Specific Binding of Human Chorionic Somatomammotropin in Rat Liver and Mammary Gland

The binding of 125I-labeled hCS was examined in cell membrane fraction obtained from rat liver and mammary gland. The specific binding of 125I-labeled hCS was determined in pregnant rat liver (binding capacity: 1.4 times 10-12 mol/mg protein), nonpregnant liver (0.4 times 10-12) and lactating mammary gland on the 7th post-partum day (0.06 times 10-12). These bindings were displaced from liver or mammary gland by hormones capable of stimulating lactation (i.e., prolactins and human growth hormone). “Lactogenic activity” was investigated in pregnant rat serum by heterologous radioreceptor assay using 125I-labeled hCS and cell membranes of the rat liver. Two peaks of “lactogenic activity” were seen in rat serum throughout pregnancy. The first peak (1321 + 503 ng hCS equivalents/ml) was at mid-pregnancy and the second peak (1232 + 819 ng) was near term. It appears that this activity in serum is mainly due to a placental lactogenic hormone in rats.