A receptor for the oocyte maturation-inducing hormone 17alpha,20beta,21-trihydroxy-4-pregnen-3-one on ovarian membranes of striped bass.
Previous studies have shown that blood plasma levels of 17alpha, 20beta-dihydroxy-4-pregnen-3-one (DHP) and 17alpha, 20beta, 21-trihydroxy-4-pregnen-3-one (20beta-S) increase in striped bass (Morone saxatilis) undergoing final oocyte maturation (FOM). Both hormones are produced by ovarian fragments undergoing hCG-induced germinal vesicle breakdown (GVBD) in vitro. In the present study, we investigated binding of DHP and 20beta-S to ovarian membranes from striped bass undergoing FOM. Saturable binding sites for DHP were not detected. Saturation of 20beta-S binding sites with 5 nM [3H]20beta-S occurred within 40 min at 0 degrees C (at 3 min, half of the maximum specific binding of steroid was calculated to have occurred), and the binding was pH-dependent. Scatchard analyses revealed the presence of a single class of high-affinity (dissociation constant [Kd] = 1.4 +/- 0.2 nM), limited-capacity (estimated concentration [Bmax] = 2.7 +/- 0.3 pmol/g ovary) 20beta-S binding sites on membranes from striped bass ovaries undergoing FOM. In contrast, only low levels of specific binding (Bmax < 0.04 pmol/g tissue) were detected on membranes from testes, liver, brain, and muscle. Ovarian membranes prepared from vitellogenic females also had low levels (Bmax < 0.1 pmol/g ovary) of specific 20beta-S binding, less than 5% of that found during FOM. Results of competition assays showed that DHP was approximately 250 times less effective than 20beta-S for displacing 20beta-S from ovarian membranes. In contrast, 20beta, 21-dihydroxy-4-pregnen-3-one was a very effective competitor, although it is only a weak inducer of oocyte GVBD in vitro. Of several other steroids tested, only progesterone showed affinity for the 20beta-S binding site within a physiological range of concentrations. Taken together with previous studies of striped bass FOM, these findings indicate that 20beta-S is the oocyte maturation-inducing steroid hormone in striped bass.
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Department of Zoology, North Carolina State University, Raleigh 27695, USA.
Ghosh
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Thomas
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Sullivan
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D000818:Animals; D001492:Bass; D001667:Binding, Competitive; D002462:Cell Membrane / Q000378:metabolism*; D003350:Cortodoxone / Q000031:analogs & derivatives* / Q000378:metabolism; D005260:Female; D006863:Hydrogen-Ion Concentration; D006908:Hydroxyprogesterones / Q000378:metabolism; D007700:Kinetics; D009865:Oocytes / Q000254:growth & development*; D010053:Ovary / Q000648:ultrastructure*; D011956:Receptors, Cell Surface / Q000378:metabolism*
D006908:Hydroxyprogesterones; D011956:Receptors, Cell Surface; C007433:17,20-dihydroxy-4-pregnen-3-one; C051954:17,20,21-trihydroxy-4-pregnen-3-one; D003350:Cortodoxone