Characterization of muscarinic acetylcholine receptors in human labial salivary glands.

Muscarinic acetylcholine receptors were studied in human labial salivary glands with (-)-[3H]MQNB. The radioligand bound with a Kd value of 150 pM. The density of muscarinic receptors was 220 fmol/mg protein. Most muscarinic antagonists bound to a homogeneous population of muscarinic receptors in this tissue. However, the inhibition curves of pirenzepine had a Hill slope of 0.57 and could be analyzed by a two site model. These results suggest that the muscarinic receptors in human labial salivary glands are a mixture of M1 and M3 types.