Central gastric antisecretory action of adenosine in the rat

The effects of intracerebroventricularly (i.c.v.) administered adenosine and some of its analogues on gastric secretion were studied in rats. The compounds inhibited the gastric output of acid, pepsin and fluid in pylorus-ligated rats in a dose-dependent manner with an order of potency: 5'-N-ethylcarboxamidoadenosine (NECA) greater than (-)N6-phenylisopropyladenosine (R-PIA) greater than (+)N6-phenylisopropyladenosine (S-PIA) greater than adenosine. Pretreatment with 10 and 30 mg/kg of theophylline i.v. or 5 mg/kg of 8-phenyltheophylline s.c. did not modify the antisecretory effect of 0.1 microgram of NECA i.c.v. NECA injected i.c.v. did not affect the secretion induced by carbachol in awake rats subjected to vagotomy or in anaesthetized rats with intact vagi. NECA i.c.v. had no effect on the serum concentration of gastrin. The depletion of brain monoamines (noradrenaline, dopamine and serotonin) with 6-OHDA i.c.v. significantly attenuated the inhibitory action of NECA. Pretreatment with 10 mg/kg of naloxone i.v. or indomethacin s.c. did not modify the antisecretory effect of NECA. The results indicate that adenosine inhibits gastric secretion in rats by a decrease in the stimulatory vagal impulses to the stomach, and that it acts in the brain via receptors insensitive to xanthines. Brain biogenic monoamines, but not opioid peptides or prostaglandins seem to be involved in the central gastric antisecretory action of adenosine.     

Central respiratory and cardiovascular effects in the rat of some putative neurotransmitter amino acids

Respiratory performance was studied in halothane anesthetized rats after intracerebroventricular (i.c.v.) injection of beta-alanine, taurine or glycine (0.01--1 mg). The amino acids induced a marked decrease in both respiratory frequency (f) and tidal volume (VT), which was immediate and longlasting. The respiratory depressant action of glycine could readily be reversed by strychnine, a glycine antagonist. Measurement of respiratory time intervals, inspiratory time (TI), expiratory time (TE) and total cycle duration (TTOT), after administration of the putative neurotransmitter amino acids revealed that the effects on f were due to prolongation of the duration of expiration. The duration of inspiration was principally unaltered, but mean inspiratory flow (VT/TI) and respiratory timing (TI/TTOT) decreased. In experiments employing the occluded breath technique, P0.1 was reduced in the same magnitude as the mean inspiratory flow (VT/TI). The results also showed a change in central (bulbopontine) setting for TE, while the setting to TI was unaltered. An inert amino acid, valine, which was administered i.c.v. in the same doses, had no effects on respiratory parameters. Apart from the effects on basal ventilation of beta-alanine, taurine and glycine, the CO2 induced respiratory response was blunted. These three amino acids also depressed heart rate and mean arterial pressure. Although relatively high doses were used to induce the respiratory effects, it may be hypothetised that the putative neurotransmitters beta-alanine, taurine and glycine may have a physiological role in the central regulation of breathing.     

Evidence that A(2a) and not A(2b) purinoceptors are coupled to production of nitric oxide in the central regulation of blood pressure

In a previous study we reported that nitric oxide (NO) partially mediates centrally the decrease of arterial blood pressure induced by adenosine A₂ subtype receptor stimulation. The present study confirms the earlier suggestion and shows that in adult male normotensive anaesthetized rats 5′-N-ethylcarboxamidoadenosine (NECA), a non-selective adenosine receptor agonist, centrally injected induced a significant decrease of arterial blood pressure. Moreover, the observation that intracerebroventricular (i.c.v.) administration of N^ω-nitro- -arginine methyl ester ( -NAME), a NO synthase inhibitor, +8-(3-chlorostyryl)-caffeine (CSC), antagonist of A_2a receptors, did not reduce furthermore the hypotensive effect induced by NECA injection, demonstrated that NO is involved only via A_2a and not via A_2b adenosine subtype receptors in the central regulation of blood pressure.     

