Selective Adenosine A1 Agonists Research Articles

Topical Trabodenoson Is Neuroprotective in a Rodent Model of Anterior Ischemic Optic Neuropathy (rNAION).

PurposeNonarteritic anterior ischemic optic neuropathy (NAION) is the leading cause of sudden optic nerve–related vision loss currently without effective treatment. We evaluated the neuroprotective potential of ocular (topical) delivery of trabodenoson, a selective A1 receptor mimetic, in a rodent model of NAION (rNAION).MethodsDaily topical delivery of 3% trabodenoson or vehicle administered in both eyes 3 days prior to rNAION induction and for 21 days post induction. Retinal appearance and optic nerve head (ONH) edema was evaluated using spectral-domain optical coherence tomography (SD-OCT). Retinal function was evaluated before and after induction by ganzfeld electroretinography (ERG). Brn3a(+) retinal ganglion cells (RGCs) were quantified by stereology. Axonal ultrastructure was evaluated by electron microscopy.ResultsTrabodenoson-treated eyes had significantly reduced optic nerve (ON) edema compared with vehicle-treated eyes (ANOVA, P < 0.05). Electrophysiologically, there was a nonsignificant trend toward b-wave and oscillatory potential (OP) preservation in the trabodenoson-treated eyes. RGC counts were higher in trabodenoson-treated eyes compared to vehicle (74% versus 47% of the contralateral eye; two-tailed t-test; P = 0.01), as were ON axons. No overt morphologic differences in cell inflammation were observed between vehicle- and trabodenoson-treated ONHs, but trabodenoson-treated ONHs revealed increased expression of astrocyte-related neuroprotective responses.ConclusionsTrabodenoson preserves RGCs in the rodent NAION model. While previous clinical trials focused on trabodenoson's ocular antihypertensive effect, our data suggest trabodenoson's primary target may be both the retina and ONH. Selective adenosine A1 agonists may prove an appropriate neuroprotective adjunctive for ischemia-related ON diseases such as NAION and glaucoma.Translational RelevanceRGC and ON neuroprotection in ischemic neuropathies may be achievable by topical administration of A1 adenosine agonists rather than by simply relying on intraocular pressure reduction.

Caffeine ameliorates hemodynamic derangements and portosystemic collaterals in cirrhotic rats.

Portal hypertension (PH), a pathophysiological derangement of liver cirrhosis, is characterized by hyperdynamic circulation, angiogenesis, and portosystemic collaterals. These may lead to lethal complications, such as variceal bleeding. Caffeine has been noted for its effects on liver inflammation, fibrogenesis, and vasoreactiveness. However, the relevant influences of caffeine in cirrhosis and PH have not been addressed. Spraque-Dawley rats with common bile duct ligation-induced cirrhosis or sham operation received prophylactic or therapeutic caffeine treatment (50 mg/kg/day, the first or 15th day since operation, respectively) for 28 days. Compared to vehicle (distilled water), caffeine decreased cardiac index, increased systemic vascular resistance, reduced portal pressure (PP), superior mesenteric artery flow, mesenteric vascular density, portosystemic shunting (PSS), intrahepatic angiogenesis, and fibrosis without affecting liver and renal biochemistry. The beneficial effects were reversed by selective adenosine A1 agonist N6-cyclopentyladenosine (CPA) or A2A agonist GCS21680. Both prophylactic and therapeutic caffeine treatment decreased portal resistance and PP in thioacetamide (200mg/kg, thrice-weekly for 8 weeks)-induced cirrhotic rats. Caffeine down-regulated endothelial nitric oxide synthase, vascular endothelial growth factor (VEGF), phospho-VEGFR2, and phospho-Akt mesenteric protein expression. Caffeine adversely affected viability of hepatic stellate and sinusoidal endothelial cells, which was reversed by CPA and GCS21680. On the other hand, caffeine did not modify vascular response to vasoconstrictors in splanchnic, hepatic, and collateral vascular beds. Caffeine decreased PP, ameliorated hyperdynamic circulation, PSS, mesenteric angiogenesis, hepatic angiogenesis, and fibrosis in cirrhotic rats. Caffeine may be a feasible candidate to ameliorate PH-related complications in cirrhosis.

