Differential response of calcium transport systems in laying hens to exogenous and endogenous changes in vitamin D status.

Changes in intestinal calcium absorption, calcium deposition into egg shell, and intestinal, renal and uterine calcium-binding protein (CaBP) in laying hens were related to changes in 25 hydroxycholecalciferol-1-hydroxylase activity (1-hydroxylase), or to the supplementation of 1alpha-hydroxycholecalciferol (1alpha-OH-CC). The onset of egg production resulted in an increased kidney 1-hydroxylase activity and intestinal and uterine CaBP. Renal concentrations of CaBP remained unchanged. Intestinal calcium and phosphorus absorption, during the period of egg shell formation, and duodenal calcium-binding protein (CaBP), were higher in 1alpha-OH-CC-fed than in cholecalciferol-fed hens. Renal or uterine CaBP and calcium deposition into the egg shell did not fluctuate with the vitamin D source or concentration. 1alpha-OH-CC injection into non-laying Nicarbazinfed hens resulted in an increase in intestinal but not renal or uterine CaBP concentrations. It is suggested that (a) CaBP in various organs responded independently to the same stimuli; and (b) calcium deposition into egg shell and uterine CaBP level are not related to kidney 1-hydroxylase activity or concentration of 1,25 dihydroxycholecalciferol.