Hypoxia enhances isoproterenol-induced increase in heart interstitial adenosine, depressing beta-adrenergic contractile responses

Extracellular metabolism of 3',5'-cyclic AMP as a source of interstitial adenosine in the rat airways

In the respiratory tract, intracellular 3',5'-cAMP mediates smooth muscle relaxation triggered by the β₂-adrenoceptor/Gs protein/adenylyl cyclase axis. More recently, we have shown that β₂-adrenoceptor agonists also increase extracellular 3',5'-cAMP levels in isolated rat trachea, which leads to contraction of airway smooth muscle. In many other tissues, extracellular 3',5'-cAMP is metabolized by ectoenzymes to extracellular adenosine, a catabolic pathway that has never been addressed in airways. In order to evaluate the possible extracellular degradation of 3',5'-cAMP into 5'-AMP and adenosine in the airways, isolated rat tracheas were incubated with exogenous 3',5'-cAMP and the amount of 5'-AMP, adenosine and inosine (adenosine metabolite) produced was evaluated using ultraperformance liquid chromatography-tandem mass spectrometry. Incubation of tracheal tissue with 3',5'-cAMP induced a time- and concentration-dependent increase in 5'-AMP, adenosine and inosine in the medium. Importantly, IBMX (non-selective phosphodiesterase (PDE) inhibitor) and DPSPX (selective ecto-PDE inhibitor) reduced the extracellular conversion of 3',5'-cAMP to 5'-AMP. In addition, incubation of 3',5'-cAMP in the presence of AMPCP (inhibitor of ecto-5'-nucleotidase) increased extracellular levels of 5'-AMP while drastically reducing extracellular levels of adenosine and inosine. These results indicate that airways express an extracellular enzymatic system (ecto-phosphodiesterase, ecto-5'-nucleotidase and adenosine deaminase) that sequentially converts 3',5'-cAMP into 5'-AMP, adenosine and inosine. The observation that extracellular 3',5'-cAMP is a source of interstitial adenosine supports the idea that the extrusion and extracellular metabolism of 3',5'-cAMP has a role in respiratory physiology and pathophysiology.

Serum adenosine deaminase activity and coronary artery disease: a retrospective case-control study based on 9929 participants

Background:

Adenosine deaminase (ADA) regulates purine metabolism through the conversion of adenosine to uric acid (UA). Adenosine and UA are closely associated with cardiovascular events, but the correlation between serum ADA activity and coronary artery disease (CAD) has not been defined.

Methods:

We performed a hospital-based retrospective case-control study that included a total of 5212 patients with CAD and 4717 sex- and age-matched controls. The serum activity of ADA was determined by peroxidase assays in an automatic biochemistry analyzer.

Results:

Serum ADA activity in the CAD group (10.08 ± 3.57 U/l) was significantly lower than that of the control group (11.71 ± 4.20 U/l, p < 0.001). After adjusting for conventional factors, serum ADA activity negatively correlated with the presence of CAD (odds ratio = 0.852, 95% confidence interval: 0.839–0.865, p < 0.001). Among the patients with CAD, serum ADA activity was lowest in patients with myocardial infarction (MI; 9.77 ± 3.80 U/l). Diabetes mellitus and hypertension increased the serum ADA activity in CAD patients.

Conclusions:

Serum ADA activity is significantly attenuated in patients with CAD, particularly in MI. We propose a mechanism by which the body maintains adenosine levels to protect the cardiovascular system in the event of CAD.

Momordica charantia polysaccharides ameliorate oxidative stress, hyperlipidemia, inflammation, and apoptosis during myocardial infarction by inhibiting the NF-κB signaling pathway

