in the guinea pig myometrium , muscarinic receptor activation leads to contraction and elicits two biochemical responses viz . an increased formation of inositol phosphates ( via a guanine nucleotide regulatory protein , distinct from the stimulatory and inhibitory g proteins of the adenylate cyclase system and a decreased synthesis of cyclic amp involving inhibitory g protein activation . we now describe two major differences in the effects of muscarinic agonists . first , the greater potency of carbachol in inhibiting cyclic amp formation ( ec50 = 8 nm ) than in stimulating the accumulation of inositol phosphates and tension ( ec50 = 15 and 2 microm , respectively ) . second , carbachol , oxotremorine and pilocarpine were equally effective in eliciting cyclic amp inhibition but the order of potency for inositol phosphate formation was carbachol greater than oxotremorine and pilocarpine was without effect . the partial agonists , pilocarpine and oxotremorine , inhibited carbachol-mediated inositol phosphate formation . pirenzepine , selective for muscarinic m1 receptor subtype , displayed a low affinity for antagonizing cyclic amp inhibition , inositol phosphate generation and tension due to carbachol ( ki = 286 , 92 and 110 nm , respectively ) . af-dx116 ( 11- [ ( 2- [ ( diethylamino ) methyl ] -1- piperidinyl ] acetyl ) -5,11-dihydro-6h-pyrido [ 2,3-b ] [ 1,4 ] benzodiazepine- 6-one ) , selective for cardiac m2 receptors blocked cyclic amp inhibition with high affinity ( ki = 1.14 nm ) while it antagonized inositol phosphate formation with low affinity ( ki = 346 nm ) .
C0034826
muscarinic receptor
C0531859
muscarinic m1 receptor