Release of two vasodilators, adenosine and prostacyclin, from isolated rabbit hearts during controlled hypoxia

Gender-specific impairment of in vitro sinoatrial node chronotropic responses and of myocardial ischemia tolerance in rats exposed prenatally to betamethasone

Excessive fetal glucocorticoid exposure has been linked to increased susceptibility to hypertension and cardiac diseases in the adult life, a process called fetal programming. The cardiac contribution to the hypertensive phenotype of glucocorticoid-programmed progeny is less known, therefore, we investigated in vitro cardiac functional parameters from rats exposed in utero to betamethasone. Pregnant Wistar rats received vehicle (VEH) or betamethasone (BET, 0.1mg/kg, i.m.) at gestational days 12, 13, 18 and 19. Male and female offspring were killed at post-natal day 30 and the right atrium (RA) was isolated to in vitro evaluation of drug-induced chronotropic responses. Additionally, whole hearts were retrograde-perfused in a Langendorff apparatus and infarct size in response to in vitro ischemia/reperfusion (I/R) protocol was evaluated. Male and female progeny from BET-exposed pregnant rats had reduced birth weight, a hallmark of fetal programming. Male BET-progeny had increased basal RA rate, impaired chronotropic responses to noradrenaline and adenosine, and increased myocardial damage to I/R. Though a 12-fold reduction in the negative chronotropic responses to adenosine, the effects of non-metabolisable adenosine receptor agonists 5'-(N-ethylcarboxamido)adenosine or 2-Chloro-adenosine were not different between VEH- and BET-exposed male rats. BET-exposed female offspring presented no cardiac dysfunction. Prenatal BET exposure engenders male-specific impairment of sinoatrial node function and on myocardial ischemia tolerance resulting, at least in part, from an increased adenosine metabolism in the heart. In light of the importance of adenosine in the cardiac physiology our results suggest a link between reduced adenosinergic signaling and the cardiac dysfunctions observed in glucocorticoid-induced fetal programming.

Role of a novel nociceptor autocrine mechanism in chronic pain

We have previously shown, in the rat, that neuropathic and inflammatory events produce a neuroplastic change in nociceptor function whereby a subsequent exposure to a proinflammatory mediator (e.g. prostaglandin E2 ; PGE2 ) produces markedly prolonged mechanical hyperalgesia. While the initial approximately 30 min of this prolonged PGE2 hyperalgesia remains PKA-dependent, it subsequently switches to become dependent on protein kinase C epsilon (PKCε). In this study we tested the hypothesis that the delayed onset, PKCε-mediated, component of PGE2 hyperalgesia is generated by the active release of a nucleotide from the peripheral terminal of the primed nociceptor and this nucleotide is then metabolized to produce adenosine, which acts on a Gi-coupled A1 adenosine receptor on the nociceptor to generate PKCε-dependent hyperalgesia. We report that inhibitors of ATP-binding cassette transporters, of ecto-5'-phosphodiesterase and ecto-5'nucleotidase (enzymes involved in the metabolism of cyclic nucleotides to adenosine) and of A1 adenosine receptors each eliminated the late, but not the early, phase of PGE2 -induced hyperalgesia in primed animals. A second model of chronic pain induced by transient attenuation of G-protein-coupled receptor kinase 2, in which the prolongation of PGE2 hyperalgesia is not PKCε-dependent, was not attenuated by inhibitors of any of these mechanisms. Based on these results we propose a contribution of an autocrine mechanism, in the peripheral terminal of the nociceptor, in the hyperalgesic priming model of chronic pain.

Direct blockade of inflammatory hypernociception by peripheral A1 adenosine receptors: involvement of the NO/cGMP/PKG/KATP signaling pathway

Through activation of the A1 adenosine receptors (A1Rs) at both the central and peripheral level, adenosine produces antinociception in a wide range of tests. However, the mechanisms involved in the peripheral effect are still not fully understood. Therefore, the mechanisms by which peripheral activation of A1Rs reduces inflammatory hypernociception (a decrease in the nociceptive threshold) were addressed in the present study. Immunofluorescence of rat dorsal root ganglion revealed significant expression of A1Rs in primary sensory neurons associated with nociceptive pathways. Functionally, peripheral activation of A1Rs reduced inflammatory hypernociception because intraplantar (i.pl.) administration of an A1R antagonist (DPCPX) enhanced carrageenan-induced hypernociception. On the other hand, local (paw) administration of CPA (a selective A1R agonist) reversed mechanical hypernociception induced by carrageenan or by the directly acting hypernociceptive mediator prostaglandin E(2) (PGE(2)). Down-regulation of A1Rs expression in primary nociceptive neurons by intrathecal treatment with antisense oligodeoxinucleotides significantly reduced peripheral antinociceptive action of CPA. Direct blockade of PGE(2) inflammatory hypernociception by the activation of A1Rs depends on the nitric oxide/cGMP/Protein Kinase G/KATP signaling pathway because the peripheral antinociceptive effect of CPA was prevented by pretreatment with inhibitors of neuronal nitric oxide synthase (N-propyl-l-arginine), guanylyl cyclase (ODQ), and Protein Kinase G (KT5823) as well as with a KATP blocker (glibenclamide). However, this effect of CPA was not reduced by naloxone, excluding the participation of endogenous opioids. These results suggest that the peripheral activation of A1R plays a role in the regulation of inflammatory hypernociception by a mechanism that involves the NO/cGMP/PKG/KATP intracellular signaling pathway.

α2-Adrenoreceptor mediated sympathoinhibition of heart rate during acute hypoxia is diminished in conscious prostacyclin synthase deficient mice

Acute hypoxia increases ventilatory drive in conscious animals, resulting in tachycardia. Sustained hypoxia changes the initial chemoreflex ventilatory increase to secondary ventilatory depression, which then evokes a gradual secondary heart rate (HR) reduction. Prostacyclin (PGI(2)) release is known to potentiate alpha(2)-adrenoreceptor (alpha(2)-AR) mediated inhibition of sympathoactivation during ischaemia and hypoxia. We examined whether alpha(2)-AR mediated sympathoinhibition was responsible for limiting hypoxic heart rate increases during initial sympathoactivation, and subsequent secondary HR depression, and if PGI(2) is required for sympathoinhibition of HR. The responses of unrestrained PGI(2) synthase deficient (PGID) and wild type (WT) mice to acute hypoxia (10% O(2) for 30 min) were investigated by simultaneous telemetry, whole body plethysmography and open-flow respirometry. PGID mice exhibited potentiated .V(E) (p < 0.007) after intraperitoneal vehicle injection (n = 8), but not so HR responses compared to WT mice during sustained hypoxia. Idazoxan (alpha(2)-AR antagonist, i.p. bolus 3 mg/kg) pretreatment did not change hypoxic ventilatory response in either group, but significantly elevated hypoxic HR in WT mice only (p < 0.013). Sodium meclofenamate (cyclooxygenase inhibition, i.p. bolus 25 mg/kg) pretreatment eliminated the potentiated .V(E) of PGID and caused significant basal hypotension that led to a transient hypertensive response to hypoxia. From these results, we suggest that alpha(2)-AR activation is required for coupling HR to central inspiratory drive during acute hypoxia, and that PGI(2) is required to enhance the inhibition of sympathoactivation.

