Adrenergic receptor activation involves ATP release and feedback through purinergic receptors

Role of purinergic receptors in cardiac sympathetic nerve injury in diabetes mellitus

Diabetic cardiac autonomic neuropathy is a common and serious chronic complication of diabetes, which can lead to sympathetic and parasympathetic nerve imbalance and a relative excitation of the sympathetic nerve. Purinergic receptors play a crucial role in this process. Diabetic cardiac sympathetic nerve injury affects the expression of purinergic receptors, and activated purinergic receptors affect the phosphorylation of different signaling pathways and the regulation of inflammatory processes. This paper introduces the abnormal changes of sympathetic nerve in diabetes mellitus and summarizes the recently published studies on the role of several purinergic receptor subtypes in diabetic cardiac sympathetic nerve injury. These studies suggest that purinergic receptors as novel drug targets are of great significance for the treatment of diabetic autonomic neuropathy. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Adenosine and Adenosine Receptors: Advances in Atrial Fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in the world. Because the key to developing innovative therapies that limit the onset and the progression of AF is to fully understand the underlying molecular mechanisms of AF, the aim of the present narrative review is to report the most recent advances in the potential role of the adenosinergic system in the pathophysiology of AF. After a comprehensive approach describing adenosinergic system signaling and the mechanisms of the initiation and maintenance of AF, we address the interactions of the adenosinergic system's signaling with AF. Indeed, adenosine release can activate four G-coupled membrane receptors, named A₁, A_2A, A_2B and A₃. Activation of the A_2A receptors can promote the occurrence of delayed depolarization, while activation of the A₁ receptors can shorten the action potential's duration and induce the resting membrane's potential hyperpolarization, which promote pulmonary vein firing, stabilize the AF rotors and allow for functional reentry. Moreover, the A_2B receptors have been associated with atrial fibrosis homeostasis. Finally, the adenosinergic system can modulate the autonomous nervous system and is associated with AF risk factors. A question remains regarding adenosine release and the adenosine receptors' activation and whether this would be a cause or consequence of AF.

Contribution of Adenosine in the Physiological Changes and Injuries Secondary to Exposure to Extreme Oxygen Pressure in Healthy Subjects

Climbers and aviators are exposed to severe hypoxia at high altitudes, whereas divers are exposed to hyperoxia at depth. The aim of this study was to report changes in the adenosinergic system induced by exposure to extreme oxygen partial pressures. At high altitudes, the increased adenosine concentration contributes to brain protection against hypoxia through various mechanisms such as stimulation of glycogenolysis for ATP production, reduction in neuronal energy requirements, enhancement in 2,3-bisphosphoglycerate production, and increase in cerebral blood flow secondary to vasodilation of cerebral arteries. In the context of mountain illness, the increased level of A_2AR expression leads to glial dysfunction through neuroinflammation and is involved in the pathogenesis of neurological disorders. Nonetheless, a high level of adenosine concentration can protect against high-altitude pulmonary edema via a decrease in pulmonary arterial pressure. The adenosinergic system is also involved in the acclimatization phenomenon induced by prolonged exposure to altitude hypoxia. During hyperoxic exposure, decreased extracellular adenosine and low A_2A receptor expression contribute to vasoconstriction. The resulting decrease in cerebral blood flow is considered a preventive phenomenon against cerebral oxygen toxicity through the decrease in oxygen delivery to the brain. With regard to lung oxygen toxicity, hyperoxia leads to an increase in extracellular adenosine, which acts to preserve pulmonary barrier function. Changes in the adenosinergic system induced by exposure to extreme oxygen partial pressures frequently have a benefit in decreasing the risk of adverse effects.

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.

