Cultured Thyroid Cell Adenosine 3′,5′-Cyclic Monophosphate Response To Thyrotropin: Loss and Restoration of Sensitivity to Iodide Inhibition

Abstract

Unlike in all other thyroid preparations, exposure of dog thyroid cells in long-term monolayer culture to I- (10-7-10-3M) for up to 19 h did not blunt the subsequent cyclic[c]AMP response to thyrotropin (TSH) stimulation. This lack of effect of I- was observed even when confluent thyroid cells were follicularized by the action of TSH in the culture medium. Preincubation of these cells in thyroxine (T4) and triiodothyronine (T3) was similarly without effect on subsequent cAMP response to TSH. Study of thyroid cells during the early phase of primary culture demonstrated that inhibition by I- (10-4M) of the cAMP response to TSH occurred after 7 h but was lost after 48 h of cell culture. This inhibitory effect of I- was prevented by the inclusion of methimazole in the preincubation medium. As with I- insensitive cells, T4 and T3 were without effect on the cAMP response to TSH in I- sensitive thyroid cells. Exposure of I- insensitive thyroid cells to I- containing medium obtained after 2 h of incubation with dog thyroid slices, and to medium enriched with the 100,000 g supernatant fraction of homogenates prepared from these thyroid slices, did not restore the inhibitory action of I-. I- sensitivity of the cAMP response to TSH was restored by preincubation of I- insensitive cells in 10-4M I- plus an H2O2-generating system (glucose-glucose oxidase). T4 and T3 are probably not organic I inhibitors of the thyroid cAMP response to TSH. They provide evidence against the existence of a soluble, freely diffusible, organic I inhibitor of thyroid adenylate cyclase. The loss of sensitivity to I- inhibition of adenylate cyclase that occurs in thyroid cells shortly after initiation of primary culture appears to be related to a defect in the cellular organification mechanism, possibly the H2O2-generating system.