Absorption of D- and L-carnitine by the intestine and kidney tubule in the rat

The process by which L- and D-carnitine are absorbed was investigated using the live rat and the isolated vascularly perfused intestine. A lumenal dose of 2-6 nmol in the perfused intestine resulted in less than 5% transport of either isomer to the perfusate in 30 min. The L-isomer was taken up by the intestinal tissue about twice as rapidly as the D-isomer by both the perfused intestine (52.8% and 21.6%, respectively) and the live animal (80% and 50%, respectively) in 30 min. After 1 h 90% of the L-carnitine had accumulated in the intestinal tissue and was released to the circulation over the next several hours. Accumulation of D-carnitine reached a maximum of 80% in 2 h and release to the circulations was similar to that of L-carnitine. Uptake of both L-[14C]carnitine and acetyl-L-[14C]carnitine was more rapid in the upper jejunal segment than in other portions of the small intestine. Acetylation occurred in all segments, resulting in nearly 50% conversion to this derivative in 5 min. Increasing the dose of L-carnitine reduced the percent acetylation. The uptake of both isomers was a saturable process and high concentrations of D-carnitine, acetyl-L-carnitine and trimethylaminobutyrate inhibited L-carnitine uptake. In the live animal after 5 h, the distribution of isotope from L-[14C]carnitine and D-[3H]carnitine differed primarily in the muscle where 29.5% of the L-carnitine and 5.3% of the D-carnitine was found and in the urine where 2.9% of the L-carnitine and 7.1% of the D-carnitine was found. The renal threshold for L-carnitine was 80 microM and for D-carnitine 30 microM, in the isolated perfused kidney. Approx. 40% of the L-carnitine but none of the D-carnitine excreted in the urine was acetylated. L-Carnitine and D-carnitine competed for tubular reabsorption.     

Tissue specific depletion of L-carnitine in rat heart and skeletal muscle by D-carnitine

L-carnitine deficiency in heart and skeletal muscle was induced by intraperitoneal injection of D-carnitine into starved or fed rats. Carnitine levels in kidney were slightly lowered, but liver, brain and plasma were unaffected. L-carnitine deficient hearts were unable to maintain normal cardiac function when perfused in an isolated working heart apparatus with palmitate as the only perfused substrate. These findings indicate that tissue levels of carnitine in heart and skeletal muscle are maintained $\text{in vivo}$ by an exchange transport mechanism. It is postulated that the depletion of L-carnitine from these tissues occurs by an exchange of the D- and L-isomer across the cell membrane. The technique may be useful for estimating the levels of carnitine required for fatty acid oxidation and normal cardiac and skeletal muscle function; however, interpretation of such tests may be complicated by the inhibitory effects of the D-isomer upon carnitine transferase enzymes.     

Properties of carnitine transport in rat kidney cortex slices

The properties of carnitine transport were studied in rat kidney cortex slices. Tissue: medium concentration gradients of 7.9 for L-[methyl-¹⁴C]carnitine were attained after 60-min incubation at 37°C in 40 μM substrate. L- and D-carnitine uptake showed saturability. The concentration curves appeared to consist of (1) a high-affinity component, and (2) a lower affinity site. When corrected for the latter components, the estimated K_m for L-carnitine was 90 μM and $\text{V = 22}\text{nmol/min}$ per ml intracellular fluid; for D-carnitine, $\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{= 166}\text{μM}$ and $\text{V = 15}\text{nmol/min}$ per ml intracellular fluid. The system was stereospecific for L-carnitine. The uptake of L-carnitine was inhibited by (1) D-carnitine, γ-butyrobetaine, and (2) acetyl-L-carnitine. γ-Butyrobetaine and acetyl-L-carnitine were competitive inhibitors of L-carnitine uptake. Carnitine transport was not significantly reduced by choline, betaine, lysine or γ-aminobutyric acid. Carnitine uptake was inhibited by 2,4-dinitrophenol, carbonyl cyanide $\text{m-}\text{chlorophenylhydrazone}$, N₂ atmosphere, KCN, $\text{N-}\text{ethylmaleimide}$, low temperature (4°C) and ouabain. Complete replacement of Na⁺ in the medium by Li⁺ reduced L- and D-carnitine uptake by 75 and 60%, respectively. Complete replacement of K⁺ or Ca²⁺ in the medium also significantly reduces carnitine uptake. Two roles for the carnitine transport system in kidney are proposed: (1) a renal tubule reabsorption system for the steady-state maintenance of plasma carnitine; and (2) maintenance of normal carnitine levels in kidney cells, which is required for fatty acid oxidation.     

