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Objective: Microdialysis and 31P-NMR spectroscopy were used to test opposing hypotheses that ischemic preconditioning inhibits adenine nucleotide degradation and purine efflux, or that preconditioning activates cardiovascular adenosine formation to provide enhanced cardioprotection., Methods: 31P-NMR spectra and matching interstitial fluid (ISF) or venous effluent samples were obtained from Langendorff perfused rat hearts. Control hearts (n = 9) underwent 30 min of global normothermic ischemia and 30 min reperfusion. Preconditioned hearts (n = 6) were subjected to a 5 min ischemic episode and 10 min reflow prior to 30 min ischemia and 30 min reperfusion. Effects of repetitive ischemia-reperfusion (3 x 5 min ischemic episodes) on adenosine levels and energy metabolism were also assessed (n = 8)., Results: Preconditioning improved post-ischemic recovery of heart rate x left ventricular developed pressure (71 +/- 5 vs 43 +/- 8%, P < 0.05) and end-diastolic pressure (14 +/- 3 vs 29 +/- 4 mmHg, P < 0.05) compared with control hearts, respectively. Preconditioning did not alter intracellular ATP, phosphocreatine (PCr), inorganic phosphate (Pi), H+ or free Mg2+ during global ischemia, but improved recoveries of PCr, Pi, and delta GATP on reperfusion. ISF adenosine increased more than 20-fold during 30 min ischemia. The 5 min preconditioning episode increased ISF adenosine 3-fold, and reduced ISF adenosine and inosine during subsequent prolonged ischemia by up to 75%. Venous purine levels during reperfusion were also reduced by preconditioning. Accumulation of adenosine in ISF and venous effluent during repetitive ischemia was progressively reduced despite comparable changes in substrate for adenosine formation via 5'-nucleotidase, (5'-AMP), and in allosteric modulators of this enzyme (Mg2+, H+, Pi, ADP, ATP)., Conclusions: (i) Ischemic preconditioning reduces interstitial and vascular adenosine levels during ischemia-reperfusion, (ii) reduced ISF adenosine during ischemia is not due to reduced ischemic depletion of adenine nucleotides in preconditioned rat hearts, (iii) preconditioning may inhibit adenosine formation via 5'-nucleotidase in ischemic rat hearts, and (iv) improved functional recovery with preconditioning is unrelated to metabolic/bioenergetic changes during the ischemic insult, but may be related to improved post-ischemic recovery of [Pi] and delta GATP in this model.

Metabolic and functional responses to extracellular Mg2+ concentration ([Mg2+]o) were studied in perfused rat heart. Elevations of [Mg2+]o from 1.2 to 2.4, 5.0, and 8.0 mM dose dependently reduced contractile function and myocardial oxygen consumption (MVO2) up to 80%. Intracellular Mg2+ concentration ([Mg2+]i) remained stable (0.45-0.50 mM) during perfusion with 1.2-5. 0 mM [Mg2+]o but increased to 0.81 +/- 0.14 mM with 8.0 mM [Mg2+]o. Myocardial ATP was unaffected by [Mg2+]o, phosphocreatine (PCr) increased up to 25%, and Pi declined by up to 50%. Free energy of ATP hydrolysis (DeltaGATP) increased from -60 to -64 kJ/mol. Adenosine efflux declined in parallel with changes in MVO2 and [AMP]. At comparable workload and MVO2, the effects of [Mg2+]o on cytosolic free energy were mimicked by reduced extracellular Ca2+ concentration ([Ca2+]o) or Ca2+ antagonism with verapamil. Moreover, functional and energetic effects of [Mg2+]o were reversed by elevated [Ca2+]o. Despite similar reductions in preischemic function and MVO2, metabolic and functional recovery from 30 min of global ischemia was enhanced in hearts treated with 8.0 mM [Mg2+]o vs. 2.0 microM verapamil. It is concluded that 1) 1.2-8.0 mM [Mg2+]o improves myocardial cytosolic free energy indirectly by reducing metabolic rate and Ca2+ entry; 2) [Mg2+]i does not respond rapidly to elevations in [Mg2+]o from 1.2 to 5.0 mM and is uninvolved in acute functional and metabolic responses to [Mg2+]o; 3) adenosine formation in rat heart is indirectly reduced during elevated [Mg2+]o; and 4) 8.0 mM [Mg2+]o provides superior protection during ischemia-reperfusion compared with functionally equipotent Ca2+ channel blockade.

