New Insights into Calcium Intake in Transition Dairy Cattle

Calcium is the major mineral component of milk and colostrum. Circulating maternal calcium pools and dietary calcium are insufficient to support maternal physiology while simultaneously supporting milk synthesis. Dairy cows are particularly susceptible to the demand by the mammary gland for calcium due to the amount of milk they produce. This creates an enormous challenge to maintain maternal calcium concentrations within a normal physiological range. Therefore, they must rely on mobilization of calcium from bone to support maternal calcium homeostasis. Approximately 50% of dairy cows will succumb to subclinical hypocalcemia and 5-10% will suffer from the clinical form of the disease, milk fever, which has major economic and animal health and production impacts. Additionally, regardless of disease state, dairy cows will lose approximately 13% of bone calcium during the first 2 months of a lactation. Serotonin has emerged as a regulator of mammary gland physiology over the last decade, modulating several aspects of milk synthesis and mammary gland involution, and has independently been shown to influence bone metabolism. More recently, using several mammalian models, we have demonstrated the importance of serotonin in regulating calcium mobilization from bone during lactation, as well as modulating calcium transport into the mammary gland from the circulation. Our research shows that serotonin is critical for the expression of key mammary gland calcium transporters and pumps, as well as the production of parathyroid hormone related-protein (PTHrP), which is the hormone necessary for the induction of calcium mobilization from bone during lactation. In dairy cows, serotonin concentrations are positively correlated with calcium and PTHrP status on day 1 of lactation. Furthermore, we have demonstrated that serotonin concentrations fluctuate over the course of an entire lactation, and decrease substantially at parturition, compared to pre-partum concentrations, similar to calcium concentrations. Recently, we demonstrated in two separate studies, that administration of 5-hydroxy-L-tryptophan pre-partum increases post-partum calcium concentrations in multiparous Holstein dairy cows. Combined, our research implicates the importance of serotonin for the regulation of maternal calcium homeostasis, as well as for the secretion of calcium into milk and colostrum.