Role of A(2A)-adenosine receptor activation for ATP-mediated coronary vasodilation in guinea-pig isolated heart

Pleiotropic effects of clopidogrel

Clopidogrel is a widely prescribed prodrug with anti-thrombotic activity through irreversible inhibition of the P2Y₁₂ receptor on platelets. It is FDA-approved for the clinical management of thrombotic diseases like unstable angina, myocardial infarction, stroke, and during percutaneous coronary interventions. Hepatic clopidogrel metabolism generates several distinct metabolites. Only one of these metabolites is responsible for inhibiting the platelet P2Y₁₂ receptor. Importantly, various non-hemostatic effects of clopidogrel therapy have been described. These non-hemostatic effects are perhaps unsurprising, as P2Y₁₂ receptor expression has been reported in multiple tissues, including osteoblasts, leukocytes, as well as vascular endothelium and smooth muscle. While the "inactive" metabolites have been commonly thought to be biologically inert, recent findings have uncovered P2Y₁₂ receptor-independent effects of clopidogrel treatment that may be mediated by understudied metabolites. In this review, we summarize both the P2Y₁₂ receptor-mediated and non-P2Y₁₂ receptor-mediated effects of clopidogrel and its metabolites in various tissues.

Coronary microvascular dysfunction in hypertrophic cardiomyopathy: Pathophysiology, assessment, and clinical impact

Myocardial ischemia constitutes one of the most important pathophysiological features in hypertrophic cardiomyopathy. Chronic and recurrent myocardial ischemia leads to fibrosis, which may culminate in myocardial dysfunction. Since the direct visualization of coronary microcirculation in vivo is not possible, its function must be studied indirectly. Invasive and noninvasive techniques allow microcirculatory dysfunction to be evaluated, including echocardiography, magnetic resonance, positron emission tomography, and cardiac catheterization. Blunted myocardial blood flow and coronary flow reserve have been suggested to associate with unfavorable prognosis. Microcirculatory dysfunction may be one additional important parameter to take into account for risk stratification beyond the conventional risk factors.

Cystic fibrosis transmembrane conductance regulator-dependent bicarbonate entry controls rat cardiomyocyte ATP release via pannexin1 through mitochondrial signalling and caspase activation

AIM

Cystic fibrosis transmembrane conductance regulator (CFTR) is expressed in the heart, but its function there is unclear. CFTR regulates an ATP release pore in many tissues, but the identity and regulatory mechanism of the pore are unknown. We investigated the role of CFTR in ATP release from primary cardiomyocytes and ventricular wall in vivo.

METHODS

Proteins involved in the signalling pathway for ATP release during simulated ischaemia (lactic acid treatment) were investigated using inhibitors and siRNA; colocalization was identified by coimmunofluorescence and proximity ligation assays; changes in near-membrane pH and calcium were identified with total internal reflection microscopy; in vivo ATP release was investigated using interstitial microdialysis of rat heart.

RESULTS

Lactic acid-induced CFTR-dependent ATP release from cultured cardiomyocytes and left ventricle in vivo. Lactic acid entry elevated near-membrane calcium, which involved Na/H- and Na/Ca-exchangers colocalized with CFTR. Calcium entry-induced CFTR activation, which involved cAMP, protein kinase A, FAK, Pyk2 and Src. Removal of extracellular bicarbonate abolished cardiomyocyte ATP release induced by lactic acid or CFTR activators. Bicarbonate stimulated cytochrome c expression, cytochrome c release and ATP release from isolated cardiomyocyte mitochondria. Pannexin 1 (Panx1) colocalized with CFTR. Lactic acid increased cardiomyocyte caspase activity: caspase inhibitors or Panx1 siRNA abolished cardiomyocyte ATP release, while pannexin inhibition abolished cardiac ATP release in vivo.

CONCLUSION

During simulated ischaemia, CFTR-dependent bicarbonate entry stimulated ATP and cytochrome c release from mitochondria; in the cytoplasm, cytochrome c-activated caspase 3, which in turn activated Panx1, and ATP was released through the opened Panx1 channel.

