Cardiothoracic Institute Workshop Series

Transcription

1 Postgraduate Medical Journal (1988) 64, Cardiothoracic Institute Workshop Series Research into lung and heart disease is rapidly accelerating and it is proving increasingly difficult to keep abreast of modern developments, particularly in basic science. We have instituted a series of one day workshops at the Cardiothoracic Institute which will be devoted to topics relevant to pulmonary and cardiac diseases. We will focus on areas of research in which there is active involvement and are particularly keen to link common areas of research in lung and heart disease. We have tried to stress the links between basic science and clinical research. The workshops are open and anyone with a relevant research interest is encouraged to attend and to actively participate. The Postgraduate Medical Journal has generously agreed to publish an introduction and the abstracts from each workshop. We hope this will provide a useful overview and summary of current research, and will also be a useful source of key references. The first workshop was held on 30th March, 1988 on the subject of adenosine. The meeting proved to be a great success, with much lively and informal discussion, which we are certain will lead to new ideas and new liaisons. Adenosine is a particularly appropriate subject, since it may be involved in regulation of cardiac function, blood flow, and vasodilatation, and is a possible mediator in asthma. Furthermore, adenosine may be useful in the therapy of certain arrhythmias and pulmonary hypertension. The workshop was superbly organised by Drs Michael Coupe and Bernard Clarke, who have also written the introduction and edited the abstracts. We hope that future workshops will cover such topics as platelet-activating factor, heart-lung transplantation, neuropeptides and eosinophils. Further information about these workshops may be obtained from the Postgraduate Secretary, Cardiothoracic Institute, London SW3. (Tel: ext 8003). Paul Oldershaw, M.A., M.R.C.P. Consultant Cardiologist Peter J. Barnes, M.A., D.M., D.Sc., F.R.C.P. Professor of Thoracic Medicine Series Editors Preface Adenosine in Heart and Lung Disease Proceedings of the first Cardiothoracic Institute Workshop, held on 30 March 1988 at the Cardiothoracic Institute, Brompton Hospital, London Interest in purine physiology and pharmacology began in 1929 when Drury and Szent-Gyorgi undertook a series of experiments using a tissue extract, the active ingredient of which was identified as adenosine.1 When injected, adenosine was able to produce slowing of the sinus rate of the heart of several species (guinea-pig, rabbit, cat) and in addition conduction through the atrioventricular node in the guinea-pig heart was impaired. Adenosine restored sinus thythm in the dog when atrial fibrillation was experimentally induced. A number of other observations were made by these workers which were later to be of great interest to physiologists and physicians alike. Adenosine had differential effects on the force of contraction of atrial muscle and ventricular muscle. Coronary blood flow was increased in the dog heart following injection of adenosine and there were effects in other organ systems including inhibition of small intestinal movements, sedation, and reduction of renal blood flow and urine output. In the 1930s several groups2'3 showed that adenosine transiently slowed the ventricular rate due to impairment of atrioventricular conduction in patients with chronic atrial flutter or fibrillation, but was unable to restore sinus rhythm. The doses employed in these studies were considerably greater than are now known to be necessary for the use of adenosine as an antiarrhythmic agent. Subsequently, it was documented that adenosine triphosphate (ATP), the parent nucleotide, could produce bradycardia and hypotension.4 The investigation of the physiological and biochemical aspects of the purines received little attention until 1963, when Robert Berne suggested that inosine and hypoxanthine, both metabolites of adenosine, were released D The Fellowship of Postgraduate Medicine, 1988

2 from the ischaemic myocardium and proposed that adenosine formation was a homeostatic mechanism for matching local tissue flow to local metabolic demand.5 The next major step in the understanding of purine physiology occurred when Burnstock proposed that separate purine receptors were present in various tissues throughout mammalian organs.6 These were classified into P1 and P2 receptors. These two cell surface purine receptors differ in their affinity for adenosine. The Pi receptor has a preference for adenosine (and is competitively inhibited by methylxanthines) and the P2 receptor for ATP. However, there are also at least two classes of extracellular receptors involved in the action of adenosine. Inhibition and stimulation of adenylate cyclase by adenosine lead to the subdivision of the Pi receptor into Al and A2 receptors respectively.7 CARDIOTHORACIC WORKSHOP 821 The agonist activity of both types of receptor depends upon the integrity of the ribofuranose ring, leading to a further alternative classification of Ra (stimulatory) and R, (inhibitory) receptors, reflecting activation or inhibition of adenylate cyclase activity.8 Although the Al and A2 receptors can be defined by these methods, an alternative scheme for their definition is based on a relative potency series of adenosine and its analogues, notably L-phenylisopropyladenosine (PIA) and 5'-N-ethylcarboxamide adenosine (NECA), which exhibit relatively greater selectivity for the Al and A2 receptors respectively.9 The agonist potency definition has some advantages over the adenylate cyclase classification, as adenosine receptors may be coupled to other second messenger systems Although there has been continuous debate in the literature over the appropriate nomenclature for the definition of adenosine receptors, the Al and A2 classification is established at present. This seems likely to remain so until protein sequence data and gene structure analysis of the receptors allows direct delineation and comparison of the different receptors at the structural level. Both Al and A2 receptors are widely distributed throughout the body. Adenosine may play a role in the pathophysiology of human diseases. It is bronchoconstrictor in asthmatics but not in normal subjects. 