Author + information
- Received March 6, 1997
- Revision received September 8, 1997
- Accepted September 29, 1997
- Published online January 1, 1998.
- Jay H Traverse, MDAB,
- James W Kinn, MDAB,
- Christopher Klassen, BSAB,
- Dirk J Duncker, MD, PhDAB and
- Robert J Bache, MD, FACCAB,* ()
- ↵*Dr. Robert J. Bache, Cardiovascular Division, Department of Medicine, University of Minnesota Medical School, Box 508 UMHC, 420 Delaware Street SE, Minneapolis, Minnesota 55455.
Objectives. We sought to determine the importance of nitric oxide (NO) production in maintaining coronary blood flow during exercise in hearts with collateral-dependent myocardium.
Background. Coronary collateral vessels demonstrate endothelium-mediated NO-dependent vasodilation in response to agonists such as acetylcholine. However, the contribution of endogenous NO production to maintaining vasodilation of coronary collateral vessels during exercise has not been previously studied.
Methods. Collateral vessel growth was induced in 13 chronically instrumented dogs by intermittent 2-min occlusions, followed by permanent occlusion of the left anterior descending coronary artery (LAD). One week after permanent LAD occlusion, myocardial blood flow was measured with microspheres during rest and treadmill exercise at 6.4 km/h at a 15% grade. Measurements were then repeated after blockade of NO production with N-nitro-l-arginine (LNNA) (20 mg/kg body weight intravenously).
Results. LNNA caused a 62 ± 4% (mean ± SEM) inhibition of the coronary vasodilation produced by acetylcholine. During rest conditions, LNNA caused a slight decrease in blood flow to the collateral region (p = NS), with no change in normal zone blood flow. During exercise, LNNA caused a decrease in mean blood flow to the collateral region (from 2.24 ± 0.19 to 1.78 ± 0.26 ml/min per g after LNNA, p < 0.05). This decrease resulted from a near doubling of the collateral vascular resistance (p < 0.05), with a trend toward an increase in small vessel resistance in the collateral zone. LNNA also reduced myocardial blood flow to the normal region during exercise (from 2.99 ± 0.24 to 2.45 ± 0.28 ml/min per g, p < 0.05) as the result of a 44 ± 13% increase in coronary vascular resistance (p < 0.05).
Conclusions. NO contributes to the maintenance of coronary collateral blood flow during exercise. In contrast to the normal heart, endogenous NO production also maintains blood flow in remote myocardial regions during exercise. These results suggest that control of blood flow during exercise in normal myocardium is altered by the presence of an occluded coronary artery.
The coronary endothelium produces a diverse group of biologically active substances that can modulate vasomotor tone. Among the most important of these is nitric oxide (NO), which is produced from l-arginine by endothelial NO synthase. NO has been shown to mediate the vasodilator responses to several endothelium-dependent agonists, including acetylcholine and bradykinin , and is responsible for flow-mediated coronary artery vasodilation that occurs during exercise in response to increased endothelial shear stress .
When occlusive coronary artery disease occurs gradually, collateral vessels develop to protect the myocardium distal to the obstruction. These collateral vessels may supply sufficient blood flow to meet the metabolic requirement of the dependent myocardium at rest, but they can become flow-limiting during periods of increased demands, resulting in angina pectoris or left ventricular dysfunction during exercise. In vitro studies using isolated vessel rings have demonstrated that collateral vessels acquire a functionally competent endothelium and muscular media early in their development and are responsive to NO-dependent agonists, such as bradykinin and acetylcholine [4–6]. Inhibition of NO synthesis has been shown to decrease retrograde blood flow from a cannulated collateral-dependent coronary artery in anesthetized open chest dogs and to significantly increase collateral vessel vascular resistance in awake dogs , demonstrating that tonic production of NO maintains collateral vasodilation. However, it is not known whether NO-mediated vasodilation contributes to the maintenance of collateral blood flow in the intact animal during exercise. Previous studies have failed to demonstrate an obligatory requirement for NO production during exercise in the normal heart because blockade of NO synthesis with N-nitro-l-arginine (LNNA) did not impair the normal increase in coronary blood flow in response to exercise . Although inhibition of NO production does not affect basal coronary blood flow in the normal heart, it does cause constriction of epicardial coronary arteries . The collateral vessels that develop in response to coronary occlusion represent a major locus of resistance to blood flow into the dependent myocardium. Furthermore, responses of collateral vessels to vasoactive agonists more closely resemble those of epicardial coronary arteries than resistance vessels. Consequently, the present study was performed to test the hypothesis that inhibition of NO production would impair coronary collateral blood flow during exercise.
