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Abstract

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. References
  7. Acknowledgements

Myocardial perfusion and contraction are closely coupled; however, the effect of recurrent no-flow ischaemia on perfusion–contraction matching remains to be established. In the present studies, we examined the influence of modulating nitric oxide availability on perfusion–contraction matching after recurrent no-flow ischaemia in acute open-chest, anaesthetized dogs. The following three groups were studied: (1) saline; (2) l-NAME (10 mg kg−1i.v.); and (3) enalaprilat (1.5 mg kg−1i.v.). Regional myocardial blood flow was measured with microspheres and contractile function with piezoelectric crystals to determine systolic wall thickening. Dogs underwent four cycles of 5 min acute ischaemia and 5 min coronary reperfusion; area at risk was similar for all groups. In all dogs, ischaemic zone contractile function was depressed after recurrent no-flow ischaemia despite increased myocardial blood flow during reperfusion; contractile function was further depressed during l-NAME and was partly restored with enalaprilat. Within the ischaemic region, blood flow in subendocardial and subepicardial layers increased significantly compared with baseline during each reperfusion period independently of treatment. Our findings suggest that reduced NO availability can significantly impair myocardial perfusion–contraction matching, which is partly restored by administration of an NO donor.

Coronary vasoregulation, myocyte structure and function can be modulated by a single ischaemic episode (DeBoer et al. 1980; Reimer et al. 1981a,b; Glower et al. 1988). Ventricular contractile dysfunction, decreased energy stores and tissue necrosis are also known consequences of recurrent ischaemia (Nicklas et al. 1985; Hoffmeister et al. 1986; Gall et al. 1993) and often occur in patients with coronary artery disease and in those undergoing cardiac interventions. Interestingly, recurrent ischaemia prior to a prolonged ischaemic event, commonly referred to as ischaemic preconditioning, has been documented to markedly lessen tissue injury in both animal (Murry et al. 1986; Kloner & Jennings, 2001a,b) and human studies (Deutsch et al. 1990; Cribier et al. 1992).

Repetitive but reversible myocardial ischaemia produces cumulative cardiac dysfunction (Gall et al. 1993) that worsens even with adequate restoration of regional blood flow to affected tissues. A number of studies document a close relation between regional blood flow and contractile function (Gallagher et al. 1984; Guth et al. 1990; Heusch, 2008). As such, the concept of perfusion–contraction matching was proposed (Ross, 1991). Diverse pharmacological interventions can affect perfusion–contraction matching (Matsuzaki et al. 1985; Guth et al. 1986; Indolfi et al. 1989; Heusch et al. 2008). Inhibition of nitric oxide synthesis has been reported to abolish perfusion–contraction matching (Kudej et al. 2000); however, overall effects on postischaemic myocardial stunning remain controversial (Duncker et al. 1999; Kudej et al. 2000). We hypothesized that decreased NO availability (blockade of NO synthase by l-NAME) could worsen myocardial perfusion–contraction matching in anaesthetized, open-chest dogs subjected to recurrent no-flow ischaemia. Particular emphasis was focused on restoration of perfusion and contraction during the reperfusion phase.

Inhibition of bradykinin breakdown by angiotensin-converting enzyme inhibitors has also been shown to protect myocardium against ischaemic injury (Richard et al. 1993; Zanzinger et al. 1994; Houel et al. 1997; Nakai et al. 1999; Ferrari, 2004); an indirect beneficial effect of treatment with these agents is enhanced NO bioavailability via bradykinin-mediated activation of NO–cyclic guanosine monophosphate (Pelc et al. 1991). We also hypothesized that increased NO availability resulting from enalaprilat treatment might improve perfusion–contraction matching.

Methods

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. References
  7. Acknowledgements

Dogs were treated in compliance with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH publication 85-23, revised 1996); Laval University is compliant with these guidelines (A5012-01). The experimental protocol was approved by the Laval University Animal Ethics Committee.

