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-Tocopherol in Adult Rats1
Department of Human Nutrition, Kansas State University, Manhattan, Kansas 66506
2To whom correspondence should be addressed. E-mail: koo{at}humec.ksu.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSExperiment 1Experiment 2RESULTSDISCUSSIONREFERENCES |
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-tocopherol (TP) in male rats. In expt.
1, bile-diverted rats with mesenteric lymph cannulas were infused
at 3.0 mL/h for 8 h with a lipid emulsion containing 5.0 µmol
TP, 565 µmol 14C-triolein (14C-OA) and 396
µmol Na+-taurocholate with 80 µmol 1,2-dipalmitoyl PC
(DPPC) or 1,2-dilinoleoyl PC (DLPC) or without PC (NoPC) in 24 mL
phosphate-buffered saline (pH 6.6). In expt. 2, the effects of
1,2-dioleoyl PC (DOPC) and 1-oleoyl-2-hydroxy-PC (lysoPC) on TP and
14C-cholesterol absorption were compared in rats with lymph
cannulas. When DPPC or DLPC was infused, the lymphatic absorption of
TP was lowered drastically. The cumulative absorptions of TP in
rats infused with DPPC and DLPC were 45 and 52%, respectively, of the
control values (NoPC). No significant difference was noted between the
PC groups. In contrast, the absorption of 14C-OA was
increased by 42 to 43% in rats infused with DPPC or DLPC compared with
that in NoPC rats. Phospholipid outputs also were significantly higher
in DPPC (34.0 ± 5.5 µmol/8 h) and DLPC (32.4 ± 2.4
µmol/8 h) rats than in NoPC rats (21.2 ± 4.2 µmol/8 h). When
lysoPC was infused, the absorptions of TP and
14C-cholesterol were increased markedly compared with those
for DOPC, with no significant difference in PL output between groups
infused with DOPC and lysoPC. These observations provide clear evidence
that PC present in a lipid emulsion inhibits TP absorption, whereas
it enhances the absorption of fat. The data also demonstrate that
lysoPC simultaneously increases the absorption of TP and
cholesterol. The findings indicate that luminal PC inhibits the
absorption of TP and that hydrolysis of PC is critical to improving
the intestinal absorption of the vitamin.
KEY WORDS: • absorption • -tocopherol • cholesterol • lysophosphatidylcholine • phosphatidylcholine • rats
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1Experiment 2RESULTSDISCUSSIONREFERENCES |
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first demonstrated that the deprivation of biliary
PC results in the impaired uptake and absorption of fatty acid (fat)
and that the provision of PC in bile-diverted rats not only
restores the absorption of fat but also stimulates protein synthesis
during active fat absorption. This observation has been confirmed in
subsequent studies by other investigators (2
,3)
. Growing
evidence, however, indicates that PC may inhibit the intestinal uptake
and absorption of certain lipids. For instance, the presence of PC in
bile-salt micelles has been shown to inhibit cholesterol uptake by
intestinal segments and everted sacs under in vitro conditions
(4
5
6
7
8)
. A marked decrease in cholesterol absorption also
has been shown in humans infused intraduodenally with purified soy PC
(9)
. More recently, a study with Caco-2 cells
(10)
showed that PC in bile-salt micelles decreased
the cell uptake, esterification and secretion of cholesterol without
affecting the uptake of fat and monoacylglycerol, whereas the
substitution of lysophosphatidylcholine (lysoPC) for the PC or the
addition of pancreatic phospholipase A2
(PLA2) reversed the inhibition of cholesterol
uptake by PC. In addition, PLA2 inhibitors or
antibodies were shown to abolish the
PLA2-dependent increase in cholesterol uptake
(10
,11)
. The inhibitory effect of PC on cholesterol uptake
also was observed when PC was incorporated into lipid emulsions
(12)
. The initial hydrolysis of the surface PC in a lipid
emulsion by pancreatic PLA2 was required for
hydrolysis of the core triacylglycerol (TG) by pancreatic
lipase/colipase and for the stimulation of cholesterol uptake by rat
intestinal cells.
