Hypoxia-induced angiogenesis in chick chorioallantoic membranes: a role for adenosine

The effect of a chronic hypoxic stimulus and of altered adenosine metabolism on vascular density was studied in the chick chorioallantoic membrane (CAM). Eggs were incubated in 15% oxygen/85% nitrogen for 3 or 7 days beginning at 7 days of age. Vessel density of the CAM was estimated by counting the number of vessels intersecting 4 concentric circles (72 mm total circumference) placed over the formalin-fixed membrane. The 15% oxygen stimulated 34-41% increases (P less than 0.001) in CAM vascularity after 3 or 7 days. Nitrobenzylthioinosine (NBTI), an adenosine re-uptake inhibitor, augmented the hypoxia-induced angiogenesis an additional 17% and 14% (P less than 0.001) at Days 10 and 14, respectively. Methyl-isobutylxanthine (MIX), an adenosine receptor blocker, reduced the vasoproliferation by 66% (P less than 0.001) at both times. Topically suffused adenosine elicited a local concentration-related increase in vascularity. This response was completely blocked by MIX. Exposure to 15% oxygen for 7 days stimulated a 13.7% increase in the hematocrit (P less than 0.001). Embryo weights were reduced 20.7% (P less than 0.001). These findings point to a modulatory role for adenosine in hypoxia-induced angiogenesis, and support the broader hypothesis that vasoactive metabolites produced in response to hypoxic conditions partially mediate a structurally based long-term autoregulatory response.