Adrenal glands of foetal sheep of 40 days gestation to term were incubated with and without ACTH or an increased [K+]. With ACTH, the 40 day foetal adrenal was capable of producing more cortisol and aldosterone per g body weight than was the term adrenal. ACTH was a potent stimulus to aldosterone and cortisol production in foetuses aged 60\p=n-\90days, and this effect declined significantly in the 91\p=n-\120day period. An increased [K+] was stimulatory to aldosterone production only after 120 days gestation. Peripheral blood levels of aldosterone, corticosterone, cortisol, 11-deoxycortisol and 11-deoxycorticosterone were measured in foetuses 60 days to term and the levels of aldosterone and cortisol were significantly lower in 90\p=n-\120day foetuses than in the younger or older ones. Direct adrenal vein cannulation proved all five steroids to be secretory products of the foetal adrenal.The ovine foetal cortex has been shown to play a significant role in the onset of parturition in this species (Liggins 1968) and for this reason its ability to
Aldosterone, corticosterone, cortisol, 11-deoxycortisol and 11-deoxycorticosterone were measured, using a double isotope dilution derivative assay, in whole blood samples of ovine foetuses bearing chronic vascular cannulae. All values are quoted as mean \m=+-\ standard error of mean. n = number of samples. Parturition occurred at 142\p=n-\152 days. The concentration of aldosterone (ng/100 ml blood) was 8.7 \ m=+-\2.5 at 100\p=n-\109days, (n = 8), 4.4 \ m=+-\1.2 at 110\p=n-\140 days (n = 32) and 11.5 \m=+-\2.0 (n = 6) at 140 days until term. The value in unstressed, sodium replete pregnant sheep was 1.2\m=+-\0.4 (n = 12). Neither corticosterone nor 11-deoxycortisol concentrations varied throughout gestation and were respectively 0.08 \m=+-\0.02 and 0.03 \ m=+-\0.04 \g=m\g/100ml blood in 56 samples. These values are essentially the same as those of pregnant, unstressed sheep. Total blood cortisol concentration was 0.10 \ m=+-\0.01 \g=m\g/100ml in 38 samples from foetuses younger than 135 days, which is lower than the value for pregnant, unstressed sheep of 0.43 \ m=+-\ 0.07 (n = 34). Cortisol values rose after 135 days to reach 0.93 \ m=+-\ 0.3 (n = 7) after 140 days. The concentration of 11-deoxycorticosterone was 33.8 \m=+-\8.0 ng/100 ml blood (n = 7) in the period 100\p=n-\109 days, then decreased to 18.7 \ m=+-\4.4 (n = 45) from 110 days to term. The concentration
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