Response of the adrenal cortex to tetracosactrin (ACTH1-24) in the premature and full-term foal.

The changes in plasma cortisol concentration in the immediate postnatal period were examined in 3 groups of newborn foals and the response of the adrenal cortex to exogenous ACTH1-24 (tetracosactrin) was tested in 2 of these groups. In full-term Thoroughbred and Pony foals a rise in plasma cortisol occurred between 0 and 30 min after birth, whereas no significant cortisol changes could be detected within 2 h of birth in the group of prematurely delivered foals. These differences in plasma cortisol between term and premature foals were accompanied by differences in blood pH and lymphocyte and neutrophil counts. The high cortisol concentrations in the term animals declined to low levels by 4-12 h after birth. There was also a clear distinction between the responsiveness of the adrenal cortex to ACTH in the term and premature animals. When short-acting tetracosactrin was given as a single i.m. injection (0.125 mg), a rapid rise in plasma cortisol occurred in all term foals whereas no response was detected in the premature group during the same 2-h test period. The adrenal cortical response was also accompanied by a significant fall in the neutrophil/lymphocyte ratio at 2 h after the injection; no such change was seen in the premature animals. After a 24-h period of treatment with depot tetracosactrin (3 doses; 0.4, 0.2, 0.2 mg i.m.), a 2nd test with short-acting tetracosactrin led to a rise in plasma cortisol in term and pre-term foals during the 2-h test period. The depot tetracosactrin also appeared to cause a 3-fold increase in basal plasma cortisol levels in both groups.