Linoleic acid absorption in the unaesthetized rat: Mechanism of transport and influence of luminal factors on absorption

Linoleic acid intestinal absorption was studied in the unanesthetized rat. At low (21–1260 μM) intraluminal concentrations, absorption took place by facilitated diffusion; while at high (1.26–2.5 mM) concentrations, simple diffusion was the predominant mechanism of transport. At low concentrations (840 μM), the equimolar additions of oleic, linolenic, and arachidonic acids or lecithin inhibited the absorption of linoleic acid. Substitution of potassium for sodium in the buffer solution, substitution of Tween 80 for sodium taurocholate, or decrease in the hydrogen ion concentration all resulted in decreased rate of linoleic acid's absorption. These experiments demonstrate that linoleic acid is absorbed by a concentration-dependent dual mechanism of transport. The absorption rate is modified by the pH, surfactant type and concentration, the simultaneous presence of other polyunsaturated fatty acids, and the thickness of the unstirred water layer.     

Arachidonic acid intestinal absorption: Mechanism of transport and influence of luminal factors on absorption in vitro

The mechanism and characteristics of intestinal absorption of arachidonic acid were studied in vitro using everted intestinal sacs of the rat. Arachidonic acid absorption was studied at concentrations of 5 μM to 8.36 mM. The plot of absorption rate vs. concentration fitted best to a rectangular hyperbola at low μM concentrations and to a straight linear relationship in the mM range of concentrations. Metabolic inhibitors and uncouplers did not change absorption in either range of concentrations. The absorption of arachidonic acid increased with thinning of the unstirred water-layer, decrease in the pH, or the substitution of sodium taurocholate by Pluronic F 68 or Tween 80. Absorption decreased following the equimolar additions of oleic, linoleic, and linolenic acids. Absorption rate did not change when the taurocholate concentration was varied from 5–15 mM or following the additions of butyric or glutamic acids, leucine, lysine, or dextrose. It was concluded that arachidonic acid is absorbed by a concentration-dependent dual mechanism of transport which is not energy dependent. At the low μM range of concentrations, facilitated diffusion is predominant, while at mM concentrations, simple diffusion is the dominant mechanism of absorption. Changes in the intestinal fluid composition, flow rate, and pH can modify the rate of absorption of arachidonic acid.     

The absorption and transport of dietary cholesterol in the presence of peanut oil or randomized peanut oil

Peanut oil has been shown to be unexpectedly atherogenic for cholesterol-fed rats, rabbits and rhesus monkeys. However, randomization (rearrangement of fatty acids to random distribution) of peanut oil significantly reduced its atherogenicity for rabbits and monkeys. This study was conducted to investigate whether the absorption and transport of dietary cholesterol was altered in the presence of peanut oil or randomized peanut oil, thereby accounting for the difference in the atherogenicity of the two diets. Intestinal lymph fistula rats were infused intraduodenally with a lipid emulsion at a rate of 3 ml/hr. The lipid emulsion contained either peanut oil (control) or randomized peanut oil (experimental) (10 mM),¹⁴C-cholesterol (1.3 mM) and sodium taurocholate (19 mM) in phosphate-buffered saline, pH 6.4. Lymph triglyceride, cholesterol and phospholipid outputs were similar in both groups of rats during fasting and subsequently during lipid infusion. Comparable recovery of¹⁴C-cholesterol from the intestinal lumen and the intestinal mucosa of the control and the experimental rats showed that the absorotion and transport of dietary cholesterol were similar in both groups of rats. Analyses of thefatty acid of both lymph and intestinal mucosal lipid again failed to reveal a difference between the 2 groups of rats. It is concluded that the difference in the atherogenicity between the peanut oil and the randomized peanut oil is probably caused by events subsequent to the release of cholesterol containing chylomicrons and very low density lipoproteins by the small intestinal epithelial cells.     

