Effects Of Adenosine Receptor Agonists Research Articles

Species-dependent effects of adenosine receptor agonists on contractile responses of vas deferens to ATP.

1. Experiments were carried out to examine the postjunctional actions of adenosine receptor agonists on the smooth muscle of the vas deferens of the guinea-pig and rabbit. 2. Although they produced neither contraction nor relaxation by themselves, adenosine analogues enhanced contractions of the guinea-pig vas deferens induced by 10 microm ATP. The rank order of potency was N6-cyclopentyladenosine (CPA) > 5'-N-ethylcarboxamidoadenosine (NECA) > adenosine > CGS 21680. Dose-response curves for NECA were shifted to the right by the nonselective adenosine receptor antagonist 8(p-sulphophenyl)theophylline (8-SPT; 100 microM) and by the selective A1-receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 1 mM). 3. In the rabbit vas deferens, contractions induced by ATP (1 mM) were inhibited rather than facilitated by NECA. Neither CPA, R(-)-N6-(2-phenyl isopropyl)-adenosine (R-PIA) nor CGS 21680 had any effect. 4. The results indicate that the smooth muscle of the guinea-pig vas deferens expresses facilitatory adenosine A1 receptors but not adenosine A2 receptors. In contrast, in rabbit there are postjunctional inhibitory adenosine A2A receptors but not adenosine A1 receptors.

Effects of adenosine receptor agonists and antagonists on audiogenic seizure-sensible DBA/2 mice

We have studied the effects of selective and non-selective adenosine receptor agonists and antagonists in audiogenic-seizure-sensitive DBA/2 mice, an animal model of generalized reflex epilepsy. With the exception of the adenosine A 3 receptor agonist, N 6-(3-iodobenzyl)-5′- N-methylcarboxamidoadenosine (IB-MECA), all the agonists studied prevented the development of audiogenic seizures in a dose-dependent manner. The ED 50 values against the clonic phase of the audiogenic seizures were low, that is: 0.06 mg/kg, i.p., for the adenosine A 1 receptor agonist, 2-chloro- N 6-cyclopentyladenosine (CCPA), 0.02 and 0.03 mg/kg, i.p., for the adenosine A 2A receptor agonists, 2-(4-(2-carboxyethyl)-phenylamino)-5′- N-ethylcarboxamidoadenosine (CGS 21680) and 2-hexynyl-5′- N-ethyl-carboxamidoadenosine (2-HE-NECA), and 0.7 mg/kg, i.p., for the adenosine A 1/A 3 receptor agonist, N 6-2-(4-aminophenyl)ethyladenosine (APNEA). Conversely, the non-selective agonist, N-ethyl-carboxamidoadenosine (NECA), was highly potent, the ED 50 being 0.0005 mg/kg, i.p. In the absence of auditory stimulation, the adenosine receptor antagonists increased the incidence of both clonic and tonic seizures in DBA/2 mice. The ED 50 values were: for caffeine, 207.5 mg/kg, i.p., for the adenosine A 1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), 327.8 mg/kg i.p., for the adenosine A 2A receptor antagonists, 3,7-dimethyl-1-propylxanthine (DPMX), 86.7 mg/kg i.p., for the ( E,18%– Z,82%)7-methyl-8-(3,4-dimethoxystyryl)-1,3-dipropylxanthine (KF 17837), 69.1 mg/kg i.p., and 5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo-(4,3- c)1,2,4-triazolo(1,5- c)-pyrimidine (SCH 58261), 321.8 mg/kg i.p. The rank order of convulsant potency in our epileptic model, following intracerebroventricular administration, was DPCPX>DMPX>1,3,7-trimethyl-8-(3-chlorostyryl)xanthine (CSC)>KF 17837>Caffeine>SCH 58261>5-amino-9-chloro-2-(2-furyl)-1,2,4-triazolo(1,5- c)quinazoline (CGS 15943). Following a subconvulsant audiogenic stimulus of 83 dB, all adenosine receptor antagonists induced both tonic and clonic seizures. The ED 50 values for such proconvulsant effects were: for caffeine 0.04 mg/kg, i.p., for the adenosine A 1 receptor antagonist, DPCPX, 5.84 mg/kg, i.p., for the adenosine A 2A receptor antagonists, DMPX, 0.02 mg/kg, i.p., CGS 15943, 0.29 mg/kg i.p., KF 17837, 0.57 mg/kg, i.p., CSC 0.12 mg/kg, i.p. and SCH 58261 0.07 mg/kg, i.p., respectively. These data suggest that stimulation of adenosine A 1 and A 2A receptors is involved in the suppression of seizures.

