Inhibition by purines of the inotropic action of isoprenaline in rat atria

Effects of adenosine and isoprenaline in left atria from both neonatal and middle-aged noninsulin-dependent diabetic rat models

1. This study examined the ability of atria from neonatal and middle-aged noninsulin-dependent diabetic rat models to respond to both adenosine and isoprenaline. 2. Cumulative additions of adenosine (1-1000 microM) produced concentration-dependent decreases in the force of contraction of rat atria that were unchanged in neonatal diabetic animals. Although direct inotropic responses to adenosine were unchanged, atria from neonatal diabetic animals exhibited an increase in maximum response to adenosine-induced antiadrenergic effect. 3. Atria from middle-aged noninsulin-dependent diabetic rats exhibited a supersensitivity to the direct inotropic effect of adenosine compared with atria from age-matched control rats. The middle-aged, noninsulin-dependent diabetic state did not alter the maximum response of atria to adenosine-induced antiadrenergic effect. 4. A comparison was made between middle-aged (10-month-old) controls and young (4-month-old) controls. Atria from middle-aged control animals exhibited a lower sensitivity and responsiveness to the direct inotropic effect of adenosine compared with those from young controls. 5. Cumulative additions of isoprenaline (10(-9)-10(-6) M) produced concentration-dependent increases in inotropy that were unchanged in atria from either neonatal or middle-aged noninsulin-dependent diabetic rats. 6. These results show that neonatal and middle-aged noninsulin-dependent diabetes and age-related factors lead to significant changes in atrial reactivity to the adenosine-induced stimulation in the absence and presence of isoprenaline. However; isoprenaline-induced positive inotropic response cannot change in each diabetic heart to an apparent extent.

Flunarizine but not theophylline modulates inotropic responses of the isolated rat heart to diazepam

Diazepam (2 x 10(-5)-6 x 10(-4) M) induced a concentration-dependent positive inotropic effect on the perfused rat heart which was preceded by a transient concentration-dependent negative inotropic response. The influence of the Ca(2+)-entry blocking drug, flunarizine, and the adenosine receptor blocking drug, theophylline on these inotropic responses was studied. Flunarizine in concentrations of 10(-9)-10(-6) M antagonized the positive inotropic response to diazepam significantly; the negative inotropic response was reduced as well. At the lower concentrations of diazepam the negative inotropic response was completely abolished in the presence of flunarizine. The actions of the Ca(2+)-entry blocker were related to the concentrations used. Theophylline in concentrations up to 5 x 10(-5) M did not interfere with either inotropic response to diazepam. The results suggest that Ca2+ currents in the myocardium are involved with the response of the isolated heart to diazepam. It is concluded that the finding that the negative inotropic effect of diazepam was almost abolished by flunarizine suggests that the site of this response most be associated with Ca(2+)-current mechanisms.

Adenosine and ventricular automaticity

Adenosine is considered a cardiodepressant agent due to its negative chronotropic and inotropic effects. However, the effect of adenosine on ventricular automaticity is less well established since both an increase and a decrease in ventricular automaticity have been reported. The experimental and clinical evidence dealing with the effects of adenosine on ventricular automaticity as well as the possible mechanisms involved, is presented in this review.

Mechanisms of adenosine-mediated actions on cellular and clinical cardiac electrophysiology

OBJECTIVE

To provide insights into the molecular mechanisms of adenosine-mediated cardiac cellular electrophysiology and how information about these mechanisms can be used to facilitate diagnostic and therapeutic approaches to various clinical arrhythmias.

DESIGN

A review of (1) adenosine metabolism and receptors in the cardiac system, (2) adenosine-mediated signal transduction pathways in the regulation of cellular electrophysiology in various cardiac cell types, and (3) the clinical usefulness of adenosine in cardiac electrophysiology is presented.

RESULTS

The effects of adenosine on cardiac electrophysiologic properties are consequences of complex interactions among the specific cardiac target structures, the density and type of adenosine receptors, and the effector systems. The easy application of adenosine and its short half-life, favorable side-effects profile, and electrophysiologic properties make it an excellent diagnostic and therapeutic tool for the initial assessment of various tachyarrhythmias.

CONCLUSION

The direct adenosine-activated KACh (potassium acetylcholine) channel signal transduction system explains the effects of adenosine on the sinus node, atrioventricular node, and atrial myocardium. The indirect adenosine-inhibited adenylate cyclase system accounts for its negative inotropic effects on the catecholamine-entrained contractility in atrial and ventricular myocardium. Because of the recent purification and cloning of adenosine receptors and subunits of G proteins, additional adenosine-mediated electrophysiologic mechanisms can be explored.

