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(Circulation. 1995;92:1254-1260.)
© 1995 American Heart Association, Inc.

Articles

Bidirectional Effects of Aminophylline on Myocardial Ischemia

Tetsuo Minamino, MD; Masafumi Kitakaze, MD; Toshikazu Morioka, MD; Koichi Node, MD; Yoshiro Shinozaki BE; Mitsuaki Chujo, MSC; Hidezo Mori, MD; Hiroshi Takeda, MD; Michitoshi Inoue, MD; Masatsugu Hori, MD; Takenobu Kamada, MD

From the First Department of Medicine, Osaka University School of Medicine, Osaka; Second Department of Physiology, Tokai University School of Medicine, Isehara (Y.S., M.C., H.M.); and Department of Information Science, Osaka (Japan) University Hospital (T. Minamino, H.T., M.I.).

Correspondence to Masafumi Kitakaze, MD, The First Department of Medicine, Osaka University School of Medicine, 2-2, Yamadaoka, Suita 565, Japan.

	Abstract

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Background Aminophylline blocks adenosine receptors and increases levels of plasma catecholamines. We investigated the effect of aminophylline on myocardial ischemia by varying its severity and attempted to identify the mechanism by which aminophylline modulates myocardial ischemia in the canine model.

Methods and Results In 41 open-chest dogs, the left anterior descending coronary artery was cannulated and perfused with blood through a bypass tube from the left carotid artery. When coronary blood flow (CBF) was reduced to 80% of the control, aminophylline increased fractional shortening (FS) from 11.0±0.4% to 18.5±1.7% (P<.05) and lactate extraction ratio (LER) from 7.5±0.1% to 13.6±1.0% (P<.01). The endocardial to epicardial flow ratio (Endo/Epi ratio) of regional myocardium was also increased. Release of adenosine was increased compared with the nonischemic condition (7±3 versus 28±5 pmol/mL). Prazosin, an ₁-adrenoceptor antagonist, blunted the aminophylline-induced improvement in contractile and metabolic function. Administration of 8-phenyltheophylline, a selective antagonist of adenosine receptors, did not increase FS, LER, or the Endo/Epi ratio when CBF was reduced to 80% of control. When CBF was reduced to 60% of control, aminophylline did not change the metabolic and contractile function. In contrast, when CBF was reduced to 33% of control, release of adenosine was increased markedly (243±19 pmol/mL) and aminophylline induced decreases in FS, LER, and Endo/Epi ratio similar to those observed with 8-phenyltheophylline.

Conclusions Aminophylline had opposite effects on the ischemic myocardium depending on the severity of ischemia. It improved mild ischemia but worsened severe ischemia. The beneficial effect of aminophylline was attributable to ₁-adrenoceptor stimulation, which improves endomyocardial flow in the ischemic myocardium. The deleterious effect was attributable to the aminophylline-induced blockade of adenosine receptors.

Key Words: catecholamines • vasoconstriction • adenosine

	Introduction

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Aminophylline exerts several potential cardiovascular actions. These actions are the result of the contribution of two major pharmacological effects: a blockade of adenosine receptors and an increase in plasma catecholamine concentrations.¹ Since adenosine is a chemical mediator of coronary hyperemic flow in the ischemic myocardium,² the blockade of adenosine receptors may reduce regional blood flow, blunting the cardioprotective effects of adenosine.³ Furthermore, aminophylline-induced increases in plasma catecholamine concentration⁴ may increase myocardial contractility, oxygen consumption, and coronary vascular resistance,^{5 6} which may worsen myocardial ischemia. Thus, aminophylline is not considered appropriate for the treatment of patients with coronary artery disease. However, recent clinical studies have shown that aminophylline improves exercise capacity in patients with angina pectoris and is associated with reducing the extent of myocardial ischemia.^{7 8} These investigators suggest that aminophylline may exert a beneficial effect by preventing redistribution of blood flow from the endomyocardium to the epimyocardium during ischemia, known as the transmural "steal" phenomenon.^{9 10} -Adrenoceptor activation during hypoperfusion is reported to favor blood flow from epicardium to endocardium,¹¹ which may contribute to the attenuation of myocardial ischemia. These two contradictory effects of aminophylline on myocardial ischemia may be related to the severity of myocardial ischemia, because adenosine release is increased in association with increased severity of ischemia. We hypothesized that if a small amount of adenosine were released, the major action of aminophylline would be to stimulate ₁-adrenoceptors. However, if release of adenosine was increased, the major action of aminophylline would be to block adenosine receptors in the ischemic myocardium.

