Adenosine reduces the Ca2+ transients of isoproterenol-stimulated rat ventricular myocytes

Adenosine A1 receptors heterodimerize with β1- and β2-adrenergic receptors creating novel receptor complexes with altered G protein coupling and signaling

G protein coupled receptors play crucial roles in mediating cellular responses to external stimuli, and increasing evidence suggests that they function as multiple units comprising homo/heterodimers and hetero-oligomers. Adenosine and β-adrenergic receptors are co-expressed in numerous tissues and mediate important cellular responses to the autocoid adenosine and sympathetic stimulation, respectively. The present study was undertaken to examine whether adenosine A1ARs heterodimerize with β1- and/or β2-adrenergic receptors (β1R and β2R), and whether such interactions lead to functional consequences. Co-immunoprecipitation and co-localization studies with differentially epitope-tagged A1, β1, and β2 receptors transiently co-expressed in HEK-293 cells indicate that A1AR forms constitutive heterodimers with both β1R and β2R. This heterodimerization significantly influenced orthosteric ligand binding affinity of both β1R and β2R without altering ligand binding properties of A1AR. Receptor-mediated ERK1/2 phosphorylation significantly increased in cells expressing A1AR/β1R and A1AR/β2R heteromers. β-Receptor-mediated cAMP production was not altered in A1AR/β1R expressing cells, but was significantly reduced in the A1AR/β2R cells. The inhibitory effect of the A1AR on cAMP production was abrogated in both A1AR/β1R and A1AR/β2R expressing cells in response to the A1AR agonist CCPA. Co-immunoprecipitation studies conducted with human heart tissue lysates indicate that endogenous A1AR, β1R, and β2R also form heterodimers. Taken together, our data suggest that heterodimerization between A1 and β receptors leads to altered receptor pharmacology, functional coupling, and intracellular signaling pathways. Unique and differential receptor cross-talk between these two important receptor families may offer the opportunity to fine-tune crucial signaling responses and development of more specific therapeutic interventions.

Crosstalk between adenosine A1 and β1-adrenergic receptors regulates translocation of PKCε in isolated rat cardiomyocytes

Adenosine A(1) receptor (A(1)R)-induced translocation of PKCε to transverse (t) tubular membranes in isolated rat cardiomyocytes is associated with a reduction in β(1)-adrenergic-stimulated contractile function. The PKCε-mediated activation of protein kinase D (PKD) by endothelin-1 is inhibited by β(1)-adrenergic stimulated protein kinase A (PKA) suggesting a similar mechanism of A(1)R signal transduction modulation by adrenergic agonists may exist in the heart. We have investigated the influence of β(1)-adrenergic stimulation on PKCε translocation elicited by A(1)R. Immunofluorescence imaging and Western blotting with PKCε and β-COP antibodies were used to quantify the co-localization of PKCε and t-tubular structures in isolated rat cardiomyocytes. The A(1)R agonist CCPA increased the co-localization of PKCε and t-tubules as detected by imaging. The β(1)-adrenergic receptor agonist isoproterenol (ISO) inhibited this effect of CCPA. Forskolin, a potent activator of PKA, mimicked, and H89, a pharmacological PKA inhibitor, and PKI, a membrane-permeable PKA peptide PKA inhibitor, attenuated the negative effect of ISO on the A(1)R-mediated PKCε translocation. Western blotting with isolated intact hearts revealed an increase in PKCε/β-COP co-localization induced by A(1)R. This increase was attenuated by the A(1)R antagonist DPCPX and ISO. The ISO-induced attenuation was reversed by H89. It is concluded that adrenergic stimulation inhibits A(1)R-induced PKCε translocation to the PKCε anchor site RACK2 constituent of a coatomer containing β-COP and associated with the t-tubular structures of the heart. In that this translocation has been previously associated with the antiadrenergic property of A(1)R, it is apparent that the interactive effects of adenosine and β(1)-adrenergic agonists on function are complex in the heart.

Differential effects of adenosine A2a and A2b receptors on cardiac contractility

The mammalian myocardium expresses four adenosine receptor (AR) subtypes: A(1)AR, A(2a)AR, A(2b)AR, and A(3)AR. The A(1)AR is well known for its profound antiadrenergic effects, but the roles of other AR subtypes in modulating contractility remain inconclusive. Thus, the objective of this study was to determine the direct and indirect effects of A(2a)AR and A(2b)AR on cardiac contractility. Experiments were conducted in paced, constant pressure-perfused isolated hearts from wild-type (WT), A(2a)AR knockout (KO), and A(2b)AR KO mice. The A(2a)AR agonist CGS-21680 did not alter basal contractility or β-adrenergic receptor agonist isoproterenol (Iso)-mediated positive inotropic responses, and Iso-induced effects were unaltered in A(2a)AR KO hearts. However, A(2a)AR gene ablation resulted in a potentiation of the antiadrenergic effects mediated by the A(1)AR agonist 2-chloro-N-cyclopentyladenosine. The nonselective AR agonist 5'-N-ethylcarboxamido adenosine and the selective A(2b)AR agonist BAY 60-6583 induced coronary flow-independent increases in contractility, but BAY 60-6583 did not alter Iso-induced contractile responses. The A(1)AR antiadrenergic effect was not potentiated in A(2b)AR KO hearts. The expression of all four AR subtypes in the heart and ventricular myocytes was confirmed using real-time quantitative PCR. Taken together, these results indicate that A(2a)AR does not increase cardiac contractility directly but indirectly alters contractility by modulating the A(1)AR antiadrenergic effect, whereas A(2b)AR exerts direct contractile effects but does not alter β-adrenergic or A(1)AR antiadrenergic effects. These results indicate that multiple ARs differentially modulate cardiac function.

