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Nutrient Metabolism

Dietary L-Carnitine Enhances the Lymphatic Absorption of Fat and -Tocopherol in Ovariectomized Rats¹

Wei Zou², Sang K. Noh³, Kevin Q. Owen and Sung I. Koo^*,4

Department of Human Nutrition, Kansas State University, Manhattan, KS 66506; ^* Department of Nutritional Sciences, University of Connecticut, Storrs, CT 06269; and Lonza, Incorporated, Fair Lawn, NJ 07410

⁴To whom correspondence should be addressed. E-mail: sung.koo{at}uconn.edu.

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
This study was conducted to determine whether the feeding of dietary L-carnitine (CN) improves the intestinal absorption of fat and -tocopherol (TOH) in ovariectomized (OX) rats. OX adult rats were weight-matched and assigned to 2 groups fed a modified AIN-93G diet containing TOH-stripped soybean oil without (–CN) or with (+CN) supplemental CN at 150 mg/kg diet. At 5 wk, each rat with a lymph cannula was infused intraduodenally at 3.0 mL/h with a lipid emulsion consisting of 565 µmol triolein labeled with ¹⁴C (¹⁴C-OA), 3.6 µmol TOH, and 396 µmol sodium taurocholate in 24 mL PBS buffer. Lymph was collected hourly for 8 h and analyzed for lipids. The lymphatic absorption of TOH for 8 h in +CN rats (899 ± 201 nmol) was higher (P < 0.05) than in –CN rats (587 ± 92 nmol). The absorption of ¹⁴C-OA in +CN rats (53.5 ± 4.0% dose/8 h) also was increased (P < 0.05) compared with –CN rats (47.6 ± 5.0% dose/8 h). Lymph flow did not differ between the groups. When bile was diverted but with infusion of sodium taurocholate, the lymphatic absorption of lipids did not differ. The present study provides evidence that dietary CN enhances the rates and amounts of lymphatic absorption of TOH and fat in OX rats. Our findings suggest that dietary CN may influence the process of lipid packaging and absorption by the enterocyte in OX rats, and may explain in part the increased status of TOH in CN-fed animals.

KEY WORDS: • carnitine • fat • intestinal absorption • -tocopherol • ovariectomy • rats

L-Carnitine (CN)⁵ plays an important role in the transfer of long-chain acyl groups into the mitochondrial matrix. Evidence suggests that CN has other biological functions (1). CN and acylcarnitines were shown to protect isolated perfused heart (2), cardiac microsomes (3), and cell membranes (4) and lipoproteins (5) from peroxidation. CN exhibits a free radical–scavenging activity (6), perhaps by directly chelating iron necessary for the generation of hydroxyl radicals (7,8). CN and acylcarnitines were also shown to be involved in reacylation and remodeling of membrane phospholipids (PL) and act as a secondary antioxidant to protect cell membranes (9,10). Several studies demonstrated that CN improves the body status of -tocopherol (TOH), a potent lipid-soluble antioxidant, protecting cellular membrane and plasma lipoproteins (11,12).

At present, little information exists on the mechanism underlying the effect of dietary CN on the nutritional status and metabolism of TOH, especially in ovariectomized (OX) animals. Recent evidence indicates that CN, as a substrate for carnitine palmitoyltransferase (CPT), plays an important role in the synthesis of triacylglycerol (TG) for chylomicron formation in the enterocyte (13), suggesting that CN may enhance other fat-soluble nutrients such as TOH. The concentrations of TOH in the liver and other tissues are significantly higher in female rats than in males (14–17), whereas ovariectomy decreases and estradiol replacement elevates the tissue concentrations of TOH in OX rats (18). Using the OX rats as a model for the state of ovarian estradiol deficiency as in postmenopausal women, the present study was conducted to determine whether dietary CN influences the lymphatic absorption of fat and TOH in OX rats, hence contributing to the improved body status of TOH in rats fed supplemental dietary CN, as reported earlier (11,12).

