Intestinal distribution of transcripts regulating calcium and phosphorus uptake in commercial laying hens

Intestinal supply of calcium and phosphorus is crucial for the maintenance of eggshell quality and skeletal integrity in laying hens; however, the hormones and transporters that govern mineral uptake and homeostasis within the small intestine and ceca have not been fully elucidated. To investigate intestinal distribution of transcripts for genes involved in the regulation of calcium and phosphorus homeostasis, the duodenum, jejunum, ileum, and ceca were collected from 21-week-old commercial laying hens with a hard-shelled egg in the shell gland (n = 7/tissue), and mRNA expression was evaluated using RT-qPCR. Genes encoding 24-hydroxylase and 25-hydroxylase were identified in all tissues, with higher levels noted in the ileum and ceca, suggesting these tissues may contribute to local vitamin D₃ availability. Expression of receptors involved in the hormonal regulation of calcium and phosphorus transport, such as calcium-sensing receptor, parathyroid hormone receptor 1, and fibroblast growth factor receptors 1, 2, 3, and 4, tended to increase in expression from the duodenum through the ileum, potentially demonstrating increased hormonal sensitivity within the distal small intestine. Similar distal increases in expression within the small intestine were observed for calcium transporters sodium-calcium exchanger 1 (NCX1), plasma membrane ATPase 2 and 4, transient receptor potential cation channels C1, V2, and M7, and inorganic phosphorus transporter 1 (P_iT-1) and 2, thereby suggesting an increased absorptive capacity by the jejunum and ileum compared to the duodenum. Interestingly, cecal expression of NCX1 and P_iT-1 was substantially higher than any location within the small intestine, identifying the ceca as a novel tissue that could be contributing to calcium and phosphorus uptake in laying hens. Together, these results highlight the importance of the distal small intestine and ceca in maintaining the homeostatic balance of calcium and phosphorus in laying hens and provide information that can be used to optimize dietary mineral delivery for robust skeletal mineralization and eggshell quality.