Adenosine and lipids: A forced marriage or a love match?

Adenosine is a fascinating compound, crucial in many biochemical processes: this ubiquitous nucleoside serves as an essential building block of RNA, is also a component of ATP and regulates numerous pathophysiological mechanisms via binding to four extracellular receptors. Due to its hydrophilic nature, it belongs to a different world than lipids, and has no affinity for them. Since the 1970's, however, new discoveries have emerged and prompted the scientific community to associate adenosine with the lipid family, especially via liposomal preparations and bioconjugation. This seems to be an arranged marriage, but could it turn into a true love match? This review considered all types of unions established between adenosine and lipids. Even though exciting supramolecular structures were observed with adenosine-lipid conjugates, as well as with liposomal preparations which resulted in promising pre-clinical results, the translation of these technologies to the clinic is still limited.

Augmentation of Muscle Blood Flow by Ultrasound Cavitation Is Mediated by ATP and Purinergic Signaling

BACKGROUND

Augmentation of tissue blood flow by therapeutic ultrasound is thought to rely on convective shear. Microbubble contrast agents that undergo ultrasound-mediated cavitation markedly amplify these effects. We hypothesized that purinergic signaling is responsible for shear-dependent increases in muscle perfusion during therapeutic cavitation.

METHODS

Unilateral exposure of the proximal hindlimb of mice (with or without ischemia produced by iliac ligation) to therapeutic ultrasound (1.3 MHz, mechanical index 1.3) was performed for 10 minutes after intravenous injection of 2×10⁸ lipid microbubbles. Microvascular perfusion was evaluated by low-power contrast ultrasound perfusion imaging. In vivo muscle ATP release and in vitro ATP release from endothelial cells or erythrocytes were assessed by a luciferin-luciferase assay. Purinergic signaling pathways were assessed by studying interventions that (1) accelerated ATP degradation; (2) inhibited P2Y receptors, adenosine receptors, or K_ATP channels; or (3) inhibited downstream signaling pathways involving endothelial nitric oxide synthase or prostanoid production (indomethacin). Augmentation in muscle perfusion by ultrasound cavitation was assessed in a proof-of-concept clinical trial in 12 subjects with stable sickle cell disease.

RESULTS

Therapeutic ultrasound cavitation increased muscle perfusion by 7-fold in normal mice, reversed tissue ischemia for up to 24 hours in the murine model of peripheral artery disease, and doubled muscle perfusion in patients with sickle cell disease. Augmentation in flow extended well beyond the region of ultrasound exposure. Ultrasound cavitation produced an ≈40-fold focal and sustained increase in ATP, the source of which included both endothelial cells and erythrocytes. Inhibitory studies indicated that ATP was a critical mediator of flow augmentation that acts primarily through either P2Y receptors or adenosine produced by ectonucleotidase activity. Combined indomethacin and inhibition of endothelial nitric oxide synthase abolished the effects of therapeutic ultrasound, indicating downstream signaling through both nitric oxide and prostaglandins.

CONCLUSIONS

Therapeutic ultrasound using microbubble cavitation to increase muscle perfusion relies on shear-dependent increases in ATP, which can act through a diverse portfolio of purinergic signaling pathways. These events can reverse hindlimb ischemia in mice for >24 hours and increase muscle blood flow in patients with sickle cell disease.

CLINICAL TRIAL REGISTRATION

URL: http://clinicaltrials.gov. Unique identifier: NCT01566890.

