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(Circulation. 1996;93:544-551.)
© 1996 American Heart Association, Inc.

Articles

Angiotensin-Converting Enzyme Inhibition Preserves Endothelium-Dependent Coronary Microvascular Responses During Short-term Ischemia-Reperfusion

Robert N. Piana, MD; Steven Y. Wang, MD, PhD ; Menachem Friedman, MD; Frank W. Sellke, MD

From the Cardiovascular Division (R.N.P.), Department of Medicine, Brigham and Women's Hospital; the Division of Cardiothoracic Surgery, Department of Surgery, the Harvard-Thorndike Research Laboratory of Beth Israel Hospital; and Harvard Medical School, Boston, Mass.

Correspondence to Frank W. Sellke, MD, Division of Cardiothoracic Surgery, Beth Israel Hospital, Dana 905, 330 Brookline Ave, Boston, MA 02215.

	Abstract

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Background Chronic angiotensin-converting enzyme (ACE) inhibition initiated days to weeks after acute myocardial infarction can reduce ventricular dilatation and improve patient survival. However, the effects on coronary vascular and myocardial function of very early ACE inhibitor therapy for acute myocardial infarction remain unresolved.

Methods and Results Hemodynamics, segmental shortening, coronary blood flow, and in vitro coronary microvascular relaxation responses were studied in noninstrumented control pigs (n=8) and pigs subjected to 30 minutes of left anterior descending ischemia followed by administration of 30 mL IV normal saline (IR-saline, n=8), 5 mg/kg IV captopril (IR-captopril, n=6), or 1.5 mg/kg IV enalaprilat (IR-enalaprilat, n=6) before 1 hour of reperfusion. Hemodynamics were similar at baseline, end of ischemia, and end of reperfusion. However, coronary blood flow immediately on reperfusion was significantly enhanced in the IR-enalaprilat cohort (59±10 mL/min) compared with the IR-saline group (32±3 mL/min, P<.05). Segmental shortening in the dyskinetic ischemic region improved only minimally at the end of reperfusion to 1±2%, -7±3%, and -2±6% for the IR-saline, IR-captopril, and IR-enalaprilat groups, respectively (P<.05, IR-captopril versus IR-saline). Arteriolar microvascular endothelium-dependent responses to ADP (P<.01) and calcium ionophore A23187 (P<.01) were impaired after ischemia-reperfusion, whereas bradykinin responses were preserved (P=.95). Endothelium-dependent venular responses to ADP and serotonin were maintained despite ischemia-reperfusion. Endothelium-independent responses to sodium nitroprusside were unaltered in arterioles and venules. Either captopril or enalaprilat restored ADP and A23187 arteriolar responses to control levels and increased bradykinin responses above control levels.

Conclusions Brief ischemia followed by reperfusion induces arteriolar microvascular endothelial dysfunction, while venular endothelial function is preserved in this porcine model. ACE inhibition enhances coronary blood flow at the time of reperfusion and can prevent impairment of endothelium-dependent arteriolar responses. However, ACE inhibition does not enhance ventricular segmental shortening acutely despite improved microvascular endothelial function and augmented postischemic coronary blood flow in this model of ischemia-reperfusion.

Key Words: microcirculation • enzymes • reperfusion • arteries

	Introduction

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ACE inhibition has rapidly become an integral component of the chronic pharmacological treatment of patients with depressed left ventricular function after acute myocardial infarction. Very early administration of ACE inhibitors in the setting of acute ischemia, however, has generated variable results. In the Cooperative New Scandinavian Enalapril Survival Study II trial,¹ early intravenous administration of enalaprilat during acute infarction yielded a nonsignificant 10% increase in mortality. By contrast, Nabel et al² found that combined administration of thrombolytic therapy and intravenous captopril for acute myocardial infarction decreased left ventricular end-diastolic volume and end-systolic volume 1 week after the event. Moreover, recent large randomized clinical trials have demonstrated a modest survival benefit of ACE inhibition begun within 24 hours after infarction.^{3 4}

In addition to providing afterload reduction and favorably influencing ventricular remodeling after myocardial infarction, experimental studies have demonstrated that ACE inhibition during acute ischemia can limit myocardial injury,^{5 6} reduce the degradation of high-energy phosphate stores,⁷ and restore myocardial contractile function.⁸ One component of these beneficial effects may involve improved myocardial perfusion with ACE inhibition. Transient ischemia has been demonstrated to impair endothelial function in the coronary microvasculature, the primary site of regulation of coronary blood flow.^{9 10} ACE inhibition could preserve microvascular endothelial function and thereby help limit myocardial injury and enhance recovery of ventricular function after successful reperfusion. In the present study, we used a porcine model of ischemia-reperfusion to investigate the influence of ACE inhibition on coronary microvascular function after ischemic insult and the relation of microvascular preservation to ventricular contractile function in this setting.

