Changes in Plasma Sex Steroid-Binding Protein Levels Associated with Ovarian Recrudescence in the Spotted Seatrout (Cynoscion nebulosus)1

Adult spotted seatrout were sampled from the wild during the period of their yearly gonadal recrudescence to study the association of plasma sex steroid-binding protein (SBP) levels with seasonal reproductive development. Plasma SBP levels were estimated by Scatchard analysis of the binding of radiolabeled testosterone to seatrout plasma. Plasma SBP levels were lowest in regressed females, with a mean binding capacity of 310 +/- 18 nM. SBP levels increased slightly with the appearance of cortical alveolar-stage oocytes and continued increasing throughout the period of early and late vitellogenesis to reach peak values of 468 +/- 46 nM in fish undergoing final oocyte maturation. The steroid dissociation constant increased from 4.80 +/- 0.19 nM in fish with regressed ovaries to 6.58 +/- 0.76 nM in fish having completed ovarian recrudescence. The male sample size was too small to permit examination of stage-dependent changes in SBP levels during testicular recrudescence. However, neither the overall mean binding capacity nor mean binding affinity differed significantly between male (n = 14) and female fish (n = 81) in this study. Plasma SBP levels were not directly correlated with plasma levels of either testosterone or estradiol, but there was a strong correlation (r2 = 0.67, p < 0.001) between plasma testosterone binding capacity and the steroid dissociation constant. Changes in mean SBP levels were associated with changes in mean gonadosomatic index, hepatosomatic index, and plasma estradiol concentration, suggesting that the plasma SBP may have an important role in the reproductive cycle of the female spotted seatrout.