Phosphatidylinositol 4,5-bisphosphate turnover is transient while phosphatidylinositol turnover is persistent in thyrotropin-releasing hormone-stimulated rat pituitary cells.

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Abstract
Stimulated inositolphospholipid turnover has been proposed to be initiated and sustained by hydrolysis of phosphatidylinositol 4,5-bisphophate [PtdIns(4,5)P2], which may be replenished by an enhanced flux of phosphatidylinositol (PtdIns) to PtdIns 4-phosphate (PtdIns4P) to PtdIns(4,5)P2. To determine whether there is continued hydrolysis and resynthesis of PtdIns(4,5)P2 in rat pituitary cells (GH3 cells) during stimulation by thyrotropin-releasing hormone (TRH), we investigated the turnover kinetics of the inositolphospholipids and of phosphatidic acid (PtdOH). In cells incubated with 32Pi for 1 min, TRH rapidly and persistently (for at least 30 min) enhanced the rate of 32P-labeling of PtdOH. After a lag time of 1 min, TRH markedly and persistenly increased 32P-labeling of PtdIns also. In contrast, TRH caused only a transient increase in 32P-labeling of PtdIns(4,5)P2 that lasted < 2 min. There was no rapid (before 10 min) effect of TRH on 32P-labeling of PtdIns4P. By 2 min of TRH stimulation, specific 32P radioactivity in PtdOH increased from 3.6% (control) of that in the .gamma.-phosphate of ATP to 15%; in PtdIns, from 0.07% to 1.3%; and in PtdIns(4,5)P2, from 3.8% to 5.4% (specific 32P radioactivity in PtdIns4P was 1.7% of that in ATP in control and TRH-stimulated cells). In cells exposed to TRH for 4 min and then to 32Pi, 32P-labeling of PtdOH and PtdIns increased, but that of PtdIns(4,5)P2 was not affected. Last, persistent turnover of PtdOH and PtdIns was not caused by initial hydrolysis of PtdIns(4,5)P2 because the turnover of PtdOH and PtdIns could be terminated by displacement of TRH from its receptor by chlordiazepoxide and restarted by reoccupying the receptors with TRH. These data demonstrate that turnover of PtdIns(4,5)P2 is stimulated only transiently, whereas turnover of PtdIns(4,5)P2 is stimulated persistently by TRH in GH3 cells. Hence, inositolphospholipid turnover in GH3 cells does not occur via continued hydrolysis of PtdIns(4,5)P2 accompanied by enhanced flux of PtdIns to PtdIns4P to PtdIns(4,5)P2, but there is direct and persistent hydrolysis of PtdIns. The dissociation of these actions suggests that there are separate mechanisms involved in coupling TRH-receptor complexes to stimulation of PtdIns(4,5)P2 and PtdIns hydrolysis in GH3 cells.