Modification of cardiovascular response of adenosine A1 receptor agonist by cyclic AMP in the spinal cord of the rats

Therapeutic potency of A1 adenosine receptor antagonists in the treatment of cardiovascular diseases, current status and perspectives

BACKGROUND

Cardiomyocytes form, transport, and metabolize the omnipresent metabolite adenosine. Depending upon the adenosine concentrations and the pharmacological properties of receptor subtypes, adenosine exerts (patho)physiological responses in the cardiovascular system. The objective of this review is to present different protective mechanisms of A1-adenosine receptor inhibitors in cardiovascular diseases.

METHODS AND RESULTS

Literature references were collected and sorted using relevant keywords and key phrases as search terms in scientific databases such as Web of Science, PubMed and Google Scholar. A1 adenosine receptor regulates free fatty acid metabolism, lipolysis, heart rate, blood pressure, and cardiovascular toxicity. The evidence clearly supporting the therapeutic potency of pharmacological A1 adenosine receptors agonists and antagonists in modulating cardiovascular risk factor parameters and treatment of cardiovascular diseases.

CONCLUSION

This review summarizes the protective role of pharmacological A1-adenosine receptor regulators in the pathogenesis of cardiovascular diseases for a better management of cardiovascular diseases.

Ketamine attenuates sympathetic activity through mechanisms not mediated by N-methyl-D-aspartate receptors in the isolated spinal cord of neonatal rats

Ketamine is believed to have sympathomimetic effects, although the central mechanism remains unclear. Using an in vitro splanchnic nerve-spinal cord preparation from neonatal rats, our previous investigations have demonstrated that tonic sympathetic activity is spontaneously generated from the thoracic spinal cord. We designed this study to investigate whether applications of ketamine to the cord would augment sympathetic activity and whether this action was dependent on N-methyl-d-aspartate receptors. Bath application of ketamine significantly reduced sympathetic activity in a concentration-dependent manner. Ketamine in 10, 20, 40, 80, and 120 microM reduced the sympathetic activity to 82.6% +/- 4.4% (P < 0.05), 61.7% +/- 5.1%, 42.8% +/- 4.2%, 24.9% +/- 4.4%, and 9.2% +/- 2.7% of the control value, respectively (P < 0.01, n = 8 for each test). The 50% inhibitory concentration of ketamine on sympathetic activity was 32 muM. Pretreatment with DL-2-amino-5-phosphonovaleric acid, a selective competitive N-methyl-d-aspartate receptor antagonist, did not alter ketamine-induced depression of sympathetic activity. These results suggest that ketamine reduces sympathetic activity by mechanisms that are independent of N-methyl-d-aspartate receptor activity.

The role of intraspinal adenosine A1 receptors in sympathetic regulation

Using a splanchnic nerve-spinal cord preparation in vitro, we have previously demonstrated that tonic sympathetic activity is generated from the thoracic spinal cord. Here, we sought to determine if adenosine receptors play a role in modulating this spinally generated sympathetic activity. Various adenosine analogs were applied. N6-Cyclopentyladenosine (CPA, adenosine A1 receptor agonist) and 5'-N-ethylcarboxamidoadenosine (NECA, adenosine A1/A2 receptor agonist) reduced, while N6-[2-(4-aminophenyl)ethyl]adenosine (APNEA, non-selective adenosine A3 receptor agonist) did not alter sympathetic activity. The inhibitory effect of CPA or NECA on sympathetic activity was reversed by 8-cyclopentyltheophylline (CPT, adenosine A1 receptor antagonist) or abolished by CPT pretreatment. In the presence of 3,7-dimethyl-1-propargylxanthine (DMPX, adenosine A2 receptor antagonist), sympathetic activity was still reduced by CPA or NECA. Sympathetic activities were not changed by applications of the more selective adenosine A2 or A3 receptor agonists or antagonists, including 4-[2-[[6-amino-9-(N-ethyl-beta-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid (CGS21680), 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385), 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Chloro-IB-MECA), and 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS1191). These findings exclude a possible involvement of A2 or A3 receptors in sympathetic regulation at the spinal levels. Interestingly, CPT alone did not affect sympathetic activity, suggesting that adenosine A1 receptors are endogenously quiescent under our experimental conditions. We conclude that intraspinal adenosine A1 receptors may down-regulate sympathetic outflow and serve as a part of the scheme for neuroprotection.