Of particular interest is the observation that micellar PC causes a
drastic decrease in the intestinal absorption of -tocopherol
(TP), a fat-soluble vitamin, as measured by using an intestinal
segment perfused in situ (13)
. At the present, direct
evidence for such an effect of PC is lacking from in vivo studies using
conscious animals. Furthermore, the exact mechanism underlying the
inhibitory effect of PC on the intestinal uptake and absorption of
TP remains to be elucidated. Thus, the present study was conducted
to: 1) determine the effects of PC differing in their fatty
acid makeup on TP absorption in bile-diverted rats with lymph
cannula and 2) examine whether lysoPC increases the
intestinal absorption of the vitamin.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1Experiment 2RESULTSDISCUSSIONREFERENCES |
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Male Sprague-Dawley rats weighing 242 ± 7 g were
purchased from Harlan Industries (Indianapolis, IN) and housed singly
in wire-bottomed plastic cages in a room of controlled temperature
(22–24°C) and lighting (light off: 0900–2100 h). On arrival, rats
had free access to a nutritionally adequate AIN-93G diet
(14)
containing egg white in place of casein (Table 1
). All rats had free access to deionized water throughout the study.
Animals were cared for in an animal care facility in the Department of
Human Nutrition, Kansas State University, that was fully accredited by
the American Association for the Accreditation of Laboratory Animal
Care. Animals were maintained in accordance with the policies and
guidelines for animal care and use procedures of the Kansas State
University Institutional Animal Care and Use Committee.
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Experiment 1 |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1Experiment 2RESULTSDISCUSSIONREFERENCES |
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Rats weighing 348–368 g were divided into three groups of five rats
each. After not being fed for 15 h, rats were anesthetized with
halothane using a vaporizer (2.0% halothane in 2.0 L oxygen/min).
Cannulation of the bile and mesenteric lymph ducts was performed as
described previously (15
,16)
. Briefly, after midline
abdominal incision, the bile duct was cannulated with PE-10 tubing
(Clay Adams, Sparks, MD), which was secured in place with suture (4-0
silk; Ethicon, Somerville, NJ). Subsequently, the major mesenteric
lymph duct was cannulated with polyethylene tubing (SV 31 tubing; Dural
Plastics, Auburn, Australia) and fixed with cyanoacrylate glue (Krazy
Glue, Columbus, OH). A silicone infusion catheter (Silastic medical
grade tubing; Dow Corning, Midland, MI) was inserted into the proximal
duodenum via the gastric fundus and secured with purse-string
suture (4-0 silk; Ethicon). The bile and lymph cannulas and the
infusion catheter were exteriorized through the right flank. The rats
were placed in individual restraining cages and allowed to recover for
20 h in a warm recovery chamber maintained at 30°C. During the
recovery period, rats were infused via the infusion catheter with
phosphate-buffered saline (PBS) buffer (277 mmol glucose, 6.75 mmol
Na2HPO4, 16.5 mmol
NaH2PO4, 115 mmol NaCl and 5 mmol KCl per L, pH
6.6) at 3.0 mL/h via a syringe pump (model 935; Harvard Apparatus,
South Natick, MA).
Composition and infusion of lipid emulsion.
After postoperative recovery, rats were infused at 3.0 mL/h with a
lipid emulsion containing 5.0 µmol TP (all-rac--tocopherol,
97%; Aldrich Chemical, Milwaukee, WI), 27.8 kBq
[carboxyl-14C]triolein
(14C-OA; specific activity, 3.8 GBq/mmol; DuPont-New
England Nuclear, Boston, MA), 565 µmol triolein (95%; Sigma
Chemical, St. Louis, MO) and 396 µmol Na+-taurocholate
with 80 µmol 1,2-dipalmitoyl PC (DPPC; 99%; Avanti Polar Lipids,
Alabaster, AL) or 1,2-dilinoleoyl PC (DLPC; 99%; Avanti Polar Lipids)
or without PC (NoPC) in 24 mL of PBS. During infusion of the lipid
emulsion, lymph was collected at hourly intervals for 8 h in
preweighed ice-chilled plastic tubes containing 30 µg
n-propyl gallate and 4 mg Na2EDTA. Lymph
samples were stored at -70°C for lipid analyses.
