Maternal age-related changes in renin-angiotensin-aldosterone system and electrolyte homeostasis during equine pregnancy

Background The renin–angiotensin–aldosterone system (RAAS) plays a key role in fluid and electrolyte homeostasis during pregnancy. In humans and laboratory animals, maternal age has been shown to modulate RAAS activity, potentially altering the secretion of renin (REN), angiotensin II (ANG-II), and aldosterone (ALD), thereby influencing sodium (Na⁺) and potassium (K⁺) regulation. Aims To evaluate whether maternal age influences RAAS activity in pregnant mares. Methods Forty-five Spanish Purebred mares were monitored monthly over 11 months of pregnancy and categorized into three age groups: Young (6–9 years; n=15), Middle-aged (10–12 years; n=15), and Older (13–16 years; n=15). Plasma REN, ANG-II, and ALD concentrations were measured using competitive immunoassays, and electrolyte concentrations were assessed using a validated analyzer. Results REN concentrations increased from mid-gestation in all age groups, peaking at month 6, and younger mares showed greater values than older ones between months 5 and 6 (p < 0.05). ANG-II concentrations fluctuated throughout gestation, and older mares exhibited greater concentrations at months 2, 3, and 5 (p < 0.05), followed by a gradual decline. ALD concentrations mirrored REN patterns, peaking at months 1, 4, 5, and 7, with the greatest values in younger mares (p < 0.05). Despite these hormonal differences, plasma Na⁺ and K⁺ concentrations remained stable across all age groups and gestational stages. Conclusion These findings demonstrate that maternal age modulates RAAS activity during equine pregnancy, with enhanced REN and ALD responses in younger mares, and elevated ANG-II concentrations during early gestation in older individuals.