DAW, K.J.; RODNICK, K.J.; Idaho State University: Effects of sex and feeding status on energy substrates in tissues of rainbow trout

Recent studies in mammals highlight sex differences in energy metabolism. Diets for carnivorous fish like rainbow trout consist primarily of proteins and lipids, and these fish are thought, unlike mammals, to be relatively intolerant of carbohydrates. Given that little attention has been paid to sex differences in substrate utilization and storage in fish, the purpose of this study was to determine the concentrations of endogenous energy substrates (glucose, glycogen and triglycerides) in the liver (L), heart (H) and axial white muscle (WM) of male and female rainbow trout under fed and fasted conditions. Fish (10-12 mo, ~34 cm, 480 g) were held at 14�C and fed either 1% of body weight (3 times a week) or fasted for 2 weeks. Tissues were rapidly sampled, frozen in liquid nitrogen, homogenized and taken through standard extraction procedures. The amount of each energy substrate was determined using colorimetric techniques. Under fed and fasted conditions, L had the highest concentration of free glucose and glycogen, followed by H and WM in both males and females. Sex and feeding status did not affect free glucose in all tissues examined, however, fasting decreased L glycogen stores in both sexes. WM had the highest concentration of triglycerides, followed by H and L. WM triglycerides were 65% higher in fed females than fed males. Heart triglycerides in females are also higher than males. Water content of L, WM and H showed no difference when feeding status and sex was considered. These results 1) suggest that tissue heterogeneity exists in terms of storage, mobilization and utilization of endogenous energy substrates and 2) stress the importance of documenting the sex of fish for studies of tissue biochemistry and energy metabolism.