Uptake of L-carnitine, D-carnitine and acetyl-L-carnitine by isolated guinea-pig enterocytes

Uptake and metabolism of L-carnitine, D-carnitine and acetyl-L-carnitine were studied utilizing isolated guinea-pig enterocytes. Uptake of the D- and L-isomers of carnitine was temperature dependent. Uptake of L-[14C]carnitine by jejunal cells was sodium dependent since replacement by lithium, potassium or choline greatly reduced uptake. L- and D-carnitine developed intracellular to extracellular concentration gradients for total carnitine (free plus acetylated) of 2.7 and 1.4, respectively. However, acetylation of L-carnitine accounted almost entirely for the difference between uptake of L- and D-carnitine. About 60% of the intracellular label was acetyl-L-carnitine after 30 min, and the remainder was free L-carnitine. No other products were observed. D-Carnitine was not metabolized. Acetyl-L-carnitine was deacetylated during or immediately after uptake into intestinal cells and a portion of this newly formed intracellular free carnitine was apparently reacetylated. L-Carnitine and D-carnitine transport (after adjustment for metabolism and diffusion) were evaluated over a concentration range of 2-1000 microM. Km values of 6-7 microM and 5 microM, were estimated for L- and D-carnitine, respectively. Ileal-cell uptake was about half that found for jejunal cells, but the labeled intracellular acetylcarnitine-to-carnitine ratios were similar for both cell populations. Carnitine transport by guinea-pig enterocytes demonstrate characteristics of a carrier-mediated process since it was inhibited by D-carnitine and trimethylaminobutyrate, as well as being temperature and concentration dependent. The process appears to be facilitated diffusion rather than active transport since L-carnitine did not develop a significant concentration gradient, and was unaffected by ouabain or actinomycin A.     

Characteristics of L-carnitine import into heart cells

L-carnitine is an essential cofactor for the transport of fatty acids across the mitochondrial membranes. L-carnitine can be provided by food products or biosynthesized in the liver. After intestinal absorption or hepatic biosynthesis, L-carnitine is transferred to organs whose metabolism is dependent upon fatty acid oxidation, such as the skeletal muscle and the heart. The intracellular transport of L-carnitine into the cell requires specific transporters and today, several of these have been characterized. Most of them belong to the solute carrier family. Heart is one of the major target for carnitine transport and use, however basic properties of carnitine uptake by heart cells have never been studied. In this paper, the transport of L-carnitine by rat heart explants has been examined and the kinetic properties of this transport determined and compared to data obtained in skeletal muscle explants. As in muscle, L-carnitine uptake by heart cells was shown to be dependent on sodium and was inhibited by L-carnitine analogues. Molecules known to interact with the skeletal muscle L-carnitine transport were studied in the heart. While trimethyl hydrazinium propionate (THP) was shown to fully inhibit the L-carnitine uptake by muscle cells, it remained inefficient in inhibiting the L-carnitine uptake by heart cells. On the other hand, compounds such as verapamil and AZT were both able to inhibit both the skeletal muscle and the cardiac uptake of L-carnitine. These data suggested that the muscle and heart systems for L-carnitine uptake exhibited different systems of regulation and these results have to be taken in consideration while administrating those compounds that can alter l-carnitine uptake in the muscle and the heart and can lead to damage to these tissues.     

Reexamination of the 1H NMR assignments and solution conformations of L-carnitine and O-acetyl-L-carnitine using C-2 stereospecifically labeled monodeuterated isomers

The two C-2 monodeuterated isomers of L-carnitine were synthesized by enzymatic hydration of crotonobetaine in D2O and by enzymatic proton exchange of L-[2-2H2]carnitine in H2O. These reactions, catalyzed by an induced Escherichia coli carnitine hydrolyase proceed stereospecifically. The two isomers of L-[2-2H]carnitine were examined by 1H NMR at 500 MHz, which allowed us to independently monitor the pD dependence and coupling constants of the H-2 protons. The results obtained indicate that there is little effect of the carboxyl charge on the conformational state(s) of L-carnitine about the C-2/C-3 bond. The NMR data obtained in this study do not support previous solution studies of the pH-dependent conformational changes for DL-carnitine nor the proposed conformation of O-acetyl-DL-carnitine in the crystalline state.     

Large variations in skeletal muscle carnitine level fail to modify energy metabolism in exercising rats

1. The importance of carnitine status in energy metabolism during exercise was studied in experimentally carnitine-depleted or supplemented rats. 2. Muscle carnitine concentration can be decreased by 40% with D-carnitine and increased by 40% with L-carnitine supplementation. 3. In spite of large variation of carnitine content, neither the exercising capacity nor the rate of muscle or liver glycogenolysis were modified during submaximal exercise. 4. The increased lipid metabolism induced by exercise can be adequately supported by endogenous levels of tissue carnitine. 5. Before any impairment in energy metabolism during exercise can be demonstrated, carnitine concentration has to be reduced to a level close to that measured with primary carnitine deficiency, i.e. less than 20 mumol/l of plasma.     

