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, Oncorhynch us mykiss, were evaluated in vitro. Livers were removed from animals f asted for 30-36h, cut into ca. 1 mm3 pieces, and incubated in the pres ence of various concentrations of salmon insulin (sINS), bovine insuli n (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 concent ration in hepatic tissue incubated for 5h. Insulin also stimulated (H2 O)-H-3 incorporation into total lipid in a dose-dependent manner. Bovi ne 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 (H2 O)-H-3 incorporation; whereas, GLU alone had no effect on lipid synthe sis in liver pieces incubated 5h. Lipid class analysis indicated that bINS significantly stimulated (H2O)-H-3 incorporation into phospholipi ds, fatty acids, and triacylglycerols. The greatest accumulation of la bel was in the triacylglycerol fraction, where incorporation was stimu lated 17-fold over control levels. Hepatic enzymatic analysis indicate d that bINS also significantly stimulated lipogenic enzyme activity 9- fold above control levels. These results indicate that INS is an impor tant regulator of lipid synthesis in the liver of trout.