Vascular Reactivity in Newly-Formed and Mature Arterialized Collateral Capillaries

Peripheral arterial occlusive disease (PAOD) is a globally-prevalent cardiovascular disease in which atherosclerotic plaques narrow arterial lumen diameters and restrict blood flow to downstream tissues. The impact of these occlusions can be mitigated by collateral vessels that connect parallel arterial branches and act as natural bypasses to maintain perfusion. In animal models that lack collateral arterioles, capillaries that connect terminal arteriolar segments can arterialize and form functional collaterals following an ischemic event; however, in the early stages of development, vasodilation is impaired. We explored the mechanism of impaired vasodilation in arterialized collateral capillaries (ACCs) and pre-existing collaterals (PECs) by evaluating endothelial-dependent vasodilation and endothelial-independent reactivity at day seven following the ischemic event. We also evaluated functional vasodilation in mature ACCs and PECs at day 21 by applying vasodilation inhibitors during the electrical stimulation of muscle contraction. Arterial occlusion was performed by ligating the cranial-lateral spinotrapezius feed artery in Balb/C mice, a strain that either lacks native arteriolar collaterals or contains a single collateral arteriole (~50% of mice), as opposed to the C57Bl/6 strain, which each contain 10 or more collateral arterioles. At seven days post-surgery, both vasodilation and vasoconstriction were impaired in ACCs when compared to terminal arterioles of similar size in unoperated limbs, but still exhibited significant changes when compared to baseline. The comparable reactivity in both endothelial-dependent and independent vasodilation at day-seven in ACCs indicates that vascular smooth muscle cells are likely responsible for the impairment, as they may still be developing, rearranging, or both, and are not yet fully capable of regulating diameter in immature ACCs. However, by 21 days post-ligation, ACCs regained the capacity to dilate in response to muscle contraction, and utilized similar vasodilation pathways as control vessels. At seven days post-ligation, PECs had impaired endothelialindependent dilation, but successful endothelial-dependent dilation, indicating the use of alternative pathways to dilate. Unlike ACCs, the PECs never completely restored vasodilation capabilities by day 21, which may be due to a variation in smooth muscle phenotype, sensitivity to vasoactive agents, and/or limited growth factor expression. For future work, evaluating collateral formation and vasodilation in a diseased model and investigating molecular variations in the smooth muscle may yield additional knowledge that can improve therapies for patients during ischemic events