Prostacyclin-mediated compensatory mechanism in the coronary circulation during acute NO synthase blockade

Nitric oxide (NO) in contrast to most prostanoids, plays a major role in the maintenance of coronary arterial tone under physiological conditions. However, in case of endothelial damage or other NO-depleting situations the importance of other vasodilating mechanisms may be increased. The aim of the present study was to investigate the crosstalk between the L-arginine - NO and the prostanoid systems in isolated rat hearts. Coronary flow and cardiac dynamics were measured in a standard Langendorff perfusion system. Application of indomethacin in the perfusion media failed to change coronary flow. Administration of L-NA, however, significantly decreased coronary flow by 24.8 +/- 2.3% (p < 0.01 vs. untreated control). In the presence of indomethacin, L-NA decreased coronary flow to an even greater extent by 35.8 +/- 5.2% (p < 0.05 vs. L-NA alone). Treatment of the preparations with L-NA or indomethacin failed to change cardiac work, coapplication of both drugs together, however, decreased cardiac work by 45 +/- 11% (p < 0.05 vs. untreated control). Heart rate remained constant throughout the experimental period and did not differ significantly between the treatment groups. The prostacyclin content of the effluent from the L-NA treated hearts was significantly higher than that of controls. We conclude that in case of decreased NO levels in the coronary circulation, arterial tone is maintained by prostacyclin production.

Visceral chest pain in unstable angina pectoris and effects of transcutaneous electrical nerve stimulation. (TENS). A review

A substantial proportion of patients with chest pain referred to hospital, show signs of coronary artery disease. Anginal pain could be conceptualized as a warning signal for coronary artery disease and impending death. But, for many reasons this theory is partly disputed. Firstly, not all ischemic episodes are accompanied by anginal pain (silent ischemia). Secondly, chest pain indistinguishable from true angina pectoris may be the result of other abnormalities of thoracic viscera. Nevertheless acute severe cardiac ischemia often gives rise to anginal chest pain. Unstable angina pectoris is carrying a higher risk for future events in spite of intensive medical treatment. A special problem are patients awaiting coronary intervention because of severe ischemia and maximum medical treatment, who experience ischemic pain. New treatment regimens are needed for these patients. This review discusses the symptom of visceral pain from the heart, angina pectoris, its relation to ischemia and unstable angina pectoris. It also addresses the role of afferent nerve stimulation (transcutaneous electrical nerve stimulation, TENS) in the treatment of severe angina pectoris as well as recent findings of TENS applicability in unstable angina.

Age-related changes in adenosine in rat coronary resistance vessels

1. The vasodilator effects of adenosine receptor agonists, isoprenaline and histamine were examined in perfused heart preparations from young (4-6 weeks) and mature (12-20 weeks) rats. 2. Adenosine induced a biphasic concentration-dependent decrease in KCl (35 mM) raised coronary perfusion pressure in hearts from young and mature rats, suggesting the presence of both high- and low-affinity sites for adenosine receptors in the two age groups tested. In heart preparations from mature rats, vasodilator responses to adenosine were significantly reduced compared with responses observed in young rats. 3. Responses to 5'-N-ethylcarboxamidoadenosine (NECA) and 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS-21680) were reduced in preparations from mature rats, whereas the vasodilator actions of N6-cyclopentyladenosine (CPA) and N6-2-(4-aminophenyl)ethyladenosine (APNEA) did not change with age. 4. The results presented in this study suggest that several adenosine receptor subtypes mediate vasodilator responses in the coronary circulation of the rat and that a reduction in response to adenosine with age may be due to changes in the high-affinity receptor site.

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

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

A role for adenosine in coronary vasoregulation in man. Effects of theophylline and enprofylline

Adenosine has been suggested to have a role in regulation of the tone of the cardiac resistance vessels. To elucidate the coronary vasoregulatory role of endogenous adenosine in man, we studied the effects of adenosine receptor antagonism by theophylline on coronary blood flow at rest and during light exercise. However, theophylline may also exert pharmacological effects not related to adenosine antagonism. To clarify the contribution of endogenous adenosine in coronary hyperaemia, the effect of theophylline was compared to that of enprofylline, a xanthine which exerts similar pharmacological effects as theophylline while lacking antagonistic action at adenosine receptors. Twenty healthy subjects (10 males) aged 22-39 years were examined. Coronary sinus (CS) blood flow and blood oxygen content were determined at rest and during supine bicycle exercise, at a load of 50 watts, for 10 min. Thereafter, stepwise infusion of adenosine (30 to 60 micrograms/kg/min into the subclavian vein) was performed. Theophylline or enprofylline treatment was instituted randomly and double-blind (10 in each group), and the procedures (i.e. determinations at rest, during exercise and during infusion of adenosine) were repeated. In all 20 subjects, basal CS flow was 70 +/- 6 ml/min and the cardiac oxygen extraction ((A-CS)O2D) was 123 +/- 3 ml/l. During exercise, CS flow and (A-CS)O2D increased to 135 +/- 17 ml/min and 132 +/- 3 ml/l, respectively. Adenosine increased CS flow dose dependently to 161 +/- 27 ml/min, while (A-CS)O2D decreased to 66 +/- 7 ml/l. The vasodilatory effect of adenosine was readily counteracted by theophylline, the increase in CS flow being 33% vs. 133% in the control situation. Enprofylline, on the other hand, enhanced the response to exogenous adenosine. Theophylline, at a dose lacking effect on heart rate and blood pressure, decreased CS flow at rest by 14% (P < 0.05) and during exercise by 18% (P < 0.05). ((A-CS)O2D increased by 14% at rest and during exercise (P < 0.001). Enprofylline, on the other hand, was without effect. The differences in responses between theophylline and enprofylline with respect to coronary flow and oxygen extraction were significant both at rest and during exercise. It is concluded that theophylline increases coronary vascular resistance, while enprofylline, lacking adenosine antagonistic properties, was without such effect. This indicates a physiological role of adenosine in regulation of coronary flow.

Theophylline increases coronary vascular tone in humans: evidence for a role of endogenous adenosine in flow regulation

To elucidate the role of adenosine in coronary vasoregulation, we studied the effects of adenosine antagonism (by theophylline) on coronary blood flow at different levels of adenosine formation (stimulated by hypoxia and exercise). Six healthy subjects were studied. Coronary sinus (CS) blood flow (thermodilution) and cardiac oxygen extraction [(A-CS)O2D] were determined while breathing room air at rest, and 12% oxygen, both at rest and during light exercise, on two occasions. One of the experiments was performed during infusion of theophylline. The basal CS flow was 118 (67-168) mL min-1 (mean and 95% confidence interval), and the (A-CS)O2D was 125 (111-142) mL L-1. Inhalation of 12% O2 decreased the arterial haemoglobin oxygen saturation to 83 (80-86)% at rest and to 77 (73-81)% during exercise. CS flow increased to 167 (93-214) and 261 (179-343) mL min-1, respectively, and (A-CS)O2D decreased to 102 (85-119) and 94 (77-111) mL L-1, respectively. Theophylline, at a dose lacking effects on myocardial work, markedly attenuated the coronary flow response to exogenous adenosine, and decreased CS flow to 89 (58-119), 120 (79-161) and 190 (162-218) mL min-1 at normoxic rest, hypoxic rest and hypoxic exercise, respectively. The overall decrease amounted to 23% (P < 0.05). The calculated coronary vascular conductance also decreased by 23% (P < 0.05) and (A-CS)O2D increased by 15% (P < 0.001). In conclusion, the data support the hypothesis that endogenous adenosine is involved in regulation of human coronary tone.