Adenosine A2A Receptors Are Upregulated in Peripheral Blood Mononuclear Cells from Atrial Fibrillation Patients

Atrial fibrillation (AF) is the most common form of cardiac arrhythmia seen in clinical practice. While some clinical parameters may predict the transition from paroxysmal to persistent AF, the molecular mechanisms behind the AF perpetuation are poorly understood. Thus, oxidative stress, calcium overload and inflammation, among others, are believed to be involved in AF-induced atrial remodelling. Interestingly, adenosine and its receptors have also been related to AF development and perpetuation. Here, we investigated the expression of adenosine A2A receptor (A2AR) both in right atrium biopsies and peripheral blood mononuclear cells (PBMCs) from non-dilated sinus rhythm (ndSR), dilated sinus rhythm (dSR) and AF patients. In addition, plasma adenosine content and adenosine deaminase (ADA) activity in these subjects were also determined. Our results revealed increased A2AR expression in the right atrium from AF patients, as previously described. Interestingly, increased levels of adenosine content and reduced ADA activity in plasma from AF patients were detected. An increase was observed when A2AR expression was assessed in PBMCs from AF subjects. Importantly, a positive correlation (p = 0.001) between A2AR expression in the right atrium and PBMCs was observed. Overall, these results highlight the importance of the A2AR in AF and suggest that the evaluation of this receptor in PBMCs may be potentially be useful in monitoring disease severity and the efficacy of pharmacological treatments in AF patients.

Role of α2-Adrenoceptors in Hypertension: Focus on Renal Sympathetic Neurotransmitter Release, Inflammation, and Sodium Homeostasis

The kidney is extensively innervated by sympathetic nerves playing an important role in the regulation of blood pressure homeostasis. Sympathetic nerve activity is ultimately controlled by the central nervous system (CNS). Norepinephrine, the main sympathetic neurotransmitter, is released at prejunctional neuroeffector junctions in the kidney and modulates renin release, renal vascular resistance, sodium and water handling, and immune cell response. Under physiological conditions, renal sympathetic nerve activity (RSNA) is modulated by peripheral mechanisms such as the renorenal reflex, a complex interaction between efferent sympathetic nerves, central mechanism, and afferent sensory nerves. RSNA is increased in hypertension and, therefore, critical for the perpetuation of hypertension and the development of hypertensive kidney disease. Renal sympathetic neurotransmission is not only regulated by RSNA but also by prejunctional α2-adrenoceptors. Prejunctional α2-adrenoceptors serve as autoreceptors which, when activated by norepinephrine, inhibit the subsequent release of norepinephrine induced by a sympathetic nerve impulse. Deletion of α2-adrenoceptors aggravates hypertension ultimately by modulating renal pressor response and sodium handling. α2-adrenoceptors are also expressed in the vasculature, renal tubules, and immune cells and exert thereby effects related to vascular tone, sodium excretion, and inflammation. In the present review, we highlight the role of α2-adrenoceptors on renal sympathetic neurotransmission and its impact on hypertension. Moreover, we focus on physiological and pathophysiological functions mediated by non-adrenergic α2-adrenoceptors. In detail, we discuss the effects of sympathetic norepinephrine release and α2-adrenoceptor activation on renal sodium transporters, on renal vascular tone, and on immune cells in the context of hypertension and kidney disease.

Adenosine and the Cardiovascular System: The Good and the Bad

Adenosine is a nucleoside that impacts the cardiovascular system via the activation of its membrane receptors, named A₁R, A_2AR, A_2BR and A₃R. Adenosine is released during hypoxia, ischemia, beta-adrenergic stimulation or inflammation and impacts heart rhythm and produces strong vasodilation in the systemic, coronary or pulmonary vascular system. This review summarizes the main role of adenosine on the cardiovascular system in several diseases and conditions. Adenosine release participates directly in the pathophysiology of atrial fibrillation and neurohumoral syncope. Adenosine has a key role in the adaptive response in pulmonary hypertension and heart failure, with the most relevant effects being slowing of heart rhythm, coronary vasodilation and decreasing blood pressure. In other conditions, such as altitude or apnea-induced hypoxia, obstructive sleep apnea, or systemic hypertension, the adenosinergic system activation appears in a context of an adaptive response. Due to its short half-life, adenosine allows very rapid adaptation of the cardiovascular system. Finally, the effects of adenosine on the cardiovascular system are sometimes beneficial and other times harmful. Future research should aim to develop modulating agents of adenosine receptors to slow down or conversely amplify the adenosinergic response according to the occurrence of different pathologic conditions.

