Release of adenosine from human hearts during angina induced by rapid atrial pacing

Purinergic signaling as a new mechanism underlying physical exercise benefits: a narrative review

In the last years, it has become evident that both acute and chronic physical exercise trigger responses/adaptations in the purinergic signaling and these adaptations can be considered one important mechanism related to the exercise benefits for health improvement. Purinergic system is composed of enzymes (ectonucleotidases), receptors (P1 and P2 families), and molecules (ATP, ADP, adenosine) that are able to activate these receptors. These components are widely distributed in almost all cell types, and they respond/act in a specific manner depending on the exercise types and/or intensities as well as the cell type (organ/tissue analyzed). For example, while acute intense exercise can be associated with tissue damage, inflammation, and platelet aggregation, chronic exercise exerts anti-inflammatory and anti-aggregant effects, promoting health and/or treating diseases. All of these effects are dependent on the purinergic signaling. Thus, this review was designed to cover the aspects related to the relationship between physical exercise and purinergic signaling, with emphasis on the modulation of ectonucleotidases and receptors. Here, we discuss the impact of different exercise protocols as well as the differences between acute and chronic effects of exercise on the extracellular signaling exerted by purinergic system components. We also reinforce the concept that purinergic signaling must be understood/considered as a mechanism by which exercise exerts its effects.

A2 Adenosine Receptor Subtypes Overproduction in Atria of Perioperative Atrial Fibrillation Patients Undergoing Cardiac Surgery: A Pilot Study

Objective: Although atrial fibrillation is a common cardiac arrhythmia in humans, the mechanism that leads to the onset of this condition is poorly elucidated. Adenosine is suspected to be implicated in the trigger of atrial fibrillation (AF) through the activation of its membrane receptors, mainly adenosine receptor (AR) subtypes A₁R and A₂R. In this study, we compared blood adenosine concentration (BAC), and A₁R, A_2AR, and A_2BR production in right (RA) and left atrium (LA), and on peripheral blood mononuclear cells (PBMCs) in patients with underlying structural heart disease undergoing cardiac surgery with or without peri-operative AF (PeOpAF).

Methods: The study group consisted of 39 patients (30 men and 9 women, mean age, range 65 [40–82] years) undergoing cardiac surgery and 20 healthy patients (8 women and 12 men; mean age, range 60 [39–72] years) as controls were included. Among patients, 15 exhibited PeOpAF.

Results: Blood adenosine concentration was higher in patients with PeOpAF than others. A_2AR and A_2BR production was higher in PBMCs of patients compared with controls and was higher in PeOpAF patients than other patients. In LA and RA, the production of A_2AR and A_2BR was higher in patients with PeOpAF than in other patients. Both A_2AR and A_2BR production were higher in LA vs. RA. A₁R production was unchanged in all situations. Finally, we observed a correlation between A₁R, A_2AR, and A_2BR production evaluated on PBMCs and those evaluated in LA and RA.

Conclusions: Perioperative AF was associated with high BAC and high A_2AR and A_2BR expression, especially in the LA, after cardiac surgery in patients with underlying structural heart disease. Whether these increases the favor in triggering the AF in this patient population needs further investigation.

Inhibition of Adenosine Pathway Alters Atrial Electrophysiology and Prevents Atrial Fibrillation

Background

Adenosine leads to atrial action potential (AP) shortening through activation of adenosine 1 receptors (A₁-R) and subsequent opening of G-protein-coupled inwardly rectifying K⁺ channels. Extracellular production of adenosine is drastically increased during stress and ischemia.

Objective

The aim of this study was to address whether the pharmacological blockade of endogenous production of adenosine and of its signaling prevents atrial fibrillation (AF).

Methods

The role of A₁-R activation on atrial action potential duration, refractoriness, and AF vulnerability was investigated in rat isolated beating heart preparations (Langendorff) with an A₁-R agonist [2-chloro-N⁶-cyclopentyladenosine (CCPA), 50 nM] and antagonist [1-butyl-3-(3-hydroxypropyl)-8-(3-noradamantyl)xanthine (PSB36), 40 nM]. Furthermore, to interfere with the endogenous adenosine release, the ecto-5′-nucleotidase (CD73) inhibitor was applied [5′-(α,β-methylene) diphosphate sodium salt (AMPCP), 500 μM]. Isolated trabeculae from human right atrial appendages (hRAAs) were used for comparison.

Results

As expected, CCPA shortened AP duration at 90% of repolarization (APD₉₀) and effective refractory period (ERP) in rat atria. PSB36 prolonged APD₉₀ and ERP in rat atria, and CD73 inhibition with AMPCP prolonged ERP in rats, confirming that endogenously produced amount of adenosine is sufficiently high to alter atrial electrophysiology. In human atrial appendages, CCPA shortened APD₉₀, while PSB36 prolonged it. Rat hearts treated with CCPA are prone to AF. In contrast, PSB36 and AMPCP prevented AF events and reduced AF duration (vehicle, 11.5 ± 2.6 s; CCPA, 40.6 ± 16.1 s; PSB36, 6.5 ± 3.7 s; AMPCP, 3.0 ± 1.4 s; P < 0.0001).

Conclusion

A₁-R activation by intrinsic adenosine release alters atrial electrophysiology and promotes AF. Inhibition of adenosine pathway protects atria from arrhythmic events.

The clinical application of purine nucleosides as biomarkers of tissue Ischemia and hypoxia in humans in vivo

During periods of ischemia and hypoxia, intracellular adenosine triphosphate stores are rapidly depleted. Its metabolism results in release of purine nucleosides into the systemic circulation. While the potential of purine nucleosides as a biomarker of ischemia has long been recognized, this has been limited by their complex physiological role and inherent instability leading to problematic sampling and prolonged, complex analysis procedures. Purine release has been demonstrated from cerebral tissue in patients undergoing carotid endarterectomy and patients presenting to hospital with stroke and transient ischemic attack. Rises in purine nucleosides have also been demonstrated in patients with angina and myocardial infarction, during systemic hypoxia, exercise, in patients with peripheral arterial disease and during surgery. This article reviews purine nucleoside production in ischemia, the development of purine analysis technology and details results of the studies investigating purine nucleosides as a biomarker of ischemia with suggestions for areas of future research.

Airway Exposure to Modified Multi-walled Carbon Nanotubes Perturbs Cardiovascular Adenosinergic Signaling in Mice

The broad list of commercial applications for multi-walled carbon nanotubes (MWCNT) can be further expanded with the addition of various surface chemistry modifications. For example, standard commercial grade MWCNT (C-grade) can be carboxylated (COOH) or nitrogen-doped (N-doped) to suite specific utilities. We previously reported dose-dependent expansions of cardiac ischemia/reperfusion (I/R) injury, 24 h after intratracheal instillation of C-grade, COOH, or N-doped MWCNT in mice. Here, we have tested the hypothesis that airway exposure to MWCNT perturbs cardiovascular adenosinergic signaling, which could contribute to exacerbation of cardiac I/R injury. 100 µL of Vehicle or identical suspension volumes containing 100 µg of C-grade, COOH, or N-doped MWCNT were instilled into the trachea of CD-1 ICR mice. 1 day later, we measured cyclic adenosine monophosphate (cAMP) concentrations in cardiac tissue and evaluated arterial adenosinergic smooth muscle signaling mechanisms related to nitric oxide synthase (NOS) and cyclooxygenase (COX) in isolated aortic tissue. We also verified cardiac I/R injury expansion and examined both lung histology and bronchoalveolar lavage fluid cellularity in MWCNT exposed mice. Myocardial cAMP concentrations were reduced (p < 0.05) in the C-grade group by 17.4% and N-doped group by 13.7% compared to the Vehicle group. Curve fits to aortic ring 2-Cl-Adenosine concentration responses were significantly greater in the MWCNT groups vs. the Vehicle group. Aortic constrictor responses were more pronounced with NOS inhibition and were abolished with COX inhibition. These findings indicate that addition of functional chemical moieties on the surface of MWCNT may alter the biological responses to exposure by influencing cardiovascular adenosinergic signaling and promoting cardiac injury.

