Coronary collateral reserve during exercise induced ischemia in swine

Acute and Chronic Exercise in Animal Models

Numerous animal cardiac exercise models using animal subjects have been established to uncover the cardiovascular physiological mechanism of exercise or to determine the effects of exercise on cardiovascular health and disease. In most cases, animal-based cardiovascular exercise modalities include treadmill running, swimming, and voluntary wheel running with a series of intensities, times, and durations. Those used animals include small rodents (e.g., mice and rats) and large animals (e.g., rabbits, dogs, goats, sheep, pigs, and horses). Depending on the research goal, each experimental protocol should also describe whether its respective exercise treatment can produce the anticipated acute or chronic cardiovascular adaptive response. In this chapter, we will briefly describe the most common kinds of animal models of acute and chronic cardiovascular exercises that are currently being conducted and are likely to be chosen in the near future. Strengths and weakness of animal-based cardiac exercise modalities are also discussed.

Pathophysiology of coronary collaterals

While the existence of structural adaptation of coronary anastomoses is undisputed, the potential of coronary collaterals to be capable of functional adaptation has been questioned. For many years, collateral vessels were thought to be rigid tubes allowing only limited blood flow governed by the pressure gradient across them. This concept was consistent with the notion that although collaterals could provide adequate blood flow to maintain resting levels, they would be unable to increase blood flow sufficiently in situations of increased myocardial oxygen demand.

However, more recent studies have demonstrated the capability of the collateral circulation to deliver sufficient blood flow even during exertion or pharmacologic stress. Moreover, it has been shown that increases in collateral flow could be attributed directly to collateral vasomotion.

This review summarizes the pathophysiology of the coronary collateral circulation, ie the functional adapation of coronary collaterals to acute alterations in the coronary circulation.

Feasibility of subendocardial and subepicardial myocardial perfusion measurements in healthy normals with (15)O-labeled water and positron emission tomography

BACKGROUND

Positron emission tomography (PET) enables robust and reproducible measurements of myocardial blood flow (MBF). However, the relatively limited resolution of PET till recently prohibited distinction between the subendocardial and the subepicardial layers in non-hypertrophied myocardium. Recent developments in hard- and software, however, have enabled to identify a transmural gradient difference in animal experiments. The aim of this study is to determine the feasibility of subendocardial and subepicardial MBF in normal human hearts assessed with (15)O-labeled water PET.

METHODS

Twenty-seven healthy subjects (mean age 41 ± 13 years; 11 men) were studied with (15)O-labeled water PET to quantify resting and hyperaemic (adenosine) MBF at a subendocardial and subepicardial level. In addition, cardiac magnetic resonance imaging was performed to determine left ventricular (LV) volumes and function.

RESULTS

Mean rest MBF was 1.46 ± 0.49 in the subendocardium, and 1.14 ± 0.342 mL · min(-1) · g(-1) in the subepicardium (P < .001). MBF during vasodilation was augmented to a greater extent at the subepicardial level (subendocardium vs subepicardium: 3.88 ± 0.86 vs 4.14 ± 0.88 mL · min(-1) · g(-1), P = .013). The endocardial-to-epicardial MBF ratio decreased significantly during hyperaemia (1.35 ± 0.23 to 1.12 ± 0.20, P < .001). Hyperaemic transmural MBF was inversely correlated with left ventricular end-diastolic volume index (LVEDVI) (r (2) = 0.41, P = .0003), with greater impact however at the subendocardial level.

CONCLUSIONS

(15)O-labeled water PET enables MBF measurements with distinction of the subendocardial and subepicardial layers in the normal human heart and correlates with LVEDVI. This PET technique may prove useful in evaluating patients with signs of ischaemia due to coronary artery disease or microvascular dysfunction.

Temporal and spatial changes in collateral formation and function during chronic myocardial ischemia

BACKGROUND

We investigated time dependence and spatial progression of cardiac function and angiogenesis signaling in a porcine model of chronic myocardial ischemia.