The polysaccharide extract of Momordica charantia has various biological activities; however, its effect on endothelial dysfunction in myocardial infarction remains unclear. To elucidate this, myocardial infarction was induced in rats using isoproterenol (ISP). Pretreatment with M. charantia polysaccharides (MCP; 150 or 300mg/kg) for 25days significantly inhibited increases in heart weight, the heart-weight-to-body-weight ratio, and infarction size, and ameliorated the increased serum levels of aspartate transaminase, creatine kinase, lactate dehydrogenase, total cholesterol, triglycerides, very-low-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and high-density lipoprotein cholesterol. In addition, MCP enhanced the activity of superoxide dismutase, catalase, and non-protein sulfhydryls, and decreased the level of lipid peroxidation. Moreover, MCP pretreatment downregulated the expression of proinflammatory cytokines (tumor necrosis factor alpha, interleukin (IL)-6, and IL-10), inflammatory markers (nitric oxide, myeloperoxidase, and inducible nitric oxide synthase), and apoptotic markers (caspase-3 and BAX), and upregulated Bcl-2 expression. Pretreatment with MCP reduced myonecrosis, edema, and inflammatory cell infiltration, and restored cardiomyocytes architecture. This myocardial protective effect could be related to the enhancement of the antioxidant defense system through the nuclear factor kappa B (NF-kB) pathways, and to anti-apoptosis through regulation of Bax, caspase-3, and Bcl-2.

Reduced adenosine release from the aged mammalian heart

Adenosine (ADO) released in the heart results in enhanced coronary blood flow and reduced catecholamine release and myocardial responsiveness to adrenergic stimulation (anti-adrenergic action). ADO release from the adrenergic-stimulated aged heart is less than that from the young adult heart. Because adrenergic signaling in the aged heart is impaired, this study was conducted to determine if reduced ADO release from the aged heart results from this reduced adrenergic responsiveness. Hearts of 3-4 months (young adult) and 21-22 months (aged) Fischer-344 rats were perfused with ADO deamination and re-phosphorylation inhibited. Coronary effluent ADO levels were determined. Cellular-free ADO levels with and without sodium acetate (NaAc)-induced mitochondrial AMP synthesis were assessed using formed S-adenosylhomocysteine (SAH) in L-homocysteine thiolactone (L-HC)-treated hearts. The activities of SAH-hydrolase were determined. Aged heart ADO release was 61% less than from young hearts. NaAc augmented young heart ADO release by 104%, while that of aged hearts remained unchanged. SAH synthesis was 51% and 56% lower in the aged heart in the absence and presence of NaAc, respectively, despite an 89% greater SAH hydrolase activity found in the aged hearts. Since synthesized AMP may be diverted to IMP and ultimately inosine by AMP deaminase, inosine release was determined. Aged heart inosine levels in the absence and presence of NaAc were 74% and 59% less than for the young hearts. It is concluded that a reduced mitochondrial AMP synthesis is in part responsible for the attenuation in ADO release from the adrenergic-stimulated aged heart.

Myocardial adenosine A(1)-receptor-mediated adenoprotection involves phospholipase C, PKC-epsilon, and p38 MAPK, but not HSP27

Adenosine via an adenosine A(1) receptor (A(1)R) is a negative feedback inhibitor of adrenergic stimulation in the heart, protecting it from toxic effects of overstimulation. Stimulation of the A(1)R results in the activation of G(i) protein, release of free Gbetagamma-subunits, and activation/translocation of PKC-epsilon to the receptor for activated C kinase 2 protein at the Z-line of the cardiomyocyte sarcomere. Using an anti-Gbetagamma peptide, we investigated the role of these subunits in the A(1)R stimulation of phospholipase C (PLC), with the premise that the resulting diacylglycerol provides for the activation of PKC-epsilon. Inositol 1,4,5-triphosphate release was an index of PLC activity. Chlorocyclopentyl adenosine (CCPA), an A(1)R agonist, increased inositol 1,4,5-triphosphate production by 273% in mouse heart homogenates, an effect absent in A(1)R knockout hearts and inhibited by anti-Gbetagamma peptide. In a second study, p38 MAPK and heat shock protein 27 (HSP27), found by others to be associated with the loss of myocardial contractile function, were postulated to play a role in the actions of A(1)R. Isoproterenol, a beta-adrenergic receptor agonist, increased the Ca(2+) transient and sarcomere shortening magnitudes by 36 and 49%, respectively. In the rat cardiomyocyte, CCPA significantly reduced these increases, an action blocked by the p38 MAPK inhibitor SB-203580. While CCPA significantly increased the phosphorylation of HSP27, this action was inhibited by isoproterenol. These data indicate that the activation of PKC-epsilon by A(1)R results from the activation of PLC via free Gbetagamma-subunits released upon A(1)R-induced dissociation of G(i)alphabetagamma. Attenuation of beta-adrenergic-induced contractile function by A(1)R may involve the activation of p38 MAPK, but not HSP27.