Age-Related Changes in Adenosine in Rat Coronary Resistance Vessels

1. The vasodilator effects of adenosine receptor agonists, isoprenaline and histamine were examined in perfused heart preparations from young (4–6 weeks) and mature (12–20 weeks) rats. 2. Adenosine induced a biphasic concentration-dependent decrease in KCl (35 mM) raised coronary perfusion pressure in hearts from young and mature rats, suggesting the presence of both high- and low-affinity sites for adenosine receptors in the two age groups tested. In heart preparations from mature rats, vasodilator responses to adenosine were significantly reduced compared with responses observed in young rats. 3. Responses to 5′- N-ethylcarboxamidoadenosine (NECA) and 2- p-(2-carboxyethyl)phenethylamino-5′- N-ethylcarboxamidoadenosine hydrochloride (CGS-21680) were reduced in preparations from mature rats, whereas the vasodilator actions of N 6-cyclopentyladenosine (CPA) and N 6-2-(4-aminophenyl)ethyladenosine (APNEA) did not change with age. 4. The results presented in this study suggest that several adenosine receptor subtypes mediate vasodilator responses in the coronary circulation of the rat and that a reduction in response to adenosine with age may be due to changes in the high-affinity receptor site.

Characterization of adenosine receptors evoking excitation of mesenteric afferents in the rat

We examined the effects of adenosine receptor agonists and antagonists on the discharge of mesenteric afferent nerves supplying the jejunum in pentobarbitone sodium-anaesthetized rats. Adenosine (0.03-10 mg kg(-1), i.v.), NECA (0.3-300 microg kg(-1), i.v.) and the A1 receptor agonist, GR79236 (0.3-1000 microg kg(-1), i.v.), each induced dose-dependent increases in afferent nerve activity and intrajejunal pressure, hypotension and bradycardia. The A1 receptor antagonist, DPCPX (3 mg kg(-1), i.v.), antagonized all the effects of GR79236 but only the haemodynamic effects of adenosine and NECA. The A2A receptor antagonist, ZM241385 (3 mg kg(-1), i.v.), antagonized the hypotensive effect of NECA but none of the effects of GR79236. The A2A receptor agonist, CGS21680 (0.3-300 microg kg(-1), i.v.), and the A3 receptor agonist, IB-MECA (0.3-300 microg kg(-1), i.v.), each induced only a dose-dependent hypotension. Subsequent administration of adenosine (3 mg kg(-1), i.v.) induced increases in afferent nerve activity and intrajejunal pressure and bradycardia. ZM241385 (3 mg kg(-1), i.v.) antagonized the hypotensive effect of CGS21680 but not the effects of adenosine. Bethanechol (300 microg kg(-1), i.v.) evoked increases in afferent nerve activity and intrajejunal pressure, hypotension and bradycardia. However, adenosine (3 mg kg(-1), i.v.) evoked greater increases in afferent nerve activity than bethanechol despite inducing smaller increases in intrajejunal pressure. In summary, A1 and A2B and/or A2B-like receptors evoke adenosine-induced increases in mesenteric afferent nerve activity and intrajejunal pressure in the anaesthetized rat. Furthermore, elevations in intrajejunal pressure do not wholly account for adenosine-evoked excitation of mesenteric afferent nerves.

A1 and A2 adenosine receptor modulation of contractility in the cauda epididymis of the guinea-pig.