12 The bronchoconstriction produced is reversed by methylxanthines.13 Adenosine injection reproduces the pain of duodenal ulceration in patients with endoscopically proven ulcers,14 and one group has suggested that adenosine may also mimic the pain of angina pectoris in normal individuals." Asphyxia occurs in around 1% of all deliveries and the degree of brain damage which ensues is related to the urinary excretion of hypoxanthine,16 although to what extent the hypoxanthine is derived from degraded adenosine in such circumstances is not known. The depression of ventilation caused by adenosine may well be responsible for the apnoea associated with hypoxia at birth,'7 although paradoxically adenosine causes tachypnoea when administered exogenously. 18 Indeed, theophylline treatment has been shown to be beneficial in animal models of apnoea. 9 The intriguing suggestions that abnormalities of adenosine feedback or metabolism might play a role in the pathophysiology of cardiovascular diseases as diverse as hypertrophic cardiomyopathy20 and the 'sick sinus' syndrome21 have yet to receive full investigation. There is a clear distinction to be drawn between the effects of adenosine when given by continuous intravenous infusion compared to when the compound is administered as an intravenous bolus. Adenosine infusion into normal subjects causes dose-related increases in heart rate and systemic vasodilatation, accompanied by minor systemic hypotension.22 Adenosine infusion has been used to induce and maintain controlled hypotension during neurosurgical operations for cerebral artery aneurysms,23 without either tachyphylaxis or rebound hypertension, possibly related to the inhibition by adenosine of renin release.24'25 The dose required ( yg/kg/min) is approximately three times the maximum whch may be comfortably tolerated by conscious subjects. In addition, adenosine reduces platelet aggregation in vitro26 and has been used to reduce platelet consumption during cardiopulmonary bypass in man.27 This may be of considerable therapeutic importance, as, during extracorporeal circulation for open heart surgery, up to 40% of circulating platelets may be lost. Finally, the use of adenosine as an antiarrhythmic agent has received considerable recent attention. This is to date the major area of therapeutic importance, as adenosine is useful both for the diagnosis28 and treatment of cardiac arrhythmias,29'30 particularly when the AV node is involved in their genesis (for example, atrioventricular reentry tachycardias associated with the Wolff-Parkinson-White syndrome). Slowing of the sinus rate and impairment of atrioventricular nodal conduction means that although adenosine may not terminate arrhythmias of atrial origin, for example atrial flutter, it may slow the ventricular response such that an accurate diagnosis of the underlying arrhythmia may be made. The very short half-life of adenosine3' means that it may be safely given repeatedly as required. Recent reports of the termination of 'catecholamine-mediated' ventricular tachycardia by

3 822 CARDIOTHORACIC WORKSHOP adenosine32 raise an interesting therapeutic use for adenosine, but also add weight to the supposition that the effects of adenosine are mediated by alterations in cyclic AMP accumulation in the myocardium. Recently, the reversal by aminophylline of atropine resistant atrioventricular (AV) block following acute myocardial infarction in man has been reported,33 which experimentally is associated with hypoxia and depressed AV nodal function, suggesting a possible therapeutic option for atropine-resistant AV block, namely the use of adenosine antagonists. Contributors PROFESSOR P.J. BARNES, Department of Thoracic Medicine, Cardiothoracic Institute, Brompton Hospital, London, UK. PROFESSOR G. BURNSTOCK, Department of Anatomy and Embryology, University College, London, UK. DR B. CLARKE, Departments of Cardiology and Clinical Pharmacology, Cardiothoracic Institute, DR M.O. COUPE, Departments of Cardiology and Clinical Pharmacology, Cardiothoracic Institute, DR M. GRIFFITH, Department of Cardiological Sciences, St George's Hospital, London, UK. DR S. HARDING, Department of Cardiac Medicine, Cardiothoracic Institute, London, UK. PROFESSOR S.T. HOLGATE, MRC Department of Immunopharmacology, University of Southampton, Southampton, UK. DR D. MAXWELL, Department of Thoracic Medicine, Guy's Hospital, London, UK. DR D. MCCORMACK, Department of Thoracic Medicine, Cardiothoracic Institute, Brompton Hospital, London, UK. Dr A. NEWBY, Department of Cardiology, University Hospital of Wales, Cardiff, Wales, UK. The physiological importance of adenosine has recently been recognized and in the last 25 years major steps in understanding the modulatory influences of purines have been made. This symposium explored various aspects of both basic effects and clinical/therapeutic applications. Bernard Clarke Michael Coupe DR P.J. OLDERSHAW, Department of Cardiology, PROFESSOR P. POOLE-WILSON, Department of Cardiac Medicine, Cardiothoracic Institute, DR P.G. REID, Department of Cardiology, Freeman Hospital, Neweastle-upon-Tyne, UK. DR E. ROWLAND, Department of Cardiology, Cardiothoracic Institute, Brompton Hospital, London, UK. DR E.A. SHINEBOURNE, Department of Paediatric Cardiology, PROFESSOR T.W. STONE, Department of Neurosciences, St George's Hospital, London, UK. DR J. TILL, Department of Paediatric Cardiology, DR D. UKENA, Department of Thoracic Medicine, Cardiothoracic Institute, London, UK. PROFESSOR T. WILLIAMS, Department of Pharmacology, Cardiothoracic Institute,

