve inhibition of intracellular Ca²⁺-extrusion by bolstering NCX function in the reverse mode. Thus, targeting the P2X4R activation may create novel well-tolerated heart-rate lowering drugs with potential benefits in patients with deteriorated ventricular function.

Tetanic Facilitation of Neuromuscular Transmission by Adenosine A2A and Muscarinic M1 Receptors is Dependent on the Uptake of Choline via High-Affinity Transporters

BACKGROUND/AIMS

In this study, we evaluated the functional impact of facilitatory presynaptic adenosine A2A and muscarinic M1 receptors in the recovery of neuromuscular tetanic depression caused by the blockage of high-affinity choline transporter (HChT) by hemicholinium-3 (HC-3), a condition that mimics a myasthenia-like condition.

METHODS

Rat diaphragm preparations were indirectly stimulated via the phrenic nerve trunk with 50-Hz frequency trains, each consisting of 500-750 supramaximal intensity pulses. The tension at the beginning (A) and at the end (B) of the tetanus was recorded and the ratio (R) B/A calculated.

RESULTS

Activation of A2A and M1 receptors with CGS21680 (CGS; 2 nmol/L) and McN-A-343c (McN; 3 μmol/L) increased R values. Similar facilitatory effects were obtained with forskolin (FSK; 3 μmol/L) and phorbol 12-myristate 13-acetate (PMA; 10 μmol/L), which activate adenylate cyclase and protein kinase C respectively. HC-3 (4 μmol/L) decreased transmitter exocytosis measured by real-time videomicroscopy with the FM4-64 fluorescent dye and prevented the facilitation of neuromuscular transmission caused by CGS, McN, and FSK, with a minor effect on PMA. The acetylcholinesterase inhibitor, neostigmine (NEO; 0.5 μmol/L), also decreased transmitter exocytosis. The paradoxical neuromuscular tetanic fade caused by NEO (0.5 μmol/L) was also prevented by HC-3 (4 μmol/L) and might result from the rundown of the positive feedback mechanism operated by neuronal nicotinic receptors (blocked by hexamethonium, 120 μmol/L).

CONCLUSION

Data suggest that the recovery of tetanic neuromuscular facilitation by adenosine A2A and M1 receptors is highly dependent on HChT activity and may be weakened in myasthenic patients when HChT is inoperative.

Is the adenosine A2B 'biased' receptor a valuable target for the treatment of pulmonary arterial hypertension?

Pulmonary arterial hypertension (PAH) is a maladaptive disorder characterized by increased pulmonary vascular resistance leading to right ventricular failure and death. Adenosine released by injured tissues, such as the lung and heart, influences tissue remodeling through the activation of adenosine receptors. Evidence regarding activation of the low-affinity A_2BAR by adenosine points towards pivotal roles of this receptor in processes associated with both acute and chronic lung diseases. Conflicting results exist concerning the beneficial or detrimental roles of the A_2B 'biased' receptor in right ventricular failure secondary to PAH. In this review, we discuss the pros and cons of manipulating A_2BARs as a putative therapeutic target in PAH.

Under stressful conditions activation of the ionotropic P2X7 receptor differentially regulates GABA and glutamate release from nerve terminals of the rat cerebral cortex

γ-Aminobutyric acid (GABA) and glutamate (Glu) are the main inhibitory and excitatory neurotransmitters in the central nervous system (CNS), respectively. Fine tuning regulation of extracellular levels of these amino acids is essential for normal brain activity. Recently, we showed that neocortical nerve terminals from patients with epilepsy express higher amounts of the non-desensitizing ionotropic P2X7 receptor. Once activated by ATP released from neuronal cells, the P2X7 receptor unbalances GABAergic vs. glutamatergic neurotransmission by differentially interfering with GABA and Glu uptake. Here, we investigated if activation of the P2X7 receptor also affects [³H]GABA and [¹⁴C]Glu release measured synchronously from isolated nerve terminals (synaptosomes) of the rat cerebral cortex. Data show that activation of the P2X7 receptor consistently increases [¹⁴C]Glu over [³H]GABA release from cortical nerve terminals, but the GABA/Glu ratio depends on extracellular Ca²⁺ concentrations. While the P2X7-induced [³H]GABA release is operated by a Ca²⁺-dependent pathway when external Ca²⁺ is available, this mechanism shifts towards the reversal of the GAT1 transporter in low Ca²⁺ conditions. A different scenario is verified regarding [¹⁴C]Glu outflow triggered by the P2X7 receptor, since the amino acid seems to be consistently released through the recruitment of connexin-containing hemichannels upon P2X7 activation, both in the absence and in the presence of external Ca²⁺. Data from this study add valuable information suggesting that ATP, via P2X7 activation, not only interferes with the high-affinity uptake of GABA and Glu but actually favors the release of these amino acids through distinct molecular mechanisms amenable to differential therapeutic control.

