Vitellogenesis and aspects of its pituitary regulation in teleosts with emphasis on winter flounder (Pseudopleuronectes americanus)

Nagler, James Joseph (1991) Vitellogenesis and aspects of its pituitary regulation in teleosts with emphasis on winter flounder (Pseudopleuronectes americanus). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The ovarian uptake of the homologous serum proteins vitellogenin (VG) and very high density lipoprotein II (VHDL II) (formerly peak A protein) were studied as potential yolk precursors involved in vitellogenesis in winter flounder (Pseudopleuronectes americanus). The major yolk precursor appears to be VG based on the quantity of yolk protein that recognizes the VG and VHDL II antisera by Western blotting. The rate of uptake of VG by the ovary is about three times greater than VHDL II. Internalized VG is processed into a 280,000 relative molecular mass (Mr) yolk protein (lipovitellin) that contributes to the major fraction (82%) of ovarian protein. Accumulation of VHDL II occurs in an unprocessed form and contributes to a fraction of ovarian protein representing 12% of the total. Phosvitin and a low Mr phosphoprotein were apparent but in small amounts. -- In vitro ovarian incubations done during the prespawning to early vitellogenic phases of the reproductive cycle in winter flounder showed that pituitary extract stimulates estradiol-17β (E₂) production only during the vitellogenic phase, while induced testosterone (T) production was greatest shortly before spawning. These observations were reflected in the seasonal pattern of serum levels of E₂ and T in female winter flounder. To investigate the effect of sockeye salmon carbohydrate-poor (Con A I) and carbohydrate- rich (Con A II) pituitary protein fractions on E₂ production,, ovarian follicles with (intact) or without the surface epithelium-thecal cell layer (defolliculated) from rainbow trout (Qncorhvnchus mykiss) were incubated in vitro. It was demonstrated that Con A I in the presence of T is capable of significantly increasing E₂ production in defolliculated ovarian follicles while under similar conditions Con A II (containing the maturational gonadotropin) was not. Purified salmonid and pleuronectid growth hormones (GHs) were tested for their ability to increase either E₂ and T production during in vitro ovarian incubations in both rainbow trout and winter flounder respectively, but were found to be inactive. -- Growth hormones were isolated from the pituitaries of sockeye salmon (Qncorhvnchus nerka) and American plaice (Hippoglossoides platessoides). A bioassay based on the increase of serum triiodothyronine in rainbow trout was developed to follow GH biological activity during pituitary fractionation. The isolation of a pituitary protein from sockeye salmon that was active in the bioassay was confirmed as monomeric GH by an amino-terminal (N-T) amino acid sequence. In plaice GH variants were isolated from two Mr regions within the pituitary, 42,000 and <33,000, that were active in the bioassay and had identical N-T amino acid sequences. The 42,000 Mr form predominates in the plaice pituitary making up 93% of the total.

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