High-affinity [3H]pirenzepine binding to putative M1 muscarinic sites in the neuroblastoma x glioma hybrid cell line (NG 108-15).

The specific binding of both the non-classical antagonist [3H] pirenzepine ( [3H]PZ) and the classical antagonist [3H](-)quinuclidinyl benzilate ( [3H](-)QNB) was determined in parallel assays of the mouse neuroblastoma x rat glioma hybrid cell line (NG 108-15). Saturation isotherms yielded a Kd = 4.0 nM and Bmax = 27.8 fmoles/mg protein for [3H]PZ and a Kd = 17.2 pM and Bmax = 53.2 fmoles/mg protein for [3H](-)QNB. The inhibition data of pirenzepine vs [3H](-)QNB was best fit to a 2-site binding model revealing both a high affinity pirenzepine site (72%, KH = 10.3 nM) and a low affinity site (28%, KL = 97.5 nM). [3H]PZ competition studies demonstrated stereospecificity, steep inhibition curves for muscarinic antagonists (Hill coefficients close to 1), and a shallow inhibition curve for a muscarinic agonist. These results indicate that muscarinic receptors on NG 108-15 cells may be subclassified (M1/M2) on the basis of the discriminative capability of [3H]PZ.