Desensitization of thyrotropin-releasing hormone receptor-mediated responses involves multiple steps.
Desensitization and recovery of the inositol 1,4,5-trisphosphate (IP3) and intracellular free calcium concentration ([Ca2+]i) responses to thyrotropin-releasing hormone (TRH) were measured in HEK293 cells stably expressing the G protein-coupled TRH receptor. TRH caused a large, rapid, and transient increase in IP3 and a biphasic increase in [Ca2+]i. Desensitization of the TRH response was measured by exposing cells to TRH, washing, and then incubating the cells in hormone-free medium before reintroducing TRH and measuring IP3, [Ca2+]i, and intracellular Ca2+ pool size. When cells were incubated with 1 microM TRH for 10 s or 10 min and reexposed to TRH, there was almost no IP3 or [Ca2+]i increase. The IP3 response recovered first, followed by the [Ca2+]i response. The ionomycin-releasable intracellular Ca2+ pool was almost completely depleted by TRH, and pool refilling was slow. Thrombin, endothelin, and carbachol, when combined, stimulated large increases in IP3 and [Ca2+]i, but did not block the IP3 or [Ca2+]i responses to TRH measured 10 min later. In contrast, cells exposed to TRH first responded to combined agonists with a nearly normal increase in IP3, but no rise in [Ca2+]i. Thus, the IP3 response to TRH displays homologous desensitization, whereas the [Ca2+]i response displays heterologous desensitization because depletion of intracellular Ca2+ pools prevents responses to other hormones.
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Department of Pharmacology and Physiology and the Cancer Center, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642, USA.
Hinkle
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D002118:Calcium / Q000378:metabolism; D002460:Cell Line; D002614:Chelating Agents / Q000378:metabolism; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000378:metabolism; D005456:Fluorescent Dyes / Q000378:metabolism; D016257:Fura-2 / Q000378:metabolism; D006801:Humans; D007202:Indicators and Reagents / Q000378:metabolism; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D007700:Kinetics; D018025:Receptors, Thyrotropin-Releasing Hormone / Q000378:metabolism*; D013973:Thyrotropin-Releasing Hormone / Q000378:metabolism
D002614:Chelating Agents; D005456:Fluorescent Dyes; D007202:Indicators and Reagents; D018025:Receptors, Thyrotropin-Releasing Hormone; D004533:Egtazic Acid; D013973:Thyrotropin-Releasing Hormone; D015544:Inositol 1,4,5-Trisphosphate; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium; D016257:Fura-2