ATP-induced cardioprotection against myocardial ischemia/reperfusion injury is mediated through the RISK pathway

The aim of the present study was to examine the post-infarct acute effect of adenosine-5'-triphosphate (ATP) on myocardial infarction (MI) size as well as its precise molecular mechanism. Sixty New Zealand white male rabbits were exposed to 40 min of ischemia followed by 180 min of reperfusion. The rabbits were intravenously administered 3 mg/kg of ATP (ATP group) or saline (control group) immediately after reperfusion and maintained throughout the first 30 min. The wortmannin+ATP, PD-98059+ATP, and 5-hydroxydecanoic acid (5-HD) sodium salt+ATP groups were separately injected with wortmannin (0.6 mg/kg), PD-98059 (0.3 mg/kg), and 5-HD (5 mg/kg) 5 min prior to ATP administration. MI size was calculated as the percentage of the risk area in the left ventricle. Myocardial apoptosis was determined using a TUNEL assay. Western blot analysis was performed to examine the levels of protein kinase B (Akt)/p-Akt and extracellular signal-regulated kinase (ERK)/p-ERK in the ischemic myocardium, 180 min after reperfusion. The infarct size was significantly smaller in the ATP group than in the control group (p<0.05). The infarct size-reducing effect of ATP was completely blocked by wortmannin, PD-98059 and 5-HD. Compared with the control group, cardiomyocyte apoptosis was significantly reduced in the ATP group, while this did not occur in the wortmannin+ATP, PD-98059+ATP and 5-HD+ATP groups. Western blot analysis revealed a higher myocardial expression of p-Akt and p-ERK 180 min following reperfusion in the ATP versus the control group. In conclusion, cardioprotection by postischemic ATP administration is mediated through activation of the reperfusion injury salvage kinase (RISK) pathway and opening of the mitochondrial ATP-dependent potassium channels.

Ecto-5'-nucleotidase, CD73, is an endothelium-derived hyperpolarizing factor synthase

OBJECTIVE

Adenosine dilates human coronary arteries by activating potassium channels in an endothelial cell-independent manner. Cell surface ecto-5'-nucleotidase (CD73) rapidly dephosphorylates extracellular adenosine 5'-monophosphate to adenosine. We tested the hypothesis that coronary vasodilation to adenine nucleotides is mediated by an endothelial CD73-dependent, extracellular production of adenosine that acts as an endothelium-derived hyperpolarizing factor.

METHODS AND RESULTS

Videomicroscopy showed that adenine nucleotides, but not inosine, potently dilated and hyperpolarized human coronary arteries independent of nitric oxide, prostacyclin, and classical endothelium-derived hyperpolarizing factors, whereas endothelial denudation, adenosine receptor antagonism, adenosine deaminase, or CD73 blockers reduced vasodilations. Liquid chromatography-electrospray ionization-mass spectrometry revealed adenosine accumulation in perfusates from arteries in the presence of adenosine 5'-diphosphate. CD73 was localized on the cell surface of endothelial cells, but not of vascular smooth muscle cells, and its deficiency suppressed vasodilation of mouse coronary arteries to adenine nucleotides and augmented vasodilation to adenosine. Adenosine dose-dependently dilated and hyperpolarized human coronary arteries to a similar extent as adenosine 5'-diphosphate.

CONCLUSIONS

Coronary vasodilation to adenine nucleotides is associated with endothelial CD73-dependent production of extracellular adenosine that acts as an endothelium-derived hyperpolarizing factor by relaxing and hyperpolarizing underlying vascular smooth muscle cells via activating adenosine receptors. Thus, CD73 is a novel endothelium-derived hyperpolarizing factor synthase in human and mouse coronary arteries.

Uridine adenosine tetraphosphate is a novel vasodilator in the coronary microcirculation which acts through purinergic P1 but not P2 receptors

