Induction of calcitonin and calcitonin receptor expression in rat mammary tissue during pregnancy.

Human breast milk samples at early time points after parturition contain high levels of calcitonin (CT) in both normal and thyroidectomized mothers, suggesting that mammary tissue produces CT. Using blot hybridization and reverse-transcriptase polymerase chain (RT-PCR) analysis of rat mammary RNA we found that CT messenger RNA is induced at midpregnancy (day 12), remains elevated through late pregnancy (day 19) but then decreases before the day of birth. RIA of mammary CT revealed that levels increase from 0.3 ng/g tissue in nonpregnant animals to peak at 1.6 ng/g on day 19 and then decline after that, paralleling messenger RNA expression. Dilution profiles for extracted mammary CT showed close parallelism with monomeric rat CT. Plasma samples from thyroparathyroidectomized rats contained 10-20 pg/ml CT that did not increase during pregnancy, suggesting that mammary CT is not released into plasma but functions locally. Consistent with this, RT-PCR detected that the CT receptor C1a isoform is expressed in rat mammary tissues during both pregnancy and lactation. This is the first report that mammary tissue expresses both CT and the CT receptor during pregnancy, suggesting that CT may have a paracrine regulatory role in the mammary gland.