Expressions of adenosine A2A receptors in coronary arteries and peripheral blood mononuclear cells are correlated in coronary artery disease patients

BACKGROUND

Altered coronary blood flow occurs in patients with coronary artery disease (CAD). Adenosine strongly impacts blood flow mostly via adenosine A_2A receptor (A_2AR) expressed in coronary tissues. As part of a systemic regulation of the adenosinergic system, we compared A_2AR expression in situ, and on peripheral blood mononuclear cells (PBMC) in CAD patients.

METHODS AND RESULTS

Aortic and coronary tissues, and PBMC were sampled in 20 CAD patients undergoing coronary artery bypass surgery and consecutively included. Controls were PBMC obtained from 15 healthy subjects. Expression and activity of A_2AR were studied by Western blotting and cAMP measurement, respectively. A_2AR expression on PBMC was lower in patients than in controls (0.83±0.31 vs 1.2±0.35 arbitrary units; p<0.01), and correlated with A_2AR expression in coronary and aortic tissues (Pearson's r: 0.77 and 0.59, p<0.01, respectively). Basal and maximal cAMP productions following agonist stimulation of PBMC were significantly lower in patients than in controls (120±42 vs 191±65 and 360±113 vs 560±215pg/10⁶ cells, p<0.05, respectively). In CAD patients, the increase from basal to maximal cAMP production in PBMC and aortic tissues was similar (+300% and +246%, respectively).

CONCLUSION

Expression of A_2AR on PBMC correlated with those measured in coronary artery and aortic tissues in CAD patients, A_2AR activity of PBMC matched that observed in aorta, and A_2AR expression and activity in PBMC were found reduced as compared to controls. Measuring the expression level of A_2AR on PBMC represents a good tool to address in situ expression in coronary tissues of CAD patients.

G-protein-coupled inward rectifier potassium current contributes to ventricular repolarization

AIMS

The purpose of this study was to investigate the functional role of G-protein-coupled inward rectifier potassium (GIRK) channels in the cardiac ventricle.

METHODS AND RESULTS

Immunofluorescence experiments demonstrated that GIRK4 was localized in outer sarcolemmas and t-tubules in GIRK1 knockout (KO) mice, whereas GIRK4 labelling was not detected in GIRK4 KO mice. GIRK4 was localized in intercalated discs in rat ventricle, whereas it was expressed in intercalated discs and outer sarcolemmas in rat atrium. GIRK4 was localized in t-tubules and intercalated discs in human ventricular endocardium and epicardium, but absent in mid-myocardium. Electrophysiological recordings in rat ventricular tissue ex vivo showed that the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) and acetylcholine (ACh) shortened action potential duration (APD), and that the APD shortening was reversed by either the GIRK channel blocker tertiapin-Q, the adenosine A1 receptor antagonist DPCPX or by the muscarinic M2 receptor antagonist AF-DX 116. Tertiapin-Q prolonged APD in the absence of the exogenous receptor activation. Furthermore, CPA and ACh decreased the effective refractory period and the effect was reversed by either tertiapin-Q, DPCPX or AF-DX 116. Receptor activation also hyperpolarized the resting membrane potential, an effect that was reversed by tertiapin-Q. In contrast, tertiapin-Q depolarized the resting membrane potential in the absence of the exogenous receptor activation.

CONCLUSION

Confocal microscopy shows that among species GIRK4 is differentially localized in the cardiac ventricle, and that it is heterogeneously expressed across human ventricular wall. Electrophysiological recordings reveal that GIRK current may contribute significantly to ventricular repolarization and thereby to cardiac electrical stability.

Blockade of A2B adenosine receptor reduces left ventricular dysfunction and ventricular arrhythmias 1 week after myocardial infarction in the rat model

BACKGROUND

Remodeling occurs after myocardial infarction (MI), leading to fibrosis, dysfunction, and ventricular tachycardias (VTs). Adenosine via the A2B adenosine receptor (A2BAdoR) has been implicated in promoting fibrosis.

OBJECTIVE

To determine the effects of GS-6201, a potent antagonist of the A2BAdoR, on arrhythmogenic and functional cardiac remodeling after MI.

METHODS

Rats underwent ischemia-reperfusion MI and were randomized into 4 groups: control (treated with vehicle), angiotensin-converting enzyme inhibitor (treated with enalapril 1 day after MI), GS-6201-1d (treated with GS-6201 1 day after MI), GS-6201-1w (treated with GS-6201 administered 1 week after MI) . Echocardiography was performed at baseline and 1 and 5 weeks after MI. Optical mapping, VT inducibility, and histologic analysis were conducted at follow-up.

RESULTS

Treatment with the angiotensin-converting enzyme inhibitor improved ejection fraction (57.8% ± 2.5% vs 43.3% ± 1.7% in control; P < .01), but had no effect on VT inducibility. Treatment with GS-6201 improved ejection fraction (55.6% ± 2.6% vs 43.3% ± 1.7% in control; P < .01) and decreased VT inducibility (9.1% vs 68.4% in control; P < .05). Conduction velocities were significantly higher at border and infarct zones in hearts of rats treated with GS-6201 than in those of other groups. The conduction heterogeneity index was also significantly lower in hearts of rats treated with GS-6201. Histologic analysis showed that while both GS-6201 and enalapril decreased fibrosis in the noninfarct zone, only GS-6201 reduced the heterogeneity of fibrosis at the border, which is consistent with its effect on VT reduction.

CONCLUSIONS

Treatment with an A2BAdoR antagonist at 1 week results in the improvement in cardiac function and decreased substrate for VT. The inhibition of fibrogenesis by A2BAdoR antagonists may be a new target for the prevention of adverse remodeling after MI.

Stress signaling by Tec tyrosine kinase in the ischemic myocardium

Nonreceptor tyrosine kinases have an increasingly appreciated role in cardiac injury and protection. To investigate novel tasks for members of the Tec family of nonreceptor tyrosine kinases in cardiac phenotype, we examined the behavior of the Tec isoform in myocardial ischemic injury. Ischemia-reperfusion, but not cardiac protective agents, induced altered intracellular localization of Tec, highlighting distinct actions of this protein compared with other isoforms, such as Bmx, in the same model. Tec is abundantly expressed in cardiac myocytes and assumes a diffuse intracellular localization under basal conditions but is recruited to striated structures upon various stimuli, including ATP. To characterize Tec signaling targets in vivo, we performed an exhaustive proteomic analysis of Tec-binding partners. These experiments expand the role of the Tec family in the heart, identifying the Tec isoform as an ischemic injury-induced isoform, and map the subproteome of its interactors in isolated cells.

