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The number of muscarinic receptors in chick amnion muscle

1 [3H]‐Propyl benzilylcholine mustard ([3H]‐PrBCM) an irreversible specific antagonist of muscarinic acetylcholine receptors, has been used to label the muscarinic receptors in the aneuronal muscle of isolated 11 day amniotic membranes of chick embryos. 2 This muscle was found to have about 9 fmol/mg dry weight of receptor material. This is only 1–2% of the amount found in intestinal muscle. 3 Pharmacological studies with isolated amniotic membranes using PrBCM show that labelling is to functional receptors and that the receptor reserve is small. 4 The significance of the difference in receptor reserve in innervated and aneuronal plain muscle is discussed.