	Methods

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Experimental Preparation
Domestic pigs of either sex weighing between 25 and 35 kg were anesthetized with ketamine (10 mg/kg IM) followed by intravenous administration of -chloralose (60 mg/kg initially and 15 mg/kg every 30 to 60 minutes as needed) and urethane (300 mg/kg initially and 60 mg/kg as needed). In control pigs (n=14), a median sternotomy was performed. The heart was removed immediately and placed in a cold Krebs' buffer of the following composition (mmol/L): NaCl 118.3, KCl 4.7, CaCl₂ 2.5, MgSO₄ 1.2, KH₂PO₄ 1.2, NaHCO₃ 25, glucose 11.1, pH adjusted to 7.4.

In the ischemia-reperfusion group (n=20), pigs were tracheally intubated and ventilated. Arterial blood gases were measured and adjusted by ventilatory rates, volumes, and concentrations of inspired oxygen to maintain PaO₂ >100 mm Hg, PaCO₂ between 30 and 45 mm Hg, and pH between 7.35 and 7.45. The internal jugular vein and the right femoral artery were isolated and cannulated for vascular access. Blood pressure was transduced from the femoral artery catheter. A sternotomy was performed, and the mid LAD was isolated with blunt and sharp dissection at the level of the second diagonal branch. An ultrasonic perivascular flow probe (Transonic Systems Inc) 1 to 2.5 mm in diameter was placed around the artery. A micromanometer-tipped catheter (Millar) was placed in the left ventricle through a direct apical puncture and secured in place with a purse-string suture. Sonomicrometer crystals (Triton Technology) were placed in the midmyocardium, perpendicular to the left ventricular long axis, in the territory subtended by the LAD. The pigs were then anticoagulated with 10 000 U IV heparin, and 30 mg IV lidocaine was administered. Baseline arterial blood pressure, heart rate, left ventricular end-diastolic pressure, segmental percent shortening, and coronary blood flow were measured.

Experimental Protocol
Ischemia-Reperfusion Studies
Pigs (n=20) were subjected to 30 minutes of ischemia by occlusion of the LAD with a vessel loop, followed by 1 hour of reperfusion. Soon after coronary occlusion, the subtended myocardium became visibly cyanotic and akinetic to dyskinetic. After 25 minutes of ischemia, hemodynamic data and coronary blood flow measurements were repeated. IR-saline pigs (n=8) then received normal saline (30 mL IV); IR-enalaprilat pigs (n=6) received enalaprilat 1.5 mg/kg IV; and IR-captopril pigs (n=6) received captopril 5 mg/kg IV. At 30 minutes, the occlusion was slowly released with resolution of cyanosis but persistent dyskinesis to akinesis in most cases. After 1 hour of reperfusion, hemodynamics and coronary blood flow measurements were recorded again. The heart was then removed and immediately placed in cold Krebs' buffer.

In Vitro Coronary Microvessel Studies
Arterial microvessels (109- to 178-µm intraluminal diameter) and venules (88- to 189-µm intraluminal diameter) from the ischemic region subtended by the LAD and arterioles subtended by the nonischemic LCx territory were carefully dissected from the epicardial surface with a x10 to x60 dissecting microscope (Olympus Optical). Microvessels were placed in an isolated Plexiglas organ chamber, cannulated with dual glass micropipettes measuring 30 to 80 µm in diameter, and secured with 10-0 nylon monofilament suture. Oxygenated Krebs' buffer aerated with 95% O₂/5% CO₂, pH=7.4, and maintained at 37°C was continuously circulated through the organ chamber and a reservoir containing a total volume of 100 mL. The arterial vessels were pressurized to 40 mm Hg in a no-flow state by two burettes filled with Krebs' buffer; venules were similarly pressurized to 10 mm Hg. With an inverted microscope (x40 to x200, IMT-2, Olympus Optical) connected to a video camera, the vessel image was projected on a television monitor (Panasonic). A video-electronic dimension analyzer (Living Systems Instrumentation) was used to measure luminal diameter, and a pressure transducer displayed distending pressures. Measurements were recorded with a Western Graphtec recorder. Vessels were allowed to bathe in the organ chamber for 30 to 60 minutes before an intervention.