Uridine adenosine tetraphosphate (Up4A) has been identified as an endothelium-derived contracting factor, which acts through purinergic P2X and P2Y receptors. Since the coronary vascular actions of Up4A are unknown, we investigated the vasoactive profile of Up4A in coronary microvessels, and studied the involvement of purinergic receptor subtypes. Studies were performed in isolated porcine coronary small arteries (diameter∼250 μm), with and without endothelial denudation, mounted on a Mulvany wire myograph. Purinergic receptor expression was assessed by real-time PCR. Up4A (10(-9)-10(-5) M) failed to induce contraction at basal tone, but produced concentration-dependent vasorelaxation in precontracted microvessels. Up4A was slightly less potent than adenosine, ATP, and ADP in producing vasorelaxation, but significantly more potent than UTP and UDP. mRNA expression of P2X(4), P2Y(1), P2Y(2), P2Y(4), P2Y(6) and A(2A), but not P2X(1), receptors was observed. Up4A-induced vasodilation was unaffected by non-selective P2 receptor antagonist PPADS, P2X(1) antagonist MRS2159, P2Y(1) antagonist MRS2179 and P2Y(6) antagonist MRS2578, but was markedly attenuated by non-selective P1 receptor antagonist 8PT and A(2A) antagonist SCH58261. Up4A-induced vasodilation was not affected by ectonucleotidase inhibitor ARL67156, suggesting that A(2A) stimulation was not the result of Up4A breakdown to adenosine. Up4A-induced vasodilation was blunted in denuded vessels; additional A(2A) receptor blockade further attenuated Up4A-induced vasodilation, suggesting that A(2A) receptor-mediated vasodilation is only partly endothelium-dependent. In conclusion, Up4A exerts a vasodilator rather than a vasoconstrictor influence in coronary microvessels, which is mediated via A(2A) receptors and is partly endothelium-dependent.

Mechanisms of metabolic coronary flow regulation

Coronary blood flow is tightly adjusted to the oxygen requirements of the myocardium. The underlying control mechanisms keep coronary venous pO(2) at a rather constant level around 20mm Hg under a variety of physiological conditions. Because coronary flow may increase more than 5-fold during exercise without any signs of under- or overperfusion, coronary flow must be controlled, at least in part, in a feed forward manner. Likely metabolic factors contributing to feed forward control are carbon dioxide and reactive oxygen species. Adaptation of coronary flow to exercise under physiological conditions involves in addition to metabolic control feed forward neuronal and endothelium-dependent control. Under pathological conditions, e.g. vessel stenosis or anemia, or specific environmental conditions, e.g. high altitude exposure, cardiac oxygenation may become critical, especially if oxygen demand is increased during physical exercise. Under such conditions the fall of coronary pO(2) may directly result in opening of oxygen sensitive potassium or closure of calcium channels. Furthermore the fall of pO(2) results in the production of vasoactive metabolites, e.g. adenosine, nitric oxide or prostaglandins, and in proton accumulation. All of these adaptations support a reduction of coronary vessel resistance. This article is part of a Special Issue entitled "Coronoray Blood Flow".

Functional contribution of P2Y1 receptors to the control of coronary blood flow

Activation of ADP-sensitive P2Y(1) receptors has been proposed as an integral step in the putative "nucleotide axis" regulating coronary blood flow. However, the specific mechanism(s) and overall contribution of P2Y(1) receptors to the control of coronary blood flow have not been clearly defined. Using vertically integrative studies in isolated coronary arterioles and open-chest anesthetized dogs, we examined the hypothesis that P2Y(1) receptors induce coronary vasodilation via an endothelium-dependent mechanism and contribute to coronary pressure-flow autoregulation and/or ischemic coronary vasodilation. Immunohistochemistry revealed P2Y(1) receptor expression in coronary arteriolar endothelial and vascular smooth muscle cells. The ADP analog 2-methylthio-ADP induced arteriolar dilation in vitro and in vivo that was abolished by the selective P2Y(1) antagonist MRS-2179 and the nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester. MRS-2179 did not alter baseline coronary flow in vivo but significantly attenuated coronary vasodilation to ATP in vitro and in vivo and the nonhydrolyzable ATP analog ATPγS in vitro. Coronary blood flow responses to alterations in coronary perfusion pressure (40-100 mmHg) or to a brief 15-s coronary artery occlusion were unaffected by MRS-2179. Our data reveal that P2Y(1) receptors are functionally expressed in the coronary circulation and that activation produces coronary vasodilation via an endothelium/nitric oxide-dependent mechanism. Although these receptors represent a critical component of purinergic coronary vasodilation, our findings indicate that P2Y(1) receptor activation is not required for coronary pressure-flow autoregulation or reactive hyperemia.