α1‐RECEPTOR OR ADENOSINE A1‐RECEPTOR DEPENDENT PATHWAY ALONE IS NOT SUFFICIENT BUT SUMMATION OF THESE PATHWAYS IS REQUIRED TO ACHIEVE AN ISCHAEMIC PRECONDITIONING EFFECT IN RABBITS

1. It is reported that alpha1-receptors and adenosine A1-receptors are involved in the ischaemic preconditioning (PC) effect on infarct size (IS). However, it is still unclear to what extent alpha1-receptors and adenosine A1-receptors contribute to the mechanism of PC. Therefore, we investigated the extent of the contribution of alpha1-receptors and adenosine A1 receptors to the PC effect on IS and examined the relationship between these receptors and protein kinase C. 2. Infarct size was measured in rabbits subjected to 30 min ischaemia and 48 h reperfusion. Tyramine (Tyr) was intravenously administered before 30 min ischaemia in the absence or presence of bunazosin (BN, alpha1-receptor blocker) and staurosporine (ST), a protein kinase C inhibitor, respectively. R(-)N6-(2-phenylisapropyl)-adenosine (PIA), a selective adenosine A1 agonist, was intravenously administered before 30 min ischaemia in the absence or presence of 8-p-sulphophenyltheophylline (8SPT), an adenosine blocker and ST, respectively. In the PC groups, BN, BN + PIA, 8SPT, 8SPT + Tyr or placebo saline was injected before or during PC. 3. Both Tyr and PIA reduced the IS, which was blocked by BN and 8SPT, respectively. The IS-reducing effect of Tyr or PIA was blocked by ST. The IS-reducing effect of PC was completely blocked by BN and 8SPT, respectively. The blocking effect of BN on the IS-reducing effect of PC was abolished by adding PIA during PC ischaemia. The blocking effect of 8SPT on the IS-reducing effect of PC was abolished by adding Tyr before PC ischaemia. 4. These data suggest that an alpha1-receptor dependent pathway exists and an adenosine A1-receptor dependent pathway, stimulation of both of which activates protein kinase C, then reduces the IS. However, exclusive stimulation of a single alpha1-receptor dependent pathway or a single adenosine A1-receptor dependent pathway alone is not sufficient but the summation of these pathways is required to achieve a PC effect on IS in rabbits.

Potent, Selective Adenosine A1 Agonists as Potential Analgesics

Potent, Selective Adenosine A1 Agonists as Potential Analgesics

Adenosine

Adenosine has been recognized as a potentially important signaling molecule in the heart for nearly a half century. The original adenosine hypothesis proposed that production of adenosine by cardiac myocytes reflects the metabolic state of the myocardium and serves to regulate vasomotor tone in the coronary resistance vessels, thereby coupling blood flow to the energetic needs of the heart.1 There are two major pathways for adenosine production in the cardiac myocyte. The transmethylation pathway involves the hydrolysis of S -adenosylhomocysteine (SAH) by SAH hydrolase to l-homocysteine and adenosine. A second pathway, the hydrolysis of AMP by 5′-nucleotidase, predominates during ischemic or hypoxic conditions.2 According to the adenosine hypothesis, when the energy requirements of the heart increase, the increased rate of ATP hydrolysis would cause cytosolic free ADP levels to rise. In this situation, the myokinase reaction can utilize two molecules of ADP to produce one molecule each of ATP and AMP, the later being the substrate for 5′-nucleotidase to produce adenosine, which can enter the interstitial space to cause coronary vasodilation. Although this hypothesis for preserving the oxygen supply/demand relationship is attractive, it does not appear to operate during physiological conditions in the normal heart, since adenosine receptor inhibition does not decrease coronary blood flow or impair the increase of coronary flow during exercise.3 Implicit in the adenosine hypothesis is the assumption that during increased cardiac work, increases of cytosolic free [ADP] would be required to drive mitochondrial respiration and would serve to increase adenosine production. In fact, over a wide range of workloads, cytosolic free [ADP] remains at a stable low level; only at extreme workloads does ADP rise significantly in the normal heart.4 Although it has been difficult to demonstrate adenosine effects during normal physiological conditions, adenosine is clearly able to exert important effects …

Electrophysiologic effects of a novel selective adenosine A1 agonist (CVT-510) on atrioventricular nodal conduction in humans.

