Changes in duodenal and nephritic Ca and P absorption in hens during different egg-laying periods.

Ca and P metabolic disorders during the egg-laying period can reduce egg production, impair eggshell quality, and even cause bone problems in hens; however, little is known regarding the capacity of duodenal and nephritic Ca and P absorption. Here, the levels of serum Ca and P metabolic indices and the expression of duodenal and renal Ca and P transporter genes were measured in hens at different egg-laying stages. The Ca, 25-(OH)-VD3, and 1,25-(OH)2-VD3 content increased during the peak (43 weeks of age) and late (72 weeks of age) egg-laying periods compared to that during the early (23 weeks of age) egg-laying period; however, there were no differences in Pi levels among the three egg-laying periods. Moreover, duodenal VDR and CaBP-D28k mRNA expression was markedly higher but NPt2b mRNA expression was markedly lower during the peak and late egg-laying periods than during the early egg-laying period. Furthermore, nephritic CaBP-D28k, PMCA1b, and FGFR1 mRNA expression was markedly higher but NPt2a and Cyp24a1 mRNA expression was markedly lower during the peak and late egg-laying periods than during the early egg-laying period. In conclusion, the present study indicated that the increased duodenal and nephritic Ca absorption during the peak and late egg-laying periods may be associated with the VD-VDR pathway, while the decreased P absorption despite relatively stable serum P levels in all three egg-laying stages may associated with osteolysis.