Intestinal absorption and lymphatic transport of fish oil (MaxEPA) in the rat

Adult male, chow-fed Sprague-Dawley rats were given intraduodenally a bolus of emulsion of 0.5 ml of fish oil (MaxEPA) or olive oil plus 0.5 ml of 20 mM sodium taurocholate. Intestinal lymph was collected from a cannula in the main intestinal lymph trunk for various times after oil administration. There was no difference in the absorption of either type of oil over 6 and 24 h, over which times about 40 and 70% of the administered dose was taken up. For MaxEPA, the flux of triacylglycerols remained at a basal level of 0.07 mumol/min for 30 min, after which it rose rapidly to a maximum of 0.87 mumol/min between 90 and 120 min. The flux was 0.4 mumol/min for the subsequent 4 h. After 30 min, the composition of the lymph triacylglycerols began to change to show the presence of large proportions of fatty acyl chains that were characteristic of fish oil, especially eicosapentaenoate (20:5(n-3] and docosahexaenoate (22:6(n-3]. The composition of the lymph remained fairly similar to that of the fish oil for up to 6 h, the last time point at which detailed analysis was done. The docosahexaenoate in the triacylglycerols of the fish oil was primarily in the sn-2 position of glycerol, whereas a more random distribution of eicosapentaenoate over all glycerol positions was found. The positional distribution of the acids in the lymph triacylglycerols was similar to that in the fish oil. There was no evidence of substantial chain elongation or shortening during absorption. The results indicate that fish oil is effectively absorbed from the rat intestine without substantial alteration in the acyl chains of the triacylglycerols.     

Intestinal absorption and lymphatic transport of cholesterol in the rat: influence of the fatty acid chain length of the carrier triglyceride

This paper deals with the effect of the fatty acid chain length of dietary triglyceride on the intestinal uptake and lymphatic transport of exogenous and endogenous cholesterol in the rat. This question seemed of interest as the chain length of the monoglyceride and fatty acids formed in the intestinal lumen from the triglyceride fed could be expected to affect the concentration of cholesterol in the micellar or isotropic phase of intestinal content. Feeding rats medium- or short-chain triglycerides (C(12) to C(2)) did not affect the lymphatic transport of endogenous cholesterol from the intestine compared to the fasting state. The extent of lymphatic transport of cholesterol added to these fats increased proportionally with chain length (C(6)-C(18)) of the component fatty acids. The uptake of exogenous cholesterol into the intestinal wall was similarly related to the chain length of the carrier triglyceride, with the exception of triacetin, which gave a much higher intestinal uptake than lymphatic transport. When cholesterol was fed in octadecane, negligible amounts only were transported to the thoracic duct lymph. This again indicates the importance of the polar split products of dietary fat for cholesterol absorption.     

Absorption and lymphatic transport of cholesterol in the rat

Rats with thoracic duct fistulae were fed triolein and triolein containing various amounts of labeled cholesterol. The analysis of the lymph lipids gave the following results. In the fasting state the cholesterol transported via the thoracic duct was 0.87 micromole/hr. Feeding 800 micromoles of triolein gave a maximum rate of transport of cholesterol of 1.65 micromoles/hr. Addition of cholesterol to the triolein further increased the cholesterol transport to a maximal rate of almost 5 micromoles/hr when 50 micromoles of cholesterol were fed per 800 micromoles of triolein. The exogenous fraction of the cholesterol transported increased linearly with increasing cholesterol load, constituting at the highest dose almost 90% of the total cholesterol transported. An almost constant fraction (about 0.4) of the dietary cholesterol was recovered in the thoracic duct lymph in 24 hr irrespective of the dose fed, from a trace up to 100 micromoles in 800 micromoles of triolein. Cholesterol absorption has the characteristics of a passive diffusion process.     

Intestinal cholesterol absorption.

The strong association between intestinal cholesterol absorption and total plasma cholesterol level has renewed interest in the absorptive process and stimulated the generation of new animal models. Increasingly, new studies suggest that cholesterol absorption is genetically controlled and supports a protein-mediated mechanism for cholesterol uptake into the intestinal mucosal cell. Insights into potential mechanisms are predicted to lead to novel pharmacological approaches to inhibit cholesterol absorption.     

Intestinal absorption of biliary and exogenous cholesterol in the rat

Non-starved rats (fed a cholesterol-free diet prior to the experiments) with common bile fistula were infused intraduodenally with rat bile labelled with [1,2-³H]cholesterol at a constant rate (0.6 ml/h) and a nutritive mixture containing, in particular, olive oil and 1 μmol [4-¹⁴C]cholesterol per ml at rates of 1 ml/h (group B) or 2.3 ml/h (group A) for 5 h. Control rats (group C) were prepared as group B rats but the nutritive mixture was free of cholesterol. 1 h after the end of infusions, the animals were killed. Biliary and exogenous cholesterol were absorbed in the upper two-thirds of the small intestine; a large proportion of ³H and ¹⁴C radioactivity was present in the mucosa, but cholesterol from exogenous origin went across the mucosa more rapidly than cholesterol from biliary source. These observations suggest the existence of a non-homogeneous luminal mixture of molecules of cholesterol from different sources. The luminal dilution of [³H]- and [¹⁴C]sterols by non-labelled sterols increased from the proximal to the distal part of the small intestine. Precursor sterols and coprosterol were present in the stomach contents and in the lumen of caecum, colon and feces.     

