THYROID-HORMONE REPLACEMENT RESTORES CIRCULATING ENKEPHALIN CONCENTRATIONS IN HYPOPHYSECTOMIZED FETAL SHEEP

We have investigated the effects of fetal hypophysectomy (HX) with or without thyroxine (T-4) replacement on the plasma concentrations of fr ee methionine-enkephalin (free Met-Enk), noradrenaline, and adrenaline in late gestation sheep fetus. Plasma adrenaline concentrations were significantly higher in intact fetal sheep (1.05 +/- 0.12 pmol/L) betw een 125 and 140 days of gestation when compared with the HX + saline ( 0.64 +/- 0.10 pmol/L) and HX + T-4(0.61 +/- 0.08 pmol/L) groups. Durin g the first 15 days of the T-4 or Saline infusion, the plasma concentr ations of free Met-Enk were significantly higher in the HX + T-4 group (392 +/- 40 pmol/L) than in the HX + saline group (299 +/- 43 pmol/L) . At this stage of gestation, however, circulating concentrations of f ree Met-Enk were significantly higher in intact fetal sheep (556 +/- 5 1 pmol/L) than in either of the HX groups. Between 125 and 140 days of gestation, plasma free Met-Enk concentrations were similar and signif icantly higher in the intact and HX + T-4 groups than those measured i n the HX + saline fetal sheep. We conclude that the decrease in circul ating free Met-Enk concentrations after removal of the fetal pituitary is primarily a consequence of functional athyroidism.