Endogenous adenosine selectively modulates oxidant stress via the A1 receptor in ischemic hearts

We tested the impact of A1 adenosine receptor (AR) deletion on injury and oxidant damage in mouse hearts subjected to 25-min ischemia/45-min reperfusion (I/R). Wild-type hearts recovered approximately 50% of contractile function and released 8.2 +/- 0.7 IU/g of lactate dehydrogenase (LDH). A1AR deletion worsened dysfunction and LDH efflux (15.2 +/- 2.6 IU/g). Tissue cholesterol and native cholesteryl esters were unchanged, whereas cholesteryl ester-derived lipid hydroperoxides and hydroxides (CE-O(O)H; a marker of lipid oxidation) increased threefold, and alpha-tocopherylquinone [alpha-TQ; oxidation product of alpha-tocopherol (alpha-TOH)] increased sixfold. Elevations in alpha-TQ were augmented by two- to threefold by A1AR deletion, whereas CE-O(O)H was unaltered. A(1)AR deletion also decreased glutathione redox status ([GSH]/[GSSG + GSH]) and enhanced expression of the antioxidant response element heme oxygenase-1 (HO-1) during I/R: fourfold elevations in HO-1 mRNA and activity were doubled by A1AR deletion. Broad-spectrum AR agonism (10 microM 2-chloroadenosine; 2-CAD) countered effects of A1AR deletion on oxidant damage, HO-1, and tissue injury, indicating that additional ARs (A(2A), A(2B), and/or A3) can mediate similar actions. These data reveal that local adenosine engages A1ARs during I/R to limit oxidant damage and enhance outcome selectively. Control of alpha-TOH/alpha-TQ levels may contribute to A1AR-dependent cardioprotection.

Adenosine A1 and A2A receptor regulation of protein phosphatase 2A in the murine heart

Adenosine plays a role in regulating the contractile function of the heart. This includes a positive ionotropic action via the adenosine A(2A) receptor (A(2A)R) and an inhibition of beta(1)-adrenergic receptor-induced ionotropy (antiadrenergic action) via the adenosine A(1) receptor (A(1)R). Phosphatase activity has also been shown to influence contractile function by affecting the level of protein phosphorylation. Protein phosphatase 2A (PP2A) plays a significant role in mediating the A(1)R antiadrenergic effect. The purpose of this study was to investigate the effects of A(2A)R and A(1)R on the activities of PP2A in hearts obtained from wild-type (WT) and A(2A)R knockout (A(2A)R-KO) mice. PP2A activities were examined in myocardial particulate and cytoplasmic extract fractions. Treatment of wild-type hearts with the A(1)R agonist CCPA increased the total PP2A activity and increased the particulate:cytoplasmic PP2A activity ratio. Treatment with the A(2A)R agonist CGS-21680 (CGS) decreased the total PP2A activity and decreased the particulate:cytoplasmic PP2A activity ratio. This indicated a movement of PP2A activity between cell fractions. The effect of CCPA was inhibited by CGS. In A(2A)R-KO hearts the response to A(1)R activation was markedly enhanced whereas the response to A(2A)R activation was absent. These data show that A(2A)R and A(1)R regulate PP2A activity, thus suggesting an important mechanism for modulating myocardial contractility.

