Investigating the role of endothelin receptor subtypes in the response to vascular injury
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Kirby2009.doc (88.17Mb)
Date
2009Author
Kirkby, Nicholas S.
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Abstract
Neointimal hyperplasia, the proliferative growth of the innermost layer of the blood
vessel wall, is a key process in the response to vascular injury, underlying conditions
such as post-interventional restenosis and vein/arterial graft disease. One of the many
mediators implicated in this process is endothelin-1 (ET-1), a potent vasoconstrictor
with pro-inflammatory and pro-mitogenic actions, which acts through ETA and ETB
receptor subtypes. It is well established that ET-1 increases, and ETA blockade
reduces, neointima formation following vascular injury. The role of ETB is less clear
because these receptors mediate potentially beneficial actions in endothelial cells
(EC; such as nitric oxide production, and ET-1 clearance) but detrimental effects
elsewhere (such as vascular smooth muscle) and it has been recently reported that
non-cell-specific ETB deficiency is associated with increased neointimal lesion size
following injury. The work described in this thesis addressed the hypothesis that
endogenous ET-1 contributes to neointimal hyperplasia by activation of the ETA
receptor, and that this action is moderated by concurrent activation of the ETB
receptor expressed in EC.
The role of ET receptors in neointimal lesion development was assessed using two
models of femoral arterial injury in the mouse: (i) an established method of intraluminal
wire-injury, and (ii) adaptation of a model of ligation injury that induces
robust neointimal lesion formation without physical damage to the endothelium.
Lesion development was assessed using standard histological techniques and this
was augmented by development of quantitative optical projection tomography (OPT)
to allow three-dimensional analysis of lesions.
The role of ETA and ETB receptors in these models was addressed using suitable
pharmacological ET receptor antagonists. Following wire-injury, selective ETB
blockade (A192621; 30mg.kg-1.day-1; 35 days) increased lesion size and blood
pressure without significant altering lesion composition. In contrast, selective ETA
blockade (atrasentan; 10mg.kg-1.day-1; 35 days) reduced lesion size and blood
pressure. Combined ETA+ETB antagonism had no effect on lesion size, despite reducing blood pressure, and reducing collagen content of the lesions. In the ligation
model, neither ETA selective, ETB selective nor ETA+ETB blockade altered lesion
size as assessed by standard histology but analysis by OPT indicated that ETA
blockade, with or without concurrent ETB blockade, reduced lesion volume.
The influence of ETB receptors expressed by ECs on lesion formation was addressed
using EC-specific ETB knockout mice. Small vessel myography indicated that
endothelium-dependent relaxation was unaltered in femoral arteries from these mice.
In addition, no effect on lesion size or rate of development was observed in either
wire- or ligation-injury models of neointima formation (although subtle effects on
lesion and medial composition were apparent after intra-luminal injury).
These results indicate that ETB receptor activation can moderate the detrimental
actions of the ETA receptor on neointimal lesion progression, and that this role is
dependent on the mode of vascular injury. Furthermore, in this setting, this beneficial
action is not primarily mediated by ETB expressed by EC, suggesting that ETB in
other cell types can reduce lesion development through another, unidentified
mechanism. Therefore, while both ETA selective and non-selective ETA/B
antagonists are currently in clinical use, in conditions where similar arterial
remodelling processes occur, selective ETA receptor antagonists might be preferred.