MAMMARY EPITHELIAL-CELLS OF LACTATING RATS EXPRESS PROLACTIN MESSENGER-RIBONUCLEIC-ACID

The presence of prolactin (PRL) mRNA in the mammary gland, placenta, a nd pituitary gland of lactating and pregnant rats was investigated by polymerase chain reaction (PCR). Polyadenylated RNA was prepared from total RNA samples by Oligo(dT)-cellulose chromatography, and complemen tary cDNAs were synthesized. A standardized amount of cDNA from each s ample was used as the template in a Taq PCR under high-stringency cond itions. PCR amplified a signal with the predicted size of approximatel y 375 bp in mammary and pituitary glands of lactating and pregnant rat s, and in placentae of pregnant rats. This band specifically hybridize d with a probe overlapping the entire sequence of the mature rat (r) P RL mRNA in Southern blot analysis. When the rPRL-specific primers were used, PCR revealed no signal in the liver or in lactating mammary gla nd explants cultured in vitro for 48 h, while the same cDNA preparatio ns gave strong signals for beta-actin. The viability of the mammary gl and explants was also suggested by their ability to secrete immunoreac tive casein in vitro. PRL mRNA was localized in the epithelium of alve oli and ducts of the lactating mammary gland by in situ hybridization. These data provide evidence that the PRL gene is expressed in the mam mary gland of pregnant and lactating rats, and suggest that the mammar y gland might contribute to PRL in milk by de novo synthesis. Thus, wh ile the placenta is an exogenous source of PRL-like activities for the fetus in utero, the mammary gland might take over this function after birth.