Temporal patterns of blood flow and nitric oxide synthase expression affect macrophage accumulation and proliferation during collateral growth

Background: The involvement of collateral blood flow/fluid shear stress, nitric oxide (NO), and macrophages during collateral growth (arteriogenesis) is established, but their interplay remains paradoxical. Methods: In order to further elucidate the "fluid shear stress/NO/macrophage" paradox, we investigated the time course of collateral blood flow (using a Doppler flow probe) and NOS expression (immunohistochemistry, Western blot) in growing rat collateral vessels after femoral artery occlusion and their impact on macrophage recruitment and collateral proliferation (immunohistochemistry, angiographies). Results: (values are given as mean +/- standard error of mean) Early after occlusion, collateral blood flow was significantly reduced (pre- 90.0 +/- 4.5 vs. post-occlusion 62.5 +/- 5.9 mu l/min; p < 0.01), and local inducible NOS (iNOS) and endothelial NOS (eNOS) expression were down-regulated (expression in % of non-occluded: eNOS 49.4 +/- 11.8% and iNOS 54.5 +/- 7.9% vs. non-occluded at 12 h after occlusion; p < 0.03). An artificial rise (induced by a peripheral vasodilatation) of the initially decreased collateral blood flow back to pre- occlusion levels reduced collateral macrophage recruitment (macrophages per collateral section: post-42.5 +/- 4.4 vs. artificial pre- occlusion 27.8 +/- 2.0; p < 0.05) and diminished collateral proliferation (proliferative index: post-0.54 +/- 0.02 vs. artificial pre-occlusion 0.19 +/- 0.04; p < 0.001) significantly 72 h after femoral artery occlusion. Conclusions: We propose the following resolution of the "fluid shear stress/NO/macrophage" paradox: Collateral blood flow and NOS expression are initially reduced during arteriogenesis allowing macrophages to accumulate and therewith enhancing collateral proliferation. After homing of macrophages (24 h after occlusion), collateral blood flow and NOS expression recover in order to join the effects of macrophages for restoring blood flow.