The Bidirectional Interaction Between Insulin and the Hypothalamus–Pituitary–Adrenal Axis in Normal Pregnant Mares

The aim of this study was to determine if the changes in plasma insulin, glucose (GLU), fructosamine (FRUCT), adrenocortical hormone (ACTH), and cortisol (CORT) concentrations in mares of different ages were substantial enough to indicate the need to also establish specific reference intervals for pregnant Spanish Purebred mares with a heterogeneous body conditional score (BCS). A total of 45 mares were used in the study, which were classified according to age into 24 <10 years (from 4 to 9 years) and 21 >10 years (from 10 to 18 years). According to the BCS, mares <10 and >10 years were distinguished into three groups as follows: underweight (BCS < 4–5; n = 8), moderate (BCS = 6–7; n = 8), and overweight (BCS = 8; n = 8) (BCS < 4–5 (n = 7), BCS = 6–7 (n = 7), and BCS = 8 (n = 7)), respectively. The main results of this study were that (I) circulating insulin, GLU, FRUCT, ACTH, and CORT concentrations were altered throughout the whole duration of pregnancy in mares; that (II) aging and BCS significantly affected insulin, ACTH, and CORT changes; and that (III) ACTH-CORT significantly correlated with insulin, FRUCT, and GLU. The results may have implications for health and disease and warrant future prospective investigations on the bidirectional interaction between insulin and the hypothalamus–pituitary–adrenal (HPA) axis in equine species, affecting the GLU and FRUCT profile through the entire physiological pregnancy.