Adenosine receptor subtypes.

The numerous and widespread effects of adenosine provide both an opportunity for the development of novel therapeutic agents acting via adenosine receptors and the challenge of achieving selectivity of action. The feasibility of achieving selectivity is enhanced if receptor subtypes can be identified. Biochemical, functional and receptor-cloning studies are beginning to provide convergent data supporting the existence of A1, A2A, A2B and A3 receptors. However, studies of the functional significance of these receptors in intact tissues both in vitro and in vivo have lagged behind the biochemical studies. In this article, Michael Collis and Susanna Hourani review the current status of adenosine receptor classification and propose that ligands with greater selectivity need to be evaluated in a wide range of functional preparations if the therapeutic potential of this area is to be realized.

The renin-angiotensin system, dietary salt, and increased sensitivity to noradrenaline in mesenteric vasculature preparations from renal/salt hypertensive rats.

Exogenous angiotensin II causes noradrenaline supersensitivity in rat mesenteric vasculature preparations. The noradrenaline supersensitivity of tissues from renal/salt hypertensive rats, with low plasma renin activity, is not caused by endogenous angiotensin II since it was unaffected by Sar1 Ileu8 angiotensin II. Dietary salt-loading caused a small increase in noradrenaline sensitivity.

Vascular reactivity to noradrenaline, potassium chloride, and angiotensin II in the rat perfused mesenteric vasculature preparation, during the development of renal hypertension.

The degree of reactivity to noradrenaline of the perfused mesenteric vasculature and the blood pressure of the renal hypertensive rat were correlated. Early (true) supersensitivity was demonstrated for noradrenaline and angiotensin but not for KCl. Later (apparent) hyperreactivity to all three substances was related to an elevated maximal response. The potentiating actions of endogenous angiotensin could cause the early (true) supersensitivity to noradrenaline.

Renal vascular reactivity in the young spontaneously hypertensive rat.

The renal resistance vessels of the mature spontaneously hypertensive rat (SHR) exhibit increased reactivity to vasoconstrictor agonists. This could be a cause or consequence of hypertension. We have compared vascular reactivity in isolated perfused kidneys from 46-day-old SHR and from normotensive control rats. The amplitude of responses in kidneys from the SHR to angiotensin II, barium chloride, or norepinephrine was not different from the control. Therefore, increased reactivity of the renal vascular smooth muscle cannot be an early pathogenic mechanism in spontaneous hypertension. Responses evoked by 5-hydroxytryptamine (serotonin) were of a greater amplitude in the SHR than in the control kidney. However, this difference was due to an interaction of serotonin with the sympathetic nerves, as it was abolished by treatment of the rats with 6-hydroxydopamine. Responses induced by electrical stimulation of the renal sympathetic nerves were also of greater amplitude in SHR than in control kidneys, both before and after the blockade of norepinephrine disposition mechanisms. Nerve stimulation evoked a greater efflux of endogenous norepinephrine from kidneys of the SHR than from those of control rats. Thus, the increased reactivity of the SHR kidney to renal nerve stimulation is due to an augmented release of endogenous norepinephrine. This could be an important factor in the early development of hypertension.

Epinephrine enhances neurogenic vasoconstriction in the rat perfused kidney.

Epinephrine has been implicated in the genesis of some forms of hypertension. We have investigated the effects of epinephrine on vasoconstrictor responses evoked by adrenergic stimuli in the isolated perfused rat kidney. Low concentrations of epinephrine (2.5 - 5 X 10(-9) M) increased the amplitude of vasoconstrictor responses evoked by electrical stimulation of the renal adrenergic nerves. These concentrations of epinephrine had no effect on the basal perfusion pressure of the kidney or on the amplitude of vasoconstrictor responses evoked by exogenous norepinephrine. The potentiating effect of epinephrine persisted after infusion of the amine had ceased. Kidneys that had been perfused with 3H-epinephrine accumulated radioactivity, which could then be released by renal nerve stimulation. Cocaine (3 X 10(-5) M) reduced the renal accumulation of 3H-epinephrine and abolished both the persistent potentiating effect of the amine and the release of radioactivity evoked by subsequent nerve stimulation. The potentiating effect of epinephrine infusion was abolished by the beta 2-selective adrenergic receptor antagonist ICI 118,551 (3 X 10(-8) M), but not by the beta 1-selective adrenergic receptor antagonist atenolol (10(-6) M). These results indicate that concentrations of epinephrine that can be achieved during acute stress can enhance the amplitude of neurogenic vasoconstrictor responses. This effect appears to be mediated via a prejunctional beta 2-adrenergic receptor. The persistent nature of this effect may be due to the neuronal accumulation and subsequent release of epinephrine.