CVT-510, N-(3(R)-tetrahydrofuranyl)-6-aminopurine riboside, is a selective A(1)-adenosine receptor agonist with potential potent antiarrhythmic effects in tachycardias involving the atrioventricular (AV) node. This study, the first in humans, was designed to determine the effects of CVT-510 on AV nodal conduction and hemodynamics. Patients in sinus rhythm with normal AV nodal function at electrophysiologic study (n = 32) received a single intravenous bolus of CVT-510. AH and HV intervals were measured during sinus rhythm and during atrial pacing at 1, 5, 10, 15, 20, 30, 45, and 60 minutes after the bolus. Increasing doses of CVT-510 (0.3 to 10 microg/kg) caused a dose-dependent increase in the AH interval. At 1 minute, a dose of 10 microg/kg increased the AH interval during sinus rhythm from 93 +/- 23 msec to 114 +/- 37 msec, p = 0.01 and from 114 +/- 31 msec to 146 +/- 44 msec during atrial pacing at 600 msec, p = 0.003). The AH interval returned to baseline by 20 minutes. CVT-510 at doses of 0.3 to 10 microg/kg had no effect on sinus rate, HV interval, or systemic blood pressure, and was not associated with serious adverse effects. At doses of 15 and 30 microg/kg, CVT-510 produced transient second/third degree AV heart block in all four patients treated. One of these patients also had a prolonged sedative effect that was reversed with aminophylline. CVT-510 promptly prolongs AV nodal conduction and does not affect sinus rate or blood pressure. Selective stimulation of the A(1)-adenosine receptor by CVT-510 may be useful for immediate control of heart rate in atrial fibrillation/flutter and to convert paroxysmal supraventricular tachycardia to sinus rhythm, while avoiding vasodilatation mediated by the A(2)-adenosine receptor, as well as the vasodepressor and negative inotropic effects associated with beta-adrenergic receptor blockade and/or calcium channel blockers.

Novel anticoagulants based on inhibition of the factor VIIa/tissue factor pathway

Prepulse inhibition (PPI) of the acoustic startle response is a behavioural tool used to assess sensorimotor gating processes and its disturbances in rats and in humans. PPI in rats is reduced by an overactivity of the dopamine (DA) system. Because there are functional interactions between DA and adenosine receptors, we tested whether PPI can be influenced by the mixed DA receptor agonist apomorphine (APO) and by the unselective adenosine antagonist theophylline (THEO). Combined administration of APO (0.5 mg/kg, i.p.) and THEO (20 mg/kg, i.p.) in doses devoid of significant effects on their own significantly reduced PPI. The PPI-disrupting effect of the combined THEO plus APO treatment was dose-dependently antagonized by co-administration of the selective adenosine A1 agonist CPA (0.15–1.5 mg/kg, i.p.), but not by the A2A agonist CGS21680 (0.1–2 mg/kg, i.p.). These data demonstrate that antagonistic interactions between DA and adenosine, involving adenosine A1 receptors, play an important role in the regulation of PPI. The possible implications of these findings for the use of adenosine agonists in the treatment of schizophrenia are discussed.