Constitutive SRC-mediated phosphorylation of pannexin 1 at tyrosine 198 occurs at the plasma membrane

Pannexin 1 (PANX1)-mediated ATP release in vascular smooth muscle coordinates α1-adrenergic receptor (α1-AR) vasoconstriction and blood pressure homeostasis. We recently identified amino acids 198-200 (YLK) on the PANX1 intracellular loop that are critical for α1-AR-mediated vasoconstriction and PANX1 channel function. We report herein that the YLK motif is contained within an SRC homology 2 domain and is directly phosphorylated by SRC proto-oncogene, nonreceptor tyrosine kinase (SRC) at Tyr¹⁹⁸ We demonstrate that PANX1-mediated ATP release occurs independently of intracellular calcium but is sensitive to SRC family kinase (SFK) inhibition, suggestive of channel regulation by tyrosine phosphorylation. Using a PANX1 Tyr¹⁹⁸-specific antibody, SFK inhibitors, SRC knockdown, temperature-dependent SRC cells, and kinase assays, we found that PANX1-mediated ATP release and vasoconstriction involves constitutive phosphorylation of PANX1 Tyr¹⁹⁸ by SRC. We specifically detected SRC-mediated Tyr¹⁹⁸ phosphorylation at the plasma membrane and observed that it is not enhanced or induced by α1-AR activation. Last, we show that PANX1 immunostaining is enriched in the smooth muscle layer of arteries from hypertensive humans and that Tyr¹⁹⁸ phosphorylation is detectable in these samples, indicative of a role for membrane-associated PANX1 in small arteries of hypertensive humans. Our discovery adds insight into the regulation of PANX1 by post-translational modifications and connects a significant purinergic vasoconstriction pathway with a previously identified, yet unexplored, tyrosine kinase-based α1-AR constriction mechanism. This work implicates SRC-mediated PANX1 function in normal vascular hemodynamics and suggests that Tyr¹⁹⁸-phosphorylated PANX1 is involved in hypertensive vascular pathology.

L-DOPA sensitizes vasomotor tone by modulating the vascular alpha1-adrenergic receptor

Blood pressure is regulated by extrinsic factors including noradrenaline, the sympathetic neurotransmitter that controls cardiovascular functions through adrenergic receptors. However, the fine-tuning system of noradrenaline signaling is relatively unknown. We here show that l-3,4-dihydroxyphenylalanine (L-DOPA), a precursor of catecholamines, sensitizes the vascular adrenergic receptor alpha1 (ADRA1) through activation of L-DOPA receptor GPR143. In WT mice, intravenous infusion of the ADRA1 agonist phenylephrine induced a transient elevation of blood pressure. This response was attenuated in Gpr143 gene-deficient (Gpr143-/y) mice. Specific knockout of Gpr143 in vascular smooth muscle cells (VSMCs) also showed a similar phenotype, indicating that L-DOPA directly modulates ADRA1 signaling in the VSMCs. L-DOPA at nanomolar concentrations alone produced no effect on the VSMCs, but it enhanced phenylephrine-induced vasoconstriction and intracellular Ca2+ responses. Phenylephrine also augmented the phosphorylation of extracellular signal-regulated kinases in cultured VSMCs from WT but not Gpr143-/y mice. In WT mice, blood pressure increased during the transition from light-rest to dark-active phases. This elevation was not observed in Gpr143-/y mice. Taken together, our findings provide evidence for L-DOPA/GPR143 signaling that exerts precursor control of sympathetic neurotransmission through sensitizing vascular ADRA1.

Early diagnosis and effective treatment regimens are the keys to tackle antimicrobial resistance in tuberculosis (TB): A report from Euroscicon's international TB Summit 2016

To say that tuberculosis (TB) has regained a strong foothold in the global human health and wellbeing scenario would be an understatement. Ranking alongside HIV/AIDS as the top reason for mortality due to a single infectious disease, the impact of TB extends far into socio-economic context worldwide. As global efforts led by experts and political bodies converge to mitigate the predicted outcome of growing antimicrobial resistance, the academic community of students, practitioners and researchers have mobilised to develop integrated, inter-disciplinary programmes to bring the plans of the former to fruition. Enabling this crucial requirement for unimpeded dissemination of scientific discovery was the TB Summit 2016, held in London, United Kingdom. This report critically discusses the recent breakthroughs made in diagnostics and treatment while bringing to light the major hurdles in the control of the disease as discussed in the course of the 3-day international event. Conferences and symposia such as these are the breeding grounds for successful local and global collaborations and therefore must be supported to expand the understanding and outreach of basic science research.