STUDY DESIGN

Yorkshire mini-swine (n = 7/group) were subjected to chronic myocardial ischemia by placing an ameroid constrictor on the left circumflex coronary artery under general anesthesia. Swine were sacrificed after either 4 or 7 weeks of ischemia. Myocardial function, angiographic evidence of angiogenesis, microvessel function, molecular signaling, and levels of apoptosis and oxidative stress were assessed.

RESULTS

Flow reserve was significantly increased at 7 versus 4 weeks. Myocardial function (+dP/dt) improved 1.5-fold by 7 weeks. In the ischemic territory, microvessels at 4 weeks displayed abnormal contraction responses to serotonin, which diminished at 7 weeks. Delta-like ligand 4 protein expression decreased at 7 weeks; expression of vascular endothelial growth factor (VEGF) and phospho-endothelial nitric acid synthase (eNOS) increased. The number of apoptotic cells was decreased at 7 weeks, and antiapoptotic markers heat shock protein (HSP) 27 and HSP 90 were upregulated at 7 weeks. There was an increase in proliferating endothelial cells at 7 weeks as compared with 4 weeks. In the adjacent normal ventricle, microvessels demonstrated smaller contraction responses to endothelin-1 and serotonin at 7 weeks. There was an increase in protein peroxidation in the ischemic territory at 7 weeks.

CONCLUSIONS

Over time, myocardial perfusion, function, and angiogenic signaling improved in the ischemic myocardium and adjacent normal territory compared with what is observed shortly after coronary occlusion.