In the present study we examined the action of native and oxidized low-density lipoproteins (LDL) on coronary vascular and cardiac function and ultrastructure in rat hearts perfused isovolumically in the Langendorff mode. Responses of the coronary resistance vessels to the endothelium-dependent vasodilator, histamine, and the endothelium-independent vasodilator, NaNO2, were measured together with contractile function (rate-pressure product) before and after perfusion for 20 min with native - or oxidized-LDL at a concentration of 100 mu g protein/ml. Ultrastructural damage was assessed via electron microscopy of perfusion-fixed heart specimens. When compared to findings in untreated, control hearts, both native and oxidized LDL significantly reduced the responsiveness of the coronary resistance vessels to histamine and NaNO2, by about 50%. The rate-pressure product was decreased more by oxidized-LDL (41%) than by native-LDL (26%). Electron microscopy showed no ultrastructural abnormalities in the vasculature or myocytes of control hearts. The administration of both native- and oxidized-LDL caused distortion of endothelial cells, increased levels of pinocytotic vesicles in both endothelial and smooth muscle cells, detachment of blood vessels from surrounding tissue, and some regions of myocyte injury with evidence of mitochondrial injury and fluid accumulation. Our results show that both native- and oxidized-LDL are toxic to the isolated heart preparation. They inhibit coronary vascular responsiveness to vasodilators, reduce contractile function, and produce damage to cardiac ultrastructure.

Objective: To examine the effect of hydrogen peroxide on the function and ultrastructure of cardiac muscle and the coronary vasculature in an isovolumic rat heart preparation perfused at constant flow., Design: Ventricular function was monitored via a balloon placed in the left ventricle and the response of the coronary vessels to vasodilators was assessed in hearts arrested with 35 mM potassium and treated with 5 microM phenylephrine to contract the coronary resistance vessels. Changes in coronary perfusion pressure reflect changes in resistance vessel tone. SETTING/ANIMALS: This experimental study consisted of 14 heart preparations, six control and eight treated hearts., Interventions: Hydrogen peroxide was included in the perfusate at a final concentration of 250 microM for 20 mins., Main Results: Hydrogen peroxide reduced rate-pressure product by 42%, caused a fivefold increase in end-diastolic pressure and increased coronary perfusion pressure by 33%. Also, the response of the coronary vasculature to the endothelium-dependent vasodilator, histamine, and endothelium-independent vasodilator, sodium nitrite, was decreased by 55% and 53%, respectively. Electron microscopy of hydrogen peroxide-treated hearts showed damage to both capillaries and arterioles. Endothelial cells were distorted and contained pinocytotic vesicles, endothelial cell junctions were disrupted and blood vessels were detached from surrounding tissue. A comparatively small amount of injury was seen in the myocyte population., Conclusions: The greater amount of ultrastructural damage seen in blood vessels compared with cardiac muscle suggests that the smooth muscle and endothelial cells of the vasculature are more susceptible to oxidant injury than the myocytes.

Control of respiration by products of ATP hydrolysis was examined in the in situ rat heart using a purpose-built nuclear magnetic resonance (NMR) coil. The in situ ratio of phosphocreatine to ATP concentrations ([PCr]/[ATP]) was 2.30 +/- 0.05, free Mg2+ concentration ([Mg2+]) was 0.57 mM, and cytosolic pH was 7.35 +/- 0.03 (n = 7). Basal inorganic phosphate concentration ([Pi]) was below NMR detection but was estimated to be 0.83 mM. The [ATP]/[ADP] [Pi] ratio, free ADP concentration ([ADP]), and free energy of ATP hydrolyses (delta GATP) were calculated to be 700,000 +/- 78,000 M-1, 18 +/- 3 microM, and -63.93 +/- 0.33 kJ/mol in situ, respectively (n = 7). In contrast, in the Langendorff perfused rat heart [ATP]/[ADP] [Pi] was only 76,140 +/- 12,830 M-1, [ADP] was 65 +/- 9 microM, and delta GATP was -59.92 +/- 0.48 kJ/mol (n = 7), all indicative of a lower energy state in vitro. Epinephrine infusion in situ (0.9 microgram.min-1.kg-1) increased the rate-pressure product 2.05-fold. During stimulation [ATP] was stable at 97 +/- 3% signal intensity, [PCr] declined by 25%, and [Pi] increased to 1.83 mM. Cytosolic pH was 7.27 +/- 0.01 and [Mg2+] was 0.64 +/- 0.05 mM. [PCr]/[ATP] declined to 1.83 +/- 0.13, and [ATP]/[ADP] [Pi] fell to 108,000 +/- 15,000 M-1. delta GATP only fell marginally to -59.56 +/- 0.49 kJ/mol. Free [ADP] increased threefold to 55 +/- 10 microM. Infusion of 2.8 +/- 0.5 microgram.min-1.kg-1 epinephrine increased the rate-pressure product 2.7-fold, further reduced [ATP]/[ADP] [Pi] (5% of basal), and elevated [ADP] more than fourfold without changing [ATP]. We conclude that the in situ heart is highly energetic compared with isolated perfused hearts and operates at a different metabolic "set-point." Because free [ADP] and [Pi] in situ approximate apparent Michaelis constants for mitochondrial respiration in vitro and increase with increased cardiac work, we conclude that each fulfills the criteria for the kinetic control of O2 consumption in the in situ rat myocardium.