Effects of short term ischemia and reperfusion on coronary vascular reactivity and myocardial function

Ischemia and reperfusion have been shown to cause damage to the endothelium as well as to the cardiac myocyte. Although the vasodilator response has been shown to be impaired following ischemia and reperfusion, the effect of a short period of global ischemia on the contractile response of the coronary vasculature is not clear. In the present study, coronary vasoconstriction in response to U46619, PGF2 alpha, 5-HT, and KCl was found to be depressed for at least 15 min following 15 min of in vitro global ischemia in rats hearts. Vasodilator blockers or inactivators were used in an effort to restore this depressed coronary response. Indomethacin (5 microM) was used to block production of vasodilator prostaglandins, L-NAME (30 microM) to block production of nitric oxide (NO), and adenosine deaminase (2.4 units/ml of coronary flow) to inactivate adenosine. None of these agents restored the normal coronary constrictor response following ischemia. When superoxide dismutase and catalase (both 20 micrograms/ml of coronary flow) were infused for 5 min before and after ischemia, the coronary response recovered more than 100% of its preischemic value by 15 min of reperfusion, but still remained depressed at 5 min reperfusion. These data suggest that free radicals produced during ischemia and/or reperfusion may be at least partly responsible for this temporary "stunning" of the coronary vasculature. Since the impaired contractile response was still present at 5 min reperfusion when the buffer was supplemented with oxygen radical scavengers, another mechanism must also be involved in this "stunning" process.

Contribution of prostaglandins in hypoxia-induced vasodilation in isolated rabbit hearts. Relation to adenosine and KATP channels

The mechanism of hypoxia-induced coronary vasodilation was studied in isolated, saline-perfused rabbit hearts under constant flow conditions. Reduction in the perfusion solution PO2 (from 520 +/- 6 to 103 +/- 9 mm Hg) under control conditions halved the coronary resistance and was accompanied by a significant release of the prostaglandin (PG) 6-keto-PGF1 alpha (from 1.8 +/- 0.3 to a maximum of 4.4 +/- 0.9 pmol min-1 g-1). The cyclooxygenase inhibitor, diclofenac (1 microM), blocked the release of PGI2 and reduced hypoxia-induced vasodilation (from 47 +/- 8% to 25 +/- 5%, P < 0.05). The relative contribution of adenosine, prostaglandins, and adenosine triphosphate (ATP)-sensitive K+ channel (KATP channel) activation in hypoxia-induced vasodilation was assessed by comparing the differential change (control response minus response after treatment) in coronary perfusion pressure (CPP) during infusion of 8-phenyltheophylline (8-PT), diclofenac, and glibenclamide, respectively. The differential change in CPP with 8-PT and diclofenac given together (-48 +/- 7%) was found to be equivalent to the sum of their respective effects (-24 +/- 7 and -19 +/- 4%, respectively). Glibenclamide (0.3 microM) reduced significantly hypoxia-induced vasodilation (differential change in CPP of -27 +/- 6%) as well as the dilator response to 10 microM adenosine and to the stable PGI2-analogue, iloprost. Forskolin-induced coronary vasodilation in arrested hearts was slightly, but significantly, reduced by glibenclamide. Our results suggest that both cyclooxygenase products and adenosine, acting independently, and concomitantly, contribute to the dilator response of coronary resistance vessels to hypoxia, in part through the activation of KATP channels.(ABSTRACT TRUNCATED AT 250 WORDS)

A model of cardiac nociception in chronically instrumented rats: behavioral and electrophysiological effects of pericardial administration of algogenic substances

This report presents evidence that pericardial administration of a mixture of algogenic substances is a potentially useful model of cardiac nociception. In awake rats in which a looped silicone catheter had been placed in the pericardial sac at least 5 days previous, administration of a mixture containing equal concentrations of bradykinin (BK), acetylcholine (ACh), adenosine (ADEN), histamine (HIST), serotonin (5-HT) and prostaglandin E2 (PGE2) (total 25 nmol in 25 microliters) led to rapid acquisition of a passive avoidance behavior. In contrast, neither BK alone (5-25 nmol) nor the same mixture of ACh, ADEN, HIST, 5-HT and PGE2 without BK led to acquisition of the behavior or produced effects significantly different than produced by saline given into the pericardial sac in the same volume (25 microliters). Both BK and the mixture containing BK produced dose-dependent cardiovascular responses (pressor response and tachycardia) of similar magnitude. Neither saline nor the mixture without BK produced significant changes in mean arterial blood pressure or heart rate. In electrophysiological experiments in the same rats, thoracic spinal cord neurons responded dose-dependently to the mixture and, except for one neuron, responded also to BK in a dose-dependent manner. However, responses to BK, when compared to a similar dosage of BK contained in the mixture, were significantly less in magnitude and duration. All units received convergent somatic input from the thorax and all neurons also received convergent input from the esophagus. Balloon distension of the esophagus excited all units. Results of the behavioral characterization of algogenic substances administered into the pericardial sac of awake rats gave evidence of differences between the effects of BK and a mixture of six substances, including BK. BK in either of two dosages tested produced effects not different than saline while the mixture containing BK was aversive. In complementary electrophysiological studies, both BK and the mixture containing BK excited thoracic spinal cord neurons, suggesting that neuron responses to putative algogenic substances are not necessarily reliable measures of cardiac nociception.

The isolated and perfused working heart of the frog, Rana esculenta: an improved preparation

1. An in vitro preparation of the intact heart of the frog Rana esculenta was set up. 2. The isolated heart, perfused at constant pressure, was spontaneously beating and able to generate physiological values of output pressure, cardiac output, ventricle work and power. It showed the typical phenomenon of the "hypodynamic state" after a relatively constant time from the onset of the perfusion. 3. Perfusion with air-saturated saline and 99.5% oxygen-saturated saline did not show significant differences in the recorded parameters. 4. This experimental model represents a useful tool for physiological and pharmacological studies, especially when the direct analysis of the effects of hormones, mediators or drugs requires an intact heart preparation.

Mechanisms of pain in angina pectoris--a critical review of the adenosine hypothesis

Clinical characteristics: Angina pectoris represents a visceral pain caused by reversible myocardial ischemia. The majority of ischemic attacks are symptomless. When pain is manifested, it appears late during the ischemic event. The pain is complex in its quality and bears little relation to the region of myocardial ischemia. Pain shows a sensitive dependence on initial conditions suggesting a mechanism with deterministic chaotic dynamics for the association between myocardial ischemia and pain. Neurophysiological substrate: Ganglia are present within the heart, particularly in epicardial fat. The blood supply of intrinsic cardiac ganglia arises primarily from branches of the proximal coronary arteries. Both afferent and efferent neurons within the intrinsic cardiac nervous system exist, while the majority of neurons in that location may be local circuit neurons. Integration takes place not only in the intrinsic cardiac nervous system, but also in mediastinal, middle cervical, and stellate ganglia. Cardiac afferent receptors are also connected to cell bodies in dorsal root and nodose ganglia, as well as intrathoracic ganglia. Myocardial regions have no spatial representation in these ganglia. Adenosine, among a number of substances, can modulate the activity generated by cardiac afferent nerve endings and intrinsic cardiac neurons. Such effects appear to be exerted at A1 receptors. Adenosine as a pain messenger: During myocardial ischemia adenosine is released in large quantities into the interstitial space. The endothelium takes up the major amount of adenosine. Thus only small increments of adenosine are detected in the blood-stream. Given as an intravenous bolus to healthy volunteers or to patients with ischemic heart disease and angina pectoris, adenosine provokes angina pectorislike pain, which is similar to habitual angina pectoris with regard to quality and location. Pain is provoked in the absence of ECG signs of ischemia. Patients with asymptomatic myocardial ischemia are less sensitive to adenosine, whereas patients with Syndrome X are more sensitive with respect to adenosine-provoked pain. When adenosine is given intraarterially, including into the coronary arteries, pain is provoked in the corresponding vascular bed. Adenosine-provoked pain and ischemic pain are counteracted by previous administration of the adenosine receptor antagonist theophylline.(ABSTRACT TRUNCATED AT 400 WORDS)