Hypotensive action of captopril in spontaneously hypertensive and normotensive rats. Interference with neurogenic vasoconstriction.

The effects of captopril and angiotensin II on adrenergic neurotransmission have been studied in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). In a pithed rat preparation, vasoconstrictor responses evoked by spinal stimulation were greater in SHR than WKY (p less than 0.01). Captopril reduced responses to electrical stimulation and this reduction was greater in the SHR (p less than 0.001). Bilateral nephrectomy reduced the vasoconstrictor responses to nerve stimulation in both strains of rat and abolished the effects of captopril. In an isolated perfused mesenteric artery WKY (p less than 0.05). Angiotensin II potentiated responses from both strains of rat, however the amplitude of the potentiation was greater in preparations from the SHR than those from WKY (p less than 0.002). Captopril (30 mg/kg by mouth) reduced blood pressure in conscious SHR over a 5-day dosing period. In WKY rats, no hypertensive action of captopril was observed. However, in another normotensive strain, the Alderley Park Wistar rat (APW), captopril lowered blood pressure. Plasma renin activity was not significantly different among these three strains of rat. The APW have previously been shown to be very sensitive to the adrenergic potentiating actions of angiotensin II. Captopril thus lowers blood pressure in SHR and APW, and both these strains are sensitive to the adrenergic potentiating actions of angiotensin II. It does not lower blood pressure in WKY, which is relatively insensitive to these actions of the octapeptide. Therefore, the hypotensive action of captopril in the rat may be due to its interference with the adrenergic potentiating effect of angiotensin II.

Decreased release of norepinephrine in the isolated kidney of the adult spontaneously hypertensive rat.

Renal resistance vessels of the mature spontaneously hypertensive rat (SHR) exhibit an increased reactivity to exogenous norepinephrine, but a normal response to renal nerve stimulation. This difference could be due either to depression of the exocytotic process or to accelerated disposition of the released transmitter. We compared the overflow of norepinephrine in isolated perfused kidneys from adult SHR and normotensive rats. After previous incubation with 3H-norepinephrine, renal nerve stimulation caused smaller increases in the overflow of intact tritiated transmitter and its metabolites in kidneys form SHR than in those from normotensive controls. A similar difference was found when the amounts of endogenous norepinephrine were measured radioenzymatically. The tissue content of norepinephrine was comparable in kidneys from both hypertensive and normotensive animals. The uptake of 3H-norepinephrine was comparable in kidneys from SHR and normotensive controls; cocaine caused a comparable depression of the 3H-uptake in which then explains the normal vasoconstrictor response to renal nerve stimulation despite the increased responsiveness of the vascular smooth muscle cells to norepinephrine.

Evidence for an A1-adenosine receptor in the guinea-pig atrium.

1 The purpose of this study was to determine whether the adenosine receptor that mediates a decrease in the force of contraction of the guinea-pig atrium is of the A1- or A2-sub-type. 2 Concentration-response curves to adenosine and a number of 5'- and N6-substituted analogues were constructed and the order of potency of the purines was: 5'-N-cyclopropylcarboxamide adenosine (NCPCA) = 5'-N-ethylcarboxamide adenosine (NECA) greater than N6cyclohexyladenosine (CHA) greater than L-N6-phenylisopropyl adenosine (L-PIA) = 2-chloroadenosine- greater than adenosine greater than D-N6-phenylisopropyl adenosine (D-PIA). 3 The difference in potency between the stereoisomers D- and L-PIA was over 100 fold. 4 The adenosine transport inhibitor, dipyridamole, potentiated submaximal responses to adenosine but had no significant effect on those evoked by the other purines. 5 Theophylline antagonized responses evoked by all purines, and with D-PIA revealed a positive inotropic effect that was abolished by atenolol. 6 The results indicate the existence of an adenosine A1-receptor in the guinea-pig atrium.

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.