Post-inflammatory Ileitis Induces Non-neuronal Purinergic Signaling Adjustments of Cholinergic Neurotransmission in the Myenteric Plexus

Uncoupling between ATP overflow and extracellular adenosine formation changes purinergic signaling in post-inflammatory ileitis. Adenosine neuromodulation deficits were ascribed to feed-forward inhibition of ecto-5′-nucleotidase/CD73 by high extracellular adenine nucleotides in the inflamed ileum. Here, we hypothesized that inflammation-induced changes in cellular density may also account to unbalance the release of purines and their influence on [³H]acetylcholine release from longitudinal muscle-myenteric plexus preparations of the ileum of 2,4,6-trinitrobenzenesulfonic acid (TNBS)-treated rats. The population of S100β-positive glial cells increase, whereas Ano-1-positive interstitial cells of Cajal (ICCs) diminished, in the ileum 7-days after the inflammatory insult. In the absence of changes in the density of VAChT-positive cholinergic nerves detected by immunofluorescence confocal microscopy, the inflamed myenteric plexus released smaller amounts of [³H]acetylcholine which also became less sensitive to neuronal blockade by tetrodotoxin (1 μM). Instead, [³H]acetylcholine release was attenuated by sodium fluoroacetate (5 mM), carbenoxolone (10 μM) and A438079 (3 μM), which prevent activation of glial cells, pannexin-1 hemichannels and P2X7 receptors, respectively. Sodium fluoroacetate also decreased ATP overflow without significantly affecting the extracellular adenosine levels, thus indicating that surplus ATP release parallels reactive gliosis in post-inflammatory ileitis. Conversely, loss of ICCs may explain the lower amounts of adenosine detected in TNBS-treated preparations, since blockade of Ca_v3 (T-type) channels existing in ICCs with mibefradil (3 μM) or inhibition of the equilibrative nucleoside transporter 1 with dipyridamole (0.5 μM), both decreased extracellular adenosine. Data indicate that post-inflammatory ileitis operates a shift on purinergic neuromodulation reflecting the upregulation of ATP-releasing enteric glial cells and the depletion of ICCs accounting for decreased adenosine overflow via equilibrative nucleoside transporters.

Methylprednisolone as a memory enhancer in rats: Effects on aversive memory, long-term potentiation and calcium influx

It is well recognized that stress or glucocorticoids hormones treatment can modulate memory performance in both directions, either impairing or enhancing it. Despite the high number of studies aiming at explaining the effects of glucocorticoids on memory, this has not yet been completely elucidated. Here, we demonstrate that a low daily dose of methylprednisolone (MP, 5mg/kg, i.p.) administered for 10-days favors aversive memory persistence in adult rats, without any effect on the exploring behavior, locomotor activity, anxiety levels and pain perception. Enhanced performance on the inhibitory avoidance task was correlated with long-term potentiation (LTP), a phenomenon that was strengthen in hippocampal slices of rats injected with MP (5mg/kg) during 10days. Additionally, in vitro incubation with MP (30-300µM) concentration-dependently increased intracellular [Ca²⁺]_i in cultured hippocampal neurons depolarized by KCl (35mM). In conclusion, a low daily dose of MP for 10days may promote aversive memory persistence in rats.

Inhibition of cholinergic neurotransmission by β3-adrenoceptors depends on adenosine release and A1-receptor activation in human and rat urinary bladders

The direct detrusor relaxant effect of β₃-adrenoceptor agonists as a primary mechanism to improve overactive bladder symptoms has been questioned. Among other targets, activation of β₃-adrenoceptors downmodulate nerve-evoked acetylcholine (ACh) release, but there is insufficient evidence for the presence of these receptors on bladder cholinergic nerve terminals. Our hypothesis is that adenosine formed from the catabolism of cyclic AMP in the detrusor may act as a retrograde messenger via prejunctional A₁ receptors to explain inhibition of cholinergic activity by β₃-adrenoceptors. Isoprenaline (1 µM) decreased [³H]ACh release from stimulated (10 Hz, 200 pulses) human (-47 ± 5%) and rat (-38 ± 1%) detrusor strips. Mirabegron (0.1 µM, -53 ± 8%) and CL316,243 (1 µM, -37 ± 7%) mimicked isoprenaline (1 µM) inhibition, and their effects were prevented by blocking β₃-adrenoceptors with L748,337 (30 nM) and SR59230A (100 nM), respectively, in human and rat detrusor. Mirabegron and isoprenaline increased extracellular adenosine in the detrusor. Blockage of A₁ receptors with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 100 nM) or the equilibrative nucleoside transporters (ENT) with dipyridamole (0.5 µM) prevented mirabegron and isoprenaline inhibitory effects. Dipyridamole prevented isoprenaline-induced adenosine outflow from the rat detrusor, and this effect was mimicked by the ENT1 inhibitor, S-(4-nitrobenzyl)-6-thioinosine (NBTI, 30 µM). Cystometry recordings in anesthetized rats demonstrated that SR59230A, DPCPX, dipyridamole, and NBTI reversed the decrease in the voiding frequency caused by isoprenaline (0.1-1,000 nM). Data suggest that inhibition of cholinergic neurotransmission by β₃-adrenoceptors results from adenosine release via equilibrative nucleoside transporters and prejunctional A₁-receptor stimulation in human and rat urinary bladder.