HPLC analysis of TP in lymph.
TP was extracted with acetone with a slight modification of the
procedure (17)
. Briefly, 100 µL lymph was placed into a
glass test tube, followed by the addition of 1 mL acetone and 150 mg
anhydrous sodium sulfate. The contents were mixed vigorously on a
vortex mixer. After centrifugation at 1000 x g at
4°C for 10 min, the upper phase was filtered through a PTFE syringe
filter (0.45 µm; Alltech Associates, Deerfield, IL), dried under
N2 and resolubilized in a defined volume of
chloroform/methanol mixture (1:3, v/v). TP acetate was added as an
internal standard into each sample. The extracts were separated with a
Beckman HPLC instrument with System Gold software (Beckman Instruments,
Fullerton, CA) equipped with a C-18 reverse-phase column (Alltima
C18, 5 µm, 4.6 x 150 mm; Alltech Associates). Methanol was used
as the mobile phase at 2 mL/min. Typical elution times were 4.1 min for
TP and 5.3 min for TP acetate. Detection was monitored at 292 nm
(Module 166; Beckman Instruments). The concentration of TP was
calculated from the peak area responses using a standard curve with
TP ranging from 110.5 to 442.3 pmol. The recovery of the internal
standard exceeded 94%.
Determination of 14C-OA absorption.
From 100-µL aliquots of fresh lymph, 14C-radioactivity was determined with a liquid scintillation counting system (Beckman LS-6500; Beckman Instruments) after mixing with scintillation liquid (ScintiVerse; Fisher Scientific, Fair Lawn, NJ). The 14C-radioactivity appearing in total hourly lymph volumes was expressed as a percentage of the total 14C-radioactivity infused.
Determination of lymphatic phospholipid (PL) and cholesterol outputs.
With 100 µL of hourly lymph samples, PL was measured colorimetrically
(UV-1201 Spectrophotometer; Shimazu Scientific Instruments, Columbia,
MD) according to the method of Raheja et al. (18)
.
Cholesterol was determined with the use of
o-phthalaldehyde (Sigma Chemical), as described by Rudel
and Morris (19)
.
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Experiment 2 |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1Experiment 2RESULTSDISCUSSIONREFERENCES |
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TP and cholesterol. The protocols
concerning diet formulation, animal care, surgical procedure and lipid
analyses were the same as described for expt. 1, except that rats
weighing 334–408 g were used for cannulation of the mesenteric lymph
duct without bile diversion. After an overnight recovery period, rats
were infused with a lipid emulsion containing 3.56 µmol TP
(all-rac--tocopherol, 97%; Aldrich Chemical), 33.3 kBq
[4-14C]-cholesterol (specific activity, 1.9
GBq/mmol; DuPont-New England Nuclear), 20.69 µmol cholesterol,
452 µmol triolein (95%; Sigma Chemical) and 396 µmol
Na+-taurocholate with either 100 µmol DOPC
(99%; Avanti Polar Lipids) or lysoPC (99%; Avanti Polar Lipids) plus
100 µmol oleic acid in 24 mL PBS (pH 6.6). Statistical analysis.
All statistical analyses were performed using PC SAS (20)
.
ANOVA and the least significant difference test were performed to
compare multiple group means and to detect time-dependent changes
within groups. Student’s t test was used to compare two
group means at designated time intervals. Differences were considered
significant at P < 0.05. Values in tables and
figures are expressed as means ± SD.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1Experiment 2RESULTSDISCUSSIONREFERENCES |
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Lymphatic absorption of TP.