To test this hypothesis, we investigated the effects of aminophylline in mild, moderate, and severe myocardial ischemia and attempted to determine the underlying mechanisms of these effects in the canine myocardium. In this study, mild, moderate, and severe myocardial ischemia were defined as reduction of coronary blood flow to 80%, 60%, and 33% of baseline, respectively.

	Methods

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Instrumentation
We anesthetized 41 mongrel dogs (weight, 15 to 21 kg) with sodium pentobarbital (30 mg/kg IV). The trachea was intubated, and the animals were ventilated with room air mixed with oxygen (100% O₂, 1 to 2 L/min). The chest was opened through the left fifth intercostal space, and the heart was suspended in a pericardial cradle. After heparin (500 U/kg) was administered intravenously, the left anterior descending coronary artery (LAD) was ligated, cannulated, and perfused with blood via the left carotid artery through an extracorporeal bypass tube. The duration of interruption of coronary blood flow by this procedure was 30 seconds or less. All of the blood flow to the LAD was supplied through this extracorporeal bypass tube. Coronary blood flow (CBF) in the perfused area was measured with an electromagnetic flow probe attached to the bypass tube, and coronary perfusion pressure (CPP) was monitored at the tip of the coronary artery cannula. A small coronary vein near the center of the perfused area was cannulated with a small, short, collecting tube (1 mm in diameter and 7 cm in length) to sample coronary venous blood. The drained venous blood was collected in a reservoir placed at the level of the left atrium and was returned to the jugular vein. A miniature pressure transducer (model P-5, Konigsberg Instruments) was inserted into the left ventricular (LV) cavity through the LV apex. The first derivative of LV pressure (LV dP/dt) was determined. A pair of ultrasonic crystals (5 MHz, 2 mm in diameter; Schuessler) was implanted in the LV anterior wall in the endomyocardial segment in the center of the perfused area to measure segmental length. The lengths of end-diastolic (EDL) and end-systolic (ESL) segments were determined at the peak of the R wave on the ECG and the minimal point of the LV dP/dt, respectively.¹² Fractional shortening [FS; FS=(EDL-ESL)/EDL] was calculated as an index of myocardial performance in the perfused area. All hemodynamic parameters were recorded on a multichannel recorder (RM-6000, Nihon-Kohden).

Measurement of Regional Myocardial Blood Flow
Regional myocardial blood flow was determined by the microsphere technique using nonradioactive microspheres (Sekisui Plastic Co, Ltd) made of inert plastic labeled with different types of stable heavy elements as previously described.¹³ Microspheres with a mean diameter of 15 µm were labeled with Br, Nb, In, and I. Specific gravities were 1.34 for Br, 1.32 for Nb, 1.20 for In, and 1.60 for I. Microspheres were suspended in isotonic saline with 0.01% Tween 80 to prevent aggregation. The microspheres were ultrasonicated for 5 minutes and then vortexed for 5 minutes immediately before injection. Approximately 1 mL of the microsphere suspension (2x10⁶ to 4x10⁶ microspheres) was injected into the perfusion line to determine the endocardial to epicardial flow ratio of each myocardial region (Endo/Epi flow ratio). The x-ray fluorescence activity of the stable heavy elements was measured by a wavelength dispersive spectrometer (PW 1480, Philips Co, Ltd). The specifications of this x-ray fluorescence spectrometer have been described previously.¹³ In brief, when the microspheres were irradiated by the primary x-ray beam, the electrons fell back to a lower orbit and emitted measurable energy. This energy level reflects the x-ray fluorescence of several differently labeled microspheres in a mixture. The Endo/Epi flow ratio was determined by calculating the ratio of endocardial tissue and epicardial tissue.