Purinergic Receptor Transactivation by the β2-Adrenergic Receptor Increases Intracellular Ca2+ in Nonexcitable Cells

The β₂ adrenergic receptor (β₂AR) increases intracellular Ca²⁺ in a variety of cell types. By combining pharmacological and genetic manipulations, we reveal a novel mechanism through which the β₂AR promotes Ca²⁺ mobilization (pEC₅₀ = 7.32 ± 0.10) in nonexcitable human embryonic kidney (HEK)293S cells. Downregulation of Gs with sustained cholera toxin pretreatment and the use of Gs-null HEK293 (∆Gs-HEK293) cells generated using the clustered regularly interspaced short palindromic repeat-associated protein-9 nuclease (CRISPR/Cas9) system, combined with pharmacological modulation of cAMP formation, revealed a Gs-dependent but cAMP-independent increase in intracellular Ca²⁺ following β₂AR stimulation. The increase in cytoplasmic Ca²⁺ was inhibited by P2Y purinergic receptor antagonists as well as a dominant-negative mutant form of Gq, a Gq-selective inhibitor, and an inositol 1,4,5-trisphosphate (IP₃) receptor antagonist, suggesting a role for this Gq-coupled receptor family downstream of the β₂AR activation. Consistent with this mechanism, β₂AR stimulation promoted the extracellular release of ATP, and pretreatment with apyrase inhibited the β₂AR-promoted Ca²⁺ mobilization. Together, these data support a model whereby the β₂AR stimulates a Gs-dependent release of ATP, which transactivates Gq-coupled P2Y receptors through an inside-out mechanism, leading to a Gq- and IP₃-dependent Ca²⁺ mobilization from intracellular stores. Given that β₂AR and P2Y receptors are coexpressed in various tissues, this novel signaling paradigm could be physiologically important and have therapeutic implications. In addition, this study reports the generation and validation of HEK293 cells deleted of Gs using the CRISPR/Cas9 genome editing technology that will undoubtedly be powerful tools to study Gs-dependent signaling.

Epilepsy-associated GRIN2A mutations reduce NMDA receptor trafficking and agonist potency - molecular profiling and functional rescue

Mutations in the N-methyl-D-aspartate receptor (NMDAR) gene GRIN2A cause epilepsy-aphasia syndrome (EAS), a spectrum of epileptic, cognitive and language disorders. Using bioinformatic and patient data we shortlisted 10 diverse missense mutations for characterisation. We used high-throughput calcium-flux assays and patch clamp recordings of transiently transfected HEK-293 cells for electrophysiological characterization, and Western blotting and confocal imaging to assay expression and surface trafficking. Mutations P79R, C231Y, G483R and M705V caused a significant reduction in glutamate and glycine agonist potency, whilst D731N was non-responsive. These mutants, along with E714K, also showed significantly decreased total protein levels and trafficking to the cell surface, whilst C436R was not trafficked at all. Crucially this reduced surface expression did not cause the reduced agonist response. We were able to rescue the phenotype of P79R, C231Y, G483R and M705V after treatment with a GluN2A-selective positive allosteric modulator. With our methodology we were not able to identify any functional deficits in mutations I814T, D933N and N976S located between the glutamate-binding domain and C-terminus. We show GRIN2A mutations affect the expression and function of the receptor in different ways. Careful molecular profiling of patients will be essential for future effective personalised treatment options.

Pannexin-1 channels and their emerging functions in cardiovascular diseases

Pannexin-1, Pannexin-2, and Pannexin-3 are three members of the Pannexin family of channel-forming glycoprotein. Their primary function is defined by their ability to form single-membrane channels. Pannexin-1 ubiquitously exists in many cells and organs throughout the body and is specially distributed in the circulatory system, while the expressions of Pannexin-2 and Pannexin-3 are mostly restricted to organs and tissues. Pannexin-1 oligomers have been shown to be functional single membrane channels that connect intracellular and extracellular compartments and are not intercellular channels in appositional membranes. The physiological functions of Pannexin-1 are to link to the adenosine triphosphate efflux that acts as a paracrine signal, and regulate cellular inflammasomes in a variety of cell types under physiological and pathophysiological conditions. However, there are still many functions to be explored. This review summarizes recent reports and discusses the role of Pannexin-1 in cardiovascular diseases, including ischemia, arrhythmia, cardiac fibrosis, and hypertension. Pannexin-1 has been suggested as an exciting, clinically relevant target in cardiovascular diseases.

