Effects of agonist efficacy on desensitization of phosphoinositide hydrolysis mediated by m1 and m3 muscarinic receptors expressed in Chinese hamster ovary cells.
Muscarinic receptor agonist-induced desensitization of phosphoinositide (PI) hydrolysis and loss of receptors were studied in Chinese hamster ovary (CHO) cells transfected with the m1 and m3 muscarinic receptor genes. Long-term exposure to the full agonist carbamylcholine (CBC) resulted in a time-dependent attenuation of the maximal PI response and a decrease in agonist potency. This desensitization was accompanied by a parallel loss of maximal ligand binding without an alteration of the binding affinity. The time course of both receptor desensitization and down-regulation was similar in m1 and m3 CHO cells. The PI response to the partial agonist McN-A-343 (McN) in m1 cells was more sensitive to desensitization by CBC than the response to the latter agonist, and this desensitization was faster than receptor down-regulation. Desensitization of the PI response to McN was reflected as a decrease in the maximal response without a marked change in potency. McN induced slow desensitization of the PI response to CBC but a much faster desensitization of its own response. Our data provide evidence that although muscarinic agonist-induced desensitization of PI hydrolysis in CHO cells is due mainly to loss of receptors, there are other important factors which play a role in this process, e.g., receptor-effector uncoupling. The relative contribution of these different mechanisms depends on the efficacy of the agonists used for the receptor desensitization and activation steps.
Hu
J
J
Department of Pharmacology and Toxicology, University of Maryland School of Pharmacy, Baltimore.
Wang
S Z
SZ
el-Fakahany
E E
EE
D008455:(4-(m-Chlorophenylcarbamoyloxy)-2-butynyl)trimethylammonium Chloride / Q000494:pharmacology*; D000818:Animals; D002217:Carbachol / Q000494:pharmacology*; D002460:Cell Line; D006224:Cricetinae; D003412:Cricetulus; D015536:Down-Regulation / Q000187:drug effects; D005260:Female; D006868:Hydrolysis; D019832:N-Methylscopolamine; D010276:Parasympatholytics / Q000378:metabolism; D010716:Phosphatidylinositols / Q000378:metabolism*; D011813:Quinuclidinyl Benzilate / Q000378:metabolism; D011869:Radioligand Assay; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D012602:Scopolamine Derivatives / Q000378:metabolism; D014316:Tritium
D010276:Parasympatholytics; D010716:Phosphatidylinositols; D011976:Receptors, Muscarinic; D012602:Scopolamine Derivatives; D014316:Tritium; D008455:(4-(m-Chlorophenylcarbamoyloxy)-2-butynyl)trimethylammonium Chloride; D011813:Quinuclidinyl Benzilate; D002217:Carbachol; D019832:N-Methylscopolamine