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 phosphatidylinosito1 (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 f 0.05 uersus 1.38 f 0.03 nmol of PI/106 cells). TRH stimulated the formation of PA which increased from 0.10 f 0.02 to 0.17 f 0.01 nmol of PA/106 cells by 2 min. In seven experiments, cellular PI was 83 f 5.0% of control ( p < 0.001) and PA was 190 f 49% ( p < 0.001) after 5 min of exposure to TRH. In cells labeled with [3H]inositol, TRH stimulated the loss of C3H]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 n~ TRH. In cells labeled with C3H]arachidonic acid.And ['"C] stearic acid, TRH caused a similar decrease in PI to 90% of control as measured as 3H or '"C mdioactivity and an increase in PA to 185% of contro) as measured as 3H radioactivity and 150% of controlas measured as 14C radioactivity. TRH caused a tra,dsient increase in 1,2-diacylglycerol to 120% of control in a time course compatible with PI to l,2-diacylglycerol to PA conversion. TRH caused an increase in the 3H/'4C radioactivity ratio in PA to a level higher than in any major lipid except PI and a transient rise in this ratio in 1,a-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 4sCa2+ 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.