Regulation of Coronary Blood Flow During Exercise

Exercise is the most important physiological stimulus for increased myocardial oxygen demand. The requirement of exercising muscle for increased blood flow necessitates an increase in cardiac output that results in increases in the three main determinants of myocardial oxygen demand: heart rate, myocardial contractility, and ventricular work. The approximately sixfold increase in oxygen demands of the left ventricle during heavy exercise is met principally by augmenting coronary blood flow (∼5-fold), as hemoglobin concentration and oxygen extraction (which is already 70–80% at rest) increase only modestly in most species. In contrast, in the right ventricle, oxygen extraction is lower at rest and increases substantially during exercise, similar to skeletal muscle, suggesting fundamental differences in blood flow regulation between these two cardiac chambers. The increase in heart rate also increases the relative time spent in systole, thereby increasing the net extravascular compressive forces acting on the microvasculature within the wall of the left ventricle, in particular in its subendocardial layers. Hence, appropriate adjustment of coronary vascular resistance is critical for the cardiac response to exercise. Coronary resistance vessel tone results from the culmination of myriad vasodilator and vasoconstrictors influences, including neurohormones and endothelial and myocardial factors. Unraveling of the integrative mechanisms controlling coronary vasodilation in response to exercise has been difficult, in part due to the redundancies in coronary vasomotor control and differences between animal species. Exercise training is associated with adaptations in the coronary microvasculature including increased arteriolar densities and/or diameters, which provide a morphometric basis for the observed increase in peak coronary blood flow rates in exercise-trained animals. In larger animals trained by treadmill exercise, the formation of new capillaries maintains capillary density at a level commensurate with the degree of exercise-induced physiological myocardial hypertrophy. Nevertheless, training alters the distribution of coronary vascular resistance so that more capillaries are recruited, resulting in an increase in the permeability-surface area product without a change in capillary numerical density. Maintenance of α- and ß-adrenergic tone in the presence of lower circulating catecholamine levels appears to be due to increased receptor responsiveness to adrenergic stimulation. Exercise training also alters local control of coronary resistance vessels. Thus arterioles exhibit increased myogenic tone, likely due to a calcium-dependent protein kinase C signaling-mediated alteration in voltage-gated calcium channel activity in response to stretch. Conversely, training augments endothelium-dependent vasodilation throughout the coronary microcirculation. This enhanced responsiveness appears to result principally from an increased expression of nitric oxide (NO) synthase. Finally, physical conditioning decreases extravascular compressive forces at rest and at comparable levels of exercise, mainly because of a decrease in heart rate. Impedance to coronary inflow due to an epicardial coronary artery stenosis results in marked redistribution of myocardial blood flow during exercise away from the subendocardium towards the subepicardium. However, in contrast to the traditional view that myocardial ischemia causes maximal microvascular dilation, more recent studies have shown that the coronary microvessels retain some degree of vasodilator reserve during exercise-induced ischemia and remain responsive to vasoconstrictor stimuli. These observations have required reassessment of the principal sites of resistance to blood flow in the microcirculation. A significant fraction of resistance is located in small arteries that are outside the metabolic control of the myocardium but are sensitive to shear and nitrovasodilators. The coronary collateral system embodies a dynamic network of interarterial vessels that can undergo both long- and short-term adjustments that can modulate blood flow to the dependent myocardium. Long-term adjustments including recruitment and growth of collateral vessels in response to arterial occlusion are time dependent and determine the maximum blood flow rates available to the collateral-dependent vascular bed during exercise. Rapid short-term adjustments result from active vasomotor activity of the collateral vessels. Mature coronary collateral vessels are responsive to vasodilators such as nitroglycerin and atrial natriuretic peptide, and to vasoconstrictors such as vasopressin, angiotensin II, and the platelet products serotonin and thromboxane A2. During exercise, ß-adrenergic activity and endothelium-derived NO and prostanoids exert vasodilator influences on coronary collateral vessels. Importantly, alterations in collateral vasomotor tone, e.g., by exogenous vasopressin, inhibition of endogenous NO or prostanoid production, or increasing local adenosine production can modify collateral conductance, thereby influencing the blood supply to the dependent myocardium. In addition, vasomotor activity in the resistance vessels of the collateral perfused vascular bed can influence the volume and distribution of blood flow within the collateral zone. Finally, there is evidence that vasomotor control of resistance vessels in the normally perfused regions of collateralized hearts is altered, indicating that the vascular adaptations in hearts with a flow-limiting coronary obstruction occur at a global as well as a regional level. Exercise training does not stimulate growth of coronary collateral vessels in the normal heart. However, if exercise produces ischemia, which would be absent or minimal under resting conditions, there is evidence that collateral growth can be enhanced. In addition to ischemia, the pressure gradient between vascular beds, which is a determinant of the flow rate and therefore the shear stress on the collateral vessel endothelium, may also be important in stimulating growth of collateral vessels.

Angiogenesis during exercise and training

In this review the factors involved in angiogenesis are discussed in their various roles in initiating angiogenesis and inducing changes in the extracellular matrix to facilitate sprouting angiogenesis which is a major part of the angiogenesis seen in exercise and exercise training. A key role in angiogenesis is played by vascular endothelial growth factor (VEGF). The regulation of blood vessel growth to match the needs of the tissue depends on the control of VEGF production through changes in the stability of its mRNA and in its rate of transcription. The detailed studies describing its characteristics and its upregulation in acute exercise are presented along with a brief overview of the changes in the extracellular matrix that facilitate sprouting angiogenesis that occurs in response to exercise and training. Although the mechanisms involved in the growth and remodeling of arterioles and larger vessels are less detailed some recent studies have provided new insights. These are presented here to show a relationship between capillary development and arteriolar growth or remodeling in exercise training that raises questions to be addressed in future studies.

Exercise training in swine promotes growth of arteriolar bed and capillary angiogenesis in heart

Exercise training induces coronary vascular adaptations. The goal of this study was to contrast the effects of training on capillary and arteriolar growth. Minipigs were trained for 1, 3, 8, and 16 wk and compared with controls. Maximal O2 consumption increased continuously throughout the study. Capillary and arteriolar densities and diameters, and proliferation of vascular cells in these vessels, were determined in perfusion-fixed tissue. The arterioles were subdivided into five groups according to diameter: 10-19.9, 20-30, 31-40, 41-70, and 71-120 microgram. The total vascular bed cross-sectional area increased by 37% at 16 wk, mainly because of an increase in the number of the small arterioles and an increase in the diameter of the larger vessels. Capillary density increased at 3 wk and then returned to control levels by 16 wk; concomitantly, the number of arterioles (20-30 microgram) increased at 16 wk. We speculate that the "extra" capillaries observed at 3 wk were the source of the new arterioles.

