EXPRESSION OF MUSCARINIC M2 RECEPTORS IN CULTURED HUMAN AIRWAY SMOOTH-MUSCLE CELLS

[H-3]-inositol phosphate formation and the inhibition of isoproterenol -induced [H-3]-cyclic adenosine monophosphate (AMP) formation in respo nse to the muscarinic agonist carbachol were studied in cultured human airway smooth muscle cells. Stimulation with carbachol produced conce ntration-dependent inhibition of isoproterenol (1 muM)-induced cyclic AMP formation (EC50, 0.15 muM; maximal inhibition, 60%). This response was itself reversed by pertussis toxin (100 ng/ml) and was competitiv ely inhibited by the muscarinic antagonists pirenzepine (pA2, 6.5), me thoctramine (pA2, 8.0), 4-diphenylacetoxy-N-methyl-piperidine (pA2, 8. 0), and parafluorohexahydrosiladifenidol (pA2, 6.5), indicating that t he M2 receptor subtype was mediating this response. In addition, carba chol also induced [H-3]-inositol phosphate formation in these cells (E C50, 11 muM; 2. 1-fold stimulation over basal), although the response observed was markedly down-regulated compared with the response seen i n noncultured airway smooth muscle preparations. Growth arrest of cell s failed to increase the magnitude of the inositol phosphate response to carbachol. These results demonstrate that cultured human airway smo oth muscle cells express functionally coupled M2 receptors and probabl y also low levels of coupled M3 receptors.