Contractile effects of adenosine A1 and A2A receptors in isolated murine hearts

The adenosine A1 receptor (A1R) inhibits β-adrenergic-induced contractile effects (antiadrenergic action), and the adenosine A2A receptor (A2AR) both opposes the A1R action and enhances contractility in the heart. This study investigated the A1R and A2AR function in β-adrenergic-stimulated, isolated wild-type and A2AR knockout murine hearts. Constant flow and pressure perfused preparations were employed, and the maximal rate of left ventricular pressure (LVP) development (+dp/d tmax) was used as an index of cardiac function. A1R activation with 2-chloro- N6-cyclopentyladenosine (CCPA) resulted in a 27% reduction in contractile response to the β-adrenergic agonist isoproterenol (ISO). Stimulation of A2AR with 2- P(2-carboxyethyl)phenethyl-amino-5′- N-ethylcarboxyamidoadenosine (CGS-21680) attenuated this antiadrenergic effect, resulting in a partial (constant flow preparation) or complete (constant pressure preparation) restoration of the ISO contractile response. These effects of A2AR were absent in knockout hearts. Up to 63% of the A2AR influence was estimated to be mediated through its inhibition of the A1R antiadrenergic effect, with the remainder being the direct contractile effect. Further experiments examined the effects of A2AR activation and associated vasodilation with low-flow ischemia in the absence of β-adrenergic stimulation. A2AR activation reduced by 5% the depression of contractile function caused by the flow reduction and also increased contractile performance over a wide range of perfusion flows. This effect was prevented by the A2AR antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM-241385). It is concluded that in the murine heart, A1R and A2AR modulate the response to β-adrenergic stimulation with A2AR, attenuating the effects of A1R and also increasing contractility directly. In addition, A2AR supports myocardial contractility in a setting of low-flow ischemia.

Beta-adrenergic and antiadrenergic modulation of cardiac adenylyl cyclase is influenced by phosphorylation

Adenosine protects the myocardium of the heart by exerting an antiadrenergic action via the adenosine A1 receptor (A1R). Because beta 1-adrenergic receptor (beta 1R) stimulation elicits myocardial protein phosphorylation, the present study investigated whether protein kinase A (PKA) catalyzed rat heart ventricular membrane phosphorylation affects the beta 1R adrenergic and A1R adenosinergic actions on adenylyl cyclase activity. Membranes were either phosphorylated with PKA in the absence/presence of a protein kinase inhibitor (PKI) or dephosphorylated with alkaline phosphatase (AP) and assayed for adenylyl cyclase activity (AC) in the presence of the beta 1R agonist isoproterenol (ISO) and/or the A1R agonist 2-chloro-N6-cyclopentyladenosine (CCPA). 32P incorporation into the protein substrates of 140-120, 43, and 29 kDa with PKA increased both the ISO-elicited activation of AC by 51-54% and the A1R-mediated reduction of the ISO-induced increase in AC by 29-50%, thereby yielding a total antiadrenergic effect of approximately 78%. These effects of PKA were prevented by PKI. AP reduced the ISO-induced increase in AC and eliminated the antiadrenergic effect of CCPA. Immunoprecipitation of the solubilized membrane adenylyl cyclase with the use of a polyclonal adenylyl cyclase VI antibody indicated that the enzyme is phosphorylated by PKA. These results indicate that the cardioprotective effect of adenosine afforded by its antiadrenergic action is facilitated by cardiac membrane phosphorylation.

Involvement of K+ channels in adenosine A2A and A2B receptor-mediated hyperpolarization of porcine coronary artery endothelial cells