A diet with lactosucrose supplementation ameliorates trinitrobenzene sulfonic acid-induced colitis in rats

Chronic intestinal inflammation contributes to an increased risk of colon cancer.

P2X receptors regulate adenosine diphosphate release from hepatic cells

Extracellular nucleotides act as paracrine regulators of cellular signaling and metabolic pathways. Adenosine polyphosphate (adenosine triphosphate (ATP) and adenosine diphosphate (ADP)) release and metabolism by human hepatic carcinoma cells was therefore evaluated. Hepatic cells maintain static nanomolar concentrations of extracellular ATP and ADP levels until stress or nutrient deprivation stimulates a rapid burst of nucleotide release. Reducing the levels of media serum or glucose has no effect on ATP levels, but stimulates ADP release by up to 10-fold. Extracellular ADP is then metabolized or degraded and media ADP levels fall to basal levels within 2-4 h. Nucleotide release from hepatic cells is stimulated by the Ca(2+) ionophore, ionomycin, and by the P2 receptor agonist, 2'3'-O-(4-benzoyl-benzoyl)-adenosine 5'-triphosphate (BzATP). Ionomycin (10 μM) has a minimal effect on ATP release, but doubles media ADP levels at 5 min. In contrast, BzATP (10-100 μM) increases both ATP and ADP levels by over 100-fold at 5 min. Ion channel purinergic receptor P2X7 and P2X4 gene silencing with small interference RNA (siRNA) and treatment with the P2X inhibitor, A438079 (100 μM), decrease ADP release from hepatic cells, but have no effect on ATP. P2X inhibitors and siRNA have no effect on BzATP-stimulated nucleotide release. ADP release from human hepatic carcinoma cells is therefore regulated by P2X receptors and intracellular Ca(2+) levels. Extracellular ADP levels increase as a consequence of a cellular stress response resulting from serum or glucose deprivation.

Pannexin-1 as a potentiator of ligand-gated receptor signaling

Pannexins are a class of plasma membrane spanning proteins that presumably form a hexameric, non-selective ion channel. Although similar in secondary structure to the connexins, pannexins notably do not form endogenous gap junctions and act as bona fide ion channels. The pannexins have been primarily studied as ATP-release channels, but the overall diversity of their functions is still being elucidated. There is an intriguing theme with pannexins that has begun to develop. In this review we analyze several recent reports that converge on the idea that pannexin channels (namely Panx1) can potentiate ligand-gated receptor signaling. Although the literature remains sparse, this emerging concept appears consistent between both ionotropic and metabotropic receptors of several ligand families.

The pannexins: past and present

The pannexins (Panxs) are a family of chordate proteins homologous to the invertebrate gap junction forming proteins named innexins. Three distinct Panx paralogs (Panx1, Panx2, and Panx3) are shared among the major vertebrate phyla, but they appear to have suppressed (or even lost) their ability to directly couple adjacent cells. Connecting the intracellular and extracellular compartments is now widely accepted as Panx's primary function, facilitating the passive movement of ions and small molecules along electrochemical gradients. The tissue distribution of the Panxs ranges from pervasive to very restricted, depending on the paralog, and are often cell type-specific and/or developmentally regulated within any given tissue. In recent years, Panxs have been implicated in an assortment of physiological and pathophysiological processes, particularly with respect to ATP signaling and inflammation, and they are now considered to be a major player in extracellular purinergic communication. The following is a comprehensive review of the Panx literature, exploring the historical events leading up to their discovery, outlining our current understanding of their biochemistry, and describing the importance of these proteins in health and disease.