Do adenosine A3 receptors exist?

1. It has been suggested that adenosine A1 receptors may be sub-divided into A1 and A3 types, based on the relative potencies of 5'-N-ethylcarboxamidoadenosine (NECA) and selected N6-substituted adenosine analogues. At A1 receptors (rat adipocytes) N6-phenylisopropyladenosine (PIA) was reported to be approx. 100-fold more potent than NECA, whereas the compounds were equipotent at A3 receptors (those in cardiac and neuronal preparations). 2. The study reported here has systematically evaluated this proposal, the rank orders of potency of NECA, R- and S-PIA, N6-cyclopentyladenosine (CPA) and N6-cyclohexyladenosine (CHA) being determined in rat adipocytes, guinea-pig ileum and rat and guinea-pig atria. 3. R-PIA, CHA and CPA were found to have consistent potencies relative to NECA across all 6 tissues, including rat adipocytes. The rank order was CPA greater than or equal to CHA, R-PIA greater than or equal to NECA greater than S-PIA. 4. We conclude that the relative potencies of these agonists do not support the concept that adenosine A1 receptors in rat adipocytes differ from those in neuronal and cardiac tissues.

Pharmacological profile of the 2-alkynyladenosine derivative 2-octynyladenosine (YT-146) in the cardiovascular system

We investigated the cardiovascular effects of 2-octynyladenosine (YT-146), an adenosine A2 agonist, in various mammalian preparations in comparison with adenosine and 2-chloroadenosine. YT-146, when intravenously administered, caused a dose-dependent decrease of blood pressure in anesthetized normotensive rats (with ED30 values of 0.4 micrograms/kg), and YT-146 was 250 times more potent than adenosine. Whereas adenosine and 2-chloroadenosine decreased heart rate at approximately equihypotensive doses, YT-146 had no negative chronotropic effects at h hypotensive doses. Orally given YT-146 (0.1 - 1 mg/kg) produced a potent and long-lasting antihypertensive effect in spontaneously hypertensive rats. YT-146 was 15.9 and 12.5 times more potent than adenosine in producing relaxation of isolated porcine coronary arteries and in increasing dog coronary blood flow, respectively. Although YT-146 was equipotent to adenosine in causing a negative inotropic effect in isolated guinea pig atria, it was less potent than adenosine in producing atrioventricular conduction block in guinea pigs. On the other hand, 2-chloroadenosine was 9.1, 1.8 and 2.4 times more potent than adenosine in lowering blood pressure, relaxing isolated porcine coronary arteries and increasing dog coronary blood flow, respectively. 2-Chloroadenosine was the most potent in producing cardiodepression, i.e., negative inotropy and atrioventricular conduction block in guinea pigs. From these results, we concluded that YT-146 is a potent coronary vasodilator and also a potent, orally active and long-acting hypotensive agent having less cardiac depressant activity.

Effect of endothelin on total and regional coronary resistance and on myocardial contractility

Endothelin is a 21-amino acid peptide produced by the endothelium and has a potent vasoconstrictor effect. Because of the importance of the endothelium on vasomotor regulation, we studied the effect of endothelin on total and regional coronary vascular resistance and on myocardial contractility in the intact heart of anesthetized dogs. Intracoronary administration of 2 to 80 pmol/kg of endothelin produced a dose-dependent increase in coronary resistance, ischaemic decrease in myocardial contractility and atrium-ventricular blockade. The increase in resistance was greater towards the outer layer of the left ventricular wall. When the coronaries were perfused at a constant rate and vasoconstriction was prevented with adenosine or nitroglycerine, endothelin did not produce inotropic changes. These results show that endothelin is a potent vasoconstrictor of the resistance coronary vessels, producing a redistribution of transmural blood flow and a decrease in myocardial contractility secondary to ischaemia.

Physiological and pharmacological properties of adenosine: therapeutic implications

Adenosine is a nucleoside which has been shown to participate in the regulation of physiological activity in a variety of mammalian tissues, and has been recognized as a homeostatic neuromodulator. It exerts its actions via membrane-bound receptors which have been characterized using biochemical, electrophysiological and radioligand binding techniques. Adenosine has been implicated in the pharmacological actions of several classes of drugs. A number of studies strongly suggest that the nucleoside may regulate cellular activity in many pathological disorders and, in that respect, adenosine derivatives appear as promising candidates for the development of new therapeutic compounds, such as anticonvulsant, anti-ischemic, analgesic and neuroprotective agents.