Adenosine receptors and membrane microdomains

Adenosine receptors are a member of the large family of seven transmembrane spanning G protein coupled receptors. The four adenosine receptor subtypes-A(1), A(2a), A(2b), A(3)-exert their effects via the activation of one or more heterotrimeric G proteins resulting in the modulation of intracellular signaling. Numerous studies over the past decade have documented the complexity of G protein coupled receptor signaling at the level of protein-protein interactions as well as through signaling cross talk. With respect to adenosine receptors, the activation of one receptor subtype can have profound direct effects in one cell type but little or no effect in other cells. There is significant evidence that the compartmentation of subcellular signaling plays a physiological role in the fidelity of G protein coupled receptor signaling. This compartmentation is evident at the level of the plasma membrane in the form of membrane microdomains such as caveolae and lipid rafts. This review will summarize and critically assess our current understanding of the role of membrane microdomains in regulating adenosine receptor signaling.

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.

The exercising heart at altitude

Maximal cardiac output is reduced in severe acute hypoxia but also in chronic hypoxia by mechanisms that remain poorly understood. In theory, the reduction of maximal cardiac output could result from: (1) a regulatory response from the central nervous system, (2) reduction of maximal pumping capacity of the heart due to insufficient coronary oxygen delivery prior to the achievement of the normoxic maximal cardiac output, or (3) reduced central command. In this review, we focus on the effects that acute and chronic hypoxia have on the pumping capacity of the heart, particularly on myocardial contractility and the molecular responses elicited by acute and chronic hypoxia in the cardiac myocytes. Special emphasis is put on the cardioprotective effects of chronic hypoxia.

Adenoprotection of the heart involves phospholipase C-induced activation and translocation of PKC-epsilon to RACK2 in adult rat and mouse

Adenosine protects the heart from adrenergic overstimulation. This adenoprotection includes the direct anti-adrenergic action via adenosine A(1) receptors (A(1)R) on the adrenergic signaling pathway. An indirect A(1)R-induced attenuation of adrenergic responsiveness involves the translocation of PKC-epsilon to t-tubules and Z-line of cardiomyocytes. We investigated with sarcomere imaging, immunocytochemistry imaging, and coimmunoprecipitation (co-IP) whether A(1)R activation of PKC-epsilon induces the kinase translocation to receptor for activated C kinase 2 (RACK2) in isolated rat and mouse hearts and whether phospholipase C (PLC) is involved. Rat cardiomyocytes were treated with the A(1)R agonist chlorocyclopentyladenosine (CCPA) and exposed to primary PKC-epsilon and RACK2 antibodies with secondaries conjugated to Cy3 and Cy5 (indodicarbocyanine), respectively. Scanning confocal microscopy showed that CCPA caused PKC-epsilon to reversibly colocalize with RACK2 within 3 min. Additionally, rat and mouse hearts were perfused and stimulated with CCPA or phenylisopropyladenosine to activate A(1)R, or with phorbol 12-myristate 13-acetate to activate PKC. RACK2 was immunoprecipitated from heart extracts and resolved with SDS-PAGE. Western blotting showed that CCPA, phenylisopropyladenosine, and phorbol 12-myristate 13-acetate in the rat heart increased the PKC-epsilon co-IP with RACK2 by 186, 49, and >1,000%, respectively. The A(1)R antagonist 8-cyclopentyl-1,3-dipropylxanthine prevented the CCPA-induced co-IP with RACK2. In mouse hearts, CCPA increased the co-IP of PKC-epsilon with RACK2 by 61%. With rat cardiomyocytes, the beta-adrenergic agonist isoproterenol increased sarcomere shortening by 177%. CCPA reduced this response by 47%, an action inhibited by the PLC inhibitor U-73122 and 8-cyclopentyl-1,3-dipropylxanthine. In conclusion, A(1)R stimulation of the heart is associated with PLC-initiated PKC-epsilon translocation and association with RACK2.

Adenosine A2A and beta-adrenergic calcium transient and contractile responses in rat ventricular myocytes

The adenosine A2A receptor (A2AR) enhances cardiac contractility, and the adenosine A1R receptor (A1R) is antiadrenergic by reducing the adrenergic beta1 receptor (beta1R)-elicited increase in contractility. In this study we compared the A2AR-, A1R-, and beta1R-elicited actions on isolated rat ventricular myocytes in terms of Ca transient and contractile responses involving PKA and PKC. Stimulation of A2AR with 2 microM (approximately EC50) CGS-21680 (CGS) produced a 17-28% increase in the Ca transient ratio (CTR) and maximum velocities (Vmax) of transient ratio increase (+MVT) and recovery (-MVT) but no change in the time-to-50% recovery (TTR). CGS increased myocyte sarcomere shortening (MSS) and the maximum velocities of shortening (+MVS) and relaxation (-MVS) by 31-34% with no change in time-to-50% relengthening (TTL). beta1R stimulation using 2 nM (approximately EC50) isoproterenol (Iso) increased CTR, +MVT, and -MVT by 67-162% and decreased TTR by 43%. Iso increased MSS, +MVS, and -MVS by 153-174% and decreased TTL by 31%. The A2AR and beta1R Ca transient and contractile responses were not additive. The PKA inhibitor Rp-adenosine 3',5'-cyclic monophosphorothioate triethylamonium salt prevented both the CGS- and Iso-elicited contractile responses. The PKC inhibitors chelerythrine and KIE1-1 peptide (PKCepsilon specific) prevented the antiadrenergic action of A1R but did not influence A2AR-mediated increases in contractile variables. The findings suggest that cardiac A2AR utilize cAMP/PKA like beta1R, but the Ca transient and contractile responses are less in magnitude and not equally affected. Although PKC is important in the A1R antiadrenergic action, it does not seem to play a role in A2AR-elicited Ca transient and contractile events.