4 Abstracts Adenosine: recognition of physiological importance G. Burnstock The potent actions of purine nucleotides and nucleosides on the heart and vasculature was first recognized by Drury and Szent-Gyorgi (1929),1 and adenosine was suggested as a physiological regulator of blood flow by Berne in The 'purinergic nerve hypothesis' was proposed in 1972,6 with evidence to suggest that ATP was the neurotransmitter in non-adrenergic, non-cholinergic nerves supplying the intestine, urinary bladder and parts of the vascular system. Later it became evident that ATP was also a co-transmitter with noradrenaline in many sympathetic nerves.3435 Purines were shown to belong to two main subtypes,36 namely P1-purinoreceptors, that are selective to adenosine, activate via adenylate cyclase and are competitively blocked by methylxanthines (such as caffeine and theophylline) in lower concentrations than produce phosphodiesterase inhibition; and P2-purinoreceptors that are selective for ATP and ADP, do not act via adenylate cyclase, are not antagonized by methylxanthines and occupation of which leads to synthesis of prostaglandins. Further division of both P1-purinoreceptors into A1 and A2 subtypes and P2-purinoreceptors into P2x and P2y subtypes is now widely accepted.37 Purines are involved in mechanisms of local blood flow, including that in heart and lung,38 as follows: (1) ATP released as a co-transmitter from sympathetic nerves produces vasoconstriction via P2x-purinoreceptors on the smooth muscle. (2) ATP released from endothelial cells and platelets produces potent vasodilatation (particularly during hypoxia and increased local blood flow) via P2y-purinoreceptors on endothelial cells that lead to release of endothelium-derived relaxing factor (EDRF). (3) Adenosine, arising largely from ectoenzymatic breakdown of ATP release from nerves, muscles or endothelial cells, produces vasodilatation via P1- purinoreceptors (A2-subtype) on smooth muscle. (4) Adenosine acts as a modulator of release of noradrenaline from perivascular sympathetic nerves via prejunctional P1-purinoreceptors (A1-subtype). (5) There is recent evidence for non-sympathetic purinergic vasoconstrictor control of intrapulmonary arteries.39. Methods in D. Ukena adenosine receptor analysis Adenosine modulates a variety of physiological functions through interactions with A1 and A2 adenosine receptors, where agonists can mediate inhibition and stimulation, respectively, of adenylate cyclase.40 Adenosine analogues, in particular the N0 substituted compounds, are more potent at A1 receptors than at A2 receptors. A set of G CARDIOTHORACIC WORKSHOP 823 compounds, including 2-chloroadenosine, the diasteromeric R- and S-N-6-phenylisopropyladenosines (R-PIA, S-PIA), N6-cyclohexyladenosine (CHA), 5'-N-ethylcarboxamidoadenosine (NECA), N6-cyclohexyl-5'-N-ethylcarboxamidoadenosine (CHNECA), and 2-phenylaminoadenosine, can be used in future efforts to define adenosine receptors subserving differing physiological functions.41 Xanthines are classic antagonists for adenosine receptors, and many of their physiological actions may be due to blockade of adenosine receptors. Caffeine and theophylline are virtually non-specific for A1 and A2 receptors. A new series of hydrophilic 1,3- dipropyl-8-phenylxanthines has been synthesized, on the basis of a Tfunctionalized congener' approach, in which a chemically active group, such as an amine or carboxylic acid, is introduced at the terminus of a chain.42 Certain conjugates of 8-[4-(carboxymethyloxy)phenyl]-1, 3- dipropylxanthine display relative A1 selectivity and in radioactive form provide an antagonist ligand for both A, and A2 receptors.43 Analogues of caffeine in which the methyl group at the 1- or 7- position is replaced with a proargyl or propyl group display some selectivity for A2 receptors.44 The profile of a series of adenosine analogues or of xanthine antagonists can then be used to define the class of adenosine receptors. Adenosine as a modulator in health and disease T.W. Stone Most tissues appear to release adenosine when stimulated either neurally or hormonally, and adenosine may then act on prejunctional and postjunctional sites to modify the degree of stimulation.45 At prejunctional sites, adenosine may alter the amount of calcium influx accompanying depolarisation or, more likely, reduce the availability of calcium to the intracellular sites of hormone or neurotransmitter release.49 The result is usually a suppression of release, mediated by an A1 or an A3 adenosine receptor although the A2 receptor can enhance release. Postjunctionally adenosine has direct inhibitory actions on some neurones and smooth muscle systems which involve adenylate cyclase, the opening of potassium channels (producing hyperpolarization and relaxation) or interference with phosphatidylinositol turnover, as when promoting vasodilatation.46 Both adenosine and ATP can also modulate the sensitivity of postjunctional receptors for acetylcholine and catecholamines. The potentiation of responses to such neurotransmitters in tracheobronchial and vascular muscle respectively47' 48 may be of particular importance in asthmatic and hypertensive conditions. Adenosine and the control of breathing D. Maxwell Amongst the many actions of adenosine, those that affect breathing have received little attention. There is now good evidence that this nucleoside has an inhibitory action on central respiratory control. Adenosine and its analogues