Coronary artery reperfusion: The ADP receptor P2Y(1) mediates early reactive hyperemia in vivo in pigs

The physiological mechanisms that regulate reactive hyperemia are not fully understood. We postulated that the endothelial P2Y(1) receptor that release vasodilatory factors in response to ADP might play a vital role in the regulation of coronary flow. Intracoronary flow was measured with a Doppler flow-wire in a porcine model. 2-MeSADP (10(-5) M), ATP (10(-4) M) or UTP (10(-4) M) alone or as co-infusion with a selective P2Y(1) receptor blocker, MRS 2179 (10(-3) M) was locally delivered through the tip of a coronary angioplasty balloon. In separate pigs the coronary artery was occluded with the balloon for 10 min. During the first and tenth minutes of coronary ischemia, 2.5 ml of MRS 2179 (10(-3) M) was delivered distal to the occlusion in 8 pigs, 10 pigs were used as controls. MRS 2179 fully inhibited the 2-MeSADP-mediated coronary flow increase (P < 0.05) with no effect on UTP, indicating selective P2Y(1) inhibition. ATP-mediated flow increase was significantly inhibited by MRS 2179. During reactive hyperemia following coronary occlusion, flow increased by nearly sevenfold. MRS 2179, however, reduced the post-ischemic hyperemia by a mean of 46% during the period 1-2.5 min following balloon deflation (P < 0.05), which corresponds to peak velocity flow during reperfusion. In conclusion, MRS 2179, a selective P2Y(1) receptor blocker, significantly reduces the increased coronary flow caused both by 2-MeSADP and reactive hyperemia in coronary arteries. Thus, ADP acting on the endothelial P2Y(1) receptor may play a major role in coronary flow during post-ischemic hyperemia.

Specificity of the ecto-ATPase inhibitor ARL 67156 on human and mouse ectonucleotidases

BACKGROUND AND PURPOSE

ARL 67156, 6-N,N-Diethyl-D-beta-gamma-dibromomethylene adenosine triphosphate, originally named FPL 67156, is the only commercially available inhibitor of ecto-ATPases. Since the first report on this molecule, various ectonucleotidases responsible for the hydrolysis of ATP at the cell surface have been cloned and characterized. In this work, we identified the ectonucleotidases inhibited by ARL 67156.

EXPERIMENTAL APPROACH

The effect of ARL 67156 on recombinant NTPDase1, 2, 3 & 8 (mouse and human), NPP1, NPP3 and ecto-5'-nucleotidase (human) have been evaluated. The inhibition of the activity of NTPDases (using the following substrates: ATP, ADP, UTP), NPPs (pnp-TMP, Ap(3)A) and ecto-5'-nucleotidase (AMP) was measured by colorimetric or HPLC assays.

KEY RESULTS

ARL 67156 was a weak competitive inhibitor of human NTPDase1, NTPDase3 and NPP1 with K(i) of 11+/-3, 18+/-4 and 12+/-3 microM, respectively. At concentrations used in the literature (50-100 microM), ARL 67156 partially but significantly inhibited the mouse and human forms of these enzymes. NTPDase2, NTPDase8, NPP3 and ecto-5'-nucleotidase activities were less affected. Importantly, ARL 67156 was not hydrolysed by either human NTPDase1, 2, 3, 8, NPP1 or NPP3.

CONCLUSIONS AND IMPLICATIONS

In cell environments where NTPDase1, NTPDase3, NPP1 or mouse NTPDase8 are present, ARL 67156 would prolong the effect of endogenously released ATP on P2 receptors. However, it does not block any ectonucleotidases efficiently when high concentrations of substrates are present, such as in biochemical, pharmacological or P2X(7) assays. In addition, ARL 67156 is not an effective inhibitor of NTPDase2, human NTPDase8, NPP3 and ecto-5'-nucleotidase.

