Mechanisms of Reduced Vascular Tone Following Collateral Arteriogenesis induced by Femoral Artery Ligation

Abstract

The presence of a well‐developed, native collateral network can decrease the severity of ischemic injury following arterial occlusion. This rescue depends on the collateral network undergoing arteriogenesis to enlarge and increase blood flow to the ischemic region. Although there has been tremendous effort attempting to understand the mechanisms of arteriogenesis, no arteriogenesis‐stimulating therapies have proved effective in clinical trials. Therefore, further research must be conducted to understand the mechanisms of arteriogenesis. Arteriogenesis, as determined by maximal collateral diameter, appears to be complete within 1 week following arterial occlusion. Recent work in our lab demonstrated that seven days post femoral artery ligation, the resting diameter in the superficial gracilis collateral was roughly 90% of the size of the maximum diameter, whereas the sham‐operated limb was only roughly 65%. Therefore, the goal of this work was to determine the mechanisms responsible for the apparent loss of resting vascular tone in the enlarged collateral at day‐7.All procedures were performed in male C57/Bl6 mice at day‐7, following femoral artery ligation between the epigastric/profunda and popliteal branches. All reagents were dissolved in a physiological salt solution and superfused over the exposed gracilis anterior. First, we determined if the apparent loss in vascular tone was due to an excess in vasodilating influence, an insufficient vasoconstricting influence, or passivity in the vascular smooth muscle cells (VSMC). Specifically, we determined if L‐NAME, an inhibitor of endothelial‐derived nitric oxide, or norepinephrine, which targets the a‐adrenergic receptors, would restore vascular tone. L‐NAME induced vasoconstriction (−47±70%), but not more than the contralateral control (−43±5%), suggesting that an excess in vasodilating influence is not the primary contributor to the loss of vascular tone. Interestingly, norepinephrine (NE) had a heterogeneous effect, in which some regions of the collateral constricted significantly more than the contralateral control (−71±7% vs −39±6%, respectively), yet other regions were minimally responsive. The greater sensitivity to NE suggests that the collateral should have greater resting vascular tone, not less. Therefore, we evaluated the role of endogenous a‐adrenergic agonists on resting vascular tone with prazosin, an a1‐adrenergic antagonist, and rauwolscine, an a2‐adrenergic antagonist. Consistent with the original finding of reduced vascular tone, prazosin reduced vascular tone in the contralateral control (73 ± 15%) increase, and had minimal effect on the enlarged collateral (16±9%). Rauwolscine had little effect on either the contralateral control or enlarged collateral (12±8% vs 1±3%, respectively). Therefore, the heterogeneous response to NE combined with the lack of response to a‐adrenergic agonists suggests that the loss of resting vascular tone is a combination of VSMC passivity combined with reduced endogenous a1‐adrenergic stimulation. Future studies may involve determining the mechanism of restored vascular tone at day‐28 or the impact of therapeutic intervention to restore resting vascular tone.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.