CALCITONIN-GENE REGULATION IN-VIVO

In view of the importance of the caliotropic hormones to calcium homeo stasis and bone strength we have studied the regulation of the express ion of the calcitonin gene in vivo in the rat. Normal rats were inject ed with 1,25-dihydroxyvitamin D3 (12.5-200 pmol/100 g body weight) and calcitonin mRNA levels measured. 1,25(OH)2D3 led to a marked decrease in calcitonin mRNA levels, which reached 4% of basal at 24 h. Nuclear transcript experiments showed that the effect was transcriptional. Af ter large changes in serum calcium for periods from 6h to 3 weeks ther e were no differences in calcitonin mRNA levels. Estrogens given to ov ariectomized rats led to increases in calcitonin mRNA. The estrogen re ceptor was present in the thyroid C-cells. Therefore the calcitonin ge ne is regulated by vitamin D and estrogens, but not by calcium.