Carbachol-induced decrease in thyroid cell adenylyl cyclase activity is independent of calcium and phosphodiesterase activation

The mechanism of adenylyl cyclase desensitization by carbachol, an agent that stimulates polyphosphoinositide hydrolysis, was studied in thyroid cells. Incubation of cultured dog thyroid cells with 10 μM carbachol for 2-4 hr reduced the subsequent thyrotropic hormone (TSH) stimulation of adenylyl cyclase activity of membrane preparations by ~40%. This inhibition was reversed by atropine, occurred even in a Ca²⁺-free medium containing ethylene glycol bis(β-aminoethylether)-N,N,N',N'-tetraacetic acid, and was not reproduced by the Ca²⁺ ionophore A23187. The carbachol effect was not prevented by simultaneous incubation of cells with either isobutylmethylxanthine, an inhibitor of phosphodiesterase, or H-7, an inhibitor of protein kinase. Pretreatment of cells with pertussis toxin to inactivate the G(i) inhibitory protein also failed to affect the carbachol inhibition. Although carbachol did not reduce the basal or the TSH-stimulated cyclase activities when added to membranes directly during the assay, exposure of cells to carbachol for 2-4 hr resulted in long lasting inhibition of TSH-stimulated cyclase activity (for at least 24 hr); recovery was seen by 48 hr after its removal. Carbachol pretreatment had no effect on ¹²⁵I-TSH binding to membranes but reduced the cyclase stimulation by not only TSH but also cholera toxin, guanosine 5'-O-(3-thio)triphosphate, and forskolin; it also significantly reduced the cholera toxin-mediated AD[³²P]-ribosylation of G(s) in membranes. These data indicate that carbachol-induced inhibition of adenylyl cyclase occurs beyond the level of TSH receptor binding and that G(s) is a possible site of its action. Thus, in dog thyroid cells, carbachol, via muscarinic receptors, can reduce the adenylyl cyclase activity by a process that does not involve Ca²⁺ or activation of phosphodiesterase.