Study Protocols
After equilibration in aerated Krebs' buffer solution, the arterial microvessels were preconstricted with acetylcholine chloride (10 nmol/L to -10 µmol/L) by a mean of 40±3% of the resting baseline diameter. After preconstriction, responses to increasing concentrations of ADP, bradykinin, the calcium ionophore A23187, and sodium nitroprusside were studied. Up to four drugs were applied to each vessel. Vessels were washed three times with Krebs' buffer and allowed to equilibrate 15 minutes between interventions. Studies were not performed on vessels after exposure to A23187.

Responses to ADP, bradykinin, and sodium nitroprusside also were studied in arterial microvessels from the nonischemic LCx region of pigs subjected to LAD occlusion. In separate experiments, preconstricted control LAD vessels also were exposed to increasing concentrations of captopril or enalaprilat. Additional experiments were performed, exposing nonprecontracted, quiescent control LAD microvessels to increasing concentrations of Ang I, Ang II, captopril, enalaprilat, acetylcholine, and the thromboxane A₂ analog U46619.

In a similar fashion, venules were harvested from the control pigs, pigs in the ischemia-reperfusion groups, and pigs subjected to ischemia-reperfusion with enalaprilat treatment. Venules were allowed to equilibrate for 30 to 60 minutes and then were preconstricted with the thromboxane A₂ analog U46619 (1 µmol/L) by a mean of 26±1%. After preconstriction, responses to increasing concentrations of ADP, serotonin, and sodium nitroprusside were studied as described above.

Drugs
Acetylcholine chloride, sodium nitroprusside, bradykinin, ADP, A23187, U46619, Ang I, and Ang II were obtained from Sigma Chemical Co. Captopril was obtained from Research Biochemical, Inc, and enalaprilat was obtained from Merck Sharp and Dohme Research Laboratory. Acetylcholine chloride, bradykinin, ADP, Ang I, Ang II, and sodium nitroprusside were dissolved in distilled water. Captopril and enalaprilat were diluted in sterile saline. A23187 was dissolved in dimethyl sulfoxide to make a 10-mmol/L stock solution. U46619 was dissolved in ethanol to make a 20-mmol/L stock solution. Stock solutions were stored at -20°C. All dilutions were prepared daily.

Statistical Analysis
All data are expressed as mean±SEM. Microvascular relaxations are expressed as the percent relaxation from the acetylcholine-induced constriction for arterioles and from the U46619-induced constriction for venules. Relaxation responses were compared between the four groups by two-factor ANOVA (concentration and treatment) for repeated measures or one-factor ANOVA (treatment). ED₅₀ also was calculated by probit transformation of the relaxation responses followed by linear regression analysis. The -log₁₀[ED₅₀] was then analyzed with one-factor ANOVA. When differences were detected with ANOVA, the Newman-Keuls test was performed to compare responses between specific groups. Hemodynamic parameters and coronary blood flow measurements were compared between groups with ANOVA and Student's t tests when appropriate. Selected data are presented in table form to conserve space. A value of P<.05 was considered significant.

	Results

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Hemodynamic Parameters
Table 1 gives the hemodynamic parameters at baseline, at the end of ischemia, and after reperfusion. There was no significant difference in blood pressure, heart rate, or left ventricular end-diastolic pressure at any time point between groups. Segmental percent shortening in the LAD region began at 18% to 20% in all groups, progressing to frank dyskinesis (-3% to -6%) in all groups during the ischemic period. Some restoration of segmental shortening occurred during reperfusion in the untreated ischemic pigs, whereas pigs treated with captopril or enalaprilat continued to demonstrate dyskinesis.