Pharmacological characterization of the receptor involved in chemoexcitation induced by adenosine.

Experiments were performed on cats anaesthetized with pentobarbitone in which carotid body chemoreceptor activity was recorded from the peripheral end of a sectioned carotid nerve. Intracarotid (i.c.) injections of adenosine and its analogues, NECA (5'-N-ethylcarboxamidoadenosine), L-PIA(L-N6-phenylisopropyladenosine), and D-PIA(D-N6-phenylisopropyladenosine), caused dose-related increases in chemosensory discharge. The rank order of potency as chemoreceptor stimulants was: NECA greater than adenosine greater than L-PIA greater than D-PIA. Infusion of theophylline antagonized the chemoexcitatory effects of NECA, and infusion of 8-phenyltheophylline (8-PT), which is a more potent adenosine antagonist with less activity as a phosphodiesterase inhibitor, reduced the chemoexcitation induced by adenosine. Infusion of 8-PT (10 micrograms min-1 i.c.), a dose which substantially reduced the effect of injected adenosine, also reduced the sensitivity of carotid chemoreceptors to hypoxia (10% O2 for 4 min). It is concluded that the adenosine receptors in the cat carotid body which mediate chemosensory excitation are xanthine-sensitive and appear to be of the A2 sub-type. Adenosine, released within the carotid body by physiological stimuli, may be involved in chemoexcitation.     

Evidence that the P1-purinoceptor in the guinea-pig taenia coli is an A2-subtype.

The effects of 5'-N-ethylcarboxamidoadenosine (NECA), L-NECA, 2-chloroadenosine, N6-phenylisopropyladenosine (L-PIA and D-PIA), cyclohexyladenosine (CHA), and adenosine were examined on the guinea-pig taenia coli. All the analogues except L-NECA caused relaxations; the order of potency for the series was: NECA greater than 2-chloroadenosine greater than L-PIA greater than CHA greater than D-PIA greater than adenosine. L-PIA was twice as potent as D-PIA in inducing relaxations of the guinea-pig taenia coli. Adenosine and its analogues that induce relaxation all caused a slow membrane hyperpolarization; differences in the rates of hyperpolarization and latencies were apparent, although not statistically significant. The duration of the response to adenosine was significantly less than that for any adenosine analogue. Ion studies, using the sucrose gap, revealed that responses to the analogues were attenuated in elevated extracellular potassium or reduced extracellular chloride. 8-Phenyltheophylline, a potent P1-purinoceptor antagonist, caused a rightward shift of all the adenosine and analogue concentration-response curves. Dipyridamole, an adenosine uptake inhibitor, potentiated the relaxations to adenosine but had no significant effect on the relaxations induced by the analogues. It is concluded that NECA, 2-chloroadenosine, L-PIA, CHA, D-PIA and adenosine mediate their relaxant effects via an extracellular P1-purinoceptor which displays characteristics of the A2-subtype as determined by the rank order of agonist potency. Electrophysiological analysis of the responses to each of the analogues did not reveal any marked differences in the modes of action even between NECA and L-PIA (preferential A2- and A1-receptor agonists, respectively).     

2-Chloroadenosine induction of vagally-mediated and atropine-resistant bronchomotor responses in anaesthetized guinea-pigs.

