Thyrotropin (TSH)-Releasing Hormone Decreases Phosphatidylinositol and Increases Unesterified Arachidonic Acid in Thyrotropic Cells: Possible Early Events in Stimulation of TSH Secretion*

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Abstract
TRH stimulated the metabolism of lipids of the phosphatidylinositol (PI)-phosphatidic acid (PA) cycle and caused an increase in the level of free or unesterified arachidonic acid in mouse pituitary thyrotropic tumor (TtT) cells. In cells labeled with [32P]orthophosphate for 45 min, TRH caused a rapid specific increase in [32P]PA to 190 .+-. 8% (.+-. SE) of the control value at 15 s (P < 0.005) and in [32P]PI to 158 .+-. 8% at 2 min (P < 0.005). In cells labeled to isotopic steady state with [3H]inositol, TRH caused a decrease in [3H]PI to 92 .+-. 1.8% of the control value at 1 min (P < 0.01) and increased the level of [3H]inositolmonophosphate. In cells labeled to isotopic steady state with [14C] stearic acid, TRH caused a transient rise in [14C]diacylglycerol and a more prolonged increase in [14C]PA. In cells labeled to isotopic steady state with [3H]arachidonic acid, TRH stimulated a rise in free [3H]arachidonic acid to 210 .+-. 8% of the control value at 15 s (P < 0.001), with a return to a level 125 .+-. 2% of the control value by 5 min. Arachidonic acid added exogenously caused efflux of 45Ca2+ from prelabeled cells and stimulated TSH secretion. Hence, in TtT cells, TRH rapidly stimulated a decrease in the level of PI and increased inositolmonophosphate, diacylglycerol and PA; and caused a rapid increase in the level of free arachidonic acid. These effects may be important in stimulation of TSH secretion by TRH. Because arachidonic acid, when added exogenously, mobilized cellular Ca2+ and stimulated TSH secretion, arachidonic acid may mediate, at least in part, TRH-stimulated TSH secretion. The action of TRH on lipid metabolism in TtT cells is different from that in mammotropic pituitary cells, since TRH does not cause an increase in the level of free arachidonic acid in GH3 cells.

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