The cardiovascular pharmacology of ICI 170777 ((6RS)-6-methyl-5-(pyrid-4-yl)-3H,6H-1,3,4- thiadiazin-2-one) a novel compound with positive inotropic and vasodilator effects.

1. This paper describes the cardiovascular effects of ICI 170777, a novel compound which enhances cardiac contractility and causes arterial and venous dilatation. 2. The positive inotropic effects of ICI 170777 on the heart were demonstrated by an increase in left ventricular dP/dtmax in the anaesthetized and conscious dog, and by an increase in tension development in isolated papillary muscles from the cat. 3. In the anaesthetized dog, the positive inotropic effects of ICI 170777 and of isoprenaline were attenuated by atenolol (5 mg kg-1, i.v.). Atenolol displaced the dose-response curve to ICI 170777 to the right by 4 fold but displaced the isoprenaline dose-response curve to the right by 247 fold. In vitro, however, atenolol (10 microM) had no significant effect on the positive inotropic response to ICI 170777. In the ganglion-blocked anaesthetized dog, infusion of a low dose of ICI 170777 which had no effect on the basal left ventricular dP/dtmax, selectively potentiated the positive inotropic effects of isoprenaline. These results indicate that ICI 170777 has both a non-adrenoceptor-mediated positive inotropic effect on the heart and also facilitates the beta-adrenoceptor-mediated control of contractility. 4. In the denervated and perfused hind-limb of the dog, ICI 170777 reduced arterial perfusion pressure and increased limb circumference at a constant arterial flow and venous pressure. This indicates that ICI 170777 has direct dilator actions on both arterial and venous vessels. In this preparation, diazoxide exerted an arterial selective vasodilator effect and sodium nitroprusside was a relatively selective venous dilator. ICI 170777 exhibited a balanced arterial and venous dilator effect which was intermediate in profile between that of diazoxide and that of sodium nitroprusside. 5. In the conscious dog, low doses (2-5 mgkg -, orally) of ICI 170777 evoked an increase in left ventricular dP/dt,,, with no significant effect on heart rate or blood pressure. At a higher dose (10mg kg 1, orally) it also reduced blood pressure and caused a significant increase in heart rate. The duration of the positive inotropic effect of 5mg kg- (orally) of ICI 170777 was 10-12 hours. This response did not diminish following repeated administration of the compound. 6. The positive inotropic action and balanced arterial and venous dilator effect of ICI 170777 indicate that the compound may be useful in the treatment of congestive heart failure, a disorder that is characterized by decreased cardiac contractility and enhanced arterial and venous constrictor tone.

Apparent affinity of some 8-phenyl-substituted xanthines at adenosine receptors in guinea-pig aorta and atria.

1 Some 8-phenyl-substituted, 1,3 dipropyl xanthines have previously been demonstrated to have a 20-400 fold greater affinity for A1 binding sites in rat CNS membranes than for A2 adenosine receptors in intact CNS cells from guinea-pigs. In the present study these compounds (1,3, dipropyl-8-phenylxanthine: DPPX; 1,3 dipropyl-8-(2 amino-4-chlorophenyl) xanthine: PACPX; 8-(4-(2-amino-ethyl)amino) carbonyl methyl oxyphenyl)-1,3-dipropylxanthine: XAC; and D-Lys-XAC) together with two that have not been reported to exhibit A1-receptor selectively (8-(p-sulphophenyl)theophylline: 8-PST; 8-(4-carboxy methyl oxyphenyl)-1,3-dipropylxanthine: XCC) have been evaluated as antagonists of the effects of 2-chloroadenosine in two isolated cardiovascular tissues. 2 The isolated tissues used were guinea-pig atria (bradycardic response) and aorta (relaxation), which are thought to possess A1 and A2 adenosine receptors, respectively. 3 All the xanthines antagonized responses evoked by 2-chloroadenosine in both tissues but did not affect responses evoked by acetylcholine (atria) or sodium nitrite (aorta). 4 The xanthines, 8-PST, XAC, D-Lys XAC, XCC and DPPX appeared to be competitive antagonists of the effects of 2-chloroadenosine, as Schild plot slopes did not differ significantly from unity. The 1,3-dipropyl substituted compounds had pA2 values from 6.5 to 7.4 and were more potent than the 1,3 dimethyl substituted 8-PST (pA2 4.9 to 5). 5 For individual xanthines, there was no difference between pA2 values obtained in the atria and in the aorta. 6 Responses evoked by 2-chloroadenosine in atria and aorta were antagonized to a similar degree by PACPX (1 microM). The agonist dose-ratio evoked by 10 microM PACPX was no greater than that evoked by 1 microM of the xanthine in both tissues. This was probably a consequence of the limited aqueous solubility of PACPX. 7. These results fail to demonstrate A, receptor selectivity for DPPX, XAC, D-Lys XAC or PACPX in tissues from the guinea-pig. The A, selectivity previously found for these compounds may therefore be due to their high affinity for binding sites in rat CNS cell membranes.

