Receptors subtypes involved in adenosine-mediated modulation of norepinephrine release from cardiac nerve terminals

Sleep fragmentation increases blood pressure and is associated with alterations in the gut microbiome and fecal metabolome in rats

The gut microbiota, via the production of metabolites entering the circulation, plays a role in blood pressure regulation. Blood pressure is also affected by the characteristics of sleep. To date, no studies have examined relationships among the gut microbiota/metabolites, blood pressure, and sleep. We hypothesized that fragmented sleep is associated with elevated mean arterial pressure, an altered and dysbiotic gut microbial community, and changes in fecal metabolites. In our model system, rats were randomized to 8 h of sleep fragmentation during the rest phase (light phase) or were undisturbed (controls) for 28 consecutive days. Rats underwent sleep and blood pressure recordings, and fecal samples were analyzed during: baseline (days -4 to -1), early sleep fragmentation (days 0-3), midsleep fragmentation (days 6-13), late sleep fragmentation (days 20-27), and recovery/rest (days 28-34). Less sleep per hour during the sleep fragmentation period was associated with increased mean arterial pressure. Analyses of gut microbial communities and metabolites revealed that putative short chain fatty acid-producing bacteria were differentially abundant between control and intervention animals during mid-/late sleep fragmentation and recovery. Midsleep fragmentation was also characterized by lower alpha diversity, lower Firmicutes:Bacteroidetes ratio, and higher Proteobacteria in intervention rats. Elevated putative succinate-producing bacteria and acetate-producing bacteria were associated with lower and higher mean arterial pressure, respectively, and untargeted metabolomics analysis demonstrates that certain fecal metabolites are significantly correlated with blood pressure. These data reveal associations between sleep fragmentation, mean arterial pressure, and the gut microbiome/fecal metabolome and provide insight to links between disrupted sleep and cardiovascular pathology.

Focusing on Adenosine Receptors as a Potential Targeted Therapy in Human Diseases

Adenosine is involved in a range of physiological and pathological effects through membrane-bound receptors linked to G proteins. There are four subtypes of adenosine receptors, described as A1AR, A2AAR, A2BAR, and A3AR, which are the center of cAMP signal pathway-based drug development. Several types of agonists, partial agonists or antagonists, and allosteric substances have been synthesized from these receptors as new therapeutic drug candidates. Research efforts surrounding A1AR and A2AAR are perhaps the most enticing because of their concentration and affinity; however, as a consequence of distressing conditions, both A2BAR and A3AR levels might accumulate. This review focuses on the biological features of each adenosine receptor as the basis of ligand production and describes clinical studies of adenosine receptor-associated pharmaceuticals in human diseases.

Adenosine, lidocaine and Mg2+ improves cardiac and pulmonary function, induces reversible hypotension and exerts anti-inflammatory effects in an endotoxemic porcine model

Introduction

The combination of Adenosine (A), lidocaine (L) and Mg²⁺ (M) (ALM) has demonstrated cardioprotective and resuscitative properties in models of cardiac arrest and hemorrhagic shock. This study evaluates whether ALM also demonstrates organ protective properties in an endotoxemic porcine model.

Methods

Pigs (37 to 42 kg) were randomized into: 1) Control (n = 8) or 2) ALM (n = 8) followed by lipopolysaccharide infusion (1 μg∙kg^-1∙h^-1) for five hours. ALM treatment consisted of 1) a high dose bolus (A (0.82 mg/kg), L (1.76 mg/kg), M (0.92 mg/kg)), 2) one hour continuous infusion (A (300 μg∙kg^-1 ∙min^-1), L (600 μg∙kg^-1 ∙min^-1), M (336 μg∙kg^-1 ∙min^-1)) and three hours at a lower dose (A (240∙kg^-1∙min^-1), L (480 μg∙kg^-1∙min^-1), M (268 μg∙kg^-1 ∙min^-1)); controls received normal saline. Hemodynamic, cardiac, pulmonary, metabolic and renal functions were evaluated.

