Characterization of low density and high density lipoprotein receptors in the rat corpus luteum and regulation by gonadotropin

Freshly prepared plasma membranes from rat corpora lutea were examined for the presence of low density lipoprotein (LDL) and high density lipoprotein (HDL) receptors by determining the specific binding of ¹²⁵I-LDL and ¹²⁵I-HDL. These membranes have two types of binding site for ¹²⁵I-LDL, one with high affinity (K(d) = 7.7 μg of LDL protein/ml), the other with low affinity (K(d) = 213 μgm of LDL protein/ml) and one type of binding site for ¹²⁵I-HDL with K(d) = 17.8 μg of HDL protein/ml. LDL receptor is sensitive to pronase and trypsin; HDL receptor, however, is resistant. The binding reaction was further characterized with respect to effect of time and temperature of incubation, requirement of divalent metal ion, influence of ionic strength, and binding specificity. In vivo pretreatment of rats with human choriogonadotropin (hCG) resulted in induction of both LDL and HDL receptors in a dose- and time-dependent manner when compared with saline-injected controls. The induction of lipoprotein receptors by hCG treatment is target organ-specific since the increase was seen only in the ovarian tissue. Membranes prepared from liver, kidney, and heart did not show an increase in lipoprotein receptors after hCG injection. An examination of the equilibrium dissociation constants for ¹²⁵I-LDL and ¹²⁵I-HDL binding after hCG administration revealed that the increase in binding activity was due to an increase in the number of binding sites rather than to a change in the binding affinity. In conclusion, rat corpus luteum possesses specific receptors for both LDL and HDL and these receptors are regulated by gonadotropins.