Transport of 17α,20β-dihydroxy-4-pregnen-3-one in brook troutSalvelinus fontinalis ovarian follicles.

Extremely low levels of the maturation inducing steroid (MIS) 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DHP) were found in the ooplasm and ovarian follicle membranes of Atlantic salmonSalmo salar ouananiche, a finding that is at variance with the elevated blood levels of the steroid. The uptake of MIS at physiological concentrations into brook trout follicles occurred by passive diffusion. Uptake of the steroid into the ovarian follicle membrane, consisting of zona radiata and the attached follicle cells, deviated from linearity in a double reciprocal plot. These results suggest that 17α,20β-DHP is binding to a receptor-like protein in the ovarian follicle or the zona radiata membrane surrounding the oocyte, and extend our previous demonstration of 17α,20β-DHP receptor-like activity in the zona radiata membrane of the late stage brook trout oocytes. An oocyte cytoplasmic receptor gave subunits on SDS PAGE that were similar to the membrane and cytosol receptors previously described.