View this table: [in this window] [in a new window]   Table 1. Hemodynamic Parameters

Coronary Blood Flow
Coronary blood flow was measured in the IR-enalaprilat and the IR-saline groups. In both groups, there was a substantial hyperemic response on reperfusion in the ischemic bed, with a rise from 12±1 mL/min before ischemia to 32±3 mL/min immediately after reperfusion in the IR-saline group, compared with a rise from 12±2 to 59±10 mL/min in IR-enalaprilat pigs (P<.05 versus IR-saline). At the end of the 60-minute reperfusion period, coronary blood flow in the ischemic bed had fallen to 23±5 mL/min in controls and to 28±4 mL/min in the IR-enalaprilat pigs.

Vessel Characteristics
Arterial microvessels harvested from the LAD region measured between 109 and 178 µm in diameter and were similar in size in all groups (control, 144±7 µm; IR-saline, 148±7 µm; IR-captopril, 151±8 µm; and IR-enalaprilat, 142±12 µm). Mean percent preconstriction was similar in all arterial vessel groups, averaging 40±3%. The mean concentration of acetylcholine chloride required to obtain this degree of contraction was 0.38, 0.11, 0.19, and 0.40 µmol/L in the control, IR-saline, and IR-captopril, and IR-enalaprilat groups, respectively. In absolute terms, the relaxation of control vessels to the maximal dose of agonist applied was 48±9 µm (90% of the amount of preconstriction) for ADP, 48±7 µm (96% of the amount of preconstriction) for bradykinin, 45±7 µm (90% of the amount of preconstriction) for A23187, and 40±6 µm (84% of the amount of preconstriction) for sodium nitroprusside. Veins harvested from the LAD region ranged from 88 to 189 µm (control, 135±18 µm; IR-saline, 138±14 µm; and IR-enalaprilat, 118±8 µm) with a mean percent preconstriction of 26±1%, similar in all experimental groups. The concentration of U46619 required to obtain this degree of contraction in venules was 1 µmol/L in all groups. In absolute terms, the relaxation of control veins to the maximal dose of agonist applied was 20±4 µm (61% of the amount of preconstriction) for ADP, 7±2 µm (23% of the amount of preconstriction) for serotonin, and 25±5 µm (81% of the amount of preconstriction) for sodium nitroprusside.

Endothelium-Dependent Responses
Thirty minutes of ischemia followed by 1 hour of reperfusion caused marked impairment of microvascular relaxations to the endothelium-dependent agents ADP (P<.05; Fig 1) and the calcium ionophore A23187 (P<.01; Fig 2). Microvascular relaxations to ADP in vessels subjected to ischemia-reperfusion were significantly improved when either captopril (P<.01 versus IR-saline) or enalaprilat (P<.01 versus IR-saline) was administered before reperfusion, restoring responses to control levels. Similarly, either enalaprilat (P<.05 versus IR-saline) or captopril (P<.01 versus IR-saline) significantly improved relaxation responses to the calcium ionophore compared with saline-treated ischemic vessels. Bradykinin responses were not significantly blunted by this relatively brief period of ischemia (P=.95; Fig 3). Interestingly, there did appear to be a generalized enhancement of bradykinin relaxation in vessels from ischemic pigs treated with captopril (P<.01 versus IR-saline) but not with enalaprilat (P=.19). The EC₅₀ (-log molar) for the dose-response curves to these endothelium-dependent agents also was calculated (Table 2), showing that for bradykinin the EC₅₀ was lowered from 9.61±0.18 for IR-saline to 10.32±0.29 for IR-captopril (P<.05 versus IR-saline). Responses of arterial microvessels from the nonischemic LCx region to the endothelium-dependent agents ADP and bradykinin were not altered compared with responses of LAD microvessels from control hearts (Table 3).

View larger version (16K): [in this window] [in a new window]   Figure 1. Plot showing responses to ADP of pressurized (40 mm Hg) porcine coronary microvessels (109 to 178 µm) from the epicardial surface of the LAD distribution in control (n=8), IR-saline (n=8), IR-captopril (n=6), and IR-enalaprilat pigs (n=6). Microvessels were preconstricted with acetylcholine chloride. Drugs were applied extraluminally. Responses are expressed as percent relaxation of preconstricted diameter vs log [ADP] (mol/L). *P<.05, **P<.01 vs IR-saline.