Phospholipids affect the intestinal absorption of carotenoids in mice

Previously, we have shown that uptake of carotenoids solubilized with mixed micelles by human intestinal Caco-2 cells is enhanced by lysophosphatidylcholine (lysoPC) and suppressed by PC. This study determined the effect of PC and lysoPC in mixed micelles on the accumulation of β-carotene and lutein in mice in order to elucidate the roles of micellar phospholipid in the intestinal uptake of carotenoids in vivo. Mixed micelles were composed of 2.5 mM monooleoylglycerol, 7.5 mM oleic acid, 12 mM sodium taurocholate, 200 μM carotenoid, and 3 mM phospholipid in PBS. The mice were fed single doses of β-carotene or lutein solubilized in PC (PC group), lysoPC (LPC group), and no phospholipid (NoPL group) micelles. The β-carotene responses in the plasma and liver of the PC group were markedly lower than those of the other two groups, whereas no differences were noticed between the LPC and NoPL groups. The average level of lutein in the plasma of the PC group after administration was significantly (P<0.05) lower than those of the other groups. Moreover, the average level of lutein in the liver was significantly (P<0.05) different among the groups in the order of LPC>NoPL>PC. Thus, the results clearly indicate that PC suppressed the accumulation of β-carotene and lutein in plasma and liver and that lysoPC enhanced the accumulation of lutein in liver. These results suggest that the hydrolysis of PC to lysoPC plays an important role in the intestinal uptake of carotenoids solubilized in mixed micelles.     

Lecithin inhibits fatty acid and bile salt absorption from rat small intestine in vivo

During digestion of a fatty meal, long chain free fatty acids (FFA) and lecithin are among the lipids solubilized in intestinal contents as mixed micelles with bile salts. We hypothesized that if lecithin were not hydrolyzed, the mixed micelles would be abnormal, and absorption of FFA and bile salts would be depressed. To test this hypothesis, isolated segments of rat small intestine were infused in vivo with micellar solutions of 2 mMolar linoleic acid and 10 mMolar taurocholate to which was added 3 mMolar 1-palmitoyl, 2-oleoyl lecithin (a common lecithin in bile and food), or 1-palmitoyl lysolecithin (the hydrolytic product of lecithin). Absorption of FFA and bile salt was measured under steady state conditions using a single-pass technique. Lecithin depressed the rate of FFA absorption by 40% (p<0.025) in jejunal and ileal segments whereas lysolecithin was associated with normal rates of FFA absorption. Lecithin also reduced taurocholate absorption from the ileum by 30% (p<0.05). These data support the idea that lecithin may depress FFA and bile salt absorption from the small intestine in pancreatic insufficiency. The following trivial names are used: lecithin (1,2-diacyl-sn-glycero-3-phosphorylcholine); lysolecithin (1-acyl-sn-glycero-3-phosphorylcholine).     

NPC1L1 and SR-BI are Involved in Intestinal Cholesterol Absorption from Small-Size Lipid Donors

In the human intestinal content after a meal, cholesterol is dispersed in a complex mixture of emulsified droplets, vesicles, mixed micelles and precipitated material. The aim of this study was to determine the contribution of the main intestinal cholesterol transporters (NPC1L1, SR-BI) to the absorption processes, using different cholesterol-solubilizing donors. Cholesterol donors prepared with different taurocholate concentrations were added to an apical medium of differentiated TC7/Caco-2 cells. As the taurocholate concentrations increased, cholesterol donor size decreased (from 712 to 7 nm in diameter), which enhanced cholesterol absorption in a dose-dependent manner (38-fold). Two transport processes were observed: (1) absorption from large donors exhibited low-capacity transport with no noticeable transporter contribution; (2) efficient cholesterol absorption occurs from small lipid donors (相似文献   

Intestinal cholesterol and oleic acid uptake from solutions supersaturated with lipids

To test the role of nonmicellar phases in lipid absorption, intestinal uptake of fatty acids and cholesterol has been studied in vitro from supersaturated and micellar solutions. The micellar solubility limit at equilibrium was established for cholesterol and oleate/monoolein (2∶1) at pH 6.7 with 10 mM taurocholate. Uptake by rat intestinal everted sacs was measured during incubation of 5 min. Cholesterol uptake increased linearly with the cholesterol content of micellar or supersaturated solutions up to a supersaturation of 150%. Oleate uptake, by contrast, remained essentially the same from either saturated or supersaturated (130–280%) mixtures. The difference between cholesterol and oleate uptake rates is explained by their distinct effects on micellar size, which is unchanged by cholesterol supersaturation but is increased by oleate. Solutions largely supersaturated (280%) with oleatemonoolein are polydisperse and contain viscous isotropic and paracrystalline phases similar to those observed during lipid absorption. These results suggest that, in the presence of such solutions, uptake occurs from both the micellar saturated and nonmicellar supersaturated phases.     

