Francis C. White

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.

Regional myocardial blood flow during acute myocardial infarction in the conscious dog.

Regional myocardial blood flow was studied in the conscious dog at periods of 5 minutes to 4 days after occlusion of a major branch of the left coronary artery. Dogs were instrumented with aortic electromagnetic flow probes, occlusive cuffs on either the anterior descending or circumflex branch of the left coronary artery, and a left atrial Silastic catheter for injection of microspheres (IS ± 5 &mgr;m) labeled with either 55Sr, 141Ce, or 51Cr. Microspheres were injected into 25 fully conscious dogs during three of the following time periods: control preocclusion and 0.1, 2, 6, 24, or 96 hours postocclusion. In the conscious dog, before occlusion, the endo-cardial half of the left ventricular myocardium received 28% more blood flow than the epicardial half. After sudden occlusion of a coronary artery branch, there was a marked reduction in blood flow as well as an alteration in distribution of blood flow within the ischemic tissue; blood flow was most severely reduced in the subendocardial center of the ischemic region, less so in the epicardial ischemic region, and least reduced in the marginal region of the infarct. Blood flow was increased to the nonischemic tissue. There was no change in this pattern of reduced blood flow by 6 hours postocclusion, but by 24 hours, flow was moderately increased to all areas except the central subendocardial core, and was further increased at 96 hours. Blood flow to the endocardial half of the left ventricular myocardium averaged 63 ml/100 g per min during the control period, was reduced to 12–18 ml/100 g per min at 0.1–6 hours in the ischemic region, increased to 29 ml/100 g per min at 24 hours, and to 48 ml/100 g per min by 96 hours. These findings indicate that there is a reversal of the flow ratio within ischemic myocardium with relative under-perfusion of the endocardial half of the wall, which is not corrected by 4 days. There is a modest increase of collateral blood flow to ischemic tissue by 24 hours and this increase is considerably augmented by % hours, apparently as a result of the growth and enlargement of collateral vessels.

Coronary collateral circulation in the pig: correlation of collateral flow with coronary bed size.

SummaryCoronary bed sizes were measured in pigs. The left anterior descending bed occupies about 31% of the heart, the right coronary bed about 38% of the heart and the left circumflex bed about 31% of the heart is (including atria). The right coronary artery supplies about 81% of the right ventricle and 23% of the left ventricle with its blood supply. The remaining portion of the right ventricle is supplied by the LAD.The collateral circulation was measured in each of the 3 coronary beds using tracer microspheres. Measurements, are in ml/min/100g. Transmural collateral flow in the LC is 5.6, the LAD 1.0, the RC in LV 3.7 and the RC in RV 4.3.Our experiments showed that the coronary bed sizes were relatively uniform in pigs, but that collateral flow was significantly different between the beds.

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.

Myocardial High-Energy Phosphate and Substrate Metabolism in Swine With Moderate Left Ventricular Hypertrophy

BACKGROUND Although left ventricular hypertrophy (LVH) is frequently associated with impaired coronary vasodilator reserve, it is uncertain whether this leads to myocardial ischemia under physiological conditions. The goal of the present study was to determine whether swine with moderate LVH exhibit metabolic evidence of ischemia when myocardial oxygen requirements are increased. METHODS AND RESULTS Myocardial metabolism was evaluated in an open-chest anesthetized preparation at baseline and during dobutamine infusion in 13 adolescent pigs with moderate LVH induced by supravalvular aortic banding and 12 age-matched control pigs. Transmural myocardial blood flow was quantified with radioactive microspheres; the ratio of phosphocreatine to ATP (PCr/ATP) in the anterior LV free wall was measured by 31P-nuclear magnetic resonance; and anterior wall lactate release was quantified from the arterial-coronary venous difference in 14C- or 13C-labeled lactate. In a subset of 5 animals from each group, the metabolic fate of exogenous glucose was determined from the transmyocardial difference in 6-14C-glucose and its metabolites 14C-lactate and 14CO2. Coronary reserve, as assessed by the ratio of blood flow during adenosine infusion to baseline blood flow, was significantly lower in the LVH pigs compared with controls (3.5 +/- 0.4 versus 5.5 +/- 0.4 mL/g.min, P < .05); however, transmural myocardial blood flow was similar in both groups of pigs, both at baseline and with dobutamine stimulation, probably reflecting the higher coronary perfusion pressure in the LVH pigs. At baseline, PCr/ATP tended to be lower in the LVH pigs (P = .09) but decreased similarly with dobutamine infusion in both groups. Isotopically measured anterior wall lactate release did not differ between the groups at baseline, nor did the increase in lactate release differ during dobutamine stimulation. The uptake of glucose, lactate, and free fatty acids did not differ between the groups in the basal state. However, during dobutamine stimulation, glucose uptake was greater in the LVH group (0.84 +/- 0.09 mumol/g.min versus 0.59 +/- 0.08 mumol/g.min, P < .05). In a subset of animals, 14C-glucose was used to assess glucose oxidation. These data showed that the LVH animals had a greater rate of glucose oxidation (0.6 +/- 0.10 versus 0.28 +/- 0.08 mumol/g.min, P < .05) and a greater rate of glucose conversion to lactate (0.20 +/- 0.04 versus 0.09 +/- 0.02 mumol/g.min, P < .05) compared with the control pigs. CONCLUSIONS These results suggest that despite their reduced coronary vasodilator reserve and the absence of a greater rise in myocardial blood flow to compensate for a substantially higher LV double product, pigs with this model of moderate LVH do not exhibit a greater susceptibility to myocardial ischemia during dobutamine stress. However, LVH pigs exhibit significantly greater use of exogenous glucose during dobutamine stress, as evidenced by increases in both glucose oxidation and anaerobic glycolysis.