Vasodilative response to hypoxia and simulated ischemia is mediated by ATP-sensitive K+ channels in guinea pig thoracic aorta

Local vasodilation in response to hypoxia or ischemia improves perfusion and O2 supply of the affected tissue. This local vasodilation thus constitutes the most important mechanism in the prevention of ischemic cell injury. The regulation of vascular tone has mainly been attributed to changes of cytoplasmatic Ca2+ ((Ca2+)i) concentrations in vascular smooth muscle cells. The mechanism underlying these changes has not, however, been elucidated so far. Using aortic strips of guinea pigs (transversally cut in spirals; normal Tyrode, in mM: NaCl 150, KCl 4.5, MgCl2 2, CaCl2 2.5, glucose 10; buffered with 10 mM HEPES at pH 7.4; equilibrated with 100% O2 at 31 degrees C) the authors could show that metabolic blockade (glucose replaced by 10 mM 2-deoxyglucose (DOG) led to a relaxation of the preparation. Thus, in four experiments, resting tension decreased from 0.75 g by 27% +/- 12% within two hours (% of maximal contractile force developed by each preparation when depolarized with 43 mM KCl and 101.5 mM NaCl). When the same experiment was carried out in the presence of 1 mM tolbutamide (a known blocker of ATP-dependent K+ channels) in vascular smooth muscle no such relaxation could be seen (n = 4). Furthermore, in the same type of preparation, similar results have been obtained upon hypoxic relaxation (100% O2 replaced by 100% N2), where 1 mM tolbutamide also prevented vasodilation. Thus, hypoxic/ischemic vasodilation in response to glycolytic inhibition (DOG) and hypoxia (N2) is based upon the opening of K+ ATP channels and hence can be prevented by sulfonylureas (the opening of K+ ATP channels would lead to hyperpolarization (increased K+ conductance, Goldmann equation), thus diminishing the open probability of voltage-gated Ca2+ channels with subsequent vasodilation). This inhibition by sulfonylureas of vasodilative response to ischemia may also constitute the so far unknown cause of the increased cardiovascular mortality seen under sulfonylurea treatment.

Lower threshold for adenosine-induced chest pain in patients with angina and normal coronary angiograms

OBJECTIVE

To investigate whether patients with angina-like chest pain and normal coronary angiograms are more sensitive to adenosine as an inducer of chest pain.

DESIGN

Increasing doses of adenosine were given in a single blind study as intravenous bolus injections. Chest pain and the electrocardiographic findings were noted.

PATIENTS

Eight patients with angina-like chest pain but no coronary stenoses (group A), nine patients with angina and coronary stenoses (group B), and 16 healthy volunteers (group C).

RESULTS

In the absence of ischaemic signs on the electrocardiogram adenosine provoked angina-like pain in all patients in groups A and B. The pain was located in the chest, and its quality and location were described as being no different from the patient's habitual angina. In group C, 14 of 16 subjects reported chest pain. The lowest dose resulting in chest pain was lower in group A (0.9 (0.6) mg) than in group B (3.1 (1.5)mg) (p < 0.005) and in group C (6.2 (3.7) mg) (p < 0.005). The maximum tolerable dose was lower in group A (4.7 (2.1) mg) than in group B (9.2 (3.8) mg) (p < 0.05) and in group C (12.0 (4.1) mg) (p < 0.005).

CONCLUSIONS

Patients with angina-like chest pain and normal coronary angiograms have a low pain threshold and low tolerance to pain induced by adenosine.

A critical review of the afferent pathways and the potential chemical mediators involved in cardiac pain

There is considerable evidence that on the anterior surface of the heart (which is usually supplied by the left anterior descending and the proximal part of the left circumflex coronary arteries), sympathetic efferent reflexes characterized by tachycardia and/or hypertension predominate following experimental or pathological perturbations. These cardiovascular reflexes are accompanied by an increase in presumed nociceptive afferent traffic and, in pathological condition, by pain. In these experiments, there is generally no effect of vagotomy on afferent nerve traffic, and lower cervical and upper thoracic sympathectomies help provide relief from angina. On the other hand, experimental or pathological perturbations involving the inferior-posterior surface of the heart (supplied by the right and distal parts of the left circumflex coronary arteries), are characterized by vagal efferent reflexes, resulting in bradycardia and/or hypotension. These reflexes are accompanied by an increase in vagal afferent nerve traffic and, in pathological conditions, by pain. In these experiments, vagotomy generally abolishes such cardiovascular reflexes, and lower cervical and upper thoracic sympathectomies are not effective in the relief from angina. Although cardiac sympathetic afferents are unquestionably involved in the central transmission of nociceptive information from the heart, it is also likely that there is a contributing role from the vagus in cardiac pain. It is important experimentally to understand the natural stimulus that gives rise to angina. In the clinical situation, a decrease in coronary blood flow or an increase in the metabolic demands of the myocardium due to increased work are obvious precipitating factors which lead to myocardial ischemia. In the experimental situation, occlusion of the coronary arteries is often used as a stimulus which mimics myocardial ischemia. As people who frequently experience angina have varying degrees of coronary artery disease, it is difficult to accept that the state of the coronary arteries of the normal experimental animal bear any resemblance to the state of the coronary arteries under pathological conditions. That is, the gain of homeostatic reflexes, the basal concentrations of neuroactive substances in the plasma, the myocardium and the afferent terminals, the excitability of the afferents, access of chemical mediators (e.g. bradykinin, 5-HT, adenosine, histamine, prostaglandins, potassium, lactate), to afferents, and the overall function of the animal are all significantly different. We have no idea how control mechanisms have been altered in the person with severe coronary artery disease compared to the normal patient or the "normal" experimental animal.(ABSTRACT TRUNCATED AT 400 WORDS)

Improved working capacity in patients with ischaemic heart disease during a 10-day treatment with oral theophylline

The objective of this study was to assess whether a 10-d treatment with oral theophylline improves the working capacity in patients with ischaemic heart disease, and to compare theophylline with conventional anti-anginal therapy. Twenty-four patients with stable effort-induced angina were included in the study. The patients received double-blind treatment in randomized order during 4 consecutive 10-d periods, separated by a 4-d wash-out period, with (a) metoprolol durules 200 mg once daily + theophylline durules 300 mg b.i.d., (b) theophylline + placebo, (c) metoprolol + placebo, and (d) placebo alone. At the end of each period a supine exercise stress test was performed. Maximal workload increased to 111 +/- 6 W during treatment with theophylline, compared to 106 +/- 6 W during placebo treatment (P = 0.01). Metoprolol increased the maximal workload to 117 +/- 6 W (P less than 0.001). The effects of metoprolol and theophylline were additive, and the working capacity increased to 123 +/- 7 W during combined therapy. Neither the degree of ST-depression nor the scoring of chest pain at maximal workload differed between the four treatment regimens. An improved working capacity was shown in patients with stable effort-induced angina pectoris during long-term theophylline treatment. The effect was additive to that of beta-blockade.

Adenosine provokes myocardial ischaemia in patients with ischaemic heart disease without increasing cardiac work

Intravenous infusion of adenosine in patients with ischaemic heart disease (IHD) has been shown to induce chest pain and ST-depression. The aim of this study was to determine whether such myocardial ischaemia could be due to an increase in myocardial work. Thus patients with stable angina pectoris (n = 8) were randomly allocated to exercise or adenosine infusion, with a 1-h rest period before the second test. The maximal tolerable work load was 120 +/- 13 W, where all patients but one experienced typical angina pectoris. ECG revealed ST-depressions in all patients. The maximal tolerable dose of adenosine was 108 +/- 6 micrograms kg-1 min-1. All patients experienced chest pain typical of habitual angina pectoris, and all but one developed ST-depressions. During exercise there was a gradual and marked increase in the rate pressure product (RPP), in parallel with the development of ST-depression. By contrast, during infusion of adenosine there was only a minor increase in RPP (P = 0.0001). In conclusion, infusion of adenosine provokes signs and symptoms of myocardial ischaemia in patients with IHD with only a minor increase in cardiac work compared to exercise. These results are consistent with the hypothesis of a myocardial steal.

