Thyrotropin-releasing hormone stimulates rapid loss of phosphatidylinositol and its conversion to 1,2-diacylglycerol and phosphatidic acid in rat mammotropic pituitary cells. Association with calcium mobilization and prolactin secretion
Thyrotropin-releasing hormone (TRH) stimulated the rapid and specific hydrolysis of phosphatidylinositol (PI) and its conversion to 1,2-diacylglycerol and phosphatidic acid (PA) in GH3 cells, cloned rat pituitary cells that secrete prolactin. The TRH induced fall in the level of PI occurred by 15 s and resulted in a lower steady state level by 2 min (1.78 ± 0.05 versus 1.38 ± 0.03 nmol of PI/106 cells). TRH stimulated the formation of PA which increased from 0.10 ± 0.02 to 0.17 ± 0.01 nmol of PA/106 cells by 2 min. In seven experiments, cellular PI was 83 ± 5.0% of control (p < 0.001) an PA was 190 ± 49% (p < 0.001) after 5 min of exposure to TRH. In cells labeled with [3H]inositol, TRH stimulated the loss of [3H]PI and increased the levels of free [3H]inositolmonophosphate and free [3H]inositol. The effect of TRH was concentration-dependent; half-maximal PI loss occurred at 3 nM TRH. In cells labeled with [3H]arachidonic acid and [14C] stearic acid, TRH caused a similar decrease in PI to 90% of control as measured as 3H or 14C radioactivity and an increase in PA to 185% of control as measured as 3H radioactivity and 150% of control as measured as 14C radioactivity. TRH caused a transient increase in 1,2-diacylglycerol to 120% of control in a time course compatible with PI to 1,2-diacylglycerol to PA conversion. TRH caused an increase in the 3H/14C radiactivity ratio in PA to a level higher than in any major lipid except PI and a transient rise in this ratio in 1,2-diacylglycerol. These data demonstrate that TRH stimulates the hydrolysis of PI by a specific phospholipase C activity and leads to the formation of 1,2-diacylglycerol and PA in GH3 cells. The rate of TRH-stimulated PI degradation was compared to our previous data on 45Ca2+ efflux and prolactin secretion. The maximum rates of PI degradation and 45Ca2+ efflux occurred by 15 s. A model of TRH action is presented in which PI breakdown and PA formation cause mobilization of cellular Ca2+ and prolactin secretion.