TRH-EVOKED ENTRY OF EXTRACELLULAR CALCIUM IN GH(4)C(1) CELLS - POSSIBLE IMPORTANCE OF ARACHIDONIC-ACID METABOLITES

Previous studies have shown that stimulating pituitary GH(4)C(1) cells with thyrotropin-releasing hormone (TRH) evoked a biphasic change in cytosolic free Ca2+ concentration ([Ca2+](i)): a rapid release of sequ estered Ca2+ due to the production of inositol-1,4,5-trisphosphate, an d Ca2+ entry via both voltage-operated Ca2+ channels and a presently u nknown voltage-independent influx pathway. The aim of the present stud y was to further evaluate to which extent the TRH-evoked changes in [C a2+](i) were dependent on entry of extracellular Ca2+, and which mecha nisms participated in regulating this Ca2+ entry. Pretreatment of the cells with 4-bromophenylacylbromid (an inhibitor of phospholipase A(2) ), nordihydroguaiaretic acid (an inhibitor of lipoxygenase), and econa zole (an inhibitor of both lipoxygenase and cytochrome P-450 enzymes), attenuated the TRH-evoked increase in [Ca2+](i), suggesting that nonc yclooxygenase metabolites of arachidonic acid or cytochrome P-450 meta bolites may participate in regulating the TRH-evoked entry of extracel lular Ca2+. Both nordihydroguaiaretic acid and econazole showed a simi lar inhibition of the Ca2+ entry, as did SKF 96365, a compound previou sly shown to inhibit receptor-activated Ca2+ entry. We also showed tha t arachidonic acid per se increased [Ca2+](i), and acidified the cytos ol in GH(4)C(1) cells in a dose-dependent manner. The effects of arach idonic acid was reversed by addition of BSA to the cell suspension. Th e calcium entry and the activation of the metabolism of arachidonic ac id may thus be important components of the TRH-evoked signal-transduct ion pathway in GH(4)C(1) cells.