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Dietary anion-cation difference and day length differently affect milk calcium secretion pathways
Milk is an important source of calcium for growth and development of children. However it has been recently shown that dairy milk calcium content decreases with long day photoperiod and varies according to the type of diet. Many proteins are involved in the secretion of calcium into milk by the mammary epithelial cell (MEC). The aim of this study was to identify the role of gene expression of these proteins in the regulation of milk calcium content. A trial was carried out according to a Latin square design using 8 dairy cows averaging 103 ± 44 DIM with two treatments in a factorial arrangement with 4 periods of 14 d. The cows received 2 levels of dietary anion-cation differences (DCAD; 0 mEq/kg DM for D0 and 400 mEq/kg for D400) and 2 d lengths (8 h of light/d for short days and 16 h/d for long days). The DCAD treatments were conceived to mimic diets based either on corn silage or on herbage. The cows were exposed to solarium lights providing UVA and UVB. Once per period, MEC were prepared after milk centrifugation by purification using an anti-cytoketatin antibody bound to magnetic beads in order to study by RT-PCR the mRNA level of genes involved in calcium secretion expressed related to RPLP0 housekeeping gene. Data were analyzed using Mixed procedure. There was no significant interaction between day length and DCAD level. Milk and calcium yields did not vary with any treatments averaging 32.7 kg.day-1.and 41.1 g.day-1, respectively. With D400 compared with D0, milk calcium content increased (P < 0.01) with no link with casein content. No significant variation was observed on gene expression with DCAD treatment and on kappa casein and a-lactalbumin mRNA levels with any treatments. Milk calcium and casein content were lower with long days compared with short days (P < 0.05).The lower calcium secretion was associated with lower mRNA levels for SPCA1, ITPR1 and PMCA1, 3 calcium transporters in milk purified MEC (P < 0.05). This work suggests that calcium secretion pathways may be downregulated with long photoperiod and that could explain a part of the seasonal decrease of milk calcium content during summer. In contrast no significant variation of gene expression could explain the increase in milk calcium content with D400.
Keywords: milk calcium, mammary epithelial cell, photoperid, feeding