Vitamin E is an endogenous antioxidant and is known to afford protection against lipid peroxidation. If lipid peroxidation was an important factor in the pathogenesis of reoxygenation injury in heart, then both the extent of lipid peroxidation and cell injury would be expected to be exacerbated in vitamin E-deficient hearts. To study reoxygenation injury in the present experiments, rat hearts were perfused in the Langendorff mode with a modified Krebs-Henseleit buffer under anoxic conditions for 60 min before resuming normoxic perfusion for 20 min. Creatine phosphokinase (CPK) activity and malonyldialdehyde (MDA), a product of lipid peroxidation, were assayed in the perfusate effluent from hearts during reoxygenation injury. Also, myocardial MDA and vitamin E contents were measured in extracts of freeze-clamped heart tissue obtained immediately before and 2 min after reoxygenation. Experiments were performed on hearts from groups of weanling rats fed either a vitamin E-deficient or vitamin E-supplemented diet (50 I.U. vitamin E/kg) for 5 to 6 weeks. After 5 weeks, the myocardial vitamin-E content was 103.8 +/- 5.3 (n = 5) and 11.5 +/- 1.6 (n = 4) ng/mg protein (mean +/- SEM) in the vitamin E-supplemented and vitamin E-deficient groups respectively. Perfused hearts from both dietary groups showed a peak of enzyme release 2 to 3 min after the reintroduction of oxygen, and enzyme release from vitamin E-deficient hearts was two-fold greater than enzyme release from vitamin E-supplemented hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

The aim of this study was to determine whether acute changes in [Mg2+]free occur during increased hydrolysis of cytosolic ATP, and whether these changes were of sufficient magnitude to be involved in the modulation of myocardial metabolism. 31P-NMR was used to estimate free cytosolic Mg2+ levels ([Mg2+]free) during hypoxia, isoproterenol stimulation, and graded low-flow ischemia in crystalloid perfused, isovolumic rat hearts. Cytosolic [Mg2+]free was calculated to be 0.73 +/- 0.12 mM in control hearts (100 mmHg hydrostatic pressure, 95% O2, n = 18). Cytosolic [Mg2+]free increased gradually during 10 min periods of hypoxia (65%, 50%, 35%, 5% O2), and 20 min infusions of isoproterenol (0.4, 3.0, 75 nM), to maximum values greater than 250% of control (P less than 0.05). During 8 min periods of graded low-flow ischemia (12.0, 7.2, 5.3, 3.4, and 1.6 ml/min/g), [Mg2+]free did not change significantly. [Mg2+]free displayed an inverse linear correlation with total cytosolic [ATP] during isoproterenol infusion (r = 0.87), and an exponential correlation during hypoxia (r = 0.82). The data indicate that acute changes in cytosolic [Mg2+]free can occur during conditions of net ATP hydrolysis although changes in ATP alone do not appear to be solely responsible for the changes in [Mg2+]free. Since the magnitude of the changes in [Mg2+]free are sufficient to alter equilibria of enzymes such as creatine kinase and myokinase, it is possible that these changes are involved in the acute modulation of myocardial metabolism.

