Physiological changes in the regulation of calcium and phosphorus utilization at the onset of egg production in commercial laying hens

Authors: GARCIA-MEJIA, ALEJANDRA - University Of Georgia; SINCLAIR-BLACK, MICAELA - University Of Georgia; BLAIR, LYSSA - University Of Georgia; ANGEL, ROSELINA - University Of Maryland; JARAMILL, BIBIANA - Iluma Alliance; REGMI, PRAFULL - University Of Georgia; NEUPANE, NABIN - University Of Georgia; Proszkowiec-Weglarz, Monika; ARBE, XABIER - H&n International; CAVERO, DAVID - H&n International; ELLESTAD, LAURA - University Of Georgia

Submitted to: Frontiers in Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/5/2024
Publication Date: 9/25/2024
Citation: Garcia-Mejia, A.R., Sinclair-Black, M., Blair, L.R., Angel, R., Jaramill, B., Regmi, P., Neupane, N., Proszkowiec-Wegla, M.K., Arbe, X., Cavero, D., Ellestad, L. 2024. Physiological changes in the regulation of calcium and phosphorus utilization at the onset of egg production in commercial laying hens. Frontiers in Physiology. vol.15-2024. https://doi.org/10.3389/fphys.2024.1465817.
DOI: https://doi.org/10.3389/fphys.2024.1465817

Interpretive Summary: At the beginning of egg production, laying hens undergo many physiological changes necessary to facilitate uptake, storage, and utilization of large amounts of calcium for eggshell calcification. These changes start to occur around two weeks prior to the onset of lay and involve specialized tissues such as intestine, kidney, and shell gland that play major roles in calcium and phosphorus absorption, retention, and utilization. This manuscript focused on revealing physiological changes associated with calcium and phosphorus uptake and utilization during the start of egg laying. Specifically, expression profiles of genes involved in calcium and phosphorus homeostasis were determined in kidney, shell gland, ileum, and liver. Moreover, circulating vitamin D3 metabolites were measured, and parameters associated with bone integrity were evaluated at 18 and 31 weeks of age in laying hens. Additionally, the effects of supplementation of feed with analogue of vitamin D3, AlphaD3, were investigated. Physiological pathways that influence functional changes associated with eggshell formation and bone mineralization in laying hens following the onset of egg production were identified. The tissues evaluated exhibited a remarkable capacity to alternate between highly demanding states of increased mineral absorption, retention, or excretion in order to maintain homeostasis between eggshell formation and bone mineralization. Key regulators of mineral homeostasis in kidney and shell gland that were identified represent potential targets for strategies aimed at enhancing mineral utilization by laying hens. In addition, targeted nutritional approaches that improve efficiency of utilization and activation of dietary vitamin D3 could be used to optimize development of medullary bone, ultimately improving skeletal integrity and hen welfare at all stages of production.

Technical Abstract: At the onset of egg production, physiological changes governing calcium and phosphorus utilization must occur to meet demands for medullary bone formation and eggshell mineralization. To understand these mechanisms, commercial laying hens fed either a Control or 1a-hydroxycholecalciferol (AlphaD3™, Iluma Alliance)-supplemented diet beginning at 18 weeks of age were sampled at 18 (n=8) and 31 weeks (n=8/diet). Expression of genes associated with calcium and phosphorus utilization were evaluated in kidney, shell gland, ileum, and liver. Circulating vitamin D3 metabolites were measured, and bone quality parameters were evaluated in humerus, tibia, and keel bone. Though diet did not heavily influence gene expression at 31 weeks, several significant differences were observed between 18- and 31-week-old hens. Heightened sensitivity to hormones regulating calcium and phosphorus homeostasis was observed at 31 weeks, indicated by increased expression of parathyroid hormone receptor 1, calcium-sensing receptor, calcitonin receptor, and fibroblast growth factor 23 receptors in several tissues. Increased renal expression of 25-hydroxylase and vitamin D binding protein (DBP) at 31 weeks suggests this tissue participates in local vitamin D3 25-hydroxylation and DBP synthesis after egg production begins. Biologically active 1,25(OH)2D3 was higher at 31 weeks, with corresponding decreases in inactive 24,25(OH)2D3. Based on increased expression of plasma membrane calcium ATPase 1 and calbindin in kidney, shell gland, and ileum, these are key facilitators of calcium uptake in these tissues. Elevated expression of renal inorganic phosphorus transporter 1 and 2, along with sodium-dependent phosphate transporter IIa, at 31 weeks suggests an increase in phosphorus excretion following hyperphosphatemia due to bone breakdown for eggshell formation. Diet did influence bone quality parameters. Bone mineral density in both humerus and tibia was higher in AlphaD3-supplemented hens at 31 weeks. Tibial bone mineral content increased between 18 and 31 weeks, with AlphaD3-supplemented hens increasing more than Control hens. Moreover, Control hens exhibited diminished tibial breaking strength at 31 weeks compared to hens at 18 weeks, while AlphaD3-supplemented hens did not. Together, these results indicate supplementation with AlphaD3 enhanced bone mineralization during the medullary bone formation period and elucidate the adaptative pathways regulating calcium and phosphorus utilization associated with the onset of lay.