Pharmacological characterization of muscarine receptors of PC12 (rat phaeochromocytoma) cells.

The muscarine receptors of PC12 (rat phaeochromocytoma) cells were studied in functional and binding experiments. The catecholamine stores of PC12 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 greater than acetylcholine greater than muscarine = methacholine greater than carbachol greater than bethanechol. The release evoked by carbachol (0.1 mmol/l) was inhibited with high potency by the M1-selective antagonist telenzepine (pKi = 8.82), with intermediate potency by pirenzepine (pKi = 7.00) and with low potency by the M2-selective antagonist AF-DX 116 (pKi = 5.74). The binding of 3H-N-methylscopolamine to PC12 membranes was inhibited by various non-selective and subtype-selective muscarinic antagonists with the following rank order of potency: telenzepine = atropine greater than 4-DAMP greater than dicyclomine greater than pirenzepine greater than HHSiD much greater than AF-DX 116. A similar rank order was obtained for the inhibition by these compounds of 3H-telenzepine binding to M1-receptors in membranes of the cerebral cortex of the guinea pig. The Hill coefficients for inhibition of 3H-N-methylscopolamine binding (to PC12 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 PC12 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). PC12 cells are presumably endowed with more than one subtype of muscarine receptors.(ABSTRACT TRUNCATED AT 250 WORDS)