Experimental Protocols
Protocol I: Reproducibility of Changes in Myocardial Contractile and Metabolic Function During Reduction of CBF to 33% of Control Flow
We evaluated the reproducibility of changes in myocardial contractile and metabolic function in the first and second episodes of severe myocardial ischemia by measuring hemodynamic and metabolic parameters without any pharmacological intervention in 5 dogs. After hemodynamic stabilization, hemodynamic parameters (LV pressure, LV dP/dt, segment length in the perfused area, CPP, and CBF) were measured. Coronary arterial and venous blood samples were obtained for blood gas analysis and determination of lactate and adenosine concentrations. After these baseline measurements were obtained, microspheres (1.0x10⁴ microspheres/mL of baseline CBF [mL/min]) were injected into the LAD through the bypass tube to determine the Endo/Epi flow ratio. CPP was reduced with an occluder attached to the extracorporeal bypass tube until CBF was decreased to 33% of control CBF (severe myocardial ischemia). When the decrease in CPP was confirmed, the occluder was manually adjusted to maintain CPP at a constant level for 10 minutes (the first ischemic episode of ischemia). All hemodynamic and metabolic parameters and the Endo/Epi flow ratio were measured 10 minutes after the onset of hypoperfusion. After these measurements were obtained, the occluder was released. After complete recovery of coronary hemodynamic parameters, CPP was again reduced so that CBF was decreased to 33% of the control flow in the same manner as above (the second episode of ischemia). All hemodynamic and metabolic parameters and the Endo/Epi flow ratio were measured again 10 minutes after the onset of hypoperfusion.

Protocol II: Effects of Aminophylline on Myocardial Contractile and Metabolic Function During Reduction of CBF to 80, 60, and 33% of Control Flow
Eighteen dogs were used in this protocol. As in protocol I, hemodynamic and metabolic parameters and the Endo/Epi flow ratio were determined in baseline conditions. CPP was reduced with an occluder attached to the extracorporeal bypass tube until CBF was decreased to 80% of control CBF (mild myocardial ischemia). When the decrease in CPP was confirmed, the occluder was manually adjusted to maintain CPP at a constant level for 10 minutes (the first episode of ischemia). All hemodynamic and metabolic parameters and the Endo/Epi flow ratio were measured 10 minutes after the onset of hypoperfusion. After these measurements were obtained, the occluder was released. After complete recovery of coronary hemodynamic parameters, aminophylline (0.8 mg · kg^-1 · min^-1) was infused intravenously for 10 minutes and hemodynamic parameters and Endo/Epi flow ratio were again measured. The dose of aminophylline used in the present study has been found to block adenosine receptors and stimulate release of catecholamines.¹ CPP was again reduced so that CBF was decreased to 80% of the control flow in the same manner as above (the second episode of ischemia). All hemodynamic and metabolic parameters and the Endo/Epi flow ratio were measured again 10 minutes after the onset of hypoperfusion (protocol IIa). To elucidate the effects of aminophylline on different degrees of myocardial ischemia, the same procedure was performed with the CBF decreased to 60% of the control flow in 6 dogs (moderate myocardial ischemia; protocol IIb) and to 33% of the control flow in 6 dogs (severe myocardial ischemia; protocol IIc).

Protocol III: Effects of 8-Phenyltheophylline on Myocardial Contractile and Metabolic Function During Reduction of CBF to 80% and 33% of Control Flow
To determine whether the beneficial effect of aminophylline is attributable to the attenuation of transmural "steal" caused by endogenous adenosine during myocardial ischemia, the same procedure as in protocol II was performed using intravenous administration of 8-phenyltheophylline (250 µg · kg^-1 · min^-1), a selective antagonist of adenosine receptors, instead of aminophylline. We investigated the effects of 8-phenyltheophylline on myocardial ischemia when CBF was decreased to 80% (mild myocardial ischemia; protocol IIIa) in 6 dogs and to 33% (severe myocardial ischemia; protocol IIIb) of the control flow in 6 dogs. 8-Phenyltheophylline was dissolved in dimethyl sulfoxide to a final concentration of 5 mmol. To adjust ionic strength and osmolarity, we added NaCl so that the solution contained 140 mmol of NaCl. In a preliminary study, we confirmed that this dose of 8-phenyltheophylline completely abolished the systemic vasodilatory effect of an intravenous infusion of exogenous adenosine (1 mg · kg^-1 · min^-1). We also confirmed that dimethyl sulfoxide, which may have cardiovascular effects such as acting as a scavenger of oxygen-derived free radicals, did not affect metabolic and hemodynamic parameters in the baseline condition or in mild and severe ischemia.