Differentiating connexin hemichannels and pannexin channels in cellular ATP release

Adenosine triphosphate (ATP) plays a fundamental role in cellular communication, with its extracellular accumulation triggering purinergic signaling cascades in a diversity of cell types. While the roles for purinergic signaling in health and disease have been well established, identification and differentiation of the specific mechanisms controlling cellular ATP release is less well understood. Multiple mechanisms have been proposed to regulate ATP release with connexin (Cx) hemichannels and pannexin (Panx) channels receiving major focus. However, segregating the specific roles of Panxs and Cxs in ATP release in a plethora of physiological and pathological contexts has remained enigmatic. This multifaceted problem has arisen from the selectivity of pharmacological inhibitors for Panxs and Cxs, methodological differences in assessing Panx and Cx function and the potential compensation by other isoforms in gene silencing and genetic knockout models. Consequently, there remains a void in the current understanding of specific contributions of Panxs and Cxs in releasing ATP during homeostasis and disease. Differentiating the distinct signaling pathways that regulate these two channels will advance our current knowledge of cellular communication and aid in the development of novel rationally-designed drugs for modulation of Panx and Cx activity, respectively.

The emerging Pannexin 1 signalome: a new nexus revealed?

Pannexins (Panxs) are a family of single-membrane, large-pore ion, and metabolite permeable channels. Of the three Panx proteins, Panx1 has been most extensively studied, and has recently emerged as an exciting, clinically relevant target in many physiological and pathophysiological settings. This channel is widely expressed across various cell and tissue types; however its links to precise signaling pathways are largely unknown. Here we review the current literature surrounding presently identified Panx1–protein interactions, a critical first step to unraveling the Panx1 signalome. First we elucidate the reported associations of Panx1 with other ion channels, receptors, and channel signaling complexes. Further, we highlight recently identified Panx1–cytoskeleton interactions. Finally, we discuss the implications of these protein–protein interactions for Panx1 function in various cell and tissue types, and identify key outstanding questions arising from this work.

Diminished adenosine A1 receptor expression in pancreatic α-cells may contribute to the pathology of type 1 diabetes

Prediabetic NOD mice exhibit hyperglucagonemia, possibly due to an intrinsic α-cell defect. Here, we show that the expression of a potential glucagon inhibitor, the adenosine A1 receptor (Adora1), is gradually diminished in α-cells of NOD mice, autoantibody-positive (AA(+)) and overtly type 1 diabetic (T1D) patients during the progression of disease. We demonstrated that islet inflammation was associated with loss of Adora1 expression through the alternative splicing of Adora1. Expression of the spliced variant (Adora1-Var) was upregulated in the pancreas of 12-week-old NOD versus age-matched NOD.B10 (non-diabetes-susceptible) control mice and was detected in the pancreas of AA(+) patients but not in control subjects or overtly diabetic patients, suggesting that inflammation drives the splicing of Adora1. We subsequently demonstrated that Adora1-Var expression was upregulated in the islets of NOD.B10 mice after exposure to inflammatory cytokines and in the pancreas of NOD.SCID mice after adoptive transfer of activated autologous splenocytes. Adora1-Var encodes a dominant-negative N-terminal truncated isoform of Adora1. The splicing of Adora1 and loss of Adora1 expression on α-cells may explain the hyperglucagonemia observed in prediabetic NOD mice and may contribute to the pathogenesis of human T1D and NOD disease.

Agonist-induced desensitization of human β3-adrenoceptors expressed in human embryonic kidney cells

β3-Adrenoceptors are resistant to agonist-induced desensitization in some cell types but susceptible in others including transfected human embryonic kidney (HEK) cells. Therefore, we have studied cellular and molecular changes involved in agonist-induced β3-adrenoceptor desensitization in HEK cells. Cells were treated with isoprenaline or forskolin, and following wash-out, cyclic adenosine monophosphate (cAMP) accumulation in response to freshly added agonist was quantified. Receptor and G protein expression were quantified by radioligand binding and immunoblot experiments, respectively. Treatment with isoprenaline induced a concentration- and time-dependent desensitization of cAMP accumulation in response to freshly added isoprenaline. This functional desensitization primarily consisted of reduced maximum responses with little change of agonist potency. Maximum desensitization was achieved by pre-treatment with 10 μM isoprenaline for 24 h. It was not accompanied by changes in β3-adrenoceptor density as assessed in saturation radioligand-binding studies. The desensitization was associated with a small reduction in immunoreactivity for α-subunits for Gs and Gi1, whereas that for Gi2, Gi3, and Gq/11 was not significantly altered. In cells treated with pertussis toxin, isoprenaline-induced cAMP accumulation as well as desensitization by isoprenaline pre-treatment remained unchanged. Isoprenaline pre-treatment also reduced forskolin-induced cAMP accumulation; conversely, pre-treatment with forskolin caused a similar desensitization of isoprenaline-induced cAMP accumulation. We conclude that agonist-induced β3-adrenoceptor desensitization in HEK cells does not involve reduced receptor numbers and small, if any, reduction of Gs expression; changes at the level of adenylyl cyclase function can fully explain this desensitization.