Effects of a single ingestion of sodium taurocholate on esterified cholesterol concentration in liver and cholesterol turnover in the rat

The overall response of the rat’s cholesterol metabolism to a single ingestion of taurocholate (80 mg) was studied with the isotopic equilibrium method. The bile acid production, measured by the daily¹⁴CO₂ output of rats in isotopic equilibrium of [26-¹⁴C]-cholesterol, initially decreased and then increased. Conversely, the hepatic concentration of esterified cholesterol first increased and then decreased. Moreover, the ingestion of taurocholate increasing the intestinal absorption coefficient of dietary cholesterol increased the abosprtion and decreased the fecal excretion and the intestinal biosynthesis of cholesterol. The balance of these last effects is an excess cholesterol inflow. The classical hypothesis of negative feedback regulation of bile acid production fails to explain the observed biphasic effect of taurocholate. This compound, when its origin is exogenous, appears to stimulate the storage of esterified cholesterol in the liver, at the expense of bile acid synthesis. This accumulation rate takes into account not only the decrease in cholesterol transformation into bile acids but also the excess inflow of cholesterol. As the exogenous taurocholate was eliminated from the body, cholesteryl ester hydrolysis occurred and provided a supplementary source of free cholesterol for bile acid synthesis.     

Role of polyphenols in the low nutritive value of Vicia faba

The inclusion of field bean (Vicia faba) in diets for growing experimental animals, reduced their growth performance in a similar way as compared with controls fed a casein diet, with 0.2% tannic acid. Hydroalcoholic extracts or isolated tannins obtained from Vicia faba seeds, decreased the intestinal transport of D-glucose. The addition of 0.05, 0.1 or 0.2% of tannic acid to the perfusion solutions containing D-glucose proportionally diminished the intestinal uptake of sugar according to its poliphenolic content. It has been postulated that polyphenols reduce, at least partially, the nutritive value of legumes, by altering monosaccharide absorption. Other constituents, however, could also be involved.     

Linoleic acid uptake by isolated enterocytes: Influence of α-linolenic acid on absorption

In a previous study we showed that intestinal uptake of α-linolenic acid (18∶3n−3) was carrier-mediated and we suggested that a plasma membrane fatty acid protein was involved in the transport of long-chain fatty acids. To further test this hypothesis, the mechanism of linoleic acid (18∶2n−6) uptake by isolated intestinal cells was examined using a rapid filtration method and 20 mM sodium taurocholate as solubilizing agent. Under these experimental conditions transport of [1-¹⁴C]linoleic acid monomers in the concentration range of 2 to 2220 nM was saturable with a V_m of 5.1±0.6 nmol/mg protein/min and a K_m of 183±7 nM. Experiments carried out in the presence of metabolic inhibitors, such as 2,4-dinitrophenol and antimycin A, suggested that an active, carriermediated mechanism was involved in the intestinal uptake of this essential fatty acid. The addition of excess unlabeled linoleic acid to the incubation medium led to a 89% decrease in the uptake of [1-¹⁴C]linoleic acid, whiled-glucose did not compete for transport into the cell. Other long-chain polyunsaturated fatty acids added to the incubation mixture inhibited linoleic acid uptake by more than 80%. The presence of α-linolenic acid (18∶3n−3) in the incubation medium caused the competitive inhibition (K_i=353 nM) of linoleic acid uptake. The data are compatible with the hypothesis that intestinal uptake of both linoleic, and α-linolenic acid is mediated by a membrane carrier common to long-chain fatty acids.     

