Uptake, metabolism and release of carnitine and acylcarnitines in the perfused rat liver.

The uptake and release of carnitine and isovalerylcarnitine have been studied in the perfused rat liver. Labelled carnitine accumulates in rat livers perfused with 50 or 500 microM [3H]carnitine. When alpha-ketoisocaproate (5 mM) is added to the perfusate after 30 min of perfusion, the net uptake of carnitine in the liver stops, and there is even a decrease in liver radioactivity. The decrease in liver carnitine can be attributed to an enhanced formation and efflux to the perfusate of short-chain acylcarnitines. Thin-layer chromatography of liver and perfusate extracts showed that efflux rates for branched-chain acylcarnitines (isovalerylcarnitine) formed are at least 2.5-fold the efflux rate for carnitine. Acetylcarnitine is released about twice as fast as carnitine from the liver. Perfusion with 50 microM [3H]isovalerylcarnitine showed that the influx rate of isovalerylcarnitine exceeds that of carnitine 1.5-fold. Since the efflux rate is still higher, a net loss of carnitine from the liver to the perfusate will result when branched-chain acylcarnitines are formed in the perfused liver. The addition of 500 microM unlabelled carnitine to the perfusate does not influence the release of labelled carnitine or acylcarnitines from the liver, showing that uptake and release are independent processes. Isovalerylcarnitine accumulates faster than carnitine does, also in the perfused rat heart. A mechanism for the development of secondary carnitine deficiencies associated with organic acidemia is proposed.