P2Y receptors and kidney function

Cellular release of nucleotides is of physiological importance to regulate and maintain cell function and integrity. Also in the tubular and collecting duct system of the kidney, nucleotides are released in response to changes in cell volume or luminal flow rate and act in a paracrine and autocrine way on basolateral and luminal P2Y receptors. Recent studies using gene knockout mice assigned a prominent role to G protein-coupled P2Y(2) receptors, which are activated by both ATP and UTP. The antidiuretic hormone, arginine-vasopressin (AVP), and possibly an increase in collecting duct cell volume induce ATP release. The subsequent activation of P2Y(2) receptors inhibits AVP-induced cAMP formation and water reabsorption, which stabilizes cell volume and facilitates water excretion. An increase in NaCl intake enhances luminal release of ATP and UTP in the aldosterone-sensitive distal nephron which by activating apical P2Y(2) receptors and phospholipase C lowers the open probability of the epithelial sodium channel ENaC, thereby facilitating sodium excretion. Thus, the renal ATP/UTP/P2Y(2) receptor system not only serves to preserve cell volume and integrity but is also regulated by stimuli that derive from body NaCl homeostasis. The system also inhibits ENaC activity during aldosterone escape, i.e. when sodium reabsorption via ENaC is inappropriately high. The P2Y(2) receptor tone inhibits the expression and activity of the Na-K-2Cl cotransporter NKCC2 in the thick ascending limb and mediates vasodilation. While the role of other P2Y receptors in the kidney is less clear, the ATP/UTP/P2Y(2) receptor system regulates NaCl and water homeostasis and blood pressure.

Pannexin 1 in the regulation of vascular tone

Pannexins are a recently discovered protein family with the isoform Panx1 ubiquitously expressed and therefore extensively studied. Panx1 proteins form membrane channels known to release purines such as ATP. Because ATP and, more generally, purinergic signaling plays an important role in the vasculature, it became evident that Panx1 could have a key role in vascular functions. This article reviews recent findings on the pivotal role of Panx1 in smooth muscle cells in the contraction of arteries as well as recent insights into Panx1 channel regulation.

Influence of erythrocyte oxygenation and intravascular ATP on resting and exercising skeletal muscle blood flow in humans with mitochondrial myopathy

Oxygen (O₂) extraction is impaired in exercising skeletal muscle of humans with mutations of mitochondrial DNA (mtDNA), but the muscle hemodynamic response to exercise has never been directly investigated. This study sought to examine the extent to which human skeletal muscle perfusion can increase without reductions in blood oxygenation and to determine whether erythrocyte O₂ off-loading and related ATP vascular mechanisms are impaired in humans with mutations of mtDNA. Leg vascular hemodynamic, oxygenation and ATP were investigated in ten patients with mtDNA mutations and ten matched healthy control subjects: 1) at rest during normoxia, hypoxia, hyperoxia and intra-femoral artery ATP infusion, and 2) during passive and dynamic one-legged knee-extensor exercises. At rest, blood flow (LBF), femoral arterial and venous blood oxygenation and plasma ATP were similar in the two groups. During dynamic exercise, LBF and vascular conductance increased 9-10 fold in the patients despite erythrocyte oxygenation and leg O₂ extraction remained unchanged (p<0.01). In the patients, workload-adjusted LBF was 28% to 62% higher during submaximal- and maximal exercises and was associated with augmented plasma ATP. The appropriate hemodynamic adjustments during severe hypoxia and ATP infusion suggest that erythrocyte O₂ off-loading and related ATP vascular mechanisms are intact in patients with mtDNA mutations. Furthermore, greater increase in plasma ATP and LBF at a given metabolic demand in the patients, in concert with unchanged oxyhemoglobin, suggest that erythrocyte O₂ off-loading is not obligatory for the exercise-induced increase in blood flow and intravascular ATP concentration.