Adenosine A2A receptor and ecto-5'-nucleotidase/CD73 are upregulated in hippocampal astrocytes of human patients with mesial temporal lobe epilepsy (MTLE)

Refractoriness to existing medications of up to 80 % of the patients with mesial temporal lobe epilepsy (MTLE) prompts for finding new antiepileptic drug targets. The adenosine A2A receptor emerges as an interesting pharmacological target since its excitatory nature partially counteracts the dominant antiepileptic role of endogenous adenosine acting via inhibitory A1 receptors. Gain of function of the excitatory A2A receptor has been implicated in a significant number of brain pathologies commonly characterized by neuronal excitotoxicity. Here, we investigated changes in the expression and cellular localization of the A2A receptor and of the adenosine-generating enzyme, ecto-5'-nucleotidase/CD73, in the hippocampus of control individuals and MTLE human patients. Western blot analysis indicates that the A2A receptor is more abundant in the hippocampus of MTLE patients compared to control individuals. Immunoreactivity against the A2A receptor predominates in astrocytes staining positively for the glial fibrillary acidic protein (GFAP). No co-localization was observed between the A2A receptor and neuronal cell markers, like synaptotagmin 1/2 (nerve terminals) and neurofilament 200 (axon fibers). Hippocampal astrogliosis observed in MTLE patients was accompanied by a proportionate increase in A2A receptor and ecto-5'-nucleotidase/CD73 immunoreactivities. Given our data, we hypothesize that selective blockade of excessive activation of astrocytic A2A receptors and/or inhibition of surplus adenosine formation by membrane-bound ecto-5'-nucleotidase/CD73 may reduce neuronal excitability, thus providing a novel therapeutic target for drug-refractory seizures in MTLE patients.

ADP-Induced Ca2+ Signaling and Proliferation of Rat Ventricular Myofibroblasts Depend on Phospholipase C-Linked TRP Channels Activation Within Lipid Rafts

Nucleotides released during heart injury affect myocardium electrophysiology and remodeling through P2 purinoceptors activation in cardiac myofibroblasts. ATP and UTP endorse [Ca²⁺ ]_i accumulation and growth of DDR-2/α-SMA-expressing myofibroblasts from adult rat ventricles via P2Y₄ and P2Y₂ receptors activation, respectively. Ventricular myofibroblasts also express ADP-sensitive P2Y₁ , P2Y₁₂ , and P2Y₁₃ receptors as demonstrated by immunofluorescence confocal microscopy and western blot analysis, but little information exists on ADP effects in these cells. ADP (0.003-3 mM) and its stable analogue, ADPßS (100 μM), caused fast [Ca²⁺ ]_i transients originated from thapsigargin-sensitive internal stores, which partially declined to a plateau sustained by capacitative Ca²⁺ entry through transient receptor potential (TRP) channels inhibited by 2-APB (50 μM) and flufenamic acid (100 μM). Hydrophobic interactions between G_q/11 -coupled P2Y purinoceptors and TRP channels were suggested by prevention of the ADP-induced [Ca²⁺ ]_i plateau following PIP₂ depletion with LiCl (10 mM) and cholesterol removal from lipid rafts with methyl-ß-cyclodextrin (2 mM). ADP [Ca²⁺ ]_i transients were insensitive to P2Y₁ , P2Y₁₂ , and P2Y₁₃ receptor antagonists, MRS2179 (10μM), AR-C66096 (0.1 μM), and MRS2211 (10μM), respectively, but were attenuated by suramin and reactive blue-2 (100 μM) which also blocked P2Y₄ receptors activation by UTP. Cardiac myofibroblasts growth and type I collagen production were favored upon activation of MRS2179-sensitive P2Y₁ receptors with ADP or ADPßS (30 μM). In conclusion, ADP exerts a dual role on ventricular myofibroblasts: [Ca²⁺ ]_i transients are mediated by fast-desensitizing P2Y₄ receptors, whereas the pro-fibrotic effect of ADP involves the P2Y₁ receptor activation. Data also show that ADP-induced capacitative Ca²⁺ influx depends on phospholipase C-linked TRP channels opening in lipid raft microdomains. J. Cell. Physiol. 232: 1511-1526, 2017. © 2016 Wiley Periodicals, Inc.

Increased Urinary Adenosine Triphosphate in Patients With Bladder Outlet Obstruction Due to Benign Prostate Hyperplasia

BACKGROUND

Diagnosis of bladder outflow obstruction (BOO) in patients with lower urinary tract (LUT) symptoms is challenging without using invasive urodynamic tests. Recently, we showed in vitro that urothelial strips from patients with benign prostatic hyperplasia (BPH) release more ATP than controls. Here, we tested whether urinary ATP can be used as a wall tension transducer non-invasive biomarker to detect BOO in patients with BPH.

METHODS

79 male patients with BOO and 22 asymptomatic controls were recruited prospectively. Patients were asked to complete the International Prostate Symptom Score (IPSS) questionnaire and to void at normal desire into a urinary flowmeter; the postvoid residual volume was determined by suprapubic ultrasonography. Urine samples from all individuals were examined for ATP, creatinine, and lactate dehydrogenase.

