Identification of three muscarinic receptor subtypes in rat lung using binding studies with selective antagonists

Heterogeneity of the muscarinic receptor population in the rat central and peripheral lung was found in competition binding experiments against [3H]quinuclidinyl benzilate [( 3H]QNB) using the selective antagonists pirenzepine, AF-DX 116 and hexahydrosiladifenidol (HHSiD). Pirenzepine displaced [3H]QNB with low affinity from preparations of central airways indicating the absence of M1 receptors in the trachea and bronchi. Muscarinic receptors in the central airways are comprised of both M2 and M3 receptors since AF-DX 116, an M2-selective antagonist, bound with high affinity to 70% of the available sites while HHSiD, an M3-selective antagonist bound with high affinity to the remaining binding sites. In the peripheral lung, pirenzepine bound with high affinity to 14% of the receptor population, AF-DX 116 bound with high affinity to 79% of the binding sites while HHSiD bound with high affinity to 18% of the binding sites. The presence of M1 receptors in the peripheral airways but not in the central airways was confirmed using [3H]telenzepine, an M1 receptor ligand. [3H]Telenzepine showed specific saturable binding to 8% of [3H]QNB labeled binding sites in homogenates of rat peripheral lung, while there was no detectable specific binding in homogenates of rat trachea or heart. The results presented here demonstrate that there are three muscarinic receptor subtypes in rat lungs, and that the distribution of the different subtypes varies within the lungs. Throughout the airways, the dominant muscarinic receptor subtype is M2. In the trachea and bronchi the remaining receptors are M3, while in the peripheral lungs, the remaining receptors are both M1 and M3.