Metabolic coronary flow regulation--current concepts

The concept of metabolic coronary flow control provides a rationale for the close relationship of coronary flow and myocardial metabolic rate of oxygen. The concept is based on the presence of an oxygen (metabolic) sensor coupled functionally to effector mechanisms, which control vascular tone. Four modes of metabolic control models have been proposed. 1) An oxygen sensor located in the wall of coronary vessels coupling to smooth muscle tension. Endothelial prostaglandin production may support this concept. 2) An oxygen sensing mechanism located in the myocardium and changing metabolism in response to changes of local pO(2). Adenosine is a metabolite produced at an accelerated rate when the supply-to-demand relationship for oxygen falls. 3) Sensing of oxygen turnover may be achieved by carbon dioxide production and, potentially, by mitochondrial production of reactive oxygen species. 4) The red blood cell might serve as an oxygen sensor in response to changes of haemoglobin oxygenation. A potential link to vessel relaxation may be red cell ATP release. A large body of experimental evidence supports the notion that K(ATP) channels play a significant role causing smooth muscle hyper-polarization. However, additional yet unknown effector mechanisms must exist, because block of K(ATP) channels does not lead to deterioration of coronary flow control under conditions of exercise. Thus, although several lines of evidence show that metabolic flow regulation is effective during hypoxic conditions,mechanisms mediating normoxic metabolic flow control still await further clarification.

ATP-loaded liposomes effectively protect mechanical functions of the myocardium from global ischemia in an isolated rat heart model

ATP-loaded liposomes (ATP-L) infused into Langendorff-instrumented isolated rat hearts protect the mechanical functions of the myocardium during ischemia/reperfusion. The left ventricular developed pressure (LVDP) at the end of the reperfusion in the ATP-L group recovered to 72% of the baseline (preservation of the systolic function) compared to 26%, 40%, and 51% in the groups treated with Krebs-Henseleit (KH) buffer, empty liposomes (EL), and free ATP (F-ATP), respectively. The ATP-L-treated group also showed a significantly lower left ventricular end diastolic pressure (LVEDP; better preservation of the diastolic function) after ischemia/reperfusion than controls. After incubating the F-ATP and ATP-L with ATPase, the protective effect of the F-ATP was completely eliminated because of ATP degradation, while the protective effect of the ATP-L remained unchanged. Fluorescence microscopy confirmed the accumulation of liposomes in ischemic areas, and the net ATP in the ischemic heart increased with ATP-L. Our results suggest that ATP-L can effectively protect myocardium from ischemic/reperfusion damage.

Antiarrhythmic drugs in atrial fibrillation: an overview of new agents, their mechanisms of action and potential clinical utility

Despite recent advances in our understanding of the mechanism of atrial fibrillation (AF), effective treatment remains difficult in many patients. Pharmacotherapy remains the mainstay of treatment and includes control of ventricular rate as well as restoration and maintenance of sinus rhythm. The currently available antiarrhythmic drugs are particularly effective in converting paroxysmal AF to sinus rhythm and in enhancing the positive effect of electrical cardioversion, but are limited in their efficacy in maintaining sinus rhythm. Moreover, there are limited options in the setting of co-existing ischaemic heart disease, left ventricular dysfunction and structural heart diseases. New drugs added to our clinical armamentarium have been, or are being, developed to combine better efficacy and lack of pro-arrhythmic effects. These developments have gained more interest particularly with the recent debate over rate control versus rhythm control for AF. Although some of these agents are promising, their uptake in clinical practice will not only depend on their efficacy as antiarrhythmic agents but also on their safety in acutely terminating AF and in long-term maintenance of sinus rhythm or rate control in the community.