RESULTS

BOO patients had significantly higher (P < 0.001) urinary ATP normalized by the voided volume (456 ± 36 nmol) than age-matched controls (209 ± 35 nmol). Urinary ATP amounts increased with the voided volume, but the slope of this rise was higher in BOO patients than in controls. A negative correlation was detected between urinary ATP and flow rate parameters, namely maximal flow rate (r = -0.310, P = 0.005), Siroky flow-volume normalization (r = -0.324, P = 0.004), and volume-normalized flow rate index (r = -0.320, P = 0.012). We found no correlation with LUT symptoms IPSS score. Areas under the receiver operator characteristics (ROC) curves were 0.91 (95%CI 0.86-0.96, P < 0.001) for ATP alone and 0.88 (95%CI 0.81-0.94, P < 0,001) when adjusted to urinary creatinine.

CONCLUSIONS

Patients with BOO release higher amounts of ATP into the urine than the control group. The high area under the ROC curve suggests that urinary ATP can be a high-sensitive non-invasive biomarker of BOO, which may have a discriminative value of detrusor competence when comparing BPH patients with low urinary flow rates. Prostate 76:1353-1363, 2016. © 2016 Wiley Periodicals, Inc.

Ion Fluxes through KCa2 (SK) and Cav1 (L-type) Channels Contribute to Chronoselectivity of Adenosine A1 Receptor-Mediated Actions in Spontaneously Beating Rat Atria

Impulse generation in supraventricular tissue is inhibited by adenosine and acetylcholine via the activation of A₁ and M₂ receptors coupled to inwardly rectifying GIRK/K_IR3.1/3.4 channels, respectively. Unlike M₂ receptors, bradycardia produced by A₁ receptors activation predominates over negative inotropy. Such difference suggests that other ion currents may contribute to adenosine chronoselectivity. In isolated spontaneously beating rat atria, blockade of K_Ca2/SK channels with apamin and Ca_v1 (L-type) channels with nifedipine or verapamil, sensitized atria to the negative inotropic action of the A₁ agonist, R-PIA, without affecting the nucleoside negative chronotropy. Patch-clamp experiments in the whole-cell configuration mode demonstrate that adenosine, via A₁ receptors, activates the inwardly-rectifying GIRK/K_IR3.1/K_IR3.4 current resulting in hyperpolarization of atrial cardiomyocytes, which may slow down heart rate. Conversely, the nucleoside inactivates a small conductance Ca²⁺-activated K_Ca2/SK outward current, which eventually reduces the repolarizing force and thereby prolong action potentials duration and Ca²⁺ influx into cardiomyocytes. Immunolocalization studies showed that differences in A₁ receptors distribution between the sinoatrial node and surrounding cardiomyocytes do not afford a rationale for adenosine chronoselectivity. Immunolabelling of K_IR3.1, K_Ca2.2, K_Ca2.3, and Ca_v1 was also observed throughout the right atrium. Functional data indicate that while both A₁ and M₂ receptors favor the opening of GIRK/K_IR3.1/3.4 channels modulating atrial chronotropy, A₁ receptors may additionally restrain K_Ca2/SK activation thereby compensating atrial inotropic depression by increasing the time available for Ca²⁺ influx through Ca_v1 (L-type) channels.

Mesenchymal Stem Cells Ageing: Targeting the "Purinome" to Promote Osteogenic Differentiation and Bone Repair

Mesenchymal stem cells (MSCs) are multipotent cells that can differentiate into bone forming cells. Such ability is compromised in elderly individuals resulting in bone disorders such as osteoporosis, also limiting their clinical usage for cell transplantation and bone tissue engineering strategies. In bone marrow niches, adenine and uracil nucleotides are important local regulators of osteogenic differentiation of MSCs. Nucleotides can be released to the extracellular milieu under both physiological and pathological conditions via (1) membrane cell damage, (2) vesicle exocytosis, (3) ATP-binding cassette transporters, and/or (4) facilitated diffusion through maxi-anion channels, hemichannels or ligand-gated receptor pores. Nucleotides and their derivatives act via adenosine P1 (A1 , A2A , A2B , and A3 ) and nucleotide-sensitive P2 purinoceptors comprising ionotropic P2X and G-protein-coupled P2Y receptors. Purinoceptors activation is terminated by membrane-bound ecto-nucleotidases and other ecto-phosphatases, which rapidly hydrolyse extracellular nucleotides to their respective nucleoside 5'-di- and mono-phosphates, nucleosides and free phosphates, or pyrophosphates. Current knowledge suggests that different players of the "purinome" cascade, namely nucleotide release sites, ecto-nucleotidases and purinoceptors, orchestrate to fine-tuning regulate the activity of MSCs in the bone microenvironment. Increasing studies, using osteoprogenitor cell lines, animal models and, more recently, non-modified MSCs from postmenopausal women, raised the possibility to target chief components of the purinergic signaling pathway to regenerate the ability of aged MSCs to differentiate into functional osteoblasts. This review summarizes the main findings of those studies, prompting for novel therapeutic strategies to control ageing disorders where bone destruction exceeds bone formation, like osteoporosis, rheumatoid arthritis, and fracture mal-union. J. Cell. Physiol. 231: 1852-1861, 2016. © 2016 Wiley Periodicals, Inc.

Inverse gradient of nitrergic and purinergic inhibitory cotransmission in the mouse colon

AIM

Gastrointestinal smooth muscle relaxation is accomplished by the neural corelease of ATP or a related purine and nitric oxide. Contractions are triggered by acetylcholine and tachykinins. The aim of this work was to study whether regional differences in neurotransmission could partially explain the varied physiological roles of each colonic area.