1. The effects of adenosine receptor agonists upon phenylephrine-stimulated contractility and [3H]-cyclic adenosine monophosphate ([3H]-cyclic AMP) accumulation in the cauda epididymis of the guinea-pig were investigated. The alpha1-adrenoceptor agonist, phenylephrine elicited concentration dependent contractile responses from preparations of epididymis. In the absence or presence of the L-type Ca2+ channel blocker, nifedipine (10 microM) the non-selective adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA, 1 microM) shifted phenylephrine concentration-response curves to the left (4 and 5 fold respectively). Following the incubation of preparations with pertussis toxin (200 ng ml(-1) 24 h) NECA shifted phenylephrine concentration-response curves to the right (5.7+/-0.9 fold). 2. In the presence of phenylephrine (1 microM), NECA and the A1 adenosine receptor selective agonists, N6-cyclopentyladenosine (CPA) and (2S)-N6-[2-endo-norbornyl]adenosine ((S)-ENBA) elicited concentration-responses dependent contractions from preparations of epididymis (pEC50 values 8.18+/-0.19, 7.79+/-0.29 and 8.15+/-0.43 respectively). The A3 adenosine receptor agonists N6-iodobenzyl-5'-N-methylcarboxamido adenosine (IBMECA) and N6-2-(4-aminophenyl) ethyladenosine (APNEA) mimicked this effect (but only at concentrations greater than 10 microM). In the presence of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 30 nM) CPA concentration-response curves were shifted, in parallel to the right (apparent pKB 8.75+/-0.88) and the maximal response to NECA was reduced. 3. In the presence of DPCPX (100 nM) the adenosine agonist NECA and the A2A adenosine receptor selective agonist, CGS 21680 (2-p-(2-carboxyethyl)-phenethylamino-N-ethylcarboxamido adenosine), but not CPA, inhibited phenylephrine (20 microM) stimulated contractions (pIC50 7.15+/-0.48). This effect of NECA was blocked by xanthine amine congener (XAC, 1 microM) and the A2A adenosine receptor-selective antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-++ +ylamino]ethyl)phenol (ZM 241385; 30 nM). 4. (S)-ENBA (in the absence and presence of ZM 241385, 100 nM), but not NECA or CPA inhibited the forskolin (30 microM)-stimulated accumulation of [3H]-cyclic AMP in preparations of the epididymis of the guinea-pig (by 17+6% of control). In the presence of DPCPX (100 nM) NECA and CGS 21680, but not (S)-ENBA, increased the accumulation of [3H]-cyclic AMP in preparations of epididymis (pEC50 values 5.35+/-0.35 and 6.42+/-0.40 respectively), the NECA-induced elevation of [3H]-cyclic AMP was antagonised by XAC (apparent pKB 6.88+/-0.88) and also by the A2A adenosine receptor antagonist, ZM 241385 (apparent pKB 8.60+/-0.76). 5. These studies are consistent with the action of stable adenosine analogues at post-junctional A1 and A2 adenosine receptors in the epididymis of the guinea-pig. A1 Adenosine receptors potentiate alpha1-adrenoceptor contractility, an effect blocked by pertussis toxin, but which may not be dependent upon an inhibition of adenylyl cyclase. The epididymis of the guinea-pig also contains A2 adenosine receptors, possibly of the A2A subtype, which both inhibit contractility and also stimulate adenylyl cyclase.

Inhibition of voltage-sensitive calcium channels by the A2A adenosine receptor in PC12 cells.

The role of the A2A adenosine receptor in regulating voltage-sensitive calcium channels (VSCCs) was investigated in PC12 cells. Ca2+ influx induced by membrane depolarization with 70 mM K+ could be inhibited with CGS21680, an A2A receptor-specific agonist. Both L- and N-type VSCCs were inhibited by CGS21680 treatment. Effects of adenosine receptor agonists and antagonists indicate that the typical A2A receptor mediates inhibition of VSCCs. Cholera toxin (CTX) treatment for 24 h completely eliminated the CGS21680 potency. Similar inhibitory effects on VSCCs were obtained by membrane-permeable activators of protein kinase A (PKA). These effects were blocked by Rp-adenosine-3',5'-cyclic monophosphothioate, a PKA inhibitor. The data suggest that activation of the A2A receptor leads to inhibition of VSCCs via a CTX-sensitive G protein and PKA. ATP pretreatment caused a reduction in subsequent rise in cytosolic free Ca2+ concentration induced by 70 mM K+, presumably by inactivation of VSCCs. Simultaneous treatment with ATP and CGS21680 produced significantly greater inhibition of VSCCs than treatment with CGS21680 or ATP alone. Furthermore, the CGS21680-induced inhibition of VSCCs was not affected by the presence of reactive blue 2. CGS21680 still significantly inhibited ATP-evoked Ca2+ influx without VSCC activity after cobalt or 70 mM K+ pretreatment. These data suggest that the A2A receptor-sensitive VSCCs differ from those activated by ATP treatment. Although A2A receptors induce inhibition of VSCCs as well as ATP-induced Ca2+ influx, the two inhibitory effects are clearly distinct from each other.