Pannexin1 regulates α1-adrenergic receptor- mediated vasoconstriction

RATIONALE

The coordination of vascular smooth muscle cell constriction plays an important role in vascular function, such as regulation of blood pressure; however, the mechanism responsible for vascular smooth muscle cell communication is not clear in the resistance vasculature. Pannexins (Panx) are purine-releasing channels permeable to the vasoconstrictor ATP and thus may play a role in the coordination of vascular smooth muscle cell constriction.

OBJECTIVE

We investigated the role of pannexins in phenylephrine- and KCl-mediated constriction of resistance arteries.

METHODS AND RESULTS

Western blot, immunohistochemistry, and immunogold labeling coupled to scanning and transmission electron microscopy revealed the presence of Panx1 but not Panx2 or Panx3 in thoracodorsal resistance arteries. Functionally, the contractile response of pressurized thoracodorsal resistance arteries to phenylephrine was decreased significantly by multiple Panx inhibitors (mefloquine, probenecid, and (10)Panx1), ectonucleotidase (apyrase), and purinergic receptor inhibitors (suramin and reactive blue-2). Electroporation of thoracodorsal resistance arteries with either Panx1-green fluorescent protein or Panx1 small interfering RNA showed enhanced and decreased constriction, respectively, in response to phenylephrine. Lastly, the Panx inhibitors did not alter constriction in response to KCl. This result is consistent with coimmunoprecipitation experiments from thoracodorsal resistance arteries, which suggested an association between Panx1 and α1D-adrenergic receptor.

CONCLUSIONS

Our data demonstrate for the first time a key role for Panx1 in resistance arteries by contributing to the coordination of vascular smooth muscle cell constriction and possibly to the regulation of blood pressure.

Immune cell regulation by autocrine purinergic signalling

Stimulation of almost all mammalian cell types leads to the release of cellular ATP and autocrine feedback through a diverse array of purinergic receptors. Depending on the types of purinergic receptors that are involved, autocrine signalling can promote or inhibit cell activation and fine-tune functional responses. Recent work has shown that autocrine signalling is an important checkpoint in immune cell activation and allows immune cells to adjust their functional responses based on the extracellular cues provided by their environment. This Review focuses on the roles of autocrine purinergic signalling in the regulation of both innate and adaptive immune responses and discusses the potential of targeting purinergic receptors for treating immune-mediated disease.

Extracellular high dosages of adenosine triphosphate induce inflammatory response and insulin resistance in rat adipocytes

Adenosine triphosphate (ATP), an important signaling molecule, participates in various pathophysiological processes via the activation of purinergic-receptors. Recent studies have shown that the expression and function of purinergic-receptors (P2-receptors) could be altered in diabetic or hyperinsulinemia conditions. To characterize the effect of ATP on insulin signaling, we treated primary rat adipocytes with varied concentrations of ATP. The pre-treatment led to impaired insulin signaling, i.e., blunted phosphorylation in Insulin Receptor Substrate-1 (IRS-1) tyrosine and Protein Kinase B (PKB) Ser473 in response to insulin treatment, when ATP concentration reached 1mM. We then observed that ATP dose-dependently reduced the level of IκB, a negative regulator of inflammatory response. Consistently, IRS-1 Ser307 phosphorylation in response to insulin treatment, a site for inflammatory pathway to interfere insulin signaling, was enhanced by ATP. Furthermore, effects of ATP on insulin signaling and IκB content were blocked by P2-receptor inhibition. Finally, insulin-stimulated glucose uptake was impaired by ATP in adipocytes but not in the L6 muscle cells. This study therefore shows for the first time the involvement of ATP-evoked P2-receptor activation in mediating the inflammatory response and the generation of insulin resistance in adipocytes.