Differential uptake of myocardial perfusion radiotracers in normal, infarcted, and acutely ischemic peri-infarction myocardium

OBJECTIVES

We measured the uptake of technetium-99m tetrofosmin (99m Tc) and thallium-201 (201 TI) in areas of healed transmural myocardial infarction and in the regions of acute peri-infarction ischemia.

METHODS

Anesthetised pigs with a 1-month old transmural infarction elicited by permanent ligature of the left anterior descending (LAD) coronary artery below the first branch underwent one hour of proximal LAD occlusion followed by injection of 99m Tc-tetrofosmin and 201TI either in the left atrium (GI, n= 19) or in the jugular vein (GII, n = 6). Twelve other pigs (GIII) with similar acute peri-infarction ischemia received 99m Tc-tetrofosmin and 201Tl into the left ventricle during cardiocirculatory arrest to rule out the effect of coronary collaterals. Radiotracer counting was determined in samples from normal, acute ischemic and necrotic regions.

RESULTS

Uptake of 99m Tc-tetrofosmin and 201 Tl was greater in the infarct scar (median % of normal tissue: 20 for 99m Tc and 8.6 for 201 Tl in GI; 22 and 15 in GII) than in acute ischemic myocardium (3.2 and 2.5 in GI; 6.4 and 3.3 in GII). Radiotracer injection in arrested hearts (GIII) depicted a similar pattern (median % of injected dose: 6.2 for 99m Tc and 10 for 201Tl in the scar; 2.3 and 4.0 in acute ischemia; 2.9 and 3.5 in normal tissue). The infarcted region showed connective tissue and lack of viable myocardium.

CONCLUSION

A 1-month old infarct scar with no viable myocardial tissue can take up significant fractions of 99mTc-tetrofosmin and 201Tl even in the absence of coronary collateral perfusion. Data suggest that the infarct scar can extract these radiotracers from the intraventricular blood.

18F-2-deoxyglucose deposition and regional flow in pigs with chronically dysfunctional myocardium. Evidence for transmural variations in chronic hibernating myocardium

BACKGROUND

Hibernating myocardium in patients with collateral-dependent myocardium is characterized by relative reductions in resting flow and increases in the uptake of 18F-2-deoxyglucose (FDG) in the fasting state. We performed the present study to examine whether these key physiological alterations could be produced in a porcine model of chronic coronary occlusion and to assess whether the adaptations consistent with hibernation varied across the myocardial wall.

METHODS AND RESULTS

We chronically instrumented pigs (n = 18) with a fixed occluder on the proximal left anterior descending coronary artery (LAD). Three months later, ventricular function, regional myocardial perfusion, and FDG deposition (by excised tissue counting or positron emission tomography) were assessed in pigs after an over-night fast in the closed-chest anesthetized state. Total LAD occlusion with angiographic collaterals was present in the majority of animals. Left ventriculography showed severe anterior hypokinesis, and resting perfusion was significantly reduced in the hibernating LAD region in comparison with the normal remote regions (subendocardium: 0.80 +/- 0.06 versus 1.07 +/- 0.06 mL.min-1.g-1, P < .001; full-thickness: 0.87 +/- 0.04 versus 0.99 +/- 0.06 mL.min-1.g-1, P < .01). There was a twofold increase in full-thickness fasting FDG uptake in the dysfunctional LAD region (1.8 +/- 0.2 by positron emission tomography versus 1.9 +/- 0.1 by ex vivo counting). Ex vivo tissue counting revealed a pronounced transmural variation in FDG uptake in the hibernating region (LAD/normal), which averaged 2.5 +/- 0.2 in the subendocardium, 1.9 +/- 0.2 in the midmyocardium, and 1.4 +/- 0.1 in the subepicardium.