Agonists of A 1 and A 2A adenosine receptors attenuate methamphetamine-induced overflow of dopamine in rat striatum

The effect of adenosine receptor agonists on the release of striatal dopamine (DA), induced by repeated doses of methamphetamine (MTH), was evaluated. Rats received three injections of MTH (5 mg/kg i.p.) at 2-h intervals. The release of DA in the striatum was measured by a microdialysis in freely moving animals. The agonist of adenosine A 1 receptor, N 6-cyclopentyladenosine (CPA) and the agonist of adenosine A 2A receptor, 2-[ p-(carboxy-ethyl)phenylethylamino]-5′- N-ethylcarboxyamidoadenosine (CGS 21680), either of them being infused locally into the striatum at concentrations of 50 and 100 μM, produced decreases in the extracellular DA level during exposure to MTH, and a weaker effect on the levels of DOPAC and HVA. The above effects were reversed by the specific antagonists of adenosine A 1 and A 2A receptors, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 3,7-dimethyl-1-propargylxanthine (DMPX), respectively. Our results indicate that both the A 1 and A 2A adenosine receptors appear to be involved in reducing the excessive release of DA in the striatum; furthermore, they suggest a neuroprotective role of adenosine in MTH neurotoxicity.

A1 adenosine receptor modulation of electrically-evoked contractions in the bisected vas deferens and cauda epididymis of the guinea-pig.

1. The effects of adenosine receptor agonists upon both electrically-evoked and phenylephrine-induced contractile responses were investigated in the bisected vas deferens and the cauda epididymis of the guinea-pig. Electrical field-stimulation (10 s trains of pulses at 9 Hz, 0.1 ms duration, supramaximal voltage) elicited biphasic and monophasic contractile responses from preparations of bisected vas deferens and cauda epididymis, respectively; these responses were abolished by tetrodotoxin (300 nM). 2. In the prostatic half of the vas deferens the A1 selective adenosine receptor agonists, N6-cyclopentyladenosine (CPA) and (2S)-N6-[2-endo-norbornyl]adenosine ((S)-ENBA) and the non-selective A1/A2 adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA) inhibited electrically-evoked contractions (pIC50+/-s.e.mean values 6.15+/-0.24, 5.99+/-0.26 and 5.51+/-0.24, respectively). The responses to CPA were blocked by the A1 adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine, DPCPX (100 nM). 3. In the epididymal half of the vas deferens NECA potentiated (at < or = 100 nM) and inhibited (at > or = 1 microM) electrically-evoked contractions. In the presence of the non-selective alpha-adrenoceptor antagonist phentolamine (3 microM), the alpha1-adrenoceptor antagonist, prazosin (100 nM), or at a reduced train length (3 s) NECA inhibited electrically-evoked contractions (pIC50 values 6.05+/-0.25, 5.97+/-0.29 and 5.71 +/-0.27, respectively). CPA (at 10 microM) also inhibited electrically-evoked contractions in this half of the vas deferens. In the presence of prazosin (100 nM), CPA also inhibited electrically-evoked contractions (pIC50 6.14+/-0.67); this effect was antagonized by DPCPX (30 nM, apparent pK(B) 8.26+/-0.88). In the presence of the P2 purinoceptor antagonist, suramin (300 microM), CPA (up to 1 microM) potentiated electrically-evoked contractions. 4. NECA, CPA and APNEA potentiated electrically-evoked contractions in preparations of cauda epididymis (pEC50 values 7.49+/-0.62, 7.65+/-0.74 and 5.84+/-0.86, respectively), the response to CPA was competitively antagonized by DPCPX (100 nM) with an apparent pK(B) value of 7.64+/-0.64. 5. The alpha1-adrenoceptor agonist phenylephrine elicited concentration-dependent contractile responses from preparations of bisected vas deferens and cauda epididymis. NECA (1 microM) potentiated responses to phenylephrine (< or = 1 microM) in the epididymal, but not in the prostatic half of the vas deferens. In preparations of epididymis NECA (1 microM) shifted phenylephrine concentration response curves to the left (4.6 fold). In the presence of a fixed concentration of phenylephrine (1 microM), NECA elicited concentration-dependent contractions of preparations of the epididymal half of the vas deferens and of the epididymis (pEC50 values 7.57+/-0.54 and 8.08+/-0.18, respectively). NECA did not potentiate responses to ATP in either the epididymal half of the vas deferens or the epididymis. 6. These studies are consistent with the action of stable adenosine analogues at prejunctional A1 and postjunctional A1-like adenosine receptors. The prejunctional A1 adenosine receptors only inhibit the electrically-evoked contractions of purinergic origin (an effect predominant in the prostatic half of the vas deferens). At the epididymis, where electrically-evoked contractions are entirely adrenergic, the predominant adenosine receptor agonist effect is a potentiation of alpha1-adrenoceptor-, but not of ATP-induced contractility.

