Cardiovascular Adaptation in Response to Chronic Hypoxia in Awake Rats

Hypoxia induces de novo formation of cerebral collaterals and lessens the severity of ischemic stroke

Pial collaterals provide protection in stroke. Evidence suggests their formation late during gestation (collaterogenesis) is driven by reduced oxygen levels in the cerebral watersheds. The purpose of this study was to determine if collaterogenesis can be re-activated in the adult to induce formation of additional collaterals ("neo-collateral formation", NCF). Mice were gradually acclimated to reduced inspired oxygen (FIO₂) and maintained at 12, 10, 8.5 or 7% for two-to-eight weeks. Hypoxemia induced "dose"-dependent NCF and remodeling of native collaterals, and decreased infarct volume after permanent MCA occlusion. In contrast, no formation occurred of addition collateral-like intra-tree anastomoses, PComs, or branches within the MCA tree. Hypoxic NCF, remodeling and infarct protection were durable, i.e. retained for at least six weeks after return to normoxia. Hypoxia increased expression of Hif2α, Vegfa, Rabep2, Angpt2, Tie2 and Cxcr4. Neo-collateral formation was abolished in mice lacking Rabep2, a novel gene involved in VEGFA→Flk1 signaling and required for formation of collaterals during development, and inhibited by knockdown of Vegfa, Flk1 and Cxcr4. Rabep2-dependent NCF was also induced by permanent MCA occlusion. This is the first report that hypoxia induces new pial collaterals to form. Hypoxia- and occlusion-induced neo-collateral formation provide models to study collaterogenesis in the adult.

Intravital Observation of Microvascular Remodeling During Chronic Exposure to Hypoxia in Mice

It is well known that chronic hypoxia elevates hematocrit levels to maintain oxygen supply to tissues. Although such high hematocrit levels might lead to hypertension due to an increase in blood viscosity, the morbidity rate of hypertension is reportedly lower in populations residing at high altitudes. The present study aimed to clarify how chronic hypoxia affects the cardiovascular system by direct observation of the microcirculation. Mouse dorsal skin chamber was used to observe arteriolar responses and capillary angiogenesis during 1-week exposure to hypoxia. Furthermore, total peripheral vascular resistance (TPR) was evaluated by measuring blood pressure (BP) and blood flow (BF) in rat carotid arteries before and after 1-week exposure to hypoxia. After 1-week exposure to hypoxia, TPR showed no significant difference compared with normoxic conditions. Observation of dorsal skin microcirculation after 1-week exposure to hypoxia, showed that the arteriolar diameter increased by 29% and the vascular area expanded by 37% compared with measures before hypoxia. These results suggest that the effects of high blood viscosity on TPR would be modified by inducing microvascular remodeling.