Protocol IV: Effects of Aminophylline on Myocardial Contractile and Metabolic Function During Reduction of CBF to 80% of Control Flow During ₁-Adrenoceptor Blockade
The Endo/Epi flow ratio in the ischemic myocardium is increased by -adrenoceptor–mediated vasoconstriction.¹¹ Furthermore, Liang and Jones reported that an ₁- but not an ₂-adrenergic constrictive tone is mainly operative in coronary circulation during hypoperfusion.¹⁴ To determine whether the beneficial effect of aminophylline is attributable to stimulation of ₁-adrenoceptors induced by an increase in the release of catecholamines, aminophylline (0.8 mg · kg^-1 · min^-1) was administered intravenously during intracoronary administration of prazosin (4 µg · kg^-1 · min^-1) in 6 dogs during reduction of CBF to 80% of control. Prazosin administration was initiated after stabilization of hemodynamic parameters and was continued throughout this protocol. Prazosin dissolved in sterile water. Because prazosin administration was slow (0.33 mL/min) compared with the baseline CBF (approximately 30 mL/min), the influence of the vehicle was negligible in the present study.

Chemical Analysis
Plasma lactate concentration was determined enzymatically,¹⁵ and the lactate extraction ratio (LER) was calculated by the following formula: (Arterial Lactate Concentration-Coronary Venous Lactate Concentration)/Arterial Lactate Concentrationx100.^{16 17 18} The coronary arterial and venous blood oxygen difference (AVO₂D) was assessed by the difference between coronary arterial and venous oxygen contents. Myocardial oxygen consumption (MVO₂) (mL/100 g per minute) was calculated as follows: CBF (mL/100 g per minute)xAVO₂D (mL/dL).

Adenosine concentrations were measured as previously described.^{16 18 19} Briefly, 1 mL of blood was drawn into a syringe containing 0.5 mL dipyridamole (0.02%) and 100 µL of 2'-deoxycoformycin (0.1 mg/mL) with EDTA (500 mmol) to block both uptake of adenosine by red blood cells and degradation of adenosine. After centrifugation, the adenosine content of the supernatant was determined by radioimmunoassay. Plasma (100 µL) was succinylated with 100 µL of dioxane containing succinic acid anhydride and triethylamine. After 20-minute incubation, the mixture was diluted with 100 µL of adenosine 2',3'-O-disuccinyl-3-[¹²⁵I]iodotyrosine methyl ester (0.5 pmol) and 100 µL of diluted anti-adenosine serum. After the mixture was kept in a cold-water bath (4°C) for 18 hours, a second antibody solution (goat anti-rabbit IgG antiserum, 500 µL) was added. After 1-hour incubation at 4°C, unreacted materials were removed by centrifugation at 2500g at 4°C for 20 minutes. The radioactivity remaining in the tube was counted by a gamma counter. The amount of adenosine degradation during this blood sampling procedure has been found to be negligible.

Data Analysis and Statistical Analysis
All values are expressed as mean±SEM unless otherwise stated. For analysis of statistical significance, Student's paired t test, unpaired t test, or paired and unpaired t tests adjusted by Bonferroni correction were used when appropriate. The baseline measurements, ischemic measurements, treatment baseline measurements, and ischemic treatment measurements were compared by repeated measures ANOVA. A value of P<.05 was considered significant.

	Results

Top Abstract Introduction Methods Results Discussion References

 
Effects of Aminophylline on Myocardial Contractile and Metabolic Function During Reduction of CBF to 80%, 60%, and 33% of Control Flow
In 5 dogs with no pharmacological intervention in protocol I, there were no significant differences between the first and second episodes of ischemia in CPP (43±4 versus 45±2 mm Hg), FS (4.1±1.1% versus 4.3±1.3%), LER (-40.1±5.1% versus -40.6±4.9%), MVO₂ (2.9±0.2 versus 2.6±0.2 mL/100 g per minute), and pH of the coronary vein (7.21±0.02 versus 7.20±0.01) at 10 minutes of ischemia, confirming that the first episode of ischemia does not influence the second episode of ischemia.