Effect of adenosine receptor agonists on release of the nucleoside analogue [ 3H]formycin B from cultured smooth muscle DDT 1 MF-2 cells

Adenosine has receptor-mediated effects in a variety of cell types and is predominantly formed from ATP by a series of nucleotidase reactions. Adenosine formed intracellularly can be released by bidirectional nucleoside transport processes to activate cell surface receptors. We examined whether stimulation of adenosine receptors has a regulatory effect on transporter-mediated nucleoside release. DDT 1 MF-2 smooth muscle cells, which possess nitrobenzylthioinosine-sensitive (ES) transporters as well as both adenosine A 1 and A 2 receptors, were loaded with the metabolically stable nucleoside analogue [ 3H]formycin B. N 6-cyclohexyladenosine (CHA), a selective adenosine A 1 receptor agonist, produced a concentration-dependent inhibition of [ 3H]formycin B release with an IC 50 value of 2.7 μM. Further investigation revealed CHA interacts directly with nucleoside transporters with a K i value of 3.3 μM. Neither 5′- N-ethylcarboxamidoadenosine (NECA), a mixed adenosine A 1 and A 2 receptor agonist, nor CGS 21680, a selective adenosine A 2A receptor agonist, affected nucleoside release. We conclude that release of the nucleoside formycin B from DDT 1 MF-2 cells is not regulated by adenosine A 1 or A 2 receptor activation.

Effect of adenosine receptor agonists and antagonists on morphine-induced catalepsy in mice

Effects of adenosine receptor agonists and antagonists on morphine-induced catalepsy in mice were investigated. The adenosine agonists, NECA (5′- N-ethylcarboxamidoadenosine) and S-PIA (S(+)- N 6-(2-phenylisopropyl)adenosine) in doses which did not induce any response, increased the cataleptogenic effect produced by morphine. However, the morphine response was decreased and increased by the lower and higher doses of the adenosine receptor agonist, CHA ( N 6-cyclohexyladenosine), respectively. The adenosine receptor antagonist, theophylline, decreased, but 8-phenyltheophylline increased, the response induced by morphine. Naloxone inhibited the catalepsy induced by morphine or morphine+NECA but not that induced by NECA alone. It is concluded that adenosine A 2 receptor activation increases, while adenosine A 1 receptor stimulation decreases, the morphine cataleptogenic response. The response to morphine may be mediated through opioid and adenosine receptor mechanisms.

Adenosine–dopamine receptor–receptor interactions as an integrative mechanism in the basal ganglia

Increasing evidence suggests that antagonistic interactions between specific subtypes of adenosine and dopamine receptors in the basal ganglia are involved in the motor depressant effects of adenosine receptor agonists and the motor stimulant effects of adenosine receptor antagonists, such as caffeine. The GABAergic striatopallidal neurons are regulated by interacting adenosine A 2A and dopamine D 2 receptors. On the other hand, the GABAergic striatonigral and striatoentopeduncular neurons seem to be regulated by interacting adenosine A 1 and dopamine D 1 receptors. Furthermore, behavioural studies have revealed interactions between adenosine A 2A and dopamine D 1 receptors that occur at the network level. These adenosine–dopamine receptor–receptor interactions might offer new therapeutic leads for basal ganglia disorders.

Adenosine receptor-mediated relaxation of rabbit airway smooth muscle: a role for nitric oxide.

In this study, we investigated the relaxant effect of adenosine receptor agonists on KCl-precontracted airway smooth muscle from rabbits and characterized the type of receptor involved in bronchorelaxation in the presence and absence of epithelium. We further defined the role of epithelium-derived relaxing factor, i.e., nitric oxide (NO), on these responses. In both epithelium-intact and -denuded tertiary airway rings from rabbits, the adenosine receptor agonists 2-[p-(2-carboxyethyl)]phenylethylamino-5-N'-ethylcarboxamidoadenos ine (CGS-21680), 5'-(N-ethyl-carboxamido)adenosine (NECA), 2-chloroadenosine (CAD), and (-)-N6-(2-phenylisopropyl)adenosine (R-PIA) relaxed airway smooth muscle with a potency order of CGS-21680 > NECA > CAD > R-PIA. A 98.5, 89.7, 73.2, and 64.7% relaxation was observed at 10(-5) M by CGS-21680, NECA, CAD, and R-PIA in the epithelium-intact bronchial rings, respectively. The 50% maximum effective concentration (EC50; x 10(-7) M) values for CGS-21680, NECA, CAD, and R-PIA were 2, 4, 9, and 80, respectively. Denuded rings, however, showed much less relaxant responses to various adenosine agonists compared with epithelium-intact rings. The adenosine receptor antagonist 8-(sulfophenyl)theophylline significantly attenuated the relaxant responses to all the agonists in the epithelium-intact and -denuded rings. The epithelium-dependent relaxant effect of the agonists in airway rings was inhibited by NG-monomethyl-L-arginine (L-NMMA; 30 microM). The EC50 (x 10(-6) M) values for CGS-21680, NECA, CAD, and R-PIA in the presence of inhibitor were 5.5, 8, 30, and 200, respectively. The L-NMMA produced an insignificant inhibitory effect in the epithelium-denuded rings. L-Arginine but not D-arginine (100 microM) reversed the inhibitory effect of L-NMMA on adenosine agonist-induced relaxation. In primary epithelial cells in culture, CGS-21680 (10(-5) M) induced a fourfold increase in NO production over the control. The CGS-21680-induced NO production in epithelial cells was significantly inhibited by NG-nitro-L-arginine methyl ester (L-NAME). Moreover, L-arginine reversed the inhibitory effect of L-NAME in the epithelial cells. The data suggest that adenosine relaxes rabbit airway smooth muscle through an A2 adenosine receptor and the epithelium serves as a source of NO.