View larger version (16K): [in this window] [in a new window]   Figure 2. Plot showing responses to calcium ionophore A23187 of pressurized (40 mm Hg) porcine coronary microvessels (109 to 178 µm) from the epicardial surface of the LAD distribution in control (n=8), IR-saline (n=8), IR-captopril (n=6), and IR-enalaprilat (n=6) pigs. Microvessels were preconstricted with acetylcholine chloride. Drugs were applied extraluminally. Responses are expressed as percent relaxation of preconstricted diameter vs log [A23187] (mol/L). *P<.05, **P<.01 vs IR-saline.

View larger version (19K): [in this window] [in a new window]   Figure 3. Plot showing responses to bradykinin of pressurized (40 mm Hg) porcine coronary microvessels (109 to 178 µm) from the epicardial surface of the LAD distribution in control (n=8), IR-saline (n=8), IR-captopril (n=6), and IR-enalaprilat (n=6) pigs. Microvessels were preconstricted with acetylcholine chloride. Drugs were applied extraluminally. Responses are expressed as percent relaxation of preconstricted diameter vs log [bradykinin] (mol/L). **P<.01 vs IR-saline.

View this table: [in this window] [in a new window]   Table 2. EC50 [-log molar] for Arteriolar Responses

View this table: [in this window] [in a new window]   Table 3. Responses of Microvessels From Nonischemic Myocardial Regions

Venular Responses
Venular responses to ADP were not significantly altered by ischemia-reperfusion, with or without enalaprilat. Relaxations to serotonin, however, were mildly enhanced by ischemia-reperfusion (P<.05 versus control) and were significantly increased after ischemia-reperfusion with enalaprilat treatment compared with control (P<.05) and IR-saline (P<.05) responses (Table 4).

View this table: [in this window] [in a new window]   Table 4. Responses of Venular Microvessels

Endothelium-Independent Responses
In arterial microvessels, relaxations to the endothelium-independent agent sodium nitroprusside were not impaired by ischemia-reperfusion in the IR-saline group (Fig 4 and Table 2). Relaxations to sodium nitroprusside also were not significantly altered in vessels from pigs treated with either captopril or enalaprilat. Similarly, venular responses to nitroprusside were not significantly altered by ischemia-reperfusion or ACE inhibitor therapy (Table 4).

View larger version (18K): [in this window] [in a new window]   Figure 4. Plot showing responses to sodium nitroprusside of pressurized (40 mm Hg) porcine coronary microvessels (109 to 178 µm) from the epicardial surface of the LAD distribution in control (n=8), IR-saline (n=8), IR-captopril (n=6), and IR-enalaprilat (n=6) pigs. Microvessels were preconstricted with acetylcholine chloride. Drugs were applied extraluminally. Responses are expressed as percent relaxation of preconstricted diameter vs log [sodium nitroprusside] (mol/L).

Responses in Nonischemic Microvessels
Fig 5 shows responses of vessels from the nonischemic LCx territory of hearts from pigs subjected to LAD territory ischemia. Endothelium-dependent responses to ADP, bradykinin, or the calcium ionophore A23187 and responses to sodium nitroprusside were not significantly altered after treatment with enalaprilat or captopril compared with the respective nontreated LCx vessels (IR-saline group; Fig 5).

View larger version (26K): [in this window] [in a new window]   Figure 5. Bar graph showing responses of pressurized (40 mm Hg) porcine coronary microvessels (109 to 178 µm) from the epicardial surface of the nonischemic LCx myocardial regions in pigs undergoing ischemia-reperfusion. Pigs received saline (n=8), enalaprilat (n=6), or captopril (n=6). Microvessels were preconstricted with acetylcholine chloride. Drugs were applied extraluminally. Responses are expressed as percent relaxation of preconstricted diameter. BK indicates bradykinin; C.I., calcium ionophore; and SNP, sodium nitroprusside.

