Functional characterization of the muscarinic receptors involved in endothelium‐dependent relaxation in isolated canine uterine artery

Abstract

Acetylcholine interacts with endothelial muscarinic receptors releasing nitric oxide and causing vasodilatation. To identify the receptor subtype responsible for acetylcholine-induced relaxation in canine uterine artery, the usual organ bath method for in vitro investigation on isolated blood vessels was applied. Using a range of muscarinic receptor antagonists such as atropine (nonselective), pirenzepine (M(1)-selective), methoctramine (M(2)-selective) and p-fluoro-hexahydro-sila-difenidol (p-FHHSiD) (M(1)/M(3)) and determining pA2 value of those antagonists through Shild analysis, we aimed at establishing a precise receptor mechanism underlying acetylcholine-induced relaxation in isolated canine uterine artery. The relaxation of uterine arterial rings in response to acetylcholine in the presence or absence of selective muscarinic receptors antagonists was calculated using concentration response curves. Acetylcholine induced concentration-dependent and endothelium-dependent relaxation of arterial rings precontracted with phenylephrine (pEC(50) = 6.90 +/- 0.02). Muscarinic receptors antagonists atropine, pirenzepine, methoctramine and p-FHHSiD competitively antagonized the response to acetylcholine and obtained pA(2) values were 9.91 +/- 0.06, 6.60 +/- 0.04, 6.21 +/- 0.08 and 8.05 +/- 0.1, respectively. This study showed that acetylcholine induced endothelium-dependent relaxation of canine uterine artery by stimulation of muscarinic receptors localized on the endothelial cells. On the basis of differential antagonist affinity, we suggest that the muscarinic receptors involved in the acetylcholine-induced relaxation of canine uterine artery are predominantly of M(3) subtype.