Hepatic palmitate metabolism in fasting Zucker rats: effect of lactate on partitioning

Hepatocytes were isolated from 48-h fasted lean and obese Zucker rats and incubated with [1-14C] palmitate. The partitioning of fatty acids to oxidation and esterification products and the effect of lactate or glucose on this partitioning were studied. With palmitate as the only substrate, the cells from the lean rats oxidized 80% of the palmitate that was metabolized compared with only 35% oxidation in the cells from the obese rats. Thus fatty acid esterification remained the predominate pathway of palmitate metabolism in the hepatocytes from fasting obese rats. The addition of lactate to the cells from the lean rats decreased oxidation and increased esterification to levels similar to those in the obese rat hepatocytes. Glucose had similar effects on esterification in lean rat hepatocytes but was less potent than lactate. The cells from the obese rats were insensitive to the addition of lactate or glucose. These studies indicate that the inability of hepatocytes from obese rats to induce ketogenesis from exogenous fatty acids after a 48-h fast and their insensitivity to added lactate or glucose may be related to the availability of endogenous energy sources in the obese liver that can be oxidized and thus inhibit ketogenesis and support fatty acid esterification.