Studies were carried out in 18 adult mongrel dogs weighing 25 to 30 kg, trained to run on a motor-driven treadmill. All studies were performed in accordance with the “Position of the American Heart Association on Research Animal Use” adopted by the Association in November 1984 and were approved by the Animal Care Committee of the University of Minnesota.
1.1 Surgical Preparation
Animals were premedicated with acepromazine (10 mg intramuscularly), anesthetized with sodium pentobarbital (30 to 35 mg/kg body weight intravenously), intubated and ventilated with room air supplemented with oxygen. A left thoracotomy was performed in the fifth intercostal space. A heparin-filled polyvinyl chloride catheter, 3.0-mm outer diameter, was introduced into the internal thoracic artery and advanced until the tip was positioned in the ascending aorta. The pericardium was then opened and the heart suspended in a pericardial cradle. A second catheter was placed into the left atrium through the appendage and secured with a purse-string suture. A similar catheter was introduced into the left ventricle at the apical dimple and secured in place. A bipolar pacing electrode was sutured to the right atrial appendage. The proximal left anterior descending coronary artery (LAD) was dissected free, and a Doppler velocity probe (Craig Hartley) was placed around the vessel. A hydraulic occluder and snare-type occluder were positioned around the artery distal to the velocity probe. A heparin-filled silicone rubber catheter (0.3-mm inner diameter) was placed into the artery distal to the occluders . The pericardium was then loosely closed, and the catheters and electrical leads were tunneled subcutaneously to exit at the base of the neck. The thoracotomy was closed in layers and the chest evacuated of air. Catheters were protected with a nylon vest and were flushed daily with heparinized saline to maintain patency.
1.2 Induction of Collateral Vessel Growth
One week after operation, 13 of the dogs were returned to the laboratory for initiation of collateral vessel development using a modification of the repetitive coronary occlusion technique of Franklin et al. . While distal coronary pressure was measured, the hydraulic occluder was inflated with saline; the flowmeter signal was monitored to ensure total occlusion. Initial distal coronary pressures during occlusion were 10 to 20 mm Hg in all dogs. A protocol of coronary occlusions 2 min in duration performed at 15-min intervals 3 h each day was then begun 5 days/week. When distal coronary pressure during occlusion exceeded 35 mm Hg (2 to 3 weeks after beginning the coronary occlusion protocol), the artery was permanently occluded by tightening the snare occluder. We have previously observed that sufficient collateral blood flow is present at this pressure to prevent infarction. Studies were performed 5 to 7 days after permanent coronary occlusion, when distal coronary pressure routinely exceeded 70 mm Hg and rest collateral blood flow is not significantly different from normal zone blood flow.
1.3 Measurement of Myocardial Blood Flow
Myocardial blood flow during exercise was measured in seven dogs with 15-μm diameter microspheres labeled with cesium-141, chromium-51, strontium-85, niobium-95 or scandium-46 (NEN Co.). In six dogs, measurements of rest myocardial blood flow before and after LNNA were made with 15-μm diameter fluorescent microspheres. Microspheres were obtained as 1.0 mCi in 10-ml low molecular weight dextran. For each measurement, 3 × 106microspheres were injected into the left atrium and flushed with normal saline. A reference sample of arterial blood was obtained from the aortic catheter at a constant rate of 15 ml/min using a peristaltic pump beginning at the time of microsphere injection and continuing for 90 s.
