In the fetal sheep, plasma Cortisol concentrations gradually increase in the last weeks of gestation and abruptly rise during the final 48-72 h preceding birth. To determine if these changes in mean circulating Cortisol concentrations result from increased pulsatile secretion and are driven by changes in ACTH pulses, blood samples from five chronically catheterized fetuses were collected every 5 min for 2 h at 133 days gestation and every 4 days thereafter until delivery at 146 ± 2 days. Volume was replaced after each blood sample and erythrocytes were returned every 20 min. Plasma Cortisol and ACTH secretion were pulsatile in fetuses at all ages. Cortisol pulse frequency increased significantly with gestation from a mean of 2.2 pulses/2 h at 133 days to 4.8 pulses/2 h at 146 days. The interpulse interval (mean ± se) decreased between 133 and 146 days from 54 ± 11 min to 23 ± 3 min, respectively. Cortisol pulse amplitude increased significantly from 10 ± 2 ng/ml at 133 days to 44 ± 13 ng/ml at 146 days. In contrast to Cortisol, ACTH pulse frequency (3 ± 0.6 pulses/2 h) and amplitude (21 ± 3 pg/ml) were similar at 133 days and 146 days. The coincidence of Cortisol and ACTH pulses did not change between 133 and 146 days. Furthermore, the number of coincident pulses failed to exceed random associations (hypergeometric probability analysis) and could have occurred by chance alone (P values ranged from 0.11-0.63). A point by point comparison of Cortisol and ACTH concentrations in fetal circulation indicate that only 36% of the variance in Cortisol concentrations could be explained by variance in ACTH (cross-correlation analysis). These data suggest that fetal Cortisol and ACTH secretion are pulsatile and that, as gestation advances, increases in constitutive Cortisol pulse amplitude and frequency may not be predominantly driven by pulsatile changes in ACTH in the ovine fetal circulation near term.
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