Surgical preparation

Adult mongrel dogs of either sex (weight 20–25 kg) were premedicated with diazepam (1 mg kg−1i.v., Sandoz Canada Inc., Montréal, Qc, Canada) and fentanyl (20 μg kg−1i.v., Sandoz Canada Inc., Montréal, Qc, Canada) and then anaesthetized using sodium pentobarbital (Vetoquinol, 30 mg kg−1i.v., Lavaltrie, Qc, Canada). Dogs were intubated and mechanically ventilated with oxygen-enriched room air; respiratory rate and tidal volume were adjusted to maintain blood gases within physiological values. A splenectomy was performed through a mid-line abdominal incision to prevent significant alterations in blood volume and haematocrit during the experiment (Sato et al. 1995). Normothermia (38 ± 1°C) was maintained with a water-jacketed Micro-Temp heating blanket (Zimmer, Dover, OH, USA); core body temperature was continuously monitored with a thermal probe positioned via the endotracheal tube, and saline was given to replace fluid loss.

In the supine position, vascular introducer sheaths (2.7 mm; Terumo Medical Corp., USA) were placed in the left and right femoral arteries; a triple-lumen central venous catheter (2.3 mm, Arrow-Howes™; Arrow International Inc., Reading, PA, USA) was placed in the right femoral vein for administration of drugs and fluids. A left lateral thoracotomy was performed in the fifth intercostal space and the heart suspended in a pericardial cradle. Polyethylene catheters (2.3 mm) were inserted into the internal thoracic artery and left atrium through the atrial appendage. Left ventricular (LV) pressure and its first derivative were measured with a 1.7 mm microtipped pressure transducer (Millar) in the LV cavity (via the apex); a 2.3 mm pigtail catheter was advanced to the aortic root via the left femoral artery. Approximately 2 cm of the left circumflex coronary artery was dissected free of surrounding tissue distal to the first marginal branch; care was exercised during dissection of this vessel to prevent undue injury to nerves within the adventitia. An electromagnetic flow probe (Transonic Systems Inc., Ithaca, NY, USA) was positioned around the vessel; space was reserved for positioning of a vascular clamp (Schwartz micro serrefine; Fine Science Tools, Vancouver, BC, Canada) for coronary artery occlusions. Circumflex artery pressure distal to the coronary flow probe was measured with a fluid-filled catheter using a modification of the Herd and Barger technique (Burattini et al. 1985); catheter patency was frequently checked by venting to the atmosphere. Piezoelectric ultrasonic dimension crystals (Triton Technologies, San Diego, CA, USA) were implanted in the LV myocardium to measure wall thickening of the anterior (non-ischaemic region) and posterior wall (ischaemic region; Gallagher et al. 1984); all contractile function measurements were performed at end-expiration (because contraction varies with the respiratory cycle). To prevent myocardial cooling, the chest cavity was covered with plastic film.

Experimental protocol

Dogs were divided into the following three groups: saline (n = 9); l-NAME (10 mg kg−1i.v., n = 8, Sigma-Aldrich Canada Ltd, Oakville, ON, Canada); and enalprilat (Vasotec, 5 mg kg−1i.v., n = 5, Merck Frosst Canada Ltd., Kirkland, Qc, Canada). A 30 min stabilization period was allowed before experiments were initiated. All dogs were subjected to four cycles of 5 min total coronary occlusion (i.e. no-flow ischaemia) followed by 5 min reperfusion. The l-NAME and enalaprilat were administered 20 min prior to onset of the first period of ischaemia using previously established dosages (Sudhir et al. 1993; Rouleau et al. 2002).

Haemodynamic measurements

Heart rate (HR), aortic and ventricular pressures, coronary artery pressure and coronary blood flow were recorded continuously during the experiments. Phasic arterial pressures were measured with Gould P23XL pressure transducers (Cleveland, OH, USA) positioned at mid-chest level; pressure transducers were calibrated to the fluid-filled pigtail catheter. Coronary blood flow (phasic and mean signals) was measured using a Doppler flowmeter system (T206 Dual Channel Flowmeter; Transonic Systems Inc.). Analog data were continuously recorded on a 12-channel direct-writing oscillograph (Yokogawa OR1200A; Electro-Meters, Dorval, QC, Canada).

