CALBINDIN-D(9K) MESSENGER-RNA IS TIGHTLY REGULATED DURING THE ESTROUS-CYCLE IN THE RAT UTERUS

Calbindin-D9k (CaBP-9k) is a cytosolic calcium binding protein with a molecular weight of 9000. CaBP-9k is mainly expressed in intestine, uterus and placenta, with intestinal levels controlled by vitamin D and uterine levels controlled by estrogens. CaBP-9k mRNA levels were measured in rat uterus throughout the estrous cycle. On the morning of proestrus, estrus and diestrus animals were sacrificed. Serum 17-beta-estradiol concentrations were determined using a radioimmunoassay. Whole uterus was used for preparation of total RNA. Northern blot analysis was performed to quantify CaBP-9k and beta-actin mRNA. CaBP-9k levels were highest at proestrus, dropped 10-fold at estrus and were not detectable at diestrus. Beta-actin levels did not change significantly throughout the estrous cycle. Peak 17-beta-estradiol concentrations coincided with maximum CaBP-9k mRNA expression at proestrus. Despite minimal concentrations of 17-beta-estradiol at estrus, CaBP-9k mRNA was still present at 10% of thc proestrus level. At diestrus, CaBP-9k mRNA was not detectable despite increasing 17-beta-estradiol. It is concluded that CaBP-9k is subject to 17-beta-estradiol regulation during the estrous cycle. Correlation between CaBP-9k mRNA and 17-beta-estradiol levels indicates a lag period for CaBP-9k induction in diestrus following a rise in steroid hormone levels.