Circulating Hypothalamic-Pituitary-Adrenal Axis Hormones and Insulin Concentrations in Horses and Ponies

Mechanisms resulting in breed predispositions to insulin dysregulation (ID) are poorly characterized. Cortisol antagonizes insulin, and free, biologically active cortisol can be increased in ID. Breed-related differences in serum free cortisol fraction (FCF) could contribute to ID, but FCF has not been quantified in equidae predisposed to ID, such as ponies. To compare FCF and other hypothalamic-pituitary-adrenal (HPA) axis hormones between horses and ponies during health and ID. We hypothesized: (1) FCF is higher in ponies than horses in health, and is higher still in ponies with ID and obesity; and (2) FCF is positively correlated with insulin in horses and ponies during health and ID. Thirty-three horses and 24 ponies were sampled before morning feeding in their normal routine. Plasma ACTH and insulin and serum total cortisol concentrations and FCF were measured. ID was defined as evidence of hyperinsulinemia at rest or after oral sugar administration. Data were compared with Mann-Whitney tests and Spearman correlation analysis (P < 0.05). Total cortisol, free cortisol, insulin concentrations, and FCF were comparable in healthy horses (n = 24) and ponies (n = 12), but ACTH concentrations were 29% higher in ponies than in horses (P = 0.016). In animals with ID, total cortisol, free cortisol, and insulin concentrations were similar between horses and ponies, but FCF was increased 40% in ponies (n = 12) compared to horses (n = 9). These data demonstrate differences in insulin, ACTH, and free cortisol during health and ID between ponies and horses.