Erythrocytic adenosine deaminase in post myocardial infarction angina patients

A comparative study on the levels of erythrocyte adenosine deaminase and lipid peroxidation has been undertaken in post myocardial infarction angina patients along with age and sex matched healthy individuals serving as control. Present findings show that levels of adenosine deaminase is highly elevated in post myocardial infarction angina patients compared to healthy persons. Malondialdehyde levels are also significantly increased in post myocardial infarction angina patients. The study shows that adenosine deaminase has an important implication in ischemic myocardial syndrome.

Adenosine increases human platelet levels of cGMP through nitric oxide: possible role in its antiaggregating effect

Adenosine is an endogenous antiaggregating substance that influences the platelet responses through specific A-type receptors that activate adenylate cyclase increasing the levels of 3',5'-cyclic adenosine monophosphate (cAMP). In this study, we investigated whether adenosine can also influence the levels of 3',5'-cyclic guanosine monophosphate (cGMP) and decrease the aggregating response of human platelets to adenosine-5-diphosphate (ADP) through this nucleotide. In platelet samples from healthy volunteers, we evaluated the effect of adenosine on ADP-induced aggregation and cyclic nucleotide synthesis. Some experiments were repeated in the presence of dipyridamole (inhibitor of adenosine uptake and phosphodiesterase activity), N(G)-monomethyl-L-arginine (L-NMMA, nitric synthase inhibitor), ionomycin (calcium ionophore), and ambroxol (2-amino-3,5-dibromo-N-[trans-4-hydroxycyclohexyl]benzylamine, inhibitor of nitric oxide (NO)-dependent activation of guanylate cyclase). Adenosine decreased the response to ADP in a concentration-dependent way (analysis of variance, ANOVA: P<.0001): cAMP levels increased from 30.0 +/- 2.0 (control) to 46.0 +/- 3.0 pmol/10(9) platelets (in the presence of 15 mumol/l adenosine) and cGMP levels increased from 5.6 +/- 1.0 (control) to 10.9 +/- 2.0 pmol/10(9) platelets (in the presence of 15 mumol/l adenosine). Also, nucleotide levels measured at the end of aggregation were higher in platelet samples exposed to adenosine than in controls. Dipyridamole at 40 mumol/l slightly increased adenosine's effects on both nucleotides. L-NMMA blunted the effect of adenosine on cGMP both in unstimulated samples and in aggregated platelets without any effect on cAMP synthesis. Platelet exposure to L-NMMA and ambroxol partially prevented adenosine's effect on ADP-induced aggregation. In conclusion, adenosine, which enhances intraplatelet cAMP levels, was determined to also cause an increase in cGMP concentrations through a mechanism that involves NO synthesis. This effect plays a direct role in the adenosine-induced antiaggregation.

Effects of percutaneous transluminal coronary angioplasty on coronary adenosine concentrations in humans

BACKGROUND

Even minimal amounts of adenosine is released during myocardial ischemia. Its role in coronary blood flow has been extensively studied, but little is known about its behaviour during percutaneous transluminal angioplasty (PTCA) in man.

MATERIAL AND METHODS

Using in situ samples the aim of this study was to evaluate adenosine plasma concentration before and after PTCA. Ten patients (8 men and 2 women, mean age 65 +/- 9 years) with a single stenosis of the left anterior descending coronary artery (LAD) of at least 70% and 10 healthy volunteers (4 men and 6 women, mean age 55 +/- 9 years) were included in the study.

RESULTS AND DISCUSSION

We found that there is a close relationship between the degree of the stenosis and the adenosine concentrations in the great cardiac vein and in the LAD, and that after PTCA there is a drop in adenosine concentration downstream from the stenosis. This study confirms the crucial role of adenosine in coronary blood flow control.

Enhanced accumulation of pericardial fluid adenosine and inosine in patients with coronary artery disease

Adenosine and inosine are believed to have cardioprotective effects. However, little is known about their possible role in the metabolic autoregulation of human coronaries and in pathologic conditions with supply/demand imbalance of the heart such as coronary artery disease. Since these low molecular weight nucleosides freely diffuse through the monolayer of the visceral pericardium, adenosine and inosine concentrations in pericardial fluid may well reflect the conditions in cardiac interstitium. The pericardial fluid and systemic venous blood adenosine and inosine concentrations were measured in 98 human subjects undergoing heart surgery for coronary artery disease or valvular heart disease. Adenosine and inosine concentrations were measured by HPLC with UV detection. In subjects with coronary artery disease pericardial fluid nucleoside concentrations were significantly higher than in patients with valvular heart disease (adenosine: 1545 (996-3146) nmol/L [median (25th-75th quartiles)] vs. 738 (390-2527) nmol/L, P<0.01; inosine: 658 (321-1331) nmol/L vs. 347 (159-1037) nmol/L, P<0.05), while in both patient groups pericardial fluid nucleoside concentrations were higher by an order of magnitude than in venous plasma. Our results show the enhanced release of adenosine and inosine by the ischemic myocardium as a marker of supply/demand imbalance and support the hypothesis that these cardiac nucleosides may have an important role in the adaptation of coronary blood flow in human coronary artery disease.

Changes in urinary uric acid excretion in obstructive sleep apnea before and after therapy with nasal continuous positive airway pressure

STUDY OBJECTIVE

To assess the utility of urinary uric acid excretion as a marker of nocturnal hypoxia in patients with obstructive sleep apnea-hypopnea syndrome (OSAHS) before and after the institution of nasal continuous positive airway pressure (CPAP).

DESIGN

Prospective, open.

SETTING

Sleep Disorders Laboratory, Veterans Affairs Medical Center.

PARTICIPANTS

Thirty consecutive male subjects, 20 with OSAHS and 10 without OSAHS.

MEASUREMENTS AND METHODS

Spot morning urine and venous blood samples were obtained in all subjects; samples were also obtained after the application of CPAP in those with OSAHS. Uric acid excretion, normalized to creatinine clearance, was calculated as the product of urinary uric acid and serum creatinine concentrations divided by urine creatinine concentration. In patients with OSAHS, uric acid excretion was 0.55+/-0.1 mg/dL before CPAP therapy and decreased to 0.30+/-0.01 mg/dL after CPAP therapy (p < 0.001). The latter value did not differ significantly from the mean value (0.32+/-0.03 mg/dL) in the control group. Uric acid excretion in OSAHS patients correlated significantly with the apnea-hypopnea index (r=0.42; p<0.0003).

CONCLUSION

Uric acid excretion is increased in OSAHS patients and normalizes after CPAP treatment, most likely reflecting differences in tissue oxygenation between the two conditions. Further studies in large number of patients may confirm the usefulness of this simple test for diagnosis and follow-up of patients with OSAHS.

Hyperpolarization-activated inward current in ventricular myocytes from normal and failing human hearts

BACKGROUND

The hyperpolarization-activated inward current (I[f]) was found to be overexpressed in hypertrophied rat ventricular myocytes, indicating that I(f) might favor arrhythmias in hypertrophied or failing ventricular myocardium. In the present study, we evaluated whether I(f) is expressed in human ventricular myocardium, if it may be increased in human heart failure, and if its autonomic modulation may be altered.

