FETAL ADRENALECTOMY DOES NOT AFFECT CIRCULATING ENKEPHALINS IN THE SHEEP FETUS DURING LATE-GESTATION

We have measured circulating catecholamines and enkephalins in intact and bilaterally adrenalectomised fetal sheep between 115 and 144 days of gestation using specific radioimmunoassays for total Met-enk-contai ning peptides (total Met-Enk), free Met-Enk and Met-Enk-arg6-phe7 (MER F). In the intact group fetal plasma concentrations of noradrenaline i ncreased from 1.7 +/- 0.4 (115-124 days) to a peak of 3.7 +/- 0.6 pmol /ml (135-144 days; all results expressed as means +/- standard error o f the mean). The mean plasma concentration and the gestational age pro file of noradrenaline were the same in the intact and adrenalectomised fetal sheep. We observed no change in the fetal plasma concentrations of adrenaline between 115 and 144 days of gestation and there was als o no effect of removal of both fetal adrenal glands on plasma adrenali ne concentrations. In the intact fetal sheep there was a significant i ncrease in the circulating concentration of free Met-Enk between 115-1 19 (497.7 +/- 128.4 pmol/1) and 125-129 days of gestation (647.8 +/- 5 9.5 pmol/1). There was a similar increase in plasma MERF concentration s between 115-119 (850.4 +/- 170.4 pmol/1) and 130-134 days (1,525.1 /- 227.0 pmol/1). There was no change, however, in the plasma concentr ations of total Met-Enk across this gestational age range. The mean ci rculating concentrations and the gestational age profiles of plasma to tal and free Met-Enk and MERF were the same in the intact and adrenale ctomised fetal sheep across the age range studied. We have demonstrate d therefore that during unstressed conditions the fetal adrenal medull a is not the major source of circulating enkephalins. There is an incr ease in the fetal plasma concentrations of free Met-Enk and MERF and a decrease in the circulating total:free Met-Enk ratio at between 115 a nd 125 days of gestation. These changes may reflect changes in the syn thesis and/or secretion of enkephalin peptides within fetal sympatheti c neurones.