Insulin induces the release of vasodilator compounds from platelets by a nitric oxide-G kinase-VAMP-3-dependent pathway

Insulin-induced vasodilatation is sensitive to nitric oxide (NO) synthase (NOS) inhibitors. However, insulin is unable to relax isolated arteries or to activate endothelial NOS in endothelial cells. Since insulin can enhance platelet endothelial NOS activity, we determined whether insulin-induced vasodilatation can be attributed to a NO-dependent, platelet-mediated process. Insulin failed to relax endothelium-intact rings of porcine coronary artery. The supernatant from insulin-stimulated human platelets induced complete relaxation, which was prevented by preincubation of platelets with a NOS inhibitor, the soluble guanylyl cyclase inhibitor, NS 2028, or the G kinase inhibitor, KT 5823, and was abolished by an adenosine A2A receptor antagonist. Insulin induced the release of adenosine trisphosphate (ATP), adenosine, and serotonin from platelet-dense granules in a NO-dependent manner. This response was not detected using insulin-stimulated platelets from endothelial NOS-/- mice, although a NO donor elicited ATP release. Insulin-induced ATP release from human platelets correlated with the association of syntaxin 2 with the vesicle-associated membrane protein 3 but was not associated with the activation of alphaIIbbeta3 integrin. Thus, insulin elicits the release of vasoactive concentrations of ATP and adenosine from human platelets via a NO-G kinase-dependent signaling cascade. The mechanism of dense granule secretion involves the G kinase-dependent association of syntaxin 2 with vesicle-associated membrane protein 3.

Nucleotide coronary vasodilation in guinea pig hearts

The role of P1 receptors and P2Y1 receptors in coronary vasodilator responses to adenine nucleotides was examined in the isolated guinea pig heart. Bolus arterial injections of nucleotides were made in hearts perfused at constant pressure. Peak increase in flow was measured before and after addition of purinoceptor antagonists. Both the P1 receptor antagonist 8-(p-sulfophenyl)theophylline and adenosine deaminase inhibited adenosine vasodilation. AMP-induced vasodilation was inhibited by P1 receptor blockade but not by adenosine deaminase or by the selective P2Y1 antagonist N6-methyl-2'-deoxyadenosine 3',5'-bisphosphate (MRS 2179). ADP-induced vasodilation was moderately inhibited by P1 receptor blockade and greatly inhibited by combined P1 and P2Y1 blockade. ATP-induced vasodilation was antagonized by P1 blockade but not by adenosine deaminase. Addition of P2Y1 blockade to P1 blockade shifted the ATP dose-response curve further rightward. It is concluded that in this preparation ATP-induced vasodilation results primarily from AMP stimulation of P1 receptors, with a smaller component from ATP or ADP acting on P2Y1 receptors. ADP-induced vasodilation is largely due to P2Y1 receptors, with a smaller contribution by AMP or adenosine acting via P1 receptors. AMP responses are mediated solely by P1 receptors. Adenosine contributes very little to vasodilation resulting from bolus intracoronary injections of ATP, ADP, or AMP.

Termination of paroxysmal supraventricular tachycardia by tecadenoson (CVT-510),a novel A1-adenosine receptor agonist

OBJECTIVES

The aim of this study was to evaluate tecadenoson safety and efficacy during conversion of paroxysmal supraventricular tachycardia (PSVT) to sinus rhythm.

BACKGROUND

Tecadenoson (CVT-510), a novel adenosine receptor (Ado R) agonist, selectively activates the A1 Ado R and prolongs atrioventricular (AV) nodal conduction at doses lower than those required to cause A2 Ado R-mediated coronary and peripheral vasodilation. Unlike adenosine, which non-selectively activates all four Ado R subtypes and produces unwanted effects, tecadenoson appears to terminate AV node-dependent supraventricular tachycardias without hypotension and bronchoconstriction.

METHODS

In this open-label, multicenter, dose escalation study, tecadenoson was administered to 37 patients (AV node re-entrant tachycardia, n = 29; AV re-entrant tachycardia, n = 8) with inducible PSVT sustained for > or =1 min during an electrophysiology study. Seven regimens (0.3 to 15 microg/kg) of up to two identical tecadenoson intravenous bolus doses were administered.