Adenosine A1 and A2A receptor effects on G-protein cycling in beta-adrenergic stimulated ventricular membranes

In the heart beta1-adrenergic (beta1R) and adenosine A1 (A1R) and A2A (A2AR) receptors modulate contractile and metabolic function. The interaction between these receptors was investigated at the level of G-protein cycling by determining the effect of receptor agonists on the binding of GTP to G-proteins and displacement of G alpha-subunit-bound GDP by GTP. Crude membranes from rat heart or brain were stimulated by agonists for beta1R (isoproterenol; ISO), A1R (chlorocyclopentyladenosine, CCPA) and A2AR (CGS-21680; CGS). GTP binding to membranes was increased by ISO (17%), CCPA (6%) and CGS (12%). Binding values observed with incubation using ISO and CCPA together were significantly less than values obtained by the incubation of individual agents alone. With ISO, GTP binding to G alpha(s) subunits as determined by immunoprecipitation was increased 79% in heart and 87% in brain. These increases were attenuated by CCPA, an effect that was inhibited by CGS. GDP release by membranes was increased 6.9% and 4.6% by ISO and CCPA, respectively. After co-incubation of these agonists, release was increased less than determined by the addition of the individual agent responses. CGS inhibited the reduced release caused by of CCPA. Adenylyl cyclase activity stimulated by ISO was attenuated 33% by CCPA, an effect inhibited by CGS. Together, these results indicate that A1R exert an antiadrenergic action at the level of beta1R stimulated G(s)-protein cycling and that A2AR reduce this action.

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.

Protein kinase Cepsilon and the antiadrenergic action of adenosine in rat ventricular myocytes

Adenosine-induced antiadrenergic effects in the heart are mediated by adenosine A(1) receptors (A(1)R). The role of PKCepsilon in the antiadrenergic action of adenosine was explored with adult rat ventricular myocytes in which PKCepsilon was overexpressed. Myocytes were transfected with a pEGFP-N1 vector in the presence or absence of a PKCepsilon construct and compared with normal myocytes. The extent of myocyte shortening elicited by electrical stimulation of quiescent normal and transfected myocytes was recorded with video imaging. PKCepsilon was found localized primarily in transverse tubules. The A(1)R agonist chlorocyclopentyladenosine (CCPA) at 1 microM rendered an enhanced localization of PKCepsilon in the t-tubular system. The beta-adrenergic agonist isoproterenol (Iso; 0.4 microM) elicited a 29-36% increase in myocyte shortening in all three groups. Although CCPA significantly reduced the Iso-produced increase in shortening in all three groups, the reduction caused by CCPA was greatest with PKCepsilon overexpression. The CCPA reduction of the Iso-elicited shortening was eliminated in the presence of a PKCepsilon inhibitory peptide. These results suggest that the translocation of PKCepsilon to the t-tubular system plays an important role in A(1)R-mediated antiadrenergic actions in the heart.

Interplay between inhibition of adenosine uptake and phosphodiesterase type 3 on cardiac function by cilostazol, an agent to treat intermittent claudication

The authors have recently shown that cilostazol, a type 3 cyclic nucleotide phosphodiesterase (PDE3) inhibitor, has a much weaker positive inotropic effect than milrinone, a PDE3 inhibitor of similar potency. They have also shown that cilostazol inhibits adenosine uptake, whereas milrinone has no such effect. This study investigated the possible cardiac functional significance of cilostazol on adenosine uptake inhibition. In isolated rabbit hearts, 10 microM of cilostazol elevated adenosine concentration in interstitial dialysate (0.16 +/- 0.01 microM, or approximately 0.81 microM in the interstitial space when adjusted for recovery rate of microdialysis) and coronary effluent (0.69 +/- 0.03 microM ). The values are significantly higher than those for 10 microM of milrinone (0.11 +/- 0.1 microM in interstitial dialysate and 0.2 +/- 0.04 microM in coronary effluent). Although cilostazol increased contractility, heart rate, and coronary flow in isolated rabbit hearts, the effect on contractility and heart rate was significantly augmented in the presence of an adenosine A 1 receptor antagonist. Conversely, an adenosine A 1 receptor agonist or an adenosine uptake inhibitor attenuated the positive inotropic effect of milrinone. These results indicate that adenosine uptake inhibition by cilostazol increases interstitial and circulatory adenosine concentration, and antagonizes PDE3 inhibition-induced contractility and heart rate increases through an adenosine A 1 receptor-mediated mechanism.

