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*

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 detailed 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 in the presence or absence of guanylylimidodiphosphate. These results suggest that stimulation of TRH release by TRH from these cells does not involve cAMP as an intracellular messenger.

Dynamics and Regulation of TSH Secretion by Superfused Anterior Pituitary Cells

“…Somatostatin is reported to act to inhibit TSH secretion via an inhibitory action on cyclic AMP accumulation in thyrotrophs [1]. It has been speculated that TRH acts to stimulate TSH secretion by increasing the activity of the enzyme adenyl cyclase leading to an increase in intracellular cyclic AMP [ 17,32], However, evidence against the direct involvement of cyclic AMP in the stimulation of TSH secretion can also be found [3,9,12,27], Using a similar preparation of superfused anterior pituitary cells Schrey et al [22,23] found that the phosphodiesterase inhibitors, theophylline and isobutylmethylxanthine, stimulate TSH release and potentiate the response to TRH. In the present study dibutyryl cyclic AMP (3.0 X 10-3 M) was ineffective in eliciting TSH secretion.…”

“…Dispersal of anterior pituitary cells permits uni form exposure to various agents and efficient oxygenation of the cells. The present study is primarily concerned with the secretion of thyrotropin 1 Preliminary reports have appeared [6,12]. …”

Role of a pituitary-specific transcription factor (pit-1/GHF-1) or a closely related protein in cAMP regulation of human thyrotropin-beta subunit gene expression.

“…A more recent study identified the neuropeptide arginine vasopressin as a possible candidate for a physiological stimulator of TSH secretion acting through the cAMP system (3). In this study the efficiency of arginine vasopressin was comparable to that of TRH, which may act independently from the cAMP system (4). Recent studies with pituitary cells in primary culture suggested that these regulators in some way also affect the production of both subunits at a pretranslational level (5,6).…”

A modulatory role for cyclic AMP in the control of thyrotrophin release: studies with forskolin and dibutyryl cyclic AMP

“…Although earlier studies showed that thyrotrophin-releasing hormone (TRH) could increase cAMP content in pituitary cells (Bowers, 1971;Labrie, Borgeat, Lemay et al 1975), it has been shown by others that TSH release by TRH can be dissociated from changes in cAMP content (Gershengorn, Rebecchi, Geras & Arevalo, 1980;Morley, 1981;Naor, 1982). Further evidence is accumulating that TRH action is mediated by hydrolysis of membrane phosphatidyl-inositol, lead¬ ing to increased intracellular free calcium levels (Geras, Rebecchi & Gershengorn, 1982;Gershengorn, 1982;Kolesnick, Musacchio, Thaw & Gershengorn, 1984), and it is now generally accepted that changes in intracellular free calcium, rather than cAMP, are the primary mechanism for stimulus-release coupling in the thyrotroph.…”