Transport and functions of carnitine in muscles.

The transport, function and metabolism of carnitine are discussed with regard to their importance in clinical chemistry. In humans carnitine is synthesized from protein-derived trimethyllysine in liver, brain and kidney. Muscles take up carnitine from the blood in an exchange-diffusion process with endogenous deoxycarnitine, the immediate precursor of carnitine. Besides catalysing the transport of long-chain acyl groups in mitochondria, carnitine is necessary for the export of intramitochondrially produced short-chain acyl residues and for the trapping and the elimination of unphysiological compounds (benzoic, pivalic, valproic acids etc.). The detection and quantitation in urine of these physiological and unphysiological carnitine esters is necessary for the diagnosis of carnitine deficiencies. The carnitine esters may be eliminated in the urine and/or distributed in tissues, where some of them (acetyl-, propionyl- and isovaleryl-carnitine) may be utilized for specific purposes. The most important carnitine-dependent metabolic disorders are listed according to their causes.

Carnitine and deoxycarnitine concentration in rat tissues and urine after their administration.

Administration of L-carnitine to rats was followed by an increase of deoxycarnitine in urine. Conversely, administration of deoxycarnitine caused an increase of carnitine. The latter treatment also produced a transient but significant diminution of L-carnitine in heart, skeletal muscle and kidney, but not in liver and plasma. Administration of D-carnitine to rats previously loaded with deoxycarnitine significantly depleted the elevated deoxycarnitine concentration in skeletal muscle and kidney while increasing it in plasma. These results suggest that the tissue exchange between L-carnitine and deoxycarnitine, already demonstrated in vitro, occurs also in vivo.

Carnitine: metabolism and clinical chemistry.

In man carnitine is synthesized from proteic trimethyllysine in liver, brain and kidney. Muscles which contain approximately 98% of carnitine must take it up from the blood in an exchange process with endogenous deoxycarnitine, the immediate precursor of carnitine. Uneven organ distribution of the enzymes catalyzing carnitine synthesis further implies an inter-organ transport of the intermediates. Assay of these intermediates in blood may assist causal definition of carnitine deficiency syndromes. Besides catalyzing the transport of long-chain acyls in mitochondria, carnitine is necessary for the export of intra-mitochondrially produced short-chain acyls and for trapping and elimination of unphysiological acyls (benzoic, pivalic, valproic acids etc.). Unlike the corresponding acyl-CoA, carnitine esters are capable of diffusing across cellular membranes, and may be eliminated in urine, distributed in tissues or both. Assay of physiological and unphysiological carnitine esters in urine is necessary for the diagnosis of carnitine insufficiencies.

Effect of diazepam on the carnitine translocation in rat heart mitochondria.

Diazepam acts as an inhibitor of the carnitine translocation through the mitochondrial inner membrane. Diazepam needs however to be added during the phase of exchange. If added during the loading phase and washed during the usual washing the diazepam still found in the mitochondrial fraction is not sufficient to exert any inhibition. Kinetic studies indicate a non-competitive inhibition and a complex carnitine-diazepam-translocase is likely to be formed.

Myocardial carnitine transport.

In mammals, carnitine is synthesized from proteic trimethyllysine in the liver, brain and (in human) kidneys. The hydroxylase catalyzing the last step (deoxycarnitine----carnitine) is missing in the remaining tissues, which are thus entirely dependent on carnitine uptake from the blood. On the basis of experimental evidence, or reasonable assumptions, an interorgan transport of carnitine, carnitine precursors and derivatives is described. In particular, evidence demonstrating a bidirectional exchange between carnitine and deoxycarnitine across cardiac sarcolemma have been provided both in vitro and in vivo experiments. It has been demonstrated that in heart slices carnitine-deoxycarnitine exchange, occurring in a close one to one ratio, is (i) insensitive to both glycolysis and oxidative phosphorylation inhibitors and (ii) sensitive to thiol reagents, such as NEM and Mersalyl. It is assumed that deoxycarnitine is released from muscles into the blood, taken up by the liver, or kidneys, to be hydroxylated to carnitine and the latter returned to the muscles. In vivo evidence for carnitine-deoxycarnitine exchange has been obtained by administering carnitine, or deoxycarnitine, to rats and measuring deoxycarnitine and carnitine, respectively, in different tissues and urine. The results clearly indicate that carnitine administration displaces endogenous deoxycarnitine from tissues and vice versa, thus further supporting the existence of a carnitine-deoxycarnitine exchange process.

