Muscarinic acetylcholine receptors in the embryonic chicken heart undergo agonist-induced internalization followed by decreases in both receptor number and mRNA expression. Muscarinic agonists cause both inhibition of adenylyl cyclase and activation of phospholipase C in chick heart cells. Treatment of cells with islet activating protein, which blocks coupling of muscarinic receptors to adenylyl cyclase but not phospholipase C, blocks muscarinic receptor-mediated regulation of receptor mRNA levels. Incubation of cells with the partial agonist pilocarpine, which causes inhibition of adenylyl cyclase but not stimulation of phospholipase C, induces less down-regulation of receptor mRNA levels than agonists which regulate both second-messenger systems. Thus, both second-messenger pathways are required for maximal regulation of muscarinic receptor mRNA levels in response to receptor activation. We also demonstrate that the regulation of receptor mRNA by agonist plays an important role in modulating the rate of recovery of muscarinic acetylcholine receptor number following agonist-induced down-regulation.
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