Modulation of sympathetic actions on the heart by opioid receptor stimulation

The sympathetic nervous system, the most important extrinsic regulatory mechanism of the heart, is inhibited postsynaptically and presynaptically by opioid peptides produced in the heart via their respective receptors. The cardiac actions of beta-adrenergic receptor (beta-AR) stimulation are attenuated by activation of the opioid receptor (OR) with OR agonist at ineffective concentrations, implying cross-talk between the OR and beta-AR. This cross-talk results from inhibition of the Gs protein and adenylyl cyclase of the beta-AR pathway by the pertussis toxin-sensitive G protein of the opioid pathway. Alterations in cross-talk between these two receptors occur in pathological situations to meet bodily needs. In myocardial ischemia, when the sympathetic activity is increased, the inhibition of beta-AR stimulation by kappa-opioid stimulation is also enhanced, thus reducing the workload, oxygen consumption and cardiac injury. Whereas cardiac responsiveness to sympathetic discharges is also reduced after chronic hypoxia, the cross-talk between kappa-OR and beta-AR is reduced to prevent undue suppression of the sympathetic influence on the heart. On the other hand, impairment of the cross-talk may result in abnormality. A lack or a significant reduction in the inhibition of beta-AR stimulation by kappa-OR stimulation may lead to an excessive increase in cardiac activities, which contribute to the maintenance of high arterial blood pressure in spontaneously hypertensive rats. Other than opioid peptides, female sex hormone and adenosine also inhibit the sympathetic actions on the heart. In addition, sympathetic action is also inhibited presynaptically by kappa-opioid peptides via their receptor.

Anti-adrenergic effect of adenosine on Na(+)-Ca(2+) exchange current recorded from guinea-pig ventricular myocytes

The Na(+)-Ca(2+) exchanger is a protein present in the cell membrane of many cell types. In heart it plays important roles in Ca homeostasis and ionic current generation. Recently, it has been reported that the beta-adrenergic agonist isoprenaline (ISO) can increase directly Na(+)-Ca(2+) exchanger activity in guinea-pig ventricular myocytes. Adenosine (ADO) exerts anti-adrenergic properties that make it effective against some arrhythmias and the aim of the present study was to determine whether or not ADO can antagonize the direct modulatory effect of ISO on the exchanger.Whole-cell patch clamp measurements of Na(+)-Ca(2+) exchanger current (I(NaCa)) were made from guinea-pig ventricular myocytes, with major interfering currents inhibited. I(NaCa) was measured at 378 degrees C as current sensitive to external nickel (Ni(2+), 10 mM) during an applied descending voltage ramp. ISO (1 microM) significantly increased both inward and outward I(NaCa). This effect was abolished in the presence of ADO (200 microM). ADO alone did not significantly alter the amplitude of I(NaCa). The effect of ADO on the response of I(NaCa) to ISO was mimicked by the A(1)ADO receptor agonist N(6)-cyclopentyladenosine (CPA, 10 microM), whereas the effect of ADO on the response of I(NaCa) to ISO was inhibited by the A(1)ADO receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 2 microM). These data suggest that the A(1)ADO receptor mediated the response. The anti-adrenergic effects on I(NaCa) of ADO were not affected by the protein kinase C (PKC) inhibitor, chelerythrine (CLT, 1 microM), nor by the nitric oxide (NO) synthase inhibitor, N (G)-nitro-L-arginine methyl ester((L)-NAME, 0.5 mM). Moreover, in the presence of PKC activator phorbol 12-myristate 13-acetate (PMA, 1 microM) or exogenous NO donor sodium nitroprusside (SNP, 100 microM), ISO preserved its stimulatory effect on I(NaCa). However, prior incubation of myocytes with pertussis toxin (PTX, 5 microg ml(-1) did prevent the effect of ADO. The anti-adrenergic effect of ADO on I(NaCa) was mimicked by externally applied carbachol (CCh, 10 microM), a muscarinic receptor agonist. We conclude that ADO antagonized the effect of beta-adrenergic stimulation of I(NaCa) by directly activating inhibitory G-protein (G(i))-linked A(1) receptors in guinea-pig ventricular myocytes. These findings may suggest a novel mechanism by which adenosine exerts some of its antiarrhythmic effects.

Phosphatase inhibitor cantharidin blocks adenosine A(1) receptor anti-adrenergic effect in rat cardiac myocytes

Experiments were performed to examine whether the protein phosphatase inhibitor cantharidin blocks the anti-adrenergic effect of adenosine A(1) receptor stimulation. In electrically stimulated adult rat ventricular myocytes loaded with the intracellular calcium concentration ([Ca(2+)](i)) indicator fluo-3, isoproterenol (10 nM) increased systolic [Ca(2+)](i) by 46%, increased twitch amplitude by 56%, and increased total cellular cAMP content by 140%. The adenosine A(1) receptor agonist 2-chloro-N(6)-cyclopentlyadenosine (CCPA) reduced isoproterenol-stimulated [Ca(2+)](i) and contractility by 87 and 80%, respectively, but reduced cAMP content by only 18%. Cantharidin had no effects on myocyte [Ca(2+)](i), contractility, or cAMP in the absence or presence of isoproterenol but blocked the effects of CCPA on [Ca(2+)](i) and contractility by approximately 44%. Cantharidin had no effect on CCPA attenuation of isoproterenol-induced increases in cAMP. Pretreatment with CCPA also reduced the increase in contractile parameters produced by the direct cAMP-dependent protein kinase A (PKA) activator 8-bromocAMP. These results suggest that activation of protein phosphatases mediate, in part, the anti-adrenergic effect of adenosine A(1) receptor activation in ventricular myocardium.