Carnitine transport in rat heart slices: I. The action of thiol reagents on the acetylcarnitine/carnitine exchange.

Rat heart slices show a permeability barrier that can be crossed by carnitine but not by sucrose and inulin. The integrity of thiol groups of heart cell membrane is essential for the uptake of carnitine. N-ethylmaleimide inhibits the transport into heart slices which is insensitive to Mersalyl. On the contrary both N-ethylmaleimide and Mersalyl inhibit acetyl carnitine/carnitine exchange. The amount of thiol groups titrated by the above reagents are related to the extent of exchange inhibition.

Carnitine transport in rat heart slices: II. The carnitine/deoxycarnitine antiport.

In rat heart slices carnitine transport occurs in an exchange process with deoxycarnitine. This has been demonstrated in double labelling experiments allowing a preloading of either 3H-carnitine or 14C-deoxycarnitine, the immediated precursor of carnitine. The stoichiometry of the carnitine/deoxycarnitine exchange resulted close to one in both directions. The relative kinetics supports the assumption that the process is mediated by a membrane bound protein. The results may rationalize the circumstance that carnitine is taken up by myocardium against a concentration gradient. The meaning of the carnitine/deoxycarnitine exchange is discussed.

On the transport mechanisms of carnitine and its derivative in rat heart slices.

The transport of L-carnitine and analogs in exchange with previously loaded 3H-carnitine has been studied in heart tissue slices. The slices are first loaded with 3H-carnitine; then they are transferred in vessels containing the same medium with possible exchangers. Acetylcarnitine, L-carnitine, D-carnitine and deoxycarnitine exchange with internal 3H-carnitine. The exchange with acetylcarnitine, the largest among the compounds tested, appears to be a saturation process and is not affected either by oxidative phosphorylation and glycolysis inhibitors. The exchange of external acetylcarnitine with internal carnitine support that also in vivo heart tissue can utilize acetylcarnitine present in blood. Finally the observed deoxycarnitine/carnitine exchange, occurring in the reverse direction, may be the mechanism by which the heart accumulate external carnitine in exchange with endogenous deoxycarnitine.

Lipid-lowering effect of carnitine in chronically uremic patients treated with maintenance hemodialysis.

Hypertriglyceridemia is often present in chronically uremic patients treated with maintenance hemodialysis and has been considered a risk factor in the accelerated development of atheroma. Muscle carnitine content is low in hemodialyzed patients. This abnormality may help to explain the myopathy and cardiomyopathy often observed in these subjects. In addition, carnitine might play a role in the hypertriglyceridemia in renal failure. Carnitine, which is necessary for fatty acid oxidation, has been recently reported to lower serum triglycerides in patients with type IV hyperlipoproteinemia. Carnitine was administered intravenously three times weekly at the end of hemodialysis in eight patients. Carnitine was given in 0.5 g doses for 8 weeks and then in 1.0 g doses for 6 additional weeks. There was a significant decrease in serum triglycerides at the end of treatment. In contrast, serum lipids in eight hemodialysis patients receiving placebo did not change significantly. Carnitine administration does not cause any side effect except some euphoria. These results suggest that carnitine may be effective in the treatment of hypertriglyceridemia in dialysis patients.

Effect of physical training on carnitine concentration in liver, heart and gastrocnemius muscle of rat.

Physical training by compulsed swimming induces in rat heart a significant increase in the concentrations of carnitine and free carnitine but no detectable changes in liver and gastrocnemius muscle are observed. These results are consistent with the increased utilisation of fatty acid and pyruvate in trained animals and with an enhanced demand for carnitine by heart muscle.

Carnitine and acetylcarnitine in skeletal and cardiac muscle.

The acetylcarnitine/carnitine ratio has been found significantly increased in tibial muscles of swimming rats in comparison with normal rats. Analogous increase occurred in electrically stimulated tibial muscle in comparison with the unstimulated controlateral muscle of the same animal. Swimming also induced a statistically significant increase of the acetylcarnitine/carnitine ratio in heart, while norepinephrine administration resulted in a decrease of carnitine not accompanied by a corresponding increase in acetylcarnitine in a severe carnitine depletion of cardiac muscle. These results are discussed in terms of carnitine function in the acetyl metabolism, assuming the acetylcarnitine as a possible acetyl reservoir in the working muscle.