Results

ALM lowered mean arterial pressure (Mean value during infusion period: ALM: 47 (95% confidence interval (CI): 44 to 50) mmHg versus control: 79 (95% CI: 75 to 85) mmHg, P <0.0001). After cessation of ALM, mean arterial pressure immediately increased (end of study: ALM: 88 (95% CI: 81 to 96) mmHg versus control: 86 (95% CI: 79 to 94) mmHg, P = 0.72). Whole body oxygen consumption was significantly reduced during ALM infusion (ALM: 205 (95% CI: 192 to 217) ml oxygen/min versus control: 231 (95% CI: 219 to 243) ml oxygen/min, P = 0.016). ALM treatment reduced pulmonary injury evaluated by PaO₂/FiO₂ ratio (ALM: 388 (95% CI: 349 to 427) versus control: 260 (95% CI: 221 to 299), P = 0.0005). ALM infusion led to an increase in heart rate while preserving preload recruitable stroke work. Creatinine clearance was significantly lower during ALM infusion but reversed after cessation of infusion. ALM reduced tumor necrosis factor-α peak levels (ALM 7121 (95% CI: 5069 to 10004) pg/ml versus control 11596 (95% CI: 9083 to 14805) pg/ml, P = 0.02).

Conclusion

ALM infusion induces a reversible hypotensive and hypometabolic state, attenuates tumor necrosis factor-α levels and improves cardiac and pulmonary function, and led to a transient drop in renal function that was reversed after the treatment was stopped.

Effect of Caffeine Chronically Consumed During Pregnancy on Adenosine A1 and A2A Receptors Signaling in Both Maternal and Fetal Heart from Wistar Rats

Background: Caffeine is the most widely consumed psychoactive substance in the world, even during pregnancy. Its stimulatory effects are mainly due to antagonism of adenosine actions by blocking adenosine A₁ and A_2A receptors. Previous studies have shown that caffeine can cross the placenta and therefore modulate these receptors not only in the fetal brain but also in the heart. Methods: In the present work, the effect of caffeine chronically consumed during pregnancy on A₁ and A_2A receptors in Wistar rat heart, from both mothers and their fetuses, were studied using radioligand binding, Western-blotting, and adenylyl cyclase activity assays, as well as reverse transcription polymerase chain reaction. Results: Caffeine did not significantly alter A₁R neither at protein nor at gene expression level in both the maternal and fetal heart. On the contrary, A_2AR significantly decreased in the maternal heart, although mRNA was not affected. Gi and Gs proteins were also preserved. Finally, A₁R-mediated inhibition of adenylyl cyclase activity did not change in the maternal heart, but A_2AR mediated stimulation of this enzymatic activity significantly decreased according to the detected loss of this receptor. Conclusions: Opposite to the downregulation and desensitization of the A₁R/AC pathway previously reported in the brain, these results show that this pathway is not affected in rat heart after caffeine exposure during pregnancy. In addition, A_2AR is downregulated and desensitized in the maternal heart, suggesting a differential modulation of these receptor-mediated pathways by caffeine.

Reduced adenosine release from the aged mammalian heart

Adenosine (ADO) released in the heart results in enhanced coronary blood flow and reduced catecholamine release and myocardial responsiveness to adrenergic stimulation (anti-adrenergic action). ADO release from the adrenergic-stimulated aged heart is less than that from the young adult heart. Because adrenergic signaling in the aged heart is impaired, this study was conducted to determine if reduced ADO release from the aged heart results from this reduced adrenergic responsiveness. Hearts of 3-4 months (young adult) and 21-22 months (aged) Fischer-344 rats were perfused with ADO deamination and re-phosphorylation inhibited. Coronary effluent ADO levels were determined. Cellular-free ADO levels with and without sodium acetate (NaAc)-induced mitochondrial AMP synthesis were assessed using formed S-adenosylhomocysteine (SAH) in L-homocysteine thiolactone (L-HC)-treated hearts. The activities of SAH-hydrolase were determined. Aged heart ADO release was 61% less than from young hearts. NaAc augmented young heart ADO release by 104%, while that of aged hearts remained unchanged. SAH synthesis was 51% and 56% lower in the aged heart in the absence and presence of NaAc, respectively, despite an 89% greater SAH hydrolase activity found in the aged hearts. Since synthesized AMP may be diverted to IMP and ultimately inosine by AMP deaminase, inosine release was determined. Aged heart inosine levels in the absence and presence of NaAc were 74% and 59% less than for the young hearts. It is concluded that a reduced mitochondrial AMP synthesis is in part responsible for the attenuation in ADO release from the adrenergic-stimulated aged heart.