Myocardial blood flow distribution

Regional myocardial blood flow was assessed using 15 μm microspheres suspended in saline (NEN, Boston, MA, USA) and labelled with 113Sn, 85Sr, 95Nb, 46Sc, 141Ce, 51Cr or 114In. Microspheres were agitated with a vortex agitator before injection into the left atrium and flushed with 15 ml warmed saline. Reference arterial blood was withdrawn from the internal thoracic artery at a rate of 7.5 ml min−1 beginning 10 s before microsphere injection and continuing for 2 min. Myocardial blood flow was evaluated at baseline, at 2 min of the first ischaemic period (IS1) and at 2 min of each reperfusion (RP) period. Regional blood flow is presented as flow per minute per gram. The relation between flow per beat per gram and function has been described for perfusion–contraction matching (Indolfi et al. 1991; Heusch, 2008); therefore, flow was also normalized for heart rate.

Postmortem analysis

At the end of each study, the left circumflex artery was reoccluded, and 10 ml of Monastral blue dye injected into the left atrium to delineate the area at risk. Under deep pentobarbital anaesthesia, cardiac arrest was induced by intra-atrial injection of saturated potassium chloride. A warmed (37°C) 1.5% solution of 2,3,5-triphenyltetrazolium chloride was infused into the ischaemic region via the coronary artery cannula over a 30 min period to identify necrotic myocardium. The heart was then rapidly excised, rinsed in saline, and the atria, large epicardial vessels and right ventricle were removed and discarded. The remaining left ventricle was fixed in 10% buffered formaldehyde for later determination of tissue necrosis and distribution of myocardial blood flow. Hearts were cut parallel to the atrioventricular groove into between five and eight transverse slices from apex to base. The outline of each ventricular slice, the necrotic area and area at risk were traced onto acetates; areas were determined using a digitizing tablet (Summagraphics II Plus, Seymour, CT, USA) interfaced with a personal computer and analysed with Sigma Scan software (SPSS Inc., CA, USA).

For myocardial blood flow analysis, tissue samples from the central portion of myocardium perfused by the left anterior descending (non-ischaemic zone) and circumflex artery beds (ischaemic zone) were subdivided into subendocardial, midmyocardial and subepicardial segments. Myocardial tissue on either side of the ischaemic–non-ischaemic interface was discarded to avoid potential cross-contamination. Tissues were weighed and placed in vials for counting of γ-activity as previously described (Kingma et al. 2000).

Data and statistical analyses

Heart rate, left ventricular systolic (LVPS) and aortic diastolic pressure (PAoD) were determined from the strip-chart recordings. Regional systolic wall thickening (SWT) was calculated as previously described (Gallagher et al. 1984). Coronary vascular conductance was calculated from mean coronary flow and the simultaneous mean arterial blood pressure and is expressed as millilitres per millimetre of mercury per minute. This index was used because it corrects for differences in arterial pressure and is linearly related to changes in blood flow (Dietz et al. 1997). Cardiac haemodynamic parameters were recorded during each microsphere injection. All data were evaluated using ANOVA for repeated measures. When a significant effect was obtained, comparisons within experimental groups were made using Scheffe's post hoc test. Results of representative measures are expressed as means ± 1SD. All statistical procedures were performed using the SAS statistical software package (SAS Inc., Cary, NC, USA). A P value less than 0.05 was used to indicate a significant difference in mean values.

Results

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. References
  7. Acknowledgements

All dogs survived the described experimental protocols. Data reported here were obtained from 22 dogs that entered the study. Anatomical risk zone size averaged 46 ± 5% (expressed as a percentage of LV area) for each experimental group; however, tissue necrosis was not detected.

Haemodynamics

Cardiac haemodynamic data are summarized in Table 1. Myocardial oxygen demand (determined using rate–pressure product) was higher in l-NAME-treated dogs; rate–pressure product decreased during ischaemia in all treatment groups.

