Correlating Changes in Follicular VLDL Binding and Follicular Aromatase Activity in the Ovulation Cycle of the Laying Hen

We examined whether very low-density lipoprotein receptors (VLDLR) on the oocyte membrane contributed to changes in VLDL uptake into the yolk by the ovarian follicle during the ovulation cycle of the laying hen. White and yellow (F5-F1) follicles were collected from laying hens at 14-16 h (when the yellow follicles actively uptake yolk) and 0-2 h (when the yolk uptake is less active) before ovulation and subjected to binding experiments with 125I-VLDL and assays for aromatase activity. The preparations from white follicles did not bind to 125I-VLDL, while yellow follicle preparations bound to the radioligand. The 125I-VLDL bindings of F3-F1 yellow follicles were higher at 14-16 h than 0-2 h before ovulation. Values of the dissociation constant (Kd) and the maximum binding capacity (Bmax) of VLDLR calculated from the 125I-VLDL bindings to the oocyte membranes showed profiles related to the changes in 125I-VLDL bindings; in F3 and F2 follicles Bmax values were higher at 14-16 h than 0-2 h before ovulation and Kd values were higher at 0-2 h than 14-16 h before ovulation. High aromatase activities of the F3-F1 yellow follicles of 14-16 h before ovulation decreased 0-2 h before ovulation, which was a change consistent with the change in 125I-VLDL bindings of the same follicles. These results suggest that the yolk VLDL uptake during the ovulation cycle is regulated by changes in VLDL binding underlined by changes in Bmax and Kd of VLDLR. Estrogen produced by the aromatase of the theca layer of the yellow follicles might facilitate VLDL binding by affecting VLDLR on the oocyte membrane of the same follicles in a paracrine manner.