Angiotensin and ACE Inhibition in Control Microvessels
Microvessels from control pigs also were exposed to increasing concentrations of Ang I, Ang II, captopril, or enalaprilat to assess any direct effect of these agents on coronary microcirculation. The percent constriction achieved with Ang I or II was minimal compared with that observed with acetylcholine chloride or U46619 in control arterioles (Fig 6). Likewise, neither captopril nor enalaprilat (maximum concentration, 100 µmol/L) contracted control coronary microvessels from baseline (0.1±0.2% and 0.3±0.4% contraction, respectively). In addition, neither captopril nor enalaprilat significantly dilated precontracted arterial microvessls (3.2±3.3% and 4.3±2.2% dilation, respectively). ACE inhibition with either enalaprilat or captopril had no significant effect on endothelium-dependent relaxations to ADP or the calcium ionophore A23187 or on cGMP-mediated endothelium-independent relaxations to sodium nitroprusside (Fig 7). However, the relaxation response to bradykinin was increased by 50% after pretreatment of vessels with either enalaprilat (P<.01) or captopril (P<.01) compared with the control response (Fig 7).

View larger version (13K): [in this window] [in a new window]   Figure 6. Bar graph showing percent constriction of baseline of control arteriolar microvessels exposed to 10 µmol/L acetylcholine chloride (ACH, n=6), 1 µmol/L U46619 (n=6), 10 µmol/L angiotensin I (A I, n=6), or 10 µmol/L angiotensin II (A II, n=6). *P<.05 vs angiotensin II response.

View larger version (27K): [in this window] [in a new window]   Figure 7. Bar graph showing responses of pressurized (40 mm Hg) porcine coronary microvessels (109 to 178 µm) from the LAD territory of noninstrumented control pigs. Vessels were pretreated with enalaprilat (n=6) or captopril (n=6) or received no pretreatment (control, n=6). Microvessels were preconstricted with acetylcholine chloride. Drugs were applied extraluminally. Responses are expressed as percent relaxation of preconstricted diameter. Abbreviations as in Fig 5. **P<.01 vs control.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
Restoration of adequate myocardial perfusion is the primary goal in the treatment of acute myocardial infarction and unstable angina. Thrombolytic therapy and direct percutaneous coronary intervention are efficacious in reestablishing epicardial coronary patency, but these modalities do not address perturbations of coronary microvascular function induced by the ischemic insult. Even after epicardial coronary patency is restored, clinical and experimental evidence indicates that myocardial perfusion often remains impaired, suggesting persistent dysfunction at the level of the coronary microvasculature.^{11 12 13 14 15} Clinically, impaired microvascular function could contribute to reduced coronary flow reserve, the "no-reflow phenomenon,"¹⁶ postinfarction angina, myocardial stunning, and arrhythmia.

The primary finding of the present study is that intravenous administration of captopril or enalaprilat 5 minutes before reperfusion in this porcine model of ischemia-reperfusion restored the blunted relaxation responses to A23187 and ADP observed after ischemia-reperfusion to control levels. In addition, treatment with either captopril or enalaprilat enhanced bradykinin responses to a level greater than that in vessels from noninstrumented control pigs. Although free radical inhibitors and anti-leukocyte antibodies have been shown to improve this type of microvascular injury and contractile dysfunction after ischemic insult, to the best of our knowledge this is the first study to examine the influence of ACE inhibition on coronary microvascular reperfusion injury.

Probably the most intriguing aspect of the present study is the disparate effects of ACE inhibitors on preservation of microvascular and myocardial function. Oxygen-derived free radical generation, the denaturing of proteins, and activation of neutrophils in mediating the detrimental effects of reperfusion injury after brief ischemia may affect functional recovery differently in blood vessels and myocardium. A recent study by Griendling and colleagues¹⁷ found that Ang II caused a marked increase in superoxide anion levels in cultured vascular smooth muscle cells owing to activation of enzyme systems that promote the generation of reactive oxygen species such as NADPH and NADH oxidases. These mechanisms may occur at different rates in vascular and myocardial tissues. Furthermore, ACE inhibitors may affect vascular function and myocardial function differently. Specifically, increased local tissue concentrations of bradykinin and nitric oxide and free radical scavenging caused by ACE inhibition may acutely alter vascular function. By contrast, improved myocardial contractile function may relate to chronic reductions in afterload, lowering of diastolic wall stress, and altered collagen deposition.