1.4 Effect of NO Blockade on Myocardial Blood Flow During Exercise (Group 1)
Aortic, left ventricular and distal coronary pressures were measured with pressure transducers (Spectramed model TNF-R) at midchest level. Left ventricular pressure was recorded both at normal and at high gain for measurement of end-diastolic pressure. Data were recorded on an eight-channel direct writing recorder (Coulbourne Instruments). After all recording instruments were connected, a 5-min period of warm-up exercise was performed while the exercise intensity was gradually increased to 6.4 km/h at a 15% grade. Heart rate was controlled during the subsequent exercise protocols by atrial pacing at a rate 10 to 20 beats/min faster than the sinus rate recorded during this warm-up exercise. The dogs were then allowed to rest for 1 h. Control rest hemodynamic measurements were then obtained with the dogs standing quietly on the treadmill. After obtaining rest measurements, exercise was begun at 6.4 km/h at a 15% grade. Hemodynamic measurements were recorded continuously to ensure that steady state conditions existed. Three minutes after beginning exercise, microspheres were injected into the left atrium for measurement of myocardial blood flow. The animals continued to exercise for 2 min after the microsphere injection.
Dogs were then allowed to rest quietly on the treadmill for 1 h. During this time, N-nitro-l-arginine (LNNA) dissolved in normal saline in a dose of 20 mg/kg was infused into the left atrium over 30 min. All hemodynamic measurements were again recorded, and the exercise protocol was repeated at the identical exercise level and pacing rate used during control conditions. Hemodynamic measurements and microsphere administration were performed 3 min after beginning exercise, and exercise continued for 2 min after microsphere injection.
1.5 Effect of NO Blockade on Rest Myocardial Blood Flow (Group 2)
The effect of inhibition of NO production on rest myocardial blood flow was assessed in a separate group of six dogs that underwent the identical surgical instrumentation and induction of collateral vessel development. With the dogs standing quietly in a sling, rest blood flow to the normal and collateral-dependent myocardium was measured with fluorescent microspheres. LNNA (20 mg/kg) was infused into the left atrium over 30 min, and blood flow measurements were repeated 30 min later.
1.6 Identification of Collateral-Dependent Myocardium
The region of collateral-dependent myocardium was identified by the shadow technique of Patterson and Kirk . After completion of the exercise protocol, dogs were premedicated with morphine sulfate (2 mg/kg intramusculary), anesthetized with alpha-chloralose (100 mg/kg intravenously, followed by an infusion of 10 mg/kg per h), intubated and ventilated with a respirator. A left thoracotomy was performed in the sixth intercostal space, and the proximal LAD was dissected free and cannulated with a thin-walled stainless steel cannula at the site of the snare occluder. A 26-gauge tube incorporated into the cannula allowed measurement of cannula tip pressure. The cannula was then perfused with nonradioactive arterial blood from a reservoir pressurized to maintain cannula tip pressure 10 to 15 mm Hg above mean aortic pressure while radioactive microspheres were injected into the left atrium. Using this technique, the collateral-dependent region was perfused with nonradioactive blood while the normal region was perfused with blood containing microspheres. The dogs were killed with pentobarbital, and the heart and kidneys were removed and placed into 10% buffered formalin.
After fixation, the left ventricle was separated from the atrium and right ventricle, and the epicardial vessels and fat were trimmed away. The left ventricle was then sectioned into five transverse rings from base to apex and inspected to ensure the absence of infarct. Each ring was then sectioned into 16 radial segments that were divided into epicardial and endocardial halves, weighed and placed into vials for counting. Myocardial and blood reference specimens were counted in a gamma-spectrometer (model 5912, Packard Instrument Co.) at window settings corresponding to the peak energies of each radionuclide. The activity in each energy window was corrected for background and overlapping counts between isotopes with a digital computer. Blood flow to each myocardial specimen (Qm) was computed using the formula Qm= Qr·Cm/Cr, where Qris the reference blood flow rate(ml/min), Cmis counts/min of the myocardial specimen, and Cris counts/min of the reference blood specimen. Collateral-dependent myocardium was identified as specimens having blood flow rates during the shadow injection >3 SD below the mean value of flow of the samples obtained from the normally perfused region.