Effects of 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a highly selective adenosine receptor antagonist, on force of contraction in guinea-pig atrial and ventricular cardiac preparations

The effects of the A1 adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) on force of contraction were examined in isolated electrically driven auricles and papillary muscles from guinea-pigs in the absence and presence of (-)-N6-phenylisopropyladenosine (PIA) and 5'-N-ethylcarboxamidadenosine (NECA). In auricles DPCPX (30-1000 mmol/l) alone increased force of contraction. DPCPX produced only a minor inhibition of phosphodiesterase I-III activity. PIA and NECA alone exerted concentration-dependent negative inotropic effects and the concentration-response curves for PIA and NECA were shifted competitively to the right by the adenosine receptor antagonist DPCPX with similar potency and efficacy. The pA2-value for the inhibition of the effects of PIA and NECA were 9.1 and 8.8, respectively. In papillary muscles DPCPX alone had no inotropic effect. In the presence of isoprenaline PIA and NECA alone exerted concentration-dependent negative inotropic effects and again DPCPX shifted the concentration-response curves for PIA and NECA competitively to the right with similar potency and efficacy. The pA2-value for the inhibition of the effects of PIA and NECA were 9.3 and 9.0, respectively. It is concluded that DPCPX is a potent competitive A1 adenosine receptor antagonist in guinea-pig atrial and ventricular cardiac preparations. Since PIA and NECA were equally potent the cardiac adenosine receptor may constitute a subtype of A1 adenosine receptors differing from the receptor in other tissues such as fat cells. Furthermore, DPCPX has a positive inotropic effect in atrial tissue which cannot be attributed to the A1 receptor antagonism.

Pronounced cholinergic but only moderate purinergic effects in isolated atrial and ventricular heart muscle from cats

1. The effects of cholinergic and purinergic stimulation on action potential, force of contraction and 86Rb efflux were investigated in cat atrial and/or ventricular heart muscle. 2. Acetylcholine and carbachol exerted a concentration-dependent negative inotropic effect in cat atrial heart muscle. Carbachol 10 mumol l-1 completely abolished the force of contraction and increased the rate constant of 86Rb efflux 2-3 fold, whereas the action potential duration was shortened to about 1/10 of its length under control conditions. 3. The effects of acetylcholine and carbachol in cat atrial heart muscle were mimicked, qualitatively, by adenosine and its analogues 5'-(N-ethyl)-carboxamido-adenosine (NECA) and (-)-N6-(R-phenyl-isopropyl)-adenosine (R-PIA). Maximal purinergic effects, however, amounted to about 15-50% in comparison to those of cholinergic stimulation. 4. In cat ventricular heart muscle, cholinergic or purinergic stimulation had no significant effects on the force of contraction in the absence of a cyclic AMP-dependent positive inotropic effect. Carbachol antagonized the positive inotropic effect elicited by either 3-isobutyl-1-methylxanthine, isoprenaline or cyclic 8-(4-chlorphenylthio)adenosine-3':5'-monophosphate; NECA and R-PIA were less effective. The inhibition by carbachol of the effects of isoprenaline was not related to a change in the rate constant of 86Rb efflux. 5. It is concluded that the effects of cholinoceptor and purinoceptor agonists in the cat heart involve a change in the potassium conductance in the atrium, whereas the effects in the ventricle may be related to changes of intracellular cyclic AMP levels. It seems reasonable to assume that, in comparison to cholinergic stimulation, a low density of purinoceptors in the cat heart is responsible for the relatively weak effects of adenosine agonists in this species.

On the adenosine receptor and adenosine inactivation at the rat diaphragm neuromuscular junction

