Coronary collateral development relies on the coordinated secretion of growth factors. However, alone they are insufficient for permanent collateral growth. We utilized proteomics to identify other important proteins in the extracellular environment that could facilitate collateralization. Chronically instrumented dogs developed coronary collaterals by the repetitive occlusion method. Subendocardial (0.19 Ϯ 0.04, 0.27 Ϯ 0.06, 0.48 Ϯ 0.10, and 0.81 Ϯ 0.11 ml ⅐ min Ϫ1 ⅐ g Ϫ1 on days 1, 7, 14, and

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... 21, respectively) and subepicardial (0.14 Ϯ 0.01, 0.36 Ϯ 0.06, 0.51 Ϯ 0.07, and 0.71 Ϯ 0.08 ml ⅐ min Ϫ1 ⅐ g Ϫ1 on days 1, 7, 14, and 21, respectively) blood flow increased in animals subjected to repetitive occlusion. Sham animals exhibited no changes in blood flow. Myocardial interstitial fluid (MIF) from both groups was analyzed by two-dimensional electrophoresis with matrix-assisted laser desorption/ionization time-of-flight identification. The acute-phase protein haptoglobin was identified in the group subjected to repetitive occlusion. ELISA of MIF showed haptoglobin to be elevated at all time points of collateral development compared with sham, with maximal production on day 7. Purified haptoglobin dose dependently stimulated endothelial cells to form tubes and vascular smooth muscle cells to migrate. Purified haptoglobin did not stimulate proliferation of either cell type. The relative contribution of haptoglobin to the chemotactic properties of MIF was tested using a neutralizing antibody. Neutralized MIF could not stimulate smooth muscle cells to migrate at any time during collateral development. Endothelial cell tube formation was inhibited after the midpoint of collateralization. Therefore, the acute-phase protein haptoglobin plays a critical role during coronary collateralization.