METHODS

We used electrophysiological and myography techniques to evaluate purinergic (L-NNA 1 mm incubated tissue), nitrergic (MRS2500 0.3 μm incubated tissue) and cholinergic neurotransmission (L-NNA 1 mm and MRS2500 0.3 μm incubated tissue) in the proximal, mid and distal colon of CD1 mice (n = 42).

RESULTS

Purinergic electrophysiological responses elicited by single pulses (28 V) were greater in the distal (IJPfMAX = -35.3 ± 2.2 mV), followed by the mid (IJPfMAX = -30.6 ± 1.0 mV) and proximal (IJPfMAX = -11.7 ± 1.1 mV) colon. In contrast, nitrergic responses decreased from the proximal colon (IJPsMAX = -11.4 ± 1.1 mV) to the mid (IJPsMAX = -9.1 ± 0.4 mV), followed by the distal colon (IJPsMAX = -1.8 ± 0.3 mV). A similar rank of order was observed in neural mediated inhibitory mechanical responses including electrical field stimulation-mediated responses and neural tone. ADPβs concentration-response curve was shifted to the left in the distal colon. In contrast, NaNP responses did not differ between regions. Cholinergic neurotransmission elicited contractions of a similar amplitude throughout the colon.

CONCLUSION

An inverse gradient of purinergic and nitrergic neurotransmission exists through the mouse colon. The proximal and mid colon have a predominant nitrergic neurotransmission probably due to the fact that their storage function requires sustained relaxations. The distal colon, in contrast, has mainly purinergic neurotransmission responsible for the phasic relaxations needed to propel dehydrated faeces.

Up-regulation of P2X7 receptor-mediated inhibition of GABA uptake by nerve terminals of the human epileptic neocortex

OBJECTIVE

Thirty percent of patients with epilepsy are refractory to medication. The majority of these patients have mesial temporal lobe epilepsy (MTLE). This prompts for new pharmacologic targets, like ATP-mediated signaling pathways, since the extracellular levels of the nucleotide dramatically increase during in vitro epileptic seizures. In this study, we investigated whether sodium-dependent high-affinity γ-aminobutyric acid (GABA) and glutamate uptake by isolated nerve terminals of the human neocortex could be modulated by ATP acting via slow-desensitizing P2X7 receptor (P2X7R).

METHODS

Modulation of [(3) H]GABA and [(14) C]glutamate uptake by ATP, through activation of P2X7R, was investigated in isolated nerve terminals of the neocortex of cadaveric controls and patients with drug-resistant epilepsy (non-MTLE or MTLE) submitted to surgery. Tissue density and distribution of P2X7R in the human neocortex was assessed by Western blot analysis and immunofluorescence confocal microscopy.

RESULTS

The P2X7R agonist, 2'(3')-O-(4-benzoylbenzoyl)ATP (BzATP, 3-100 μm) decreased [(3) H]GABA and [(14) C]glutamate uptake by nerve terminals of the neocortex of controls and patients with epilepsy. The inhibitory effect of BzATP (100 μm) was prevented by the selective P2X7R antagonist, A-438079 (3 μm). Down-modulation of [(14) C]glutamate uptake by BzATP (100 μm) was roughly similar in controls and patients with epilepsy, but the P2X7R agonist inhibited more effectively [(3) H]GABA uptake in the epileptic tissue. Neocortical nerve terminals of patients with epilepsy express higher amounts of the P2X7R protein than control samples.

SIGNIFICANCE

High-frequency cortical activity during epileptic seizures releases huge amounts of ATP, which by acting on low-affinity slowly desensitizing ionotropic P2X7R, leads to down-modulation of neuronal GABA and glutamate uptake. Increased P2X7R expression in neocortical nerve terminals of patients with epilepsy may, under high-frequency firing, endure GABA signaling and increase GABAergic rundown, thereby unbalancing glutamatergic neuroexcitation. This study highlights the relevance of the ATP-sensitive P2X7R as an important negative modulator of GABA and glutamate transport and prompts for novel antiepileptic therapeutic targets.

Impairment of ATP hydrolysis decreases adenosine A1 receptor tonus favoring cholinergic nerve hyperactivity in the obstructed human urinary bladder

This study was designed to investigate whether reduced adenosine formation linked to deficits in extracellular ATP hydrolysis by NTPDases contributes to detrusor neuromodulatory changes associated with bladder outlet obstruction in men with benign prostatic hyperplasia (BPH). The kinetics of ATP catabolism and adenosine formation as well as the role of P1 receptor agonists on muscle tension and nerve-evoked [(3)H]ACh release were evaluated in mucosal-denuded detrusor strips from BPH patients (n = 31) and control organ donors (n = 23). The neurogenic release of ATP and [(3)H]ACh was higher (P < 0.05) in detrusor strips from BPH patients. The extracellular hydrolysis of ATP and, subsequent, adenosine formation was slower (t (1/2) 73 vs. 36 min, P < 0.05) in BPH detrusor strips. The A(1) receptor-mediated inhibition of evoked [(3)H]ACh release by adenosine (100 μM), NECA (1 μM), and R-PIA (0.3 μM) was enhanced in BPH bladders. Relaxation of detrusor contractions induced by acetylcholine required 30-fold higher concentrations of adenosine. Despite VAChT-positive cholinergic nerves exhibiting higher A(1) immunoreactivity in BPH bladders, the endogenous adenosine tonus revealed by adenosine deaminase is missing. Restoration of A1 inhibition was achieved by favoring (1) ATP hydrolysis with apyrase (2 U mL(-1)) or (2) extracellular adenosine accumulation with dipyridamole or EHNA, as these drugs inhibit adenosine uptake and deamination, respectively. In conclusion, reduced ATP hydrolysis leads to deficient adenosine formation and A(1) receptor-mediated inhibition of cholinergic nerve activity in the obstructed human bladder. Thus, we propose that pharmacological manipulation of endogenous adenosine levels and/or A(1) receptor activation might be useful to control bladder overactivity in BPH patients.