1. The bronchoconstrictor effects of intravenous administration of adenosine derivatives in anaesthetized non-curarized guinea-pigs have been studied. 2. 2-Chloroadenosine (2-Cl-Ade), 5'-N-ethylcarboxamideadenosine (NECA) and L-N6-phenylisopropyl-adenosine (L-PIA) all produced dose-dependent, transient increases in tracheal insufflation pressure, with an order of potency (NECA greater than or equal to 2-Cl-Ade much greater than L-PIA) typical of A2-receptor mediated biological responses. 3. 2-Chloradenosine-induced bronchoconstrictor responses disappeared after vagotomy or topical application of tetrodotoxin (TTX) on cervical vagal trunks. 4. 2-Chloradenosine-induced bronchospasm was unaffected by atropine (1 mg kg-1 i.v.), physostigmine (50 micrograms kg-1 i.v.) and hexamethonium (30 mg kg-1 i.v.) but was significantly reduced by theophylline (25 mg kg-1 i.v.). 5. The magnitude of 2-Cl-Ade-induced bronchospasm was significantly reduced by acute (10 micrograms kg-1 i.v.) or chronic (55 mgkg-1 s.c. four days before the experiment) pretreatment with capsaicin. 6. Guanethidine (20 mg kg-1 s.c on two consecutive days), prazosin (10 micrograms kg-1 i.v.), diphenhydramine (1 mg kg-1 i.v.) and indomethacin (1 mg kg-1 i.v.) failed to block the bronchomotor response to 2-Cl-Ade. In contrast, cyproheptadine (1-5 mgkg-1 i.v.) markedly reduced, but did not abolish the bronchospasm elicited by the purine derivative. 7. We conclude that in anaesthetized non-curarized guinea-pigs, a transient vagally-mediated bronchospasm can be induced by stimulation of A2-purinoceptors. This effect is complex and involves, at least in part, stimulation of capsaicin-sensitive sensory nerves and 5-hydroxytryptamine release.(ABSTRACT TRUNCATED AT 250 WORDS)     

The adenosine receptor activity of EMD 28422, a purine derivative with reported actions on benzodiazepine receptors.

The effects of a novel purine derivative, N6-[2-(4-chlorophenyl)-bicyclo-2.2.2.octyl-(3)]-adenosine (EMD 28422), that has been found to influence central benzodiazepine receptors, has been compared to those of other adenosine analogues such as L-phenylisopropyladenosine (L-PIA), cyclohexyladenosine (CHA) and adenosine-5'-N-ethyl-carboxamide (NECA). EMD 28422 was about 30 times less potent than CHA and 4 times less potent than NECA in displacing bound [3H]-L-PIA from specific binding sites in the rat brain, presumably reflecting adenosine A1-receptors. A similar relative potency was found using depression of field e.p.s.p. in the hippocampal slice in vitro. In isolated fat cells EMD 28422 was antilipolytic, but some 1000 times less potent than L-PIA. In rat isolated hippocampal slices, which have adenosine A2-receptors, EMD 28422 was more than 300 times less potent than NECA and in guinea-pig thymocytes, which similarly have A2-receptors, EMD 28422 was about 60 times less potent. The results are compatible with the opinion that EMD 28422 is a rather weak agonist at adenosine receptors, with limited selectivity for A1- or A2-receptors. The compound is highly lipophilic, which plays a role in determining its potency in a given biological system. The results are discussed in relation to reported adenosine modulation of benzodiazepine receptors.     

Characterization of the adenosine receptors mediating hypothermia in the conscious mouse.