Conversion of D-carnitine into L-carnitine with stereospecific carnitine dehydrogenases

D-Carnitine was converted to L-carnitine by resting and permeabilized cells as well as with purified stereospecific carnitine dehydrogenases from Agrobacterium sp. With permeabilized cells only 11% of D-carnitine was converted into L-carnitine. Using highly stereospecific D- and L-carnitine dehydrogenases from Agrobacterium sp. (pH 8.5, 50 mM D-carnitine, 1 mM NAD + , 0.1 mM NADH, 25-fold excess of L-carnitine dehydrogenase) almost 50% of the D-carnitine could be converted into L-carnitine.     

Does carnitine have a role in fat absorption?

The effect of D-carnitine and tetradecylglycidic acid (TDGA), an inhibitor of carnitine palmitoyltransferase, on intestinal absorption of palmitic acid was determined. The proximal intestinal segment was ligated in adult male rats and filled with 0.5 microCi of 14C-palmitic acid alone or with either D-carnitine or TDGA. Thirty minutes later the radioactivity was determined in the intestinal lumen, intestinal wall and plasma. The absorption of palmitic acid was decreased in the presence of D-carnitine (10 mg/ml) as evidenced by significantly lower levels of radioactivity in the gut wall and the plasma and by significantly greater residual radioactivity in the lumenal contents. L-carnitine had no effect on plasma radioactivity but if D- and L-carnitine were given together the effect of D-carnitine was still in evidence. TDGA also inhibited intestinal absorption of palmitic acid.     

Carnitine transport and exogenous palmitate oxidation in chronically volume-overloaded rat hearts

L-Carnitine transport and free fatty acid oxidation have been studied in hearts of rats with 3-month-old aorto-caval fistula. For carnitine transport experiments, the hearts were perfused via the ascending aorta with a bicarbonate buffer containing 11 mM glucose and variable concentrations L-[14C]carnitine (10-200 microM). In some experiments, the active component of carnitine transport was suppressed by the adjunction of 0.05 mM mersalyl acid. The subtraction of passive from total transport allowed reconstruction of the saturation curves of the carrier-mediated transport of L-carnitine. Our data suggest that at a physiological carnitine concentration (50 microM), the rate of [14C]carnitine accumulation was significantly depressed in mechanically overloaded hearts. In addition, according to Lineweaver-Burk analysis, the affinity of the membrane carrier for L-carnitine was considerably diminished (Km carnitine 125 instead of 83 microM, Vmax unchanged). The above alterations of L-carnitine transport did not result from a decrease of the transmembrane gradient of sodium, since the intracellular Na+ content of the hypertrophied hearts was quite similar to that of control hearts. The ability of atrially perfused, working hearts to oxidize the exogenous free fatty acids was assessed from 14CO2 production obtained in the presence of [U-14C]palmitate or [1-14C]octanoate. The total 14CO2 production, expressed per min per g dry weight, was significantly diminished in hearts from rats with the aorto-caval fistula if 1.2 mM palmitate was used. On the other hand, in the presence of 2.4 mM octanoate, a substrate which circumvents the carnitine-acylcarnitine translocase, no such reduction of the 14CO2 production could be detected. Our results suggest that the decrease of L-carnitine transport, resulting in a significant depression of tissue carnitine, may impair long-chain fatty acid activation and/or translocation into mitochondria. In contrast, the oxidation of short-chain fatty acids, the activation of which takes place directly in mitochondrial matrix, is not limited in volume-overloaded hearts.     

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.     

Butyrobetaine is equal to L-carnitine in elevating L-carnitine levels in rats

This work shows that butyrobetaine administered to rats in a single dose can be highly effective in elevating L-carnitine levels in all tissues. This ability of butyrobetaine was compared to that of L-carnitine. In an experiment with tracer dose of the compounds, 12 h following administration of [3H]butyrobetaine plasma and tissues contained radioactivity exclusively in L-carnitine and in similar amounts as in the other group of animals receiving L-[3H]carnitine. This was observed both after intraperitoneal and oral administration of the compounds. In the loading experiments 100 mumol [3H]butyrobetaine was administered orally to one group and 100 mumol L-[3H]carnitine to the other group of animals and 12 h later it was found that butyrobetaine caused the same increments in L-carnitine as L-carnitine administration. The increments in the organs of the butyrobetaine-treated group (in decreasing order) were as follows: kidney, 1227 nmol/g vs. 652 nmol/g; liver, 469 nmol/g vs. 258 nmol/g; muscle, 1043 nmol/g vs. 881 nmol/g; plasma, 79.4 nmol/ml vs. 39.3 nmol/ml. Butyrobetaine (100 mumol) caused similar increments when it was administered intraperitoneally. Based on these results butyrobetaine can be considered as a potential agent for L-carnitine supplementation therapy.     

