Abstract
Simple SummaryThis study provides new evidence on the physiological mechanisms involved in the electrolyte balance during periovulatory period in cycling mares. The interrelationships among adrenocorticotropic hormone (ACTH), cortisol (CORT), aldosterone (ALD) and electrolytes (sodium—Na+, potassium—K+ and chloride—Cl−) were evaluated. The simultaneous increase in ACTH, CORT and ALD toward the time of ovulation could suggest the involvement of the adrenocortical pituitary axis in the ovulatory mechanisms, contributing at the same time to the maintenance of electrolyte homeostasis.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.
Highlights
Stress, inducing an increase of adrenocorticotropic pituitary hormone (ACTH) and cortisol (CORT) responses, exerts negative effects on reproduction; it suppresses the release of gonadotropin-releasing hormone (GnRH), interferencing with the pulsatility and the peak of luteinizing hormone (LH) and changing the secretion of oestradiol-17β (E2)
The concentrations of ACTH and CORT showed similar biphasic patterns; the first highest hormonal peak occurred on day of ovulation and the second at day +5
ALD concentrations significantly increased at the time of ovulation, simultaneously to ACTH and CORT, they continued increasing significantly reaching maximum values on day +5
Summary
Studies on the hypothalamic pituitary adrenal (HPA) axis response to stress under different physiological conditions such, transport [1], and gynecological manipulations for artificial insemination [2], pregnancy [3], weaning [4], nutritional imbalances [5], exercise [6], pain [7], and social stress [8] have been widely documented. Stress, inducing an increase of adrenocorticotropic pituitary hormone (ACTH) and cortisol (CORT) responses, exerts negative effects on reproduction; it suppresses the release of gonadotropin-releasing hormone (GnRH), interferencing with the pulsatility and the peak of luteinizing hormone (LH) and changing the secretion of oestradiol-17β (E2). The baseline ACTH concentrations in cyclic mares in non-stressful conditions are unknown, and the scarce evidence on CORT is contradictory, such as an increase along luteal [10,11] and ovulatory periods [12,13].
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