Table 1.  Summary of cardiac haemodynamic data
 Heart rate (beats min−1)Left ventricular systolic pressure (mmHg)Diastolic aortic pressure (mmHg)Rate–pressure product) (beats per minute × mmHg × 10−3)
  1. Data are means ± 1SD. Abbreviations: IS1, first coronary occlusion period; and RP1, RP2, RP3 and RP4, first, second, third and fourth reperfusion periods. The P values were obtained from repeated-measures analyses.

Saline (n = 9)
Baseline114 ± 18113 ± 1988 ± 2112.9 ± 3.1
IS1115 ± 21102 ± 1482 ± 1411.7 ± 2.4
RP1112 ± 21110 ± 2286 ± 2012.2 ± 2.8
RP2115 ± 21111 ± 2689 ± 2212.6 ± 3.0
RP3112 ± 23108 ± 2991 ± 2612.0 ± 3.5
RP4110 ± 22104 ± 2785 ± 2311.3 ± 3.2
l-NAME (n = 8)
Baseline136 ± 10112 ± 1692 ± 1215.1 ± 1.9
IS1133 ± 11105 ± 2190 ± 2214.0 ± 2.4
RP1129 ± 9119 ± 19102 ± 1715.2 ± 1.9
RP2130 ± 12119 ± 18102 ± 1415.4 ± 1.9
RP3133 ± 13117 ± 19100 ± 1615.4 ± 1.8
RP4135 ± 13115 ± 18101 ± 1315.3 ± 1.6
Enalaprilat (n = 5)
Baseline132 ± 31100 ± 1084 ± 1013.3 ± 3.7
IS1134 ± 3088 ± 1073 ± 1212.0 ± 3.9
RP1132 ± 3194 ± 779 ± 912.6 ± 3.9
RP2130 ± 3495 ± 580 ± 512.4 ± 3.9
RP3122 ± 2894 ± 778 ± 511.6 ± 3.2
RP4124 ± 2691 ± 974 ± 711.3 ± 3.2
P values
Groups0.1100.1400.0900.070
Interventions0.0010.0010.0030.001
Groups × Interventions0.0030.180.0010.200
Within saline0.0880.050.0080.030
Within l-NAME0.0020.0050.0010.050
Within enalaprilat0.0050.0830.0050.008

Blood flow distribution

Regional blood flow distribution was evaluated in ischaemic and non-ischaemic myocardium at baseline, during the first coronary occlusion and during each reperfusion period (see Table 2). Within the ischaemic zone, myocardial blood flow was markedly reduced during each period of coronary occlusion in all experimental groups (P = n.s. between groups). Upon removal of the vascular clamp on the circumflex artery, reactive hyperaemia responses were observed, and diastolic coronary artery pressure was restored. Blood flow during each reperfusion period was consistently higher (compared with baseline values) in the subendocardial tissue layer and also increased, albeit to a lesser extent, within the subepicardium. In l-NAME-treated dogs, higher observed subendocardial/subepicardial blood flow ratios within the ischaemic zone were consistent with elevated subendocardial blood flow levels. In enalaprilat-treated dogs, lower subendocardial/subepicardial ratios were observed, consistent with uniform increases in blood flow in both subendocardial and subepicardial regions. Non-ischaemic zone subendocardial and subepicardial blood flows were also higher in enalaprilat-treated dogs (compared with either saline or l-NAME). As a result, subendocardial/subepicardial blood flow ratios in non-ischaemic myocardium of this group were lower.

Table 2.  Summary of myocardial blood flow distribution data
 Ischaemic zoneNon-ischaemic zone
EndoEpiEndo/EpiEndoEpiEndo/Epi
  1. Data are means ± 1SD. Abbreviations: Endo and Epi, subendocardial and subepicardial blood flow (in ml min−1g−1); Endo/Epi, subendocardial/subepicardial blood flow ratio; IS1, first coronary occlusion period; and RP1, RP2, RP3 and RP4, first, second, third and fourth reperfusion periods. The P values were obtained from repeated-measures analyses.

