Endocrine and Electrolyte Balances during Periovulatory Period in Cycling Mares

In cycling females, the periovulatory period is characterized by stimulation of the hypothalamic pituitary adrenal (HPA) axis. The aim of present study was to analyze the pattern and interrelationships among adrenocorticotropic hormone (ACTH), cortisol (CORT), aldosterone (ALD) and electrolytes (sodium-Na⁺, potassium-K⁺ and chloride-Cl^-) during periovulatory period in cycling mares. Venous blood samples were obtained daily from a total of 23 Purebred Spanish broodmares, aged 7.09 ± 2.5 years, from day -5 to day +5 of estrous cycle, considering day 0, the day of ovulation. Plasma ACTH was measured by a fluorescent immunoassay kit, serum CORT and ALD by means of a competitive ELISA immunoassay, and plasma Na⁺, K⁺ and Cl^- were quantified by an analyzer with selective electrodes for the three ions. ACTH showed higher concentrations at day 0 compared to days -5 to -1 and +1 to +3 (p < 0.05). CORT showed higher concentrations at day 0 compared to days -5 to -2 and +1 to +5 (p < 0.05). ALD showed higher concentrations at day 0 compared to days -5 to -2 (p < 0.05) and +2 (p < 0.05). Na⁺ and Cl^- showed higher concentrations at day 0, compared to day -5 and +5. K⁺ showed lower concentrations at day 0 compared to day +1 (p < 0.05). The significant correlations obtained between ACTH and CORT (r = 0.20) and between ACTH and ALD (r = 0.32) suggest that although ACTH may have an effect both on CORT and ALD, there are other very important determinants that could be considered. Hence, it is possible to presume that the pituitary adrenocortical response and ALD may be involved in the ovulatory mechanisms without a direct relation with electrolyte pattern.