Pharmacological characterization of muscarine receptors of PC 12 (rat phaeochromocytoma) cells

Summary The muscarinereceptors of PC12 (rat phaeochromocytoma) cells were studied in functional and binding experiments. The catecholamine stores of PC 12 cells were labelled by incubation of the cells with tritiated noradrenaline. Muscarinic agonists elicited concentration-dependent release of tritium which consisted overwhelmingly of unchanged ³H-noradrenaline. The rank order of potency was: oxotremorine > acetylcholine > muscarine = methacholine > carbachol > bethanechol. The release evoked by carbachol (0.1 mmol/l) was inhibited with high potency by the M₁-selective antagonist telenzepine (pK_i = 8.82), with intermediate potency by pirenzepine (pK_i = 7.00) and with low potency by the M₂-selective antagonist AF-DX 116 (pK_i = 5.74).The binding of 3H-N-methylscopolamine to PC 12 membranes was inhibited by various non-selective and subtype-selective muscarinic antagonists with the following rank order of potency: telenzepine = atropine > 4-DAMP > dicyclomine > pirenzepine > HHSiD > AF-DX 116. A similar rank order was obtained for the inhibition by these compounds of 3H-telenzepine binding to Mi-receptors in membranes of the cerebral cortex of the guinea pig. The Hill coefficients for inhibition of 3H-N-methylscopolamine binding (to PC 12 membranes) by pirenzepine, telenzepine and AF-DX 116 were below unity. Specific binding of both ³H-telenzepine and 3H-N-methylscopolamine to muscarine receptors of PC 12 membranes was saturable and of high affinity; the maximal number of binding sites was higher for ³H-N-methylscopolamine than for 3H-telenzepine (calculated for the active (+)enantiomer).PC 12 cells are presumably endowed with more than one subtype of muscarine receptors. The predominant receptor is an atypical receptor; it is neither a M₂- nor a M₃-receptor, and in spite of the high affinity of telenzepine for this receptor it is probably also not an M₁-receptor.