Saline (n = 9)
Baseline1.11 ± 0.270.95 ± 0.361.25 ± 0.331.15 ± 0.330.85 ± 0.251.38 ± 0.24
IS10.09 ± 0.040.19 ± 0.110.65 ± 0.321.15 ± 0.320.87 ± 0.301.39 ± 0.28
RP13.54 ± 1.772.25 ± 0.991.82 ± 0.991.11 ± 0.270.82 ± 0.241.41 ± 0.27
RP22.83 ± 0.921.88 ± 0.941.80 ± 0.741.08 ± 0.270.83 ± 0.351.38 ± 0.25
RP32.35 ± 1.171.71 ± 0.751.51 ± 0.601.05 ± 0.230.79 ± 0.231.38 ± 0.25
RP42.63 ± 1.041.69 ± 0.721.64 ± 0.391.04 ± 0.150.73 ± 0.161.47 ± 0.28
l-NAME (n = 8)
Baseline1.30 ± 0.481.09 ± 0.451.22 ± 0.191.16 ± 0.490.99 ± 0.401.17 ± 0.20
IS10.06 ± 0.060.19 ± 0.120.23 ± 0.191.19 ± 0.350.85 ± 0.271.42 ± 0.18
RP15.33 ± 1.172.88 ± 1.012.17 ± 1.111.19 ± 0.240.82 ± 0.171.47 ± 0.22
RP24.57 ± 0.982.19 ± 1.062.48 ± 1.071.12 ± 0.230.80 ± 0.231.44 ± 0.15
RP34.37 ± 1.492.29 ± 0.942.03 ± 0.591.18 ± 0.260.81 ± 0.211.47 ± 0.19
RP44.09 ± 1.062.25 ± 0.681.93 ± 0.621.19 ± 0.310.85 ± 0.211.40 ± 0.20
Enalaprilat (n = 5)
Baseline1.33 ± 0.211.27 ± 0.261.08 ± 0.251.34 ± 0.181.20 ± 0.291.15 ± 0.21
IS10.08 ± 0.040.17 ± 0.150.55 ± 0.351.44 ± 0.301.14 ± 0.141.28 ± 0.24
RP13.43 ± 0.533.70 ± 0.830.95 ± 0.201.34 ± 0.241.10 ± 0.161.23 ± 0.27
RP23.99 ± 0.633.37 ± 1.031.26 ± 0.381.33 ± 0.231.15 ± 0.281.20 ± 0.27
RP34.23 ± 0.903.13 ± 0.731.39 ± 0.361.29 ± 0.211.10 ± 0.251.22 ± 0.29
RP43.71 ± 1.102.94 ± 0.971.28 ± 0.271.17 ± 0.321.03 ± 0.341.20 ± 0.28
P values
Groups0.3850.0850.5200.2660.0800.320
Interventions0.0010.0010.0010.5190.0400.001
Groups × Interventions0.0140.6220.0270.9310.6500.024
Within saline0.0010.0010.0860.8050.6100.950
Within l-NAME0.0010.0010.0010.9140.0260.001
Within enalaprilat0.0010.0010.0590.5570.6250.080

Ischaemic zone coronary vascular conductance during reperfusion was significantly higher in enalaprilat-treated dogs (Fig. 1), but was similar for all treatment groups in non-ischaemic myocardium.

image

Figure 1. Changes in coronary vascular conductance in ischaemic and non-ischaemic zones for dogs treated with saline (circles), l-NAME (squares) or enalaprilat (diamonds) Data are means ± 1SD; *P ≤ 0.01 versus saline-treated group.