The rates of lymph flow or total lymph volumes did not differ
regardless of whether PC was infused. The hourly rates of lymph flow
were 2.1 ± 0.5 mL when infused with DPPC, 2.0 ± 0.4 mL with
DLPC and 2.0 ± 0.2 mL with NoPC. Figure 1
shows the hourly lymphatic absorption of TP in rats with bile
diversion. The intraduodenal infusion of either DPPC or DLPC in a lipid
emulsion drastically lowered the hourly rates of TP absorption at
4 h and thereafter, with no significant difference between DPPC-
and DLPC-infused rats (Table 2
). The hourly rates of TP absorption rose slowly but steadily in all
groups during the first 3 h of lipid infusion (Fig. 1)
. At 4 h and thereafter, however, the hourly rates of absorption did not rise
further in DPPC and DLPC rats and remained at 50 to 60 nmol/h. In
contrast, the rate of TP absorption in NoPC rats increased rapidly
beginning at 4 h and reached 122.8 ± 23.4 nmol/h at 8 h. The rates of TP absorption in DPPC, DLPC and NoPC groups were
45.1 ± 10.7, 48.0 ± 7.9 and 86.5 ± 16.9 nmol/h,
respectively. The cumulative absorptions of TP at 8 h were
361.0 ± 85.3 nmol (7.2 ± 1.7% dose) in DPPC, 383.8 ± 63.4 nmol (8.5 ± 1.4% dose) in DLPC and 692.2 ± 135.0 nmol
(13.9 ± 2.7% dose) in NoPC rats, with no significant difference
between the DPPC and DLPC groups. The total absorptions of TP in
DPPC and DLPC groups represented 52 and 56%, respectively, of the
controls (NoPC).
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TP absorption,
luminal infusion of DPPC or DLPC markedly increased the absorption of
14C-OA, as provided in the form of triolein. The
hourly rates of 14C-OA absorption rose rapidly in
rats infused with DPPC or DLPC starting at 3 h and continued to
rise above the rates observed in NoPC rats until 8 h (Fig. 2
). The rates of 14C-OA absorption were 6.6 ± 0.9% dose/h in DPPC, 6.7 ± 0.5% dose/h in DLPC and 4.7 ± 0.6% dose/h in NoPC rats. The hourly rates of
14C-OA absorption in both DPPC and DLPC rats
steadily increased in close parallel, with no significant difference at
any hourly intervals (Fig. 2)
. The cumulative absorptions of
14C-OA for 8 h in DPPC and DLPC rats were
53.1 ± 7.4% dose and 53.5 ± 4.2% dose, respectively,
which were significantly higher than in NoPC rats (37.5 ± 5.8%
dose).
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). After rapid increases for the initial 2 h, the rates of PL
output in DPPC and DLPC rats remained constant at 4.0–5.3 µmol/h. In
rats infused with no PC, the rates of PL output after 2 h ranged
from 2.6 to 3.0 µmol/h. The total amounts of PL released for 8 h
were 34.0 ± 5.5 µmol in DPPC, 32.4 ± 2.4 µmol in DLPC
and 21.2 ± 4.2 µmol in NoPC rats, with no significant
difference between the PC groups (Table 2)
.
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Lymphatic absorption of TP and 14C-cholesterol
(14C-CH).
Rates of lymph flow in lysoPC and DOPC rats were 2.6 ± 0.4 and
2.5 ± 0.6 mL/h, respectively. The total volumes of lymph secreted
for 8 h were 20.5 ± 3.5 mL in lysoPC rats and 20.1 ± 5.1 mL in DOPC rats, with no significant difference between groups.
However, the luminal infusion of lysoPC significantly increased the
lymphatic absorption of TP compared with that of DOPC (Fig. 4A
, P < 0.05). The hourly rate of TP absorption was
increased sharply with lysoPC infusion, peaking at 78.9 ± 17.6
nmol/h at 3 h, and then declined to a rate of 73.0 nmol/h
thereafter. In rats infused with DOPC, however, the absorption of
TP, after peaking at 69.5 ± 20.7 nmol/h at 3 h, decreased
to 53.0 ± 12.2 nmol/h at 4 h and remained at that level. The
rates of TP absorption were 73.0 ± 8.3 nmol/h in lysoPC rats
and 51.1 ± 8.3 nmol/h in DOPC rats. The cumulative absorptions of
TP for 8 h were 583.7 ± 12.8 nmol (16.4 ± 0.4%
dose) in lysoPC rats and 408.5 ± 66.5 nmol (11.5 ± 1.9%
dose) in DOPC rats (Table 3
). Similarly, the lymphatic absorption of 14C-CH
was significantly higher in lysoPC rats than in DOPC rats at 3 h
and thereafter (Fig. 4B
, P < 0.05). The hourly rates
of 14C-CH absorption were 4.2 ± 0.1%
dose/h in lysoPC rats and 3.1 ± 0.3% dose/h in DOPC rats. The
cumulative absorption of 14C-CH for 8 h also
was significantly higher in lysoPC rates (33.7 ± 1.1% dose) than
in DOPC rats (24.8 ± 2.1% dose) (Table 3)
.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1Experiment 2RESULTSDISCUSSIONREFERENCES |
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TP
and cholesterol, whereas it enhances the absorption of fat (or fatty
acid). In addition, the present data demonstrate that the substitution
of lysoPC for PC simultaneously increases the absorption of TP and
cholesterol. Data show that the inhibitory effects of PC on TP and
cholesterol absorption are unrelated to the chain length or
saturation/unsaturation of the acyl groups of the PC infused. To
compare the effects of exogenous PC (DPPC versus DLPC), the influence
of biliary PC was eliminated by bile diversion, but
Na+-taurocholate was provided at 49.5 µmol/h to
ensure an adequate supply of bile salts (21)
.