Myocardial specimens and reference blood samples containing fluorescent microsphere were processed using the method of Glenny et al. . Tissue was weighed and then digested in 4N KOH in a heated 50°C bath for 48 h. Microspheres were recovered in 10 μm filters after vacuum filtration. The microspheres were digested in 2-ethoxy-ethyl acetate. Fluorescence measurements were made with a fluorescent spectrophotometer (Perkin Elmer Co.) using a FAC7 software package (Fluorescent Microsphere Resource Center, University of Washington). Myocardial blood flow was calculated in the same manner as with radioactive microspheres.
1.7 Degree of Blockade of Endothelium-Dependent Vasodilation
Responses of coronary blood flow to endothelium-dependent and independent vasodilators could not be directly assessed in our model because permanent occlusion of the collateral-dependent artery prevented intraarterial administration of agonist. Consequently, to assess the ability of LNNA to inhibit endothelium-dependent, NO-mediated vasodilation, five additional dogs with the identical surgical instrumentation but without coronary occlusion were studied. With the dogs standing quietly in a sling, LAD coronary artery blood flow was measured with a Doppler velocity probe. Acetylcholine dissolved in normal saline was infused into the coronary artery catheter at rates of 1.5, 3.75, 7.5 and 15 μg/min (infusion rates between 0.15 to 1.5 ml/min). Coronary blood flow was recorded continuously during each infusion, and measurements were taken after blood flow had achieved a steady state. Normal saline was infused at the same rates to determine the effect of vehicle on coronary blood flow. Thirty minutes later, the response to the endothelium-independent vasodilator sodium nitroprusside infused into the coronary artery at rates of 0.6, 1.5 and 3.0 μg/kg per min was examined. LNNA at a dose of 20 mg/kg was then infused into the left atrium over 30 min. The responses to acetylcholine and sodium nitroprusside were again measured beginning 30 min after administration of LNNA.
1.8 Data Analysis
Heart rate and pressures were measured directly from the strip-chart recordings. Transcollateral vessel resistance was calculated as the pressure drop from mean aortic pressure to pressure in the LAD coronary artery distal to the occlusion divided by mean collateral zone blood flow. Small vessel resistance in the collateral-dependent region was calculated as (Distal coronary pressure− Left ventricular end-diastolic pressure)/Mean collateral zone blood flow. Total vascular resistance in the normal zone was calculated as (Mean aortic pressure− Left ventricular end-diastolic pressure)/Mean normal zone blood flow. Resistance calculations were expressed as mm Hg·ml−1·min·g. In one animal, measurement of left ventricular end-diastolic pressure could not be obtained and was not included in the calculation of resistance.
For dogs in which dose–response curves to acetylcholine and sodium nitroprusside were obtained, the coronary Doppler shift in kHz was converted to blood flow using the equation q= 2.5 × d2× f, where q is coronary blood flow in ml/min, d is the internal diameter of the vessel in mm, and f is the Doppler frequency shift measured in kHz . Because the artery becomes adherent to the flow probe after operation, coronary artery diameter is fixed at the site where the velocity signal is obtained so that the external diameter of the artery is equal to the internal diameter of the flow probe. Based on previous observations, the internal diameter of the artery was taken as 80% of its external diameter.
Data were compared within individual groups using analysis of variance for repeated measures; a value of p < 0.05 was required for statistical significance. When a statistically significant result was found, individual comparisons were performed using the Wilcoxon signed-rank test with a Bonferroni correction. Data are expressed as mean value ± SEM.
2.1 Group 1: Response to NO Blockade During Exercise
2.1.1 Hemodynamic Data
Hemodynamic data during treadmill exercise before and after administration of LNNA in the seven dogs in group 1 are shown in Table 1. Both mean aortic pressure and peak left ventricular systolic pressure increased significantly in response to LNNA. Heart rate was maintained constant by atrial pacing. Distal coronary pressure during exercise was unchanged after administration of LNNA, whereas the transcollateral pressure gradient increased from 43 ± 3 to 60 ± 7 mm Hg (p < 0.08).