1. The effects of a range of adenosine receptor-selective ligands on body temperature were investigated following intracerebroventricular (i.c.v.) and intraperitoneal (i.p.) injection in conscious mice. The compounds tested were the non-selective adenosine receptor agonist 5'-N-ethyl-carboxamidoadenosine (NECA), the adenosine A1 receptor-selective agonists cyclopentyl-adenosine (CPA), N6-(9R-phenyl-isopropyl)-adenosine (R-PIA) and N-(1S,trans)-[2-hydroxyclopentyl]-adenosine (GR79236), the A2a receptor selective agonist 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxyamidoaden osine (CGS-21680), the A2b receptor agonist N-[(2-methylphenyl)methyl[adenosine (metrifudil) and the A3 receptor agonist N6-(4-aminophenylethyl)adenosine (APNEA). 2. NECA (0.01-1 microgram, i.c.v.), all of the A1-selective agonists (0.01-1 microgram, i.c.v.) and APNEA (0.1-3 micrograms i.c.v.) produced profound and dose-related hypothermia and sedation. However, CGS-21680 (0.1-10 micrograms i.c.v.) and metrifudil (0.01-1 microgram i.c.v.), produced only mild hypothermia at the highest doses tested. 3. The hypothermic response to the A1 receptor-selective agonists, GR79236 and R-PIA was dose-dependently antagonized by peripheral administration of either the non-selective adenosine receptor antagonist, 8-phenyltheophylline (8-PT, approximately 40 and 30 fold rightward shifts of the dose-response curves respectively at 10 mg kg-1, i.p.), or the adenosine A1 receptor-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, approximately 20 fold shift of the GR79236 dose-response curve at 1 mg kg-1, i.p.). The hypothermic response to APNEA was similarly dose-dependently antagonized by the A1 receptor-selective antagonist, DPCPX (5 fold shift at 0.1 mg kg-1, i.p.).(ABSTRACT TRUNCATED AT 250 WORDS)     

Central effects of the cannabinoid receptor agonist WIN55212-2 on respiratory and cardiovascular regulation in anaesthetised rats

1 The primary aim was to study the central respiratory effects of cannabinoids (CB). To this end, the cannabinoid receptor agonist WIN55212-2 was injected into the cisterna magna of urethane-anaesthetised rats and changes in respiratory parameters were observed. The secondary aim was to observe the centrally elicited cardiovascular actions of WIN55212-2. Involvement of opioid mechanisms in the central effects of WIN55212-2 was also studied. 2 Intracisternal (i.c.) application of WIN55212-2 (1, 3, 10 and 30 microg kg(-1)) dose-dependently decreased the respiratory rate and minute volume. Tidal volume was slightly increased, whereas peak inspiratory flow remained unchanged. In addition, WIN55212-2 increased mean arterial pressure and the plasma noradrenaline concentration and decreased heart rate. 3 I.c. injection of WIN55212-3 (1, 3, 10 and 30 microg kg(-1)), an enantiomer of WIN55212-2 lacking affinity for cannabinoid receptors, elicited no effects. All effects of WIN55212-2 were prevented by the CB1 receptor antagonist SR141716 (2 mg kg(-1) i.v.). I.c. administration of the opioid receptor agonist DAMGO (0.1, 0.3, 1 and 3 microg kg(-1)) markedly lowered the respiratory rate, tidal volume, minute volume and peak inspiratory flow. These effects were attenuated by the opioid receptor antagonist naloxone (0.2 mg kg(-1) i.v.). In contrast, naloxone did not affect the respiratory and cardiovascular effects of i.c. administered WIN55212-2. 4 Our results show that activation of CB1 cannabinoid receptors in the brain stem depresses respiration and enhances sympathetic tone and cardiac vagal tone. Opioid mechanisms are not involved in these central cannabinoid effects.     

Ventrolateral medullary respiratory neurons and peripheral chemoreceptor stimulation by almitrine

In anaesthetized, paralysed and artificially ventilated dogs, activities were recorded from the phrenic nerve and from respiratory units within the nucleus ambiguus (nA) and within the nucleus retroambigualis (nRA). The respiratory neurons were classified according to their discharge pattern. Almitrine (50 micrograms/kg i.v.) induced an increase in the firing rate of late peak inspiratory neurons (40 units) and in early burst inspiratory cells (10 units) of the nA. In the nRA, almitrine (50 micrograms/kg i.v.) also stimulated late peak expiratory neurons (17 units) and early burst expiratory cells (9 units). It is concluded that the chemoreceptor pathway is not restricted to the inspiratory neurons of the nTS and that both inspiratory and expiratory neurons of the nA and nRA are involved in the chemoreceptor pathway activated by almitrine.     

