Thyrotropin-Releasing Hormone (TRH) Action in Mouse Thyrotropic Tumor Cells in Culture: Evidence against a Role for Adenosine 3′,5′-Monophosphate as a Mediator of TRH-Stimulated Thyrotropin Release*

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Abstract
It has been suggested that TRH stimulation of TSH release is mediated by the adenylate cyclase-cAMP system. To determine whether cAMP is a necessary intracellular messenger for TRH stimulation of TSH release, we have performed etailed studies of the TRH effect employing a nearly homogeneous population of mouse thyrotropic tumor cells in culture. Dibutyryl cAMP, methylisobutylxanthine, and cholera toxin caused an increase in TSH release which was additive to that of TRH. TRH stimulated TSH release in a dose-dependent fashion; half-maximal stimulation occurred at approximately 0.6 nM but had no effect on total intracellular cAMP levels measured in the presence or absence of methylisobutylxanthine. There was no correlation between total intracellular cAMP levels and TSH release after 1 h. Moreover, there was no effect of TRH on protein kinase-bound or total intracellular cAMP levels at 1, 5, or 60 min of incubation. Lastly, TRH had no effect on adenylate cyclase activity in homogenates of thyrotropic cells n the presence or absence of guanylylimidodiphosphate. These results suggest that stimulation of TSH release by TRH from these cells does not involve cAMP as an intracellular messenger.

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