Effect of Adenosine Receptor Agonists and Antagonists on the Expression of Opiate Withdrawal in Rats

The effects of the selective A 1 adenosine receptor agonist N 6-cyclopentyladenosine (CPA) and the selective A 2a agonist 2-[p-(2-carboxethyl)phenylethyl-ethylamino]-5′-ethylcarboxamidoadenosine (CGS 21680) (each at 0.03, 0.1 and 0.3 mg/kg, SC) as well as the selective A 1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), non-selective antagonists 3-isobutyl-1-methylxanthine (IBMX), aminophylline, 3,7-dimethyl-1-propargyl-xanthine (DMPX) and 8(p-sulfophenyl)-theophylline (8-SPT) were investigated (each at 5, 10 and 30 mg/kg, SC) for their ability to alter the naloxone-precipitated opiate withdrawal syndrome in morphine-dependent rats. Effects of CPA and CGS 21680 on opiate withdrawal in the presence of aminophylline were also investigated. Both CPA and CGS 21680, caused a significant reduction in the incidence of body shakes, teeth chatter and paw shakes and decreased the amount of faecal matter produced. DPCPX, IBMX, DMPX, 8-SPT and aminophylline significantly increased the incidence of jumps and decreased the amount of faecal matter produced. The incidence of body shakes was significantly increased by DMPX, 8-SPT and IBMX. Neither CPA nor CGS 21680 were able to reverse the significant increase in the incidence of jumps caused by aminophylline. These data suggest that there is a role for endogenous adenosine in the modulation of the opiate abstinence syndrome and both A 1 and A 2a adenosine receptors are involved in this phenomenon.

Adenosine A3 receptors promote degranulation of rat mast cells both in vitro and in vivo.

To investigate the effects of adenosine receptor agonists and antagonists on 5-HT release from rat isolated pleural mast cells and on plasma protein extravasation in the skin of conscious rats. In vitro Rat isolated pleural mast cells were loaded with [14C] 5-HT, sensitised with mouse monoclonal anti-DNP and then challenged with human serum albumin-DNP. DNP-stimulated 5-HT release from mast cells was determined. In vivo Rats, loaded intravenously with [125I] human serum albumin, were injected intradermally with adenosine agonists at sites on the back. 30 min later plasma protein extravasation at each injection site was determined. In isolated mast cells, each adenosine agonist enhanced DNP-induced 5-HT release, N6-(3-iodobenzyl)-5-(N-methyl-carboxamidoadenosine), (IB-MECA), being the most potent agonist. The adenosine A1/A2 antagonist, 8-phenyltheophylline (8-PT), had no effect on the response to IB-MECA. In contrast, 3-(4-amino-iodobenzyl)-8-[4-[[[carboxy]methyl]oxy]phenyl]-1-propylxanthi ne, (I-ABOPX), inhibited (pA2 6.2) the IB-MECA responses. In the skin of conscious rats, intradermal IB-MECA produced a marked plasma protein extravasation (PPE) which was mimicked by N6-2-(4-aminophenyl)-ethyladenosine (APNEA). The PPE produced by IB-MECA was not affected by either 8-PT or CGS15943A, but was virtually abolished by cyproheptadine and in rats pre-treated with Compound 48/80. These results indicate that stimulation of adenosine A3 receptors both enhances degranulation in vitro and directly produces degranulation of rat mast cells in vivo.

