HUMAN MUSCARINIC RECEPTORS EXPRESSED IN A9L AND CHO CELLS - ACTIVATION BY FULL AND PARTIAL AGONISTS

1 A comparative study of receptor activation by ten full and partial muscarinic agonists was undertaken on the five subtypes of human muscarinic receptors expressed at similar receptor densities in Chinese hamster ovary (CHO-K1) cells. In addition, m(1), m(2) and m(3) receptors were expressed in mouse fibroblast A9L cells in order to compare the influences of cell type on agonist activation of these receptors. 2 Receptor-effector coupling efficiencies were greater in CHO than A9L cells and agonists displayed greater potencies and similar or greater intrinsic activities at CHOm(1) and CHOm(3) than A9Lm(1) and A9Lm(3) receptors. Although mt receptor density was 6 fold higher in A9L than CHO cells, carbachol elicited significantly greater inhibition of adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation in CHOm(2) cells. These data suggest that not only receptor density but receptor-effector coupling and/or coupling efficiencies play significant roles in agonist-induced responses. 3 In CHO cells, receptor-effector coupling efficiencies were m(3)=m(1)>m(5). Although CHOm(5) receptors were the least efficiently coupled, some partial agonists displayed higher intrinsic efficacies at m(5) than m(3) receptors suggesting that, in CHO cells, m(5) and m(3) receptors may activate different G proteins and/or effecters to stimulate inositol monophosphate (IP1) formation. 4 McN-A-343 was a functionally selective m(4) agonist. It had little or no agonist activity at m(3) receptors expressed in either A9L or CHO cells. The slopes of McN-A-343 concentration-response curves in CHOm(2) cells were significantly lower than the slopes obtained with this compound in CHOm(4) cells suggesting that the mode of activation by McN-A-343 differed between the two muscarinic receptors negatively coupled to adenylyl cyclase. 5 Cloned receptors provide valuable tools for the study of agonist-receptor interaction and agonist-receptor activation but caution should be applied in assuming that the results are valid for all cell types or for tissue-expressed receptors.