RESULTS

After the first or second bolus, PSVT converted to sustained sinus rhythm for > or =5 min in 86.5% (32/37) of the patients, with 91% (29/32) of the conversions occurring after the first bolus (most within 30 s), coincident with anterograde conduction block in the AV node. No effects on sinus cycle length (SCL) or systolic blood pressure were observed. The atrial-His (AH), but not the His-ventricular (HV) interval was prolonged up to 5 min after the final tecadenoson bolus, returning to baseline by 10 min. Tecadenoson was generally well tolerated.

CONCLUSIONS

In this study, tecadenoson rapidly terminated sustained PSVT by depressing AV nodal conduction without causing hypotension. After sinus rhythm restoration, there was minimal AH interval prolongation without HV interval or SCL prolongation.

Strain-dependent variation in vascular responses to nitric oxide in the isolated murine heart

Numerous studies in the literature have employed gene-modified mice to investigate vascular function. However, only very limited information exists on baseline murine vascular physiology or on potential variations between different strains. We therefore compared coronary and aortic vascular responses to endothelium-derived vasodilators and exogenous nitric oxide (NO) in three commonly used mouse strains and correlated these data with expression of eNOS, NADPH oxidase subunits, gp91(phox) and p67(phox), and superoxide production. Isolated perfused hearts from MF1, 129sv and C57BL/6J mice were subjected to: (a) increasing doses of bradykinin, acetylcholine and sodium nitroprusside, and (b) bolus doses of adenosine and the NO synthase inhibitor, N(G)-monomethyl- L -arginine. Vascular responses of thoracic aortic rings were assessed for comparison. Expression of eNOS and NADPH oxidase subunits was assessed by immunoblotting, and superoxide production by lucigenin-enhanced chemiluminescence. Coronary vasodilator responses to bradykinin, acetylcholine and sodium nitroprusside were significantly attenuated in MF1 compared with C57BL/6J and 129sv hearts. Similarly, aortic relaxation to acetylcholine was significantly impaired in MF1 aortic rings compared with in C57BL/6J aortae; these differences were reversed by Tiron. N(G)-monomethyl- L -arginine induced significantly less vasoconstriction in MF1 and 129sv hearts compared with C57BL/6J. No differences in aortic relaxation to A23187 or sodium nitroprusside were observed. Cardiac and aortic superoxide production and cardiac expression of p67(phox) and gp91(phox) were significantly greater in MF1 mice compared with the other strains. There is significant strain-dependent variation in coronary and aortic vascular responsiveness in mice, which may reflect differences in the balance between NO and superoxide generation.

A partial agonist of the A(1)-adenosine receptor selectively slows AV conduction in guinea pig hearts

The use of full agonists of the A(1)-adenosine receptor (A(1)-ADOR) as antiarrhythmic agents is limited by their actions to cause high-grade atrioventricular (AV) block, profound bradycardia, atrial fibrillation, and vasodilation. It may be possible to avoid these undesired actions by use of partial agonists. We determined the effects of CVT-2759, a potential partial agonist of A(1)-ADORs, on guinea pig hearts. CVT-2759 (0.1-100 microM) increased the S-H interval of the isolated heart from 45 +/- 1 to 60 +/- 3 ms (P < 0. 01) with a half-maximal effect at 3.1 microM. CVT-2759 did not cause second-degree AV block. CVT-2759 significantly attenuated the actions of the full agonists N(6)-cyclopentyladenosine and adenosine. CVT-2759 caused a moderate slowing of atrial rate by </=13% and did not shorten the durations of either the atrial or the ventricular monophasic action potential. Coronary conductance was increased by CVT-2759 only at concentrations >10 microM. In contrast, CVT-2759 was a full agonist to decrease cAMP content of rat adipocytes and Fischer rat thyroid line 5 cells. Results of radioligand binding assays indicated that CVT-2759 stabilized a high-affinity, G protein-coupled state of the A(1)-ADOR in membranes prepared from rat adipocytes but not in membranes prepared from the guinea pig brain. The results suggest that a weak A(1)-ADOR agonist, such as CVT-2759, may be useful to slow AV nodal conduction and thereby ventricular rate without causing AV block, bradycardia, atrial arrhythmias, or vasodilation.