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The effect of shell calcification and 1,25-dihydroxyvitamin D₃ (1,25-(OH)₂D₃) on calbindin-D_28k (previously known as vitamin D-dependent calcium-binding protein) and calbindin mRNA was investigated in the intestine and eggshell gland (ESG) of juvenile female chicks, laying hens and non-laying female birds with active gonads. Increasing amounts of 1,25-(OH)₂D₃ were fed to laying hens and juvenile birds treated with oestradiol to develop the ESG. The intestinal concentration of calbindin was increased 30-fold by 1,25-(OH)₂D₃ in chicks treated with oestradiol and fed a vitamin D-deficient diet. In these same animals, 1,25-(OH)₂D₃ had no effect on the formation of calbindin mRNA or calbindin in the ESG even though fully viable 1,25-(OH)₂D₃ receptors are present in this tissue. In laying birds fed adequate amounts of vitamin D₃, intestinal, but not ESG, calbindin was increased by the addition of 1,25-(OH)₂D₃ to the diet. At the onset of egg production the concentrations of calbindin and calbindin mRNA were increased in the intestine and ESG. This increase occurred within the period of calcification of the first egg, through a process unaffected by vitamin D. Calcification of the first egg increased the concentration of calbindin in the ESG by eight- to tenfold, although the concentration of calbindin mRNA was increased by only two- to threefold.

These results suggest that the induction of calbindin synthesis by 1,25-(OH)₂D₃ or by the egg calcification process is associated with an increase in the concentration of calbindin mRNA in the ESG and intestine. They also suggest that the vitamin D-dependent physiological changes in the ESG occurring during the calcification process do not include an increase in calbindin synthesis, that calbindin is induced and/or regulated by a mechanism requiring additional factors besides 1,25-(OH)₂D₃, and that post-transcriptional regulation of calbindin synthesis in these tissues may be possible.