Regulation of palmitate metabolism by carnitine and glucagon in hepatocytes isolated from fasted and carbohydrate refed rats

Abstract

1. 1. Glucagon stimulates the oxidation of palmitate in the liver cells isolated from carbohydrate-refed rats and incubated in a simple salt medium. The maximal stimulation was observed at the concentration of glucagon of 4 · 10 −8 M and was already noticeable after 5 min of incubation. The stimulation of the oxidation was balanced mainly by the inhibition of triacylglycerol synthesis. The extent of stimulation was not dependent on the concentration of intracellular carnitine, but was decreased at higher concentrations of palmitate in the medium. 2. 2. Glucagon did not have any significant effect on palmitate metabolism in the cells isolated from fasted rats. It seems probable that those cells are isolated in the activated state as far as it concerns their capacity to oxidize fatty acids. 3. 3. Under optimal conditions (in the presence of glucagon and an excess of carnitine), the refed cells (cells isolated from livers of rats fasted for 48 h and refed with carbohydrate for 48 h) oxidized palmitate at approximately the same rate as fasted cells (cells isolated from livers of rats fasted for 48 h). However, the refed cells still differed from fasted cells in that they required a much higher intracellular carnitine concentration and a higher palmitate concentration in the medium to reach the maximal level of oxidation. 4. 4. Carnitine decreased the intramitochondrial redox potential ( E = E 0 − 2.3 RT/ nF log[red]/[ox] in the refed cells which indicated more specific stimulation of β-oxidation. Glucagon increased the redox potential under all conditions used. 5. 5. Glucagon slightly stimulated carnitine transport into the cells and acetylcarnitine formation, and had a very pronounced stimulatory effect on the synthesis of long-chain acylcarnitines in refed cells. 6. 6. The level of total long-chain acyl-CoA was increased by glucagon in refed cells. Carnitine increased the level of total long-chain acyl-CoA in fasted cells. It seems probable that in the presence of glucagon, the extramitochondrial acyl-CoA level is increased, which may indicate direct inhibition of the triacylglycerol synthesizing enzymes. 7. 7. It is concluded that glucagon acts at least in part at one of the early stages in fatty acid metabolism, i.e., carnitine acyltransferase and/or glycerophosphate acyltransferase.