At the present, the mechanism underlying such dual effects of PC on
lipid absorption is far from clear. Although intact PC is taken up via
the intestinal brush border membrane (22)
, its direct
uptake is thought to be minimal (23)
. Considerable
evidence indicates that the action of intact PC is largely
intraluminal. Studies, as mostly conducted under in vitro conditions,
suggest that PC may affect the intestinal absorption of lipids by
influencing the following intraluminal events: 1) the rates
of lipolysis, 2) formation and diffusion of mixed micelles
across the unstirred water layer and 3) transfer of micellar
lipids to the brush border membrane. Earlier studies have shown that
the presence of PC on the surface of lipid emulsions hinders the
hydrolysis of the core TG by pancreatic lipase even in the presence of
bile salts and colipase, whereas a limited initial hydrolysis of PC to
lysoPC by pancreatic PLA2 facilitates the binding
of lipase/colipase to the substrate interface, resulting in a rapid
hydrolysis of TG to fatty acids and monoacylglycerol
(24
25
26)
. More recently, a study using rat IEC-6
intestinal cells (12)
also showed that pancreatic
lipase/colipase was ineffective in hydrolyzing TG in PC-containing
lipid emulsions and that the initial hydrolysis of the surface PC by
the addition of pancreatic PLA2 significantly
increased the ability of pancreatic lipase/colipase to hydrolyze the
core TG of the emulsion. Minimal hydrolysis of TG was required for
stimulation of the cell uptake of cholesterol, suggesting that fatty
acid and monoacylglycerol liberated from TG are key determinants in
facilitating cholesterol transfer to the enterocyte. Thus, the combined
actions of pancreatic PLA2 and lipase/colipase
are critical for support of the normal rates of luminal lipolysis and
subsequent formation of mixed micelles. The inclusion of PC in mixed
micelles also has been shown to increase the solubility of lipids and
the size of micelles in the presence of bile salts, which results in
slowing of the rate of micellar diffusion across the unstirred water
layer (6
,8
,27)
. Thus, these observations suggest that
before its hydrolysis to lysoPC, PC may affect the intestinal uptake of
lipids by slowing the rates of lipolysis and micelle formation in the
intestinal lumen.
An important observation from the present study is that exogenous PC in
bile-diverted rats inhibits the absorption of TP and
cholesterol, whereas it enhances the absorption of fat under in vivo
conditions. It should be pointed out that the amount of PC infused was
more than adequate to provide the amount of biliary PC lost by bile
diversion, which was determined to be 3–4 µmol/h (unpublished data).
In the present study, the amount of PC infused in the bile-diverted
rats was set at 10 µmol/h to provide the amount of PC lost (4.0
µmol) by bile diversion and the estimated intake of PC through a
typical diet. On a daily basis, this amount is equal to an intake of
113 mg PC and equivalent to 0.38 mg/kJ based on the average daily food
intake of 20 g (AIN-93G diet), providing 297 kJ. For a human who
consumes 10,450 kJ (2500 kcal)/d, it would be equivalent to a daily
intake of 4.0 g PC, which is within the range (4–8 g) of daily PC
intake estimated for adult humans (28)
. Thus, the observed
inhibitory effect of PC on TP and cholesterol absorption is not
attributable to a lack or excess of PC supply. The inhibition of TP
absorption by PC also was demonstrated previously by using the rat
small intestine perfused in situ with egg PC incorporated into bile
salt micelles (13)
. However, the mechanism underlying the
dual effects of PC (i.e., inhibition of TP and cholesterol
absorption and enhancement of fat absorption) has yet to be understood.