Input-specific modulation of neurotransmitter release in the lateral horn of the spinal cord via adenosine receptors

Activation of adenosine A2A receptors (A2ARs) in the CNS produces a variety of neuromodulatory actions dependent on the region and preparation examined. In autonomic regions of the spinal cord, A1R activation decreases excitatory synaptic transmission, but the effects of A2AR stimulation are unknown. We sought to determine the location and function of the A2ARs in the thoracic spinal cord, focusing on the intermediolateral cell column (IML). A2AR immunoreactivity was observed throughout the gray matter, with particularly dense immunostaining in regions containing sympathetic preganglionic neurons (SPNs), namely, the IML and intercalated nucleus. Electron microscopy revealed A2AR immunoreactivity within presynaptic terminals and in postsynaptic structures in the IML. To study the functional relevance of these A2ARs, visualized whole-cell patch-clamp recordings were made from electrophysiologically identified SPNs and interneurons within the IML. The A2AR agonist c2-[p-(carboxyethyl)phenethylamino]-5'-N-ethylcarboxyamidoadenosine (CGS 21680) had no significant effect on EPSPs but increased the amplitude of IPSPs elicited by stimulation of the lateral funiculus. These effects were attributable to activation of presynaptic A2ARs because CGS 21680 application altered the paired pulse ratio. Furthermore, neurons in the IML that have IPSPs increased via A2AR activation also receive excitatory inputs that are inhibited by A1R activation. These data show that activating A2ARs increase inhibitory but not excitatory transmission onto neurons in the IML. Simultaneous activation of A1Rs and A2ARs therefore could facilitate inhibition of the postsynaptic neuron, leading to an overall reduction of sympathetic nervous activity.

Modification of cardiovascular response of posterior hypothalamic adenosine A(2) receptor stimulation by adenylate cylase, guanylate cyclase and by K(ATP) channel blockade in anesthetized rats

Cardiovascular inhibitory effects induced by posterior hypothalamic adenosine A(2) receptors and their modulation by nitric oxide were suggested by our previous report. In this experiment, we examined the modulation of cardiovascular effects of adenosine A(2) receptor stimulation by adenylate cyclase, guanylate cyclase and ATP-sensitive K(+) channel in the posterior hypothalamus. Posterior hypothalamic injection of drugs was performed in anesthetized, artificially ventilated male Sprague-Dawley rats. Injection of adenosine A(2) receptor agonist 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA; 1, 2 and 5 nmol) produced a dose-dependent decrease of blood pressure and heart rate. Pretreatment with adenosine A(2) receptor antagonist 3,7-dimethyl-1-propargylxanthine (10 nmol) blocked the depressor and bradycardiac effects of CPCA (5 nmol). Pretreatments with adenylate cyclase inhibitor MDL-12330 (10 nmol) and guanylate cyclase inhibitor LY-83583 (5 nmol) attenuated the depressor and bradycardiac effects of CPCA (5 nmol). In addition, pretreatment with ATP-sensitive K(+) channel blocker glipizide (20 nmol) attenuated the depressor and bradycardiac responses of CPCA (5 nmol). These results suggest that posterior hypothalamic adenosine A(2) receptors play an inhibitory role in the central cardiovascular regulation and that both adenylate cyclase and guanylate cyclase mediate the depressor and bradycardiac actions of adenosine A(2) receptors. Also, ATP-sensitive K(+) channel mediates the posterior hypothalamic cardiovascular regulations of adenosine A(2) receptors.