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

BackgroundCoronary 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 modelof gradual coronary artery occlusion, we investigated the effect of continuous heparin infusion on coronary collateral development. Methods and ResultsWe placed ameroid constrictors on the left circumflex coronary artery of 16 minipigs; the ameroid constrictors completely occluded the left circumflex coronary artery at 10

Regional redistribution of myocardial blood flow after coronary occlusion and reperfusion in the conscious dog

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. ConclusionsThese 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.

Myocardial lactate release during ischemia in swine. Relation to regional blood flow.

Early and late changes in regional myocardial blood flow distribution within the left circumflex coronary arterial bed after occlusion and after occlusion and reperfusion were compared with the extent of myocardial tissue necrosis. Radiolabeled microspheres, 15 micrometer, were used to study regional myocardial blood flow in conscious dogs at 5 minutes, 2 and 6 hours and 1 month after coronary occlusion. Blood flow was measured in conscious dogs whose hearts were reperfused for 72 hours after 2,6 and 24 hours of occlusion. Blood flow was measured in four distinct transmural myocardial zones delineated by dye injections and gross infarct features of the occluded left circumflex coronary bed. After occlusion, myocardial flow was redistributed from deep layers to outer layers, and within 6 hours after occlusion collateral flow was increased to the outer zones in excess of redistributed flow. After reperfusion, blood flow greately increased to regions containing predominantly normal tissue, and flow was redistributed away from the necrotic zones. The indigenous collateral circulation was a major determinant of infarct size in the occluded and reperfused myocardium. The concept of a migrating and narrowing marginal zone is discussed.

VEGF mRNA is Stabilized by Ras and Tyrosine Kinase Oncogenes, as well as by UV Radiation—Evidence for Divergent Stabilization Pathways

To determine the relation between regional myocardial blood flow, contractile function, and myocardial lactate release during mild-to-moderate regional myocardial ischemia, nine open-chest swine were instrumented for measurement of regional myocardial blood flow (microsphere method), contractile function (sonomicrometry), and hemodynamics. L-[1-14C]Lactate or L-[U-13C]lactate was infused intravenously using a primed continuous infusion technique to quantify regional myocardial lactate release. D-[U-13C]glucose or D-[6-14C]glucose was simultaneously infused to determine the contribution of exogenous glucose to lactate release. Graded coronary ischemia (two to three levels) was created in the left anterior descending coronary arterial distribution by mechanically constricting the artery in five animals or by decreasing flow through a cannulated left anterior descending artery in four animals. In all nine animals, subendocardial blood flow was 0.99 +/- 0.21 (ml/min)/g during control and 0.34 +/- 0.14 (ml/min)/g during the most severe grade of underperfusion (p less than 0.001) in the left anterior descending coronary arterial distribution. Regional myocardial lactate release was 0.15 +/- 0.09 and 1.19 +/- 0.75 mumols/ml, respectively (p less than 0.003). A highly significant inverse correlation was observed between subendocardial blood flow and myocardial lactate release during the graded reductions in blood flow (r = -0.71, p less than 0.001). Results from sonomicrometry showed a significant reduction in contractile ventricular function in the anterior wall during the graded reductions in blood flow. The regional arterial-venous glucose difference increased significantly with underperfusion in the left anterior descending coronary arterial distribution, from 0.14 +/- 0.15 to 0.56 +/- 0.37 mumols/ml (p less than 0.003). The contribution of exogenous glucose to lactate release also increased significantly; 0.04 +/- 0.03 mumols/ml of the lactate came from exogenous glucose during control compared with 0.64 +/- 0.59 mumols/ml during the most severe underperfusion (p less than 0.02). A significant positive correlation exists between lactate release and lactate from exogenous glucose during graded underperfusion (r = 0.96, p less than 0.001). In summary, these data demonstrate a close inverse relation between regional myocardial lactate release and regional subendocardial blood flow during graded ischemia.

VEGF mRNA is Reversibly Stabilized by Hypoxia and Persistently Stabilized in VEGF-Overexpressing Human Tumor Cell Lines

Vascular Endothelial Growth Factor (VEGF) is a pivotal endothelial cell mitogen that mediates both normal and pathological angiogenesis. Although expressed at very low levels in cells not undergoing vascularization, VEGF mRNA is transiently upregulated and stabilized by a variety of extracellular stimuli, and is persistently upregulated and stabilized in many human tumor cell lines (White et al., 1995). Here we demonstrate that oncogenic activation of tyrosine protein kinases and Ras proteins induce a 6- to 16-fold increase in the abundance of VEGF mRNA and a 3- to 5-fold increase in the stability of VEGF mRNA, suggesting that persistent activation of signaling pathways induced by these oncoproteins accounts for overexpression of VEGF in a significant fraction of human tumors. In addition to these oncoproteins, ultraviolet (UV) radiation upregulated and stabilized VEGF mRNA 15- and 5-fold, respectively. While the tyrosine kinase inhibitor, genistein, blocked VEGF upregulation by activated tyrosine protein kinases, and the Ras inhibitor, N-Acetyl-S-trans-farnesyl-L-cysteine (AFC), eliminated VEGF expression in cells transformed by v-Ras, neither agent blocked upregulation by hypoxia or UV radiation. These data argue that multiple divergent pathways upregulate and stabilize VEGF mRNA.