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·kg^-1 body weight) or infused with lipid emulsions at rates of 0, 0.39 and 0.77 mg TG·min^-1 ·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.