5 824 CARDIOTHORACIC WORKSHOP cause respiratory depression in animals through central mechanisms. 0 Hypoxia, which increases adenosine levels in brain tissue, causes neonatal apnoea and in adults causes mild respiratory depression. Theophylline, an adenosine antagonist, stimulates ventilation, prevents the central depressant effects of both adenosine and hypoxia in animals50 and is used in the management of neonatal apnoea. If the respiratory effects of theophylline are due to central antagonism of adenosine, this nucleoside may have a tonic modulatory role in the central control of breathing. In contrast, exogenous adenosine in the cat stimulates peripheral ventilatory control structures such as the carotid body,5' and there is now evidence that this occurs in man. Adenosine infusions in man stimulate respiration, an effect which is inhibited by hyperoxia, and augment hypoxic ventilatory sensitivity.52 In addition, bolus injections of adenosine near the carotid artery stimulate breathing.53 Further work is needed, however, to establish whether endogenous adenosine plays a role in the mechanism of peripheral chemoreception. Adenosine in S.T. Holgate asthma The observation that adenosine and its parent nucleotide adenosine 5'-monophosphate (AMP) could cause bronchoconstriction when inhaled by atopic or non-atopic asthmatic patients, raised the possibility that this ubiquitous autacoid could contribute as a mediator of airways obstruction.54 After inhalation of a single concentration, bronchoconstriction reaches a maximum after 3-5 minutes, is prolonged but, in contrast to allergen, is not accompanied either by late phase response or increase in non-specific bronchial responsiveness.55 In common with many other pharmacological activities of adenosine, the airways response to adenosine in asthma is preferentially antagonized by theophylline but not enprofylline, and enhanced by the adenosine uptake blocker dipyridamole, when administered either by inhalation or intravenously. Airways responsiveness to inhaled adenosine and AMP is only weakly reflected by indices of non-specific bronchial responsiveness such as the PC20 histamine or methacholine, and in this respect it can be considered to be a measure of 'indirect' airways responsiveness. This implies an intermediary cell between the challenge and contraction of airways smooth muscle. The capacity of adenosine and related analogues to enhance the release of preformed mediators from mast cells and for the airways response with AMP to be almost totally inhibited by prior administration of terfenadine and astemizole (potent H - receptor antagonists) and sodium cromoglycate and nedocromil sodium suggest that bronchial mast cells are involved in the response. This has already been confirmed by demonstrating increased concentrations of histamine in plasma following a single airway challenge with AMP. A mast cell mechanism with enhanced release of preformed mediators as the underlying cause for bronchoconstriction provoked by adenosine and AMP also explains the ease with which the airways can be rendered tachyphylactic to repeated challenge, and the cross tachyphylaxis that occurs between the airways response to AMP and exercise in asthma.56 Thus the airways responses to adenosine have provided an interesting insight into the role of the mast cell in asthma. It raises the interesting possibility that this nucleoside released into the inflammatory environment of the airways mucosa might play an important regulatory role on mast cell function. Adenosine and the pulmonary circulation D. McCormack In addition to its other known physiological functions adenosine is known to be a vasodilator of systemic vessels There have also been reports that adenosine vasodilates small coronary arteries to a greater extent than larger ones.59 We evaluated the effects of adenosine on small human pulmonary vessels. From fresh human lung tissue small pulmonary arteries ( ym internal diameter) were dissected free of surrounding pulmonary tissue, suspended on fine wires and suspended in an organ bath of Krebs solution maintained at 37 C and gassed continually with 95% 02 and 5% CO2. Tension was recorded with a force transducer and recorded on a polygraph. The arteries were precontracted with 1 pm 5-hydroxytryptamine (5-HT) and adenosine, phenylisopropyladenosine (PIA) or 5'-N-ethylcarboxamidoadenosine (NECA) were added in a cumulative-dose manner. Adenosine, NECA and PIA all relaxed the arteries in a dose-dependent fashion with EC30 values of 5.0 x 10-5M, 5.0 x 10-5 M, and 2.9 x 10-4 M respectively. 