Desensitization of thyrotropin-releasing hormone receptor-mediated responses involves multiple steps

Area of Science:

• Cellular signaling and endocrinology.
• G protein-coupled receptor (GPCR) desensitization mechanisms.

Background:

• Thyrotropin-releasing hormone (TRH) triggers signaling via its receptor, a GPCR.
• TRH stimulates inositol 1,4,5-trisphosphate (IP3) production and intracellular calcium release ([Ca2+]i).
• Understanding TRH receptor desensitization is crucial for cellular response regulation.

Purpose of the Study:

• To investigate the desensitization and recovery kinetics of TRH-induced IP3 and [Ca2+]i responses.
• To determine the mechanisms underlying homologous and heterologous desensitization in this system.
• To assess the role of intracellular calcium pool depletion in TRH response recovery.

Main Methods:

• HEK293 cells stably expressing the TRH receptor were used.
• Cells were exposed to TRH, washed, and re-exposed to measure IP3 and [Ca2+]i.
• Intracellular calcium pool size was assessed using ionomycin.
• Responses to TRH were compared with combined stimulation by thrombin, endothelin, and carbachol.

Main Results:

• TRH induced rapid and transient IP3 and biphasic [Ca2+]i increases.
• Pre-exposure to TRH caused significant desensitization of both IP3 and [Ca2+]i responses.
• The IP3 response recovered before the [Ca2+]i response.
• TRH treatment depleted intracellular calcium pools, with slow refilling.
• TRH-pre-treated cells showed homologous IP3 desensitization but heterologous [Ca2+]i desensitization due to pool depletion.

Conclusions:

• The TRH receptor exhibits homologous desensitization for IP3 production.
• The [Ca2+]i response to TRH displays heterologous desensitization, primarily due to intracellular calcium pool depletion.
• Intracellular calcium pool dynamics significantly influence the recovery and cross-desensitization of cellular responses.