TRH-evoked entry of extracellular calcium in GH4C1 cells: possible importance of arachidonic acid metabolites.
Previous studies have shown that stimulating pituitary GH4C1 cells with thyrotropin-releasing hormone (TRH) evoked a biphasic change in cytosolic free Ca2+ concentration ([Ca2+]i): a rapid release of sequestered Ca2+ due to the production of inositol-1,4,5-trisphosphate, and Ca2+ entry via both voltage-operated Ca2+ channels and a presently unknown voltage-independent influx pathway. The aim of the present study was to further evaluate to which extent the TRH-evoked changes in [Ca2+]i were dependent on entry of extracellular Ca2+, and which mechanisms participated in regulating this Ca2+ entry. Pretreatment of the cells with 4-bromophenylacylbromide (an inhibitor of phospholipase A2), nordihydroguaiaretic acid (an inhibitor of lipoxygenase), and econazole (an inhibitor of both lipoxygenase and cytochrome P-450 enzymes), attenuated the TRH-evoked increase in [Ca2+]i, suggesting that noncyclooxygenase metabolites of arachidonic acid or cytochrome P-450 metabolites may participate in regulating the TRH-evoked entry of extracellular Ca2+. Both nordihydroguaiaretic acid and econazole showed a similar inhibition of the Ca2+ entry, as did SKF 96365, a compound previously shown to inhibit receptor-activated Ca2+ entry. We also showed that arachidonic acid per se increased [Ca2+]i, and acidified the cytosol in GH4C1 cells in a dose-dependent manner. The effects of arachidonic acid was reversed by addition of BSA to the cell suspension. The calcium entry and the activation of the metabolism of arachidonic acid may thus be important components of the TRH-evoked signal-transduction pathway in GH4C1 cells.
Törnquist
K
K
Department of Zoology, University of Helsinki, Finland.
Ekokoski
E
E
Forss
L
L
D000818:Animals; D016718:Arachidonic Acid / Q000378:metabolism* / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D004464:Econazole / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007093:Imidazoles / Q000494:pharmacology; D009637:Masoprocol / Q000494:pharmacology; D010741:Phospholipases A / Q000096:biosynthesis; D054467:Phospholipases A2; D010902:Pituitary Gland / Q000166:cytology; D051381:Rats; D013973:Thyrotropin-Releasing Hormone / Q000187:drug effects / Q000378:metabolism*
D007093:Imidazoles; D016718:Arachidonic Acid; D013973:Thyrotropin-Releasing Hormone; D004464:Econazole; D009637:Masoprocol; D010741:Phospholipases A; D054467:Phospholipases A2; C063159:1-(2-(3-(4-methoxyphenyl)propoxy)-4-methoxyphenylethyl)-1H-imidazole; D002118:Calcium