INSULIN STIMULATES HEPATIC LIPOGENESIS IN RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

The effects of the pancreatic hormones, insulin and glucagon, on rates of lipid biosynthesis in liver removed from rainbow trout, Oncorhynchus mykiss, were evaluated in vitro. Livers were removed from animals fasted for 30-36h, cut into ca. 1 mm3 pieces, and incubated in the presence of various concentrations of salmon insulin (sINS), bovine insulin (bINS), or a combination of bINS and bovine/porcine glucagon (GLU). Lipid synthesis was evaluated by total lipid concentration, (H2O)-H-3 incorporation into total lipid, and by fatty acid synthetase activity. Both mammalian and sINS tended to increase tissue total lipid concentration in hepatic tissue incubated for 5h. Insulin also stimulated (H2O)-H-3 incorporation into total lipid in a dose-dependent manner. Bovine INS (2 x 10(-6) M) stimulated de novo synthesis nearly 6-fold over control rates; sINS (2 x 10(-6) M) stimulated label incorporation more than 7-fold over control rates. Glucagon inhibited INS-stimulated (H2O)-H-3 incorporation; whereas, GLU alone had no effect on lipid synthesis in liver pieces incubated 5h. Lipid class analysis indicated that bINS significantly stimulated (H2O)-H-3 incorporation into phospholipids, fatty acids, and triacylglycerols. The greatest accumulation of label was in the triacylglycerol fraction, where incorporation was stimulated 17-fold over control levels. Hepatic enzymatic analysis indicated that bINS also significantly stimulated lipogenic enzyme activity 9-fold above control levels. These results indicate that INS is an important regulator of lipid synthesis in the liver of trout.