The Timing of Progesterone-Induced Ribonucleic Acid and Protein Synthesis for Augmentation of Luteinizing Hormone Secretion*

Abstract

Progesterone addition to pituitary cells pretreated with estradiol leads within 45 min to an unambiguous augmentation of pulsatile GnRH-stimulated LH secretion. To investigate this rapid action, we established the kinetics of early events through manipulation of RNA synthesis, protein synthesis, and progesterone-receptor binding. Female rat pituitary cells cultured in medium containing charcoal-treated serum plus 0.2 nM estradiol were changed to 0.1% BSA-medium +/- 200 nM progesterone at time 0; at 90 and 150 min the cells were challenged with 1 nM GnRH 15-min pulses. The 3-fold augmentation of GnRH-stimulated LH secretion induced by progesterone was inhibited completely by simultaneous addition of 1 microM actinomycin D or emetine as was GnRH self-priming. In another series, the ability of cycloheximide to completely block progesterone augmentation was gradually diminished with delay of addition, but even 90 min after progesterone (30 min before GnRH pulse) cycloheximide resulted in 50% blockade of augmentation. In contrast, inhibition of RNA synthesis 60-90 min after progesterone introduction had little or no effect on progesterone augmentation. The temporal profile of inhibition by the progesterone antagonist RU486 was indistinguishable from that resulting from blockade of RNA synthesis and suggests that continual activation of the receptor is required for continued RNA synthesis. In summary: 1) both RNA and protein synthesis are required for GnRH self-priming; and 2) progesterone augmentation of GnRH-stimulated LH secretion requires RNA synthesis and synthesis of protein(s) which appear to be turning over rapidly, accumulating slowly, or both.