In women and females of different species of laboratory animals, oestrogens stimulate the renin-angiotensin-aldosterone system (RAAS) by increasing tissue and circulating levels of angiotensinogen and renin during the preovulatory period. Progesterone and cortisol compete with aldosterone for mineralocorticoid receptors, which results in increased Na+ reabsorption during the postovulatory period. The purpose of the current research was to analyse the relationship of oestradiol-17β, progesterone and cortisol with RAAS in 23 mares during an oestrous cycle. During the preovulatory period, significant positive correlations of oestradiol-17β with renin and aldosterone concentrations and negative correlations of progesterone with renin and aldosterone concentrations were found. In contrast, during the postovulatory period, oestradiol-17β concentrations were positively correlated with angiotensin concentrations and progesterone was negatively correlated with this component of the RAAS. Cortisol concentrations were not correlated with the hormones of the RAAS, neither before nor after ovulation. This research demonstrates that, as occurs in other species, changes in the RAAS during the periovulatory period in mares may be modulated by variations in the concentrations of steroid hormones.
Keywords: mare; oestrus; renin–angiotensin–aldosterone system.