Identification of specific high density lipoprotein-binding sites in rat testis and regulation of binding by human chorionic gonadotropin.

Freshly prepared rat testicular membranes bind iodinated rat high density lipoprotein (HDL) with high affinity (Kd = 32 microgram of HDL protein/ml). This high density lipoprotein binding differs from low density lipoprotein binding by cultured human fibroblast cells in two ways; it is not affected by Ca2+ or ethylenediaminetetraacetic acid, and it is not sensitive to pronase and trypsin. Testicular binding activity is primarily found in interstitial tissue containing Leydig cells and can be modulated by human chorionic gonadotropin administration in vivo (2-fold increase of binding capacity, with no affinity change) with 250 units/kg of human chorionic gonadotropin injection daily for 4 days. The interstitial high density lipoprotein binding site also recognizes rat very low density lipoprotein, but not rat low density lipoprotein, as shown by displacement experiments. When membrane preparations of various other tissues were assessed for their high density lipoprotein binding, we found that the adrenal density lipoprotein binding, we found that the adrenal gland binds rat high density lipoprotein with similar affinity and capacity, while spleen, kidney, and heart showed no high affinity binding. In addition, when iodinated rat low density lipoprotein was tested for its ability to bind to testicular membranes, no high affinity saturable binding was observed. We conclude that there are specific high density lipoprotein-binding sites present in steroidogenic tissues; these binding sites are not found in the nonsteroidogenic tissues tested. Furthermore, no high affinity low density lipoprotein-binding sites can be demonstrated in the testis; thus it appears that high density lipoprotein, rather than low density lipoprotein, is the major cholesterol-carrying lipoprotein recognized by the rat testis.