Selective attenuation of norepinephrine release and stress-induced heart rate increase by partial adenosine A1 agonism

The release of the neurotransmitter norepinephrine (NE) is modulated by presynaptic adenosine receptors. In the present study we investigated the effect of a partial activation of this feedback mechanism. We hypothesized that partial agonism would have differential effects on NE release in isolated hearts as well as on heart rate in vivo depending on the genetic background and baseline sympathetic activity. In isolated perfused hearts of Wistar and Spontaneously Hypertensive Rats (SHR), NE release was induced by electrical stimulation under control conditions (S1), and with capadenoson 6 · 10(-8) M (30 µg/l), 6 · 10(-7) M (300 µg/l) or 2-chloro-N(6)-cyclopentyladenosine (CCPA) 10(-6) M (S2). Under control conditions (S1), NE release was significantly higher in SHR hearts compared to Wistar (766+/-87 pmol/g vs. 173+/-18 pmol/g, p<0.01). Capadenoson led to a concentration-dependent decrease of the stimulation-induced NE release in SHR (S2/S1  =  0.90 ± 0.08 with capadenoson 6 · 10(-8) M, 0.54 ± 0.02 with 6 · 10(-7) M), but not in Wistar hearts (S2/S1  =  1.05 ± 0.12 with 6 · 10(-8) M, 1.03 ± 0.09 with 6 · 10(-7) M). CCPA reduced NE release to a similar degree in hearts from both strains. In vivo capadenoson did not alter resting heart rate in Wistar rats or SHR. Restraint stress induced a significantly greater increase of heart rate in SHR than in Wistar rats. Capadenoson blunted this stress-induced tachycardia by 45% in SHR, but not in Wistar rats. Using a [(35)S]GTPγS assay we demonstrated that capadenoson is a partial agonist compared to the full agonist CCPA (74+/-2% A(1)-receptor stimulation). These results suggest that partial adenosine A(1)-agonism dampens stress-induced tachycardia selectively in rats susceptible to strong increases in sympathetic activity, most likely due to a presynaptic attenuation of NE release.

Adenosine and its receptors in the heart: regulation, retaliation and adaptation

The purine nucleoside adenosine is an important regulator within the cardiovascular system, and throughout the body. Released in response to perturbations in energy state, among other stimuli, local adenosine interacts with 4 adenosine receptor sub-types on constituent cardiac and vascular cells: A(1), A(2A), A(2B), and A(3)ARs. These G-protein coupled receptors mediate varied responses, from modulation of coronary flow, heart rate and contraction, to cardioprotection, inflammatory regulation, and control of cell growth and tissue remodeling. Research also unveils an increasingly complex interplay between members of the adenosine receptor family, and with other receptor groups. Given generally favorable effects of adenosine receptor activity (e.g. improving the balance between myocardial energy utilization and supply, limiting injury and adverse remodeling, suppressing inflammation), the adenosine receptor system is an attractive target for therapeutic manipulation. Cardiovascular adenosine receptor-based therapies are already in place, and trials of new treatments underway. Although the complex interplay between adenosine receptors and other receptors, and their wide distribution and functions, pose challenges to implementation of site/target specific cardiovascular therapy, the potential of adenosinergic pharmacotherapy can be more fully realized with greater understanding of the roles of adenosine receptors under physiological and pathological conditions. This review addresses some of the major known and proposed actions of adenosine and adenosine receptors in the heart and vessels, focusing on the ability of the adenosine receptor system to regulate cell function, retaliate against injurious stressors, and mediate longer-term adaptive responses.

Purinergic receptors in the splanchnic circulation

There is considerable evidence that purines are vasoactive molecules involved in the regulation of blood flow. Adenosine is a well known vasodilator that also acts as a modulator of the response to other vasoactive substances. Adenosine exerts its effects by interacting with adenosine receptors. These are metabotropic G-protein coupled receptors and include four subtypes, A(1), A(2A), A(2B) and A(3). Adenosine triphosphate (ATP) is a co-transmitter in vascular neuroeffector junctions and is known to activate two distinct types of P2 receptors, P2X (ionotropic) and P2Y (metabotropic). ATP can exert either vasoconstrictive or vasorelaxant effects, depending on the P2 receptor subtype involved. Splanchnic vascular beds are of particular interest, as they receive a large fraction of the cardiac output. This review focus on purinergic receptors role in the splanchnic vasomotor control. Here, we give an overview on the distribution and diversity of effects of purinergic receptors in splanchnic vessels. Pre- and post-junctional receptormediated responses are summarized. Attention is also given to the interactions between purinergic receptors and other receptors in the splanchnic circulation.

