PORCINE M2 MUSCARINIC ACETYLCHOLINE RECEPTOR-EFFECTOR COUPLING IN CHINESE-HAMSTER OVARY CELLS

The relationship between porcine ma muscarinic receptor coupling to in hibition of cAMP formation and stimulation of phosphatidylinositol met abolism in Chinese hamster ovary cells was examined. Reduction of the number of receptors per cell with the slowly dissociating antagonist ( -)-quinuclidinyl benzilate caused a decrease in maximal response with no effect on EC(50) for coupling to phosphatidylinositol metabolism. I nhibition of cAMP formation showed the opposite dependence with no eff ect on maximal response but an increase in EC(50) value as receptor de nsity decreased, Pilocarpine appeared to be a partial agonist at low c ell receptor density but displayed full agonism at higher receptor den sity. These results are compatible with a two-state model describing m 2 muscarinic receptor acting via two different G proteins, This model is compatible with ob servations of negative antagonism where antagoni sts stimulated cAMP formation in adenylyl cyclase inhibition assays, a nd can also be used to estimate receptor affinities for G proteins in systems which display negative antagonism.