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Investigates adenosine stimulation of DNA synthesis in human endothelial cells. Stimulation of DNA synthesis by adenosine and adenine nucleotides; Mechanism of adenosine-induced DNA synthesis; Stimulation of adenosine synthesis by adenosine and adenine nucleotides.

Studies the role of interstitial adenosine in the reduced metabolic and mechanical responsiveness of the aged heart to beta-adrenergic stimulation. Comparison between young and aged rat hearts; Effect of isoproterenol; Effect of sulfophenyl-theophyline; Effect of exogenously administered adenosine deaminase.

Focuses on a study of the myocardial adenosine A1-receptor sensitivity during juvenile and adult states of heart maturation. Reference to previous reports; Purpose of the present study; Detailed information on the methods and materials used; Results of the study; Conclusion reached based on the results.

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.

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.

Myocardial contractile responsiveness to beta-adrenoceptor stimulation is known to be reduced with maturation or aging. The present study was undertaken to determine the role of antiadrenergic A1 and stimulatory A2 adenosine receptors in the modulation of beta-adrenergic-elicited contractile performance of the heart at juvenile (approximately 25 days) and adult (approximately 79 days) stages of maturation. Isoproterenol, a beta-adrenergic agonist, at 10(-7) M produced a greater maximal increase in contractility, assessed as the maximal rate of left ventricular pressure development (+dP/dtmax), in immature than in mature hearts (104 and 80%, respectively), but produced a greater increase in venous adenosine concentration in the mature than in the immature hearts (738 and 277 nM, respectively). Isoproterenol at 10(-9) to 10(-8) M produced similar increases in contractility in the absence or presence of the A1 adenosine receptor antagonist xanthine amine congener (XAC; 0.5 microM) for both immature and mature hearts. In addition, XAC did not alter the isoproterenol-elicited contractile response in the immature heart during hypoperfusion induced by 50% reduction of coronary flow. However, in the mature heart, 10(-8) M isoproterenol elicited a significantly greater increase in +dP/dtmax during hypoperfusion in the presence (79%) vs. the absence (60%) of XAC. In both immature and mature hearts, hypoperfusion enhanced isoproterenol-elicited venous adenosine concentration by similar magnitudes of 76 and 72%, respectively. In further studies, the A2 adenosine receptor antagonist 9-chloro-2-(2-furyl)[1,2,4]-triazolo[1,5-c]quinazolin-5-amine (CGS-15943; 1 microM) reduced the isoproterenol-elicited contractile response of mature but not immature hearts during normal perfusion. These results suggest that myocardial adenosine modulates the beta-adrenergic-elicited contractile response of the adult heart via activation of both A1 and A2 adenosine receptors and that these functions of adenosine become expressed with myocardial maturation.

Adenosine per se is a potent vasodilator of vascular smooth muscle. Endothelial cells modulate vascular tone via the release of nitric oxide (NO), which also elicits vasodilation. This study was undertaken to determine whether adenosine could directly stimulate endothelial cells to enhance NO production, which could subsequently reduce vascular tone. NO production was evaluated in porcine carotid artery endothelial cells (PCAEC) and human saphenous vein endothelial cells (HSVEC) seeded on multiwell plates, grown to confluence, and treated with adenosine for 1 h. The bathing medium was collected, and the NO production was determined as reflected by the formation of NO2- and NO3-. NO production by PCAEC was significantly increased by adenosine in a dose-dependent manner, whereas there was only an insignificant tendency for an increase by HSVEC. The addition of the NO synthase competitive inhibitor, NG-monomethyl-L-arginine (NMMA), or the adenosine receptor antagonist, theophylline, prevented the increase in NO production by adenosine. The results suggest that adenosine stimulates, by a receptor-mediated mechanism, the production of NO by arterial, but not by venous, endothelial cells.

The effect of adenosine on proliferation of human endothelial cells was investigated by adding adenosine to the medium of cultures derived from human umbilical veins. Cell counts on cultures grown in 10 microM adenosine for 4-7 days were 41-53% greater than counts from control cultures. In contrast, 10 microM adenosine had no effect on growth of a human fibroblast cell strain (IMR-90). Neither inosine nor 2',5'-dideoxyadenosine influenced endothelial cell growth at concentrations of 0.1 or 10 microM. Addition of adenosine deaminase abolished the proliferative effect of added adenosine and inhibited proliferation by 16% in control cultures, suggesting that endogenous adenosine may enhance proliferation in culture. The adenosine receptor antagonist, 8-phenyltheophylline, at 0.1 and 1.0 microM blocked the enhanced proliferation caused by 10 microM adenosine. Addition of 10 microM adenosine enhanced DNA synthesis in endothelial cell cultures as indicated by an increased incorporation of [3H]thymidine into acid-insoluble cell material. The results indicate that addition of physiological concentrations of adenosine to human umbilical vein endothelial cell cultures stimulates proliferation, possibly via a surface receptor, and suggest that adenosine may be a factor for human endothelial cell growth and possibly angiogenesis.

