Abstract
Cholinergic angiogenesis is mediated by an endothelial nicotinic acetylcholine receptor (EC nAChR). Short-term administration of nicotine stimulates angiogenesis via EC nAChRs.The long-term effects of nicotine upon cholinergic angiogenesis are unknown. The objective of this study was to determine whether chronic nicotine exposure blunts angiogenesis. We exposed C57/Bl6 male mice (n = 42) to nicotine (200 microg/ml drinking water) or vehicle for 8 or 16 weeks. Subsequently, hindlimb ischemia was induced by ligation of the left femoral artery. After surgery, animals in the vehicle-treated group were re-randomized to vehicle (vehicle group) or nicotine (acute exposure group) for 2 weeks; whereas animals that had been previously treated (for 8 or 16 weeks with nicotine) continued to receive nicotine (8 WK or 16 WK groups). After 2 weeks, animals were sacrificed for immunohistochemical, gene expression, and angiogenesis studies. Capillary density of the ischemic hindlimb was increased by nicotine in naïve animals (vehicle vs acute exposure: 2.40 +/- 0.09 vs 2.82 +/- 0.10 capillaries/myocyte, p < 0.05). However, prior exposure to nicotine for 16 weeks (16 WK) abolished the effects of nicotine to increase capillary density in the ischemic hindlimb (acute vs 16 WK: 2.82 +/- 0.10 vs 2.47 +/- 0.03 capillaries/ myocyte; p < 0.05). The impairment of cholinergic angiogenesis was associated with a reduction in nAChR expression and plasma VEGF levels. Chronic exposure to nicotine impaired capillary sprouting of aortic segments ex vivo (vehicle vs 16 WK: 0.303 +/- 0.029 vs 0.204 +/- 0.017 mm(2), p < 0.05, n = 3 in each group). In conclusion, the current study shows for the first time that chronic exposure to nicotine impairs cholinergic angiogenesis, an effect mediated by downregulation of the vascular nAChR, and attenuation of nicotine-induced VEGF release. These studies may explain the impairment in angiogenic processes observed in long-term smokers.