Release of endothelial mediators and sympathetic transmitters at different coronary flow rates in rabbit hearts

1. The release of three endothelial mediators, namely, endothelial-derived relaxing factor (EDRF), prostacyclin (PGI2) and endothelin, and of two sympathetic neurotransmitters, noradrenaline and neuropeptide Y (NPY), from resting or sympathetically stimulated rabbit Langendorff hearts was investigated at normal or elevated coronary flow. The sympathetic nerves to the hearts were stimulated at 5 Hz for 30 s and the cardiac effluent was analysed for nitrite (metabolite of EDRF) with electron paramagnetic resonance spectrometry, for 6-keto-PGF1 alpha (metabolite of PGI2) with gas chromatography/mass spectrometry, for endothelin- and NPY-like immunoreactivity with radioimmunoassay, and for noradrenaline and purines with liquid chromatography. 2. During perfusion of the hearts at normal flow (35 +/- 1.4 ml min-1) the effluent concentration of nitrite was 0.15 +/- 0.02 microM, that of 6-keto-PGF1 alpha 0.74 +/- 0.08 nM, and that of endothelin-like immunoreactivity 0.18 +/- 0.01 pM. Nerve stimulation augmented the release of 6-keto-PGF1 alpha from 76 +/- 8 to 99 +/- 10 pmol (3 min)-1 (P less than 0.05), but did not affect the release of nitrite or endothelin-like immunoreactivity. Nerve stimulation also facilitated the outflow of noradrenaline and of NPY-like immunoreactivity by 52 +/- 11 pmol (3 min)-1 and 19 +/- 7 fmol (3 min)-1, respectively. 3. Elevation of the coronary flow to 79 +/- 3.2 ml min-1 did not affect the effluent concentrations of nitrite, 6-keto-PGF1 alpha and endothelin-like immunoreactivity, implying that their outflows were augmented. Sympathetic stimulation at elevated coronary flow did not further augment the outflow of endothelial mediators or of NPY-like immunoreactivity, but increased the outflow of noradrenaline by 62 +/- 12%, in comparison to stimulation at normal flow. Perfusion of the heart with the noradrenaline uptake blocker desipramine (5 microM) completely abolished the promoting effecting of elevated coronary flow on noradrenaline outflow during sympathetic stimulation. 4. These data indicate that an increase in coronary flow in perfused rabbit hearts is paralleled by a corresponding facilitation of the formation of the endothelial mediators, EDRF, prostacyclin and endothelin. Such an elevation of mediator formation does not affect nerve stimulation-induced release of sympathetic transmitters in the heart.

Direct cutaneous hyperalgesia induced by adenosine

The intradermal injection of adenosine produces a dose-dependent decrease in mechanical nociceptive threshold in the hindpaw of the rat that is not attenuated by elimination of indirect pathways for the production of hyperalgesia. Adenosine-induced hyperalgesia is mimicked by the A2-agonists, 5'-(N-ethyl)-carboxamido-adenosine and 2-phenylaminoadenosine but not by the A1-agonist, N6-cyclopentyladenosine and antagonized by the adenosine A2-receptor antagonist, PD 081360-0002 but not by the A1-antagonist, 1,3-dipropyl-8-(2-amino-4-chlorophenyl)xanthine. The latency to onset of adenosine and 2-phenylaminoadenosine hyperalgesia is similar to that produced by prostaglandin E2, a directly acting hyperalgesic agent but shorter than that produced by leukotriene B4, which acts indirectly. 2-Phenylaminoadenosine hyperalgesia is prolonged by rolipram, a phosphodiesterase inhibitor. Both 2-phenylaminoadenosine and prostaglandin E2 hyperalgesia are antagonized by the A1-agonist N6-cyclopentyladenosine and the mu-agonist, [D-Ala2, NMe-Phe4, Gly-ol]enkephalin. However, 1-acetyl-2-(8-chloro-10,11-dihydrodibenz[b,f]oxazepine-10-ca rbonyl) hydrazine, a prostaglandin-receptor antagonist, inhibits prostaglandin E2 (Taiwo and Levine, Brain Res. 458, 402-406, 1988) but not 2-phenylamino-adenosine hyperalgesia and PD 081360-0002, the adenosine receptor antagonist, inhibits 2-phenylamino-adenosine but not prostaglandin E2 hyperalgesia. These data suggest that adenosine is a directly acting agent that produces hyperalgesia by an action at the A2-receptor and that this hyperalgesia is mediated by the cAMP second messenger.

Epicardial oxygen tensions during changes in arterial PO2 in pigs

Arterial hypoxemia decreased epicardial tissue PO2, measured by means of a multiwire surface electrode, as well as coronary venous PO2 and myocardial lactate extraction. Left ventricular blood flow increased, O2 delivery, O2 demand and O2 consumption of the left ventricle remained unchanged. Thus, epicardial and coronary venous PO2 indicated decreased capillary and interstitial PO2 rather than cellular hypoxia. A linear relation between mean epicardial PO2 and coronary venous PO2 proves both parameters equally effective in reflecting changes in myocardial tissue oxygenation. However, PO2 distribution curves provide additional information and epicardial PO2 is superior in models with regional changes of myocardial oxygenation.

Effects of exogenous adenosine in a patient with transplanted heart. Evidence for adenosine as a messenger in angina pectoris

In this pilot study some cardiac effects of exogenous adenosine on the denervated heart were studied in a patient with transplanted heart since 3 years. He was instrumented with catheters into the left coronary artery, the coronary sinus and the right ventricle. Adenosine was given in increasing doses intracoronarily, into the aorta at the diaphragmal level and into a peripheral vein. When given into the aorta pain was provoked dose-dependently and not different from a reference group. When given intracoronarily no pain was provoked except at the highest dose when a slight discomfort of the chest was provoked. After intravenous injection no pain was provoked in the chest or in adjacent structures. Coronary sinus flow increased dose-dependently and not different from the reference group. No increased heart rate response occurred after intravenous or intracoronary injections. Extensive degrees of sinus and AV nodal blockade occurred. In conclusion, the results are in keeping with a role for adenosine as a messenger between myocardial ischaemia and angina pectoris and cardiac sympathetic pressure response. The importance of innervation for proper sinus and AV nodal function was also illustrated.

Atrial bioenergetic variations in moderate hypoxia: danger or protective defense?

