[3H]oxotremorine-M binding to membranes prepared from rat brain and heart: evidence for subtypes of muscarinic receptors.

[3H]Oxotremorine-M has been used as a ligand to label muscarinic binding sites on membranes prepared from rat fore-brain and heart. In rat brain membranes, two binding sites could be identified: a high affinity low capacity site and a low affinity high capacity site. In contrast, only a high affinity site could be labelled in heart membranes. The potency order of agonists for the high affinity site in brain membranes and heart membranes was the same, suggesting that these binding sites represent the same subtype of muscarinic receptors. However, the affinity of pirenzepine for the brain high affinity site was higher than that for the heart high affinity site suggesting that these sites may represent different receptor populations. The potency order of agonists for the high affinity and low affinity sites in brain membranes were significantly different suggesting that these binding sites represent pharmacologically distinct binding sites. GTP abolished the high affinity sites in heart and brain membranes, but the low affinity site in brain membranes appeared unaffected. These results are consistent with the hypothesis proposing subtypes of muscarinic receptors.