The effects of central injections of adenosine analogs on blood pressure and heart rate in the rat

Rats were implanted with chronic indwelling cannulae into the lateral cerebral ventricle. After recovery from surgery, acute experiments on blood pressure were conducted under methoxyflurane/nitrous oxide anesthesia. Rats were injected intracerebroventricularly with two adenosine analogs, 5'-N-ethylcarboxaminidoadenosine (NECA) and (-)-N-(1-methyl-2-phenylethyl)adenosine(L-phenylisopropyladenosine) (L-PIA), and the effects on blood pressure and heart rate recorded. Both analogs produced dose-related reductions in blood pressure and heart rate with L-PIA producing a more potent depression of heart rate than NECA. These effects on blood pressure and heart rate were antagonized by parenteral injections of caffeine. In separate experiments, the responses of blood pressure and heart rate to microinjection of NECA into the brainstem of rats anaesthetized with methoxyflurane/nitrous oxide were also examined. Microinjection of 2.7 nmol/kg into the fourth ventricle in the region of the area postrema produced a profound and long-lasting depression of blood pressure and heart rate. These results show that central injections of analogs of adenosine can influence the areas of the central nervous system involved in the control of cardiovascular function.     

Central endothelin-B receptor stimulation does not affect morphine analgesia in rats

Several neurotransmitter mechanisms have been proposed to play a role in the actions of morphine. We reported that centrally administered endothelin A (ETA) receptor antagonists potentiate morphine analgesia in rats. It has also been reported that ETB agonist, IRL1620, has antinociceptive action mediated through opiate receptors in the periphery. The present study was conducted to determine if central ETB receptors are involved in analgesic actions of morphine. The effect of intracerebroventricular (i.c.v.) administration of ETB receptor agonist, IRL1620, on morphine-induced analgesia and hyperthermia was determined in the rat. Morphine (4 mg/kg, s.c.) produced a significant increase (84%) in tail-flick latency compared to the control group and the analgesic response lasted for 4 h. IRL1620 (30 microg, i.c.v.) did not produce any increase (16%) in tail-flick latency over the 5-hour observation period in vehicle-treated rats. Pretreatment with IRL1620 (3, 10, and 30 microg, i.c.v.) did not have any significant effect on the intensity and duration of morphine (4 mg/kg, s.c.)-induced analgesia. Morphine (4 mg/kg, s.c.) administration produced an increase in body temperature compared to the control group. In vehicle-pretreated rats, IRL1620 (30 microg, i.c.v.) did not produce any change in body temperature. The morphine-induced hyperthermic effect was not altered in IRL1620-pretreated rats. These studies demonstrate that IRL1620, a specific ETB receptor agonist, did not affect the morphine-induced analgesic and hyperthermic effect in rats. It can be concluded that central ETB receptors are not involved in modulation of pharmacological actions of morphine.     

Rabbit isolated vas deferens possess A1 and A2 adenosine receptors.

1. The effects of adenosine, 5'-N-ethylcarboxamidoadenosine (NECA), 2-chloroadenosine, N6-phenylisopropyladenosine (L-PIA and D-PIA) and N6-cyclohexyladenosine (CHA) were examined on the rabbit isolated vas deferens. 2. All the analogues in a concentration-dependent manner inhibited contractile response to electrical stimulation. 3. 8-Phenyltheophylline caused a rightward shift of all the adenosine and its analogues' concentration-response curves. 4. The order of potency for the adenosine and its analogues on the rabbit isolated vas deferens was: CHA = NECA greater than L-PIA greater than 2-chloroadenosine greater than D-PIA greater than adenosine. 5. It is concluded that CHA, NECA, L-PIA, 2-chloroadenosine, D-PIA and adenosine mediate their inhibitory effects on the rabbit isolated muscle via both A1 and A2 adenosine receptors.     