Adenosine in the spinal cord and periphery: release and regulation of pain

In the central nervous system (CNS), adenosine is an important neuromodulator and regulates neuronal and non-neuronal cellular function (e.g. microglia) by actions on extracellular adenosine A(1), A(2A), A(2B) and A(3) receptors. Extracellular levels of adenosine are regulated by synthesis, metabolism, release and uptake of adenosine. Adenosine also regulates pain transmission in the spinal cord and in the periphery, and a number of agents can alter the extracellular availability of adenosine and subsequently modulate pain transmission, particularly by activation of adenosine A(1) receptors. The use of capsaicin (which activates receptors selectively expressed on C-fibre afferent neurons and produces neurotoxic actions in certain paradigms) allows for an interpretation of C-fibre involvement in such processes. In the spinal cord, adenosine availability/release is enhanced by depolarization (K(+), capsaicin, substance P, N-methyl-D-aspartate (NMDA)), by inhibition of metabolism or uptake (inhibitors of adenosine kinase (AK), adenosine deaminase (AD), equilibrative transporters), and by receptor-operated mechanisms (opioids, 5-hydroxytryptamine (5-HT), noradrenaline (NA)). Some of these agents release adenosine via an equilibrative transporter indicating production of adenosine inside the cell (K(+), morphine), while others release nucleotide which is converted extracellularly to adenosine by ecto-5'-nucleotidase (capsaicin, 5-HT). Release can be capsaicin-sensitive, Ca(2+)-dependent and involve G-proteins, and this suggests that within C-fibres, Ca(2+)-dependent intracellular processes regulate production and release of adenosine. In the periphery, adenosine is released from both neuronal and non-neuronal sources. Neuronal release from capsaicin-sensitive afferents is induced by glutamate and by neurogenic inflammation (capsaicin, low concentration of formalin), while that from sympathetic postganglionic neurons (probably as adenosine 5'-triphosphate (ATP) with NA) occurs following more generalized inflammation. Such release is modified differentially by inhibitors of AK and AD. Following nerve injury, there is an alteration in capsaicin-sensitive adenosine release, as spinal release now is less responsive to opioids, while peripheral release is less responsive to inhibitors of metabolism. Following inflammation, adenosine is released from a variety of cell types in addition to neurons (e.g. endothelial cells, neutrophils, mast cells, fibroblasts). ATP is released both spinally and peripherally following inflammation or injury, and may be converted to adenosine by ecto-5'-nucleotidase contributing an additional source of adenosine. Release of adenosine from both spinal and peripheral compartments has inhibitory effects on pain transmission, as methylxanthine adenosine receptor antagonists reduce analgesia produced by agents which augment extracellular levels of adenosine spinally (morphine, 5-HT, substance P, AK inhibitors) and peripherally (AK inhibitors, AD inhibitors). Increases in extracellular adenosine availability also may contribute to antiinflammatory effects of certain agents (methotrexate, sulfasalazine, salicylates, AK inhibitors), and this could have secondary effects on pain signalling in chronic inflammation. The purpose of the present review is to consider: (a). the factors that regulate the extracellular availability of adenosine in the spinal cord and at peripheral sites; and (b). the extent to which this adenosine affects pain signalling in these two distinct compartments.

The involvement of nitric oxide on the adenosine A(2) receptor-induced cardiovascular inhibitory responses in the posterior hypothalamus of rats

This study was performed to investigate the putative relationship between nitric oxide (NO) and adenosine A(2) receptors on central cardiovascular regulation in the posterior hypothalamus of rats. Posterior hypothalamic injection of drugs was performed in anesthetized, artificially ventilated male Sprague-Dawley rats. Injection of adenosine A(2) receptor agonist 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA; 1, 2 and 5 nmol) produced a dose-dependent decrease of blood pressure and heart rate. Pretreatment with adenosine A(2) receptor antagonist 3,7-dimethyl-1-propargylxanthine (10 nmol) blocked the depressor and bradycardiac effects of CPCA (5 nmol). Pretreatment with soluble guanylate cyclase inhibitor LY-83,583 (5 nmol) attenuated the depressor and bradycardiac effects of CPCA (5 nmol). In addition, pretreatment with NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (40 nmol) attenuated the depressor and bradycardiac responses of CPCA (5 nmol). These results suggest that adenosine A(2) receptor in the posterior hypothalamus plays an inhibitory role in central cardiovascular regulation and that NO participates in the inhibitory response induced by adenosine A(2) receptor stimulation in the posterior hypothalamus.

