Regulation of calbindin D 28K and its mRNA in the intestine of the domestic hen

Abstract

Intestinal calbindin synthesis in laying hens was analyzed to assess controlling factors operating during egg formation. In the absence of vitamin D, calbindin was not induced by estrogen and testosterone. In immature vitamin D-replete pullet, blood levels of 1,25(OH) 2D 3 increased in response to estrogen but the duodenal concentration of calbindin and its mRNA were increased only when testosterone was given together with estrogen. The plasma concentration of 1,25(OH) 2D 3 and the duodenal levels of calbindin and its mRNA were substantially higher in laying hens than in immature pullets. No differences in these parameters were observed between the stages of the ovulatory cycle. Suppression of shell formation for a week decreased the concentration of 1,25(OH) 2D 3 and of duodenal calbindin but did not affect the level of its mRNA. When egg shell formation resumed in hens previously laying shell-less eggs, the concentrations of 1,25(OH) 2D 3 and of calbindin and its mRNA increased toward the end of shell formation. A most important factor regulating intestinal calbindin synthesis in laying hens turned out to be 1,25(OH) 2D 3. Intestinal calbindin mRNA is more stable in laying hens than in young birds as its concentration declines more slowly when the stimulation provided by 1,25(OH) 2D 3 is withdrawn, as occurs following suppression of shell formation and after parathyroidectomy in laying birds. Intestinal calbindin mRNA is therefore increased by a process other than increasing 1,25(OH) 2D 3 formation. The factor influencing the stability of this mRNA in laying hens could be calcium. It is concluded that in hens the increased duodenal calbindin synthesis elicited by plasma 1,25(OH) 2D 3 at sexual maturity primarily involves a transcriptional process and the stabilization of the mRNA. Shell formation further increases the calbindin concentration, possibly by more effective translation of the calbindin mRNA.