Tissue distribution and regulation of rat prolactin receptor gene expression. Quantitative analysis by polymerase chain reaction.

The rat prolactin receptor (PRLR) exists as two forms, short and long. We have developed a quantitative polymerase chain reaction (Q-PCR) in order to measure the absolute number of mRNA molecules encoding both forms of PRLR in 16 tissues of adult female rats at two stages of the estrous cycle (proestrus and diestrus I) and in the mammary gland of 20-day pregnant and 7-day lactating rats. Using this technique, it was possible to detect as few as 500 molecules of a target mRNA per micrograms of total RNA. All tissues examined expressed the two forms of receptor transcripts, ranging from 1.8 x 10(3) molecules/micrograms of total RNA in the skeletal muscle to 2.9 x 10(7) molecules/micrograms of total RNA in the ovary. Fourteen tissues expressed the long form mRNA predominantly, the thymus and the kidney expressed both forms equally, and the liver expressed the short form predominantly. In the liver, the level of mRNA expression of the short form was approximately 2-fold higher in proestrus than in diestrus. In the ovary, uterus, and cerebral cortex, the expression of the long form transcript was higher in proestrus than in diestrus: 4-fold in the ovary, 2.8-fold in the uterus, and 1.4-fold in the cerebral cortex. In contrast, the hypothalamus and the pituitary expressed 1.6-fold more long form transcript in diestrus than in proestrus. These results indicate that PRLR mRNA is ubiquitously but variably expressed in a tissue-specific manner and is clearly regulated by the hormonal environment associated with the stage of the estrous cycle, pregnancy, and lactation.