Modification of cardiovascular responses to adenosine A1 receptor stimulation in the posterior hypothalamus of anaesthetized rats by cAMP and by GABA(B) receptor blockade

1 Injection of N(6)-cyclohexyladenosine (CHA; 1, 5 and 10 nmol), an adenosine A1 receptor agonist, into the posterior hypothalamus of rats produced a dose-dependent decrease in blood pressure (BP) and heart rate (HR). 2 Pretreatment with 8-cyclopentyl-1,3-dimethylxanthine (CPDX; 50 nmol), an adenosine A1 receptor antagonist, blocked the depressor and bradycardic effects of CHA (10 nmol). 3 Pretreatment with 8-bromo-cyclic adenosine monophosphate (AMP) (10 nmol), a cAMP analogue, attenuated the depressor and bradycardic effect of CHA (10 nmol); 8-bromo-cyclic guanosine monophosphate (GMP) (10 nmol), a cGMP analogue, did not modify those effects of CHA. 4 In addition, pretreatment with 5-aminovaleric acid (25 nmol), a gamma-aminobutyric acid (GABA)(B) receptor antagonist, attenuated the depressor and bradycardic effects of CHA (10 nmol). 5 These results suggest that adenosine A1 receptors in the posterior hypothalamus have an inhibitory role in the central cardiovascular regulation and that these vasodepressive and bradycardic actions are modified by raised cAMP and by GABA(B) receptor inhibition.

Adenosine A1 receptors reduce release from excitatory but not inhibitory synaptic inputs onto lateral horn neurons

Although adenosine is an important neuromodulator in the CNS, its role in modulating sympathetic outflow at the level of the spinal cord has not been studied. Because very little is known about adenosine A1 receptors (A1Rs) in the spinal cord, we determined their location and role with particular reference to the control of sympathetic preganglionic activity and interneuronal activity in the rat. High levels of immunoreactivity for A1Rs were observed throughout the spinal cord. Immunostaining was dense in the intermediolateral cell column (IML) and intercalated nucleus, regions containing retrogradely labeled sympathetic preganglionic neurons (SPNs). Electron microscopy revealed A1R immunoreactivity (A1R-IR) within presynaptic terminals and (to a lesser extent) postsynaptic structures in the IML, as well as the luminal membrane of endothelial cells lining capillaries. Using double-labeling techniques, some presynaptic terminals were observed to synapse onto SPNs. To investigate the effects of activating these A1Rs, visualized whole-cell patch-clamp recordings were made from electrophysiologically and morphologically identified SPNs and interneurons. Applications of the A1R agonist cyclopentyladenosine (CPA) reduced the amplitude of EPSPs elicited by stimulation of the lateral funiculus, an effect blocked by the A1R antagonist 8-cyclopentyl-1,3-dipropylxanthine. These effects were attributable to adenosine acting at a presynaptic site because CPA application increased the paired-pulse ratio. CPA did not affect evoked IPSPs. These data show that activating A1Rs reduces fast excitatory, but not inhibitory, transmission onto SPNs and interneurons in the IML and that A1Rs may play a protective role on neurons involved in the control of sympathetic outflow.