This study investigated the effects of the following adenosine agonists: 5;-ethylcarboxamidoadenosine (NECA), N6-cyclopentyadenosine (CPA) 2-[p-(2-carboxyethyl)]phenylamino-5;N-ethylcarboxamidoadenosine (CGS-21680), and 2-chloroadenosine (CAD) and its antagonist, 4-(2-[7-amino-2-[2-furyl]]1,2,4-triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM-241385), a selective A2A adenosine receptor antagonist, and the involvement of the K+ATP and KCa channels on the resting membrane potential (RMP) of confluent monolayers of cultured porcine coronary artery endothelial cells (PCAECs). Adenosine agonists and K+ATP channel openers (pinacidil, cromakalim) hyperpolarized cultured PCAECs. The average RMP was -32.31 +/- 1.2 mV. Adenosine agonists at 10-5 M caused a significant increase in RMP to -65.0 +/- 1.5 mV for CAD (a nonselective adenosine receptor agonist) to -75.9 +/- 1.6 mV for CGS-21680 (a selective A2A receptor agonist) and to -87.0 +/- 3.5 mV for NECA (a nonselective A1/A2A/A2B receptor agonist). Pinacidil and cromakalim at 10 microM increased the membrane potential to -76.2 +/- 1.2 mV and -75.22 +/- 0.12 mV, respectively. The hyperpolarization induced by adenosine receptor agonists and KATP openers was inhibited by an application of the K+ATP channel blocker glibenclamide (10 microM), indicating the involvement of the K+ATP channel in the adenosine-mediated hyperpolarization of PCAECs. Moreover, 1-EB10, a selective opener of the maxi-KCa channel, hyperpolarized PCAECs, and the effect of 1-EB10 was completely blocked by a selective, irreversible blocker of the high conductance KCa (maxi-K) channels (penitrem A), but it only partially blocked the effect of NECA. ZM-241385 has no effect on hyperpolarization elicited by K+ATP and KCa channel openers. However, ZM-241385 significantly blocked the hyperpolarization effect of CAD and CGS-21680. ZM-241385 partially blocked the hyperpolarizing effect of NECA, and a combination of ZM-241385 and penitrem A further blocked the hyperpolarizing effect of NECA. These results further support the involvement of K+ channels in adenosine A2A and A2B receptor-mediated hyperpolarization of PCAECs.

Adenosine A(2A) and A(2B) receptors mediated nitric oxide production in coronary artery endothelial cells

The present study further examined the functional presence and the signal transduction mechanism(s) for adenosine A(2A) and A(2B) receptors through nitric oxide (NO) and the guanosine 3', 5'-cyclic monophosphate (cGMP) pathway in cultured porcine coronary artery endothelial cells (PCAEC). The application of adenosine receptor agonists, NECA, CGS-21680 and CAD between 10(-7) and 10(-4) M, enhanced the production of NO (measured as nitrite) in a dose-dependent manner. On the basis of EC(50) values, these agonists showed the following order of potency: NECA>CGS-21680>CAD. This order appears to be of the A(2) adenosine receptor subtype. Similarly, the same concentrations of adenosine agonists evoked the production of cGMP in a dose-dependent manner, exhibiting a rank order that is similar to that of NO production. NO synthase inhibitor, N-nitro-L-arginine methylester (L-NAME, 10(-5) M), inhibited the production of NO and cGMP, which was reversed by L-arginine (10(-4) M). Selective A(2A) adenosine receptor antagonists, ZM-241385 and SCH-58261, at 10(-7) M, significantly inhibited the effects of CGS-21680, but only partly inhibited the effect of NECA on NO and cGMP production. Along with the earlier molecular evidence from this laboratory [Am. J. Physiol. 279 (2000) H650], the present data further support the presence of both A(2A) and A(2B) receptors in PCAEC. These results further support that coronary endothelial cells express functional A(2A) and A(2B) adenosine receptors, leading to GMP production through the NO-synthase-linked mechanism. This is the first direct evidence where an A(2B) adenosine receptor has been linked to NO production in cultured endothelial cells and could play a role in coronary artery physiology and pathophysiology.