CONCLUSIONS

These results demonstrate that pigs instrumented with a proximal LAD stenosis develop hibernating myocardium characterized by relative reductions in resting function and perfusion in association with increased uptake of FDG in the fasting state. The transmural variations in relative resting flow and FDG uptake suggest that myocardial adaptations consistent with hibernation are most pronounced in the subendocardial layers and vary in relation to local coronary flow reserve.

Inotropic reserve and histological appearance of hibernating myocardium in conscious pigs with ameroid-induced coronary stenosis

Inotropic reserve, demonstrated with administration of sympathomimetic amines, is characteristic of hibernating myocardium. The goal of this study was to determine whether inotropic reserve was present following chronic coronary artery constriction in the pig, which is one potential model of hibernating myocardium. The effects of isoproterenol were examined in five conscious pigs 21 +/- 2.1 days after ameroid implantation on the left circumflex coronary artery on measurements of left ventricular (LV) pressure, LV dP/dt, and regional wall thickening in the ameroid-dependent zone (posterior wall) and contralateral non-ischemic zone (anterior wall). Isoproterenol, 0.1 microgram/kg/min, increased LV dP/dt by 96 +/- 11%, heart rate by 43 +/- 13 beats/min, and normalized systolic wall thickening, slightly, but not significantly more in the ameroid-dependent zone (+1.57 +/- 0.31 mm) than in the contralateral non-ischemic zone (+1.04 +/- 0.31 mm), although the baseline wall thickening was reduced significantly in the ameroid-dependent zone. This occurred at a time when baseline myocardial blood flow was preserved and myocardial perfusion in the ameroid-dependent zone was derived in part from the native coronary circulation and also through collateral channels. Two weeks later histological evidence of lesions characteristic of hibernating myocardium, i.e., myofibrolysis and increased glycogen deposition, were observed. Thus, these histological changes and the confluence of chronically depressed regional function and residual inotropic reserve in the conscious pig with chronic ameroid-induced coronary constriction support this model for further study of hibernating myocardium.

Mechanism of impaired myocardial function during progressive coronary stenosis in conscious pigs. Hibernation versus stunning?

The major goal of this study was to determine whether impaired myocardial contractile function during the development of progressive coronary artery stenosis induced by ameroid constriction in conscious pigs reflected myocardial "hibernation" or "stunning." Minipigs were instrumented with a coronary ameroid constrictor and hydraulic occluder, regional wall thickness crystals, a left ventricular (LV) pressure gauge, and aortic and left atrial catheters. In the seven pigs in which it was measured, systolic wall thickening (WT) distal to the ameroid fell by a maximum of 56 +/- 6% at 20 +/- 3 days after ameroid implantation and then began to recover. At 1 day after ameroid implantation, brief complete coronary artery occlusion (CAO) resulted in wall thinning distal to the ameroid (-113 +/- 4%) and transmural decreases in myocardial blood flow in endocardial (from 0.82 +/- 0.08 to 0.02 +/- 0.01 mL/min per gram) and epicardial (from 0.73 +/- 0.13 to 0.03 +/- 0.02 mL/min per gram) layers. At 20 +/- 3 days, baseline myocardial blood flow was not altered either in endocardial (0.92 +/- 0.10 mL/min per gram) or epicardial (0.85 +/- 0.12 mL/min per gram) layers, whereas brief complete coronary artery occlusion still reduced WT (-83 +/- 12%) and myocardial blood flow in endocardial (to 0.21 +/- 0.03 mL/min per gram) and epicardial (to 0.43 +/- 0.12 mL/min per gram) layers, indicating that the coronary artery was not totally occluded. Pathology in four pigs demonstrated no gross necrotic myocardium shortly after this time point. Transient reductions in WT distal to the ameroid were observed during progressive coronary artery stenosis in response to spontaneous increases in activity. Beat-by-beat analysis of these episodes revealed that acute reductions in WT followed increases in LV dP/dt and heart rate and exhibited delayed recovery. These data suggest that the reduced function during ameroid-induced coronary stenosis reflected cumulative myocardial stunning rather than a primary deficit in coronary blood flow or "hibernating myocardium."

