Effects of 11-ketotestosterone and temperature on inhibin subunit mRNA levels in the ovary of the shortfinned eel, Anguilla australis.

Members of the transforming growth factor-b (TGFb) superfamily are important during early oogenesis in mammals. In this study, we tested whether documented effects of 11-ketotestosterone (11KT) on previtellogenic eel ovaries are mediated through affecting the expression of key ovarian TGFb genes. Furthermore, we investigated whether 11KT effects interacted with temperature. Accordingly, three thermal regimes were compared and their interaction with 11KT-mediated actions on expression of TGFb superfamily genes (chiefly inhibin subunits) evaluated in the eel (Anguilla australis). Inhibin subunit mRNA levels were also measured in ovarian explants cultured in vitro with 11KT and in ovaries from eels collected from the wild. In wild eels, inhibin-bA mRNA levels were higher in early than in previtellogenic eels; inhibin-a expression did not differ between stages, whereas that of inhibin-bB first decreased, then recovered with advanced developmental stage. Temperature was ineffective in modulating any of the end points, at least as long as a Q10 adjustment was made to correct for 'metabolic dose'. However, 11KT affected the expression of inhibin-a compared to control fish, while those of inhibin-b subunit genes remained unaffected. In contrast, 11KT dramatically reduced mRNA levels of inhibin-b subunits in vitro, but had inconsistent effects on inhibin-a transcript abundance. We conclude that 11KT affects ovarian inhibin subunit gene expression, but effects are not in keeping with the changes seen during early oogenesis in eels from the wild. We further contend that in vivo temperature experiments are easily biased and that Q10 corrections may be required to identify 'true' temperature effects.

Triacylglyceride physiology in the short-finned eel, Anguilla australis­changes throughout early oogenesis.

During certain stages in an animal's life cycle, energy requirements may exceed energy intake from the diet. The spawning migration of temperate eels is a textbook example of negative energy balance, forcing these fish to rely on stored fats (triacylglycerides) to provide their muscles with energy for swimming and their growing oocytes with the nutrients needed to develop and support healthy offspring. We predicted broad implications of this great need for endogenous triacylglycerides in terms of their packaging, transport, and ovarian uptake. To test this, serum lipid concentrations and transcript abundances of intestinal and hepatic triacylglyceride packagers and ovarian triacylglyceride modifiers and receivers were investigated throughout previtellogenesis (feeding phase) and into early vitellogenesis (fasting phase) in short-finned eels. A switch from exogenous to endogenous triacylglyceride packaging was seen as the liver upregulated transcript levels of apolipoprotein B and microsomal triacylglyceride transport protein and downregulated those of apolipoprotein E and lipoprotein lipase. In the intestine, the reverse response was observed. Furthermore, ovarian transcript abundances of triacylglyceride modifiers and receivers increased (apolipoprotein E, lipoprotein lipase, and vitellogenin receptor), indicative of increased triacylglyceride uptake during previtellogenesis. We propose that increased hepatic apolipoprotein B production is a conserved vertebrate response to prolonged periods of negative energy balance.