Triiodothyronine (T₃) Enhances the Stimulatory Effect of 1, 25-Dihydroxyvitamin D₃ on Calbindin-D_28K mRNA Expression in the Kidney and Intestine but Not in Cerebellum of the Chick

The present study was conducted to investigate the role of thyroid hormones in the regulation of gene expression of calbindin-D_28k (CaBP-D28K) in the chicken. By employing slot blot and RIA analyses, levels of CaBP-D28K mRNA and CaBP-D28K protein in the intestine, kidney, cerebellum and liver were measured 6 and 12 h after i.m. injection of 1, 25-dihydroxyvitamin D₃ [1, 25 (OH)₂D₃; 250 ng/chick] and 3, 5, 3′-triiodothyronine (T₃; 500 ng/chick) in one-day-old chicks. The abundant messages of CaBP-D28K mRNA were detected in the intestine, kidney and cerebellum while there was little message in the liver. After 1, 25 (OH)₂D₃ treatment (6 + 12 hours), levels of CaBP-D28K mRNA increased in the intestine, but there was no change in the mRNA levels in the kidney and cerebellum. Although T₃ alone had no effect on CaBP-D28K mRNA levels, simultaneous administration of T₃ enhanced the. 1, 25 (OH)₂D₃ effect on levels of CaBP-D28K mRNA in the intestine both 6 and 12 h post-treatment, and in the kidney 12 h post-treatment. At a protein level, co-treatment with 1, 25 (OH)₂D₃ and T₃ elicited a significant increase in CaBP-D28K expression in the intestine 12 h post-treatment, as compared to treatment with only 1, 25 (OH)₂D₃, whereas no differences were observed in the CaBP-D28K protein levels in the kidney and cerebellum. These results suggest that thyroid hormones may play a synergistic role with 1, 25 (OH)₂D₃ for CaBP-D28K gene expression in the intestine and kidney in chicks.