Expression of inducible nitric oxide synthase in mice: Pharmacological evaluation of adenosine receptor agonists

Inhibition of inducible nitric oxide (NO) synthase during endotoxaemia may be of therapeutic value. We have previously shown that pretreatment of mice with adenosine receptor agonists 1 h before lipopolysaccharide administration results in a dose-dependent reduction of plasma nitrite and nitrate (NO − x ) levels. This report examines the effects of adenosine receptor agonists. 5′- N-ethylcarboxamidoadenosine (NECA), N 6-cyclohexyladenosine (CHA), R-phenylisopropyl-adenosine ( R-PIA) and 5′-( N-cyclopropyl)carboxamidoadenosine (CPCA), on the level of inducible NO synthase expression in a model of liver inflammation induced by lipopolysaccharide. Following lipopolysaccharide administration (10 mg/kg, i.p.), liver mRNA expression peaked at 3 h and declined to 35% of maximal level after 24 h. Pretreatment with adenosine receptor agonists (0.001 mg/kg to 5 mg/kg, i.p.) depressed inducible NO synthase mRNA expression significantly. Down-regulation of inducible NO synthase mRNA expression corresponded with changes in plasma NO − x level as well as activity of NO synthase in the liver. Administration of R-PIA (5 mg/kg, i.p.) increased the survival of animals injected with a lethal dose of lipopolysaccharide. Thus adenosine receptor agonists may useful as anti-inflammatory agents in the treatment of endotoxaemia.

P1-purinoceptor-mediated vasodilatation and vasoconstriction in hypoxia.

1. The effects of adenosine receptor agonists were examined on isolated rings of guinea-pig pulmonary artery under normoxic and hypoxic conditions. The rings were denuded of endothelium and tissues were precontracted with phenylephrine (3 x 10(-6) M) before constructing cumulative concentration-response curves to the agonists. 2. 5'-(N-ethylcarboxamido)adenosine (NECA) caused concentration-dependent contractions of the pulmonary artery which were not different between hypoxia and normoxia. The contractions were converted to a relaxation in the presence of the cyclooxygenase inhibitor, indomethacin, and again these were unaffected by hypoxia. 3. Examination of a range of agonists under normoxic conditions in the presence of indomethacin revealed relaxations, except for the A2a receptor-selective agonist, CGS 21680. The vasorelaxation was therefore A2b receptor-mediated. 4. In hypoxia, however, in the presence of indomethacin, vasoconstriction occurred to R(-)-N(6)-(2-phenylisopropyl)adenosine (R-PIA) and, to a greater extent, to Nb-cyclopentyladenosine (CPA). In the absence of indomethacin, the constriction by CPA during hypoxia was significantly greater. 5. The indomethacin-resistant contraction by CPA was abolished by the A1 receptor antagonist, 8-cyclopentyltheophylline (CPT, 3 x 10(-6) M). 6. This study has demonstrated cyclooxygenase-dependent and-independent vasoconstrictions to adenosine agonists in guinea-pig pulmonary artery under hypoxic conditions. The cyclooxygenase-independent contraction is mediated via A1 receptors. 7. These results suggest that endogenous adenosine released in the pulmonary circulation under hypoxic conditions will cause vasoconstriction and may contribute to the pulmonary hypertension associated with acute respiratory failure.

Adenosinergic mechanisms in anticonvulsant action of diazepam and sodium valproate

The effects of adenosine receptor agonists and antagonists were studied in pentylenetetrazole (PTZ)-induced seizures in rats. Animals were pretreated with the non-specific adenosine receptor antagonist, theophylline (50 and 100 mg/kg, i.p.), or the specific A 1 adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), in a dose of 1 mg/kg, i.p., followed by 100% anticonvulsant doses of diazepam (4 mg/kg)/sodium valproate (300 mg/kg, i.p.). Subsequently, they were challenged with convulsant doses of PTZ i.e. 60 mg/kg, i.p. It was seen that while DPCPX could not reverse the protection of both the antiepileptic drugs, theophylline significantly reversed this protection, as assessed by percent incidence of seizures and change in latency parameters. In another set of experiments, the rats were pretreated with a combination of subanticonvulsant doses of adenosine (500 mg/kg) or specific adenosine A 1 receptor agonist, cyclopentyladenosine (CPA) and diazepam (0.5 and 1 mg/kg)/sodium valproate (150 mg/kg), prior to PTZ challenge. We observed a decrease in incidence and increase in latency of seizures following either combination. The protection observed was independent of the hypothermic and hypotensive effects of adenosine and CPA. These results indicate that though A 1 agonist enhances the protection of diazepam and sodium valproate, a direct involvement of adenosine A 1 receptor in anticonvulsant action of these drugs is doubtful.