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Regional LV function

Regional LV function (expressed as percentage change from baseline values) is illustrated in Fig. 2. Within the ischaemic zone, significant hypokinesia occurred during all coronary occlusion periods. In saline-treated dogs, SWT was restored to near baseline levels during RP1 but progressively decreased to 80% of baseline values by the end of the experiment. In l-NAME-treated dogs, recovery of SWT was significantly less (∼40% of baseline values; P = 0.04 versus saline) during each reperfusion period. For enalaprilat-treated dogs, contractile dysfunction was less pronounced (∼60% of baseline values; P = n.s. versus saline). In non-ischaemic myocardium, SWT did not change relative to baseline conditions for either treatment group.

image

Figure 2. Regional wall thickening in ischaemic and non-ischaemic zones for dogs treated with saline (circles), l-NAME (squares) or enalaprilat (diamonds) Contractile dysfunction is only shown for IS1; however, a similar level of hypokinesia within the ischaemic zone was observed for each ischaemic period. Results are means ± 1SD; *P ≤ 0.04versus saline-treated group.

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Systolic wall thickening for ischaemic and non-ischaemic myocardium versus corresponding subendocardial blood flow per beat is shown in Fig. 3. In the ischaemic zone, flow–function relations, comprising data at baseline and during 5 min ischaemia, were not different for either treatment group. However, during reperfusion, a downward and rightward shift of the flow–function relation was observed. In l-NAME-treated dogs, this relation was further shifted downwards and rightwards (P = 0.001 versus saline); the shift in the flow–function relation was less pronounced for enalaprilat-treated dogs. Thus, while postischaemic blood flow per beat per gram improved markedly in all experimental groups, the recovery of contractile function was lower. In non-ischaemic myocardium, no change in flow–function relations were detected for either experimental group.

image

Figure 3. Relation between subendocardial blood flows normalized to heart rate (in ml beat−1g−1) and systolic wall thickening (SWT) in ischaemic and non-ischaemic zones Subendocardial blood flow per beat per gram allows prediction of wall function independent of changes in heart rate. Data for saline (circles), l-NAME (squares) and enalaprilat (diamonds) treatment groups are shown at baseline (open symbols) and during the first period of ischaemia (crossed symbols); as no statistical differences in either SWT or subendocardial blood flow per beat per gram were detected within groups during each reperfusion period, the data for each group were pooled (closed symbols). Results are means ± 1SD; *P ≤ 0.001 versus saline treatment.

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Discussion

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. References
  7. Acknowledgements

The present study in a canine model of recurrent no-flow ischaemia demonstrates that blockade of NO synthesis with l-NAME significantly lowers regional contractile function, with little effect on coronary vascular conductance. Enalaprilat (indirect NO donor) treatment significantly enhanced coronary vascular conductance, with little effect on regional contractile function. This resulted in improved subendocardial flow–function matching within the ischaemic myocardium.

Our findings indicate that recurrent no-flow ischaemia in the absence of tissue necrosis produces cumulative contractile dysfunction. These data are in agreement with an earlier study in dogs documenting a progressive worsening of postischaemic function with multiple, reversible ischaemic episodes (Gall et al. 1993). The cause of the persistent contractile dysfunction could be related to myocardial creep (Gall et al. 1993), regional depletion of high-energy phosphates, alterations in calcium flux or increased levels of reactive oxygen intermediates (Reimer et al. 1981b; Braunwald & Kloner, 1982; Jolly et al. 1984). However, mechanisms responsible for the impaired myocardial function after recurrent, reversible ischaemia remain to be established. Endogenous NO released from ischaemic myocardium has been documented to protect against ischaemia–reperfusion injury (Hoshida et al. 1995; Pabla et al. 1996; Wang et al. 1997); indeed, upregulation of NO synthesis is postulated to mediate preconditioning protection (Bolli et al. 1997; Qiu et al. 1997; Takano et al. 1998). While NO produces significant recovery of postischaemic cardiac contractile function (Depre et al. 1995; Naseem et al. 1995; Schulz & Wambolt, 1995), it has also been reported to exacerbate tissue injury (Yasmin et al. 1997; Wildhirt et al. 1999) and reduce myocardial contractility (Node et al. 1996). In the study by Node et al. (1996), contractile function, measured by fractional shortening, decreased during coronary occlusion in dogs given the NO synthase blocker, l-NAME. Results of the present study concur with the findings of Node et al. (1996) even though different experimental protocols (low-flow versus recurrent no-flow ischaemia) were used.