8-Phenyl theophylline antagonized the effects of adenosine with a pkb value of 6.1. Removal of the endothelium from the arteries by gentle abrasion did not significantly affect the relaxation by adenosine. Additionally adenosine relaxation of large (segmental or lobar) and small pulmonary arteries was compared, and the EC30 values for the two sizes of arteries were not different. The relative potencies of adenosine, NECA and PIA suggest that the pulmonary vasodilator effects of adenosine are mediated through the A2 receptor. This effect is not endothelial-dependent and large and small pulmonary arteries are equally sensitive to the effects of adenosine. Adenosine may function as a regulatory of pulmonary vascular tone in animals60 and in man.61 The antiadrenergic effects of adenosine on intact ventricular myocardium and isolated cardiac myocytes S.E. Harding Adenosine receptor agonists prevented or reduced the positive inotropic response of guinea-pig papillary muscle to the beta-adrenoreceptor stimulant, isoprenaline. The effect of adenosine itself was small but statistically significant. More pronounced antiadrenergic effects were observed when 2-chloroadenosine was used in the presence of adenosine deaminase. Adenosine deaminase alone enhanced the response to isoprenaline. This suggests

6 the presence of endogenously produced adenosine in the papillary muscle preparation. The effect of 2- chloroadenosine was inhibited by 8-phenyltheophylline, confirming the involvement of cell surface adenosine receptors. 2-Chloroadenosine reduced the force of contraction of guinea-pig papillary muscle by a small amount in the absence of beta-adrenoceptor stimulation. This effect was prevented by prior reserpinization of the animals, and may therefore be caused by 2-chloroadenosine reversing tonic stimulation by endogenous noradrenaline. Pertussis toxin treatment of guinea-pigs reduced the antiadrenergic effect of 2-chloroadenosine on papillary muscle, suggesting the involvement of a guanine-nucleotide binding protein, probably Ni.62 Little or no antiadrenergic effect was detectable on rabbit papillary muscle.63 There was a pronounced antiadrenergic effect at adenosine receptors on isolated rat myocytes, although evidence of cell heterogeneity was found. Adenosine receptors in M.O. Coupe the human heart In tissue such as platelets and fat cells it is clear that adenosine, acting through specific receptors, modulates the rate of cyclic AMP formation.9 However, in cardiac tissue the mechanism of action of adenosine remains controversial. Various workers have found either an increase, a decrease or no change in the rate of cyclic AMP formation after the addition of adenosine in myocardial slice preparations, membrane preparations or whole heart preparations The discrepancies in findings are likely to be due in part to the low density of myocardial Al receptors,67 in part to substantial species differences, and also to the presence of A2 receptors from vessels in the homogenates. Using a myocardial slice preparation from human heart removed at transplantation, we have studied the effects of a series of adenosine analogues on the rate of cyclic AMP formation on both resting and isoprenaline-stimulated cardiac tissue. Adenosine analogues decreased the rate of isoprenalinestimulated cyclic AMP formation in both atrium and ventricle, in a dose-dependent manner. Agonist potencx ratios confirmed the presence of the A, receptor. The resting rate of cyclic AMP accumulation was decreased by adenosine in the atrium but not in the ventricle. This work confirms that adenosine alters the rate of cyclic AMP formation in human heart and this is likely to be the mechanism of the adenosine/catecholamine antagonism. The cardiovascular actions of adrenosine in man B. Clarke It is nearly 60 years since the cardiovascular actions of adenosine in man were first explored.' Much of the confusion regarding the cardiovascular actions of adenosine has been due to a failure to recognize that bolus injection produces different effects from an infusion. In CARDIOTHORACIC WORKSHOP 825 normal subjects, adenosine infusion ( mg/kg/min) causes dose-related increases in heart rate and skin temperature, which are not accompanied by a fall in blood 2 2 pressure. These responses are potentiated by the adenosine uptake inhibitor dipyridamole,6' but unaffected by beta-adrenergic blockade.69 Attenuation of the positive chronotropic effect of adenosine occurs by the xanthine theophylline, but not by enprofylline (which acts without adenosine antagonism). Complete pharmacological autonomic blockade completely abolishes the positive chronotropic effect of adenosine infusion, in keeping with this effect of the nucleoside in patients with autonomic failure.22 Therapeutic uses for adenosine infusion include controlled hypotension during cerebral artery aneurysm surgery,23 and to increase coronary artery graft flow and reduce platelet losses during cardiopulmonary bypass.27 The antagonism of catecholamine release during phaeochromocytoma surgery suggests other therapeutic uses for the nucleoside besides afterload reduction and arrhythmia control.7' The role of adenosine in coronary regulation A. Newby The presence of adenosine receptors on the coronary vasculature, on atrial and ventricular myocytes, on endogenous nerve endings and on the impulse conducting tissues implies that adenosine plays an important role at all these sites. Elucidating the mechanisms by which adenosine arises at its receptors remains, however, the key to understanding its physiological and pathological roles. We have tested four hypotheses that adenosine is formed from: (A) cytosolic AMP by cytosolic 5'-nucleotidase; (B) cytosolic AMP by ecto-5'-nucleotidase; (C) by ecto-5'- nucleotidase action on selectively released ATP or AMP, or (D) nucleotides released during cell