The Effect of Ischemia and Reperfusion on Enteric Glial Cells and Contractile Activity in the Ileum

BACKGROUND

We investigated the effects of ischemia followed by different periods of reperfusion (I/R) on immunoreactive S100β-positive glial and Hu-immunoreactive neurons co-expressing the P2X2 receptor in the myenteric plexus of the rat ileum.

METHODS

The ileal artery was occluded for 35 min with an atraumatic vascular clamp. The animals were killed 24 h, 72 h, and 1 week after ischemia. Sham animals were not submitted to ileal artery occlusion. The relative density, size, and co-localization of P2X2 receptor-expressing cells in relation to S100β-immunoreactive glial and Hu-immunoreactive neuronal cells were evaluated. Additionally, we analyzed the effects of I/R on gastrointestinal transit and ileum contractile activity.

RESULTS

The cellular density of P2X2 receptor and neuronal Hu immunoreactivity/cm(2) decreased after I/R, whereas glial S100β immunoreactivity/cm(2) increased. No significant differences between sham and I/R groups were observed regarding the perikarya area of Hu-positive neurons. The area of S100β-immunoreactive glial cells increased by 24.1 % 1 week after I/R compared with the 24 h group. Methylene blue progression along the small intestine decreased (P < 0.05) from 24.5 ± 2.3 % in the sham group to 17.2 ± 2.0 % 1 week post-ischemia. We noted a significant (P < 0.05) decrease in the maximal contraction amplitude triggered by electrical field stimulation in the presence of ATP in preparations submitted to 24 h of I/R.

CONCLUSIONS

Changes in the P2X2 receptor density parallel myenteric neuronal loss following I/R of the rat ileum. This, together with the increase in the activated (oversized) glial cells, may contribute to decreased GI motility after I/R.

P2X7 receptor activation downmodulates Na(+)-dependent high-affinity GABA and glutamate transport into rat brain cortex synaptosomes

Sodium-dependent high-affinity amino-acid transporters play crucial roles in terminating synaptic transmission in the central nervous system (CNS). However, there is lack of information about the mechanisms underlying the regulation of amino-acid transport by fast-acting neuromodulators, like ATP. Here, we investigated whether activation of the ATP-sensitive P2X7 receptor modulates Na(+)-dependent high-affinity γ-aminobutyric acid (GABA) and glutamate uptake into nerve terminals (synaptosomes) of the rat cerebral cortex. Radiolabeled neurotransmitter accumulation was evaluated by liquid scintillation spectrometry. The cell-permeant sodium-selective fluorescent indicator, SBFI-AM, was used to estimate Na(+) influx across plasma membrane. 2'(3')-O-(4-benzoylbenzoyl)ATP (BzATP, 3-300 μM), a prototypic P2X7 receptor agonist, concentration-dependently decreased [(3)H]GABA (14%) and [(14)C]glutamate (24%) uptake; BzATP decreased transport maximum velocity (Vmax) without affecting the Michaelis constant (Km) values. The selective P2X7 receptor antagonist, A-438079 (3 μM), prevented inhibition of [(3)H]GABA and [(14)C]glutamate uptake by BzATP (100 μM). The inhibitory effect of BzATP coincided with its ability to increase intracellular Na(+) and was mimicked by Na(+) ionophores, like gramicidin and monensin. Increases in intracellular Na(+) (with veratridine or ouabain) or substitution of extracellular Na(+) by N-methyl-D-glucamine (NMDG)(+) all decreased [(3)H]GABA and [(14)C]glutamate uptake and attenuated BzATP effects. Uptake inhibition by BzATP (100 μM) was also attenuated by calmidazolium, which selectively inhibits Na(+) currents through the P2X7 receptor pore. In conclusion, disruption of the Na(+) gradient by P2X7 receptor activation downmodulates high-affinity GABA and glutamate uptake into rat cortical synaptosomes. Interference with amino-acid transport efficacy may constitute a novel target for therapeutic management of cortical excitability.

Activation of P2Y6 Receptors Facilitates Nonneuronal Adenosine Triphosphate and Acetylcholine Release from Urothelium with the Lamina Propria of Men with Bladder Outlet Obstruction

PURPOSE

Deregulation of purinergic bladder signaling may contribute to persistent detrusor overactivity in patients with bladder outlet obstruction. Activation of uridine diphosphate sensitive P2Y6 receptors increases voiding frequency in rats indirectly by releasing adenosine triphosphate from the urothelium. To our knowledge this mechanism has never been tested in the human bladder.

