Sex differences in binding of human growth hormone to isolated rat

Since liver is a target for growth hormone action, binding of 125I-labeled human growth hormone to en- zymatically isolated rat hepatocytes was studied. Specific binding was shown with hepatocytes from both male and fe- male animals. There was a single class of receptors for human growth hormone on cells from males (affinity con- stant, Ka = 1.16 X 109 liters/mole; sites per cell, q = 6200). In males, bovine growth hormone was almost as potent as human growth hormone in displacing bound 125I-labeled human growth hormone, while ovine prolactin was about 1000 times less potent. Cells from female rats bound more I251-labeled human growth hormone than cells from males. The cells from fe- males contained at least two classes of receptors for human growth hormone. The receptor of highest affinity had the same affinity for human growth hormone as the single recep- tor found in males (Ka = 0.96 X 109 liters/mole). However, there were three to four times as many of these receptors per cell in females (q = 21,000). In females, bovine growth hor- mone and ovine prolactin were both about 20 times less po- tenit than human growth hormone. Treatment of male rats with estrone produced cells that show the same binding char- acteristics as females. These results indicate that human growth hormone binds to a somatogenic receptor in hepatocytes from male rats. In females and estrogen-treated males, the receptors that bind human growth hormone recognize lactogenic as well as so- matogenic properties. This suggests that the lactogenic and growth-promoting effects of human growth hormone in the rat are mediated by different receptors.