Basic fibroblast growth factor improves myocardial function in chronically ischemic porcine hearts

The effect of basic fibroblast growth factor (bFGF) administration on regional myocardial function and blood flow in chronically ischemic hearts was studied in 26 pigs instrumented with proximal circumflex coronary artery (LCX) ameroid constrictors. In 13 animals bFGF was administered extraluminally to the proximal left anterior descending (LAD) and LCX arteries with heparin-alginate beads and 13 other animal served as controls. bFGF-treated pigs showed a fourfold reduction in left ventricular infarct size compared to untreated controls (infarct size: 1.2 +/- 0.4% vs. 5.1 +/- 1.3% of LV mass, mean +/- SEM, P < 0.05). Percent fractional shortening (% FS) in the LCX area at rest was reduced compared with the LAD region in both bFGF and control pigs. However, there was better recovery in the LCX area after rapid pacing in bFGF-treated pigs (% FSLCX/% FSLAD, 22.9 +/- 7.3%-->30.5 +/- 8.5%, P < 0.05 vs. prepacing) than in controls (16.0 +/- 7.8%-->14.3 +/- 7.0%, P = NS). Furthermore, LV end-diastolic pressure rise with rapid pacing was less in bFGF-treated than control pigs (pre-pacing; pacing; post-pacing, 10 +/- 1; 17 +/- 3; 11 +/- 1* mmHg vs 10 +/- 1; 24 +/- 4; 15 +/- 1 mmHg, *P < 0.05 vs. control). Coronary blood flow in the LCX territory (normalized for LAD flow) was also better during pacing in bFGF-treated pigs than in controls. Thus, periadventitial administration of bFGF in a gradual coronary occlusion model in pigs results in improvement of coronary flow and reduction in infarct size in the compromised territory as well as in prevention of pacing-induced hemodynamic deterioration.

Ischemia in collateral-dependent myocardium: effects of nifedipine and diltiazem in man

It has recently been shown that ischemia in collateral-dependent myocardium may develop at a very variable threshold in anginal patients; accordingly, the aim of this study was to assess whether nifedipine and diltiazem can increase blood flow to collateralized myocardium in man. Nine patients with complete coronary occlusion filled by collaterals, with no other coronary stenosis, normal left ventricular function, and reproducibly positive exercise tests were studied. They underwent exercise tests off therapy and after acute randomized administration of nifedipine (10 mg sublingually), diltiazem (120 mg orally), and nitroglycerin (0.5 mg sublingually), the latter a drug known to increase blood flow to collateralized myocardium. Following nifedipine, time to 1 mm ST segment depression increased significantly (from 430 +/- 176 to 576 +/- 205 seconds, p < 0.01), while heart rate and rate-pressure product remained unchanged (115 +/- 16 vs 121 +/- 17 beats/min and 199 +/- 29 vs 204 +/- 44 beats/min.mm Hg.10(2), respectively, p = NS for both). Similarly, diltiazem significantly increased time to ischemic threshold from baseline to 638 +/- 125 seconds (p < 0.01), but did not change heart rate and rate-pressure product at 1 mm ST segment depression. Submaximal rate-pressure products were significantly lowered by both nifedipine and diltiazem. Nitroglycerin not only significantly improved time to ischemic threshold (from baseline to 666 +/- 76 seconds, p < 0.01), but also increased heart rate (from baseline to 137 +/- 16 beats/min, p < 0.01) and rate-pressure product (from baseline to 242 +/- 48 beats/min.mm Hg.10(2), p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Heparin accelerates coronary collateral development in a porcine model of coronary artery occlusion

BACKGROUND

Coronary collaterals develop in response to an ischemic stimulus. However, collateral growth is not sufficient to result in the complete recovery of coronary reserves. Using a porcine model of gradual coronary artery occlusion, we investigated the effect of continuous heparin infusion on coronary collateral development.