METHODS AND RESULTS

The whole-cell patch-clamp technique was used to record I(f) in isolated ventricular myocytes from 34 failing (dilated [DCM] or ischemic [ICM] cardiomyopathy) and 13 donor hearts (NF). I(f) was observed in all myocytes showing typical current properties, ie, time and voltage dependence, block by [Cs+]o, permeability for K+ and Na+, and current increase with raising [K+]o. There was a trend toward larger current densities in myopathic (at -130 mV in [K+]o 25 mmol/L; DCM: -1.37 +/- 0.12 pA/pF, n = 50; ICM: -1.39 +/- 0.24 pA/pF, n = 30) than in nonfailing cells (-1.18 +/- 0.21 pA/pF, n = 24), although this difference did not reach statistical significance (P=.23). Boltzmann distributions yielded an activation threshold of -80 mV and half-maximal activation at -110.96 +/- 0.06 mV in myopathic and normal myocytes. Isoproterenol (10(-5) mol/L) shifted the current activation by 10 mV (31 myopathic, 5 NF). Carbachol and adenosine had no direct effect on I(f) (6 and 12 myopathic, 3 and 3 NF, respectively) but reversibly antagonized beta-adrenergic stimulation (5 and 7 myopathic, 2 and 2 NF, respectively). Autonomic modulation was similar in failing and nonfailing cells.

CONCLUSIONS

In end-stage heart failure, no significant change of I(f) could be found, although there was a trend toward increased I(f). Together with an elevated plasma norepinephrine concentration and a previously reported reduction in I(K1) in human heart failure, I(f) might favor diastolic depolarization in individual myopathic cells.

Interplay between milrinone and adenosine in the inhibition of human platelet response

1. In this study, we investigated the influence of the inotropic agent and coronary vasodilator milrinone on platelet aggregation and intracellular levels of 3',5' cyclic adenosine monophosphate (cAMP) in human platelet-rich plasma (PRP) and whole blood (WB). Furthermore, we evaluated the influence of milrinone on the effects of adenosine, which reduces the platelet aggregation through an elevation of intraplatelet cAMP levels. 2. Milrinone decreased the platelet aggregation in response to agonists in both PRP and WB. A dose-dependent increase of intraplatelet cAMP levels was demonstrated: this result is in accordance with an effect on platelet phosphodiesterases. 3. Milrinone at low concentration and adenosine exerted additive effects on platelet aggregation and intraplatelet cAMP levels. 4. An interplay between milrinone and adenosine was shown in WB. Furthermore, dipyridamole, which prevents the uptake of endogenous adenosine, markedly enhanced the milrinone antiaggregating effect, whereas the adenosine receptor blocker, theophylline, decreased it. 5. The present data provide evidence that milrinone modulates the platelet function through an influence on intraplatelet levels of cAMP and it is able to interplay with substances stimulating adenylyl cyclase. 6. The interplay between milrinone and adenosine in the inhibition of the human platelet function could be effective during milrinone administration in the treatment of heart failure, when blood adenosine levels are significantly increased. These milrinone effects could be advantageous from a therapeutic point of view, since patients with heart failure are at risk of thrombosis and ischemic heart disease.

Glyceryl trinitrate enhances the adenosine-induced inhibition of platelet responses: a mechanism potentially involved in the in vivo anti-aggregating effects of organic nitrates

1. The present study investigated the influence of the organic nitrate glyceryl trinitrate (GTN) on the anti-aggregating effects of adenosine. We determined the effects of adenosine, GTN and their combination on platelet responses in platelet-rich plasma and whole blood, and on intracellular levels of 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP). 2. Adenosine inhibited the in vitro platelet aggregation in response to different agonists in a dose-dependent way through an elevation of intraplatelet cAMP levels. Effective adenosine concentrations were higher than those detectable under physiological conditions, but very close to levels achieved during myocardial ischaemia or haemorrhagic shock. 3. GTN was able to decrease platelet responses influencing intraplatelet cGMP levels. Furthermore, the drug increased the inhibitory effects of adenosine and enhanced its effects on intraplatelet cAMP levels. 4. The present data provides further evidence that compounds that increase intraplatelet levels of cGMP and cAMP act synergistically on the inhibition of platelet aggregability through the influence of increased cGMP levels on cAMP accumulation. The interplay between GTN and adenosine in the inhibition of platelet function could be effective during nitrate administration in the treatment of acute myocardial ischaemia when blood adenosine levels are significantly increased.

Attenuation of hypoxia-induced increases in ventilation by adenosine antagonists in rhesus monkeys

The respiratory effects of caffeine and paraxanthine, two xanthine adenosine antagonists with phosphodiesterase (PDE) activity, CGS 15943, a non-xanthine adenosine antagonist lacking PDE inhibitory activity, and rolipram, a non-xanthine PDE inhibitor lacking adenosine antagonist activity, were characterized in unanesthetized, seated rhesus monkeys exposed to 10% O2 balanced in N2 (hypoxia). Ventilation was measured continuously by enclosing the monkey's head in a fitted helmet and using a pressure-displacement plethysmographic technique. Respiratory frequency (f) and minute volume (VE) increased during 15-minute periods of hypoxia, and intramuscular administration of caffeine (0.3 and 1.0 mg/kg), paraxanthine (0.3 and 1.0 mg/kg) and CGS 15943 (0.03 and 0.1 mg/kg) attenuated the ventilatory response to hypoxia. In contrast, rolipram (0.003-0.03 mg/kg) did not significantly alter the ventilatory response to hypoxia. Drug effects also were characterized in monkeys exposed to air (normoxia) or 3%, 4% and 5% CO2 balanced in air (hypercapnia). Doses of caffeine, paraxanthine or CGS 15943 that attenuated the ventilatory response to hypoxia had no significant effect on f or VE during conditions of normoxia or hypercapnia. The results indicate that adenosine may play a major role in the function of peripheral, O2-sensitive mechanisms during hypoxia.

Pericardial fluid adenosine in ischemic and valvular heart disease

Adenosine released by ischemic myocardial cells stimulates coronary artery vasodilation. Measurement of adenosine concentrations in pericardial fluid in animal models of myocardial ischemia has been used to study the process of adenosine release. To determine whether pericardial fluid adenosine concentrations are increased in human ischemic heart disease, adenosine concentrations were measured in pericardial fluid in 23 subjects undergoing open-heart surgery for coronary artery disease. The results were compared with adenosine concentrations measured in pericardial fluid obtained from 20 subjects undergoing surgery for valvular heart disease. Adenosine concentrations also were measured in pleural fluid obtained during internal mammary artery bypass grafting. Adenosine concentrations were significantly increased in subjects with coronary artery disease compared with fluid obtained from subjects with valvular heart disease (2.47 +/- 0.24 vs 1.36 +/- 0.21 [SEM] microM [p = 0.0013]). Adenosine concentrations were higher in pleural fluid than pericardial fluid from the same individuals. Adenosine concentrations were significantly correlated with pericardial fluid cell counts and lactate dehydrogenase concentrations (r = 0.48; p = 0.0012 and r = 0.77, p = 0.0001, respectively). The results are consistent with myocardial release of adenosine in ischemic heart disease. If adenosine concentrations in pericardial fluid approximate those in myocardial interstitial fluid, sufficient adenosine is present to stimulate adenosine receptor activation in coronary artery smooth muscle.