2.1.2 Myocardial Blood Flow During Exercise
Left ventricular mass ranged from 79 to 158 g (mean 114 ± 9). The mass of collateral-dependent myocardium ranged from 19.6 to 37.1 g (mean 25.8 ± 2.2) and comprised 23.3 ± 2.2% of the left ventricle. Myocardial blood flow measurements are shown in Tables 2 and 3. ⇓⇓Mean myocardial blood flow in the normal zone during control exercise was 2.99 ± 0.24 ml/min per g and decreased to 2.45 ± 0.28 ml/min per g after NO synthase (NOS) inhibition with LNNA (p < 0.05). This decrease in blood flow in response to LNNA was uniform throughout all regions of the normal zone. When blood flow was examined by layers, subendocardial blood flow (ENDO) was significantly higher than subepicardial blood flow (EPI) during control exercise (p < 0.05), with an ENDO/EPI flow ratio of 1.15 ± 0.04 during control exercise. The ENDO/EPI blood flow ratio was unchanged during exercise after LNNA.
Mean myocardial blood flow to the collateral-dependent region was 2.24 ± 0.19 ml/min per g during control exercise and was significantly lower than normal zone blood flow (p < 0.05). After LNNA, mean blood flow in the collateral-dependent region decreased by 20 ± 7% (p < 0.05). When blood flow was examined by layers, there was a tendency toward a greater decrease in subendocardial flow than in subepicardial flow in the collateral zone (ENDO/EPI flow ratio decreased from 1.02 ± 0.06 to 0.89 ± 0.07 after LNNA, p < 0.07).
Coronary vascular resistance values are shown in Table 3and Figs. 1 and 2. ⇓⇓After LNNA, the aortic to distal coronary pressure gradient during exercise increased by 42 ± 15% (p < 0.05), and blood flow significantly decreased, resulting in a near doubling of the transcollateral resistance (p < 0.05). Small vessel resistance in the collateral zone tended to increase after LNNA but did not achieve statistical significance (Fig. 1). Coronary vascular resistance in the normal zone (Fig. 2), which reflects the contributions of both conduit and resistance vessels, increased by 44 ± 13% during exercise after LNNA (p < 0.05).
2.2 Group 2: Response to NO Blockade at Rest
Responses to LNNA in six awake dogs at rest are shown in Table 4. LNNA caused a significant increase in aortic pressure and a significant decrease in heart rate. During control conditions, blood flow tended to be less in the collateral zone than in the normally perfused region and was of borderline significance (p < 0.08). Blood flow in the collateral-dependent region decreased from 1.20 ± 0.17 to 0.94 ± 0.10 ml/min per g in response to LNNA but did not achieve statistical significance (p = 0.18). The trend toward a decrease in collateral zone blood flow in conjunction with the increase in arterial pressure in response to LNNA resulted in a 65 ± 26% increase in vascular resistance (p < 0.07). Mean myocardial blood flow at rest in the normal zone was 1.35 ± 0.19 ml/min per g during control conditions and was unchanged after LNNA.
2.3 Effect of LNNA on Coronary Responses to Acetylcholine and Sodium Nitroprusside
Coronary blood flow responses to intracoronary infusions of acetylcholine and sodium nitroprusside before and after LNNA in five normal dogs are shown in Fig. 3. Heart rate was 119 ± 7 beats/min, and mean aortic pressure was 96 ± 3 mm Hg during control conditions. Intracoronary infusions of acetylcholine and sodium nitroprusside had no effect on heart rate or arterial pressure. Infusion of vehicle did not change coronary blood flow. Baseline coronary blood flow was 44 ± 3 ml/min and increased to 89 ± 12 ml/min in response to the largest dose of acetylcholine (15 μg/min) and to 110 ± 13 ml/min in response to the largest dose of sodium nitroprusside (3.0 μg/kg per min). After administration of LNNA, mean aortic pressure rose to 125 ± 11 mm Hg, and heart rate fell to 101 ± 9 beats/min (both p < 0.05). LNNA significantly blunted the response of coronary blood flow to acetylcholine, with 62 ± 4% inhibition of the increase in coronary blood flow caused by acetylcholine at a dose of 15 μg/min (p < 0.01). In contrast, LNNA had no effect on the increase in coronary flow produced by sodium nitroprusside.
