Herawi M, Lambrecht G, Mutschler E, Moser U, Pfeiffer A
Medizinische Klinik II, Universität München, Federal Republic of Germany.
Gastroenterology. 1988 Mar;94(3):630-7. doi: 10.1016/0016-5085(88)90233-8.
The hypothesis that muscarinic receptors on smooth muscle differ from those in epithelial glands was tested by comparing the properties of muscarinic binding sites in porcine fundic smooth muscle with those in mucosal membranes. The binding of agonists and of antagonists was assessed by displacement of [3H]N-methylscopolamine. Pirenzepine (M1-antagonist) labeled low-affinity binding sites in smooth muscle (KD = 229 nM) and in mucosa (KD = 124 nM) consistent with the presence of M2 sites. Carbachol interacted with a high-affinity (KD = 164 nM) and a low-affinity (KD = 18.2 microM) state of binding sites in smooth muscle. Guanyl 5'-yl-imidodiphosphate converted all sites to the low-affinity state. N-ethylmaleimide pretreatment increased the affinity of carbachol and the proportion of high-affinity sites. In clear contrast, only low-affinity sites of carbachol were detectable in mucosa (KD = 17 microM) that were not modulated by N-ethylmaleimide or guanyl 5'-yl-imidodiphosphate. The cardioselective antagonist AF-DX 116 displayed low affinity to mucosal binding sites (KD = 3.4 microM), whereas its affinity to smooth muscle was 503 nM. The antagonist hexahydro-sila-difenidol had a very high affinity (KD = 2.9 nM) to mucosal receptors, whereas its affinity to smooth muscle sites was 88 nM. These data show that muscarinic M2 binding sites in mucosa and smooth muscle can be distinguished by both agonist and antagonist binding experiments, and suggest the existence of different subtypes of M2-binding sites in these tissues.
通过比较猪胃底平滑肌与粘膜膜中毒蕈碱结合位点的特性，对平滑肌上的毒蕈碱受体与上皮腺中的受体不同这一假设进行了检验。通过[3H]N-甲基东莨菪碱的置换来评估激动剂和拮抗剂的结合。哌仑西平（M1拮抗剂）标记了平滑肌（KD = 229 nM）和粘膜（KD = 124 nM）中的低亲和力结合位点，这与M2位点的存在一致。卡巴胆碱与平滑肌中高亲和力（KD = 164 nM）和低亲和力（KD = 18.2 microM）的结合位点状态相互作用。鸟苷5'-基-亚氨基二磷酸将所有位点转变为低亲和力状态。N-乙基马来酰亚胺预处理增加了卡巴胆碱的亲和力和高亲和力位点的比例。与之形成鲜明对比的是，在粘膜（KD = 17 microM）中仅可检测到卡巴胆碱的低亲和力位点，这些位点不受N-乙基马来酰亚胺或鸟苷5'-基-亚氨基二磷酸的调节。心脏选择性拮抗剂AF-DX 116对粘膜结合位点的亲和力较低（KD = 3.4 microM），而其对平滑肌的亲和力为503 nM。拮抗剂六氢硅二苯醇对粘膜受体具有非常高的亲和力（KD = 2.9 nM），而其对平滑肌位点的亲和力为88 nM。这些数据表明，粘膜和平滑肌中的毒蕈碱M2结合位点可通过激动剂和拮抗剂结合实验加以区分，并提示在这些组织中存在不同亚型的M2结合位点。