Functional studies in atrium overexpressing A1-adenosine receptors

1. Adenosine and the A1-adenosine receptor agonist R-PIA, exerted a negative inotropic effect in isolated, electrically driven left atria of wild-type mice. 2. In left atria of mice overexpressing the A1-adenosine receptor, adenosine and R-PIA exerted a positive inotropic effect. 3. The positive inotropic effect of adenosine and R-PIA in transgenic atria could be blocked by the A1-adenosine receptor antagonist DPCPX. 4. In the presence of isoprenaline, adenosine exerted a negative inotropic effect in wild-type atria but a positive inotropic effect in atria from A1-adenosine receptor overexpressing mice. 5. The rate of beating in right atria was lower in mice overexpressing A1-adenosine receptors compared with wild-type. 6. Adenosine exerted comparable negative chronotropic effects in right atria from both A1-adenosine receptor overexpressing and wild-type mice. 7. A1-adenosine receptor overexpression in the mouse heart can reverse the inotropic but not the chronotropic effects of adenosine, implying different receptor-effector coupling mechanisms.

Anti-arrhythmic effect of kappa-opioid receptor stimulation in the perfused rat heart: involvement of a cAMP-dependent pathway

During myocardial ischaemia the beta-adrenoceptor is activated, which contributes, at least partly, to cardiac arrhythmias via inducing [Ca2+]i oscillations. Since beta-adrenoceptor is negatively modulated by the kappa-opioid receptor in the heart, the present study attempted to determine if kappa-opioid receptor stimulation modulates the arrhythmogenic action of beta-adrenoceptor stimulation and to delineate the underlying mechanism. The effect of U50,488H, a selective kappa-opioid agonist, on arrhythmias in the isolated perfused rat heart subjected to low flow and 10(-6)mol/l norepinephrine (NE) were determined. Low flow induced arrhythmias, which were potentiated by NE, but not by 10(-6)mol/l U50,488H. The arrhythmia-potentiating effect of NE was antagonized by 10(-6)mol/l propranolol, a beta-adrenoceptor antagonist. U50,488H at 10(-6)mol/l also abolished the potentiation in arrhythmias by NE without affecting the arrhythmias induced by low flow. The anti-arrhythmic action of the kappa-opioid receptor agonist was abolished by 10(-6)mol/l nor-binaltorphimine, a selective kappa-opioid receptor antagonist, but not by 10(-7)mol/l calphostin C, an inhibitor of protein kinase C. Similarly, kappa-opioid receptor stimulation with U50,488H also abolished the NE-induced [Ca2+]i oscillations which are believed to cause cardiac arrhythmias, in ventricular myocytes. To determine whether the inhibitory actions of U50,488H against the effects of beta-adrenoceptor stimulation was via a cAMP-dependent or a cAMP-independent pathway, we determined the effects of U50,488H on NE-enhanced cAMP production and [Ca2+]i oscillations induced by either forskolin, an activator of adenylate cyclase, or Bay K-8644, a selective L-type Ca2+ channel agonist, in the ventricular myocytes. We found that U50,488H abolished the effect of forskolin, but did not alter the effect of Bay K-8644, on [Ca2+]i oscillations in the ventricular myocyte. In addition, U50, 488H also attenuated significantly the NE-induced elevation in cAMP in the heart. The observations suggest that kappa-opioid receptor stimulation abolishes the effect of beta-adrenoceptor stimulation on arrhythmias and [Ca2+]i oscillation via a cAMP-dependent pathway. The finding may be useful for the prevention and treatment of ischaemic heart diseases.

Adenosine A2a-receptor activation enhances cardiomyocyte shortening via Ca2+-independent and -dependent mechanisms

Adenosine A2a receptor (A2aR) stimulation enhances the shortening of ventricular myocytes. Whether the A2aR-mediated increase in myocyte contractility is associated with alterations in the amplitude of intracellular Ca2+ transients was investigated in isolated, contracting rat ventricular myocytes using the Ca2+-sensitive fluorescent dye fura 2-AM. In the presence of intact inhibitory G protein pathways, 10(-4) M 2-p-(2-carboxyethyl)phenethyl-amino-5'-N-ethylcarboxamidoadenosine (CGS-21680), an A2aR agonist, insignificantly increased Ca2+ transients by 8 +/- 5%, whereas myocyte shortening increased by 54 +/- 1%. In contrast, 2 x 10(-7) M isoproterenol, a beta-adrenergic receptor agonist, increased Ca2+ transients by 104 +/- 15% and increased myocyte shortening by 61 +/- 6%. When A2aR were stimulated in myocytes that had the antiadrenergic actions of adenosine (Ado) abolished by either treatment with pertussis toxin (PTx) or the presence of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an adenosine A1-receptor antagonist, the maximum increases in Ca2+ transients were similarly nominal (with PTx: 10(-4) M CGS-21680, 14 +/- 6% and 10(-4) M Ado, 15 +/- 4%; without PTx: 10(-5) M Ado + 2 x 10(-7) M DPCPX, 19 +/- 1%). These results indicate that compared with beta-adrenergic stimulation, which markedly increases myocyte Ca2+ transients and shortening, A2aR-mediated increases in myocyte shortening are accompanied by only modest increases in Ca2+ transients. These observations suggest that the A2aR-induced contractile effects are mediated predominantly by Ca2+-independent inotropic mechanisms.

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.