The Uptake of Carnitine by Slices of Rat Cerebral Cortex

Abstract: The properties of carnitine transport were studied in rat brain slices. A rapid uptake system for carnitine was observed, with tissue-medium gradients of 38 ± 3 for L-[¹⁴CH₃]carnitine and 27 ± 3 for D-[¹⁴CH₃]carnitine after 180 min incubation at 37°C in 0.64 mM substrate. Uptake of L- and D-carnitine showed saturability. The estimated values of K _m for L- and D-carnitine were 2.85 mM and 10.0 mM, respectively; but values of V _max (1 μmol/min/ml in-tracellular fluid) were the same for the two isomers. The transport system showed stereospecificity for L-carnitine. Carnitine uptake was inhibited by structurally related compounds with a four-carbon backbone containing a terminal carboxyl group. L-Carnitine uptake was competitively inhibited by γ-butyrobetaine ( K _i= 3.22 mM), acetylcarnitine ( K _i= 6.36 mM), and γ-aminobutyric acid ( K _i= 0.63 mM). The data suggest that carnitine and γ-aminobutyric acid interact at a common carrier site. Transport was not significantly reduced by choline or lysine. Carnitine uptake was inhibited by an N₂ atmosphere, 2,4-dinitrophenol, carbonylcyanide- N -chlorophenylhydrazone, potassium cyanide, n-ethylmaleimide, and ouabain. Transport was abolished by low temperature (4°C) and absence of glucose from the medium. Carnitine uptake was Na⁺-dependent, but did not require K⁺ or Ca²⁺.     

Influence of carnitine supplementation on muscle substrate and carnitine metabolism during exercise

We examined 1) the effect of L-carnitine supplementation on free fatty acid (FFA) utilization during exercise and 2) exercise-induced alterations in plasma levels and skeletal muscle exchange of carnitine. Seven moderately trained human male subjects serving as their own controls participated in two bicycle exercise sessions (120 min, 50% of VO2max). The second exercise was preceded by 5 days of oral carnitine supplementation (CS; 5 g daily). Despite a doubling of plasma carnitine levels, with CS, there were no effects on exercise-induced changes in arterial levels and turnover of FFA, the relation between leg FFA inflow and FFA uptake, or the leg exchange of other substrates. Heart rate during exercise after CS decreased 7-8%, but O2 uptake was unchanged. Exercise before CS induced a fall from 33.4 +/- 1.6 to 30.8 +/- 1.0 (SE) mumol/l in free plasma carnitine despite a release (2.5 +/- 0.9 mumol/min) from the leg. Simultaneously, acylated plasma carnitine rose from 5.0 +/- 1.0 to 14.2 +/- 1.4 mumol/l, with no evidence of leg release. Consequently, total plasma carnitine increased. We concluded that in healthy subjects CS does not influence muscle substrate utilization either at rest or during prolonged exercise and that free carnitine released from muscle during exercise is presumably acylated in the liver and released to plasma.     

L-Carnitine transport in human placental brush-border membranes is mediated by the sodium-dependent organic cation transporter OCTN2

Maternofetal transport of L-carnitine, a molecule that shuttles long-chain fatty acids to the mitochondria for oxidation, is thought to be important in preparing the fetus for its lipid-rich postnatal milk diet. Using brush-border membrane (BBM) vesicles from human term placentas, we showed that L-carnitine uptake was sodium and temperature dependent, showed high affinity for carnitine (apparent K_m = 11.09 ± 1.32 µM; V_max = 41.75 ± 0.94 pmol·mg protein^–1·min^–1), and was unchanged over the pH range from 5.5 to 8.5. L-Carnitine uptake was inhibited in BBM vesicles by valproate, verapamil, tetraethylammonium, and pyrilamine and by structural analogs of L-carnitine, including D-carnitine, acetyl-D,L-carnitine, and propionyl-, butyryl-, octanoyl-, isovaleryl-, and palmitoyl-L-carnitine. Western blot analysis revealed that OCTN2, a high-affinity, Na⁺-dependent carnitine transporter, was present in placental BBM but not in isolated basal plasma membrane vesicles. The reported properties of OCTN2 resemble those observed for L-carnitine uptake in placental BBM vesicles, suggesting that OCTN2 may mediate most maternofetal carnitine transport in humans. membrane transport; valproate; maternofetal; xenobiotics; acylcarnitine     

Catabolic pathways for high-dosed L(-)- or D(+)-carnitine in germ-free rats?