ACE inhibitors administered during acute ischemia could preserve arteriolar microvascular endothelial function through several mechanisms. By lowering preload and afterload, ACE inhibition could favorably influence the balance of oxygen supply and demand during acute ischemia. This mechanism, however, probably is not a significant component of the microvascular preservation that we observed, for blood pressure, heart rate, and end-diastolic pressure were not significantly altered by either enalaprilat or captopril administration in our study. Similar protective effects of ACE inhibition in the absence of hemodynamic changes have been observed by others.^{18 19 20}

Although hemodynamics were not altered by ACE inhibition, the hyperemic response immediately on reperfusion was markedly enhanced by ACE inhibition. Ischemia is known to activate the renin-angiotensin system, and blockade of the enhanced Ang II production, either systemically or at the local tissue level, could thus contribute to improved coronary blood flow after temporary coronary occlusion. Augmented vasodilation of epicardial conduit vessels may in part explain this phenomenon, but the primary site of regulation of coronary blood flow lies instead in the smaller resistance vessels.^{9 10} Interestingly, we found only a small direct effect of Ang I or II on such resistance vessels. A significant component of the enhanced hyperemia after transient ischemia may therefore relate to impaired degradation of bradykinin in the presence of ACE inhibition rather than to reduced local levels of Ang II. By stimulating local release of prostacyclin and nitric oxide, potent vasodilators at the levels of both conduit and resistance vessels, bradykinin may improve microvascular perfusion, limit the ischemic insult to the microvasculature, and thus help preserve a functional endothelium.

In addition to improving coronary blood flow, increased local concentrations of bradykinin can indirectly (through nitric oxide and prostacyclin) enhance cardiac glucose uptake,²¹ reduce neutrophil activation and adhesion,²² attenuate free radical production,^{23 24 25} and inhibit platelet aggregation and adhesion,^{26 27 28} thereby removing other potential contributors to the endothelial dysfunction observed after ischemia-reperfusion. Although the direct role of local bradykinin levels in preventing microvascular endothelial dysfunction following ischemia has not been studied, it is intriguing to note that the beneficial effects of ACE inhibition on myocardial function after an ischemic insult can be suppressed by coadministration of bradykinin antagonists or inhibitors of nitric oxide production.²⁹

Whereas ACE inhibition could enhance blood flow to the ischemic myocardium, it was demonstrated previously that the reperfusion process may actually exacerbate microvascular endothelial dysfunction, primarily attributable to the introduction of cytotoxic oxygen-derived free radicals.^{14 30 31} Several investigators have shown that sulfhydryl-containing ACE inhibitors effectively scavenge these free radicals in vitro.^{32 33 34} Importantly, others have found that even nonsulfhydryl-containing ACE inhibitors can prevent electrolysis-induced vascular endothelial dysfunction and microsomal peroxidation, postulated to be attributable to a free radical scavenging effect.^{34 35 36} In our study, ACE inhibition restored impaired endothelium-dependent microvascular responses to control levels regardless of the presence of a sulfhydryl moiety. We demonstrated previously that antioxidants and free radical scavengers limit microvascular endothelial injury after ischemic insult in a model of ischemic cardioplegia,³⁰ and a similar effect may explain the cyto-protective benefits of ACE inhibition in the present study. As discussed previously, Ang II may cause an increase in vascular tissue levels of superoxide anion,¹⁷ which may be prevented with ACE inhibition. Nevertheless, the clinical relevance of the free radical scavenging properties of ACE inhibitors remains controversial.^{18 37 38}

The effects of ischemia-reperfusion on the coronary venous microcirculation have not been previously examined in a porcine model. Venular function may be important in the setting of ischemia because under conditions of maximum vascular dilation, coronary veins may provide up to 31% of resistance to myocardial perfusion.³⁹ Using a canine model, Lefer et al⁴⁰ found that 60 minutes of ischemia and 270 minutes of reperfusion resulted in attenuated relaxation of large coronary venous rings to ADP and A23187. In our model, venular responses to ADP and serotonin were not significantly reduced following briefer periods of ischemia and reperfusion. These disparate results may relate to the differences in the protocols and animal models used.