The pathogenesis of post-ischaemic depression of contractility in myocardium was examined in isovolumic rat heart. 31P-NMR was used to monitor changes in ATP, creatine phosphate (CrP), inorganic phosphate (Pi), and [H+] during brief periods of ischaemia and reperfusion with and without allopurinol treatment. During 5, 10, or 15 min of total global ischaemia, the decline in function (rate-pressure product) correlated inversely with [Pi] (r = 0.92, P less than 0.01). Cardiac function exhibited a slow progressive recovery during 20 min of reperfusion, ultimately reaching only 85%, 78%, and 69% of its pre-ischaemic value following 5, 10, and 15 min of global ischaemia respectively. Following each ischaemic period [ATP], [CrP], [Pi], and [H+] all recovered to control levels within 5-10 min of initiating reperfusion. Allopurinol (2 mM) treatment of hearts made ischaemic for 15 min significantly improved contractile recovery to 89 +/- 7%. Allopurinol also exhibited significant anti-arrhythmic activity during the reperfusion period, decreasing the incidence of premature contractions and the duration of tachy-arrhythmias. Allopurinol had no effect on the final repletion of [ATP] and [CrP], or the recovery of [Pi] and [H+], although the rate of ATP repletion was elevated in the initial 5 min of reperfusion. These results show that neither depletion of the cytosolic high-energy phosphate pool, nor sustained elevations in [Pi] or [H+] are important in the production of post-ischaemic contractile impairment. The beneficial action of allopurinol suggests that xanthine oxidase derived oxygen free-radicals may be involved in the sustained contractile dysfunction following brief ischaemic episodes.

Temporal and quantitative relations between cytosolic energy metabolism, adenosine efflux, and coronary flow were examined during 10 min of isoproterenol (ISO) infusion (60 nM) or hypoxia (5% O2) in isolated isovolumic rat heart. Myocardial metabolism was monitored using 31P-nuclear magnetic resonance spectroscopy, and venous effluent was collected and assayed for adenosine. During ISO infusion, coronary flow increased to approximately 170%, and [ATP]/[ADP] [Pi] (cytosolic phosphorylation potential) declined to less than 25% of preinfusion levels, respectively (P less than 0.001). During hypoxia, coronary flow increased to 190%, and [ATP]/[ADP] [Pi] declined to less than 25% of normoxic levels (P less than 0.001). Release of adenosine into the coronary venous effluent increased greater than 10-fold and displayed significant inverse linear correlations with log[ATP]/[ADP] [Pi] and positive linear correlations with free cytosolic [AMP] and coronary flow during ISO infusion and hypoxia. Adenosine deaminase (ADA) treatment reduced coronary vasodilation by approximately 30% during ISO infusion and 40% during hypoxia (P less than 0.001) and augmented chronotropic and inotropic responses to ISO infusion (P less than 0.01). Infusion of ADA potentiated changes in [ATP]/[ADP] [Pi] and [AMP] observed during ISO infusion and hypoxia (P less than 0.05). These results indicate that 1) endogenous adenosine mediates metabolic vasodilation in the heart, 2) adenosine modulates the response of isolated myocardium to catecholamines, 3) myocardial adenosine formation appears to be linked to cytosolic metabolism via changes in [ATP]/[ADP] [Pi] and [AMP], and 4) endogenous adenosine provides a significant, metabolically beneficial action in isolated hearts during hypoxia and inotropic stimulation.

Alterations in cytosolic metabolites and [Mg2+]i were monitored using 31P magnetic resonance spectroscopy during inotropic stimulation of isolated rat heart [10 nM isoproterenol, 0.6 microM isobutyl-1-methylxanthine (IBMX), 5 microM ouabain]. All drugs significantly elevated contractile function (rate-pressure product) and MVO2 by approximately 100-150% (P less than 0.001), decreased cytosolic [creatine phosphate] ([CrP]) and [ATP] (approximately 65 and 80% of control values, respectively) (P less than 0.001), increased [Pi] to more than 180% of pretreatment values, and decreased [H+] by less than 15% (P less than 0.05). A significant relative shift in the alpha-P and beta-P resonances of ATP (P less than 0.01) occurred with inotropic stimulation. [Mg2+]i calculated on the basis of these shifts was found to be 0.78 +/- 0.1 mM in control hearts, and increased to maxima of 1.9 +/- 0.2, 2.0 +/- 0.2, and 2.9 +/- 0.2 mM during infusion of isoproterenol, IBMX, and ouabain, respectively. Changes in [Mg2+]i correlate with cytosolic [ATP] + [CrP] in all hearts (r = 0.89, 0.91, and 0.88 in isoproterenol-, IBMX-, and ouabain-treated hearts, respectively). The significantly higher [Mg2+]i with ouabain infusion (P less than 0.01) at similar workloads and [ATP] + [CrP] supports the proposal that a ouabain-inhibited Mg2+ pump exists in the plasma membrane. The data support acute changes in [Mg2+]i during alterations in inotropic state that may be important in modulating metabolic and contractile function.

