[The study of direct effect of methimazole on thymidine incorporation in FRTL-5 cells]

During the course of treatment of Graves' disease with the anti-thyroid drug, methimazole (MMI), a decrease in a patient's goiter size is sometimes observed. Using rat thyroid cell strain, FRTL-5, the direct effect of MMI on thyroid cell growth was investigated. FRTL-5 cells (2 X 10(5)) were cultured for 48 hours with TSH, (Bu)2 cAMP or forskolin in the presence of [3H]-thymidine. All three stimulators increased cell growth, expressed as [3H]-thymidine incorporation into DNA in a dose-dependent fashion. When FRTL-5 cells were cultured for 48 hours in the presence of MMI at 10(-6)-10(-3) M with these stimulators (TSH 250 microU/ml, (Bu)2 cAMP 10(-3) M, forskolin 10(-5) M), [3H]-thymidine incorporation was suppressed in dose-dependent fashions (MMI 10(-5) M-10(-3) M). From the present study, it is suggested that methimazole directly modulates thyroid cell growth induced by thyroid growth stimulators which are involved in adenylate cyclase-cyclic AMP system and that the site of its action exists beyond cAMP production.