The results of the present study indicate that NO production contributes to the maintenance of blood flow to collateral-dependent myocardium during exercise. Inhibition of NO production by LNNA resulted in a near doubling of the transcollateral resistance during exercise. Furthermore, small vessel resistance in the collateral zone failed to decrease after LNNA at a time when the fall in tissue blood flow might have been expected to cause metabolic vasodilation. Although LNNA caused no change in normal zone blood flow during rest conditions, it significantly decreased blood flow during exercise. This result is in contrast to previous studies in which LNNA did not decrease coronary blood flow during exercise in the normal heart and suggests that control of vasomotor tone is altered in remote regions of normal myocardium after chronic coronary artery occlusion. The possible mechanisms and implications of these changes caused by blockade of NO synthesis are discussed in detail.
3.1 Endothelium-Dependent Coronary Vasodilation
Previous studies have documented that endogenous NO can influence coronary vasomotor tone in the normal heart. Chu et al. found that blocking NO synthesis with l-monomethyl arginine (LNMMA) produced dose-related decreases in epicardial coronary artery diameter but had little effect on rest coronary blood flow in awake dogs. Jones et al. observed that N-nitro-l-arginine methyl ester (l-NAME) caused constriction of small arteries but compensatory vasodilation of arterioles, resulting in preservation of coronary blood flow. Parent et al. demonstrated that the increase in coronary blood flow produced by acetylcholine in dogs was attenuated by LNNA and was partially restored after administration of l-arginine. These results were specific for endothelium-dependent vasodilation because the response to the endothelium-independent vasodilator nitroglycerin was unchanged by LNNA. Komaru et al. used intravital microscopy to study the effect of NO production in the microcirculation of open chest dogs. Acetylcholine produced dose-dependent vasodilation of small arteries (>120 μm) and arterioles (<120 μm). The response to acetylcholine was completely inhibited by LNMMA in the small arteries but was only partially blocked in the arterioles, in agreement with our findings that demonstrated incomplete inhibition of the coronary blood flow response to acetylcholine by LNNA and suggests that other vasodilator substances not derived from l-arginine, such as the endothelium-derived hyperpolarizing factor , may also be involved in the vasodilator response to acetylcholine. In addition to agonist-mediated, endothelium-dependent vasodilation, Kuo et al. demonstrated that flow-mediated vasodilation also occurs in isolated porcine coronary resistance vessels (40 to 80 μm). This response is dependent on the production of NO because dilation was abolished by LNMMA.
3.2 Endothelial Function in Collateral Vessels
In studies of preconstricted rings of epicardial collateral vessels obtained 12 weeks after placement of an ameroid constrictor on the LAD of dogs, Flynn et al. found that both acetylcholine and bradykinin produced dose-dependent relaxation that was attenuated by inhibition of NO synthesis with LNMMA. Angus et al. observed that acetylcholine caused equivalent relaxation of preconstricted rings of collateral vessels and normal coronary artery segments of similar size obtained from dogs 6 months after ameroid occlusion of the left circumflex coronary artery. Altman et al. examined responses of retrograde blood flow from the cannulated collateralized LAD 4 to 6 months after coronary artery occlusion. Both acetylcholine and bradykinin significantly increased collateral blood flow; the increase in collateral flow was equivalent to that produced by nitroglycerin, a known potent vasodilator of coronary collateral vessels. Frank et al. demonstrated that NO production is important in maintaining tonic collateral vasodilation in awake dogs at rest. In that study, LNNA caused a significant increase in coronary collateral vascular resistance that was accompanied by a small decrease in collateral zone blood flow at rest. These results are in agreement with our findings in which LNNA tended to decrease rest collateral zone blood flow at rest. Overall, these studies indicate that endothelium-dependent vasodilator mechanisms acting through NO contribute to maintenance of collateral vessel dilation during rest conditions.
