1. Muscarinic cholinergic receptors have been identified and characterized by radioligand binding studies in human peripheral lung tissue. The tissue was obtained at thoracotomy of 12 patients, of whom four had chronic obstructive lung disease.

2. The radioligand l-quinuclidinyl[phenyl-4-3H]benzilate (3H-QNB) was used to label the muscarinic cholinergic receptors. Binding was saturable, protein dependent and showed a high affinity and stereospecificity. Specific binding could be inhibited by agonists and antagonists; molar inhibition constants determined for the agents used were of the same order of magnitude as those reported for 3H-QNB inhibition in various tissues of laboratory animals. Inhibition experiments with agonists resulted in Hill slopes which were significantly different from unity, indicating multiple binding sites. The stable GTP analogue guanyl-5′-imidodiphosphate had no effect on the Hill slopes of agonists or antagonists.

3. The number of binding sites was significantly less in lung tissue from patients with chronic obstructive lung disease.

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