Binding of oestradiol, progesterone and prolactin in rat lung

Binding of [3H]oestradiol and of [3H]progesterone in the cytosol from lungs of adult male rats was suppressible, dependent on incubation time and on protein concentration and was protein in nature. Suppressible binding of oestradiol consisted of a high affinity site (dissociation constant (Kd), 1 x 10(-9) +/- 0.2 x 10(-9) mol/l; maximum number of binding sites (Bmax), 0.7 +/- 0.2 pmol/mg protein) and a lower affinity site (Kd, 2.4 x 10(-8) +/- 0.6 x 10(-8) mol/l; Bmax, 6.3 +/- 0.4 pmol/mg protein) and showed evidence of positive co-operation. Suppressible binding of progesterone consisted of a single site with a Kd of 6 x 10(-9) +/- 1 x 10(-9) mol/l and Bmax of 44.5 +/- 8 fmol/mg protein. Binding of 125I-labelled ovine prolactin was found in homogenates of fetal lung (20 days of gestation) but not of adult lung (80 days of age). Treatment of adult rats with ovine prolactin was associated with an increase in the number of binding sites of high affinity for 125I-labelled ovine prolactin but these sites showed an altered specificity. This 'up-regulation' of the prolactin binding may provide a mechanism by which prolactin stimulates surfactant production in lung. These results, together with the known effects of these hormones on certain lung functions, provide further evidence that lung is a target organ for oestradiol, progesterone and prolactin.