The importance of NO in maintaining collateral blood flow during exercise has not been previously studied. The collateral vasoconstriction that occurred after administration of LNNA in the present study most likely resulted from inhibition of tonic and flow-mediated release of NO. The increase in blood flow through the collateral vessels during exercise would be expected to augment shear stress on the endothelial cells. In canine epicardial coronary arteries, an increase in shear results in vasodilation through release of NO . Although the response of collateral vessels to increasing shear has not been studied, the increase in blood flow during exercise would most likely also result in shear-mediated vasodilation of the collateral vessels.
3.3 Response of Resistance Vessels in the Collateral Zone During Exercise
Mean myocardial blood flow in the collateral-dependent region was ∼75% of normal zone blood flow during control exercise. This disparity in blood flow was not unexpected because the study was performed relatively early during collateral vessel development. However, our findings suggest that impaired vasodilation of the small vessels in the collateral region also contributed to the observed hypoperfusion because distal coronary pressure failed to decrease below its rest value during exercise. This finding suggests that metabolic vasodilation in the collateral region was impaired during exercise and is in agreement with previous studies that have demonstrated reduced receptor-mediated, endothelium-dependent vasodilation in the microvessels of collateral-dependent myocardium .
LNNA caused a significant increase in the rate–pressure product during exercise, implying increased myocardial oxygen demands. An increase in metabolic demands, in conjunction with the collateral constriction produced by LNNA would be expected to cause metabolic vasodilation of the resistance vessels in the collateral zone. However, this did not occur; small vessel resistance was actually 21 ± 10% higher after LNNA than during control exercise. Because heart rate and left ventricular diastolic pressures were unchanged from control exercise, the decrease in collateral zone blood flow during exercise after LNNA most likely reflects impaired NO production rather than metabolic or hemodynamic influences on myocardial perfusion. In addition to a direct coronary vasodilator effect during exercise, NO has been shown to contribute to beta-adrenergic vasodilation of coronary resistance vessels . Blocking NO production could thus impair beta-adrenergic vasodilation resulting from sympathetic venous system activation during exercise. Furthermore, impaired beta-adrenergic vasodilation could unmask alpha-adrenergic vasoconstriction in the coronary resistance vessels during exercise.
3.4 Effect of NO Inhibition on Blood Flow to Normal Myocardium During Exercise
Inhibition of NO production by LNNA resulted in a significant decrease in normal zone flow during exercise. This finding was unexpected because previous studies have failed to demonstrate an obligatory role for NO production in maintaining coronary blood flow during exercise in the normal heart . The results suggest that inhibition of NO production in collateralized hearts unmasks an impairment of resistance vessel dilation during exercise in the remote normal region. It is possible that repetitive episodes of ischemia in the LAD territory during collateral development could induce changes in the resistance vessels of the normal region. Lust et al. observed that vasodilator responses to acetylcholine and adenosine were significantly reduced in LAD epicardial artery segments 3 h after circumflex occlusion, whereas the responses to endothelium-independent vasodilators were unaltered. Further support for the influence of regional ischemia on remote regions was demonstrated by Przyklenk et al. in preconditioning experiments. They observed that brief periods of ischemia in one territory conferred protection in remote regions during a subsequent occlusion. The mechanism by which regional ischemia can influence remote regions is not known, but might involve the production of catecholamines or heightened sympathetic tone . Because NO has been demonstrated to contribute to beta-adrenergic vasodilation of coronary resistance vessels , it is possible that the increase in sympathetic activation during exercise coupled with impaired beta-adrenergic receptor-mediated vasodilation after LNNA could unmask alpha-adrenergic vasoconstriction, thereby causing a reduction in myocardial blood flow in the normal region.