METHODS AND RESULTS

We placed ameroid constrictors on the left circumflex coronary artery of 16 minipigs; the ameroid constrictors completely occluded the left circumflex coronary artery at 10 +/- 1 days. Half of the animals also were instrumented with subcutaneously placed osmotic pumps and catheters that delivered heparin (300 units/h) into the external jugular vein. At 2, 3, and 4 weeks, we assessed blood flow at rest and during vasodilation using radioactive microspheres. Our results indicate that the animals receiving heparin restored resting myocardial blood flow to normal levels at or before 2 weeks; in contrast, we did not see normal resting myocardial blood flow levels in the untreated-ameroid animals until 3 weeks. Under vasodilated conditions, untreated-ameroid animals experienced a severe loss of coronary reserves at 2 weeks. Although this improved with time, these animals still were significantly underperfused at 4 weeks. In contrast, in the heparin-treated animals, coronary reserves returned to near-normal levels between 3 and 4 weeks. In addition, infarct size was significantly smaller in the heparin-treated animals.

CONCLUSIONS

These experiments suggest that the administration of heparin in the early phases of gradual coronary occlusion accelerates the rate of return of normal blood flow under resting conditions, substantially increases the recovery of coronary reserve, and reduces the risk of infarction.

Coronary collateral development in swine after coronary artery occlusion

We have quantified the development of the coronary collateral circulation in the pig. The collateral circulation was induced to grow by placing an ameroid occluder on the left circumflex coronary artery. Two to 16 weeks after ameroid placement, the coronary collateral circulation was identified after the injection of several colors of a silicone polymer into the coronary arteries and the aorta. We identified intercoronary and extracardiac collaterals and quantified their number, location, size, and wall thickness. Intercoronary collaterals grew to a level that represents a 14-fold increase in normal collateral blood flow under resting conditions compared with the values in an animal not subjected to coronary artery occlusion. Extracardiac collaterals could potentially supply approximately 30% of resting flow. The sources of the extracardiac collaterals were the bronchial and internal mammary arteries. Coronary collateral morphometry and DNA synthesis in the pig heart also were examined. Coronary collaterals had significantly less smooth muscle than did normal arterioles. This may account, in part, for the reduced response of the coronary collaterals to vasodilators. We observed intense DNA synthesis in endothelial and smooth muscle cells in the first 2 or 3 weeks of ischemia. However, DNA synthesis rapidly ceased after this time, coincident with coronary collateral reserve values (ischemic/nonischemic regional blood flow ratios during maximal vasodilation) reaching their maximum level. This suggests that failure of the vessels to continue proliferating accounts for the occurrence of the plateau in blood flow levels.

Microvascular pressures and resistances in the left ventricular subepicardium and subendocardium

