STIMULATION OF CYCLIC-AMP ACCUMULATION AND PHOSPHOINOSITIDE HYDROLYSIS BY M(3) MUSCARINIC RECEPTORS IN THE RAT PERIPHERAL LUNG

The effects of oxotremorine-M (oxo-M), a muscarinic agonist, on cyclic AMP (cAMP) accumulation in slices of the rat peripheral lung were inv estigated. Oxo-M stimulated cAMP accumulation in a concentration-depen dent manner with an EC(50) value of 4.2 mu M and a maximal effect of 2 .4 +/- 0.39-fold over basal. In the presence of forskolin (25 mu M), t he maximal effect of oxo-M was increased to 14.1 +/- 4.0-fold over bas al. Forskolin alone caused a 5.9 +/- 2.2-fold increase in cAMP relativ e to basal; therefore, the combination of both drugs was more than add itive. The effects of oxo-M on cAMP accumulation were unaffected by te trodotoxin, indicating that the action of oxo-M was not mediated by ne uronal release of neurotransmitters. Oxo-M had a small inhibitory effe ct on cAMP in a homogenate preparation, indicating that the stimulator y response to oxo-M in slices of the lung is not due to direct stimula tion of adenylyl cyclase. Characterization of the oxo-M potentiation o f forskolin-stimulated cAMP accumulation using different muscarinic an tagonists yielded calculated pK(B) values that agreed with binding aff inities for the M(3) subtype. Oxo-M elicited phosphoinositide hydrolys is in the lung, and the nature of the antagonism of this response was also consistent with that expected for an M-mediated response. cAMP ac cumulation in the presence of oxo M (100 mu M), forskolin (12 mu M), o r both drugs combined was inhibited by indomethacin (1 mu M). These re sults demonstrate that the M(3) receptor stimulates cAMP accumulation and phosphoinositide hydrolysis in the rat peripheral lung, and the me chanism for cAMP stimulation may involve arachidonic acid metabolites.