In this work we determined progesterone receptor (PR) mRNA content in female rabbit lung during the first 5 days of pregnancy and in ovariectomized animals after subcutaneous injection of oestradiol benzoate (25 micrograms/kg) for 2 days or oestradiol benzoate (25 micrograms/kg) for 2 days plus a single dose of progesterone (5 mg/kg) on day three. On each day (0-5) of pregnancy and 24 h after the last dose in the case of the treated animals, animals were killed and lung was excised; total RNA was extracted and processed for Northern blot analysis. The results showed three main PR mRNA transcripts (6.1, 4.4 and 1.8 kb) in rabbit lung. The 4.4 kb species was the most abundant. PR mRNA content was markedly increased by oestradiol benzoate and downregulated by progesterone. It significantly increased on the first day of pregnancy and then diminished progressively, reaching its lowest value on day 5. These findings suggest that PR mRNA content in the rabbit lung is regulated by sex steroid hormones and changes according to the physiological concentrations of oestradiol and progesterone.

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