The effects of hypoxia on contractile tension and on tissue adenylate pool content, nicotinamide adenine nucleotide, NAD, nicotinamide adenine dinucleotide phosphate, NADP, and creatine phosphate, CrP, were investigated in isolated, spontaneously beating, guinea pig atria. When two different degrees of hypoxia were induced by lowering oxygen tension from 95% O2 (control) to 40% (moderate hypoxia) and 20% (severe hypoxia) for 30 min, contractile tension slowly decreased to 60% and 40% of control, respectively. In 40% O2 hypoxic atria, ATP was not significantly decreased, AMP slightly increased, TAN (total adenylate nucleotides) and adenylate energy charge [(ATP + 0.5 ADP)/(ATP + ADP + AMP)] did not change and creatine phosphate was decreased down to 53%. Hypoxic atria in 20% O2 showed a significant decrease of 26% in ATP, while ADP and AMP increased four and seven times, respectively. The adenylate energy-charge value was reduced from 0.93 to 0.70. Creatine phosphate decreased to below the analytical detection limit. Moderate hypoxia (40% O2), which induced a significant decrease of contractile tension but only minor changes of energetic tissue metabolism, was further investigated 2, 5, and 10 min after low oxygen tension was applied. Two stages of variations were evident during 30 min of experimental hypoxia. Within the first 10 min, concomitantly with atrial tension decrease, ATP, NAD, NADP, ATP/AMP, ATP/ADP, and TAN decreased, CrP began to decrease, inosine and xanthine showed no significant change. During the following 20 min of hypoxia, all parameters returned to the control levels with the exception of creatine phosphate. Adenylate energy charge did not change. The electrophysiological analysis of atrial cells did not show any major change in action potential configuration and resting potential, during 40% O2 hypoxia. The differences at metabolic level between moderate and more severe hypoxia suggest that the energetic state may be extremely unbalanced, in atrial tissue, as long as hypoxia is aggravated. Moreover, the time-course study, during 30 min of 40% O2, suggests that the early decrease of contractile tension does not depend on lowered energy availability, instead it might be, at least in part, a preventive measure to maintain energy balance in myocardial tissue to counteract hypoxic damage and, in this mechanism of defense, creatine phosphate shuttle seems to play a relevant role.

Production of AMP and adenosine in the interstitial fluid compartment of the isolated perfused normoxic guinea pig heart

The pathway of production of AMP and adenosine in the myocardial interstitial fluid compartment was studied in the isolated perfused normoxic guinea pig heart by collecting the transmyocardial effluent (t.m.e.) with the method of De Deckere and Ten Hoor (1977). Besides adenosine and inosine, AMP was found in t.m.e. Infusion of alpha,beta-methylene adenosine 5'-diphosphate (AOPCP), a specific inhibitor of the ecto 5'-nucleotidase, resulted in increases in t.m.e. AMP and inosine and a decrease in adenosine. Infusion of acetate producing a nearly twofold increase in myocardial AMP content did not increase the t.m.e. AMP even in the presence of AOPCP. In preparations made from 6-OH dopamine treated animals, the t.m.e. adenosine and inosine were reduced and AOPCP produced smaller increases in AMP and inosine, indicating that most if not all of the t.m.e. AMP originated from the sympathetic nerve terminals. Infusions of beta,gamma-imidoadenosine and beta,gamma-methylene adenosine 5'-triphosphate (AMPPNP and AMPPCP), non-hydrolysable analogs of ATP, resulted in dose-dependent increases in the t.m.e. AMP, which were much augmented in the presence of AOPCP. AMPPNP produced similar effects in 6-OH dopamine-treated preparations. As AMPPNP and AMPPCP are good substrates of ATP pyrophosphohydrolase, these findings indicate the presence of ATP pyrophosphohydrolase in the myocardial interstitial space.

Coffee, catecholamines and cardiac arrhythmia

It is generally agreed that a high release of noradrenaline in the heart may elicit cardiac arrhythmia. Furthermore, administration of aminophylline to patients with ischaemic heart disease frequently elicits ventricular premature beats and ventricular tachycardia. Recent data from animal experiments demonstrate that methylxanthines (aminophylline, caffeine, theophylline) can facilitate noradrenaline release from sympathetic nerve endings. Such facilitation of transmitter release is amplified when myocardial oxygenation is impaired, stressing its clinical significance in relation to ischaemic heart disease. If a similar amplification is also operative in man, a dose of methylxanthine causing a very modest increase in myocardial sympathetic transmitter release in the heart of a healthy subject may be sufficient to cause a three- to six-fold elevation in the local myocardial concentration of noradrenaline in a patient with ischaemic heart disease. We hypothesize that such hypoxia-induced facilitation of sympathetic transmitter release lies behind the reported connection between cardiac events and methylxanthines, for instance sudden cardiac death following coffee consumption.

Differential release of calcitonin gene-related peptide and neuropeptide Y from the isolated heart by capsaicin, ischaemia, nicotine, bradykinin and ouabain

The influence of various drugs as well as total ischaemia on the outflow of calcitonin gene-related peptide (CGRP), which is present in sensory nerves, and neuropeptide Y (NPY), which is co-stored with noradrenaline (NA), from the isolated guinea-pig heart, was studied in vitro. Capsaicin exposure and total ischaemia for 5-30 min induced a Ca2+-dependent increase in the outflow, suggesting release, of CGRP- but not NPY-like immunoreactivity (LI) from the heart. When characterized by high performance liquid chromatography (HPLC), the CGRP-LI present in heart extracts and the released CGRP-LI by capsaicin eluted in a major peak corresponding to synthetic CGRP. Incubation with morphine, indomethacin or reserpine pretreatment did not influence the capsaicin-evoked release of CGRP-LI. Capsaicin pretreatment depleted the cardiac content of CGRP-LI but not NPY-LI. The increase in perfusate volume observed after 30 min ischaemia in controls was reduced after capsaicin pretreatment. Nicotine exposure induced release of CGRP- as well as NPY-LI in a concentration- and Ca2+-dependent manner. The increased outflow of NPY-LI was not influenced by capsaicin pretreatment. Among other agents tested, bradykinin and ouabain caused increased outflow of CGRP but not of NPY-LI. Noradrenaline, tyramine, histamine, vasopressin, alpha,beta methylene ATP, ATP or adenosine induced changes in cardiac contractility or flow but did not evoke any detectable release of CGRP- or NPY-LI. In conclusion, the release of multiple neuropeptides can be studied in combination with contractile recordings using the isolated perfused guinea-pig whole heart preparation. Activation of cardiac sensory nerves by capsaicin, nicotine, bradykinin and ouabain, as well as ischaemia, induced release of CGRP while nicotine also evoked NPY release.

The role of adenosine and prostacyclin in coronary flow regulation in healthy man

Coronary flow regulation in man is incompletely understood. We addressed the hypothesis that endogenous adenosine and prostacyclin (PGI2) affect basal coronary tone and/or the coronary flow response to increased myocardial work. In healthy volunteers coronary sinus flow and cardiac oxygen extraction were measured at rest and during leg exercise (40-50 W), before and after i.v. administration of theophylline (to block adenosine receptors) and oral administration of ibuprofen (to inhibit prostaglandin synthesis). Before drug, the basal coronary flow was 118 +/- 23 ml min-1 and the cardiac extraction of oxygen 122 +/- 3 ml l-1. Leg exercise elevated coronary flow by 89 +/- 16 ml min-1. Theophylline, at a dose blocking the coronary flow response to dipyridamole (an adenosine-dependent mechanism), induced a moderate increase in myocardial oxygen extraction (by 11%, P less than 0.05), but failed to affect either the basal coronary flow (105 +/- 16 ml min-1) or the increase during exercise (88 +/- 25 ml min-1). Ibuprofen inhibited the urinary excretion of PGI2 metabolite by about 90%, but did not affect basal coronary tone or the coronary response to exercise. These data indicate that endogenous adenosine is of limited importance for normal coronary regulation in man, and that PGI2 has no effect.