ABCG5/G8 Deficiency in Mice Reduces Dietary Triacylglycerol and Cholesterol Transport into the Lymph

The adenosine triphosphate‐binding cassette (ABC) transporter G5/G8 is critical in protecting the body from accumulating dietary plant sterols. Expressed in the liver and small intestine, it transports plant sterols into the biliary and intestinal lumens, thus promoting their excretion. The extent to which G5/G8 regulates cholesterol absorption remains unclear. G5/G8 is also implicated in reducing the absorption of dietary triacylglycerols (TAG) by unknown mechanisms. We hypothesized that G5/G8 suppresses the production of chylomicrons, and its deficiency would enhance the absorption of both dietary TAG and cholesterol. The aim of this study was to investigate the effects of G5/G8 deficiency on lipid uptake and secretion into the lymph under steady‐state conditions. Surprisingly, compared with wild‐type mice (WT) (n = 9), G5/G8 KO (n = 13) lymph fistula mice given a continuous intraduodenal infusion of [3H]‐TAG and [14C]‐cholesterol showed a significant (P < 0.05) reduction in lymphatic transport of both [³H]‐TAG and [¹⁴C]‐cholesterol, concomitant with a significant (P < 0.05) increase of [³H]‐TAG and [¹⁴C]‐cholesterol accumulated in the intestinal lumen. There was no difference in the total amount of radiolabeled lipids retained in the intestinal mucosa between the two groups. G5/G8 KO mice given a bolus of TAG showed reduced intestinal TAG secretion compared with WT, suggesting an independent role for G5/G8 in facilitating intestinal TAG transport. Our data demonstrate that G5/G8 deficiency reduces the uptake and secretion of both dietary TAG and cholesterol by the intestine, suggesting a novel role for the sterol transporter in the formation and secretion of chylomicrons.     

Effect of lecithin on jejunal absorption of micellar lipids in man and on their monomer activity in vitro

The effect of lecithin on jejunal absorption of fatty acids and octadecenoylglycerol was studied in healthy volunteers with a jejunal perfusion system which excluded pancreatic and biliary secretions from the test segment. Lecithin significantly reduced the absorption of oleic acid (P<0.05) and octadecenoylglycerol (P<0.01), while it had no effect on the absorption of ricinoleic acid. In vitro, lecithin reduced monomer activities of all three lipids; the changes were greater for oleic acid and octadecenoylglycerol than for ricinoleic acid (P<0.02). From these data it is concluded that lecithin reduces monomer activity of fatty acids in mixed micellar solutions and that it can thereby reduce the absorption rates of micellar lipids. Intact lecithin is not absorbed under these conditions. Maldigestion of lecithin in pancreatic insufficiency may, therefore, aggravate the steatorrhea observed in this condition.     

Absorption of triglycerides with defined or random structure by rats with biliary and pancreatic diversion

Fat absorption may be compromised by pancreatic or bile insufficiency, resulting in low uptake of essential fatty acid and energy. Using a rat model of malabsorption, we examined the absorption of defined triglycerides with medium-chain fatty acids (MCFA) in thesn-1,3 positions and essential fatty acids in thesn-2 position (MLM) compared to other fats. The thoracic duct was cannulated for collection of lymph, and the common bile and pancreatic duct was cannulated to divert both the pancreatic juice and bile. The rats were given a single bolus of triglyceride as a taurocholate emulsion. Fat absorption was measured from collected lymph samples. The triglycerides administered were a defined triglyceride, MLM [mainly (8∶0/10∶0)-(18∶2n−6)-(8∶0/10∶0)], a similar triglyceride subjected to chemical randomization, a mixture of medium-chain triglycerides and soybean oil, and soybean oil, respectively. The first three triglycerides had approximately 36 wt% linoleic acid (18∶2n−6) content. Administration of defined triglyceride was followed by significantly higher lymphatic level (wt%) of 18∶2n−6 (P<0.01) as well as a relative enhancement in mol% of 18∶2n−6 (P<0.05) compared to the other triglycerides. Lymphatic absorption of MCFA was similar in the three first groups but not as efficient as for long-chain fatty acids. Our results indicate that defined triglycerides thus may provide a means to increase absorption of essential fatty acids in fat malabsorption, such as that seen in cystic fibrosis, or for pre-term infants. Honored Student presentation, 84th AOCS Annual Meeting & Expo, Anaheim, California.     

Site of bile acid absorption in the rat

Bile acid absorption was measured in the small intestine of the rat using⁹¹Y as a nonabsorbed reference substance. Some 50% of the secreted bile acids were absorbed in the proximal half of the small intestine. In situ incubations of ligated intestinal segments into which tauro(¹⁴C-carbonyl)cholic acid was introduced confirmed the considerable uptake of bile acids in the jejunum. The in situ experiments indicated that serosal transport is the limiting stage of bile acid absorption in the jejunum but not in the ileum. Increasing bile acid concentrations in the in situ experiments did not affect the percentage disappearance of dose from the jejunum but reduced the percentage mucosal uptake in the ileum. It is concluded that, in the rat, the proximal small intestine is as important in the absorption of bile acids as the distal small intestine.     

