BIOACTIVE-TO-IMMUNOREACTIVE ACTH ACTIVITY CHANGES WITH SEVERITY OF STRESS IN LATE-GESTATION OVINE FETUS

The late-gestation ovine fetus (>130 days, 0.90 gestation, full term 1 45 days) is capable of increased pituitary-adrenal activity following a variety of stressors. To examine the relationship between plasma imm unoreactive adrenocorticotropic hormone (iACTH) and bioactive ACTH-lik e activity (bACTH), late-gestation fetuses were studied under nonstres s (group I, chronically catheterized), ''mild'' stress (group II, acut ely exteriorized), or marked stress (group III, acutely exteriorized a nd hemorrhaged 30% of blood volume). Plasma iACTH was determined by st andard radioimmunoassay. Plasma bACTH was determined by a bioassay tha t utilized dispersed rat adrenal cells. Plasma iACTH was lower in grou p I than in groups II or III (78.4 +/- 16.3 vs. 320 +/- 116 and 622 +/ - 144 pg/ml, respectively, mean +/- SE, P < 0.05 group I vs. III). Pla sma bACTH was significantly lower in groups I and II than in group III (13.8 +/- 2.7 and 52 +/- 22 vs. 601 +/- 106 pg/ml, respectively, P < 0.05). The ratio of bACTH to iACTH was low in groups I and II but elev ated in group III (0.188 +/- 0.028 and 0.091 +/- 0.07 vs. 0.996 +/- 0. 122, P < 0.05). We conclude that the amount of bACTH compared with iAC TH in the late-gestation fetus is low under nonstress states or mild s tress and that bACTH increases disproportionately compared with iACTH after marked stress. This suggests that a change in ACTH processing an d/or secretion is associated with stimulus intensity in the late-gesta tion ovine fetus.