MATERIALS AND METHODS

We examined the role of the uridine diphosphate sensitive P2Y6 receptor on tetrodotoxin insensitive nonneuronal adenosine triphosphate and [(3)H]acetylcholine release from the human urothelium with the lamina propria of control organ donors and patients with benign prostatic hyperplasia.

RESULTS

The adenosine triphosphate-to-[(3)H]acetylcholine ratio was fivefold higher in mucosal urothelium/lamina propria strips from benign prostatic hyperplasia patients than control men. The selective P2Y6 receptor agonist PSB0474 (100 nM) augmented by a similar amount adenosine triphosphate and [(3)H]acetylcholine release from mucosal urothelium/lamina propria strips from both groups of individuals. The facilitatory effect of PSB0474 was prevented by MRS2578 (50 nM) and by carbenoxolone (10 μM), which block P2Y6 receptor and pannexin-1 hemichannels, respectively. Blockade of P2X3 (and/or P2X2/3) receptors with A317491 (100 nM) also attenuated release facilitation by PSB0474 in control men but not in patients with benign prostatic hyperplasia. Immunolocalization studies showed that P2Y6, P2X2 and P2X3 receptors were present in choline acetyltransferase positive urothelial cells. In contrast to P2Y6 staining, choline acetyltransferase, P2X2 and P2X3 immunoreactivity decreased in the urothelium of benign prostatic hyperplasia patients.

CONCLUSIONS

Activation of P2Y6 receptor amplifies mucosal adenosine triphosphate release underlying bladder overactivity in patients with benign prostatic hyperplasia. Therefore, we propose selective P2Y6 receptor blockade as a novel therapeutic strategy to control persistent storage symptoms in obstructed patients.

Impact of ectonucleotidases in autonomic nervous functions

Adenine and uracil nucleotides play key functions in the autonomic nervous system (ANS). For instance, ATP acts as a neurotransmitter, co-transmitter and neuromodulator in the ANS. The purinergic system encompasses (1) receptors that respond to extracellular purines, which are designated as P1 and P2 purinoceptors, (2) purine release and uptake, and (3) a cascade of enzymes that regulate the concentration of purines near the cell surface. Ectonucleotidases and adenosine deaminase (ADA) are enzymes responsible for the hydrolysis of ATP (and other nucleotides such as ADP, UTP, UDP, AMP) and adenosine, respectively. Accordingly, these enzymes are expected to play an important role in the control of neuro-effector transmission in tissues innervated by both the sympathetic and parasympathetic divisions of the ANS. Indeed, ectonucleotidases have the ability to either terminate P2 receptor responses initiated by nucleoside triphosphates (ATP and UTP), and/or to favor the activation of ADP (e.g. P2Y1,12,13) and UDP (e.g. P2Y6) and/or adenosine (P1) specific receptors. In addition, ectonucleotidases can also importantly protect some P2 receptors from desensitization (e.g. P2X1, P2Y1). In this review, we present the (putative) roles of ectonucleotidases and ADA in the ANS with a focus on their regulatory activity at neuro-effector junctions in the following tissues: heart, vas deferens, urinary bladder, salivary glands, blood vessels and the intestine. We also present their implication in nociceptive transmission.

Amplification of neuromuscular transmission by methylprednisolone involves activation of presynaptic facilitatory adenosine A2A receptors and redistribution of synaptic vesicles

The mechanisms underlying improvement of neuromuscular transmission deficits by glucocorticoids are still a matter of debate despite these compounds have been used for decades in the treatment of autoimmune myasthenic syndromes. Besides their immunosuppressive action, corticosteroids may directly facilitate transmitter release during high-frequency motor nerve activity. This effect coincides with the predominant adenosine A2A receptor tonus, which coordinates the interplay with other receptors (e.g. muscarinic) on motor nerve endings to sustain acetylcholine (ACh) release that is required to overcome tetanic neuromuscular depression in myasthenics. Using myographic recordings, measurements of evoked [(3)H]ACh release and real-time video microscopy with the FM4-64 fluorescent dye, results show that tonic activation of facilitatory A2A receptors by endogenous adenosine accumulated during 50 Hz bursts delivered to the rat phrenic nerve is essential for methylprednisolone (0.3 mM)-induced transmitter release facilitation, because its effect was prevented by the A2A receptor antagonist, ZM 241385 (10 nM). Concurrent activation of the positive feedback loop operated by pirenzepine-sensitive muscarinic M1 autoreceptors may also play a role, whereas the corticosteroid action is restrained by the activation of co-expressed inhibitory M2 and A1 receptors blocked by methoctramine (0.1 μM) and DPCPX (2.5 nM), respectively. Inhibition of FM4-64 loading (endocytosis) by methylprednisolone following a brief tetanic stimulus (50 Hz for 5 s) suggests that it may negatively modulate synaptic vesicle turnover, thus increasing the release probability of newly recycled vesicles. Interestingly, bulk endocytosis was rehabilitated when methylprednisolone was co-applied with ZM241385. Data suggest that amplification of neuromuscular transmission by methylprednisolone may involve activation of presynaptic facilitatory adenosine A2A receptors by endogenous adenosine leading to synaptic vesicle redistribution.