1. The effects of adenosine and adenosine analogues 2-chloroadenosine (CADO), L-N6-phenylisopropyladenosine (L-PIA), D-N6-phenylisopropyladenosine (D-PIA), N6-cyclohexyladenosine (CHA) and 5'-N-ethylcarboxamide adenosine (NECA) on evoked endplate potentials (e.p.ps) and on twitch tension were investigated in innervated diaphragms of the rat. 2. Adenosine and its analogues decreased, in a concentration-dependent manner, the amplitude of both the e.p.ps and the twitch responses evoked by nerve stimulation. The order of potency in decreasing the twitch tension was CHA, L-PIA, NECA greater than D-PIA greater than CADO greater than adenosine. L-PIA was about 8 times more potent than D-PIA. Neither adenosine nor the adenosine analogues affected the twitch responses of directly stimulated tubocurarine-paralysed muscles. 3. 8-Phenyltheophylline (8-PT), theophylline and isobutylmethylxanthine (IBMX), in concentrations virtually devoid of effect on neuromuscular transmission, antagonized the inhibitory effect of 2-chloroadenosine. The order of potency of the alkylxanthines as antagonists of the adenosine receptor at the rat diaphragm neuromuscular junction was 8-PT greater than IBMX greater than theophylline. The antagonism by these xanthines was shown to be competitive, the pA2 value for 8-PT being 7.16. In concentrations slightly higher than those used to test its ability to antagonize the adenosine receptor, IBMX and 8-PT increased the amplitude of e.p.ps without modifying their decay phase or the resting membrane potential of the muscle fibre. 4. The adenosine uptake inhibitor, nitrobenzylthioinosine (NBI) and the adenosine deaminase inhibitor, erythro-9(2-hydroxy-3-nonyl)adenine (EHNA), in concentrations virtually devoid of effect on neuromuscular transmission, potentiated the inhibitory effect of adenosine at the rat diaphragm neuromuscular junction. The potentiation factors were about 2.6 for NBI (5 microM), 2.2 for EHNA (25 microM) and 4.6 for the combination of NBI (5 microM) and EHNA (25 microM). 5. It is concluded that both uptake and deamination contribute to the inactivation of adenosine at the rat diaphragm neuromuscular junction and that in this preparation the inhibitory effect of adenosine on transmission is mediated by a xanthine-sensitive adenosine receptor with an agonist profile which does not fit the criteria for its classification either as an A1 or A2-adenosine receptor.

Adenosine inhibits the positive inotropic effect of 3-isobutyl-1-methylxanthine in papillary muscles without effect on cyclic AMP or cyclic GMP

1 Adenosine and the adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA) produced a small positive and negative inotropic effect, respectively, in isolated electrically driven papillary muscles of guinea-pigs. 2 Adenosine (100 mumol l-1) had no effect on cyclic AMP or cyclic GMP content. PIA (100 mumol l-1) slightly increased cyclic AMP. 3 In the presence of 3-isobutyl-1-methylxanthine (IBMX; 60 mumol l-1), which increased force of contraction 2 fold, adenosine and PIA exerted strong negative inotropic effects. PIA was more potent than adenosine (mean IC25 2.1 and 168 mumol -1, respectively). 4 In contrast, the nucleosides did not affect the increase in force of contraction produced by elevating extracellular Ca2+ concentration. 5 The IBMX-antagonistic effects of adenosine and PIA were not accompanied by modification of the IBMX-induced increase in cyclic AMP and cyclic GMP. 6 The effects of adenosine and PIA on force of contraction were accompanied by a partial reversal of the IBMX-induced increase in the maximal rate of depolarization of slow action potentials. 7 It is concluded that adenosine and PIA are able to attenuate the positive inotropic effect of a phosphodiesterase inhibitor. This effect is unlikely to be due to a reduction of the IBMX-induced increase in cyclic AMP content. It is conceivably due to an inhibition of the stimulant action of cyclic AMP on slow Ca2+ channels leading to the reduction of the slow inward current which in turn reduces force of contraction.

[Cardiac effects of adenosine. Mechanism of action, pathophysiologic and clinical significance]

Adenosine has a negative inotropic effect in cardiac atrial preparations ("direct" negative inotropic effect). This effect is probably due to an activation of a potassium outward current which shortens the action potential duration and hence reduces the force of contraction. A pertussis toxin-sensitive N-protein is involved in the signal transduction from the adenosine receptor to atrial potassium channels. In ventricular cardiac preparations adenosine has no negative or even a weak positive inotropic effect, but it reduces the force of contraction in the presence of cAMP-increasing agents such as isoprenaline ("indirect" negative intropic effect). This effect is due to an inhibition of the slow Ca2+ inward current which has previously been enhanced by an increase in the cellular cAMP content. This "indirect" negative inotropic effect of adenosine is also present in the human heart. Since increased amounts of adenosine are released during cardiac stimulation via beta-adrenoceptors, the "indirect" effect might protect the heart against excessive stimulation by catecholamines. In addition, adenosine has negative chronotropic actions and prolongs AV conduction by an activation of potassium channels or an inhibition of the slow Ca2+ inward current (AV node). Cardiac bradyarrhythmias in hypoxia have been attributed to an increased formation and release of adenosine. Furthermore, adenosine has been shown to terminate supraventricular tachycardias involving the AV node. Since it has a very short duration of action it might prove safe and hence advantageous to conventional therapy in the treatment of supraventricular tachycardias.