Studies of the nucleoside transporter inhibitor, draflazine, in the human myocardium

1. The aim of the present study was to determine the effect of the nucleoside transporter inhibitor, draflazine, on the force of contraction in human myocardium and the affinity of the compound for the nucleoside transporter. Nucleoside transport inhibitors, like draflazine, are of potential importance for cardiopreservation of donor hearts for heart transplantation. 2. Functional experiments were performed in isolated electrically driven (1 Hz, 1.8 mmol l-1 Ca2+) human atrial trabeculae and ventricular papillary muscle strips. The affinity of draflazine for the myocardial nucleoside transporter was studied in isolated membranes from human ventricular myocardium and human erythrocytes in radioligand binding experiments using [3H]-nitrobenzylthioinosine ([3H]-NBTI). Dipyridamole was studied for comparison. 3. In membranes from human myocardium and erythrocytes, [3H]-NTBI labelled 1.18 pmol mg-1 protein and 23.0 pmol mg-1 protein, respectively, nucleoside transporter molecules with a KD value of 0.8 nmol l-1. Draflazine concentration-dependently inhibited binding of [3H]-NBTI to myocardial and erythrocyte membranes with a K(i)-value of 4.5 nmol l-1. The potency as judged from the K(i) values was ten times greater than that of dipyridamole in both myocardial and erythrocyte membranes. 4. Draflazine, at concentrations up to 100 mumol l-1, did not produce negative inotropic effects in atrial and ventricular myocardium. (-)-N6-phenylisopropyladenosine (R-PIA) and carbachol did not reduce force of contraction in ventricular myocardium, but exerted concentration-dependent direct negative inotropic effects in atrial myocardium. 5. The data provide evidence that draflazine specifically binds to the nucleoside transporter of the human heart and erythrocytes with high affinity. The compound does not produce negative inotropic effects at concentrations as high as 100 micromol 1-1.6. Draflazine could be a useful agent for cardio preservation because it does not produce cardio depressant effects. Thus, it may be possible to perfuse explanted hearts directly with this agent without the hazard of cardiodepression.

Antiadrenergic effect of carbachol but not of adenosine on contractility in the intact human ventricle in vivo

OBJECTIVES

The purpose of this study was to investigate the antiadrenergic effects of adenosine and carbachol on beta-adrenoceptor-stimulated human ventricular contractility in vivo. In addition, the antiadrenergic effects of adenosine and carbachol were compared in vitro.

BACKGROUND

Adenosine is reported to exhibit an antiadrenergic negative inotropic response in the beta-adrenergic-stimulated ventricular myocardium in vitro. The effect of adenosine is similar to the antiadrenergic effect of m-cholinoceptor stimulation in vitro.

METHODS

The inotropic response in vivo was assessed in seven healthy volunteers by M-mode echocardiography and simultaneous blood pressure monitoring. It was calculated as the increase in the rate-corrected velocity of circumferential fiber shortening and in the systolic pressure/dimension ratio. All volunteers received pretreatment with 450 mg of dipyridamole/day for 48 h. In addition, the effects of adenosine and carbachol in the presence of 0.03 mumol/liter of isoproterenol on cumulative concentration-response curves of isolated, electrically driven human ventricular muscle strips were compared in vitro (n = 13).

RESULTS

The positive inotropic response to continuous infusion of 20 ng/kg per min of isoproterenol (increase of rate-corrected velocity of circumferential fiber shortening [10.2 +/- 2.1% x square root of beats/min per ms] and increase of systolic pressure/dimension ratio 1.09 +/- 0.3 mm Hg/mm) was significantly (p < 0.01) reduced by 3.6 micrograms/kg body weight of intravenous carbachol (4.2 +/- 1.2% x square root of beats/min per ms, 0.21 +/- 0.18 mm Hg/mm) but not by 50 micrograms/kg of intravenous adenosine (8.2 +/- 3.1% x square root of beats/min per ms, 1.35 +/- 0.42 mm Hg/mm), although adenosine induced a significant negative dromotropic effect. In vitro comparison of force of contraction with cumulative concentration-response curves in the presence of 0.03 mumol/liter of isoproterenol demonstrated an EC50 value (concentration producing half-maximal effect) for adenosine 466 times higher than that for carbachol (65.3 vs. 0.14 mumol/liter, p < 0.001).

CONCLUSIONS

In contrast to carbachol, adenosine does not attenuate the catecholamine-induced increase in contractility in the human ventricle in vivo. These differences between the A1-adenosine receptor- and m-cholinoceptor-mediated effects could be due to fewer A1-adenosine receptors or a less efficient receptor-effector coupling, or both.

Systemic and cardiac neuroendocrine activation and severity of myocardial ischemia in humans

OBJECTIVES

The purpose of this study was to assess the effect of different degrees of ischemia on circulating and cardiac neurohormones and vasotone.

BACKGROUND

Neuroendocrine activation and subsequent systemic vasoconstriction may complicate ischemia. Whether this relates to severity of ischemia and subsequent cardiac dysfunction, and whether neurohormonal balance in the ischemic area changes, is unknown.

METHODS

Fifty-six normotensive patients with coronary artery disease were evaluated during incremental atrial pacing. On the basis of ST segment changes, patients were classified in a nonischemic (n = 11) or ischemic group (n = 45), the latter patients were subsequently classified as lactate (n = 28) or nonlactate (n = 17) producing, to identify neurohormonal changes in the effluent of the ischemic myocardium.

RESULTS

Angina occurred in 55%, 82% and 82% of patients in the nonischemic, lactate- and nonlactate-producing groups, respectively. Baseline hemodynamic variables and neurohormones were comparable in all groups, as were heart rate, rate-pressure product and coronary hemodynamic variables during pacing. In lactate producers, contractility did not improve, relaxation deteriorated, left ventricular filling pressure increased and cardiac output decreased during pacing, indicating more severe ischemia compared with that in nonlactate producers. Neurohormones did not change in the nonischemic group. In contrast, arterial and coronary venous catecholamines increased significantly more in lactate producers than in nonlactate producers (arterial norepinephrine by 68% vs. 36%, respectively). Moreover, arterial angiotensin II increased in lactate producers from a baseline mean +/- SEM of 6.8 +/- 0.9 to 9.7 +/- 1.6 pmol/liter (p < 0.05), accompanied by a sustained 23% increase in systemic resistance and arterial pressures. In lactate producers, baseline net cardiac norepinephrine release changed to net uptake during pacing (-0.05 +/- 0.02 vs. 0.06 +/- 0.05 nmol/min, p < 0.05). Epinephrine uptake increased in all patients with ischemia, albeit more in lactate producers.

CONCLUSIONS

Circulating catecholamines and renin-angiotensin levels are activated, and systemic vasotone is increased in relation to the degree of ischemia. Cardiac epinephrine uptake increases, whereas net baseline norepinephrine release from the ischemic myocardium changes to net uptake. Modulation of this neurohormonal activation may provide an alternative mode to limit ischemia.