Evidence of the involvement of D1 dopamine receptors in PCP-induced stereotypy and ataxia in rabbits

A behavioural study on the effects of D1 and D2 dopamine receptor antagonists (SCH 23390 and sulpiride respectively) and of an A1 adenosine receptor agonist (N6-L-phenylisopropyladenosine, L-PIA) against phencyclidine (PCP)-induced effects was assessed in adult male rabbits. SCH 23390 (0.003-0.01 mg/kg i.v.) and sulpiride (12.5 mg/kg i.v.) were able to significantly prevent PCP-induced stereotypy. Ataxia was reduced by SCH 23390 (0.003 mg/kg i.v.), while it was potentiated by sulpiride (12.5 mg/kg i.v.). Given alone at 12.5 mg/kg, sulpiride induced some EEG and behavioural effects in rabbits, while SCH 23390 (0.003 and 0.01 mg/kg) did not. L-PIA prevented both PCP-induced stereotypy and ataxia at the dose (0.1 mg/kg i.v.) devoid of behavioural or EEG effects by itself. Our results suggest that D1 dopamine receptors might play a more important role than D2 receptors in the expression of PCP-induced behaviour. They also propose that A1 adenosine receptors might be involved (e.g. via an influence on the dopamine release) in the behavioural effects of PCP.     

The plasma protein extravasation induced by adenosine and its analogues in the rat dorsal skin: evidence for the involvement of capsaicin sensitive primary afferent neurones and mast cells

1. The contribution of sensory neurons and mast cells to the oedema evoked by adenosine A1 (N(6)-cyclopentyladenosine, CPA, 3 - 30 nmol site(-1)), A2 (5'N-ethylcarboxamidoadenosine, NECA, 1 - 10 nmol site(-1)) and A3 receptor agonists (N6-[3-iodobenzyl]-N-methyl-5'-carboxiamidoadenosine, IB-MECA, 0.01 - 3 nmol site(-1)) was investigated in the rat skin microvasculature, by the extravascular accumulation of intravenously-injected (i.v.) 125I-albumin. 2. Intradermal (i.d.) injection of adenosine and analogues induced increased microvascular permeability in a dose-dependent manner (IB-MECA > NECA > CPA > adenosine). The non-selective adenosine receptor antagonist theophylline (5 - 50 nmol site(-1)) markedly inhibited adenosine, CPA or NECA but not IB-MECA-induced plasma extravasation. The A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 0.3 - 3 micromol kg(-1), i.v.) significantly reduced CPA-induced plasma extravasation whereas responses to adenosine, NECA or IB-MECA were unchanged. The A2 receptor antagonist 3,7-dymethyl-1-proprargylxanthine (DMPX, 0.5 - 50 nmol site(-1)) significantly reduced NECA-induced plasma extravasation without affecting responses to adenosine, CPA and IB-MECA. 3. The tachykinin NK1 receptor antagonist (S)-1-[2-[3-(3,4-dichlorphenyl)-1 (3-isopropoxyphenylacetyl) piperidin-3-yl] ethyl]-4-phenyl-1 azaniabicyclo [2.2.2]octane chloride (SR140333), but not the NK2 receptor antagonist (S)-N-methyl-N[4-acetylamino-4-phenyl piperidino)-2-(3,4-dichlorophenyl)butyl]-benzamide (SR48968), significantly inhibited the plasma extravasation evoked by higher doses of adenosine (100 nmol site(-1)), CPA (100 nmol site(-1)), NECA (1 nmol site(-1)) and IB-MECA (0.1 - 1 nmol site(-1)). In rats treated with capsaicin to destroy sensory neurons, the response to higher doses of adenosine, CPA and NECA, but not IB-MECA, was significantly inhibited. 4. The effects of adenosine and analogues were largely inhibited by histamine and 5-hydroxytryptamine (5-HT) antagonists and by compound 48/80 pretreatment. 5. In conclusion, our results provide evidence that adenosine A1 and A2, but not A3, receptor agonists may function as cutaneous neurogenic pro-inflammatory mediators; acting via microvascular permeability-increasing mechanisms that can, depending on dose of agonist and purine receptor under study, involve the tachykinin NK1 receptor and mast cell amines.     