Mediation by adenosine of bradycardia in rat heart during graded global ischaemia

The role of adenosine as a mediator of the bradycardia associated with graded global ischaemia in rat heart was examined. Hearts were perfused at 37 degrees C in the isovolumic mode with Krebs-bicarbonate medium at 12.0 ml/min/g. After equilibration, the coronary flow was reduced to 0.5, 2.5, or 5.0 ml/min/g for 20 min. Effluent was collected and assayed for adenosine and inosine by HPLC. Heart rate was measured and bipolar electrograms were obtained in severely ischaemic hearts. Basal adenosine release was 124 +/- 15 pmol/min/g. Adenosine release increased by approximately 50% in hearts perfused at 5.0 ml/min/g. In hearts perfused at 2.5 and 0.5 ml/min/g, adenosine release increased by approximately 1300 and 2300% respectively. The pattern of adenosine release at 0.5 and 2.5 ml/min/g was phasic, with adenosine release rate increasing to a maximum after about 10 min then dropping to values slightly higher than initial values. Ischaemia produced significant bradycardia and first degree AV block. Adenosine antagonism with 5 micron 8-phenyltheophylline blocked up to 25% of this bradycardia and significantly reduced the conduction delay. Adenosine release rate correlated closely with that component of heart rate slowing which was inhibited by 8-phenyltheophylline. It is concluded that adenosine released during graded global ischaemia mediates up to a quarter of the associated bradycardia. The effect of adenosine is phasic. Adenosine acts primarily to depress the sinus pacemaker. First degree AV block also occurs. These effects were only apparent at coronary flow rates below 5.0 ml/min/g.

Improved working capacity following theophylline infusion in patients with ischaemic heart disease

The effect of theophylline on the working capacity of patients with ischaemic heart disease was evaluated in a double-blind, randomized cross-over study. Eight patients, receiving no medication, with stable effort-provoked angina pectoris and typical exercise-induced ST depressions were studied. Following intravenous administration of theophylline or placebo, the patients did a supine leg exercise limited by intolerable chest pain. The workload was continuously increased by 10 W/min. Following theophylline treatment the workload at the onset of chest pain increased from 71 +/- 9 to 114 +/- 14 W (P less than 0.002). The ST depression was less pronounced following theophylline at submaximal exercise (-0.01 +/- 0.00 vs. -0.09 +/- 0.02 mV, P less than 0.005, at 70 W). The maximum tolerable workload increased from 129 +/- 15 after placebo infusion to 153 +/- 12 W after theophylline infusion P less than 0.01). It is speculated that this beneficial effect of the adenosine receptor antagonist theophylline may possibly be due to inhibition of a pathophysiological coronary steal induced by elevated levels of adenosine during ischaemia.

Effect of hypoxia on nerve-stimulation-induced release of noradrenaline from the rabbit heart

The present study addressed the hypothesis that cardiac production of adenosine (ADO) and/or prostacyclin (PGI2) during hypoxia is augmented to a level sufficient to affect nerve-stimulation-induced release of noradrenaline (NA). Innervated rabbit hearts were perfused at high (95% O2) or low (8% O2) oxygen pressure. The effluxes of NA and purines from the heart were determined by HPLC and that of the PGI2 metabolite by radioimmunoassay. Five minutes of hypoxia elevated effluent purines (sum of ADO, inosine, and hypoxanthine) from 1.1 microM to 6.2 microM, but did not affect the outflow of NA. The ADO receptor antagonists THEO (100-200 microM) and 8PSOT (100 microM) given during hypoxia increased the evoked outflow of NA by 77% (P less than 0.01) and 37% (P less than 0.05), respectively. Indomethacin (30 microM, a prostaglandin synthesis inhibitor) reduced the efflux of PGI2 metabolite by 93% but did not per se affect NA outflow during simultaneous administration of THEO, either under normoxia or hypoxia. It is concluded that ADO, but not PGI2, plays a role in reducing transmitter release during hypoxia. In addition, hypoxia leads to an enhancement of transmitter release, probably unrelated to ADO or purines. The lack of effect of hypoxia alone on evoked outflow of transmitter seems to be the result of a combination of these two processes.

Absolute rates of adenosine formation during ischaemia in rat and pigeon hearts

1. The activities of ecto- and cytosolic 5'-nucleotidase (EC 3.1.3.5), adenosine kinase (EC 2.7.1.20), adenosine deaminase (EC 3.5.4.4) and AMP deaminase (EC 3.5.4.6) were compared in ventricular myocardium from man, rats, rabbits, guinea pigs, pigeons and turtles. The most striking variation was in the activity of the ecto-5'-nucleotidase, which was 20 times less active in rabbit heart and 300 times less active in pigeon heart than in rat heart. The cytochemical distribution of ecto-5'-nucleotidase was also highly variable between species. 2. Adenosine formation was quantified in pigeon and rat ventricular myocardium in the presence of inhibitors of adenosine kinase and adenosine deaminase. 3. Both adenosine formation rates and the proportion of ATP catabolized to adenosine were greatest during the first 2 min of total ischaemia at 37 degrees C. Adenosine formation rates were 410 +/- 40 nmol/min per g wet wt. in pigeon hearts and 470 +/- 60 nmol/min per g wet wt. in rat hearts. Formation of adenosine accounted for 46% of ATP plus ADP broken down in pigeon hearts and 88% in rat hearts. 4. The data show that, in both pigeon and rat hearts, adenosine is the major catabolite of ATP in the early stages of normothermic myocardial ischaemia. The activity of ecto-5'-nucleotidase in pigeon ventricle (16 +/- 4 nmol/min per g wet wt.) was insufficient to account for adenosine formation, indicating the existence of an alternative catabolic pathway.

NADH content and lactate production in the perfused rabbit heart

The influence of oxygen availability and absence of contractile activity on the NADH content and lactate production were investigated in the rabbit heart. Isolated hearts were perfused according to Langendorff with a modified Tyrode solution, saturated with a gas mixture containing either 95% O2:5% CO2 (control), 50% O2:5% CO2 in N2 (hypoxia), or 5% CO2 in N2 (anoxia). In another series of hearts cardiac arrest was induced by perfusion with Tyrode solution (95% O2:5% CO2) where the KCl concentration was increased to 15 mmol l-1 (hyperkalemia). Oxygen uptake (VO2) was similar in hypoxic and control hearts (P greater than 0.05), whereas lactate production was four-fold higher during hypoxia vs. control (P less than 0.01). Hyperkalemia resulted in a 60% decrease in VO2 (P less than 0.05), and no significant change in lactate production vs. control (P greater than 0.05). Both PCr and ATP were substantially decreased only during anoxia. Muscle NADH, whose changes reflect those within the mitochondria, averaged (+/- SE) 0.074 +/- 0.010, 0.153 +/- 0.016, 0.486 +/- 0.162 and 1.771 +/- 0.091 mmol kg-1 dry wt during control, hyperkalemia, hypoxia and anoxia, respectively. It is concluded that: muscle contraction during conditions of adequate oxygen supply results in an oxidation of mitochondrial NADH (presumably due to ADP stimulation of respiration), and a decreased oxygen availability results in an increase in NADH and an accelerated lactate production, although the VO2 is not affected.

