The pivotal interaction between serotonin and calcium shifts in lactating pregnant Spanish Purebred mares: the aging effect

During the pregnancy and lactation phases, physiological adaptations occur in the mother to cope with the additional nutritional demands of the fetus and newborn. In experimental animals and cows, serotonin (5-HT) induces augmented bone mobilization by increasing calcium (Ca²⁺) concentrations in blood and milk during pregnancy and/or lactation. These interactions between 5-HT and Ca²⁺ homeostasis remain unknown in mares. Hence, the hypothesis of this study was that, as in other species, mares’ 5-HT and Ca²⁺ concentrations are influenced by pregnancy and lactation and that this relationship could be influenced by age. The aim was to verify the existence of a bidirectional interaction between circulating 5-HT and ionized (ICa²⁺) and total (TCa²⁺) Ca²⁺ shifts in thirty-one healthy lactating Spanish Purebred mares during pregnancy, evaluating the effect of different ages (<10- and >10 years old). Compared to >10-year-old mares, those aged <10 years old showed a greater 5-HT concentration from the 3rd to the 8th month of pregnancy (p < 0.05), a greater ICa²⁺ concentration from the 5th to the 8th month (p < 0.05), a lower TCa²⁺ concentration from the 1st to the 3rd month (p < 0.05), a greater concentration at the 7th, 8th, and 11th month (p < 0.05), and a greater ICa²⁺/TCa²⁺ ratio from the 5th to the 7th month (p < 0.05). The data obtained show an interesting and significant relationship between circulating 5-HT and both ICa²⁺ and TCa²⁺, as well as between ICa²⁺ and TCa²⁺. Moreover, aging appears to reduce the secretory tone of 5-HT, with a concurrent large shift in Ca²⁺ metabolism in lactating pregnant mares.