Adenosine in the heart attenuates the contractile and metabolic effects of beta-adrenergic stimulation. The effect of adenosine on changes in intracellular Ca2+ concentration [( Ca2+]i) elicited with electrical stimulation was studied in rat ventricular myocytes in the absence and presence of isoproterenol (ISO). Fura-2 was utilized as a Ca2+ indicator. Autofluorescence was determined, and in vivo calibration was conducted, for each myocyte. Phenylisopropyladenosine (PIA; 10(-7) M; 5 min), an adenosine A1 receptor agonist, had no effect on the Ca2+ transient magnitude (TM) or the rate of Ca2+ transient decline determined at 150 nM Ca2+(i) (RD150). ISO (10(-8) M; 1 min) in the continued presence of PIA resulted in a 16% increase in the TM, but no change in the RD150. Inhibiting the PIA with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10(-7) M; 3 min) in the continued presence of ISO plus PIA resulted in a further 51% increase in the TM and a 57% increase in the RD150. In PIA-treated myocytes, ISO-induced spontaneous high-frequency Ca2+ transients occasionally were observed after the inhibition of PIA by DPCPX. The results of this study suggest that adenosine attenuates myocardial contractile responses to beta-adrenergic stimulation, in part, by reducing the beta-adrenergic-induced changes in the Ca2+ transients occurring in the contracting ventricular myocyte.

The antiadrenergic action of adenosine was examined in open- and closed-chest preparations of anesthetized rats. The positive inotropic effects of a jugular vein infusion of either isoproterenol or epinephrine were attenuated by phenylisopropyladenosine, an adenosine A1-receptor agonist. 1,3-Dipropyl,8-cyclopentylxanthine, a specific A1-receptor antagonist, inhibited the action of phenylisopropyladenosine. The results indicate that adenosine receptor-mediated mechanisms are functional in the blood-perfused rodent heart and support the possibility of a physiological role for adenosine in modulating cardiac contractility.

The effects of adenosine analogues on beta-adrenergic receptor and receptor-independent elicited increases in adenylate cyclase activity were investigated using membranes obtained from primary cultures of adult rat ventricular myocytes. Phenylisopropyladenosine, an A1-receptor agonist, at concentrations of 0.1, 1.0, and 10 microM, maximally inhibited isoproterenol-stimulated adenylate cyclase activity by 35, 55, and 41%, respectively. The inhibition by phenylisopropyladenosine was antagonized by 10 microM theophylline. One micromolar phenylisopropyladenosine was much less effective at attenuating forskolin-stimulated activity, such that the maximum inhibition was 26%. Phenylisopropyladenosine had no effect on adenylate cyclase stimulation by 5'-guanylylimidodiphosphate. Phenylaminoadenosine, an A2 agonist, at 10 microM or greater stimulated adenylate cyclase activity. This effect was not significantly inhibited by theophylline or 0.1 microM 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), which antagonized phenylisopropyladenosine inhibition of isoproterenol-stimulated adenylate cyclase activity. Additionally, N-ethylcarboxamidoadenosine, a nonselective adenosine-receptor agonist, had no effect on adenylate cyclase activity in the absence of DPCPX but stimulated adenylate cyclase activity in the presence of DPCPX. These results indicate that both A1 and A2 receptors exist on the ventricular myocyte sarcolemma. More importantly, the findings suggest that adenosine inhibition of catecholamine-stimulated adenylate cyclase activity is primarily due to an alteration in beta-adrenergic receptor-mediated transduction and perhaps in part by a direct inhibition of the catalytic component.