disruption. Studies with cultured heart cells72 and ischaemic hearts73 support mechanism A and identify a novel, active, AMP preferring 5'-nucleotidase; they discount mechanism B. Studies with blood platelets74 and neurones7' which have established secretory pathways support mechanism C. Recent experiments with globally ischaemic hearts indicate operation of mechanism A together with mechanism C or D. The data support the hypothesis76 that adenosine released from cardiac myocytes functions normally to limit net ATP depletion but they suggest that pathological ischaemia may open up additional sources of adenosine production. Electrophysiological effects of adenosine B. Clarke Adenosine fulfils many of the criteria of the ideal antiarrhythmic drug. It has a low level of toxicity, rapid action, and may be safely used in a wide range of cardiac rhythm disorders, with few major interactions with other drugs.77

7 826 CARDIOTHORACIC WORKSHOP The cellular electrophysiological actions of adenosine are diverse and reflect differences between species and the nature of the preparation in question. It seems likely that increased potassium conductance and antagonism of the effects of catecholamines are major facets of the action of adenosine.78 In human subjects, endocardial recordings of monophasic action potentials are possible during cardiac catheterization. Adenosine causes dose-dependent shortening of the atrial action potential duration, but has only a minor effect on ventricular action potential duration, confirming in vitro findings (B. Clarke & E. Rowland, personal communication). The regional electrophysiological effects of adenosine in atrioventricular nodal re-entrant tachycardia (AVNRT) and atrioventricular re-entrant tachycardia (AVRT) include gradual AH interval prolongation, without any effect on the HV or VA' intervals in the majority of instances. Termination of AVRT in the Wolff-Parkinson- White syndrome is readily achieved by doses of adenosine below 5mg; termination occurs within 6-30 seconds (median 18s) of injection of the nucleoside. Pre-excitation may be transiently revealed by adenosine after injection in either sinus rhythm or during termination of AVRT. The proarrhythmic effects of adenosine include sinus bradycardia, re-initiation of tachycardia, complete heart block, and both atrial and ventricular extrasystoles. Despite the shortening of atrial action potential, which could theoretically facilitate the induction of atrial fibrillation, this arrhythmia is only rarely seen after adenosine administration. The rapidity of action, side effect profile and almost exclusive AV nodal action of adenosine make it an important addition to currently available antiarrhythmic agents. References 1. Drury, A.N. & Szent-Gyorgi, A. The physiological activity of adenine compounds with especial reference to their action upon the mammalian heart. J Physiol 1929, 68: Honey, R.M., Ritchie, W.T. & Thomson, W.A.R. The action of adenosine upon the human heart. Q J Med 1930, 23: Jezer, A., Oppenheimer, B.S. & Schwartz, S.P. The effect of adenosine on cardiac irregularities in.,man. Am Heart J 1933, 9: Wayne, E.J., Goodwin, J.F. & Stoner, H.B. The effect of adenosine triphosphate on the electrocardiogram of man and animals. Br Heart J 1949, 11: Berne, R.M. Cardiac nucleotides in hypoxia: possible role in regulation of coronary blood flow. Am J Physiol 1963, 204: Burnstock, G. Purinergic nerves. Pharmacol Rev 1972, 24: Von Calker, D., Muller, M. & Hamprecht, B. Adenosine regulates via two different types of receptors the accumulation of cyclic AMP in cultured brain cells. J Neurochem 1979; 33: Londos, C., Cooper, D.M.F. & Wolff, J. Subclasses of external adenosine receptors. Proc Natl Acad Sci USA 1980, 77: Adenosine in the treatment of supraventricular tachycardia in children J.A. Till Small children are less able to withstand tachyarrhythmias than their adult counterparts. The ventricular muscle of the neonate or infant is less compliant; the rate of tachycardia is usually greater because the young A-V node will allow more rapid conduction and they have less cardiac reserve. Congestive cardiac failure develops more quickly. The requirements of a therapeutic agent in this clinical situation include a rapid onset of action and an absence of further cardiac compromise.29 We have treated 72 episodes of supraventricular tachycardia in 27 children with intravenous adenosine. These have included a wide range of mechanisms. In re-entry supraventricular tachycardia utilizing the A-V node as one limb of the circuit (AVRT, AVNRT, and long R-P tachycardia), adenosine successfully terminated 53 of 62 episodes (85%). Adenosine was less effective in the termination of atrial flutter, ectopic atrial tachycardia or His bundle tachycardia. Side effects were transient (up to 4 seconds) and included complete A-V block, sinus bradycardia, ventricular ectopic beats, flushing and respiratory disturbance. Repeated re-initiation of tachycardia (within 4 seconds) limited therapeutic efficiency in some cases but adenosine offers a safe and rapid method of termination in the majority of episodes of supraventricular tachycardia in childhood Ukena, D., Bohme, E. & Schwabe, U. Effects of several 5-carboxamide derivatives of adenosine on adenosine receptors of human platelets and rat fat cells. Naunyn-Schmiedeberg's Arch Pharmacol 1984, 327: Rubio, R., Bianchi, J. & Berne, R.M. Atrial negative inotropic effect of acetylcholine (ACh) and adenosine (ADO) are the result of the initial trigger of a positive inotropic mechanism. Pflugers Archiv 1986, 407: Stiles, G.L. Adenosine receptors: Structure, function and regulation. Tr Pharmacol Sci 1986, 7: Cushley, M.J., Tattersfield, A.E. & Holgate, S.T. Inhaled adenosine and guanosine on airway resistance in normal and asthmatic subjects. Br J Clin Pharmacol 1983, 15: Cushley, M.J., Tattersfield, A.E. & Holgate, S.T. Adenosine-induced bronchoconstriction in asthma: antagonism by inhaled theophylline. Am Rev Respir Dis 1984, 129: Watt, A.H., Lewis, D.J.M., Horne, J.J. & Smith, P.M. Reproduction of epigastric pain of duodenal ulceration by adenosine. Br Med J 1987, 294:

8 15. Sylven, C., Beermann, B., Jonzon, B. & Brandt, R. Angina-pectoris like pain provoked by intravenous adenosine in healthy volunteers. Br Med J 1986; 293: Saugstad, O. Hypoxanthine as a measurement of hypoxia. Pediatr Res 1975, 9: Hedner, T., Hedner, J., Jonason, J. & Wessberg, P. Effects of theophylline on adenosine induced respiratory depression in the preterm rabbit. Eur J Respir Dis 1984, 65: Fuller, R.J., Maxwell, D.L., Conradson, T.-B., Dixon, C.M.S. & Barnes, P.J. Circulatory and respiratory effects of infused adenosine in conscious man. Br J Clin Pharmacol 1987, 24: Lagerkrantz, H., Yamamoto, Y., Fredholm, B.B., Prabhakar, N. & von Euler, C. Adenosine analogues produce apnoea antagonised by theophylline in rabbit neonates. Pediatr Res 1984, 18: Watt, A. Hypertrophic cardiomyopathy: a disease of impaired adenosine-mediated autoregulation of the heart. Lancet 1984, i: Watt, A. Hypothesis. Sick sinus syndrome: An adenosine mediated disease. Lancet 1985, i: Biaggioni, I., Olafsson, B., Robertson, R.M., Hollister, A.S. & Robertson, D. Cardiovascular and respiratory effects of adenosine in conscious man. Evidence for chemoreceptor activation. Circ Res 1987, 61: Sollevi, A., Lagerkranser, M., Irestedt, L., Gordon, E. & Lindquist, C. Controlled hypotension with adenosine during cerebral aneurysm surgery. Anesthesiology 1984, 61: Lagerkranser, M., Sollevi, A., Irestedt, L., Tidgren, B. & Andreen, M. Renin release during controlled hypotension with nitroprusside, nitroglycerin and adenosine: a comparative study in the dog. Acta Anaesth Scand 1985, 29: Fukanaga, A.F., Flacke, W.E. & Bloor, B.C. Hypotensive effects of adenosine and adenosine triphosphate compared with sodium nitroprusside. Anaes Anal 1982; 61: Born, G.V.R., Honour, A.J. & Michell, J.R.A. Inhibition by adenosine and by 2 chloroadenosine of the formation and embolization of platelet thrombi. Nature 1964, 202: Sollevi, A., Torsell, L., Fredholm, B.B. Settergren, G. & Blomback, M. Adenosine spares platelets during cardiopulmonary bypass in man without causing systemic vasodilatation. Scand J Thor Cardiovasc Surg 1985, 19: Watt, A.H., Bernard, M.S., Webster, J., Passani, S.L., Stephens, M.R. & Routledge, P.A. Intravenous adenosine in the treatment of supraventricular tachycardia: a dose-ranging study and interaction with dipyridamole. Br J Clin Pharmacol 1986, 21: Clarke, B., Till, J., Rowland, E., Ward, D.E., Barnes, P.J. & Shinebourne, E.A. Rapid and safe termination of supraventricular tachycardia in children by adenosine. Lancet 1987, i: DiMarco, J.P., Sellers, T.D., Berne, R.M., West, G.A. & Belardinelli, L. Adenosine: electrophysiological effects and therapeutic use for terminating paroxysmal supraventricular tachycardia. Circulation 1983, 68: CARDIOTHORACIC WORKSHOP Klabunde, R.E. Dipyridamole inhibition of adenosine metabolism in human blood. Eur J Pharmacol 1983, 93: Lerman, B.B., Belardinelli, L., West, A., Berne, R.M. & DiMarco, J.P. Adenosine sensitive ventricular tachycardia; evidence suggesting cyclic-amp mediated triggered activity. Circulation 1986, 74: Wesley, R.C., Lerman, B.B., DiMarco, J.P., Berne, R.M. & Belardinelli, L. Mechanism of atropineresistant atrioventricular block during inferior myocardial infarction: possible role of adenosine. J Am Coil Cardiol 1986, 8: Burnstock, G. Do some nerve cells release more than one transmitter. Neurosci Lett 1976, 1, Burnstock, G. & Kennedy, C. A dual function for adenosine 5 triphosphate in the regulation of vascular tone: excitatory cotransmitter with noradrenaline from perivascular nerves and locally released inhibitory intravascular agent. Circ Res 1986, 58: Burnstock, G. In: Straub, R.W. & Bolis, L. (eds) A Basis for Distinguishing Two Types of Purinergic Receptor. Cell Membrane Receptors for Drugs and Hormones; a Multidisciplinary Approach. Raven Press, New York, 1978, pp Burnstock, G. & Kennedy, C. Is there a basis for distinguishing two types of P2-purinoreceptor. Gen Pharmacol 1985, 16: Hopwood, A.M. & Burnstock, G. ATP mediates coronary vasoconstriction via P2x receptors and coronary vasodilatation via P2y purinoreceptors in the isolated perfused rat heart. Eur J Pharmacol 1987, 136: Inoue, T. & Kannan, M.S. Evidence for purinergic neurotransmission in the rat intrapulmonary artery. Biophys J 1988, 53: Daly, J.W. Adenosine receptors: targets for future drugs. J Med Chem 1982, 25: Ukena, D., Olsson, R.A. & Daly, L.W. Definition of subclasses of adenosine receptors associated with adenylate cyclase interaction of adenosine analogs with inhibitory A1 receptors and stimulatory A2 receptors. Can J Physiol Pharmacol 1987, 65: Jacobson, K.A., Kirk, K.L., Padgett, W.L. & Daly, J.W. A functionalised congener approach to adenosine receptor antagonists: amino acid conjugates of 1,3- dipropylxanthine. Mol Pharmacol 1986, 29: Jacobson, K.A., Ukena, D., Kirk, K.L. & Daly, J.W. 3H Xanthine amine congener of 1,3-dipropyl-8- phenylxanthine: An antagonist radioligand for adenosine receptors. Proc Natl Acad Sci USA 1986, 83: Ukena, D., Shamim, M.J., Padgett, W.L. & Daly, J.W. Analogs of caffeine antagonists with selectivity for A2 adenosine receptors. Life Sci 1986, 39: Stone, T.W. Physiological roles for adenosine and ATP in the nervous system. Neuroscience 1981, Long, C.J. & Stone, T.W. Adenosine reduces agonistinduced production of inositol phosphate in rat aorta. J Pharm Pharmacol 1987, 39:

9 828 CARDIOTHORACIC WORKSHOP 47. Gustaffson, L.E., Wiklund, N.P. & Cederqvist, B. Apparent enhancement of cholinergic transmission in rabbit bronchi via A2 receptors. Eur J Pharmacol 1986, 120: Jackson, E.K. Role of adenosine in noradrenergic transmission in spontaneously hypertensive rats. Am J Physiol 1987, 253: Ribeiro, J.A. & Sebastiao, A.M. Adenosine receptors and calcium. Basis for proposing a third (A3) adenosine receptor. Neurobiol 1986, 26: Eldridge, F.L., Millhorn, D.E. & Kiley, J.P. Antagonism by theophylline of respiratory inhibition induced by adenosine. J Appl Physiol 1985, 59: McQueen, D.S. & Ribeiro, J.A. Pharmacological characterisation of the receptor involved in chemoexcitation induced by adenosine. Br J Pharmacol 1986, 88: Maxwell, D.L., Fuller, R.W., Nolop, K.B., Dixon, C.M.S. & Hughes, J.M.B. The effects of adenosine on ventilatory responses to hypoxia and hypercapnia in man. J Appl Physiol 1986, 61: Watt, A.H., Reid, P.G., Stephens, M.R. & Routledge, P.A. Adenosine-induced respiratory stimulation in man depends upon the site of infusion. Evidence for an action on the carotid body. Br J Clin Pharmacol 1987, 23: Holgate, S.T., Mann, J.S., Church, M.K. & Cushley, M.J. Mechanisms and significance of adenosineinduced bronchoconstriction in asthma. Allergy 1987, 42: Mann, J.S., Holgate, S.T. Renwick, A.G. & Cushley, M.S. Airway effects of purine nucleosides and release with bronchial provocation in asthma. J Appl Physiol 1986, 61: Pauwels, R. The role of adenosine in bronchial asthma. Bull Eur Physiopathol Respir 1987, 23: Collis, M.G. & Brown, C.M. Adenosine relaxes the aorta by interacting with an A2 receptor and an intracellular site. Eur J Pharmacol 1983, 96: Herlihy, J.T., Bockman, E.L., Berne, R.M. & Rubio, R. Adenosine relaxation of isolated vascular smooth muscle. Am J Physiol (H) 1976, 230: Schnaar, R.L. & Sparks, H.V. Responses of large and small coronary arteries to nitroglycerin, NaNO2, and adenosine. Am J Physiol 1972, 223: Mentzer, R.M., Rubio, R. & Berne, R.M. Release of adenosine by hypoxic canine lung tissue and its possible role in the pulmonary circulation. Am J Physiol 1975, 229: McCormack, D.G., Clarke, B. & Barnes, P.J. Characterisation of adenosine receptors on human pulmonary arteries. J Physiol 1987, 396: Hopwood, A.M., Harding, S.E. & Harris, P. Pertussis toxin reduces the antiadrenergic effect of 2-chloroadenosine on papillary muscle and the direct negative inotropic effect of 2-chloroadenosine on atrium. Eur J Pharmacol 1987, 141: Hopwood, A.M., Harding, S.E. & Harris, P. An antiadrenergic effect of adenosine on guinea pig but not rabbit ventricles. Eur J Pharmacol 1987, 137: Huang, M. & Drummond, G.I. Effect of adenosine on cyclic AMP accumulation in ventricular myocardium. Biochem Pharmacol 1976, 25: Dobson, J.G. Reduction by adenosine of the isoproterenol induced increase in cyclic adenosine 3,5- monophosphate formation and glycogen phosphorylase activity in rat heart muscle. Circ Res 1978, 43: Bruckner, R., Fenner, A., Meyer, W., Nobis, T.M., Schmitz, W. & Scholz, H. Cardiac effects of adenosine and adenosine analogs in guinea-pig atrial and ventricular preparations: Evidence against a role of cyclic AMP and cyclic GMP. J Pharmacol Exp Ther 1985, 234: Lohse, M.J., Ukena, D. & Schwabe, U. Demonstration of Ri type adenosine receptors in bovine myocardium by radioligand binding. Naunyn- Schmiedeberg's Arch Pharmacol 1985, 328: Conradson, T.-B., Dixon, C.M.S., Clarke, B. & Barnes, P.J. Cardiovascular effects of infused adenosine in man: potentiation by dipyridamole. Acta Physiol Scand 1987, 129: Clarke, B., Conradson, T.B., Dixon, C.M.S. & Barnes, P.J. Influence of autonomic blockade on cardiovascular effects of infused adenosine in man. Clin Sci 1986, 71: 45p. 70. Maxwell, D.L., Fuller, R.W., Conradson, T.B. et al. Contrasting effects of two xanthines, theophylline and enprofylline, on the cardiorespiratory stimulation of infused adenosine in man. Acta Physiol Scand 1987, 131: Sollevi, A. Cardiovascular effects of adenosine in man: possible clinical implications. Prog Neurobiol 1986, 27: Meghji, P., Holmquist, C.A. & Newby, A.C. Adenosine formation and release from neonatal rat heart cells in culture. Biochem J 1985, 229: Meghji, P., Middleton, K.M. & Newby, A.C. Absolute rates of adenosine formation in rat and pigeon hearts. Biochem J 1988, 249: Gordon, J.L. Extracellular ATP; effects, sources and fate. Biochem J 1986, 223: Richardson, P.J., Brown, S.J., Bailyes, E.M. & Luzio, L.P. Ecto-enzymes control adenosine modulation of immuno-isolated cholinergic synapses. Nature 1987, 327: Newby, A.C. Adenosine and the concept of retaliatory metabolites. Tr Biochem Sci 1984; 9: DiMarco, J.P., Sellers, T.D., Lerman, B.B., Greenberg, M.L., Berne, R.M. & Belardinelli, L. Diagnostic and therapeutic use of adenosine in patients with supraventricular tachyarrhythmias. J Am Coll Cardiol 1985, 6: Pelleg, A. Cardiac cellular electrophysiologic actions of adenosine and adenosine triphosphate. Am Heart J 1985, 110: Overholt, E.D., Rheuban, K.S., Gutgesell, H.P., Lerman, B.B., & DiMarco, J.P. Usefulness of adenosine for arrhythmias in infants and children. Am J Cardiol 1988, 61: