Muscarinic Receptor Subtypes Mediating Vasorelaxation of the Perforating Branch of the Human Internal Mammary Artery

The effect of acetylcholine (ACh) on the isolated perforating branch of the human internal mammary artery (HIMA) was investigated. ACh induced concentration- and endothelium-dependent relaxation of arterial rings precontracted with phenylephrine (pEC 50 = 6.93 ± 0.01). The muscarinic receptor antagonist atropine (no selectivity), pirenzepine (M 1 ), methoctramine (M 2 ), and p-fluoro-hexahydro-siladifenidol (M 1 /M 3 ) competitively antagonized the response to ACh. The pA 2 values were 9.81 ± 0.15, 7.74 ± 0.08, 6.27 ± 0.08, and 7.88 ± 0.04, respectively. In conclusion, this study has shown that ACh induced an endothelium-dependent relaxation of the perforating branch of the HIMA by stimulation of muscarinic receptors on the endothelial cells. On the basis of differential antagonist affinity, we suggest that the muscarinic receptors involved in the ACh-induced relaxation of the isolated perforating branch of HIMA are predominantly of M 1 subtype.