JG. ATP as a source of interstitial adenosine in the rat heart

The contribution of neuronal ATP to interstitial adenosine levels was investigated in isolated perfused rat hearts. Ventricular surface transudates, representing interstitial fluid, were analyzed for norepinephrine, ATP, and adenosine. Exocytotic release of norepinephrine was induced by electrical stimulation of cardiac efferents emanating from the stellate ganglion. Ganglion stimulation increased contractility, interstitial norepinephrine, ATP, and adenosine. Interstitial adenosine was 11- to 27-fold higher than interstitial ATP, suggesting that the released ATP is unlikely the only source of adenosine. In the presence of AOPCP (α,β-methyleneadenosine 5'-diphosphate), an ecto-5'-nucleotidase inhibitor, the ganglion-stimulated increase in interstitial ATP and adenosine reached levels similar to those in the absence of AOPCP, also suggesting that adenosine does not derive from extracellular ATP. The perfusate Ca2+ was raised from 1 to 4 mM to determine the importance of the enhanced contractile function on the levels of norepinephrine, ATP, and adenosine. The results were increases in contractility and interstitial norepinephrine, ATP, and adenosine, which were not suppressed with atenolol, indicating a norepinephrine-independent release of ATP and adenosine. Reserpine treatment and administration of guanethidine depleted the catecholamine stores and diminished the catecholamine release, respectively. However, neither agent altered Ca2+-induced increases in ATP and adenosine. It is concluded that the amount of neuronal-derived ATP is low and most likely does not contribute significantly to interstitial levels of adenosine. Furthermore, elevations in interstitial norepinephrine, ATP, and adenosine are associated with neuronal-independent increases in contractile function.Key words: perfused heart, stellate ganglion, co-transmission, calcium, and contractility.

Adenosine A1 receptor-mediated antiadrenergic effects are modulated by A2a receptor activation in rat heart

Presently, the physiological significance of myocardial adenosine A2a receptor stimulation is unclear. In this study, the influence of adenosine A2a receptor activation on A1 receptor-mediated antiadrenergic actions was studied using constant-flow perfused rat hearts and isolated rat ventricular myocytes. In isolated perfused hearts, the selective A2a receptor antagonists 8-(3-chlorostyryl)caffeine (CSC) and 4-(2-[7-amino-2-(2-furyl)[1,2, 4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM-241385) potentiated adenosine-mediated decreases in isoproterenol (Iso; 10(-8) M)-elicited contractile responses (+dP/dtmax) in a dose-dependent manner. The effect of ZM-241385 on adenosine-induced antiadrenergic actions was abolished by the selective A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (10(-7) M), but not the selective A3 receptor antagonist 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1, 4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS-1191, 10(-7) M). The A2a receptor agonist carboxyethylphenethyl-aminoethyl-carboxyamido-adenosine (CGS-21680) at 10(-5) M attenuated the antiadrenergic effect of the selective A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA), whereas CSC did not influence the antiadrenergic action of this agonist. In isolated ventricular myocytes, CSC potentiated the inhibitory action of adenosine on Iso (2 x 10(-7) M)-elicited increases in intracellular Ca2+ concentration ([Ca2+]i) transients but did not influence Iso-induced changes in [Ca2+]i transients in the absence of exogenous adenosine. These results indicate that adenosine A2a receptor antagonists enhance A1-receptor-induced antiadrenergic responses and that A2a receptor agonists attenuate (albeit to a modest degree) the antiadrenergic actions of A1 receptor activation. In conclusion, the data in this study support the notion that an important physiological role of A2a receptors in the normal mammalian myocardium is to reduce A1 receptor-mediated antiadrenergic actions.

Endogenous adenosine reduces depression of cardiac function induced by beta-adrenergic stimulation during low flow perfusion

High levels of norepinephrine in the heart are cardiotoxic resulting in contractile dysfunction and arrhythmic activity via beta-adrenoceptor mediated mechanisms. A low flow heart model perfused with physiological saline containing glucose and bubbled with an O2 gas mixture was used to determine whether adenosine, a nucleoside with antiadrenergic properties, could reduce the functional manifestations of catecholamine cardiotoxicity. Isolated rat hearts were treated with dipropylcyclopentylxanthine (DPCPX; 0.1 microM; A1 receptor antagonist) to block endogenous adenosine. In DPCPX-treated hearts stimulated with isoproterenol (ISO; 1 microM) during 45 min of low flow (0.5 ml/min) perfusion, the recovery of contractile function (ConF) at 15 min after the restoration of normal flow was 64% of control (before low flow) values as compared to 110% recovery of ConF in the absence of ISO. The incidence of arrhythmias observed upon restoration of control flow was increased by ISO when the action of endogenous adenosine was blocked with DPCPX. In the absence of DPCPX both the functional depression and arrhythmias induced by ISO were prevented in the presence of phenylisopropyladenosine (PIA; 1 microM; A1 receptor agonist). At 15 min after normal flow was restored. ConF in ISO-treated hearts with PIA was 53% greater than in the absence of PIA and presence of DPCPX. This enhancement of ConF by PIA was significantly reduced by DPCPX. By 30 min after flow restoration, these significant differences were absent. DPCPX reversed the PIA-induced reduction in arrhythmias observed upon restoration of normal flow. PIA and DPCPX alone in the absence of ISO, and ISO in the absence of PIA and DPCPX, did not result in altered ConF upon restoration of normal flow. These findings indicate that intense beta-adrenergic stimulation of the heart during low-flow perfusion in the absence of adenosine A1 receptor activity induces contractile depression and arrhythmicity subsequent to restoration of control perfusion. It is concluded that endogenous adenosine protects the heart against catecholamine toxicity via stimulation of adenosine A1 receptors.

