Mechanoadaptation, arteriogenesis, and vascular reactivity in male and female C57Bl/6 mice
Authors: Cardinal Trevor R , Burckhardt Laura L , Chu Megan T , Krall Amanda M , Gallagher Ryan R , Bynum Alex J 
Published: November 2023

Women have a higher incidence of peripheral arterial occlusive disease (PAOD) and a greater likelihood for developing critical limb ischemia. The importance of the collateral circulation in maintaining tissue blood flow following arterial occlusion suggests that this sexually‐dimorphic response may be due to sex differences in collateral development and function. In experimental animals, females exhibit impaired regional recovery (i.e. limb pressure or resistance) that may not may not require estrogen deficiency. Although the collateral circulation is the primary site of blood flow resistance following arterial occlusion and the likely explanation of any impairment in hindlimb recovery, neither of these conflicting reports directly evaluated collateral enlargement or function. Therefore, we tested the hypothesis that collateral enlargement and reactivity were impaired in female mice.

All experiments were performed in C57Bl/6 mice at day‐7 or day‐28 following femoral artery ligation (FAL), between the epigastric/profunda and popliteal branches. In the stem to the collateral circulation (i.e. the profunda femoris), enlargement does not involve cellular proliferation, so we evaluated mecahnoadaptive outward remodeling by measuring vascular smooth muscle cell length and overlap with immunofluorescence and confocal microscopy. At day‐7 following FAL, vascular smooth muscle cell (VSMC) length increased 25% in both male and female mice. Similarly, VSMC overlap decreased by 30% in both male and female mice‐indicating roughly equivalent mechanoadaptive outward remodeling. At the midzone of the gracilis anterior collateral, inner and outer diameter increased 100% in both males and females, while wall thinkness increased over 200%, indicating similar levels of hypertrophic outward remodeling in both sexes. Given the lack of difference in enlargement between male and female collaterals, we next evaluated reactivity, to determine if female mice exhibit impaired collateral function. As expected, reactivity was substantially impaired at day‐7 following FAL, with collateral midzone diameter increasing 15% during functional vasodilation, as compared to nearly 60% increases in sham‐operated contralateral collaterals; responses were not different between males and females. At day‐28 following FAL, collateral midzone reactivity in males was restored, and no longer different from the contralateral control, with both increasing 75% during functional vasodilation. Surprisingly, collateral midzone reactivity was enhanced in females, as compared to both males and the contralateral control, increasing 150% during functional vasodilation.

If these data are representative of all collateral vessels, they suggest that impairments in flow recovery following arterial occlusion in female mice with intact ovaries are likely due to dysfunction in the distal microvasculature. Since female mice exhibit more impaired vasodilation during hypercholesterolemia, we will evaluate the collateral enlargement and function in a more clinically‐relevant animal model with vascular disease risk factors in future animal studies aimed at understanding the worse prognosis of females patients with PAOD.