Agonist-induced reduction in both the number of ml muscarinic receptors and the mRNA coding for the receptor protein was investigated in Chinese hamster ovary (CHO) cells which were transfected with the ml muscarinic receptor gene. Receptor concentration was measured by the specific binding of the muscarinic ligand, [3H]quinuclidinyl benzilate ([3H]QNB), and Northern blot hybridization analysis was used to evaluate the levels of receptor mRNA. Incubation of cells with 1 mM of the muscarinic agonist, carbamylcholine (CBC), for 24 h decreased receptor density and mRNA levels in cells by 65% and 73%, respectively. These results indicate that agonist-induced down-regulation of ml muscarinic receptors might be due to, at least in part, a decrease in receptor synthesis resulting from a reduction in the steady-state level of their mRNA.