Antiarrhythmic effects of BN‐063, a newly synthesized adenosine A1 agonist, on myocardial ischaemia in rats

1. It has been shown that adenosine is able to reduce the severity of arrhythmias induced by myocardial ischaemia. In isolated preparations, the antiarrhythmic effect of adenosine on ventricular myocardium is known to antagonize the catecholamine-induced stimulation of intracellular cyclic AMP production, an effect mediated via adenosine A1 receptors. 2. The aim of this study was to evaluate the antiarrhythmic effect of BN-063 (1-cyclopropylisoguanosine), a newly synthesized selective adenosine A1 agonist, on ventricular arrhythmias in rats. 3. Arrhythmias were induced by left coronary artery ligation or by administration of isoprenaline (7 mg kg-1) subcutaneously. Pretreatment with BN-063 (0.25, 0.5 and 1.0 mg kg-1) 10 min prior to occlusion significantly delayed the onset of ventricular arrhythmias, reduced the total number of ventricular premature contraction (VPC) and ventricular tachycardia (VT), decreased the incidence of VT and ventricular fibrillation (VF) and mortality during the first 30 min following left coronary artery ligation. In contrast, pretreatment with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an adenosine A1 antagonist, was arrhythmogenic during the ischaemic period. The rate-pressure product, an index for indirect measurement of myocardial oxygen consumption, was also significantly reduced by BN-063 during ligation time. 4. The incidence of VT, VF and mortality was also significantly reduced when BN-063 was administered after left coronary artery ligation. 5. BN-063 converted the VF induced by isoprenaline to normal sinus rhythm and improved the survival rate. 6. It is concluded that, through activation of adenosine A1 receptors, BN-063 can suppress ventricular arrhythmias induced by myocardial ischaemia and catecholamines. The antiarrhythmic actions of BN-063 may be mediated by reducing heart rate and antagonizing the stimulatory effects of catecholamine in myocardial ischaemia.

Modulation of vasodilatation to levcromakalim by adenosine analogues in the rabbit ear: an explanation for hypoxic augmentation

1. We have used a rabbit isolated ear, buffered-perfused preparation to investigate the effects of adenosine analogues on the vasodilatation to the potassium channel opener, levcromakalim (the active (-)-enantiomer of cromakalim). We have examined the effects of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective adenosine A1 antagonist, on vasodilatation to levcromakalim under hypoxic conditions and also following inhibition of nitric oxide synthesis. 2. Levcromakalim relaxed preconstricted preparations with an EC50 = 369 +/- 48 nM and maximum relaxation of tone (Rmax) = 81.0 +/- 3.2%. In the presence of 1 microM N6-cyclohexyladenosine (CHA) a selective adenosine A1 agonist, there was a significant (P < 0.01) leftward shift in the concentration-response curve with an EC50 = 194 +/- 54 nM and Rmax = 93.2 +/- 2.0%. Conversely, the presence of CHA did not influence vasodilatation to either pinacidil or sodium nitroprusside. 3. Hypoxia also significantly (P < 0.001) increased the vasodilator potency of levcromakalim (EC50 = 134 +/- 22 nM), and this enhancement was completely reversed (EC50 = 380 +/- 107 nM, P < 0.01) by pretreatment of the preparations with 5 microM DPCPX, a selective A1 adenosine antagonist. However, under normoxic conditions DPCPX did not influence vasodilatation to levcromakalim. 4. Inhibition of nitric oxide synthesis with 100 microM NG-nitro-L-arginine methyl ester (L-NAME) caused a significant (P < 0.001) leftward shift in the concentration-response curve to levcromakalim (EC50 = 73.0 +/- 7.6 nM). Pretreatment of preparations with DPCPX partially reversed the increase in potency found in the absence of nitric oxide synthesis (EC50 = 153 +/- 18 nM, P < 0.001). 5. We have shown that an adenosine Al agonist may increase the potency of levcromakalim indicating that adenosine receptor activation may augment the vasodilator activity of levcromakalim. That responses to levcromakalim but not those to pinacidil were affected by CHA points to further differences in the pharmacology of these potassium channel openers. The reversal by the adenosine Al antagonist of the hypoxic-potentiation of vasodilatation to levcromakalim, and also augmentation following inhibition of nitric oxide synthesis, suggests that under these conditions there is an endogenous release of adenosine which may enhance responses to levcromakalim. The findings of this study suggest that levcromakalim may selectively dilate vessels where there is elevated adenosine release.