Evidence that A2 purinoceptors are involved in endothelium-dependent relaxation of the rat thoracic aorta.

1. The effect of adenosine and some adenosine analogues on the isolated thoracic aorta from rats was compared with the effect of adenosine 5'-triphosphate (ATP) and adenosine 5'-diphosphate (ADP). 2. Both ATP and adenosine analogues caused relaxation of the noradrenaline (30 nM)-contracted thoracic aorta. 3. The order of potency for adenosine analogues was 5'-(N-ethyl) carboxamidoadenosine (NECA) greater than L-N6-phenylisopropyladenosine (L-PIA), adenosine 5'-monophosphate (AMP), adenosine indicating the presence of adenosine A2 receptors. 4. Removal of the endothelium or prior treatment with haemoglobin (10 microM) attenuated relaxant responses to both ATP and NECA, attenuation being greater for ATP than NECA. 5. 8-Phenyltheophylline (10 microM) reduced relaxant responses to NECA but not to ATP in the intact tissue. 6. These results provide evidence that there are two components to relaxation of the rat thoracic aorta induced by purinoceptor agonists. The first is an endothelium-dependent mechanism involving release of endothelium-derived relaxant factor (EDRF) and the second is due to a direct effect on smooth muscle.     

Adenosine analogues stimulate cyclic AMP-accumulation in cultured neuroblastoma and glioma cells

The effect of three stable adenosine analogues, L-phenylisopropyl-adenosine (L-PIA), 2-chloroadenosine, and adenosine 5'-ethylcarboxamide (NECA), on cyclic AMP accumulation was studied in five different cell lines derived from the nervous system. In N18-neuroblastoma cells, with cholinergic properties, all three analogues caused an increased accumulation of cyclic AMP with the following relative order of potency: NECA greater than 2-chloroadenosine greater than L-PIA. The half maximal effect of NECA was obtained at close to 10(-8) M concentration. In the two other neuroblastoma cell lines, 41A3 with cholinergic and NIE115 with adrenergic properties, the two analogues NECA and PIA had similar effects. In glioma C6 and 138 MG cells NECA was also found to be more potent that PIA in elevating cyclic AMP levels. However, the absolute potency of NECA in these cell lines was almost 100 times lower. Phosphodiesterase (PDE) activity in crude homogenates of the five cell lines showed essentially similar Km and Vmax, with the exception that the three neuroblastoma cell lines showed biphasic, the glial cell lines monophasic Eadie-Hofstee plots. Theophylline was equally potent as an inhibitor of PDE in all cell lines, but the non-xanthine, inhibitor rolipram, was more potent against neuroblastoma than glial cell PDE. These results indicate that all five cell lines have adenosine receptors of the A2-subtype. However, the apparent affinity of the adenosine analogues to these receptors was markedly different between the neuroblastoma and glial cell lines. The absolute potency of adenosine analogues may be a poor criterion to classify adenosine receptors, into A1 and A2 subtypes, especially when intact cells are used.     

On the type of receptor involved in the inhibitory action of adenosine at the neuromuscular junction

