This study was designed to investigate the effects of pulsatile infusion of ACTH into ovine fetuses on the endocrine changes that precede parturition, the timing of birth and the subsequent survival of the lamb. Where appropriate, these parameters were compared with fetuses infused with pulses of saline and uninfused normal term fetuses. Ten fetuses received a 15-min infusion of synthetic ACTH(1–24) (79 ng/min) from day 125 (n=9) or day 126 (n=1) of gestation. Seven fetuses were born prematurely within 174±14 h (mean ± s.e.m.) after the commencement of the infusion, i.e. at 132 ± 0·6 days, whilst three died in utero at 130–131 days. When born all lambs could breath, walk and suckle. Of the seven premature lambs, four died 2–10 days after parturition but three survived for at least 12 months after birth. Fetuses infused with pulses of ACTH exhibited intermittent but very large increases in plasma ACTH values, with the first pulse, on day 1, increasing ACTH values from 5·1 ± 1·1 to 140 ± 31·3 pmol/l (P<0·001). At the next sampling time, ACTH values were not significantly different from preinfusion values. A similar plasma ACTH profile was observed on each subsequent day of ACTH treatment. In contrast, fetuses (n=4) infused with pulses of saline between 125 and 131 days exhibited fetal plasma concentrations of ACTH which ranged between 2 and 12 pmol/l for the majority of the time. Of the uninfused fetuses (n=8) that were studied during the last week of normal gestation, seven were born alive at 148·9± 1·0 days of gestation, whilst one lamb was stillborn at 146 days. In these fetuses, plasma concentrations of ACTH increased slowly to 35·6 ±2·4 pmol/l on the day before delivery with a further increase to 76·4± 3·9 pmol/l occurring on the day of delivery. In fetuses infused with pulses of ACTH there was also a significant (P< 0·001) increase in the fetal cortisol to corticosterone ratio from a value of 2·9 before the commencement of the infusion to 69·1 just before birth. In ewes bearing uninfused fetuses born at normal term, maternal plasma concentrations of progesterone on day 4 before delivery were significantly (P<0·05) lower than on day 5 before delivery. In comparison, in ewes bearing fetuses infused with pulses of ACTH, a significant (P<0·05) decrease from maternal plasma concentrations of progesterone on day 5 before delivery did not occur until day 1 before delivery. In ewes bearing uninfused or prematurely delivered fetuses infused with pulses of ACTH, maternal plasma concentrations of oestrogen did not significantly (P<0·01) increase until the day of parturition. It is concluded that a minimum of 6–7 days of ACTH treatment is required by the fetal adrenal for the induction of cortisol synthesis sufficient to produce the birth of viable lambs. However, premature lambs have a 57% mortality rate in the 2- to 10-day period after birth.
Journal of Endocrinology (1990) 124, 99–107