DAW, K.J ; RODNICK, K.J.; Idaho State University: Does Exogenous Glucose Prevent Glycogen Depletion in Ventricle strips of Rainbow Trout Oncorhynchus mykiss?

Due to their relative intolerance of carbohydrate, diets for carnivorous fish like rainbow trout consist primarily of proteins and lipids. Carbohydrate is important in cardiac metabolism, however little is known about the relative importance of glycogen versus exogenous glucose for contractility of cardiac tissue in trout. Most physiological studies have paid little or no attention to the sex of fish. However, recent data from our lab demonstrated 1) sex differences in cardiac glycogen and 2) the requirement of glucose for inotropic actions of sex steroids. The aim of this study was to determine whether exogenous glucose modifies the use of glycogen during contractile activity in ventricle strips. Immature rainbow trout (10-12 mo, n = 16, 31 � 0.4 cm, 377 � 19 g) were held at 14�C. Fish were killed, the ventricle was excised, rinsed in teleost Ringers, and cut into uniform strips. Strips were then attached to isometric force transducers, bathed in oxygenated (99.5% O₂:0.5% CO₂) Ringers with 5 mM glucose and electrically stimulated at 0.5 Hz for 2 h. Rates of glycogen depletion was determined by substrate differences in cardiac tissue 1) frozen immediately; 2) incubated in Ringers solution (resting); and 3) stimulated to contract in Ringers (working). Glycogen was measured using acidified tissue extracts and standard colorimetric techniques. RESULTS: Exogenous glucose did not prevent glycogen depletion under the working experimental conditions and the extent of glycogen depletion was similar between males (71%) and females (62%). There were no sex differences in baseline concentrations of free glucose or glycogen. CONCLUSION: In immature rainbow trout, energy production to promote contractility requires glycogen in the presence of physiological glucose.