Calbindin-D9K gene expression in human myometrium during pregnancy and labor.

The 9-kilodalton vitamin D-dependent calcium-binding protein (CaBP9K) is expressed in the intestine and uterus of mammals. In this study, we demonstrated the intracellular location of CaBP9K and quantified its expression in human myometrial tissues from nonpregnant and pregnant women (before and after the onset of labor). By Western blot analysis, we found that antiserum raised against bovine intestinal CaBP9K was specific for CaBP9K in human myometrium. By Northern blot analysis, with an oligodeoxynucleotide probe specific for human CaBP9K, we identified a single 0.7-kilobase messenger ribonucleic acid (mRNA) species in myometrial tissues from pregnant women before and after the onset of labor. CaBP9K mRNA and immunoreactive protein were localized within myometrial smooth muscle cells by in situ hybridization and immunohistochemistry. The highest levels of CaBP9K mRNA and immunoreactive protein were found in myometrial tissues obtained from pregnant women at term before the onset of labor. CaBP9K mRNA and immunoreactive levels of CaBP9K were decreased significantly in myometrial tissues obtained after the onset of labor (2- and 8-fold, respectively). These findings demonstrated for the first time that CaBP9K was present in human myometrium and suggested that it may play a role in regulating uterine smooth muscle function during pregnancy.