Adenosine may modulate blood flow and electrical activity in heart in response to changes in myocardial energy metabolism. In the present study, 31P NMR spectroscopy was used to examine the relation between cytosolic phosphate metabolite levels and release of adenosine into the venous effluent of isovolumic heart during graded low-flow ischaemia or metabolic stimulation with isoproterenol. When coronary flow rate was varied in steps between 1.6 and 12 ml/min/g, cytosolic ATP levels did not change significantly but the phosphorylation potential exhibited a linear correlation with flow rate below approximately 7 ml/min/g. Purine release (adenosine and inosine) correlated linearly with the cytosolic phosphorylation potential and free AMP concentration. Metabolic stimulation of hearts with isoproterenol (0.4, 3.0, and 60 nM), produced a significant fall in cytosolic ATP levels and decreased the cytosolic phosphorylation potential. Purine release in these hearts increased exponentially as the cytosolic phosphorylation potential dropped, and as cytosolic free AMP increased. These results support a link between the phosphorylation potential and the mechanism of adenosine production during ischaemia and metabolic stimulation. Presumably, this link is the activity of the enzyme 5'-nucleotidase, which is responsible for converting AMP to adenosine, together with the concentration of its substrate, AMP. In low-flow ischaemia, cytosolic AMP may control adenosine formation. With isoproterenol stimulation, a more complex relationship exists, indicating possible allosteric regulation of the enzyme(s) responsible for adenosine formation, in addition to changes in AMP concentration.

Changes in 5'-nucleotidase activity were calculated on the basis of alterations in ATP, ADP, phosphocreatine, Pi, Mg2+, IMP and AMP, determined by using 31P n.m.r. spectroscopy and h.p.l.c., during isoprenaline infusion, graded hypoxia and graded underperfusion in isolated rat heart. Calculated activity changes were compared with the total efflux of purines (adenosine + inosine + hypoxanthine) in order to assess the involvement of various 5'-nucleotidases in formation of adenosine. Purine efflux exhibited an exponential relation with cytosolic [AMP] during isoprenaline infusion and hypoxia (r = 0.92 and 0.95 respectively), supporting allosteric activation of 5'-nucleotidase under these conditions. Purine efflux displayed a linear relation with cytosolic [AMP] during graded ischaemia (r = 0.96), supporting substrate regulation in the ischaemic heart. The calculated activities of membrane-bound ecto-5'-nucleotidase were similar to the observed relations between purine efflux and cytosolic [AMP] in all hearts. The calculated activities of the ATP-activated cytosolic and lysosomal enzymes and of the ATP-inhibited cytosolic 5'-nucleotidase could not explain the observed release of purines under the conditions examined. These results indicate that the kinetic characteristics of the membrane-bound ecto-enzyme are consistent with an important role in the formation of extracellular adenosine, whereas the characteristics of the other 5'-nucleotidases are inconsistent with roles in adenosine formation under the conditions of the present study.

Isolated hearts repleted with calcium after a short period of perfusion with a calcium-free medium undergo the injury of the calcium paradox and release cellular protein. In the present experiments, 15 min perfusion with a calcium containing HEPES solution subsequent to 10 min calcium-free perfusion resulted in the loss of 42.7 +/- 3.9 mg of protein/g heart. If hearts were repleted with calcium for 30 s only, then returned to calcium-free perfusion, resultant protein loss was the same. When calcium repletion was further decreased to 20 s, 23.9 +/- 1.3 mg/g of protein was lost. This critical period coincided with the onset of contracture but was prior to major ion movements. It is concluded that the transition to irreversible injury occurs within 30 s of calcium repletion and that subsequent removal of extra-cellular calcium does not alter the course of events leading to cell death.

The relation between the PC/ATP ratio and left ventricular function was examined in the Langendorff-perfused guinea-pig heart over a range of perfusion flow rates. PC/ATP ratios were determined from the 31P-nuclear magnetic resonance spectra of hearts obtained at 80.98 MHz and ventricular function estimated by measuring pressure in the left ventricle. When flow rates were increased over the range 0.6 to 6.0 ml/min, the PC/ATP ratio increased from 0.64 +/- 0.05 at 0.6 ml/min to 1.82 +/- 0.12 at 3.8 ml/min with no further increase up to a flow rate of 6.0 ml/min. Developed pressure (DP) increased with the flow rate up to 6.0 ml/min but the end diastolic pressure (EDP) also increased. The DP/EDP ratio was found to correlate closely with the PC/ATP ratio over the range of flow rates examined. The PC/ATP ratio may be a practical index of myocardial function available to the clinician when the topical magnetic resonance technique is fully developed.