It is generally believed that TP and other fat-soluble vitamins
enter the intestinal cell via passive diffusion, moving along with
absorbed lipids (29
30
31)
. However, a recent in vitro study
using Caco-2 cells (10)
demonstrated that the presence of
intact PC in bile salt micelles differentially affects the cell uptake
of lipids from the micellar matrix. The data showed that PC markedly
reduced the uptake of cholesterol by Caco-2 cells, whereas it did not
interfere with the cell uptake of oleic acid and monoacylglycerol
(10)
. The investigators postulated that the presence of
intact PC in bile salt micelles slows the transfer (desorption) of more
hydrophobic lipids such as cholesterol (a 27-carbon lipid), without
affecting the transfer of other less hydrophobic lipids, such as fatty
acid and monoacylglycerol, products of TG hydrolysis by pancreatic
lipase/colipase. This hypothesis is supported by the earlier
observation by Pownall et al. (32)
that the rate of
transfer of a hydrophobic molecule from PC single bilayer vesicles
decreases with increasing hydrophobicity of the molecule. Consistent
with these observations is the present finding that luminally infused
PC slows the lymphatic absorption of TP, an extremely hydrophobic
29-carbon lipid, whereas it enhances the absorption of fat (oleic
acid).
The present data clearly demonstrate that the inhibitory effect of PC
on the absorption of TP and cholesterol is abolished when lysoPC is
substituted for PC. Several studies previously have shown that the
inclusion of lysoPC in mixed micelles or addition of pancreatic
PLA2 in mixed micelles or a lipid emulsion
enhances cholesterol uptake by intestinal cells under in vitro
conditions (10
11
12)
. The present study is the first to
show under in vivo conditions that the substitution of lysoPC for PC in
a lipid emulsion simultaneously enhances the absorption of TP and
cholesterol. This effect of lysoPC may be mediated not only by its
favorable effect on TG hydrolysis and micellar formation in the
intestinal lumen but also by its stimulation of intracellular
processing of lipids within the enterocyte. It is well documented that
once lysoPC is taken up by the enterocyte, it is reconverted to PC and
facilitates the intracellular reacylation and packaging of lipids and
the formation and secretion of chylomicrons (1
,21
,33
34
35
36)
.
In summary, the present study provides evidence that the luminal
infusion of PC in bile-diverted rats with lymph cannulas
drastically lowers the absorption of TP but markedly increases the
lymphatic absorption of fat and the output of PL. The substitution of
lysoPC for PC in a lipid emulsion significantly enhances the intestinal
absorption of TP and cholesterol. Thus, pancreatic
PLA2, which hydrolyzes PC to lysoPC, may play an
important role in regulating the absorption of lipids and
lipid-soluble vitamins. Our data here, as obtained under in vivo
conditions, suggest that exogenous or dietary PC may be used to lower
cholesterol absorption but may adversely affect the body or nutritional
status of vitamin E by decreasing its absorption. Further investigation
is required to delineate the impacts of dietary PC intake on TP
status and cholesterol metabolism.
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FOOTNOTES |
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3 Abbreviations used: TP, -tocopherol; 14C-CH, 14C-cholesterol; 14C-OA, 14C-oleic acid; DLPC, 1,2-dilinoleoyl phosphatidylcholine; DOPC, 1,2-dioleoyl phosphatidylcholine; DPPC, 1,2-dipalmitoyl phosphatidylcholine; lysoPC, 1-oleoyl-2-hydroxy phosphatidylcholine; PC, phosphatidylcholine; PL, phospholipid.
Manuscript received September 18, 2000. Initial review completed November 16, 2000. Revision accepted December 11, 2000.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSExperiment 1Experiment 2RESULTSDISCUSSIONREFERENCES |
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