ONTOGENY AND ULTRADIAN RHYTHMS OF ADRENOCORTICOTROPIN AND CORTISOL INTHE LATE-GESTATION FETAL HORSE

Fetal maturation and the timing of parturition in both sheep and prima tes are thought to be controlled by the hypothalamic-pituitary-adrenal axis but little is known about the endocrinology of the equine fetus. We investigated the ontogeny of plasma concentrations of adrenocortic otropic hormone (ACTH), cortisol and corticosteroid binding capacity i n the late-gestation fetal horse. We also wished to determine whether there is ultradian rhythmic release of ACTH and cortisol in fetal hors es and we compared fetuses to maternal and non-pregnant adult horses. Six fetuses, 278-304 days gestation (term approximate to 335), were ca theterized and sampled daily until delivery. Mean (+/-S.E.M.) ACTH con centrations increased significantly from 159+/-21 to 246+/-42 pg/ml ov er the last 2 days before parturition. Fetal cortisol increased signif icantly from 3.1+/-1.0 to 13.4+/-3.7 ng/ml (mean+/-S.E.M.) over the la st 9 days before delivery. The slope of regressions for ACTH and corti sol concentrations with respect to time were positive in all subjects and statistically significant in 3 of 6 for ACTH and 5 of 6 for cortis ol. Fetal corticosteroid binding capacity declined from 49.5+/-20.5 to 16.1+/-2.2 ng/ml (mean+/-S.E.M.) over the last 10 days before parturi tion. However, the greatest changes in ACTH, cortisol and corticostero id binding capacity occurred very late in gestation, during the last 4 8 to 72 h before parturition. Significant peaks and nadirs in plasma A CTH concentration were detected in all 20 experiments and in plasma co rtisol concentration in 17 of 20 experiments using Cluster analysis. W e found statistically significant periods of oscillation between 11 an d 64 min in plasma ACTH (19 of 20 experiments) and cortisol (15 of 20 experiments) using power spectral density analysis. Statistically sign ificant periods between 11 and 17 min were detected in 11 of 20 experi ments for ACTH and in 8 of 20 for cortisol. We conclude that: 1) at th e end of gestation, equine fetal plasma ACTH and cortisol concentratio ns increase while corticosteroid binding capacity decreases suggesting that there is a disproportionately large increase in unbound cortisol at this time; 2) the secretion of ACTH and cortisol is rhythmic in bo th fetal and adult horses; 3) most animals exhibit a period of oscilla tion between 11 and 17 min; and 4) there is no apparent developmental change from late gestation to adulthood in the ultradian oscillator in fluencing ACTH and cortisol secretion in this species.