The effect of a selective α7-nicotinic acetylcholine receptor antagonist on endothelium-dependent relaxation in rat mesenteric and pulmonary arteries.

1Preetpal S. Chadha, 2James D. Moffatt & 1Rebecca Lever, 1Department of Pharmacology, The School of Pharmacy, University of London, WC1N 1AX. 2The Royal Veterinary College, London NW1 0TU.

Neuronal type nicotinic acetylcholine receptors (nAChR), in particular of the α7 subtype, are expressed on vascular endothelial cells where their function remains unclear. Endothelial cells have also been shown to possess all the components of an endogenous cholinergic system including acetylcholinesterase (AChE), choline acetyltransferase (ChAT) and the vesicular ACh transporter (VAChT) (Reviewed by Kirkpatrick et al., 2001). Previously, we have found that nicotine exposure has differential effects in mesenteric and pulmonary arteries where it leads to improved maximum endothelium-dependent relaxation in mesenteric vessels and an impaired response in pulmonary vessels (Chadha et al., 2005a). Furthermore, the effect of nicotine in mesenteric arteries was shown to be sensitive to α7 nAChR-selective antagonists (Chadha et al., 2005b). In the present study, we have investigated acetylcholine-induced relaxation in rat mesenteric and pulmonary arteries, following exposure of vessels to the α7 nAChR-selective antagonist methyllycaconitine (MLA) alone.

Male Wistar rats (240-260g) were killed by CO2 asphyxiation. Third order mesenteric and intrapulmonary arteries were isolated and treated with 6-hydroxydopamine (2 mM) and capsaicin (0.1 mM) for 30 minutes, in order to remove neuronal influences. Following this, some vessels were incubated in DMEM containing MLA (10-7 M) both alone and in the presence of the VAChT inhibitor vesamicol (100 μ M) and AChE (0.1 units/ml). Following 24 or 48 hours incubation of pulmonary or mesenteric vessels respectively, 2 mm segments of artery, were mounted on a wire myograph under normalised tension in oxygenated (95% O2/5% CO2) Krebs’ buffer maintained at 37°C. Maximum contraction to KCl (120 mM) was initially determined and ~50% maximal tone subsequently induced using phenylephrine or U46619, in the presence of nifedipine (0.3 μM), before assessment of ACh-induced relaxation. Maximal responses to ACh (Emax) are expressed as percent relaxation of active tone (mean ± s.e. mean) and differences determined by ANOVA, followed byDunnett’spost test. n = 4-6 per group.

Exposure of rat mesenteric arteries to the α7 nAChR-selective antagonist MLA augmented endothelium-dependent relaxation in response to ACh compared to vehicle control (Emax: 87.4 ± 2.1% vs 69.5 ± 5.1%; pEC50: 8.0 ± 0.1 vs 7.1 ± 0.2, respectively. P < 0.05). The effects of MLA were completely abolished following co-exposure to vesamicol and AChE (Emax: 65.6 ± 5.9; pEC50: 7.1 ± 0.1), whilst vesamicol/AChE treatment alone was without effect. Conversely, maximum relaxation of pulmonary arteries in response to ACh, following treatment with MLA for 24 hours, was impaired (Emax: 50.0 ± 7.1% vs 81.9 ± 5.0%, respectively). Vesamicol/AChE treatment had no effect on pulmonary artery responses under any conditions.

These data suggest that in mesenteric arteries, an autocrine effect of endogenous ACh may be in place, mediated in part by α 7 nAChR.

Chadha P.S et al. (2005a). http://www.pa2online.org/abstracts/Vol3Issue2abst057P.pdf
Chadha P.S et al., (2005b). http://www.pa2online.org/abstracts/Vol3Issue4abst145P.pdf
Kirkpatrick C.J et al., (2001). Jpn. J. Pharmacol. 85, 24-28.