Inhibition of renin release by analogues of adenosine in rabbit renal cortical slices

Renal cortical slices obtained from male New Zealand rabbits were used to investigate the role of adenosine in the regulation of renin release. Isoproterenol produced a significant (p less than 0.01), twofold to threefold increase in renin release, that was both dose-dependent and time-dependent. Addition of either the l-phenylisopropyl or the N6-ethylcarboxamido derivative of adenosine attenuated this stimulation at concentrations as low as 10(-9) M or 10(-8) M, respectively. Higher doses of d-phenylisopropyladenosine (10(-6) M) or adenosine (10(-5) M) were necessary to significantly reduce the beta-adrenergic response (p less than 0.01). Inhibition was absent in slices preincubated with 10(-5) M 8-phenyltheophylline, a concentration that had no effect on either basal or stimulated renin release. The site of inhibition appeared to be distal to beta-adrenergic and prostaglandin receptors since l-phenylisopropyladenosine (10(-8) M) blocked stimulation by selective beta-adrenergic receptor agonists, prenalterol (10(-6) M) or salbutamol (10(-5) M), and by prostaglandin E1. These data suggest that adenosine and its analogues inhibit renin release and that this inhibition may be mediated by a receptor-dependent action on a common point in the pathway leading to release.

Comparative studies with the enantiomers of the glycol metabolite of propranolol and their effects on the cardiac beta-adrenoceptor

The two enantiomers ((R)- and (S)-) of propranolol glycol, a metabolite of propranolol, have been synthesized, and their effects upon the beta-adrenoceptor studied by two methods. The ability of these compounds to antagonize the inotropic actions of isoprenaline was examined on spontaneously beating rat atrial preparations. Also, the effects of these enantiomers upon the binding of [3H]dihydroalprenolol to beta-receptors in rat cardiac ventricular membranes was studied. Experiments with the atria indicated that the (S)-glycol was a reversible competitive antagonist of isoprenaline with a potency approximately one thousand times lower than that of (+/-)-propranolol. In contrast, the (R)-glycol appeared to act as an irreversible antagonist, producing complex dose-response curves. The effects of these compounds to cause displacement of alprenolol binding were consistent with the organ bath data. The interaction of the (S)-glycol with the beta-receptor binding site was reversible (Ki of 27.6 +/- 4.2 microM) but less potent than that of (+/-)-propranolol (Ki of 0.99 +/- 0.07 nM). On the other hand, pretreatment of ventricular membranes with the (R)-glycol, followed by extensive washing techniques, resulted in alprenolol binding which did not regain control values, providing further evidence for an irreversible effect upon the beta-receptor. The possible significance of these pharmacological actions of the two enantiomers is discussed in terms of the in vivo metabolic pathways for propranolol.

Antagonism between (-)-N6-phenylisopropyladenosine and the calcium channel facilitator Bay K 8644, on guinea-pig isolated atria

Antagonism between (-)-N6-phenylisopropyladenosine (PIA) and the dihydropyridine calcium channel facilitator Bay K 8644 was investigated in guinea-pig spontaneously beating or electrically driven isolated atria, taken from normal and from reserpine-treated animals. PIA (3-100 nM) produced a dose-dependent decrease in contractile tension and frequency in spontaneously beating atria being more effective in reserpinized preparations. Bay K 8644 (5-200 nM) produced an increase in contractile tension in both normal and reserpinized atria. In electrically driven left atria the positive inotropic effect of Bay K 8644 was similar to that in spontaneously beating preparations. The positive chronotropic effect of Bay K 8644 was slight and variable. PIA produced a rightward parallel shift of the concentration-response curves for the positive inotropic effects of Bay K 8644 in all experimental conditions. In spontaneously beating atria from normal guinea-pigs, the Schild regression plot was linear and its slope near to unity; pA2 of PIA 8.63 +/- 0.05 (IC50 2.35 +/- 0.25 nM). In electrically driven atria the antagonism by PIA of the effects of Bay K 8644 was apparently competitive, and the IC50 of PIA was 18.6 +/- 0.4 nM. PIA antagonized the positive chronotropic effect of Bay K 8644 in spontaneously beating preparations, both from normal and from reserpine-treated animals. Carbachol did not modify the positive inotropic effects of Bay K 8644. These data indicate that PIA may interact with Bay K 8644 at the level of the slow calcium channels, and may decrease the transmembrane calcium flux into the cell.