	MATERIALS AND METHODS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
    Animals. Female Sprague-Dawley rats (Harlan Sprague Dawley) weighing 208 ± 8 g were placed individually in stainless steel wire-bottomed plastic cages in a windowless room at a controlled temperature (21–23°C) and subjected to a daily 12-h light:dark cycle with the dark period from 0330 to 1530 h. The rats were maintained in an animal care facility in the Department of Human Nutrition, Kansas State University, and cared for in accordance with the animal care guidelines and protocols approved by the Institutional Animal Care and Use Committee.

    Dietary treatment and ovariectomy. During wk 1 of acclimation, rats consumed ad libitum a nutritionally adequate AIN-93G diet (19) formulated by Dyets containing egg white in place of casein as the protein source. The mineral contents of the diet were adjusted according to the AIN recommendations with the use of egg white (20). The soybean oil of the diet contained 0.02% tert-butylhydroquinone and tocopherols (2.7 mg RRR d--tocopherol, 0.3 mg ß-tocopherol, 23.2 mg -tocopherol, and 6.7 mg -tocopherol per 70 g). The vitamin mix of the diet contained all-rac dl--tocopherol as the source of vitamin E. The rats had free access to deionized water (Millipore) delivered via a stainless steel watering system. Rats weighing 240 ± 9 g were food deprived for 16 h and ovariectomized under halothane anesthesia (2.0% halothane in 2.0 L oxygen/min) using a halothane vaporizer (Ohio Medical Products). After 1 wk postoperative recovery, OX rats were weight matched and assigned to 2 groups: one fed the AIN-93G diet containing TOH-stripped soybean oil without L-carnitine (–CN) or with (+CN) supplemental CN (Carniking®, Lonza) at 150 mg/kg diet. Carniking contained 50% CN in an approved food additive (Sipernat 50, a silica-derived anticaking agent) and the control (–CN) diet was supplemented with Sipernat 50 (the vehicle). In the present study, the dosage of CN was set at 150 mg/kg diet. This amount of CN was estimated to be 0.01 mg/kJ on the basis of the rat’s daily food intake of 20 g (334 kJ), which is equivalent to a daily intake of 104.5 mg for a human consuming 10,450 kJ (2500 kcal)/d and is within the range of a typical diet providing 95–135 mg CN/d (1).

    Animal surgery. After 5 wk of dietary treatment, rats weighing 325 g from each group were used. The effect of dietary CN on the intestinal absorption of TOH and other lipids was examined in bile-intact and bile-diverted rats (21). After 16-h of food deprivation, rats were anesthetized with halothane. After a midline abdominal incision, the major mesenteric lymph duct was cannulated with vinyl tubing (0.50 mm i.d., 0.80 mm o.d.; SV 31, Dural Plastics & Engineering) and fixed with cyanoacrylate glue (Krazy Glue). An intraduodenal infusion catheter was installed by inserting silicone tubing (1.02 mm i.d., 2.16 mm o.d.; Silastic, Dow Corning Medical Products) through the gastric fundus into the proximal duodenum and secured with a purse-string suture. For the experiment with bile-diverted rats, the common bile duct was cannulated with PE-10 polyethylene tubing (0.28 mm i.d., 0.61 mm o.d.; Becton Dickinson, Clay Adams Brand), secured in situ with a purse-string suture. The lymph and bile duct cannulae and the infusion catheter were exteriorized through the right flank. After the incision was closed, rats were placed in individual restraining cages and allowed to recover for 20–24 h in a recovery chamber maintained at 30°C. Immediately after surgery, glucose-PBS [in mmol/L: 277.0 D-(+)-glucose, 6.8 Na₂HPO₄, 16.5 NaH₂PO₄, 115.0 NaCl, and 5.0 KCl; pH 6.7] was continuously infused through the infusion catheter at 3.0 mL/h via a syringe infusion pump (Harvard Apparatus, Model 935).