Inotropic effects of diadenosine tetraphosphate in isolated canine cardiac preparations

We studied the effects of diadenosine tetraphosphate (AP4A) on the force of contraction in canine preparations. The force of contraction was measured in isolated electrically driven (1 Hz) atrial and ventricular cardiac trabeculae from adult dogs. AP4A (100 microM) alone and after prestimulation with 10 nM isoproterenol reduced force of contraction in atrial preparations by approximately 24%. Moreover, AP4A (100 microM) alone and after prestimulation with 10 nM isoproterenol reduced the force of contraction in ventricular preparations by 29 and 29%, respectively. The negative inotropic effects of AP4A were abolished by the A1-adenosine receptor antagonist 1,3-dipropyl-cyclopentyl-xanthine (DPCPX). In summary, in canine myocardium, AP4A alone and after prestimulation with a beta-adrenoceptor agonist exerts negative inotropic effects, which are probably mediated via A1-adenosine receptors.

Adenosine A2a receptors increase arterial endothelial cell nitric oxide

BACKGROUND

Adenosine is a potent vasodilator of vascular smooth muscle. Endothelium-derived nitric oxide (NO) elicits vasodilation. We have previously reported that adenosine stimulates the production of NO from porcine carotid arterial endothelial cells (PCAEC) via a receptor-mediated mechanism. This study was to determine whether adenosine also enhances NO production from human arterial endothelium and to define the involvement of adenosine A1 and A2 receptors.

MATERIALS AND METHODS

Human iliac arterial endothelial cells (HIAEC) and PCAEC were harvested and cultured in dishes. NO production was evaluated with a NO electrode sensor which measured continuously real-time NO production.

RESULTS

NO content of the medium bathing HIAEC and PCAEC was significantly increased with adenosine (100 micromol/L). Ethylcarboxamidoadenosine (NECA), a nonselective adenosine receptor agonist, and carboxyethyl-phenethylamino-ethylcarboxamidoadenosine (CGS-21680), a selective adenosine A2a receptor agonist, increased NO production by HIAEC and PCAEC with respective EC50 values of 3.32 and 6.96 nmol/L for NECA and 30.97 and 29.47 nmol/L for CGS-21680. Chlorofuryl-triazolo-quinazolinamine (CGS-15943; 1 micromol/L), an adenosine A1 and A2 receptor antagonist, and aminofuryltriazolotriazinyl-aminoethylphenol (ZM-241385; 1 micromol/L), a selective adenosine A2a receptor antagonist, inhibited the effect of CGS-21680. Chlorocyclopentyl-adenosine (CCPA; 1 micromol/L), an adenosine A1 receptor agonist, significantly depressed NO production by both HIAEC and PCAEC: This effect was inhibited by cyclopentyl-dipropylxanthine (DPCPX), a selective adenosine A1 receptor antagonist.

CONCLUSIONS

The results demonstrate that adenosine A2a receptors increase, and adenosine A1 receptors decrease, the production of NO by human and porcine arterial endothelial cells.

Reduced responsiveness of [Ca2+]i to adenosine A1- and A2-receptor stimulation in the isoproterenol-stimulated ventricular myocytes of spontaneously hypertensive rats

To determine the modulatory action of adenosine-receptor stimulation on [Ca2+]i responses to beta-adrenoceptor stimulation in the heart of the spontaneously hypertensive rat (SHR), the electrically induced [Ca2+]i transient in response to isoproterenol (ISO) in single ventricular myocytes pretreated with adenosine agonists in SHRs and its normotensive control Wistar-Kyoto (WKY) rats was measured with a spectrofluorometric method by using fura-2/AM as the calcium indicator. In both types of rat, ISO at 0.001-1 microM augmented the electrically induced [Ca2+]i transient, and the effect was blocked by a beta-adrenoceptor blocker, propranolol. In SHRs that did not exhibit cardiac hypertrophy, the resting level of [Ca2+]i and the amplitude of the electrically induced [Ca2+]i transient were the same as those in WKY rats, whereas the augmentation of the electrically induced [Ca2+]i transient in response to ISO was significantly lower than that in WKY rats. In WKY rats, the effects of ISO on the electrically induced [Ca2+]i transient were inhibited by the adenosine A1-receptor agonist, R(-)-N6-(2-phenylisopropyl)adenosine (R-PIA) at 0.01-10 microM. In contrast, the effects of ISO were further enhanced by the adenosine A2-receptor agonist, N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl)]adenosine (DPMA) at 1-10 microM. In SHRs, the inhibitory effect of R-PIA was significantly reduced, whereas the excitatory effect of DPMA was absent. The effects of both adenosine-receptor agonists in both types of rat were abolished by the respective adenosine-receptor antagonists, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 3,7-dimethyl-1-propargylxanthine (DMPX). The results indicate that the modulatory actions of adenosine-receptor stimulation on [Ca2+]i response to beta-adrenoceptor stimulation in the hearts of SHRs are reduced, which is independent of cardiac hypertrophy.