Effect of the adenosine antagonist 8-phenyltheophylline on glycerol-induced acute renal failure in the rat.

8-Phenyltheophylline (8-PT)(10 mg kg-1) or its vehicle(1 ml kg-1) were administered intravenously or intraperitoneally twice daily over 48 h to rats with acute renal failure (ARF) induced by intramuscular (i.m.) injection of glycerol. Rats treated with 8-PT i.v. had significantly lower plasma urea and creatinine levels at 24 and 48 h compared to untreated animals. The vehicle also reduced plasma urea and creatinine when compared to untreated controls. However, plasma urea levels in 8-PT-treated rats were significantly lower than in vehicle-treated animals at 24 and 48 h after both i.v. and i.p. administration. Plasma creatinine concentrations also tended to be lower in the 8-PT-treated group. [3H]-inulin clearance at 48 h after i.m. glycerol was significantly greater in rats dosed i.p. with 8-PT compared to either untreated or vehicle treated rats. Examination of kidneys taken from rats 48 h after i.m. glycerol showed that 8-PT treatment significantly reduced renal damage and kidney weight compared to the untreated or vehicle-treated groups. In a 7 day study all the rats which received 8-PT i.p. survived whilst in the vehicle and untreated groups the mortality rates were 12 and 21% respectively. In a separate series of experiments 8-PT (10 mg kg-1, i.v. or i.p.) was found to antagonize adenosine-induced bradycardia in conscious rats for up to 5 h. There is no clear explanation for the partial protection afforded by the vehicle but it may be related to either its alkalinity or an osmotic effect produced by the polyethylene glycol component. 9 The protective effect of 8-PT in rats with ARF was probably the result of adenosine antagonism.

Amelioration of glycerol-induced acute renal failure in the rat with 8-cyclopentyl-1,3-dipropylxanthine.

1. Previous studies have shown that 8-phenyltheophylline (8-PT), a non-selective antagonist at adenosine A1- and A2-receptors, can ameliorate the severity of glycerol-induced acute renal failure (ARF) in the rat. In the present study we have examined the effects of an antagonist with selectivity for adenosine A1-receptors (8-cyclopentyl-1,3-dipropylxanthine, CPX) on the development of ARF. 2. In the anaesthetised rat 8-PT (4 mg kg-1, i.v.) and CPX (0.1 mg kg-1, i.v.) antagonised adenosine-evoked responses which are thought to be mediated via A1-receptors (bradycardia and decrease in renal blood flow). The agonist dose-ratio produced by CPX was equal to or greater than that found with 8-PT (heart rate and renal blood flow respectively). The hypotensive response to adenosine which is predominantly due to A2-receptor activation was also antagonised by 8-PT, whereas CPX was a much less effective antagonist of this response. 3. Administration of CPX (0.1 mg kg-1, i.v.; twice daily for two days) significantly attenuated the increase in plasma levels of urea and creatinine, the increased kidney weight and the renal tubule damage observed in rats 2 days following induction of ARF with intramuscular glycerol injection. In addition treatment with CPX significantly enhanced the clearances of inulin and p-aminohippurate. 4. After glycerol injection, the mortality rate over 7 days in untreated and vehicle-treated rats was 43% and 21% respectively. In contrast, all animals treated with CPX survived over the 7 day observation period. 5. These results support the suggestion that adenosine is an important factor in the development of ARF and indicate that this effect of the purine is likely to be mediated via an adenosine A1-receptor.

Apparent affinity of 1,3-dipropyl-8-cyclopentylxanthine for adenosine A1 and A2 receptors in isolated tissues from guinea-pigs.

