By 94 days of gestation plasma cortisol increases block ACTH response to hypotension in lamb fetuses

By use of a crossover design, we studied the effects of increasing plasma cortisol concentration on ACTH responses to a standardized stress in 14 lamb fetuses between 94 and 108 days gestation. On a random basis we assigned the animals into two groups of seven. Animals in groups I and II received infusions of cortisol (5 and 1 microgram/min, respectively) or saline for 4 h. After the cortisol or saline pretreatment, we reduced arterial pressure approximately 40-50% in both groups of animals with nitroprusside. After saline pretreatment, hypotension in the group I animals produced an increase in the fetal plasma ACTH from 15 +/- 3 to 200 +/- 20 pg/ml (P less than 0.001), and in the group II animals pretreated with saline plasma ACTH increased from 21 +/- 4 to 141 +/- 19 pg/ml (P less than 0.001) with hypotension. Cortisol pretreatment elevated fetal plasma cortisol levels from 7 +/- 3 to 36 +/- 5 ng/ml in group I and from 8 +/- 4 to 20 +/- 2 ng/ml in group II. The ACTH response to hypotension in both groups was abolished by the cortisol pretreatment. We conclude that by 94 days gestation increases in plasma cortisol within a physiological range block ACTH responses to hypotension in lamb fetuses.