Regulation of gonadotropin subunit messenger ribonucleic acid expression by gonadotropin-releasing hormone pulse amplitude in vitro.

The present study examined the effect of GnRH pulse amplitude on alpha, LH beta and FSH beta mRNAs using an in vitro perfusion model. Pituitaries from 30-day-old female rats were dissociated and the cells plated for 48 h to allow attachment to collagen-coated microcarrier beads. The beads were loaded into perifusion chambers, preperifused for 1 h, and then given GnRH pulses (17.5-175 pg/ml) every 30 min for 24 h. Perifusate LH was measured after 2 h and 22 h of perifusion and alpha LH and FSH beta messenger RNAs (mRNAs) were determined by hybridization to complementary DNA (cDNA) probes. All doses of GnRH acutely stimulated LH release, and responses were similar after 2 h and 22 h. LH release increased as a function of GnRH pulse dose with maximal increases seen following 70 pg/ml pulses. alpha mRNA levels (control = 0.73 +/- 0.1 fmol cDNA bound/100 micrograms pituitary DNA) were increased 30% and 40% after 24 h of 35 and 70 pg/ml pulses, respectively (P < 0.05 vs. media controls). LH beta mRNA concentrations (control = 0.44 +/- 0.08 fmol cDNA bound) were only elevated after 35 pg/ml GnRH pulses (36% increase). FSH beta mRNA showed the largest responses to GnRH pulses, increasing by 45% and 84% after 35 and 70 pg pulses, respectively (control = 0.14 +/- 0.02 fmol bound). The highest GnRH pulse dose (175 pg/ml) was ineffective in stimulating an increase in FSH beta mRNA levels. These results show that all three gonadotropin subunit mRNA concentrations increase after 24 h of GnRH pulses, but the pattern of individual subunit mRNA responses was dependent upon the amplitude of the GnRH pulse stimulus. These data support earlier results in vivo, in that the subunit responses to GnRH pulse dose were similar. Thus, alterations in the amplitude of pulsatile GnRH secretion from the median eminence may be one mechanism by which the expression of gonadotropin subunit genes are regulated.