Intravenously administered aminophylline (0.8 mg · kg^-1 · min^-1) increased heart rate slightly from 151±12 to 178±11 (P<.05), from 141±10 to 160±10 (P<.01), and from 154±8 to 185±11 beats per minute (bpm) (P<.01) in protocols IIa, IIb, and IIc, respectively. Aminophylline also increased LV dP/dt from 3033±133 to 3533±80 (P<.05), from 2633±84 to 3217±98 (P<.01), and from 2853±125 to 3440±165 bpm (P<.05) in protocols IIa, IIb, and IIc, respectively. Aminophylline did not alter mean blood pressure in protocols IIa through IIc.

There were no significant changes in CPP, CBF, FS, LER, MVO₂, or pH of the coronary venous blood or in the Endo/Epi flow ratio in baseline conditions (control, after the end of the first episode of ischemia) in protocols IIa through IIc. Aminophylline treatment caused no significant change in either CPP, CBF, LER, MVO₂, or pH of the coronary vein or in the Endo/Epi flow ratio in protocols IIa, IIb, and IIc. Reduction of the control CBF to 80% of control (from 29±3 to 23±2 mL/min in the first episode of ischemia and from 29±2 to 23±2 mL/min in the second episode) resulted in a decrease in CPP from 102±2 to 58±6 mm Hg in the first episode of ischemia and from 98±3 to 56±3 mm Hg in the second episode. Aminophylline increased FS, LER, MVO₂, and pH of the coronary vein in mild myocardial ischemia (Fig 1). When CBF was reduced to 60% of the control flow (from 27±3 to 16±5 mL/min in the first episode of ischemia and from 28±4 to 16±2 mL/min in the second episode), CPP was reduced from 102±5 to 48±3 mm Hg in the first episode of ischemia and from 104±4 to 53±4 mm Hg in the second episode. Aminophylline caused no change in FS (8.5±0.4% versus 9.2±1.2%), LER (-4.8±3.0% versus -0.3±0.9%), MVO₂ (4.7±0.3 versus 5.0±4.7 mL/100 g per minute), or pH of the coronary vein (7.24±0.03 versus 7.33±0.02). When CBF was reduced to 33% of the control flow (from 30±3 to 10±1 mL/min in the first episode of ischemia and from 31±3 to 10±1 mL/min in the second episode), CPP was reduced from 102±5 to 41±3 mm Hg in the first episode of ischemia and from 97±4 to 46±2 mm Hg in the second episode. Aminophylline significantly reduced FS, LER, MVO₂, and pH of the coronary vein in severe ischemia (Fig 2). Aminophylline increased the Endo/Epi flow ratio in mild myocardial ischemia; however, it reduced this ratio in severe myocardial ischemia (Fig 3). Aminophylline did not change the Endo/Epi flow ratio in moderate ischemia (Fig 3).

View larger version (41K): [in this window] [in a new window]   Figure 1. Changes in fractional shortening (A), lactate extraction ratio (B), pH of the coronary venous blood (C), and myocardial oxygen consumption (D) in untreated (first episode) and aminophylline (Am)-treated (second episode) ischemia during reduction of coronary blood flow to 80% of the control.

View larger version (37K): [in this window] [in a new window]   Figure 2. Changes in fractional shortening (A), lactate extraction ratio (B), pH of the coronary venous blood (C), and myocardial oxygen consumption (D) in untreated (first episode) and aminophylline (Am)-treated (second episode) ischemia during reduction of coronary blood flow to 33% of the control.

View larger version (39K): [in this window] [in a new window]   Figure 3. Changes in endocardial to epicardial flow ratio (Endo/Epi Flow Ratio) between untreated (first episode) and aminophylline (Am)-treated (second episode) ischemia during reduction of coronary blood flow (CBF) to 80%, 60%, and 33% of the control.

In the nonischemic condition, the difference between coronary arterial and venous adenosine concentrations (AVAdD) was 7±3 pmol/mL. AVAdD increased in a stepwise manner as the severity of ischemia increased (Fig 4).

