Cytosolic acetylation of sulfamethazine decreases hepatic release of ketone bodies in vivo in fasting rats

Saturating concentration for the uptake of free fatty acids and release of ketone bodies in vivo in rat liver

The aim of this study is to determine how hepatic uptake of free fatty acids (FFA) and release of ketone bodies (KB) in vivo are influenced by plasma concentration of FFA. Anesthetized fasted rats were cannulated (aorta, portal vein, hepatic vein and inferior vena cava) and were intravenously injected with heparin (1000 U x kg(-1) body weight) or infused with lipid emulsions at rates of 0, 0.39 and 0.77 mg TG x min(-1) x rat(-1). Heparin injection elevated systemic concentration of plasma FFA significantly to 1.0 mM from 0.7 mM in the saline-injected rats. The arteriovenous (AV) gradient of FFA across the liver in the heparin-injected rats was, however, unchanged from the saline-injected rats. Hepatic release of KB in the heparin-injected rats, as indicated by the negative gradient across the liver, was found to be 83 % greater than the saline-injected rats. In the rats infused with lipid emulsions, plasma FFA concentrations were found to elevate further to 2.2 mM. However, no further increases in hepatic uptake of FFA and release of KB were found. Hepatic gradients of FFA and KB in the rats infused with lipid emulsions and injected with heparin were found to be the same. It may be concluded, therefore, that the uptake of FFA and release of KB in vivo in rat livers may be saturated at a plasma FFA concentration <1.0 mM, much lower than reported previously in in vitro studies.

The effect of xenobiotic acetylation on interorgan metabolism of glucose in fasted rats

The effect of cytosolic acetylation on interorgan glucose metabolism was studied by arteriovenous (AV) difference and tracer kinetic techniques in fasted, ketamine-anesthetized rats. The administration of sulfamethazine (SMZ, 2 mmol/kg, i.p.) resulted in 39% and 313% increases in hepatic contents of glucose and lactate, respectively. Plasma concentrations of glucose and lactate in the aorta, portal vein and hepatic vein also increased (33-43% for glucose, and 86-200% for lactate). Net hepatic release of glucose was not significantly different from the control. Net hepatic uptake of lactate increased 72-151% in the SMZ-treated rats. The concentration and hepatic gradient of alanine were little changed in the SMZ-treated rats. The rates of turnover for plasma glucose, estimated from [3-3H]-glucose, were not significantly different between the control and SMZ-treated rats. The rate of glucose recycling, estimated from the difference in the rates of turnover between [3-3H]-and[U-14C]-glucose, decreased by 42% in the SMZ-treated rats. Muscle glycogen in the SMZ-treated rats also decreased (33%). In conclusion, our data indicate that cytosolic acetylation can affect not only ketogenesis, as we have previously reported, but also interorgan metabolism of glucose. Although direct evidence is not available, increases in the levels of plasma and liver glucose suggest that gluconeogenesis is increased in the SMZ-treated rats. A net loss of lactate from muscle glycogen store to the liver is indicated by the increase in hepatic uptake of lactate and the decrease in the rate of glucose recycling in the rats treated with SMZ.

Sources of triacylglycerol accumulation in livers of rats fed a cholesterol-supplemented diet

The source of free fatty acids (FFA) and the pathways contributing to the accumulation of neutral fats in livers of rats fed a cholesterol-enriched diet were investigated in this report. Supplementation with 1% cholesterol in the diet for four weeks resulted in hepatomegaly in the rats. The contents of cholesterol and triacylglycerols (TG) per gram liver measured in rats fasted overnight increased by 48 mg (approximately tenfold) and 66 mg (approximately fourfold), respectively. The activities of glycerophosphate acyltransferase and diacylglycerol acyltransferase, the two key enzymes for TG synthesis in liver microsomes, were found to increase by 23 and 19%, respectively, in the cholesterol-fed rats. The secretion of plasma TG present predominantly in very low density lipoprotein was found to decrease by approximately 30%. The incorporation of tritium from tritiated water in liver FFA increased by twofold in rats fed the cholesterol-supplemented diet, whereas the activity of CPT I in liver mitochondria decreased by 23%. The uptake of plasma FFA in vivo in livers of fasted rats maintained on the cholesterol-supplemented diet decreased by 60%. Our data thus indicate that the excess TG accumulated in livers of rats fed the cholesterol-enriched diet resulted from increased synthesis and decreased secretion of TG. To meet the demand of fatty acids for this purpose, de novo lipogenesis increased, whereas beta-oxidation decreased. Although difference in the uptake of extrahepatic FFA may be discounted, a difference in the uptake of chylomicron remnants between the control and cholesterol-fed rats may not be ruled out.