Increased urinary loss of uric acid in adults with acute respiratory failure requiring mechanical ventilation

INTRODUCTION

The purpose of this study was to test a hypothesis of increased urinary excretion of uric acid as an indicator of adenosine triphosphate (ATP) degradation in adult patients with acute respiratory failure, and to look for a correlation to the clinical outcome.

STUDY DESIGN

Prospectively 31 patients with acute respiratory failure were studied. The patients were divided into two groups according to the clinical outcome: the need for solely supplemental oxygen (group 1), death or mechanical ventilation (group 2).

METHODS

Uric acid was determined by spectrophotometry.

RESULTS

Mean uric acid excretion was 39 mumol/kg (range, 7 to 92 mumol/kg) body weight/per 24 h in group 1 (16 patients) compared with 65 mumol/kg/24 h (range, 8 to 253 mumol/kg/24 h) in group 2 (13 patients were mechanically ventilated, and two patients died). The difference was highly significant (p less than 0.0001).

CONCLUSION

Increased amount of urinary uric acid was related to the severity of acute respiratory failure in adults.

Adenosine, the heart, and coronary circulation

Adenosine is known to regulate myocardial and coronary circulatory functions. Adenosine not only dilates coronary vessels, but attenuates beta-adrenergic receptor-mediated increases in myocardial contractility and depresses both sinoatrial and atrioventricular node activities. The effects of adenosine are mediated by two distinct receptors (i.e., A1 and A2 receptors). A1 adenosine receptors, located in atrial and ventricular myocardium and sinoatrial/atrioventricular nodes, are responsible for inhibition of adenylyl cyclase activity. A2 adenosine receptors, located in coronary endothelial and smooth muscle cells, are responsible for stimulation of this enzyme activity. During increased myocardial oxygen demand due to rapid pacing and exercise, although both coronary blood flow and adenosine concentrations in the myocardium and coronary efflux increased, there is no clear consensus explaining its cause and effect relation at present. However, ischemia/reperfusion-induced coronary hyperemia is believed to be mostly attributed to released adenosine, and it has been proven that adenosine attenuates the severity of ischemia due to its coronary vasodilatory action. The beneficial effects of adenosine during ischemia/reperfusion processes do not seem simple. This is because myocardial ischemia and reperfusion injury is caused by 1) activated leukocytes and platelets, 2) ATP depletion and calcium overload of myocardium, and 3) catecholamine release from the presynaptic nerves as well as 4) the impaired coronary circulation. Intriguingly adenosine attenuates all of these deleterious actions and thereby attenuates ischemia/reperfusion injury. Indeed, adenosine attenuates the severity of contractile dysfunction (myocardial stunning) and limits the infarct size. Thus, administration of adenosine or potentiators of adenosine production in the ischemic myocardium may be beneficial for the attenuation of ischemic and reperfusion injuries, although further clinical investigations are necessary.

Pharmacodynamic interactions between infused adenosine and oral theophylline

1. Five healthy human subjects were given, in single-blind fashion, either (a) 625 mg theophylline orally, followed 4 h later, by a 40 min infusion of adenosine (40 micrograms kg-1 min-1 for 5 min, 60 micrograms kg-1 min-1 for 5 min and 80 micrograms kg-1 min-1 for 30 min), or (b) 625 mg theophylline orally followed by 0.9% sodium chloride infusion, or (c) placebo theophylline tablets followed by adenosine infusion. 2. All five subjects experienced adverse effects during adenosine infusion, mainly at the higher infusion rates; two subjects also experienced chest pain but not during combined treatment with theophylline and adenosine. 3. Diastolic blood pressure (DBP) rose by 16.5 mmHg (P less than 0.001) following treatment with theophylline only, fell by 24.5 mmHg (P less than 0.001) during the adenosine infusion after placebo theophylline and remained unchanged during the adenosine infusion following theophylline. Pulse rate rose by 12 min-1 (P less than 0.01) during adenosine infusion following placebo, but not after theophylline alone or theophylline and adenosine combined. 4. The respiratory rate fell by 6 min-1 (P less than 0.01) during treatment with adenosine only, being lower than for the two treatments containing theophylline (P less than 0.05). 5. Plasma potassium and magnesium fell by 0.25 mmol l-1 (P less than 0.001) and 0.037 mmol l-1 (P less than 0.05), respectively, during treatment with theophylline only, but these effects were not altered by infusion of adenosine. 6. This study has demonstrated interactions between theophylline and adenosine on diastolic blood pressure and respiratory rate, but no interaction on metabolic parameters.(ABSTRACT TRUNCATED AT 250 WORDS)

Increase of Gi alpha in human hearts with dilated but not ischemic cardiomyopathy

In myocardial membranes from hearts with dilated cardiomyopathy (DCM), there was a 37% increase of the Gi alpha-protein as measured by 32P-ADP-ribosylation of a approximately 40 kDa pertussis toxin substrate. Immunoblotting techniques also showed increased amounts of Gi alpha in DCM. In hearts with ischemic cardiomyopathy (ICM), Gi alpha was not altered compared with nonfailing myocardium (NF). Basal and Gpp(NH)p-stimulated adenylate cyclase activity was reduced in DCM but not in ICM. The number of beta-adrenoceptors was similarly reduced both in DCM and ICM compared with NF. Alterations of m-cholinoceptors or A1-adenosine receptors did not occur. Consistently, "indirect" negative inotropic effects of the m-cholinoceptor agonist carbachol and the A1-adenosine receptor agonist R-PIA were not different in ICM, DCM, and nonfailing myocardium. In ICM and DCM, there was a marked reduction of the positive inotropic responses to isoprenaline and milrinone. However, there was a further reduction in DCM compared with ICM. It is concluded that the increase of Gi alpha is accompanied by a reduction of basal and guanine-nucleotide-stimulated adenylate cyclase activity. Alterations of m-cholinoceptors and A1-adenosine receptors do not appear to be involved. The further decrease of the positive inotropic effects of isoprenaline and milrinone in DCM provides evidence that the increase of Gi alpha is functionally relevant in DCM but not ICM and hence might contribute to the reduced effects of endogenous catecholamines and exogenous cAMP-dependent positive inotropic agents in the former but not the latter condition.

[Alterations of the cAMP-adenylate cyclase system in the failing human heart. Consequences for the therapy with inotropic drugs]

In heart failure, an increase in the activity of the sympathetic nervous system takes place to maintain perfusion pressure to vital organs, resulting in increased levels of noradrenaline in the blood of these patients. This permanent stimulation produces a down-regulation of cardiac beta-adrenoceptors. Since noradrenaline acts primarily on the cardiac beta 1-adrenoceptor subtype, beta 1-adrenoceptors decrease in number, whereas the beta 2-adrenoceptor subpopulation remains unchanged in most instances. Consequently, the positive inotropic response to beta-adrenoceptor agonists is diminished. However, there is also a decrease in the positive inotropic effect of beta 2-adrenoceptor agonists, histamine and cAMP-phosphodiesterase inhibitors such as milrinone, whereas the positive inotropic effect of cAMP-independent Na(+)-channel activators such as DPI 206-106 and the effects of cardiac glycosides are not diminished. These observations suggest a more generalised alteration of the cAMP-adenylate cyclase system in the failing heart. Stimulatory guanine nucleotide-binding protein (Gs) couples receptors to adenylate cyclase that stimulate cAMP formation, such as beta-adrenoceptors, histamine receptors and glucagon receptors. In the failing human heart, Gs content has been reported to remain unchanged as compared with that in non-failing myocardium. However, there is a 35%-40% increase in inhibitory guanine nucleotide-binding proteins (Gi), which are involved in the receptor-mediated inhibition of adenylate cyclase. Taken together, two defects of the cAMP-adenylate cyclase system have been identified: an increase in Gi content and a decrease in the number of beta-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine potentiates the inhibitory effects of calcium channel antagonists on human platelet aggregation induced by thromboxane A2 or U46619

