Inhibition of stimulated and basal ACTH by cortisol during ovine pregnancy

In pregnant ewes, as in pregnant women, plasma adrenocorticotropic hormone (ACTH) and cortisol concentrations are increased. Inhibition of free cortisol concentrations by dexamethasone, a synthetic glucocorticoid, is reduced in pregnant women compared with nonpregnant women. These experiments were designed to test the hypothesis that basal and stimulated ACTH concentrations are less sensitive to negative feedback inhibition by cortisol in pregnant ewes than in nonpregnant ewes. Ewes were infused with vehicle and with cortisol at two different rates (1 and 2 micrograms.kg-1.min-1) for 1 h; plasma ACTH concentrations during and after the infusion and after subsequent stimulation by hypotension were measured. Basal plasma ACTH concentrations during a 2-h infusion of cortisol (2 micrograms.kg-1.min-1) were also measured in undisturbed ewes. Cortisol significantly inhibited both stimulated and basal ACTH. The degree of suppression of ACTH was not reduced in the pregnant ewes compared with the nonpregnant ewes. The results indicate that both basal and stimulated ACTH are sensitive to negative feedback inhibition by cortisol during ovine pregnancy.