1. The classification of adenosine receptor subtypes (A1 and A2) in intact tissues has been based on the order of agonist potency. In this study the apparent affinity of 1,3-dipropyl-8-cyclopentylxanthine (CPX), an antagonist which has been reported to be A1 selective, and the non-selective antagonist 1,3-dimethyl-8-phenylxanthine (8PT) has been evaluated on isolated tissues from the guinea-pig. 2. The isolated tissues used were atria (bradycardic response, proposed A1 sub-type), aorta and trachea (relaxant response, proposed A2 sub-type). 3. Both the xanthines antagonized responses to adenosine in the three tissues but had little or no effect on responses to carbachol (atria), sodium nitrite (aorta) or isoprenaline (trachea). 4. pA2 values for 8PT were similar on the three tissues (6.3-6.7), however, the pA2 value for CPX on the atria (7.9-8.4) was greater than that on the aorta (6.6) or trachea (6.6). 5. These results support the suggestion that the adenosine receptors which mediate bradycardia in the atrium are of the A1 sub-type and that those which mediate relaxation in the aorta and trachea are of the A2 type.

Evidence that the positive inotropic effects of the alkylxanthines are not due to adenosine receptor blockade.

We investigated the possibility that the positive inotropic effects of the alkylxanthines are due to adenosine receptor blockade. The potency of 8-phenyltheophylline, theophylline and enprofylline as adenosine antagonists was assessed in vitro, using the guinea-pig isolated atrium, and in vivo, using the anaesthetized dog. The order of potency of the alkylxanthines as antagonists of the negative inotropic response to 2-chloroadenosine in vitro, and of the hypotensive response to adenosine in vivo was 8-phenyltheophylline greater than theophylline greater than enprofylline. The order of potency of the alkylxanthines as positive inotropic and chronotropic agents in the anaesthetized dog was enprofylline greater than theophylline greater than 8-phenyltheophylline. The results of this study indicate that the inotropic effects of the alkylxanthines in the anaesthetized dog are not due to adenosine receptor blockade.

Amelioration of cisplatin-induced acute renal failure with 8-cyclopentyl-1,3-dipropylxanthine.

1. The effect of the selective adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (CPX), on the development of cisplatin-induced acute renal failure was investigated in the rat. 2. CPX at doses of 0.03, 0.1 and 0.3 mg kg-1, i.v. caused increasing degrees of antagonism of adenosine-induced bradycardia in anaesthetized rats. The magnitude of antagonism was not directly proportional to the increment in dose, but for each dose, it was similar in rats injected with either saline or cisplatin. CPX at a dose of 0.03 mg kg-1 significantly antagonized adenosine-induced bradycardia for up to 2.5 h, while doses of 0.1 and 0.3 mg kg-1 produced significant blockade for periods longer than 5 h. 3. Administration of cisplatin (6 mg kg-1, i.v.) caused acute renal failure characterized by decreased inulin and p-aminohippurate clearances, increased urine volume but decreased excretion of Na+, K+ and Cl- ions and by increased plasma levels of urea and creatinine. Kidney weight was increased in cisplatin-treated rats and renal tubule necrosis occurred. 4. Administration of CPX (0.03 mg kg-1, i.v.; twice daily for two days) to rats given cisplatin did not reduce the severity of the resultant renal failure. However, treatment with 0.1 mg kg-1 CPX attenuated the increases in plasma creatinine/urea levels observed in rats on days 3 and 7 after induction of renal failure. In addition, this dose significantly reduced renal tubule damage and increased inulin and p-aminohippurate clearances. A similar pattern of protection was noted with CPX at a dose of 0.3 mg kg-1 although the increase in inulin clearance was not statistically significant. However, this higher dose of CPX significantly increased Na+ and K+ excretion compared to vehicle-treated rats. 5 CPX at doses of 0.03, 0.1 and 0.3 mgkg- produced blockade of an A1-receptor mediated response i.e. adenosine-induced bradycardia, but only treatment with the higher doses of CPX (0.1 and 0.3mgkg-1) ameliorated nephrotoxicity produced by cisplatin. The lack of any protective effect afforded by the lowest dose of CPX could be a result of its shorter duration of action.6. This study indicates that adenosine plays a significant role in the pathophysiology of cisplatin-induced acute renal failure.

Antidepressant drug action and presynaptic alpha-receptors.

