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

The research investigates the functioning of the adrenal gland in premature and full-term foals immediately after birth. The study reveals that premature foals show less adrenal activity and cortisol secretion as compared to full-term foals, although both groups have high levels of adrenocorticotrophic hormone (ACTH). Catecholamine levels are also explored, showing a correlation between high levels and acidotic conditions.

Adrenal Function in Full-Term and Premature Foals

• The study scrutinizes the functioning of the adrenal gland, responsible for releasing hormones in response to stress, in both premature and full-term foals during their immediate post-natal period. The researchers particularly focus on cortisol, a stress hormone secreted by the adrenal cortex.
• Term foals observed in the study showed high plasma cortisol concentrations within the first two hours after birth, implying an enhanced adrenal function even before delivery. This was supported by significant arteriovenous differences in plasma cortisol detected across the umbilical circulation at birth.
• However, no such arteriovenous differences were detected in the premature foals, and their post-natal alterations in plasma cortisol were minimal, indicating a reduced adrenal activity.

Endogenous and Exogenous ACTH Response

• The inability of the adrenal gland in premature foals to secrete cortisol was not due to a lack of endogenous adrenocorticotrophic hormone (ACTH) as high levels of this hormone were detected in both groups immediately after birth. ACTH stimulates the adrenal cortex to release cortisol.
• The researchers also tested the adrenal gland’s sensitivity to an external or ‘exogenous’ dose of ACTH1-24. The results showed that term foals could elicit a strong response to this hormone on the day of birth, but this response declined over time. However, the same dose resulted in a slight response in the premature foals.

Catecholamine Levels

• The study also investigates the total plasma catecholamine concentrations in the newborn foals. Catecholamines are hormones produced by the adrenal glands that are involved in the body’s stress and fight-or-flight response.
• There was a wide range of these levels in both groups, but the highest values were seen in acidotic animals, suggesting a significant inverse relationship between blood pH (acidotic conditions) and the catecholamine level at delivery.