Association of pre-ischaemic disturbances in energy metabolism with postischaemic dysfunction of the rat isolated working heart

1. Metabolic and functional effects of two protocols of preconditioning were compared in rat isolated hearts subjected to 20 min global ischaemia (37 degrees C) and reperfusion (30 min Langendorff + 15 min working). Prior to the ischaemic period, hearts were perfused according to Langendorff (control group) or were preconditioned by three 5 min cycles or two 10 min cycles of ischaemia and reperfusion (PC-I and PC-II groups, respectively). 2. There was no difference in the contractile function between the two preconditioned groups at the onset of sustained ischaemia, although the PC-II group showed enhanced release of adenosine (Ado), inosine, hypoxanthine and xanthine into the interstitium accompanied by losses of tissue adenine nucleotides (sigmaAN = ATP + ADP + AMP), total creatine (sigmaCr = phosphocreatine + creatine) and activation of glycolysis following the preconditioning period. During reperfusion, the PC-I group showed enhanced functional recovery, higher contents of sigmaAN and sigmaCr, and the smallest lactate dehydrogenase release compared with these indices in the control and PC-II groups. Postischaemic myocardial dysfunction was similar in the control and PC-II groups. 3. Functional recovery of hearts in both preconditioned groups was positively correlated with myocardial contents of ATP, sigmaAN and sigmaCr at the end of reperfusion, but not with pre-ischaemic Ado release into the interstitium. The results suggest that pre-ischaemic disturbances of energy metabolism, rather than activation of Ado receptors or stunning, may contribute to efficacy of multiple preconditioning in the rat isolated heart.

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.

Kinetics of intracellular Ca2+ concentration changes and cell contraction of electrically stimulated cardiomyocytes as analysed by automated digital-imaging microscopy

Enzymatically disaggregated, electrically stimulated cardiomyocytes from adult rats were examined by television-mediated vital microscopy for intracellular Ca2+ concentration and contractile activity. Using an inverted microscope in the epifluorescence mode, the Ca2+ signal was imaged with a low-light-level CCD camera and traced by means of the intracellular concentration of the fluorescent complex of Ca2+ with its indicator Fluo-3. Using the transmitted-light mode, cardiomyocytes that were not loaded were imaged with a conventional CCD camera with automatic gain control and traced by length measurements. Optical images of at least 40 cardiomyocytes per batch of cells from one heart were recorded in up to 20 microscopic fields of observation on videotape within 20 min. They were consecutively analysed by a personal computer installed with an image analysis card at a time-resolution of 20 ms, employing a discrete convolution operation, filtering and threshold setting for fluorescence measurements, and contour description and vectorial analysis for length measurements. Frames of fluorescent images were corrected for the halo effect caused by the increase in the Ca(2+)-dependent fluorescence signal after electrical stimulation. The cell contraction had to be measured in the transmission mode without Fluo-3 due to the inhibition caused by the intracellular Fluo-3. The following coefficients of variation (V) were determined: Vfluorescence < 0.033 and Vtransmission < 0.003 for the precision of measurement, and Vfluorescence < 0.05 and Vtransmission < 0.04 for the reproducibility. The system was validated with isoprenaline and ouabain as agents to modify the Ca(2+)-signal and the contraction. The response of cardiomyocytes of various rats to electrical stimulation, with respect to amplitude and its time point, had a V < 0.08 for both the Ca(2+)-signal and the contraction.

Ca2+ preconditioning elicits a unique protection against the Ca2+ paradox injury in rat heart. Role of adenosine. Fixed

Repeated Ca2+ depletion and repletion of short duration, termed Ca2+ preconditioning (CPC), is hypothesized to protect the heart from lethal injury after exposing it to the Ca2+ paradox (Ca2+ PD). Hearts were preconditioned with five cycles of Ca2+ depletion (1 minute) and Ca2+ repletion (5 minutes). These hearts were then subjected to Ca2+ PD, ie, one cycle of Ca2+ depletion (10 minutes) and Ca2+ repletion (10 minutes). Hearts subject to the Ca2+ PD underwent rapid necrosis, and myocytes were severely injured. CPC hearts showed a remarkable preservation of cell structure; ie, 65% of the cells were normal in CPC hearts compared with 0% in the Ca2+ PD hearts. LDH release was significantly reduced in CPC hearts compared with Ca2+ PD hearts (2.45 +/- 0.18 and 8.02 +/- 0.7 U.min-1 x g-1, respectively). ATP contents of CPC hearts were less depleted compared with the Ca2+ PD hearts (5.9 +/- 0.8 and 3.0 +/- 0.16 mumol/g dry weight, respectively). Addition of the adenosine A1 receptor agonist R-phenylisopropyl adenosine before and during Ca2+ PD provided protection similar to that in CPC hearts, whereas the nonselective adenosine A1 receptor antagonist, 8-(p-sulfophenyl)-theophylline, blocked the beneficial effects of CPC. CPC-mediated protection was aborted when hearts subjected to CPC were treated with pertussis toxin (the guanine nucleotide or G-protein inhibitor). The present study suggests that Ca2+ preconditioning confers significant protection against the lethal injury of Ca2+ PD in rat hearts. Cardioprotection appears to result from adenosine release during preconditioning and by Gi-protein-modulated mechanisms.

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

Endogenous interstitial adenosine may protect the hypoxic heart by attenuating beta-adrenergic-induced contractile and metabolic responses, thereby reducing energy utilization. Constant-flow perfused rat hearts were used to study: 1) the effect of hypoxia on isoproterenol (ISO)-induced increase in interstitial adenosine, as estimated with epicardial surface transudates, and 2) the role of endogenous adenosine in hypoxic depression of ISO-induced contractile responses. ISO (1 nM for 10 minutes) in the normoxic heart increased transudate adenosine 114% from a pre-ISO normoxic value of 343 pmol/ml. ISO administered to the hypoxic heart increased transudate adenosine 357% from a pre-ISO hypoxic value of 797 pmol/ml. The absolute magnitude of the ISO-induced increase in transudate adenosine was 625% greater during hypoxia than during normoxia. This was associated with a reduction in the ISO-induced contractile response during hypoxia. In other experiments, with normoxia ISO (10 nM for 10 seconds) increased developed left ventricular pressure by 140 mm Hg, and the maximum rates of left ventricular pressure development and relaxation by 5,860 and 2,771 mm Hg/sec, respectively, above control values of 90 mm Hg, 2,250 mm Hg/sec, and 1,875 mm Hg/sec. Hypoxia reduced the three ISO-induced contractile responses by 50%, 56%, and 36%. However, 1,3-dipropyl-8-cyclopentylxanthine (5 x 10(-7) M), an adenosine A1-receptor antagonist, added to the hypoxic hearts resulted in only a 31%, 39%, and 9% reduction in the ISO-induced responses in developed left ventricular pressure and the maximum rates of left ventricular pressure development and relaxation, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine and the stunned heart

