A B S T R AThis case is also compared with previous reports in the literature of patients with elevated serum levels of immunoreactive TSH in the presence of elevated total and free thyroid hormones. A classification of these cases, termed "inappropriate secretion of TSH," is proposed.

Endocytosis and recycling of both thyrotropin-releasing hormone (TRH) and its G-protein-coupled receptor were visualized by conventional and confocal fluorescence microscopy in pituitary cells using a rhodaminelabeled TRH analog (Rhod-TRH) and indirect immunofluorescent staining of cells stably transfected with an epitopetagged TRH receptor (TRHR). The epitope-tagged TRHR was confined to the cell surface prior to agonist treatment. Both Rhod-TRH and TRHR were also localized on the plasma membrane after agonist binding at 0°C. Ligand binding at 37C resulted in rapid endocytosis, and both Rhod-TRH and the epitope-tagged TRHR appeared in cytoplasmic vesicles within 5 min. Fluorescently labeled TRH and transferrin colocalized in the same endocytotic vesicles, and internalization of Rhod-TRH and TRHR was inhibited by hypertonic medium, suggesting that endocytosis occurred by a clathrindependent mechanism. Internalized TRHRs returned to the membrane within 20 min after removal of TRH, and cycloheximide did not block receptor recycling. A mutant TRHR truncated at Cys335 signaled but did not internalize Rhod-TRH, confirming the importance of the carboxyl terminus of the TRfIR in receptor-mediated endocytosis. Thus, the TRH-TRHR complex is endocytosed via clathrin-coated vesicles and the receptor is recycled to the plasma membrane.

A B S T R A C T Receptors for thyrotropin-releasing hormone (TRH) are present on mouse pituitary thyrotropic tumor cells. Incubation of thyrotropes with 100 nM TRH or 4 nM L-triiodothyronine (T3) for 48 h decreased the number of TRH receptors to -50 and 20% of control, respectively. There was no effect on the equilibrium dissociation constant which was 3-5 nM. The depletion in the number of available TRH receptors was time-and dose-dependent. TRH, 100 nM, decreased the receptor number to 70% after 24 h, 50% after 48 h, and 45% ofcontrol after 72 h. T3, 4 nM, decreased the receptor number to 52% after 24 h, 20% after 48 h, and 17% of control after 72 h. After 48 h, half-maximal depletion occurred with 1-2 nM TRH and -0.15 nM T3. Incubation with 100 nM TRH and 4 nM T3 caused a significantly greater reduction in the receptor level than either hormone alone. The decrease in the receptor level was reversible within 72 h after removal of TRH, 100 nM, but was only partially reversed, from 20 to 40% of control, after removal of T3, 4 nM, after 120 h. By regulating the number of available TRH receptors on the thyrotrope, TRH and T3 interact to control thyrotropin release.

No abstract

Consecutive challenges with thyrotropin-releasing hormone (TRH) of oocytes expressing the TRH receptor (TRH-R) resulted in a pronounced desensitization, manifested as a decrease in chloride current amplitude and an increase in response latency. Exposure to low concentrations of TRH resulted in a marked decrease in the amplitude of the subsequent response to a higher concentration of the agonist, even though the second challenge was given before the onset of the response to the first challenge (within 3 - 15 s). Cellular calcium concentration ([Ca]i) did not increase within this interval, suggesting that calcium was not involved in the desensitization process. The latency of the second response, however, was either unchanged or shortened, implying additive effects of processes initiated by the first challenge. A longer interval (30 s) between the two challenges brought about a more pronounced decrease in amplitude and a prolongation of response latency. The calcium mobilization initiated by a second challenge with a high concentration of the agonist exhibited a longer latency, a lower rate of [Ca]i increase and a lower amplitude. Stimulation of co-expressed cholinergic-muscarinic ml receptors with a low concentration of acetylcholine resulted in a pronounced desensitization of the TRH response (heterologous desensitization). Activation of protein kinase C by beta-phorbol 12-myristate, 13-acetate resulted in a dose-dependent inhibition of the response to TRH, suggesting that protein kinase C was involved in desensitization. Chelerythrine, a specific inhibitor of protein kinase C, abolished a large part of the desensitization. A mutant of the TRH-R that lacks protein kinase C consensus phosphorylation sites in the C-terminal region, exhibited desensitization.(ABSTRACT TRUNCATED AT 250 WORDS)