    Preparation and infusion of lipid emulsion. After postoperative recovery, each rat was infused with a lipid emulsion at 3.0 mL/h via the duodenal catheter under subdued light. The lipid emulsion consisted of 3.6 µmol of TOH (all-rac--tocopherol, 97%, Aldrich Chemical), 27.8 kBq of [carboxy-¹⁴C]-triolein (¹⁴C-OA; specific activity, 3.0–4.4 GBq/mmol, DuPont NEN), 565.0 µmol of triolein (95%, Sigma Chemical), and 396.0 µmol of sodium taurocholate in 24 mL of PBS. The amounts of triolein and TOH provided in the lipid emulsion for an 8-h infusion were 33% of the daily intakes of a rat consuming 20.0 g/d of the AIN-93G diet that contains 7.0% fat (19). The amount of bile salt (sodium taurocholate) in the emulsion was set at 0.89%, which is adequate to emulsify the lipids, but considered safe with no detrimental effect on the intestinal absorptive function (22). Lymph was collected hourly under subdued light for 8 h in preweighed, ice-chilled, conical centrifuge tubes containing 30 µg of n-propyl gallate as an antioxidant and 4 mg of Na₂EDTA as an anticoagulant. The tubes for bile collection contained 10 µg of n-propyl gallate. All samples were stored at –70°C until analysis.

    TOH and lipid analysis. Lymph or bile samples (100 µL), mixed with TOH acetate as an internal standard, were extracted by 2 mL of acetone, and TOH was analyzed by HPLC (21,23) using methanol as the mobile phase (24). Absorbance was monitored at 292 nm (Module 166, Beckman Instruments). Typical retention times were 4.1 min for TOH and 5.3 min for TOH acetate. TOH was quantified based on the TOH standard curve with a linear range between 110.5 and 442.3 pmol (r = 0.999). Total PL was measured colorimetrically (UV-1201 Spectrophotometer; Shimadzu Scientific Instruments) by the method of Reheja et al. (25). Total cholesterol (CH) was determined using o-phthalaldehyde (26).

    Determination of ¹⁴C-OA absorption. Fresh lymph (100 µL) was mixed with scintillation liquid (ScintiVerse, Fisher Scientific) and ¹⁴C-radioactivity was determined by scintillation spectrometry (Beckman LS-6500, Beckman Instruments). The total ¹⁴C-radioactivity appearing in the lymph collected hourly was used to determine the amount of ¹⁴C-OA absorbed. The hourly rate of ¹⁴C-OA absorption was expressed as the percentage of the total dose of ¹⁴C-OA infused.

    Statistical analysis. Values are presented as means ± SD unless otherwise stated. Repeated-measures ANOVA was performed by PC SAS to compare group means and to detect time-dependent changes within groups (27). When there was a significant treatment effect from repeated-measures ANOVA, differences (P < 0.05) among means were determined by Fisher’s protected least significant difference test.

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
    Body weight and food intake. Body weights and food intakes, as determined weekly, did not differ between groups throughout the experiment (initial body weights: 208.1 ± 8.4 g; final body weights: 324.0 ± 8.3 g for +CN vs. 323.9 ± 4.2 g for –CN. The daily food intakes were 15.7 ± 1.1 g in +CN and 15.9 ± 1.2 g in –CN).

    Lymph flow and lymphatic absorption of TOH. In bile duct–intact rats, the rates of lymph flow or total lymph volumes did not differ between groups (Table 1). The rates of lymph flow in +CN and -CN rats were 2.8 ± 0.7 mL/h and 3.1 ± 0.4 mL/h, respectively. At 2 h and thereafter, the rates of TOH absorption differed significantly between groups (3.5 ± 0.3% dose/h in +CN and 2.3 ± 0.4% dose/h in –CN rats) (Fig. 1A). The total absorption of TOH for 8 h in +CN rats was significantly higher than in –CN rats (Table 1).