Prostacyclin as neuromodulator in the sympathetically stimulated rabbit heart

Prostaglandin E2 (PGE2) has previously been shown to inhibit sympathetic neurotransmission in different organs and species. Based on this inhibitory effect and on its reversal by cyclo-oxygenase inhibitors, PGE2 has been claimed to be a physiological modulator of in vivo release of norepinephrine (NE) from sympathetic nerves. It is now recognized that prostacyclin (PGI2) is the main cyclo-oxygenase product in the heart. We therefore addressed the question whether PGI2, within the same preparation, is formed in increased amounts during sympathetic nerve stimulation and has neuromodulatory activity. The effluent from isolated rabbit hearts subjected to sympathetic nerve stimulation or to infusion of NE or adenosine (ADO) was collected, and its content of PGE2 and 6-keto-PGF1 alpha (dehydration product of PGI2) was analyzed using gas chromatography/mass spectrometry, operated in the negative ion/chemical ionization mode. Other hearts were infused with PGI2 and nerve stimulation induced outflow of endogenous NE into the effluent was analyzed using HPLC with electrochemical detection. Nerve stimulation at 5 or 10 Hz (before but not after adrenergic receptor blockade), as well as infusion of NE (10(-6)-10(-5)M) or ADO (10(-4)M) increased the cardiac outflow of 6-keto-PGF1 alpha. Basal and nerve stimulation induced efflux of 6-keto-PGF1 alpha was approximately 5 times higher than the corresponding efflux of PGE2. PGI2 dose-dependently inhibited the outflow of NE from sympathetically stimulated hearts, the inhibition at 10(-6)M being approximately 40%. On the basis of these observations we propose that PGI2 is a more likely candidate than PGE2 as a potential modulator of neurotransmission in cardiac tissue in vivo.

An improved isolated working rabbit heart preparation using red cell enhanced perfusate

The performance of isolated working rabbit hearts perfused with Krebs-Henseleit (KH) buffer was compared with those in which the buffer was supplemented with washed human red blood cells (KH + RBC) at a hematocrit of 15 percent. When perfused with KH alone at 70 cm H2O afterload and paced at 240 beats/minute, coronary flow was more than double, whereas aortic flow was 40-60 percent of that in hearts perfused with KH + RBC, regardless of left atrial filling pressures (LAFP). Peak systolic pressure reached a plateau at 120 mm Hg in KH + RBC, but at 95 mm Hg in the KH group. Stroke work, however, was similar in the two groups. Despite the high coronary flow, oxygen uptake by hearts perfused with KH was substantially less and did not respond to increases in LAFP as in those perfused with KH + RBC. There was a 20 percent drop in ATP and glycogen content after 90 minutes' perfusion. In contrast, isolated hearts perfused with RBC-enriched buffer remained stable for at least 150 minutes. Irrespective of the perfusate, triacylglycerol content of the muscle remained at similar levels throughout the course of study. Increasing RBC in the perfusate from 15 percent to 25 percent had no additional effect on cardiac performance or oxygen consumption. Our findings demonstrate that in the isolated working rabbit heart inclusion of RBC in the perfusate improves mechanical and metabolic stability by providing an adequate oxygen supply.

How does dipyridamole elevate extracellular adenosine concentration? Predictions from a three-compartment model of adenosine formation and inactivation

Steady-state mathematical models are developed according to which adenosine is formed in the cytoplasm of a group of cells, arises in the extracellular space via the symmetric nucleoside transporter and is inactivated in the adenosine forming cells and after rate-limiting transport into other cell-types. Dipyridamole increases the Km and Vmax. of the transporter symmetrically with respect to influx and efflux. Models incorporating differing degrees of compartmentation are used to predict intracellular and extracellular adenosine concentration as a function of dipyridamole concentration and adenosine formation rate. The vasodilator action of dipyridamole is explained since it is predicted to elevate interstitial fluid adenosine concentrations at all rates of adenosine formation provided that washout of the interstitial compartment is restricted.

Cardiovascular effects of adenosine

The results, briefly summarized above, indicate that adenosine could be a physiologically important modulator of several aspects of cardiovascular regulation. Most cells are equipped with adenosine receptors. These receptors are of at least two subtypes which can be defined by the relative agonist potency. At these adenosine receptors, methylxanthines, including caffeine and theophylline, act as competitive antagonists. The role of adenosine antagonism, as a mechanism behind the cardiovascular effects of these xanthines, was recently reviewed (Fredholm, 1984). The concentrations of adenosine are low during resting conditions, but may be raised substantially by, for example, hypoxia, ischaemia and increased mechanical or biochemical work. The adenosine levels can also be raised by drugs, including uptake inhibitors such as dipyridamole. Already the concentrations of adenosine that occur during basal conditions are sufficient to produce significant effects, for example, on blood-flow. When the concentrations are raised the importance of endogenous adenosine becomes even greater. Adenosine may not only be of physiological significance but may also be pharmacologically important. First, there are several drugs that may act by affecting the levels of adenosine or by influencing its receptors. Second, the possibility exists that adenosine itself could be used clinically. For example, adenosine may be an attractive alternative to sodium nitroprusside or nitroglycerin when controlled hypotension is to be achieved. Adenosine may also be used to preserve blood platelets during extracorporal circulation or to produce selective regional vasodilatation. Both the physiological and pharmacological aspects are subject to intense study in several laboratories.

Inhibition by AD6 (8-monochloro-3-beta-diethylaminoethyl-4-methyl-7-ethoxycarbonyl methoxy coumarin) of platelet aggregation in dog stenosed coronary artery

The action of AD6 as an anti-thrombotic agent was studied in a model of coronary artery thrombosis and on platelet aggregation in the dog. AD6 (10-100 microM) in vitro inhibited aggregation induced by ADP, epinephrine, collagen and PAF (platelet aggregating factor) used at their threshold concentration for maximal aggregation. Arterial thrombosis was induced in a coronary vessel by critically reducing (about 70%) the vessel lumen. Thrombus formation was estimated by measuring coronary flow in the stenosed vessel. Using this procedure on the left descending coronary artery (LAD), we obtained reproducible blood flow changes in 18 dogs. AD6 was given i.v. at three different doses. At 0.25 mg/kg two out of four dogs showed decreased thrombus formation at the stenosis site. Seven out of eleven dogs treated with 0.5 mg/kg and two out of three treated with 1.5 mg/kg showed decreased thrombus formation. Major decreases in coronary resistance, evaluated by measuring blood flow in the unstenosed left circumflex artery (LCX), were evident only after the highest dose. We conclude that AD6 has an inhibitory action on dog platelet aggregation and reduces thrombus formation in a stenosed coronary vessel.

Adenosine-induced coronary release of prostacyclin at normal and low pH in isolated heart of rabbit

Rabbit hearts were perfused by the Langendorff method with drug-free perfusion medium or with a medium containing adenosine (10(-7) M-10(-4)M) and the coronary and transmyocardial efflux rates of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) were measured. Perfusion was performed both at pH 7.4 and 6.9. In other experiments the hearts were pre-labelled with [14C]-arachidonic acid and the coronary efflux of radioactivity and of labelled lipids and 6-keto-PGF1 alpha were determined. The basal coronary flow was elevated by almost 70% during tissue acidosis, in comparison to control. Adenosine induced a dose-dependent increase in the coronary flow, amounting to about 75% at normal pH and a drug concentration of 10(-5)M. The adenosine-induced increase in coronary flow was not facilitated by low pH. The base coronary efflux of 6-keto-PGF1 alpha from the hearts was 2.5--3.6 ng min-1. Adenosine (10(-6)-10(-5)M) significantly facilitated this efflux, up to 6.5 ng min-1. The efflux of 6-keto-PGF1 alpha was not changed by perfusion with acidic medium, either in the basal state or during perfusion with adenosine. The basal interstitial efflux of 6-keto-PGF1 alpha was 4.5-5.5 ng 3 min-1. This efflux was not affected by perfusion of the heart with adenosine-containing medium. In hearts pre-labelled with [14C]-arachidonic acid, adenosine (10 microM) induced a specific liberation of labelled lipid-extractable substances, including 6-keto-PGF1 alpha. 6 From these data we conclude that adenosine stimulates the liberation of 6-keto-PGFI. from the rabbit heart by increasing precursor availability and subsequent formation of prostacyclin in the coronary vessels. Furthermore, the increase in coronary flow induced by tissue acidosis is not related to an augmented formation of prostacyclin.