These experiments tested the hypothesis that differences in the distribution of subepicardial and subendocardial microvascular resistances may alter the transmural distributions of microvascular pressures. Isolated blood- and physiological saline-perfused porcine hearts were surgically incised to enable exposure of the subendocardial and subepicardial microcirculations. Microvascular pressures were measured during cardiac arrest and maximal vasodilation at various perfusion pressures to formulate relations between perfusion pressure and microvascular pressure in the different subendocardial (both free wall and papillary muscle) and subepicardial segments. Measurements of arteriolar and venular pressures in both myocardial regions were performed in comparably sized vessels (80-120 microns in diameter). At a coronary perfusion pressure of 100 mm Hg, subendocardial arteriolar and venular pressures were 60 +/- 4 and 33 +/- 3 mm Hg, respectively. In contrast, at the same coronary perfusion pressure, arteriolar and venular pressures in the subepicardial microcirculation averaged 80 +/- 6 and 22 +/- 3 mm Hg, respectively (p less than 0.05 versus subendocardium). At all levels of coronary perfusion pressure, arteriolar pressures were significantly lower in the subendocardium than in the subepicardium (p less than 0.05). Venular pressures were also higher in the subendocardial microcirculation than in the subepicardial microcirculation at all but the lowest perfusion pressure (p less than 0.05). The relative distribution of resistances in arteries, microvessels, and veins was also different between the subepicardium and subendocardium. Specifically, in the subendocardium, arterial and venous resistances were higher, percentage-wise, but microvascular resistance was proportionately lower than that in the subepicardium (p less than 0.05). From these data, it is concluded that the distribution of microvascular resistances and pressures is different during maximal vasodilation in the subepicardial and subendocardial microcirculations of the left ventricle. It is also speculated that differences in autoregulatory capacity and vulnerability to ischemia may be partially related to unequal distribution of microvascular resistances across the wall of the left ventricle.

The coronary collateral circulation in normal goats

This study was undertaken to evaluate the coronary collateral circulation of the goat. Year old, castrated male goats were anesthetized with sodium pentobarbital. A branch of the left circumflex coronary artery was dissected and a snare placed around it. Regional myocardial blood flow was measured by injecting colored microspheres into the left atrium before and during 3 hr of occlusion of this coronary artery. The Area At Risk for infarction, as defined by a left atrial injection of Brilliant Green dye, was divided into a central Ischemic zone and a peripheral Ischemic Border zone. The degree of overlap in the blood flow distributions between the risk and the nonrisk areas was quantitatively assessed by injecting microspheres directly into the artery to the Area At Risk. Baseline blood flow to the normally perfused goat myocardium was 1.13 +/- 0.18 ml/min/g (mean +/- SE). Following occlusion, the flow to the Ischemic zone was 0.07 +/- 0.03 ml/min/g and to the Ischemic Border zone was 0.31 +/- 0.18 ml/min/g. Flow to either zone did not increase during the 3-hr observation period. Overlap at the perimeter of the risk and nonrisk areas was approximately 22%. We conclude that flow to the Ischemic zone is low because the goat has few native collateral blood vessels, and that flow to the Ischemic Border zone is significantly affected by overlap with normal myocardium.

Effect of long-term exercise on regional myocardial function and coronary collateral development after gradual coronary artery occlusion in pigs

The effect of myocardial ischemia, induced by long-term exercise, on regional myocardial function and coronary collateral development was examined in pigs after gradual occlusion of the left circumflex coronary artery (LCx) with an ameroid occluder. Thirty days after surgery, regional myocardial function and blood flow were assessed during exercise in 22 pigs separated into exercise (n = 12) and sedentary groups (n = 10). The exercise group trained on a treadmill for 25 +/- 1 days, 30-50 min/day, at heart rates of 210-220 beats/min. After 5 weeks, another exercise test was performed. In the exercise group, after training, we observed an improvement in systolic wall thickening, expressed as a percentage of rest, in the collateral-dependent LCx region from 64 +/- 8% to 87 +/- 6% (p less than 0.01) at moderate exercise levels (220 beats/min) and from 45 +/- 7% to 73 +/- 7% (p less than 0.01) at severe exercise levels (265 beats/min). Transmural myocardial blood flow in the LCx region expressed as a ratio of flow in the nonoccluded region of the left ventricle also increased significantly (p less than 0.01) during severe exercise after 5 weeks. The sedentary group showed an improvement in systolic wall thickening in the LCx region during moderate exercise compared with the initial exercise test (p less than 0.05) but no significant change in systolic wall thickening or myocardial blood flow ratios during severe exercise after 5 weeks. We conclude that long-term exercise after gradual LCx coronary artery occlusion in pigs improves myocardial function and coronary collateral reserve in collateral-dependent myocardium during exercise.