Adenosine is one agent under investigation as a therapeutic intervention of myocardial stunning. Adenosine caused numerous effects on the cardiovascular system through its interaction with A1 and A2 receptors. We investigated adenosine A1 receptor mediated mechanisms of cardiac protection in the stunned rat myocardium. Previous studies showed that both adenosine and R-phenylisopropyladenosine (PIA), an A1 receptor agonist, prolonged the time to onset of ischemic contracture in ischemic isolated rat hearts. Phenylaminoadenosine, an A2 receptor agonist, did not have any effect, while receptor antagonists blocked adenosine and PIA action. Direct attenuation of the effects of myocardial stunning was observed by altering levels of interstitial fluid adenosine. Our laboratory has shown that administration of erythro-9(2-hydroxy-3-nonyl) adenine (EHNA; an adenosine deaminase inhibitor) to dogs subjected to left anterior descending coronary artery (LAD) occlusion followed by reperfusion results in dramatic increases in ischemic levels of interstitial fluid adenosine and postischemic myocardial function. Using a similar model in dogs, we have shown that exogenous intracoronary adenosine (50 micrograms/kg per min) augmented postischemic recovery of function, as assessed by significant enhancement (p < 0.01) of systolic wall thickness (7.0 +/- 3.0 pretreatment vs -5.7 +/- 1.7 controls). These data support the role for an adenosine A1 receptor mediated mechanism for protection against myocardial stunning.

Fluorometric quantitation of adenosine concentration in small samples of extracellular fluid

Adenosine is a naturally occurring nucleoside which regulates many physiological processes by interacting with adenosine-specific receptors. Knowledge of the extracellular adenosine concentration at the site of adenosine receptors on target cells is required for an understanding of mechanisms involving the action of the nucleoside. Samples of extracellular fluid which reside in close proximity to the surface of target cells are frequently small in volume. This report describes improvements in accuracy and reliability of a fluorometric assay designed for determining the concentration of adenosine in microliter samples of extracellular fluids. The utility of the assay is demonstrated by determining adenosine concentrations in interstitial and coronary effluent samples from normoxic perfused rat hearts. The assay also clearly detects changes in the interstitial and coronary effluent adenosine levels produced by isoproterenol stimulation or hypoxia. Thus, this assay is useful for determining the adenosine concentration in microliter samples of extracellular fluid and should facilitate investigations dealing with the functions of adenosine.

Adenosine-sensitive afterdepolarizations and triggered activity in guinea pig ventricular myocytes

This study examines the cellular basis and specificity of the effects of adenosine on early afterdepolarizations (EADs), delayed afterdepolarizations (DADs), and triggered activity (TA) induced by various drugs with different mechanisms of action. Membrane potential and currents were measured in isolated guinea pig ventricular myocytes. Adenosine (10-100 microM) significantly (p less than 0.05) reduced the amplitude of DADs and suppressed TA induced by isoproterenol (10-50 nM) and forskolin (1 microM) but not those induced by dibutyryl cAMP (1 microM), ouabain (1-5 microM), and 7.2 mM [Ca2+]o. Adenosine also abolished EADs and TA induced by isoproterenol. In contrast, adenosine failed to abolish EADs and TA induced by quinidine (3 microM) or those that occurred spontaneously (i.e., in the absence of drugs). Transient inward current (ITi) was induced on repolarization after 2-second-long single depolarizing voltage steps or after 12-second-long trains of 300-msec depolarizing pulses. Concomitant with the attenuation of DADs, adenosine suppressed ITi caused by isoproterenol and forskolin but not those induced by ouabain, dibutyryl cAMP, and elevated [Ca2+]o. The amplitude of ITi was dependent on the magnitude of the activating voltage step, but the suppression of ITi by adenosine was not. The selective A1-adenosine receptor antagonist N-0861 (9-methyladenine derivative) antagonized the effects of adenosine on afterdepolarizations, ITi, and TA. In myocytes from guinea pigs treated with pertussis toxin, adenosine failed to attenuate DADs and ITi or abolish TA induced by isoproterenol or forskolin. In parallel experiments, isoproterenol (10 nM) raised cellular cAMP from 5.7 +/- 0.2 to 8.1 +/- 0.1 pmol and the selective A1 receptor agonist cyclopentyladenosine (5 microM) reduced it to 6.5 +/- 0.2 pmol (p less than 0.05). Thus, adenosine specifically attenuates afterdepolarizations and abolishes TA by suppressing ITiS that are associated with stimulation of adenylate cyclase via a pertussis toxin-sensitive A1 receptor-mediated action. In conclusion, the response of TA to adenosine may identify a mechanism of afterdepolarization related to stimulation of adenylate cyclase.