Developmental expression and modulation of the vitellogenin receptor in ovarian follicles of the rainbow trout, Oncorhynchus mykiss

Ovarian uptake of vitellogenin (VTG) accounts for a considerable proportion of oocyte growth in many oviparous species. The mechanisms controlling the expression and modulation of the VTG receptor, therefore, will be key determinants in effecting oocyte growth. This study documents the expression and modulation of the VTG receptor in follicles of the rainbow trout (Oncorhynchus mykiss) during ovarian development. SDS‐PAGE and ligand blotting with 125I‐VTG demonstrated the presence of the VTG receptor in rainbow trout follicles throughout the period of ovarian development, known as vitellogenesis. The vitellogenin receptor was also shown to be present in very small follicles (those measuring 0.3 mm or less in diameter) and in ovulated eggs. Filter binding assays followed by Scatchard analyses revealed developmental changes in both the receptor equilibrium dissociation constant, Kd, and the maximum number of binding sites, Bmax. The binding affinity of the VTG receptor for VTG in follicles was highest in the smallest follicles examined (0.4 mm in diameter, Kd = 1.3 × 10−9 M) and in those approaching ovulation (1.8 × 10−9 M). Ovulated eggs also had a high binding affinity for VTG (1.2 × 10−9 M). The Bmax increased almost 4‐fold during early development from 70.8 fmol/mg protein in 0.4 mm follicles to 260.5 fmol/mg protein in 0.6 mm follicles, but subsequent to this there was little change in the Bmax throughout follicle growth. Ovulated eggs had a relatively low binding capacity for VTG. The number of VTG receptors per follicle increased 100‐fold with increasing follicle size, but dropped dramatically at ovulation. The data presented also suggests that the density of VTG receptors (numbers per mm2 follicle surface) changed significantly during follicular development.