Coronary vascular remodeling and coronary resistance during chronic ischemia

Our previous observations that minimal coronary resistance (MCR) decreases by 60% in a model of chronic ischemia suggest that angiogenesis and vascular remodeling occurred. To test this hypothesis we conducted quantitative morphometry on the arteriolar (ART) and capillary (CAP) beds of 14 pigs subjected to chronic ischemia. We induced chronic ischemia by ameroid occlusion of the left circumflex coronary artery for 2 to 8 weeks. We measured numerical densities (ND) and total cross sectional areas (CSA) of the ART and CAP in the ischemic regions. In the same pigs minimal coronary resistance (MCR) was measured during adenosine infusion, using a constant pressure coronary perfusion pump. In 8 other pigs we gave tritiated thymidine to determine the extent of DNA synthesis in smooth muscle and endothelial cells. At autopsy we injected colored silicone into the vessels of these pigs so we could evaluate the coronary collateral vessels as well as the other arterioles. After 3 weeks of ameroid occlusion, ART ND increased 45% above control, while ART CSA increased 21% above control. After 8 weeks of ameroid occlusion, there were further significant increases in ART CSA, but not in their ND. These changes corresponded to a decrease in MCR to 35% of control after 3 weeks of ameroid occlusion. After 8 weeks of ameroid occlusion, MCR had increased to 49% of control, however there was a further increase in ART CSA to 58% above control. DNA synthesis was occurring since endothelial and smooth muscle cells had total DNA labeling indexes of 1.2% (compared to 0.01% for controls) 2-5 days after ameroid occlusion, but were near control levels by 8 weeks. Many arterioles showed endothelial cell denuding and medial damage. Also capillaries showed degenerative changes and new sprouts. Silicone casts of the left circumflex bed vessels showed increased volumes averaging 55% more than controls. These data imply that angiogenesis is partly responsible for the decreased minimal coronary resistance seen in chronic ischemia. The loss of wall integrity in old arterioles and increased compliance in newly formed arterioles and capillaries also may contribute to the reduced resistance. Newly formed vessels are a significant portion of the total number of vessels. The combined effect of these changes is a reduced minimal coronary resistance that is modulated by 8 weeks. Measurements of coronary collateral growth paralleled the changes in the ischemic bed suggesting that angiogenesis factors controlling growth in both the bed at risk and its periphery are controlled by similar temporal events.