Interactions of edrophonium with neostigmine in the rat trachea

The muscarinic M, receptor of airway smooth muscle has both an orthosteric binding site and an allosteric binding site. Edrophonium may bind to the allosteric site, resulting in the inhibition of the action of the orthosteric site. Therefore, we examined the effects of edrophonium on neostigmine-induced contractile and phosphatidylinositol responses of rat trachea. Neostigmine (100 AM in final concentration) was added, and ring tension was examined by the addition of edrophonium. After the completion of the experiment, KrebsHenseleit (K-H) solution containing both edrophonium and neostigmine was changed three times with fresh K-H solution, and the tension was recorded. Tracheal slices were incubated with [H-3]myo-inositol and 100 muM neostigmine in the presence or absence of edrophonium. The [H-3]inositol monophosphate (IP1) was measured. Data were expressed as mean +/- SE. Statistical significance (P < 0.05) was determined with analysis of variance. Neostigmine-induced tension and IP1 accumulation were attenuated by edrophoniurn at concentrations of 100 muM or more. This attenuation was reversed to more than 80% of control levels by washing with fresh K-H solution. The results suggest that edrophonium would bind to the allosteric site, resulting in the inhibition of the action of the orthosteric site of muscarinic M-3 receptors of rat trachea.