Studies on equine prematurity 2: Post natal adrenocortical activity in relation to plasma adrenocorticotrophic hormone and catecholamine levels in term and premature foals.

Adrenocortical and medullary function was investigated during the immediate post natal period in premature and full term foals. High plasma cortisol concentrations were characteristic of the term foals in the first 2 h after birth and these were accompanied by significant arteriovenous differences in plasma cortisol across the umbilical circulation at birth, indicating enhanced adrenal activity before delivery. No such arteriovenous differences were detected in the premature group and post natal changes in plasma cortisol were minimal. The apparent inability of the premature foal adrenal to secrete cortisol was not due to the lack of endogenous adrenocorticotrophic hormone (ACTH) because high levels of this hormone were found immediately after birth in both groups of foals. Tests on the sensitivity of the foal adrenal to exogenous ACTH1-24 (0.125 mg intramuscularly [im]) showed that a maximum response to this hormone could be elicited in term foals on the day of birth. Subsequently basal cortisol levels and the response of the adrenal to ACTH1-24 declined. By contrast, only a slight response was observed following the same dose of ACTH1-24 in the premature group. Exposure to Depot ACTH1-24 over 24 h enhanced the basal secretion of cortisol in both premature and term foals but no consistent response to the same ACTH test dose could be elicited in the former. A wide range of total plasma catecholamine concentrations was observed in both groups of newborn foals. The highest values were seen in acidotic animals and there was a significant inverse relationship between blood pH and total plasma catecholamine level at delivery.(ABSTRACT TRUNCATED AT 250 WORDS)