Evidence for adenosine receptor-mediated isoprenaline-antagonistic effects of the adenosine analogs PIA and NECA on force of contraction in guinea-pig atrial and ventricular cardiac preparations

The effects of the adenosine agonists (-)-N6-phenylisopropyladenosine (PIA) and 5'-N-ethylcarboxamideadenosine (NECA) on force of contraction, adenylate cyclase activity and normal as well as slow action potentials were studied in guinea-pig isolated atrial (left auricles) and ventricular preparations (papillary muscles). In auricles PIA and NECA exerted concentration-dependent negative inotropic effects with similar potencies (mean EC50:0.05 mumol l-1 for PIA and 0.03 mumol l-1 for NECA). Similar results were obtained in the presence of isoprenaline. In papillary muscles PIA and NECA alone had no effect on force of contraction but produced negative inotropic effects in the presence of isoprenaline (mean EC50:0.19 mumol l-1 for PIA and 0.10 mumol l-1 for NECA). In both preparations, the negative inotropic effects of PIA and NECA in the presence of isoprenaline were antagonized by the adenosine receptor antagonist 8-phenyltheophylline. In both preparations, PIA and NECA did not affect adenylate cyclase activity, both in the absence and presence of isoprenaline. In auricles the negative inotropic effects of both nucleosides were accompanied by a shortening of the action potential. This effect was also observed in the presence of isoprenaline. In papillary muscles the adenosine analogs did not detectably alter the shape of the normal action potential. Ca2+-dependent slow action potentials elicited in potassium-depolarized preparations also remained unaltered in the presence of PIA or NECA alone. However, the isoprenaline-induced enhancement of the maximal rate of depolarization of slow action potentials was attenuated by PIA or NECA.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Demonstration of Ri-type adenosine receptors in bovine myocardium by radioligand binding

Adenosine has been shown to have negative inotropic, chronotropic and dromotropic effects on the heart. The pharmacological profiles of these effects suggest that they are mediated via Ri (A1) adenosine receptors, but a direct demonstration of these receptors is still missing. In the present study we report direct labelling of these receptors with (-)N6-[125I]-p-hydroxyphenylisopropyladenosine [( 125I]HPIA)1. The radioligand bound in a saturable and reversible manner to a crude membrane preparation, the Bmax-value was 30.5 fmol/mg protein and the KD-value 1.1 nmol/l. A similar affinity of the ligand was obtained in kinetic and competition experiments. Competition experiments with a variety of adenosine analogues gave a pharmacological profile characteristic of Ri adenosine receptors with high affinities of N6-substituted derivatives and a marked stereospecificity for N6-phenylisopropyladenosine (PIA). Purification of the membrane preparation by density gradient centrifugation resulted in a 30-fold increase in the number of binding sites which was paralleled by a similar increase in the number of binding sites for [3H]ouabain. Guanine nucleotides decreased binding of [125I]HPIA in a dose-dependent manner, but the IC50-values were considerably higher than those reported in other tissues. Finally, binding of [125I]HPIA appeared to be entropy-driven which has been shown to be characteristic of agonist binding to Ri adenosine receptors. These results suggest the presence of Ri adenosine receptors in ventricular myocardium which may be responsible for the mediation of the effects of adenosine and its analogues.

Electrophysiologic effects of adenosine triphosphate and adenosine on the mammalian heart: clinical and experimental aspects

Adenosine triphosphate (ATP) and adenosine have strong negative chronotropic and dromotropic effects on the mammalian heart. The sensitivity of the sinus node and the atrioventricular node to ATP and adenosine manifests pronounced variability among species. For more than three decades, ATP has been used routinely in Europe in the acute therapy of paroxysmal supraventricular tachycardia. Preliminary clinical trials with adenosine in the United States suggest that this compound may have a similar therapeutic value. The exact mechanisms of action of ATP and adenosine on the mammalian heart are still not fully known. However, the vast clinical experience indicates that ATP, and probably also adenosine, can be safely and repetitively used in the acute therapy of paroxysmal supraventricular tachycardia.