View larger version (30K): [in this window] [in a new window]   Figure 4. Adenosine release in the control condition and during ischemia induced by reduction of coronary blood flow (CBF) to 80%, 60%, and 33% of the control.

Effects of 8-Phenyltheophylline on Myocardial Contractile and Metabolic Function During Reduction of CBF to 80% and 33% of Control Flow
Administration of 8-phenyltheophylline did not change heart rate, LV dP/dt, or mean blood pressure in protocols IIIa and IIIb. There were no significant changes in CBF, FS, LER, pH of the coronary vein, MVO₂, or Endo/Epi flow ratio in baseline conditions (control, 10 minutes after the end of the first episode of ischemia) in protocols IIIa and IIIb. When CBF was reduced to 80% of the control (from 31±4 to 25±3 mL/min in the first episode of ischemia and from 30±5 to 25±3 mL/min in the second episode), there were no significant differences in FS (10.2±0.8% versus 12.1±1.3%), LER (7.2±0.8% versus 6.8±0.9%), MVO₂ (4.7±0.3 versus 5.0±4.7 mL/100 g per minute), or pH of the coronary vein (7.35±0.02 versus 7.37±0.03) with or without 8-phenyltheophylline. When CBF was reduced to 33% of the control (from 29±3 to 10±1 mL/min in the first episode of ischemia and from 29±5 to 10±1 mL/min in the second episode), 8-phenyltheophylline significantly reduced FS, LER, MVO₂, and pH of the coronary vein (Fig 5). The effects of 8-phenyltheophylline in severe myocardial ischemia were similar to those of aminophylline. Administration of 8-phenyltheophylline did not increase the Endo/Epi flow ratio in mild myocardial ischemia but decreased the Endo/Epi flow ratio in severe myocardial ischemia (Fig 6).

View larger version (36K): [in this window] [in a new window]   Figure 5. Changes in fractional shortening (A), lactate extraction ratio (B), pH in coronary venous blood (C), and myocardial oxygen consumption (D) in the untreated (first episode) and 8-phenyltheophylline (8PT)-treated (second episode) ischemia during reduction of coronary blood flow to 33% of the control.

View larger version (31K): [in this window] [in a new window]   Figure 6. Changes in the endocardial to epicardial flow ratio (Endo/Epi Flow Ratio) in the untreated (first episode) and 8-phenyltheophylline (8PT)-treated (second episode) ischemia during reduction of coronary blood flow (CBF) to 80% and 33% of the control.

Effects of Aminophylline on Myocardial Ischemia During ₁-Adrenoceptor Blockade
Intracoronary administration of prazosin did not change systemic hemodynamic parameters. There were no significant changes in CBF, FS, LER, pH of the coronary vein, MVO₂, or Endo/Epi flow ratio in baseline conditions (control, 10 minutes after the end of the first episode of ischemia). When CBF was reduced to 80% of the control (from 28±3 to 23±2 mL/min in the first episode of ischemia and from 29±3 to 23±2 mL/min in the second episode), prazosin abolished the beneficial effects of aminophylline on FS (9.7±1.7% versus 11.7±0.6%), LER (7.2±.0.9% versus 8.4±0.9%), MVO₂ (6.2±0.3 versus 6.1±0.3 mL/100 g per minute), and pH of the coronary vein (7.36±0.01 versus 7.41±0.12) in the second episode of ischemia compared with the first episode. Furthermore, prazosin abolished the aminophylline-induced increase in the Endo/Epi flow ratio of regional myocardial blood (0.83±0.03 versus 0.85±0.03) in the second episode of ischemia compared with the first.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
In the present study, aminophylline improved mild ischemia by stimulating ₁-adrenoceptors, which favors endomyocardial blood flow during flow reduction when adenosine release is not prominent. However, when adenosine release was increased in severe ischemia, the inhibitory effect of aminophylline on adenosine receptors predominated over the beneficial effects of ₁-adrenoceptor stimulation, worsening the myocardial ischemia. These two opposite effects of aminophylline on myocardial ischemia are attributable to its two major pharmacological actions: stimulation of ₁-adrenoceptors due to increased norepinephrine release and blockade of adenosine receptors.