The isolated, perfused heart is known to exhibit a linear relation between aortic pressure, coronary flow rate, oxygen consumption and contractile function (rate-pressure product) over a wide range of aortic pressures. Our study sought to determine whether the cytosolic phosphorylation potential [( ATP]/[ADP][Pi]) is the link between mitochondrial respiration and contractile function in this preparation. 31P NMR spectroscopy was used to measure phosphate metabolite levels in isovolumic rat hearts during graded perfusion from 1.6 to 12.8 ml/min/g. It was found that an increase in contractile function paralleled the increase in flow rate, but that marked changes in creatine phosphate, inorganic phosphate and hydrogen ion concentration occurred only at lower flow rates. The cytosolic phosphorylation potential showed a high, positive correlation with contractile function at flow rates below 7.2 ml/min/g, which suggested that mitochondrial respiration was oxygen-limited and that the heart was ischemic. Thus, when oxygen limits myocardial oxidative phosphorylation, cytosolic energy metabolite levels may limit contractile function. At the higher flow rates studied, other metabolic controls may operate to link mitochondrial respiration and workload.

Significant progress in two areas will be required before the application of magnetic resonance spectroscopy in clinical cardiology is realized. On the technical side, methods must be developed to allow NMR spectra to be obtained non-invasively from the heart uncontaminated by surrounding tissue and with sufficient spatial resolution and signal to noise to be of practical use. On the application side, the potential value of the information derived from the NMR spectrum to diagnosis must be demonstrated. This review addresses the latter problem by evaluating the information content of the 31P-spectrum of heart within the framework of the practical constraints imposed by its possible clinical application as opposed to its acknowledged research application.

The 31P NMR spectrum of freshly prepared guinea-pig heart perfused in the Langendorff mode at 37 degrees C with medium containing either glucose or pyruvate as an energy source exhibited no inorganic phosphate peak making the determination of intracellular pH from the chemical shift of this peak impossible. By incorporating 2-deoxyglucose-6-phosphate into the heart to act as an alternative pH indicator, intracellular pH was determined to be 6.98 +/- 0.02 (mean +/- S.E.M. for eight animals). 2-Deoxyglucose was not taken up and phosphorylated by guinea-pig heart in the presence of glucose. Uptake and phosphorylation did proceed if glucose was omitted from the perfusion medium, but a large inorganic phosphate peak was present in the initial spectrum prior to 2-deoxyglucose addition. With pyruvate as sole energy source, an inorganic phosphate peak was absent initially and 2-deoxyglucose was taken up and phosphorylated. During a 12 min period of global ischaemia at 20 degrees C, when inorganic phosphate was visible together with 2-deoxyglucose-6-phosphate, a close correlation was found between the pH estimated from the chemical shift of inorganic phosphate and 2-deoxyglucose-6-phosphate over the pH range 6.3 to 6.9.

This study examines the possibility that endogenous adenosine modulates efficiency in isovolumic perfused rat hearts stimulated with isoproterenol or norepinephrine. Efficiency in these hearts is calculated as the rate of pressure work divided by the myocardial oxygen consumption. Within 2 min of infusion of isoproterenol (50 nM), heart rate increased by 35%, the rate pressure product by 290%, oxygen consumption by 142%, and efficiency by 67%. Infusion of adenosine deaminase (2-4 IU/ml), or 8-phenyltheophylline (5 microM), into stimulated hearts augmented the increase in heart rate by 40-45%, rate-pressure product by 18-20%, and oxygen consumption by 50-55%. Efficiency was reduced by 30-35%. Adenosine release into the coronary venous effluent increased from 195 +/- 20 pmol/min/g to 2400 +/- 180 pmol/min/g after 5 min. A similar pattern of results was observed when norepinephrine (0.1 mM) was used. The results indicate that extracellular adenosine, released by catecholamine treatment, inhibits the effects of the catecholamines on rate and contractility. Consequently, adenosine reduces cardiac work (rate-pressure product), but in so doing, improves efficiency.