Calcium channel antagonists inhibit platelet function in vitro and ex vivo, but the mechanism responsible has not been clearly defined. The concentrations of these agents required to inhibit platelet aggregation in vitro are several fold higher than those attained in vivo. Adenosine, a known inhibitor of platelet function, is produced in large quantities in ischemic myocardium. In order to test the hypothesis that adenosine may potentiate the platelet-inhibitory effects of calcium channel antagonists, we studied the effect of adenosine plus nifedipine, verapamil or diltiazem on human platelet aggregation induced by thromboxane A2 or the stable endoperoxide/thromboxane A2 mimic, U46619 +/- epinephrine. Adenosine, in concentrations achieved in the plasma during myocardial ischemia (0.01-0.1 microM), enhanced the inhibitory effects of nifedipine, verapamil and diltiazem on platelet aggregation 5-100 fold. The same concentrations of adenosine alone did not inhibit platelet aggregation. In the presence of non-inhibitory concentrations of adenosine, nifedipine, in concentrations approaching those attained in vivo following standard therapeutic doses (as low as 0.29 microM), significantly inhibited thromboxane A2-induced platelet aggregation. Therefore, adenosine potentiates the in vitro inhibitory effects of calcium channel antagonists on platelet aggregation induced by thromboxane A2 or thromboxane A2 plus epinephrine. These results suggest that adenosine production by ischemic myocardium may augment the inhibitory effect of calcium channel antagonists on platelets.

Antagonism of novel inotropic agents at A1 adenosine receptors and m-cholinoceptors in human myocardium

The effects of the new inotropic agents saterinone, sulmazole, UD-CG 212.Cl and milrinone at A1 adenosine receptors and m-cholinoceptors were evaluated in human myocardium from patients with heart failure. At A1 adenosine receptors, all compounds inhibited 3H-DPCPX-binding to ventricular membrane preparations at micromolar concentrations. As judged from the K1-values, the rank order of potency was saterinone greater than sulmazole greater than UD-CG 212.Cl greater than milrinone. The new inotropic agents also displaced the binding of 3H-QNB at m-cholinoceptors. Except for saterinone, the concentration ranges of mean Ki-values were considerably higher at m-cholinoceptors than at A1 adenosine receptors. The rank order of potency was saterinone greater than sulmazole greater than UD-CG 212.Cl greater than milrinone. Competition of the A1 adenosine receptor agonist R-PIA to 3H-DPCPX-binding showed a biphasic curve with a shallow slope (Hill coefficient nH = 0.63) and revealed two affinity states of the A1 adenosine receptor. In the presence of guanine nucleotides [Gpp(NH)p], the competition curve showed one low affinity class of binding sites and was shifted to the right. In contrast, the competition curves of the new inotropic agents were characterized by a monophasic, steeper slope (mean Hill coefficient nH = 0.98). Guanine nucleotides had no effect. Similar results were obtained with saterinone and carbachol at m-cholinoceptors. Competition with carbachol revealed three affinity states of the m-cholinoceptor, the super-high affinity binding was reversed by Gpp(NH)p. Competition with saterinone revealed one class of binding sites which was not influenced by Gpp(NH)p.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical indicators of myocardial ischaemia during coronary artery bypass grafting

Metabolic indicators of myocardial ischaemia were measured in coronary sinus blood in six patients undergoing coronary artery bypass grafting (CABG). Five arterial and coronary sinus blood samples were taken in each case--one before cardiopulmonary bypass (CPB), and three during and one after CPB. Moderate hypothermia with topical cardiac cooling and cold cardioplegia were used. Myocardial infarction occurred perioperatively in two patients. Myocardial lactate production was not found before CPB in any patient, but it was common during CPB. Adenosine, inosine and hypoxanthine were released into the coronary sinus blood, but their release did not correlate significantly with lactate production. Myocardial noradrenaline production showed positive correlation with lactate levels (p less than 0.05). Release of adrenaline from the myocardium during CABG was also demonstrated. Myocardial catecholamine production was especially seen in the patients with myocardial infarction. Myocardial catecholamine release seemed to be the most sensitive of the studied biochemical indicators of myocardial ischaemia during CABG.

Characterization of A1 adenosine receptors in atrial and ventricular myocardium from diseased human hearts

The purpose of the present study was to characterize adenosine receptors in human atrial and ventricular myocardium. In isolated electrically driven preparations, adenosine produced "direct" negative inotropic effects in atrial myocardium (AT). In ventricular myocardium (VE), it only had negative inotropic properties when force of contraction had been stimulated with isoprenaline ("indirect" effect), but it has no inotropic effect alone. The adenosine receptor antagonist 8-phenyltheophylline antagonized the "direct" and "indirect" effects; these findings indicated that both effects were mediated by adenosine receptors. In cardiac membranes from human AT and VE, adenosine receptors were characterized with [3H]-8-cyclopentyl-1,3-dipropylxanthine (DPCPX) binding. The effects of agonists R-(-)-N6-phenylisopropyladenosine (R-PIA), S-(+)-N6-phenylisopropyladenosine (S-PIA), and 5'-(N-ethylcarboxamido) adenosine (NECA) and the effects of guanine nucleotides [Gpp(NH)p] were studied also. The antagonist affinities as judged from the apparent affinity, Kd, of [3H]DPCPX were similar in AT (2.2 nmol/l; 95% confidence limits, 1.4-3.7) and VE (1.8 nmol/l; 95% confidence limits, 1.0-3.0). The number of adenosine receptors was 1.7 times greater in AT (26.9 +/- 2.33 fmol/mg protein; n = 5) than in VE (16.2 +/- 2.3 fmol/mg protein; n = 5). High and low affinity states of adenosine receptors evaluated with the influence of Gpp(NH)p on agonist competition with R-PIA were similar in AT or VE. The rank orders of potency for agonists (R-PIA greater than S-PIA greater than NECA) and antagonists (DPCPX greater than 8-phenyltheophylline greater than theophylline) were characteristic for the A1 receptor subtype. It is concluded that A1 adenosine receptors exist in the human myocardium. Since binding properties were similar in AT and VE, the same A1 adenosine receptor probably couples to different effectors in a similar guanine nucleotide-dependent way. [3H]DPCPX is the first radiolabeled antagonist ligand that allows detection of A1 adenosine receptors and their coupling in the human myocardium.