Because adenosine has been shown to attenuate the catecholamine-induced increase in myocardial cAMP formation and glycogen phosphorylase activity (Circ. Res. 43: 785-792, 1978), the present study was undertaken to determine whether the nucleoside inhibits the catecholamine-elicited increase in cardiac contractile state. Isolated rat atria were bathed in oxygenated physiologic saline and stimulated to contract isometrically at 2/s. Isoproterenol (0.1 microM) increased peak contractile force (PCF) by 96% and the rate of force development (+dF/dt) by 107%. Adenosine (10 microM) alone had no effect on these contractile parameters. Isoproterenol in the presence of adenosine increased PCF and +dF/dt only 15 and 14%, respectively. Elevation of bathing medium Ca2+ or administration of dibutyryl cAMP (DBcAMP) increased PCF and +dF/dt, but these responses were not decreased by adenosine. Inosine, adenine, adenosine 5'-monophosphate, and guanosine inhibited the isoproterenol-induced responses 5-22%. The results indicate that adenosine markedly inhibits, whereas some related purines only mildly attenuate, the catecholamine-elicited, but not the Ca2+- or DBcAMP-elicited, increases in contractility. Thus, adenosine may antagonize catecholamine-elicited glycogenolysis and enhanced contractile state in the heart by exerting an effect at the level of, or proximal to, cAMP formation.

Adenosine inhibition of hormone-sensitive adenylate cyclase activity was investigated using isolated myocardial membranes prepared from rat hearts. When cyclase activity was determined in membranes, using [alpha-32P]ATP as substrate, 10(-5) M adenosine inhibited isoproterenol-stimulated adenylate cyclase activity by 25% but did not inhibit basal activity or fluoride (5 mM) activation of the enzyme. The adenosine reduction of isoproterenol-sensitive cyclase activity was dependent on GTP but was not prevented by 10(-3) M theophylline. Adenosine neither appeared to compete with ATP for the substrate converting site of the enzyme nor reduced 5'-guanylyl imidodiphosphate activation of the enzyme. Inasmuch as lower concentrations of adenosine had no influence on enzyme activity, endogenous adenosine may be present in the adenylate cyclase assay. To obviate the effects of endogenous adenosine, the adenylate cyclase assay was then modified to a 2'-deoxy system with [alpha-32P]dATP used as the substrate in the presence of adenosine deaminase. With this assay system, the 15% inhibition of isoproterenol-stimulated adenylate cyclase activity produced by the adenosine receptor agonists, 10(-8) M 2-chloroadenosine or phenylisopropyladenosine, was prevented by 10(-4) M 8-phenyltheophylline or isobutylmethylxanthine (IBMX), respectively. While under these assay conditions, 10(-7) M 2',5'-dideoxyadenosine, a P-site analogue, did not influence the hormone-sensitive cyclase activity. The 35% reduction of the hormone-sensitive enzyme produced by this analogue at 10(-5) M was not prevented by IBMX. These results suggest that nanomolar concentrations of adenosine analogues interact with a methylxanthine-sensitive adenosine receptor that mediates the attention of membrane hormone-sensitive adenylate cyclase activity.

The divalent cation ionophore A23187, when added to guinea pig papillary muscle, produced contractile effects that were similar to those produced by isoproterenol or histamine, but the ionophore's effects did not appear to result from the release of endogenous transmitters or prostaglandin production. Optimally effective concentrations of A23187 (6 microM) and isoproterenol (1 microM) more than doubled the peak contractile force and the rates of force development and relaxation and caused a 25% decrease in the duration of the contraction. Both A23187 and isoproterenol decreased the resting tension by approximately 0.15 g and significantly diminished the magnitude of a potassium-induced contracture. The positive inotropic effect of A23187 was prevented by incubating the tissue in calcium-depleted medium and antagonized by D 600, a blocker of sarcolemmal calcium influx, and acetylcholine. The contractile effects of A23187 appear to be related, in part, to its ability to increase the movement of calcium across the sarcolemma electroneutrally, since no change of the action potential occurred. In addition, possible intracellular actions of this ionophore may produce contractile effects that resemble those produced by isoproterenol and that reflect an increased sequestration of calcium within the myocardial cell.

