Specific prolactin binding in the rat adrenal gland: its characterization and hormonal regulation

Rat adrenal prolactin receptors possess the same hormonal specificity as those in the prostate gland and liver, but are less stable during storage and after freezing. There is a gradual decrease in specific prolactin binding to the adrenal during sexual maturation in male rats; maximum binding capacity of 980 fmol/mg protein is at 25 days of age decreasing to approximately 100 fmol/mg protein at day 90. Prolactin receptors in the prostate are high at 25 days of age (700 fmol/mg protein), decrease sharply by day 30 (180 fmol/mg protein) and then gradually increase. Ovariectomy resulted in a significant rise in total prolactin binding in the adrenal gland, while the administration of oestradiol or testosterone reduced the binding, the reverse of changes in prolactin binding in the liver. Only oestrogen increased serum levels of prolactin in female rats. Ovine prolactin (500 micrograms) given to female rats resulted in a rapid increase over a period of 2-8 h total prolactin receptors in the adrenal, and these then decreased to normal levels, indicating a possible positive regulation of prolactin receptors by homologous hormone.