P2X7-induced zeiosis promotes osteogenic differentiation and mineralization of postmenopausal bone marrow-derived mesenchymal stem cells

Polymorphisms of the P2X7 receptor have been associated with increased risk of fractures in postmenopausal women. Although both osteoblasts and osteoclasts express P2X7 receptors, their function in osteogenesis remains controversial. Here, we investigated the role of the P2X7 receptor on osteogenic differentiation and mineralization of bone marrow mesenchymal stem cell (BMSC) cultures from postmenopausal women (age 71±3 yr, n=18). We focused on the mechanisms related to intracellular [Ca(2+)]i oscillations and plasma membrane-dynamics. ATP, and the P2X7 agonist BzATP (100 μM), increased [Ca(2+)]i in parallel to the formation of membrane pores permeable to TO-PRO-3 dye uptake. ATP and BzATP elicited reversible membrane blebs (zeiosis) in 38 ± 1 and 70 ± 1% of the cells, respectively. P2X7-induced zeiosis was Ca(2+) independent, but involved phospholipase C, protein kinase C, and Rho-kinase activation. BzATP (100 μM) progressively increased the expression of Runx-2 and Osterix transcription factors by 452 and 226% (at d 21), respectively, alkaline phosphatase activity by 88% (at d 28), and mineralization by 329% (at d 43) of BMSC cultures in a Rho-kinase-dependent manner. In summary, reversible plasma membrane zeiosis involving cytoskeleton rearrangements due to activation of the P2X7-Rho-kinase axis promotes osteogenic differentiation and mineralization of BMSCs, thus providing new therapeutic targets for postmenopausal bone loss.

Attenuated aortic vasodilation and sympathetic prejunctional facilitation in epinephrine-deficient mice: selective impairment of β2-adrenoceptor responses

It has been suggested that there is a link between epinephrine synthesis and the development of β2-adrenoceptor-mediated effects, but it remains to be determined whether this development is triggered by epinephrine. The aim of this study was to characterize β-adrenoceptor-mediated relaxation and facilitation of norepinephrine release in the aorta of phenylethanolamine-N-methyltransferase-knockout (Pnmt-KO) mice. Catecholamines were quantified by reverse-phase high-performance liquid chromatography-electrochemical detection. Aortic rings were mounted in a myograph to determine concentration-response curves to selective β1- or β2-adrenoceptor agonists in the absence or presence of selective β1- or β2-adrenoceptor antagonists. Aortic rings were also preincubated with [(3)H]norepinephrine to measure tritium overflow elicited by electrical stimulation in the presence of increasing concentrations of nonselective β- or selective β2-adrenoceptor agonists. β2-Adrenoceptor protein density was evaluated by Western blotting and β2-adrenoceptor localization by immunohistochemistry. Epinephrine is absent in Pnmt-KO mice. The potency and the maximal effect of the β2-adrenoceptor agonist terbutaline were lower in Pnmt-KO than in wild-type (WT) mice. The selective β2-adrenoceptor antagonist ICI 118,551 [(±)-erythro-(S*,S*)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride] antagonized the relaxation caused by terbutaline in WT but not in Pnmt-KO mice. Isoproterenol and terbutaline induced concentration-dependent increases in tritium overflow in WT mice only. β2-Adrenoceptor protein density was decreased in membrane aorta homogenates of Pnmt-KO mice, and this finding was supported by immunofluorescence confocal microscopy. In conclusion, epinephrine is crucial for β2-adrenoceptor-mediated vasodilation and facilitation of norepinephrine release. In the absence of epinephrine, β2-adrenoceptor protein density was decreased in aorta cell membranes, thus potentially hindering its functional activity.

Lack of endogenous adenosine tonus on sympathetic neurotransmission in spontaneously hypertensive rat mesenteric artery

Background

Increased sympathetic activity has been implicated in hypertension. Adenosine has been shown to play a role in blood flow regulation. In the present study, the endogenous adenosine neuromodulatory role, in mesenteric arteries from normotensive and spontaneously hypertensive rats, was investigated.

Methods and Results

The role of endogenous adenosine in sympathetic neurotransmission was studied using electrically-evoked [³H]-noradrenaline release experiments. Purine content was determined by HPLC with fluorescence detection. Localization of adenosine A₁ or A_2A receptors in adventitia of mesenteric arteries was investigated by Laser Scanning Confocal Microscopy. Results indicate a higher electrically-evoked noradrenaline release from hypertensive mesenteric arteries. The tonic inhibitory modulation of noradrenaline release is mediated by adenosine A₁ receptors and is lacking in arteries from hypertensive animals, despite their purine levels being higher comparatively to those determined in normotensive ones. Tonic facilitatory adenosine A_2A receptor-mediated effects were absent in arteries from both strains. Immunohistochemistry revealed an adenosine A₁ receptors redistribution from sympathetic fibers to Schwann cells, in adventitia of hypertensive mesenteric arteries which can explain, at least in part, the absence of effects observed for these receptors.

Conclusion

Data highlight the role of purines in hypertension revealing that an increase in sympathetic activity in hypertensive arteries is occurring due to a higher noradrenaline/ATP release from sympathetic nerves and the loss of endogenous adenosine inhibitory tonus. The observed nerve-to-glial redistribution of inhibitory adenosine A₁ receptors in hypertensive arteries may explain the latter effect.