The relationship between cAMP-dependent protein kinase activity and epinephrine-produced activation of phosphorylase and increase in contractility was investigated in the intact working rat heart. Epinephrine was administered as a bolus into the superior vena cava of open-chest preparations and the hearts were rapidly frozen. cAMP increased within 5 s and returned to control within 20-30 s. Protein kinase and phosphorylase kinase activity ratios increased transiently with the same time course as that for cAMP. The phosphorylase activity ratio and the rate of left ventricular pressure development increased maximally within 15 s and returned to control in 30-60 s. Continuous infusion of epinephrine caused a sustained elevation of the protein kinase. Free catalytic protein kinase activity increased proportionately with the dose of epinephrine. The beta-adrenergic blocking agent, practolol, had no effect on the basal levels of the five parameters studied, but did prevent the epinephrine-produced increases. The results suggest that the time course of cAMP-dependent protein kinase activation is appropriate if this enzyme is to play a role in the catecholamine-induced increase in both glycogenolysis and contractility in the in vivo heart.

Adenosine reduces cardiac mechanical and metabolic manifestations of catecholamine stimulation possibly by attenuating catecholamine-enhanced adenylate cyclase activity and sarcolemmal Ca2+ flux. The effects of adenosine and Ca2+ on catecholamine-induced myocardial protein phosphorylation were investigated using isolated rat hearts perfused with a 32P-enriched medium. Isoproterenol (10(-7) M, 1 min) elicited a 107-379% increase in 32P incorporation into proteins having molecular weights of 155, 92, 30, 28, 22, and 20 kdaltons. The left ventricular pressures and maximum rates of ventricular pressure development and ventricular relaxation were significantly elevated. These effects of isoproterenol were inhibited by propranolol (10(-5) M). Adenosine (10(-5) M, 2 min) decreased the isoproterenol-elicited increases in 32P incorporation by 50-86% and decreased the contractile responses but had no effect in the absence of isoproterenol. Raising the perfusion Ca2+ concentration from 1 to 4 mM did not alter the 32P incorporations but increased contractile parameters. The increase in Ca2+ augmented the isoproterenol 32P responses but not the contractile responses to isoproterenol. These results are consistent with the proposal that catecholamines augment cardiac metabolism and contractility by enhancing myocardial protein phosphorylation. Adenosine and Ca2+ modulate these responses.

The importance of endogenous myocardial adenosine in attenuating catecholamine-elicited contractile responses was investigated in perfused oxygenated rat hearts. Perfusion of the isolated hearts with adenosine deaminase potentiated the isoproterenol-induced increases of three contractile variables (left ventricular pressure development and rates of both left ventricular pressure development and relaxation). The peak (maximal, within 30 s) and maintained (after 1 min) increases of the contractile variables caused by 10(-8) M isoproterenol were enhanced by 15-22 and 31-43%, respectively. Adenosine deaminase appeared in epicardial surface transudates of similarly perfused hearts, indicating that the enzyme had entered the myocardial interstitial space. Isoproterenol alone elevated the release of adenosine into coronary effluents of isoproterenol-stimulated hearts, and adenosine deaminase prevented the release of the nucleoside. The higher the level of adenosine in the effluent, the greater the reduction of the peak contractile variables. Phenylisopropyladenosine at 10(-8) M prevented the adenosine deaminase potentiation of 10(-9) M isoproterenol-induced contractile responses. The adenosine analogue at 10(-6) M blocked completely the isoproterenol-produced increases in the contractile variables. These results suggest that endogenous adenosine prevents full mechanical responsiveness to beta-adrenoceptor stimulation in the oxygenated myocardium. In addition, the findings support the notion that adenosine serves as an important negative feedback modulator in the oxygenated heart.

Isolated guinea pig atria stimulated to contract isometrically were used to determine whether adenosine at a concentration that does not cause a direct depressant effect on peak contractile force, rate of force development, and rate of relaxation was capable of influencing the elevation in these contractile parameters caused by an increase in preload, paired electrical stimulation, an increase in contraction frequency, and catecholamine stimulation in K+-depolarized and nondepolarized atrial muscle. Adenosine had no effect on the contractile parameters that were enhanced by an increase in preload or paired electrical stimulation. The nucleoside reduced the increases in the contractile parameters produced by isoproterenol stimulation, an increase in contraction frequency, and isoproterenol-induced contractions in depolarized atria. All adenosine reductions were inhibited by theophylline, an antagonist of adenosine actions. The adenosine reduction of the elevated contractile parameters caused by increasing contraction frequency was not prevented by atropine (a muscarinic antagonist) or propranolol (a beta-adrenergic blocking agent). These results suggest that adenosine at a concentration that does not produce direct negative inotropic responses is capable of attenuating the elevation in contractility elicited by catecholamine stimulation, an increase in contraction frequency, and catecholamine-induced contractions in depolarized atria. However, the reduction by adenosine of the contractile responses elicited by an increase in contraction frequency appears to be independent of catecholamines.