Gnotobiotic rats received up to 3 mmol L-carnitine/day with the drinking water during 9 days. They excreted about a quarter of the administered dose with the urine, partially in form of acetyl-L-carnitine, but trimethylamine, trimethylamine N-oxide or gamma-butyrobetaine were not detectable in urine or faeces in contrast to conventional animals. After oral loading with D-carnitine the unphysiological isomer was absorbed and either excreted unchanged in urine or metabolized to acetonyltrimethylammonium. With regard to the development of carnitine deficiency syndromes and the degradation of nutritional carnitine the conclusion has to be drawn, that the bacteria of the gastro-intestinal tract, but not the tissues of the mammals, are responsible for the metabolization of L-carnitine to gamma-butyrobetaine or trimethylamine.     

Effects of L-carnitine on mechanical recovery of isolated rat hearts in relation to the perfusion with glucose and palmitate

The purpose of this study was to investigate the effects of L-carnitine on the hemodynamic parameters of Langendorff hearts. Isolated rat hearts were perfused with various solutions containing high or low concentrations of fatty acids, additional glucose or no glucose, and L-carnitine or no L-carnitine. The most interesting part of the experiments was the behaviour of the hearts in the reperfusion period after no-flow ischemia of 20 min. The results were: (1) With glucose and high fatty acid concentrations the hearts showed an improved recovery of the left ventricular functions in the reperfusion period compared with low fatty acid concentrations. Without glucose the left ventricular pressure is much lower in the reperfusion period. (2) Addition of L-carnitine improved the recovery of the ischemically damaged hearts. This improvement is especially impressive at low fatty acid concentrations. L-carnitine addition at high fatty acid concentrations but without glucose strongly improved reperfusion behaviour. (3) The coronary flow is increased by 2 experimental conditions: (i) perfusion at low levels of fatty acids, carnitine and with glucose and (ii) high levels of fatty acids and carnitine but without glucose. These findings suggest that supplementation of L-carnitine has a beneficial effect on the isolated heart under various conditions, and possibly on specific human heart diseases.     

How important are carnitine and ketones for the newborn infant?

The newborn oxidizes a large amount of fat. This is reflected in the slow rise of plasma levels of ketones and of total carnitines and acylcarnitines. Feeding a diet devoid of carnitine (soy-based formulas, total parenteral nutrition [TPN] ) rapidly results in a fall in plasma total carnitine levels, whereas in the adult such a fall is observed only after a prolonged time of TPN. This suggests that carnitine synthesis in the newborn is less efficient than in the adult. Gluteal adipocytes in the newborn show a rise in carnitine content and in the activity of carnitine transferases soon after birth, when values are higher than in the adult. Their respiration, lipolysis, and triglyceride formation are enhanced by L-carnitine and inhibited by D-carnitine. This is not so in the adult. Addition of L-carnitine to soybean-based formulas decreases plasma triglyceride and free fatty acid levels in premature infants, who have lower carnitine levels at birth than full-term babies. In pregnant women plasma total carnitine levels are significantly depressed. maternal urinary excretion of total carnitine decreases as gestational age increases, and less is also found in amniotic fluid. Plasma levels of total carnitines and acylcarnitine are the same (or higher) in fetal as in maternal plasma. It is concluded that carnitine may be of particular importance to the neonate and that adding it to foods lacking this substance may be advantageous.     

Aggregation of submitochondrial particles by heparin and its application to the study of carnitine transport.

A novel technique for the separation of submitochondrial particles from the external medium, an essential procedure in transport studies, was devised. Very low concentrations of heparin (5-10 micrograms/ml) aggregate the particles and permit their rapid sedimentation in a micro-centrifuge. The transfer of activated fatty acids into mitochondria for oxidation depends on the exchange of matrix carnitine for external fatty-acylcarnitine. To study the matrix face of the carnitine/acylcarnitine translocase, inverted submitochondrial particles were prepared and loaded with L-[14C]carnitine. As found in intact mitochondria, the Km value for L-carnitine was 8 mM, that for palmitoyl-L-carnitine was two orders of magnitude lower, and 11-trimethylaminoundecanoyl-DL-carnitine was a competitive inhibitor. The properties of the carrier exposed to the outer and to the matrix sides of the mitochondrial inner membrane are thus similar.