View this table: [in this window] [in a new window]   TABLE 1 Cumulative lymphatic outputs of 14C-OA, CH, PL, and TOH in bile-intact rats fed a diet without (–CN) or with (+CN) supplemental L-carnitine at 150 mg/kg diet1

View larger version (15K): [in this window] [in a new window]   FIGURE 1 Hourly rates of lymphatic TOH absorption during luminal infusion of a lipid emulsion in bile-intact (A) and bile-diverted rats (B) fed a diet with no L-carnitine (–CN) or a diet with CN at 150 mg/kg (+CN). Values are means ± SEM, n = 6. *Different from –CN group, P < 0.05.

    Lymphatic absorption of ¹⁴C-OA and outputs of PL and CH. In bile duct-intact rats, the absorption of ¹⁴C-OA in +CN was higher (P < 0.05) compared with –CN rats (Table 1). The lymphatic output of PL in +CN increased (P < 0.05) compared with –CN rats (Table 1), whereas CH output did not differ between +CN and –CN rats(Table 1).

In bile-diverted rats, the lymphatic outputs of ¹⁴C-OA, PL, and CH were significantly lowered. With bile diversion, ¹⁴C-OA absorption did not differ between +CN rats (27.7 ± 8.3% dose/8 h) and –CN rats (25.5 ± 6.9% dose/8 h). PL output did not differ between +CN (11.2 ± 2.5 µmol/8 h) and –CN rats (11.0 ± 2.0 µmol/8 h). Similarly, CH output did not differ between +CN (3.5 ± 1.2 µmol/8 h) and –CN rats (3.6 ± 0.6 µmol/8 h).

    Biliary secretion of TOH, PL, and CH. The biliary outputs of TOH in +CN and –CN rats were 32.7 ± 6.6 and 33.7 ± 5.4 nmol/8 h, respectively, with no difference between groups. Also, biliary outputs of PL did not differ between the groups (18.0 ± 3.0 in +CN vs. 17.7 ± 1.4 µmol/8 h in –CN) or CH (4.7 ± 0.3 in +CN vs. 4.8 ± 0.8 µmol/8 h in –CN).

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Previous studies demonstrated that CN enhances the body status of TOH in rats (11,12), although the mechanism underlying such an effect of CN is far from clear. Our data here provide convincing evidence that supplemental dietary CN significantly increases the lymphatic absorption of TOH and fat in OX rats. This observation may explain in part the increased status of TOH in CN-fed animals.

CN serves as a substrate for CPTI, which facilitates the transport of long-chain fatty acyl moiety into the mitochondria and peroxisomes for ß-oxidation. Considerable evidence suggests that CN plays another important role in transferring long-chain acyl moiety for the synthesis of TG in such tissues as liver and intestine, which synthesize TG for export. In a recent study (28), CPT isoforms were identified in liver microsomes. TG synthesis, which occurs in the liver microsomal lumen, is CN dependent because CPT mediates the transport of the acyl group into the microsomal lumen where diacylglycerol acyltransferase II esterifies diacylglycerol to TG (29,30). Furthermore, a recent study (13) demonstrated that a similar CN-dependent pathway for TG synthesis exists in the rat intestine. When etomoxir, a specific inhibitor of CPT, was infused intraduodenally, the lymphatic output of TG via chylomicrons was dramatically reduced. A previous study using ligated rat intestinal segments showed that the absorption of palmitic acid was significantly decreased in the presence of D-carnitine, a competitive inhibitor of CPT (31). Our finding that dietary CN significantly enhances the intestinal absorption of fat is consistent with the concept that the intestinal synthesis of fat is a CN-dependent process, which may influence the subsequent formation of VLDL or chylomicrons. Although it is unknown how dietary CN may influence TOH absorption, it is generally accepted that the absorptive process for TOH follows the path of TG movement in the enterocyte (32).