Modulation of erythropoietin production by selective adenosine agonists and antagonists in normal and anemic rats

Hypoxia or anemia is the fundamental stimulus for erythropoietin (EPO) production. Recent in vitro studies suggest that EPO secretion in response to hypoxia is regulated by adenosine in the kidney. In order to examine the in vivo effect of adenosine on EPO production, we determined the effects of adenosine receptor agonists and antagonists on serum EPO concentration in normal and anemic rats. In normal rats, intravenous injection of adenosine agonists (NECA, CHA and CGS-21680) dose-dependently stimulated EPO production. Pretreatment with KW-3902, an adenosine A1 antagonist with modest A2b antagonistic action, or KF17837, an adenosine A2a antagonist, inhibited the NECA (0.1 mg/kg, i.v.)-stimulated EPO production. Anemic hypoxia, induced by 2% (v/w body weight) blood withdrawal, increased serum EPO concentration from 38 ± 2 to 352 ± 76 mU/ml, with the increased serum adenosine concentration in the renal vein. KF17837 (0.1 mg/kg, i.v.), but not KW-3902 (0.1 mg/kg, i.V.), inhibited the anemic hypoxia-induced increase in EPO production. The present findings support the notion that adenosine mediates the EPO production in response to hypoxia in the kidney.

Changes in adenosine receptors mediating hypotension in morphine-dependent rats

The hypotensive actions of morphine have been shown to be mediated by adenosine. Since tolerance has been reported to the hypotensive effects of morphine, this study was designed to determine whether morphine dependence altered adenosine receptor-mediated decreases in blood pressure in the Hooded Wistar rat. Following the induction of morphine dependence, the effects of adenosine receptor agonists and antagonists were studied in intact and pithed rat preparations. The hypotensive effects of adenosine were significantly less in morphine-dependent rats when compared to opiate naive rats. The adenosine A 1 receptor agonist cyclohexyladenosine induced decreases in diastolic blood pressure which were significantly reduced in morphine-dependent rats when compared to opiate naive rats. However, the adenosine A 2 a receptor agonist 2- p-(car☐yethyl)-phenylamino-5′- N-ethylcar☐amidoadenosine (CGS 21680) had a greater effect on blood pressure in morphine-dependent rats compared to opiate naive rats. The effects of adenosine receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine, 8-phenyltheophylline, 8-( p-sulfophenyl)theophylline and 3,7-dimethyl-1-propargylxanthine infused at 50 μg/kg per min on the hypotensive actions of adenosine were studied in opiate naive and morphine-dependent rats. In intact rats the induction of morphine dependence reduced the potency of these antagonists at inhibiting adenosine-induced decreases in blood pressure. The same series of experiments was conducted in the pithed rat preparation. In this case the hypotensive actions of both cyclohexyiadenosine and CGS 21680 were greater in morphine-dependent rats than opiate naive rats. In pithed rats, morphine dependence did not change the potencies of adenosine receptor antagonists on he hypotensive actions of adenosine. These results suggest that adenosine A 1 receptors are downregulated in morphine-dependent rats, and that adenosine A 2 receptors are upregulated in morphine-dependent rats.

Effects of adenosine-receptor agonists on renin release in anaesthetized rats

To investigate the effects of the interaction between adenosine receptors and renal nerves on renin release. The effects on renin secretion of A1 (2-chloro-N6-cyclopentiladenosine) and A2 (2-hexynil-5'-N-ethyl-carboxamido-adenosine) adenosine-receptor agonists were studied in two groups of anaesthetized rats, each with one kidney surgically denervated. Arterial blood pressure and the renal blood flow of innervated and denervated kidneys were continuously recorded. Cannulae were inserted into both renal veins through femoral veins. After 1h of rest, A1 and A2 agonists were intravenously infused for 30 min in the two groups of rats. Plasma renin activity was measured by radioimmunoassay in blood samples drawn simultaneously from both renal veins and the femoral artery before and after the drug infusion. Infusions of A1 and A2 agonists produced comparable hypotensive effects. During A1 agonist administration, the heart rate decreased significantly, but it did not change after A2 agonist treatment. Renal blood flow was reduced by administration of the A1 agonist in both kidneys, while A2 agonist administration significantly reduced the renal blood flow of the innervated kidney only. The veno-arterial difference in plasma renin activity decreased after the A1 agonist infusion in both kidneys, but after the A2 agonist infusion it increased significantly in the innervated kidney only. Renal nerves do not influence the inhibition of renin release mediated by A1 adenosine receptors. In vivo, A2-receptor agonist administration can stimulate renin release only in the presence of intact renal nerves.