Evidence for a M1 muscarinic receptor on the endothelium of human pulmonary veins

To characterize the muscarinic receptors on human pulmonary veins associated with the acetylcholine (ACh)‐induced relaxation, isolated venous and arterial preparations were pre‐contracted with noradrenaline (10 μM) and were subsequently challenged with ACh in the absence or presence of selective muscarinic antagonists. ACh relaxed venous preparations derived from human lung with a pD2 value of 5.82±0.09 (n=16). In venous preparations where the endothelium had been removed, the ACh relaxations were abolished (n=4). ACh relaxed arterial preparations with a pD2 value of 7.06±0.14 (n=5). Atropine (1 μM), the non selective antagonist for muscarinic receptors, inhibited ACh‐induced relaxations in human pulmonary veins. The affinity value (pKB value) for atropine was: 8.64±0.10 (n=5). The selective muscarinic antagonists (darifenacin (M3), himbacine (M2,M4), methoctramine (M2) and pFHHSiD (M1,M3)) also inhibited ACh‐induced relaxations in venous preparations. The pKB values obtained for these antagonists were not those predicted for the involvement of M2–5 receptors in the ACh‐induced relaxation in human pulmonary veins. The pKB value for darifenacin (1 μM) was significantly greater in human pulmonary arterial (8.63±0.14) than in venous (7.41±0.20) preparations derived from three lung samples. In human pulmonary veins, the pKB values for pirenzepine (0.5 and 1 μM), a selective antagonist for M1 receptors, were: 7.89±0.24 (n=7) and 8.18±0.22 (n=5), respectively. In the venous preparations, the pKB values derived from the functional studies with all the different muscarinic antagonists used were correlated (r=0.89; P=0.04; slope=0.78) with the affinity values (pKi values) previously published for human cloned m1 receptors in CHO cells. These results suggest that the relaxations induced by ACh are due to the activation of M1 receptors on endothelial cells in isolated human pulmonary veins. British Journal of Pharmacology (2000) 130, 73–78; doi:10.1038/sj.bjp.0703301