Regulation of net triacylglycerol synthesis by metabolic substrates in isolated rat liver cells.

Triacylglcerol metabolism was studied in isolated rat hepatocytes using a fully enzymatic method to masure cellular glyceride-glycerol. With this method 40 and 50 MUmol triacylglycerol were found per g cellular protein in liver cells from fed and starved rats, respectively, comparable to values obtained after organic solvent extraction and alkaline hydrolysis of neutral lipids. Carbohydrate refeeding of animals increased triacylglycerol levels in hepatocytes to 80 mumol. Upon incubation without fatty acids a 15% decrease in cellular triacylglycerol was found in 60 min. When 1mM oleate or palmitate were added cellular triacylglycerol increased. The rates of net triacylglycerol increased. The rates of net triacylglycerol synthesis were not significantly different with oleate and palmitate. Starvation reduced the rates from both fatty acids, whereas carbohydrate refeeding led to a marked increase in net triacylglycerol synthesis. Besides 20mM glucose, 5mM L-lactate and 5mM fructose stimulated triacylglycerol synthesis from fatty acids. THe stimulatory effect of lactate was higher in hepatocytes from starved animals, so that the differences in triacylglycerol synthesis between liver cells from fed and starved rats were abolished. Fatty acids taken up and not recovered in newly formed triacylglycerol were released as ketone bodies. When radioactive lectate was offered to cells from starved rats, label incorporated into neutral lipids was exclusively recovered from the glycerol moiety of triacylglycerols. 5mM ethanol which alone increased fatty acid esterification, reduced the stimulatory effect of lactate but increased the effect of fructose on net triacylglycerol formation. These findings indicate that esterification rate in liver cells from starved rats can be limited by availability of alpha-glycerophosphate, which is provided by glyceroneogenesis. The possible physiological significance of these findings is discussed with regard to liver cell heterogeneity and nutritional adaptation of liver triacylglycerol formation.