Inotropic response to DPI 201-106 in the failing human heart

1. The present study was designed to characterize the positive inotropic response to DPI 201-106 in isolated papillary muscle strips obtained from heart failure patients undergoing surgery. 2. The positive inotropic responses to isoprenaline and milrinone and cardiac beta-adrenoceptor density were also determined. 3. DPI 201-106 increased the force of contraction in papillary muscle strips from patients with moderate (NYHA II-III) and severe (NYHA IV) heart failure, in a concentration-dependent manner. This positive inotropic effect was more pronounced in tissues from NYHA IV patients. Furthermore, these responses were greater than those produced by milrinone or isoprenaline. The positive inotropic effects of isoprenaline and milrinone were reduced in NYHA IV compared to NYHA II-III. Consistently, there was also a smaller density of beta-adrenoceptors in myocardium from NYHA IV than in NYHA II-III. The positive inotropic effect of Ca2+ was similar in tissues from both groups of patients. 4. The positive inotropic effect of DPI 201-106 was not antagonized by adenosine or carbachol, whereas both compounds reduced the positive inotropic effect of isoprenaline. 5. DPI 201-106 did not increase the Ca2+ -sensitivity of chemically skinned ventricular fibres, whereas a significant increase of the Ca2+ -sensitivity was obtained with trifluoperazine. 6. It is concluded that DPI 201-106 produces significant positive inotropic effects in tissue excised from the failing human heart. The lack of inhibition by adenosine and carbachol might contribute to its greater effectiveness in NYHA IV than NYHA II-III and indicates that its mechanism of action is cyclic AMP-independent. A sensitization of the contractile proteins to Ca2+ does not appear to be important for the positive inotropic action of DPI 201-106 in the failing human heart.

Adenylate degradation products release from the human myocardium during open heart surgery

Purine degradation products were determined in the human heart coronary sinus effluent collected from patients undergoing cardiac surgery, during infusion of a cardioplegic solution. At the onset of cardiopulmonary bypass the mean concentrations of adenosine, inosine and hypoxanthine were 0.1, 0.5 and 0.3 mumol/l, respectively. Ischemic arrest leads to a progressive increase of the respective levels to 1.4 17.8 and 9.6 mumol/l after 60-80 min of ischemia. Xanthine concentration was undetectable (less than 0.2 mumol/l) throughout. A substantial urate release (20 mumol/l) was observed which decreased with the duration of ischemia. Xanthine oxidoreductase activity in human myocardium was found to be below the detection limit (0.1 mU/g wet weight). Thus, urate release represented wash out of urate which had accumulated in the tissue.

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.

Release of adenosine and lactate from human hearts during atrial pacing in patients with ischemic heart disease

Thirty-eight patients treated by atrial pacing were divided into three groups (Group I, patients with neither coronary stenosis nor anginal pain during pacing; Group II, patients with no coronary stenosis but having anginal pain during pacing; Group III, patients with coronary stenosis). The concentrations of adenosine and lactate were measured in the coronary sinus blood and in the arterial blood before, during, and after atrial pacing. During atrial pacing, significant levels of adenosine were released from the heart of patients in Group III, whereas significant lactate release was observed in Groups II and III. In Group II, the concentration of adenosine in coronary sinus blood appeared to increase during pacing, but not significantly. There was no significant correlation between the release of adenosine and that of lactate. A significant release of adenosine due to atrial pacing may be observed only in patients with coronary artery disease.

Human basophil/mast cell releasability. VII. Heterogeneity of the effect of adenosine on mediator secretion

5'-N-ethylcarboxamideadenosine (NECA) greater than 2-chloroadenosine greater than adenosine greater than N6-(R-phenyl-isopropyl)-adenosine (R-PIA) inhibited in vitro anti-IgE-induced histamine and peptide leukotriene C4 (LTC4) release from human basophils in a concentration-dependent fashion. Micromolar concentrations of adenosine, NECA and R-PIA potentiated the anti-IgE-stimulated release of histamine and LTC4 from human lung parenchymal mast cells. Submillimolar concentrations of adenosine, NECA and R-PIA inhibited in a concentration dependent manner the release of histamine and prostaglandin D2 (PGD2) from skin mast cells challenged with anti-IgE. These results demonstrate marked heterogeneity of the modulatory effect exerted by adenosine on mediator release from human basophils and mast cells.

Reaction kinetics and inhibition of adenosine kinase from Leishmania donovani

The reaction kinetics and the inhibitor specificity of adenosine kinase (ATP:adenosine 5'-phosphotransferase, EC 2.7.1.20) from Leishmania donovani, have been analysed using homogeneous preparation of the enzyme. The reaction proceeds with equimolar stoichiometry of each reactant. Double reciprocal plots of initial velocity studies in the absence of products yielded intersecting lines for both adenosine and Mg2+-ATP. AMP is a competitive inhibitor of the enzyme with respect to adenosine and noncompetitive inhibitor with respect to ATP. In contrast, ADP was a noncompetitive inhibitor with respect to both adenosine and ATP, with inhibition by ADP becoming uncompetitive at very high concentration of ATP. Parallel equilibrium dialysis experiments against [3H]adenosine and [gamma-32P]ATP resulted in binding of adenosine to fre enzyme. Tubercidin (7-deazaadenosine) and 6-methyl-mercaptopurine riboside acted as substrates for the enzyme and were found to inhibit adenosine phosphorylation competitively in vitro. 'Substrate efficiency (Vmax/Km)' and 'turnover numbers (Kcat)' of the enzyme with respect to specific analogs were determined. Taken together the results suggest that (a) the kinetic mechanism of adenosine kinase is sequential Bi-Bi, (b) AMP and ADP may regulate enzyme activity in vivo and (c) tubercidin and 6-methylmercaptopurine riboside are monophosphorylated by the parasite enzyme.

Adenosine causes transient dilatation of coronary arteries in man

We investigated the effects of i.v. adenosine on coronary blood flow in 10 normal subjects undergoing investigation for chest pain. Coronary flow transiently doubled after greater than or equal to 3.5 mg adenosine without increase in perfusion pressure, systolic load or inotropic state at a constant, paced heart rate. The data provide direct evidence that adenosine dilates coronary arteries in man. The transience of the effect suggests a possible role for adenosine in repeated estimations of coronary flow reserve.

Plasma hypoxanthine and exercise

During exercise, ATP is converted to ADP and AMP to supply energy for muscular contraction. It is then regenerated via various pathways of intermediary metabolism. However, with high levels of exercise, net ATP degradation in muscle occurs. In exercise and other clinical situations, adenine nucleotide degradation leads to an accumulation of degradative purine products including hypoxanthine. In an effort to monitor events of energy metabolism, we examined plasma hypoxanthine levels at various exercise intensities. Peak plasma hypoxanthine levels after maximal exercise (18.9 +/- 2.6 microM, mean +/- SEM) were significantly greater than resting levels (1.1 +/- 0.1 microM; p less than 0.001). Hypoxanthine levels after steady state exercise at 52, 76, and 97% of ventilatory threshold did not exceed resting levels. However, plasma hypoxanthine rose significantly after exercise at 124% of ventilatory threshold (6.3 +/- 1.0 microM; p less than 0.01) and at 152% of ventilatory threshold (17.0 +/- 3.6 microM; p less than 0.001). Exercise at subventilatory threshold intensity (74% of ventilatory threshold) for a prolonged time period, such that total work equaled that performed at 152% of ventilatory threshold, did not elevate hypoxanthine levels (0.46 +/- 0.1 microM) above resting values. We conclude that elevation of plasma hypoxanthine levels occur during exercise at intensities that exceed the ventilatory threshold and indicate that net adenine nucleotide degradation has occurred.