Prepartum adrenocortical maturation in the fetal foal: responses to ACTH.

The present study was carried out on 19 chronically catheterized mares and fetuses in late gestation (term > 320 days). In six animals which were monitored up to the time of delivery of a live foal, plasma and amniotic fluid cortisol concentrations remained low until 4-5 days before parturition when there was a rapid, significant rise (P < 0.05) which was not accompanied by any corresponding changes in maternal plasma cortisol. Circulating fetal ACTH concentrations became more variable close to delivery and ANOVA revealed no significant increases during this critical period, although a negative correlation between plasma ACTH and time to delivery was observed (P < 0.05). Tests on fetal adrenal responsiveness to exogenous ACTH1-24 were carried out on ten animals. Before 295 days of gestation no significant increases in fetal plasma cortisol above its basal level of 20-30 nmol/l could be elicited by ACTH, administered as a single i.v. injection (1-2 micrograms/kg). By 304 +/- 3 days (mean +/- S.E.M.) small but significant (P < 0.05) increments in plasma cortisol were detected after ACTH, while in the oldest group (313 +/- 2 days) significant (P < 0.01) 50-60% increments were seen throughout the test period (2 h). Only one fetus was tested within 3 days of delivery and here a fourfold rise in plasma cortisol was evoked by ACTH.(ABSTRACT TRUNCATED AT 250 WORDS)