Homeostasis of mucosal cholesterol in the small intestine of the rat

This study was undertaken to investigate the capacity of the intestinal mucosa to maintain a constant cholesterol content under conditions where mucosal uptake or cholesterol transport into the lymph were manipulated. Two series of bile-diverted unanaesthetised rats were infused intraduodenally with saline, triolein emulsified with Pluronic F68, or taurocholate with or without added tomatine. Pluronic F68 is a nontoxic detergent which promotes mucosal uptake of polar lipids but not cholesterol. Tomatine is a cholesterol-binding saponin. One series of rats was used for measuring mucosal cholesterol content, DNA and protein after the test infusions. A second series of rats had the thoracic lymph duct cannulated but otherwise remained the same as the first series. The second series was used for measuring the effect of the different infusions on mass cholesterol output into lymph. Mucosal cholesterol content of rats that were not fed decreased with bile-diversion and was restored with taurocholate infusion. This suggested a contribution of luminal cholesterol to the mucosal cholesterol pool. However, evidence for a contribution from the lumen was provided by only one of two groups of rats given infusions which did not promote mucosal uptake of cholesterol. First, addition of tomatine to the taurocholate infusate prevented both the increase in lymph output of cholesterol and the increased mucosal cholesterol content shown in rats given taurocholate alone. Second, in another group of rats in which mucosal uptake of cholesterol was prevented, i.e. in rats given Pluronic F68-triolein emulsions, the increased fat absorption was accompanied by a marked increase in cholesterol output into lymph without a concomitant decrease in mucosal cholesterol content. These results would be consistent with increased mucosal synthesis of cholesterol as a possible source of endogenous cholesterol absorbed into lymph.     

Effect of peanut oil and randomized peanut oil on cholesterol and oleic acid absorption, transport, and distribution in the lymph of the rat

Peanut oil was shown to be atherogenic in cholesterol-fed rats, rabbits, and monkeys. However, after randomization, a process in which the fatty acids in peanut oil are randomly rearranged, its atherogenicity was significantly reduced in cholesterol-fed rabbits and monkeys. The mechanism for this effect remains unknown. This study was designed to investigate whether the absorption, transport and distribution of dietary cholesterol and oleic acid in the lymph were altered in the presence of peanut oil or randomized peanut oil. Previous investigators collected lymph through the mesenteric duct for 6 h and analyzed lymph for cholesterol. In the present study, lymph fluids were collected at timed intervals for up to 8 h and then at 24 h via the thoracic duct. Cholesterol and oleic acid (fatty acid) were estimated not only in the whole lymph but also in lymph lipoprotein fractions and in major lipid fractions. A 24-h lymph collection will enhance accuracy as short-term fluctuations in lipid absorption will not affect the results. Thoracic duct lymph collection is quantitative compared to mesenteric duct lymph collection, which provides only a fraction of the total lymph. Rats were given a lipid emulsion containing either peanut oil or randomized peanut oil. The emulsion also contained cholesterol, oleic acid, and sodium taurocholate in saline and was given through a duodenal catheter. Results show that absorption, transport, and distribution of cholesterol and oleic acid in the lymph fluids were similar in both dietary groups. These results suggest that the atherogenicity of peanut oil may be due to other events taking place subsequent to the release of cholesterol-containing chylomicrons and very low density lipoprotein by the small intestinal epithelial cells into the blood or may be due to the triglyceride structure itself.     

