Dexamethasone induces advanced growth hormone expression in the fetal rat pituitary gland in vivo.

The effects of dexamethasone (DEX) on the onset of GH expression in the fetal rat pituitary gland were studied. Pregnant rats received oral administration of DEX dissolved in drinking water (25 mg/liter) for 40 h before killing on days 16-19 of pregnancy. The number of immunoreactive GH cells in the control fetus on day 18 was very small, whereas it was remarkably increased by DEX to a compatible value obtained from control fetuses on day 19. The effect of DEX on the induction of immunoreactive GH cells was found to be less potent on day 17 and not evident on day 16. The population of GH cells on day 19 was doubled, and they stained more intensely by GH antiserum after DEX treatment. In situ hybridization revealed that there were many GH messenger RNA (mRNA)-positive cells in the pituitary gland of a DEX-treated fetus at day 18, whereas they were negligible in number in the age-matched control. Northern blot analysis revealed that DEX induced accumulation of small amounts of GH mRNA in the fetal pituitary gland on day 17, whereas it induced a marked elevation of GH mRNA level on day 18. Reducing the levels of endogenous glucocorticoids by the administration of metyrapone (0.5 mg/ml in drinking water) to the pregnant rats from day 17-19 resulted in the significant reduction in the number of GH cells at day 19. Short-term treatment of DEX (duplicate sc injections at 18 h and 6 h before killing) also induced immunoreactive GH cells on day-18 fetuses but not on day-17 fetuses. Immunoreactive GH cells appeared on day 17 only after quadruplicate injections of DEX during last 40 h, suggesting that there is a difference in fetal sensitivity to DEX between days 17 and 18. These results indicate that immunoreactive GH cells and GH mRNA are inducible by DEX at an earlier stage of the fetal life than they normally appear. It is suggested that glucocorticoids play an important role in the development of GH cells in the fetal pituitary gland in vivo.