Acetylcholine-induced contractions in the porcine internal mammary artery: possible role of muscarinic receptors.

The effect of acetylcholine on the isolated, non-precontracted, porcine internal mammary artery (IMA) was investigated. Acetylcholine induced concentration-dependent contractions of non-precontracted IMA rings with denuded endothelium (pEC50 = 5.80 +/- 0.04) and was without effect on arterial segments with intact endothelium. The muscarinic receptor antagonists atropine, pirenzepine, methoctramine and p-fluoro-hexahydro-sila-diphenidol (pFHHSiD) antagonized the response to acetylcholine. The constrained pA2 values were 10.14, 7.74, 7.34 and 10.5, respectively. It is concluded that acetylcholine induces concentration-dependent contractions of porcine internal mammary artery rings on basal tone and that this contractile effect is probably due to direct cholinergic stimulation of smooth muscle cells, maybe including activation of muscarinic M1 receptors.