Validation of the Experimental Model Used in the Present Study
When we reduced CBF to 80%, 60%, and 33% of baseline CBF in the present study, the extent and pathophysiology of myocardial ischemia seemed dramatically different. When CBF was reduced to 80% of baseline, LER decreased from 24.7±0.7% to 7.5±0.1%. In contrast, when CBF was reduced to 60% of baseline, LER decreased from 24.9±1.1% to -0.3±0.9%. Parker et al²⁰ reported that LER fell from 13.6±12.2% to -20.2±39.9% (mean±SD) in patients with coronary artery disease who developed angina pectoris during a pacing stress test. In patients without angina pectoris, LER fell from 25.0±7.4% to 14.2±13.8%.²⁰ Yamada et al²¹ reported that an exercise stress test caused LER to decrease from 22.8±19.0% to 16.6±12.8% (mean±SD) in association with ECG changes in patients with coronary artery disease, while a pacing stress test caused LER to decrease from 20.2±12.0% to 0.2±33.1% (mean±SD) in association with ECG changes. Release of adenosine in the 80% and 60% ischemic models in the present study was similar to that observed during exercise stress tests in a previous human study.²² These findings suggest that the extent of ischemia caused by CBF reduction to 80% and 60% of the control in the present study reflected ischemic events in patients with coronary artery disease.

When brief periods of ischemia have been found to lessen the extent of a subsequent episode of ischemia, a phenomenon called "ischemic preconditioning" occurs. In a study by Okazaki et al,²² effort angina induced by an exercise stress test made the myocardium resistant to a stress test performed 15 minutes later. In the present study, we induced two episodes of flow reduction to 33% in a model of ischemia in the same group of dogs at approximately 30-minute intervals. The severity of ischemia, gauged by FS and LER, was similar in both episodes, indicating that preconditioning or cardioprotective effects of prior ischemia did not occur.

Pharmacological Effects of Aminophylline on Contractile and Metabolic Function in Ischemic Myocardium
Aminophylline increases levels of plasma catecholamines.^{1 4} In the present study, intravenous aminophylline administration increased heart rate and LV dP/dt by stimulating ß-adrenoceptors. Ten minutes after aminophylline, CBF was increased slightly, but not significantly, because of the synergistic effects of - and ß-adrenoceptor stimulation and adenosine receptor blockade. Because the second episode of ischemia was produced by reducing CBF to 80%, 60%, or 33% of baseline CBF, it is unlikely that a slight increase in total CBF was responsible for the beneficial effects of aminophylline.

Buffington and Feigl²³ reported that when coronary perfusion pressure was reduced to 75 and 50 mm Hg, norepinephrine preserved endomyocardial flow, suggesting that -adrenoceptor stimulation may attenuate myocardial ischemia by increasing endocardial flow during mild coronary hypoperfusion. In the present study, aminophylline improved contractile and metabolic function when coronary perfusion pressure was mildly reduced to approximately 58±6 mm Hg, which is similar to the extent of coronary hypoperfusion reported in the study by Buffington and Feigl.²³ Prazosin, an ₁-adrenoceptor antagonist, blunted the beneficial effects of aminophylline, suggesting that the beneficial effects of aminophylline were attributable to ₁- adrenoceptor stimulation. Heusch et al²⁴ reported that ₁-adrenoceptors are located in the large coronary arteries and ₂-adrenoceptors in the small coronary arteries. Thus, increases in endocardial coronary flow due to -adrenoceptor stimulation may be attributable to increased tone of the large coronary arteries. Feigl⁵ suggested that changes in capacitance in large coronary arteries due to ₁-adrenoceptor activation predominantly favor endomyocardial flow. Furthermore, Liang and Jones¹⁴ reported that an ₁- but not an ₂-adrenergic constrictor tone was operative in coronary circulation during hypoperfusion. The present findings confirm the important role of ₁-adrenoceptor activity in maintaining endomyocardial flow during myocardial ischemia.