Receptors have been demonstrated on the terminations of the sympathetic adrenergic nerves. One type, the so-called alpha 2-receptors, are activated by the norepinephrine that is released from the nerve terminals into the synaptic cleft; this activation causes a reduction in the output of the transmitter (negative feedback). Recent studies have demonstrated that certain antidepressant drugs can block these alpha 2-receptors and thus prevent their inhibitory action on the release of norepinephrine. If this occurs in the brain, the increases in norepinephrine levels could help explain the antidepressant action of these agents.

Adenosine relaxes the aorta by interacting with an A2 receptor and an intracellular site.

The purpose of this study was to determine whether the adenosine receptor that mediates relaxation of the noradrenaline-contracted guinea-pig aorta is of the A1 or A2 subtype. 5'-N-ethylcarboxamide adenosine (NECA) and 5'-N-cyclopropylcarboxamide adenosine (NCPCA) were about 100 times more potent as relaxants of the aorta than L-N6-phenylisopropyladenosine (L-PIA) and N6-cyclohexyladenosine. L-PIA was 3 times more potent than D-PIA. These relaxations were not altered by the purine transport inhibitor dipyridamole, but were attenuated by the cell surface adenosine receptor antagonist 8-phenyltheophylline. Adenosine and 2-chloroadenosine differed from NECA and NCPCA since they evoked greater maximal relaxations and their submaximal responses were less sensitive to blockade by 8-phenyltheophylline. These differences were abolished by dipyridamole which indicates that they were due to an intracellular action of adenosine and 2-chloroadenosine. The intracellular 'P-site' agonist, 9-beta-D-xylofuranosyladenine evoked small relaxations that were attenuated by dipyridamole but were unaffected by 8-phenyltheophylline. These results indicate that adenosine can relax the aorta via interactions with a cell surface A2 receptor and with an intracellular site.

Adenosine A1 receptor mediated inhibition of nerve stimulation-induced contractions of the rabbit portal vein.

The aim of this study was to determine whether the adenosine receptor that inhibits adrenergic neurotransmission in the rabbit portal vein is of the A1 or the A2 subtype. Isometric contractions of the isolated vein were evoked by electrical field stimulation and by exogenous noradrenaline. Low concentrations of adenosine, and a number of analogues inhibited the response evoked by field stimulation but had no effect on those evoked by noradrenaline. The order of inhibitory potency was: L-N6-phenylisopropyladenosine (L-PIA) = N6-cyclohexyladenosine (CHA) = 5'-N-cyclopropylcarboxamide adenosine (NCPCA) greater than or equal to 5'-N-ethylcarboxamide adenosine (NECA) = 2-chloroadenosine greater than adenosine greater than D-PIA. The difference in potency between the stereoisomers L- and D-PIA was about 60 fold. The purine transport inhibitor dipyridamole potentiated the inhibitory effect of adenosine but not that of its analogues. The inhibitory responses evoked by adenosine and its' analogues were attenuated by the adenosine antagonist theophylline. These results indicate that adenosine selectively inhibits contractions of the rabbit portal vein evoked by adrenergic nerve stimulation via activation of an adenosine A1 receptor.

Can ATP stimulate P1-receptors in guinea-pig atrium without conversion to adenosine?

The aim of this study was to determine whether ATP must be hydrolysed to adenosine in order to activate the P1-purinoceptor. Isometric contractions of electrically paced guinea-pig isolated left atria were recorded. Purines evoked negative inotropic responses that were competitively antagonised by theophylline. The order of agonist potency was 2-chloroadenosine greater than adenosine greater than beta, gamma-methylene ATP greater than ATP. Adenosine deaminase alone, or combined with 5'-nucleotidase, attenuated responses to adenosine and 5' AMP, respectively, but did not decrease those to ATP or beta, gamma-methylene ATP. Inhibition of 5'-nucleotidase did not alter responses to ATP. Dipyridamole potentiated responses to ATP both in the absence and in the presence of adenosine deaminase. Alpha, beta-methylene ATP had little agonist activity, however this was not due to its resistance to hydrolysis as the stable beta, gamma-methylene isostere of ATP was a potent agonist. These results indicate that hydrolysis of ATP to adenosine or 5' AMP is not a pre-requisite for activation of the P1-receptor in the guinea-pig atrium.