Mediation of the cardiovascular response to spinal gamma-aminobutyric acid(B) receptor stimulation by adenosine A(1) receptors in anesthetized rats

Cardiovascular inhibitory effects induced by intrathecal (i.t.) administration of adenosine A(1) receptor agonist and its modulation by gamma-aminobutyric acid(B) (GABA(B)) receptor was suggested by our previous report. In this experiment, we examined the mediation of cardiovascular effects of GABA(B) receptor stimulation by adenosine A(1) and A(2) in the spinal cord. I.t. administration of GABA(B) receptor agonist, baclofen (30, 60 and 100 nmol) produced a dose dependent decrease of blood pressure and heart rate. Pretreatment with adenosine A(1) receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine (50 nmol), attenuated the depressor and bradycardiac effects of baclofen (100 nmol), but not with adenosine A(2) receptor antagonist, 3, 7-dimethyl-1-propargylxanthine (25 nmol). These results suggest that GABA(B) receptors in the spinal cord play an inhibitory role in the central cardiovascular regulation and that the depressor and bradycardiac actions are mediated by adenosine A(1) receptors.

Modification of cardiovascular response of adenosine A2 receptor agonist by adenylate cyclase in the spinal cord of rats

This study was performed to investigate the influence of spinal adenosine A2 receptors on the central regulation of blood pressure (BP) and heart rate (HR), and to define whether its mechanism is mediated by adenylate cyclase or guanylate cyclase. Intrathecal (i.t.) administration of drugs at the thoracic level were performed in anesthetized, artificially ventilated male Sprague-Dawley rats. Injection (i.t.) of adenosine A2 receptor agonist, 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA; 1, 2 and 3 nmol) produced a dose dependent decrease of BP and HR. Pretreatment with adenylate cyclase inhibitor, MDL-12,330, attenuated the depressor and bradycardiac effects of CPCA (2 nmol), but not with guanylate cyclase inhibitor, LY-83,583. These results suggest that adenosine A2 receptor in the spinal cord plays an inhibitory role in the central cardiovascular regulation and that the depressor and bradycardiac actions are mediated by adenylate cyclase.

Modification of cardiovascular responses to spinal GABA(B) receptor stimulation by cAMP and by K(ATP) channel blockade in anaesthetized rats

1. Intrathecal (i.t.) injection of baclofen (30, 60 and 100 nmol), a GABA(B) receptor agonist, produced a dose-dependent decrease in blood pressure (BP) and heart rate (HR). 2. Pretreatment with 5-aminovaleric acid (50 nmol), a GABA(B) receptor antagonist, blocked the depressor and bradycardic effects of baclofen (100 nmol). 3. Pretreatment with 8-bromo-cAMP (10 nmol), a cAMP analogue, attenuated the depressor and bradycardic effects of baclofen (100 nmol), but not with 8-bromo-cGMP (10 nmol), a cGMP analogue. 4. In addition, pretreatment with glipizide (20 nmol), an ATP-sensitive K+ channel (K(ATP)) blocker, attenuated the depressor and bradycardic effects of baclofen (100 nmol). These results suggest that GABA(B) receptors in the spinal cord have an inhibitory role in the central cardiovascular regulation and that these depressive and bradycardic actions are modified by cAMP and by K(ATP) channel blockade.

Mediation of the cardiovascular response of adenosine A1 receptor through a GABA(B) receptor in the spinal cord of the rat

Cardiovascular inhibitory effects induced by intrathecal (i.t.) administration of adenosine A1 receptor agonist and its modulation by cyclic AMP was suggested by our previous report. In this experiment, we examined the mediation of cardiovascular effects of adenosine A1 receptor by gamma-aminobutyric acid receptors A and B [GABA(A) and GABA(B)] in the spinal cord. I.t. administration of 10 nmol of N6-cyclohexyladenosine (CHA), an adenosine A1 receptor agonist, and pretreatment with bicuculline (10 nmol, i.t), a GABA(A) receptor antagonist, and 5-aminovaleric acid (50 nmol, i.t.), a GABA(B) receptor antagonist, prior to injection of CHA were performed in anesthetized, artificially ventilated Sprague-Dawley rats. I.t. injection of 50 nmol of 5-aminovaleric acid significantly attenuated the inhibitory cardiovascular effects of CHA but 10 nmol of bicuculline did not alter CHA-induced cardiovascular actions. It is suggested that cardiovascular responses of adenosine A1 receptor is mediated by GABA(B) receptor in the spinal cord.