Several studies report that transient ischaemia produces persistent regional myocardial dysfunction even after blood flow in the infarct-related artery is restored (Heyndrickx et al. 1978; DeBoer et al. 1980; Vatner, 1980). In the present study during reperfusion, both subendocardial and subepicardial layer blood flow within the ischaemic zone increased substantially for all groups studied; subendocardial/subepicardial blood flow ratios also increased. These data differ from those reported by Node et al. (1996), where l-NAME did not markedly influence subendocardial/subepicardial blood flow ratios either before or after coronary hypoperfusion. After recurrent no-flow ischaemia, we observed reperfusion hyperaemia; in the absence of transmural autoregulation, subendocardial/subepicardial blood flow distribution is directly dependent on coronary perfusion pressure (Rouleau et al. 1979) and effective coronary back pressure in different myocardial layers (Cantin & Rouleau, 1992). Higher blood flow in l-NAME-treated dogs in the present study could also be due to increased diastolic arterial perfusion pressure or level of oxygen demand. In the absence of tissue injury, blood flow and contractile function decrease proportionately along a consistent flow–function relation (Gallagher et al. 1984); variables such as increased heart rate significantly influence this flow–function relation (Heusch, 2008; Heusch et al. 2008). In the perfusion–contraction matching concept proposed by Ross (1991), there is no imbalance between regional supply (blood flow) and demand (contractile function) in the absence of postischaemic tissue injury, but rather a preserved balance between supply and demand that allows myocardium to further adapt to more prolonged myocardial ischaemia (Heusch et al. 2008). This hypothesis may be important to explain how ischaemic preconditioning confers protection against cellular injury. Our data support this concept; however, we also demonstrate that during reperfusion the flow–function relations shift downwards and rightwards (indicative of moderate dysfunction) despite the absence of identifiable macroscopic tissue injury (by tetrazolium staining) or significant variations in heart rate. As such, in the acute setting, contractile function remained depressed after recurrent no-flow ischaemia despite higher blood supply during reperfusion; this relation was further depressed in the presence of NO synthase blockade. While ischaemic zone contractile function was improved in dogs treated with enalaprilat relative to those given l-NAME, the level of recovery was inferior to that obtained in dogs given saline only. Inhibition of bradykinin breakdown (Erdos & Skidgel, 1987), an indirect consequence of angiotensin-converting enzyme inhibition, which has been reported to protect myocardium against ischaemic injury (Pelc et al. 1991; Richard et al. 1993; Kitakaze et al. 1995; Nakai et al. 1999) could explain the beneficial effects observed with enalaprilat. Overall, these data highlight the multifactorial influences of NO on postischaemic myocardial flow–function relations.

Limitations

Normalization of regional blood flow to a single cardiac cycle is often used for evaluation of the relation between myocardial flow and function (Heusch, 2008). When blood flow is normalized to heart rate, the flow–function relation at rest and during exercise is superimposable (Indolfi et al. 1989). Changes in heart rate markedly affect perfusion–contraction matching (Heusch et al. 2008); pharmacological interventions could also affect these relations. Plasma or tissue NO levels were not determined in these studies because blockade of NO synthase using l-NAME at the dosage used markedly affects NO availability in dogs (Kudej et al. 2000); likewise, angiotensin-converting enzyme inhibitors are known to facilitate release of NO and thereby affect myocardial flow and function (Mombouli & Vanhoutte, 1991; Zanzinger et al. 1994).

In conclusion, in an acute canine preparation of recurrent no-flow ischaemia we document a significant myocardial perfusion–function mismatch during reperfusion. Reduced NO availability significantly impaired myocardial perfusion–contraction matching, which can be partly restored by administration of NO donors.

References

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. References
  7. Acknowledgements
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Acknowledgements

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. References
  7. Acknowledgements

The authors thank André Blouin and Lynn Atton for their technical expertise and Guy Noel for preparation of experimental animals. We also thank Serge Simard for assistance with the statistical analyses. These studies were performed with financial support from the Heart and Stroke Foundation of Quebec and Fonds de la Recherche en Santé de Québec.