The effects of adenosine and adenosine analogues (L-N6-phenylisopropyladenosine (L-PIA), D-N6-phenylisopropyladenosine (D-PIA), N6-cyclohexyladenosine (CHA), N6-methyladenosine, 5'-N-ethylcarboxamide adenosine (NECA) and 2-chloroadenosine) on evoked endplate potentials (e.p.ps) and on twitch tension were investigated in innervated sartorius muscles of the frog. Adenosine and its analogues decreased, in a concentration-dependent manner, the amplitude of both the e.p.ps and the twitch responses evoked by indirect stimulation. The order of potencies in decreasing twitch tension was: L-PIA, CHA, NECA greater than 2-chloroadenosine greater than D-PIA greater than N6-methyladenosine, adenosine. L-PIA was about ten fold more potent than D-PIA. None of the adenosine analogues tested affected the twitch responses of directly stimulated tubocurarine-paralyzed muscles. In concentrations that did not modify neuromuscular transmission, theophylline and 8-phenyltheophylline (8-PT) but not isobutylmethylxanthine (IBMX), antagonized the inhibitory action of 2-chloroadenosine at the neuromuscular junction. 8-PT behaved as a competitive antagonist and was about forty fold more potent than theophylline. It is concluded that the R-type adenosine receptor at the neuromuscular junction should not be classified in the A1/A2 system. The possibility of calcium-linked adenosine receptors having pharmacological profiles distinct from those originally defined as modulating adenylate cyclase is discussed.     

Evidence for an A2/Ra adenosine receptor in the guinea-pig trachea

1 An attempt was made to determine whether the extracellular adenosine receptor that mediates relaxation in the guinea-pig trachea is of the A(1)/R(i) or A(2)/R(a) subtype.2 Dose-response curves to adenosine and a number of 5'- and N(6)-substituted analogues were constructed for the isolated guinea-pig trachea, contracted with carbachol.3 The 5'-substituted analogues of adenosine were the most potent compounds tested, the order of potency being 5'-N-cyclopropylcarboxamide adenosine (NCPCA) > 5'-N-ethylcarboxamide adenosine (NECA) > 2-chloroadenosine > L-N(6)-phenylisopropyladenosine (L-PIA) > adenosine > D-N(6)-phenylisopropyladenosine (D-PIA).4 The difference in potency between the stereoisomers D- and L-PIA on the isolated trachea was at the most five fold.5 Responses to low doses of adenosine and its analogues were attenuated after treatment with either theophylline or 8-phenyltheophylline. The responses to 2-chloroadenosine were affected to a lesser extent than were those to the other purines.6 Adenosine transport inhibitors, dipyridamole and dilazep, potentiated responses to adenosine, did not affect those to NCPCA, NECA, L-PIA and D-PIA but significantly reduced the responses to high doses of 2-chloroadenosine.7 Relaxations evoked by 9-beta-D-xylofuranosyladenosine which can activate intracellular but not extracellular adenosine receptors, were attenuated by dipyridamole but unaffected by 8-phenyltheophylline.8 The results support the existence of an extracellular A(2)/R(a) subtype of adenosine receptor and an intracellular purine-sensitive site, both of which mediate relaxation.     

Adenosine modulation of adrenergic neurotransmission in the human fallopian tube

The effects of adenosine and adenosine analogues on nerve-induced contractile responses and [3H]noradrenaline ([3H]NA) release, were studied in the isthmic part of human oviducts. Adenosine and L-N6-phenylisopropyladenosine (L-PIA) could enhance neurogenic contractile responses in preparations obtained mainly in the proliferative phase. At higher concentrations, adenosine derivatives inhibited contractile responses to nerve stimulation in both proliferative and secretory phase, with the potency order: 5'-N-ethylcarboxamideadenosine (NECA) greater than or equal to L-PIA much greater than D-PIA. This indicated actions at both stimulatory A1- and inhibitory A2-receptors. Adenosine, L-PIA and NECA but not D-PIA inhibited [3H]NA release during nerve stimulation. The relative potency order for the prejunctional inhibition was compatible with an action at A1-receptors. Furthermore, adenosine was found to modulate nerve-induced contractions via postjunctional stimulatory A1- and inhibitory A2-like receptors. The postjunctional effects may be influenced by cyclic hormonal changes. The adenosine antagonist 8-p-sulfophenyltheophylline (PS?T) reversibly antagonized the stimulatory and inhibitory effects by adenosine and analogues.