The cellular mechanism of the enhanced release of catecholamines caused by aminophylline is not clearly understood. Blockade of adenosine A₁ receptors has been found to increase norepinephrine release.²⁵ Because aminophylline is a nonspecific adenosine receptor antagonist, aminophylline-induced antagonism of adenosine A₁ receptors may increase catecholamine release. However, because 8-phenyltheophylline, a specific antagonist of adenosine receptors, did not improve myocardial ischemia, adenosine A₁ receptor–mediated inhibition of norepinephrine release was not responsible for the aminophylline-induced improvement in myocardial ischemia observed in the present study. Aminophylline is known to cause direct stimulation of the sympathetic nervous system^{26 27} and the central nervous system.²⁸ Although we did not identify the cellular mechanism of the stimulatory effect of aminophylline on the sympathetic nervous system of the ischemic myocardium, our results suggest that the effect of aminophylline on catecholamine release improves endomyocardial flow distribution, resulting in improvement in myocardial ischemia.

To test whether beneficial and deleterious effects of aminophylline in mild and severe myocardial ischemia are attributable to the antagonism of adenosine receptors, we administered 8-phenyltheophylline, which blocks only adenosine receptors, in mild and severe myocardial ischemia. Administration of 8-phenyltheophylline did not improve mild myocardial ischemia, indicating that the beneficial effect of aminophylline in mild ischemia is not attributable to an antisteal effect caused by blockade of adenosine receptors. A xanthine derivative, bamiphylline, which is the most selective antagonist of A₁ adenosine receptors available for clinical use, has improved exercise-induced myocardial ischemia, possibly by redistribution of coronary flow toward the underperfused subendocardium.²⁹ This study showed that the anti-ischemic effect of this xanthine derivate is unlikely to be mediated by antagonism of adenosine A₂ receptors. As the vascular effects of adenosine are mediated by A₂ receptors, this clinical study also suggested that the anti-ischemia effects of xanthines are not mediated by the antisteal effects caused by adenosine.

When we reduced CBF to 33% of the control flow, CPP was reduced to 41±3 mm Hg in the first episode of ischemia. Buffington and Feigl²³ reported that when coronary perfusion pressure is reduced to 38±4 mm Hg, the role of improvement in the Endo/Epi flow ratio due to -adrenoceptor stimulation becomes smaller relative to the effects of metabolic coronary vasodilation related to adenosine, indicating that endogenous adenosine may play a crucial role in regulating coronary blood flow and the Endo/Epi flow ratio in severe ischemia. Adenosine release, which can induce a potent coronary metabolic vasodilatory action, was markedly increased in severe ischemia in this study. Administration of 8-phenyltheophylline worsened myocardial ischemia to the same extent as aminophylline, indicating that the deleterious effects of aminophylline in severe myocardial ischemia were due to inhibition of adenosine receptors. Thus, the present results revealed that aminophylline had bidirectional effects on myocardial ischemia: a beneficial effect attributable to ₁-adrenoceptor activation in mild myocardial ischemia and a deleterious effect attributable to blockade of the adenosine receptors in severe myocardial ischemia. The amount of adenosine released appears to determine which pharmacological action of aminophylline will predominate in the ischemic myocardium.

We did not measure collateral flow from the nonischemic area to the ischemic area. Harrison et al³⁰ have found that collateral vessels are not constricted by stimulation of -adrenoceptors. Thus, changes in collateral flow from the nonischemic area may have only a small effect during administration of aminophylline.

Pathophysiological and Clinical Relevance
Aminophylline attenuates the severity of myocardial ischemia or prolongs the occurrence of myocardial ischemia without causing dilation of epicardial coronary arteries in patients with coronary artery disease.^{7 8} This beneficial effect of aminophylline in latent myocardial ischemia induced by an exercise test, which is similar in extent to the 80% and 60% ischemic models in this study, may be attributable to ₁-adrenoceptor activation of coronary vessels. In severe ischemia, such as acute myocardial infarction, administration of aminophylline may worsen myocardial ischemia caused by inhibition of adenosine receptors because adenosine mediates cardioprotection in the ischemic and reperfused myocardium. We must be careful when we use aminophylline for the treatment of ischemic heart diseases.

	Acknowledgments

 
The authors wish to thank Yoshitomo Edahiro, Shinya Suzuki, and Noriko Tamai for their valuable assistance with experimental preparations.

Received February 6, 1995; accepted February 27, 1995.

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