正渗透过程中水与溶质的传递现象

正渗透是一种利用渗透原理的新兴膜技术,近年来在国内外受到了广泛的关注。解析该过程中溶剂水的传递和驱动溶质的反向传递对其发展和应用至为关键。首先开展了两种膜的取向下,正渗透过程中的水通量和溶质反向摩尔通量的实验研究。当驱动溶液在膜分离层侧时,水通量更高,而溶质反向摩尔通量更低,表明水的传递对溶质的反向传递有限制作用。而后分别考察了不同的单一溶质和二元混合溶质作为驱动溶质时,水和溶质的传递现象。当单一中性溶质或电解质作为驱动溶质时,水通量和溶质反向摩尔通量均随驱动溶液浓度的升高而增大;在相同操作条件下,驱动溶质的扩散系数越小,溶质反向摩尔通量越小;中性溶质与电解质混合溶液为驱动溶液时,溶质分子之间存在耦合传递效应。     

Ursolic acid and other pentacyclic triterpenoids stimulate intestinal alkaline sphingomyelinase in vitro

Purpose: Alkaline sphingomyelinase (alk‐SMase) is an enzyme that hydrolyses sphingomyelin in a bile salt‐dependent manner in the gastrointestinal tract, and has been proposed as an inhibitor of colon carcinogenesis. Ursolic acid (UA) is a plant‐derived pentacyclic triterpenoid that has been shown to have anti‐proliferative and apoptotic effects on HT29 human colon adenocarcinoma cells, with activation of alk‐SMase as an early event. The aim of this study was to study the in vitro effects of UA and its analogues on the activity of purified rat intestinal alk‐SMase. Methods: Rat intestinal alk‐SMase activity was determined after incubation with UA in the presence and absence of taurocholate (TC). The effect was compared with boswellic acids, another group of pentacyclic triterpenoids. Results: UA enhanced the activity of rat intestinal alk‐SMase in a dose‐dependent manner, without a similar effect on bacterial neutral SMase. Four types of boswellic acid also increased the enzyme activity, with the effect of acetyl‐keto‐β‐boswellic acid being most potent. Activation of alk‐SMase by TC at a low concentration (0.4 mM), but not at a high concentration, was enhanced by UA. Conclusions: Ursolic acid and four types of boswellic acid, all pentacyclic triterpenoids, have a stimulatory effect on the activity of intestinal alk‐SMase.     

Effect of plant sterols,fatty acids and lecithin on cholesterol absorption in vivo in the rat

The inhibitory effect of plant sterols, fatty acids and lecithin on cholesterol intestnal absorption was studied in the unanesthetized rat using a single pass perfusion technique. Bile was excluded from the perfused intestine. Cholesterol absorption did not change following the additions of cholestanol, cholestanone, lanosterol, stigmasterol and β-sitosterol. A 3-fold increase in the molarity of cholestanol and β-sitosterol or the separate additions of the saturated short and medium chain fatty acids, butyric and octanoic, also did not change cholesterol absorption. The unsaturated long chain fatty acids, oleic, linoleic, linolenic and arachidonic, inhibited cholesterol absorption. Lecithin additions at concentrations of 0.1–1.5 mM caused a progressive, dose-related inhibition of cholesterol absorption. The inhibitory effect of these agents on cholesterol absorption is likely to have been caused by changes in cholesterol solubility in the micelle and shifts in the partition coefficient of cholesterol away from the cell membrane to the micelle.     

Effects of rice bran oil on the intestinal microbiota and metabolism of isoflavones in adult mice

This study examined the effects of rice bran oil (RBO) on mouse intestinal microbiota and urinary isoflavonoids. Dietary RBO affects intestinal cholesterol absorption. Intestinal microbiota seem to play an important role in isoflavone metabolism. We hypothesized that dietary RBO changes the metabolism of isoflavonoids and intestinal microbiota in mice. Male mice were randomly divided into two groups: those fed a 0.05% daidzein with 10% RBO diet (RO group) and those fed a 0.05% daidzein with 10% lard control diet (LO group) for 30 days. Urinary amounts of daidzein and dihydrodaidzein were significantly lower in the RO group than in the LO group. The ratio of equol/daidzein was significantly higher in the RO group (p < 0.01) than in the LO group. The amount of fecal bile acids was significantly greater in the RO group than in the LO group. The composition of cecal microbiota differed between the RO and LO groups. The occupation ratios of Lactobacillales were significantly higher in the RO group (p < 0.05). Significant positive correlation (r = 0.591) was observed between the occupation ratios of Lactobacillales and fecal bile acid content of two dietary groups. This study suggests that dietary rice bran oil has the potential to affect the metabolism of daidzein by altering the metabolic activity of intestinal microbiota.