Ecto-5'-nucleotidase is expressed by pericytes and fibroblasts in the rat heart

Ecto-5'-nucleotidase is anchored at the outer surface of cell membranes and thus its reaction product adenosine is released into the extracellular space. Extracellular adenosine displays via specific receptors a wide range of physiological effects in heart. There are discrepancies in the literature concerning the distribution of ecto-5'-nucleotidase in heart. Since we suspected that these may be due to technical problems, in the present study on ecto-5'-nucleotidase in rat heart we attempted to circumvent some technical pitfalls. Good preservation of the tissue with open capillary lumina, providing a clear identification of endothelium, was obtained by perfusion fixation. At the light microscopic level, the distribution of ecto-5'-nucleotidase studied by enzyme histochemistry and immunohistochemistry using a monoclonal and a polyclonal antibody yielded congruent results. The enzyme was rather homogeneously distributed throughout the myocardium, with a slightly higher incidence of stained cells in the outer thirds than in the inner third of the wall. Consistently high levels of ecto-5'-nucleotidase were seen only in interstitial cells. The walls of large vessels and heart muscle cells were constantly negative for ecto-5'-nucleotidase. The endothelia of capillaries were mostly negative but a few profiles occasionally displayed a weak immunoreaction. The interstitial cells staining positive for ecto-5'-nucleotidase could be identified as pericytes and as fibroblasts according to their shapes and localizations. The immunoreactivity of fibroblasts was confirmed by electron microscopy. These data indicate that adenosine may be formed extracellularly in the interstitium of the myocardium, where it would have direct access to important targets such as myocytes, arterioles and nerve endings.

Dual action of angiotensin II on coronary resistance in the isolated perfused rabbit heart

We studied the functional role of angiotensin II (AII) receptor subtypes and vasodilatory endothelial autacoid release in response to AII in isolated perfused rabbit hearts. AII infusion induced biphasic changes in coronary perfusion pressure (CPP): an initial increase was followed by a decrease until a plateau was reached. At higher concentrations of AII (> or = 10 nmol/l) this plateau phase was lower than the initial CPP level. AII infusion elicited inverse changes in peak left ventricular pressure (LVP): coronary constriction was associated with a transient decline, and during the plateau phase LVP was clearly increased. AII also moderately augmented prostacyclin (PGI2) release from the coronary vascular bed. The AII-induced changes in CPP, LVP, and PGI2 release were effectively inhibited by the AT1 receptor subtype antagonist ICI D8731 (30 nmol/l), but not by the AT2 receptor antagonist CGP 42112 (30 nmol/l). The adenosine A1 receptor antagonist 8-phenyltheophylline (0.1 mumol/l) attenuated the decline in CPP following the constriction phase without affecting the changes in LVP during AII infusion. The cyclooxygenase inhibitor diclofenac (1 mmol/l) had no effect on the AII-induced changes in CPP, whereas the nitric oxide-synthase inhibitor NG-nitro-L-arginine (30 mumol/l) markedly potentiated the vasoconstriction but was without effect on the plateau phase of the response. In contrast to AII, the thromboxane analogue U46619 elicited sustained increases in CPP which were associated with slight decreases in LVP.(ABSTRACT TRUNCATED AT 250 WORDS)