At present, no information is available on whether dietary CN also influences intestinal synthesis of PL, which is needed for chylomicron formation. An earlier study (33) showed that rat liver microsomes utilize acylcarnitine as a substrate for PL synthesis. Our data here indicate that dietary CN significantly increases the lymphatic output of PL. The increased lymphatic output of PL is not attributable to an increase in biliary PL secretion because there was no change in the biliary secretion of PL in the +CN rats. This observation suggests that the increase in lymphatic PL output by dietary CN may be associated with an increase in intestinal synthesis and/or mobilization of PL during lipid packaging and chylomicron formation in the enterocyte.

Our data here showed that bile diversion abolishes the stimulatory effect of CN on lipid absorption. Previous studies showed that dietary CN is absorbed and transported to the liver via the portal vein, and excreted into the bile, mostly as acylcarnitine (34,35). Most (80%) of the biliary CN in rats and humans is acylcarnitine, one third of which is long-chain acylcarnitine (36). Long-chain acylcarnitine, incorporated into mixed micelles with bile acids (37), may increase micellar solubilization of fat and TOH. Our preliminary data showed that a lipid emulsion containing propionyl- and lauroyl-CN did not significantly affect lipid absorption in rats under similar conditions (unpublished data). It remains to be determined whether longer-chain acylcarnitine improves the micellar solubilization and subsequent transfer of lipids to the enterocyte.

It is also possible that supplemental dietary CN may increase the biliary secretion of acylcarnitine, which is then hydrolyzed in the intestinal lumen and absorbed into the enterocyte. We observed a significant increase in the biliary output of total CN in rats fed the same level of dietary CN in a separate experiment (unpublished data). The increased availability of CN via this route may facilitate TG synthesis in the enterocyte via the process involving the microsomal CPT system (28–30). It was shown that dietary CN increases the concentrations of free CN and acylcarnitine in the proximal segments of the small intestine in rats (38). Thus, dietary CN may improve the intestinal concentrations of CN and increase CPT activity, in particular, in OX rats with compromised CN status (39). It is possible that the intestinal availability of CN might have been affected by the overnight food deprivation before surgery and low energy intake during the postoperative recovery period in the present study. Whether the contribution of biliary CN to the intestine is altered under these conditions remains to be determined.

In summary, the present study provides the first evidence that dietary CN significantly enhances the lymphatic absorption of fat and TOH, a fat-soluble vitamin, in OX rats. Further studies are required to elucidate the mechanism underlying the stimulatory effect of dietary CN on lipid absorption, and to determine whether dietary CN improves the status of TOH and other fat-soluble vitamins in postmenopausal women or individuals with compromised CN status.

	FOOTNOTES

 
¹ Presented in abstract form at Experimental Biology 00, April 2000, San Diego, CA [Zou, W., Koo, S. I., Noh, S. K., Gross, K. L. & Owen, K. Q. (2000) Dietary carnitine enhances the lymphatic absorption of -tocopherol in ovariectomized rats. FASEB J. 14: A505 (abs.)]. This work is part of [Koo, S. I., Gross, K. L. & Owen, K. Q. (2002) Method for increasing intestinal absorption of fat soluble vitamins in post-menopausal women and lower animals. U.S. Patent no. 6,476,010].

² Current address: Division of Endocrinology, Clinical Nutrition, and Vascular Medicine, School of Medicine, University of California, Davis, CA 95617.

³ Current address: Department of Food and Nutrition, Changwon National University, Changwon, Kyongnam, 641–773, Korea.

⁵ Abbreviations used: TOH, -tocopherol; CH, cholesterol; CN, L-carnitine; +CN, diet with CN; –CN, diet without CN; CPT, carnitine palmitoyltransferase; ¹⁴C-OA, triolein labeled with ¹⁴C; OX, ovariectomized; PL, phospholipids; TG, triacylglycerol.

Manuscript received 19 November 2004. Initial review completed 7 January 2005. Revision accepted 20 January 2005.

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TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

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