# The Pivotal Interaction Between Serotonin and Calcium Shifts in Lactating Pregnant Spanish Purebred Mares: The Aging Effect

## TL;DR

This study explores how serotonin and calcium levels change in pregnant and lactating mares, and how aging affects these interactions.

## Contribution

The study reveals a novel relationship between serotonin and calcium metabolism in lactating pregnant mares, influenced by age.

## Key findings

- Younger mares (<10 years) had higher serotonin and ionized calcium levels during pregnancy.
- Aging reduces serotonin secretion and alters calcium metabolism in lactating pregnant mares.
- Serotonin is positively linked to both ionized and total calcium concentrations in mares.

## Abstract

This is an experimental study carried out on thirty-one healthy lactating and pregnant Spanish Purebred mares to evaluate if, in equines, like other species, serotonin (5-HT) induces increased bone mobilization by augmenting calcium (Ca2+) concentrations in blood and milk during pregnancy and/or lactation. The hypothesis of this study was that plasma 5-HT and ionized (ICa2+) and total Ca2+ (TCa2+) could vary under pregnancy and lactation conditions in mares of advancing age, providing information not only from a physiological but also from a clinical point of view in the equine species. Positive relationships between circulating 5-HT and both ICa2+ and TCa2+, as well as between ICa2+ and TCa2+, were observed. Aging appears to reduce the secretory tone of 5-HT, with a concurrent large shift in Ca2+ metabolism in lactating pregnant mares.

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.