We have reported that the divalent cation ionophore A23187, like the beta-adrenergic agonist isoproterenol, increased the force of contraction and rate of relaxation and shortened the duration of contraction of papillary muscles isolated from guinea pigs. A23187 produced a fall in resting tension and decreased the contracture tension of K +/- depolarized muscles, as did isoproterenol. In the present studies, isoproterenol produced a concentration-dependent, rapid, and sustained increase in the cyclic AMP (cAMP) content of papillary muscle. In contrast, A23187 had no detectable effect on cAMP levels, even in the presence of the phosphodiesterase inhibitor, papaverine. Neither drug, at concentrations maximal for contractile effects, altered cyclic GMP (cGMP). Isoproterenol increased the cAMP-dependent protein kinase activity ratio, whereas A23187 did not change the activity of this enzyme. However, both A23187 and isoproterenol produced a concentration-dependent increase in phosphorylase activity. Concentrations of A23187 or isoproterenol that enhanced contractility maximally increased the alkali-labile phosphate (by ca. 35%) but were without effect on the acid-labile, alkali-stable phosphate in the total acid precipitable protein. Contractile effects of isoproterenol, which reflect activated Ca2+ uptake, and the increase in phosphorylase activity produced by this agent are believed to be due to an increase in cAMP with subsequent activation of cAMP-dependent protein kinases and phosphorylation of proteins. A23187 may produce similar contractile effects without an increase in cAMP or cAMP-dependent protein kinase activity by activating other protein kinases and/or inhibiting phosphoprotein phosphatases, most likely by its effects on intracellular calcium.

The properties of adenosine attenuation of catecholamine-elicited increases in peak contractile force, rate of force development, and rate of relaxation were studied in isolated rat atria. Adenosine, at a concentration that did not cause a direct depressant effect by itself, was capable of reducing by approximately 15% the increase in the contractile parameters elicited by isoproterenol. This reduction was not overcome by elevating the catecholamine concentration. The adenosine reduction was prevented by theophylline or the presence of adenosine deaminase. The reduction appears to be independent of the acetylcholine-mediated reduction of catecholamine responses. Adenosine reduced the positive inotropic responses elicited by norepinephrine and epinephrine but not phenylephrine. Adenosine deaminase in oxygenated atria potentiated the catecholamine-elicited contractile responses and reduced the progressive fall of the elevated contractile responses observed with continual catecholamine stimulation. In hypoxic atria adenosine deaminase potentiated the positive inotropic responses observed with catecholamine stimulation. The results suggest that an adenosine-specific mechanism is capable of attenuating the elevation in contractility elicited by beta-adrenergic stimulation. In addition, endogenous adenosine may be responsible, in part, for the reduction of catecholamine-mediated contractile responses in oxygenated and hypoxic myocardial tissue.

The effect of nicotine on adenosine release, oxygen consumption, and contractility was investigated in perfused rat hearts. Continuous infusion of nicotine into the perfusing physiological saline (PS) elicited a propranolol (10(-6) M) sensitive transient elevation of developed left ventricular pressure (LVP) and maximum rates of left ventricular pressure development and relaxation (+/- dP/dtmax) within 20 s, which subsequently declined to maintained elevated plateau levels by 1 min. The continuous infusions of nicotine to achieve PS concentrations of 5 X 10(-4), 1 X 10(-4), or 5 X 10(-5) M, respectively resulted in significant increases in the mean plateau levels of LVP (33.4, 10.1, or 6.3%), +dP/dtmax (26.3, 10.8, or 6.9%) and-dP/dtmax (35.0, 11.9, or 9.0%) at 1 min. The inclusion of propranolol (10(-6) M) with or without atropine (10(-6) M) did not alter these maintained plateau responses to nicotine. During the plateau phase of the contractile response oxygen consumption of the hearts was significantly elevated by 36, 19, or 11%, and mean levels for adenosine in the coronary effluent rose by 261, 76, or 74% in response to 5 X 10(-4), 1 X 10(-4), or 5 X 10(-5) M nicotine, respectively. Nicotine did not influence [14C]adenosine uptake by the hearts. These results suggest that nicotine is capable of 1) augmenting cardiac contractility and oxygen consumption independent of beta-adrenergic or muscarinic influence, and 2) elevating the appearance of adenosine in the coronary circulation presumably by enhancing myocardial production of the nucleoside.