Adenosinergic cardioprotection: Multiple receptors, multiple pathways

Adenosine, formed primarily via hydrolysis of 5'-AMP, has been historically dubbed a "retaliatory" metabolite due to enhanced local release and beneficial actions during cellular/metabolic stress. From a cardiovascular perspective, evidence indicates the adenosinergic system is essential in mediation of intrinsic protection (e.g., pre- and postconditioning) and determining myocardial resistance to insult. Modulation of adenosine and its receptors thus remains a promising, though as yet not well-realized, approach to amelioration of injury in ischemic-reperfused myocardium. Adenosine exerts effects through A(1), A(2A), A(2B), and A(3) adenosine receptor subtypes (A(1)AR, A(2A)AR, A(2B)AR, and A(3)AR), which are all expressed in myocardial and vascular cells, and couple to G proteins to trigger a range of responses (generally, but not always, beneficial). Adenosine can also enhance tolerance to injurious stimuli via receptor-independent metabolic effects. Given adenosines contribution to preconditioning, it is no surprise that postreceptor signaling typically mimics that associated with preconditioning. This involves activation/translocation of PKC, PI3 kinase, and MAPKs, with ultimate effects at the level of mitochondrial targets-the mitochondrial K(ATP) channel and/or the mitochondrial permeability transition pore (mPTP). Nonetheless, differences in cytoprotective signaling and actions of the different adenosine receptor subtypes have been recently revealed. Our understanding of adenosinergic cytoprotection continues to evolve, with roles for the A(2) subtypes emerging, together with evidence of essential receptor "cross-talk" in mediation of protection. This review focuses on current research into adenosine-mediated cardioprotection, highlighting recent findings which, together with a wealth of prior knowledge, may ultimately facilitate adenosinergic approaches to clinical cardiac protection.

Impact of chronic beta-adrenoceptor activation on neurotensin-induced myocardial effects in rats

In heart failure chronic sympathetic activation results in contractile dysfunction in part through down-regulation of the beta-adrenoceptor-cAMP system. However, the impact of chronic adrenergic activation on cardiac sympathetic neuromodulator systems is unclear. In this study, we sought to determine whether chronic adrenergic activation modifies myocardial norepinephrine release and contractile responses elicited by neurotensin, a neuropeptide found in cardiovascular system. Chronic administration of isoproterenol, a beta-adrenoceptor agonist, to rats (0.05 mg/kg daily for 1 month, i.p.), produced cardiac hypertrophy with preserved baseline ventricular systolic function, but reduced contractile responses to exogenous norepinephrine as shown in isolated, isovolumically-contracting heart preparations. Neurotensin produced a marked increase in coronary effluent norepinephrine release, an effect abolished by SR 48692, a specific neurotensin receptor antagonist. In isoproterenol-treated rats, neurotensin has no significant impact on myocardial norepinephrine release. Consistently, concentration-dependent positive inotropic responses elicited by neurotensin in control rat hearts were blunted over a wide range of neurotensin concentrations (10(-10)-10(-5.5) M) in isoproterenol-treated rats. In conclusion, these data indicate that following chronic beta-adrenoceptor activation, neurotensin-induced effects on norepinephrine release and subsequent contractile changes are markedly down-regulated.

Adenosine A1 receptor-mediated inhibition of myocardial norepinephrine release involves neither phospholipase C nor protein kinase C but does involve adenylyl cyclase

Stimulation of adenosine A1 receptors in the heart exerts cardioprotective effects by inhibiting norepinephrine (NE) release from sympathetic nerve endings. The intraneuronal signal transduction triggered by presynaptic adenosine A1 receptors is still not completely understood. The objective of the present study was to determine whether phospholipase C (PLC), protein kinase C (PKC), and adenylyl cyclase (AC) are involved in the adenosine A1 receptor-mediated inhibition of endogenous (stimulation-induced) NE release in isolated Langendorff-perfused rat hearts as an approach to elucidate their role in the cardiovascular system. Activation of adenosine A1-receptors with 2-chloro-N6-cyclopentyladenosine (CCPA) decreased cardiac NE release by ~40%. Inhibition of PLC with 1-[6-[[(17b)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U 73122) as well as inhibition of PKC with 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide (GF 109203X) slightly but significantly decreased NE release; however, the suppressive effect of CCPA on NE release was not modulated by U 73122 or GF 109203X. Blockade of AC with 9-(tetrahydro-2′-furyl)adenine (SQ 22536) reversed the inhibitory effect of CCPA on sympathetic neurotransmitter release irrespective of whether PKC was pharmacologically activated by phorbol 12-myristate 13-acetate or was not activated, indicating a PKC-independent but AC-dependent mechanism. Direct stimulation of AC with forskolin increased NE release by ∼20%; an effect that was antagonized by either CCPA or SQ 22536. These data suggest that the adenosine A1 receptor-mediated inhibition of NE release does not involve PLC or PKC but does involve AC.