Despite the preservation of arteriolar microvascular endothelial function by ACE inhibition, in the present study we were unable to demonstrate that treatment translated into improved myocardial function. The regional myocardial dyskinesis (measured by segmental shortening) induced by temporary coronary occlusion was not reversed after 1 hour of reperfusion in this model with either captopril or enalaprilat therapy. Previous studies of ischemia-reperfusion in other animal models have reported conflicting data regarding the effect of ACE inhibition on the restoration of contractile function. Westlin and Mullane³² found that captopril significantly improved contractile function in a canine model of brief (15-minute) ischemia followed by prolonged (3-hour) reperfusion. However, enalaprilat administration actually yielded less recovery compared with that seen in saline controls. In a similar canine model, Przyklenk and Kloner⁴¹ reported positive results with both the sulfhydryl-containing compound zofenopril and enalapril. Ehring et al⁶ recently reported attenuation of myocardial stunning with the administration of ramiprilat. The failure of ACE inhibition to enhance segmental shortening following reperfusion in the present study may reflect the model, experimental design, or limited duration of reperfusion. The porcine model used here may be more representative of human coronary physiology than the canine model in that dogs' extensive collateral network renders them less susceptible to transmural myocardial infarction after temporary coronary occlusion. Dogs also lack coronary endothelial xanthine oxidase, an enzyme postulated to contribute significantly to local free radical production in humans. By contrast, the porcine heart mimics human physiology in these respects. In light of these physiological differences and the more prolonged ischemic time in our study, recovery of myocardial contractility after reperfusion may be less likely. Although preserved microvascular endothelial function did not correlate with improved myocardial contractile function in this short-term study, it is possible that the longer-term beneficial effect of chronic ACE inhibition on ventricular remodeling after infarction may be mediated in part through such microvascular preservation and enhanced vasodilator responses. Furthermore, microvascular endothelial cell functional preservation may reduce the incidence of coronary spasm and improve and maintain myocardial perfusion.

Study Limitations
Whereas the in vitro method used in this study allows us to avoid the autoregulatory and metabolic influences involved in in vivo studies, direct inferences regarding the in vivo responses to ischemia and reperfusion cannot be made. All pharmacological agents in this study were applied extraluminally to avoid the flow-mediated dilation that accompanies intraluminal administration. The agents must therefore diffuse across the vascular wall to affect smooth muscle or endothelium. However, because the vessel sizes within the various groups were similar, our results should not be biased by this need for diffusion. We examined microvascular function after 30 minutes of ischemia and 1 hour of reperfusion. Although myocardial ischemia as short as 30 minutes has been associated with subendocardial tissue injury,⁴² most of the damage is reversible. Shorter periods of ischemia may have resulted in more reversibility of the induced myocardial dyskinesis and allowed us to better determine whether ACE inhibition could exert a protective effect on myocardial contractility. However, 30 minutes of coronary occlusion probably is the shortest interval before successful reperfusion in the clinical setting. Finally, additional studies with bradykinin or Ang II receptor antagonists could elucidate the importance of increased local levels of bradykinin or reduced levels of Ang II to the microvascular endothelial preservation we observed with ACE inhibition.

Conclusions
In a porcine model of ischemia-reperfusion mimicking human physiology, endothelium-dependent responses are impaired in the arteriolar microcirculation, whereas microvascular venular responses are not significantly reduced. The administration of an ACE inhibitor minutes before reperfusion helps preserve coronary arteriolar microvascular endothelial function without altering systemic hemodynamics. This salutary effect is observed with ACE inhibition independent of the presence of a sulfhydryl moiety. Potential mechanisms of this endothelial preservation may involve improved coronary blood flow, free radical scavenging, inhibition of Ang II–induced generation of oxygen free radicals, potentiation of local microvascular effects of bradykinin as mediated by nitric oxide and prostacyclin, or direct blockade of a cytotoxic effect of Ang II. Despite the beneficial influence of ACE inhibition on endothelial function, myocardial contractile function does not appear to be enhanced in this model 1 hour after reperfusion.

	Selected Abbreviations and Acronyms

 

ACE = angiotensin-converting enzyme Ang = angiotensin IR-captopril = ischemia-reperfusion treated with captopril IR-enalaprilat = ischemia-reperfusion treated with enalaprilat IR-saline = ischemia-reperfusion treated with saline LAD = left anterior descending artery LCx = left circumflex artery

	Acknowledgments

 
This work was supported by NHLBI grant HL-46716 from the NIH, Bethesda, Md, and the American Heart Association, Massachusetts Affiliate Grant 13-501-912. This work also was given fellowship training support by the American College of Chest Physicians. Dr Piana was supported by a Hewlett-Packard Medical Fellowship Award.

Received July 5, 1995; revision received August 31, 1995; accepted September 14, 1995.

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