Although the microvessels in the normal region are the major determinant of coronary vascular resistance, it is possible that the contribution of the conduit vessels to total coronary resistance is increased in this model. After occlusion of the proximal LAD, the dependent myocardium derives its blood supply principally from collateral vessels originating from the circumflex and septal arteries, with a small contribution from the right coronary artery . Consequently, the circumflex and septal arteries must conduct sufficient additional blood flow to maintain perfusion of the collateralized region. Longhurst et al. measured myocardial blood flow during exercise in dogs 2 months after placement of an ameroid constrictor on the left circumflex artery. During moderately heavy exercise, the increases in both normal zone and collateral zone blood flow were significantly less in the ameroid group than in a sham-operated control group. They concluded that the limited conductance of the conduit vessels supplying the left ventricle impaired blood flow to the normal region during exercise. Because our animals were studied at an earlier phase of collateral development that resulted in lower flow rates in the collateral zone during exercise, it is less likely that inadequate epicardial artery conductance would have limited blood flow to the normal region.
Recent evidence indicates that chronic elevations of blood flow can increase NOS gene expression . In our model, the increased flow rates in the epicardial arteries supplying the collateral vessels might be expected to cause enhanced production of NO. Increased dependence on NO production could then augment the vasoconstriction that occurred when NO production was inhibited. Miller et al. found that chronic increases in blood flow produced by a femoral arteriovenous fistula increased both tonic and receptor-mediated synthesis of NO in vessel rings. Furthermore, inhibition of NOS with LNMMA resulted in enhanced vasoconstriction in vessels with increased flow compared with that in sham-operated control vessels. If these findings can be extended to the coronary circulation, then the increased flow in the proximal arteries from which the collateral vessels arise could enhance NO production, thereby augmenting the increase in resistance after LNNA.
An increased pressure drop across the conduit vessels might encroach on the lower limit of autoregulation of the distal vessels. Smith and Canty reported that NO contributes to coronary autoregulation in normal awake dogs. They determined that the lower limit of autoregulation in normal dogs was 45 mm Hg, whereas after inhibition of NOS with l-NAME, the lower autoregulatory break point increased to 61 mm Hg. It is likely that this break point would be even higher during exercise , thereby increasing the likelihood that perfusion pressure could fall below the autoregulatory plateau. Thus, the combination of an increased pressure drop across the proximal arteries and alterations in autoregulation of the resistance vessels might account for the reduction of myocardial blood flow in the normal region during exercise after LNNA.
3.5 Clinical Implications
A recent study using positron emission tomography in patients with single-vessel coronary artery disease documented impaired vasodilator responses in remote regions of normal myocardium . The investigators found that myocardial blood flow at rest in the collateral-dependent region was reduced compared with that in a group of patients without coronary artery disease. Unexpectedly, rest blood flow in the remote “normal” region was also reduced, and the increases in blood flow in response to pacing and dipyridamole were significantly impaired. Although endothelial dysfunction can precede the appearance of angiographically evident atherosclerotic disease and could have contributed to the impaired microvascular responses in that study, our results demonstrate that coronary occlusion can cause impairment of vasodilation in response to exercise in remote myocardial regions, even in the absence of atherosclerotic disease.
NO production contributes to the maintenance of coronary collateral blood flow during exercise in the dog. In contrast to the normal heart, endogenous NO is also important in maintaining blood flow to regions of normally perfused myocardium during exercise in this model of single-vessel coronary artery occlusion. This finding suggests that control of vasomotor tone in normal myocardium is altered by the presence of an occluded coronary artery with a region of collateral-dependent myocardium.
We gratefully acknowledge the expert technical assistance provided by Todd Pavek, Melanie Crampton and Paul Lindstrom. Secretarial assistance was provided by Carol Quirt.
☆ This work was supported by U.S. Public Health Service Grants HL32427, HL20598 and HL21872 from the National Heart, Lung, and Blood Institute, National Institutes of Health, Betheda, Maryland and an individual National Research Service Award HL09128 (Dr. Traverse).
- subendocardial blood flow
- subepicardial blood flow
- left anterior descending coronary artery
- N-nitro-l-arginine methyl ester
- l-monomethyl arginine
- nitric oxide
- nitric oxide synthase
- Received March 6, 1997.
- Revision received September 8, 1997.
- Accepted September 29, 1997.
- The American College of Cardiology
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