Evidence for a M-1 muscarinic receptor on the endothelium of human pulmonary veins

1 To characterize the muscarinic receptors on human pulmonary veins associa ted with the acetylcholine (ACh)-induced relaxation, isolated venous and ar terial preparations were precontracted with noradrenaline (10 mu M) and wer e subsequently challenged with ACh in the absence or presence of selective muscarinic antagonists. 2 ACh relaxed venous preparations derived from human lung with a pD(2) valu e of 5.82+-0.09 (n = 16). In venous preparations when the endothelium had b een removed, the ACh relaxations were abolished (n = 4). ACh relaxed arteri al preparations with a pD(2) value of 7.06+/-0.14 (n = 5). 3 Atropine (1 mu M), the non selective antagonist for muscarinic receptors, inhibited ACh-induced relaxations;in human pulmonary veins. The affinity v alue (pK(B) value) for atropine was: 8.64+/-0.10 (n = 5). The selective mus carinic antagonists (darifenacin (M-3), himbacine (M-2,M-4), methoctramine (M-2) and pFHHSiD (M-1,M-3)) also inhibited ACh-induced relaxations in veno us preparations. The pKB values obtained for these antagonists were not tho se predicted for the involvement of M2-5, receptors in the ACh-induced rela xation in human pulmonary veins. 4 The pK(B) value for darifenacin (1 mu M) was significantly greater in hum an pulmonary arterial (8.63+/-0.14) than in venous (7.41+/-0.20) preparatio ns derived from three lung samples. 5 In human pulmonary veins, the pKB values for pirenzepine (0.5 and 1 mu M) , a selective antagonist for M-1 receptors, were: 7.89+/-0.24 (n = 7) and 8 .18+/-0.22 (n = 5), respectively. In the venous preparations, the pK(B) val ues derived from the functional studies with all the different muscarinic a ntagonists used were correlated (r = 0.89; P = 0.04; slope = 0.78) with the affinity values (pK(i) values) previously published for human cloned mi re ceptors in CHO cells. 6 These results suggest that the relaxations induced by ACh are due to the activation of M-1 receptors on endothelial cells in isolated human pulmonar y veins.