Absorption and lymphatic transport of cholesterol and sitosterol in the rat

An attempt was made to determine the mechanism for the greater absorbability of cholesterol as compared to sitosterol. Sitosterol-22,23-(3)H in different combinations with cholesterol-4-(14)C, dissolved in 0.8 ml of triolein, was fed to rats with lymph fistulae. Feeding 1.5, 50, or 100 micro moles of sitosterol resulted in a transfer to the lymph in 24 hr of 3-6% of the sitosterol, largely independent of the dose fed. The total amount of sitosterol transferred to the lymph was therefore almost linearly related to the dose fed. 30% of a tracer dose of cholesterol-4-(14)C fed together with the sitosterol was transferred to the lymph in 24 hr. When a total of 50 micro moles of sterol, containing cholesterol-(14)C and sitosterol-(3)H in the proportions 1:3, 1:1, and 3:1, was similarly fed, we found that sitosterol had no significant effect on the lymphatic transport of the simultaneously fed cholesterol. The ratio of (3)H to (14)C in the lymph was between 0.1 and 0.2 (the ratio in each fed mixture being taken as 1.0). The ratio was constant during the absorption period and independent of the ratio of sterols in the fed sterol mixture. Thus the same percentage of each sterol was always absorbed, and the sterols exerted no mutual interference in each others' absorption. We conclude that the mechanism for specificity in sterol absorption must be located early in the transport of the sterols within the intestinal mucosa cell.     

Intestinal lymph flow and lymphatic transport of protein during fat absorption

A substantial increase in intestinal lymph flow and protein content follows fat ingestion. The effect of intraduodenal feeding of fats was studied in the rat to define the mechanisms responsible. The change appears to be largely independent of the route of fat absorption, that is, whether by the portal venous route or, alternatively, by the lymphatic route. It must be presumed that it is related to events unconnected with the route taken by absorbed fat leaving the intestinal cell.     

Intestinal cholesterol absorption in the chyluria model

Isotopic methods for the measurement of dietary cholesterol absorption were compared with the lymph cholesterol balance procedure in filarial chyluria patients. After a single intravenous injection of radioactive cholesterol, absorption was found to be 746 +/- 136 mg/day by method I, which is based upon the fecal endogenous neutral steroid mass measurement, and 471 +/- 135 mg/day by the simultaneously measured lymph/plasma ratio of cholesterol specific activity (dpm/mg). The corresponding value, determined as the difference between lymph cholesterol transport on a cholesterol-containing diet (1500 mg) and on a cholesterol-free diet, was 622 mg/day. When radioactive cholesterol (1487 mg/day) was fed daily to a second patient, absorption determined by isotopic fecal recovery (353 mg/day) matched that obtained by the lymph balance procedure (326 mg/day). Transudation of plasma cholesterol into the intestinal lymph, estimated by the single intravenous injection of radioactive beta-sitosterol, was independent of both the luminal content of plant sterols and the absorption of dietary cholesterol. The absorption of endogenous cholesterol was calculated by: 1) subtracting the cholesterol originating from plasma (transudation) together with the absorbed dietary cholesterol found in lymph from the total mass of cholesterol transported in lymph, and 2) the lymph balance method, i.e., after interrupting the endogenous cholesterol mucosal uptake by beta-sitosterol feeding (9 g/day) while on a cholesterol-free diet. Endogenous cholesterol was preferentially absorbed compared to dietary cholesterol, but there was no competition for absorption. The major portion of dietary cholesterol found in lymph was esterified, but esterification was not a prerequisite for absorption.     

Development and physiological regulation of intestinal lipid absorption. III. Intestinal transporters and cholesterol absorption

Intestinal cholesterol absorption is modulated by transport proteins in enterocytes. Cholesterol uptake from intestinal lumen requires several proteins on apical brush-border membranes, including Niemann-Pick C1-like 1 (NPC1L1), scavenger receptor B-I, and CD36, whereas two ATP-binding cassette half transporters, ABCG5 and ABCG8, on apical membranes work together for cholesterol efflux back to the intestinal lumen to limit cholesterol absorption. NPC1L1 is essential for cholesterol absorption, but its function as a cell surface transporter or an intracellular cholesterol transport protein needs clarification. Another ATP transporter, ABCA1, is present in the basolateral membrane to mediate HDL secretion from enterocytes.     

Intestinal cholesterol absorption: identification of different binding proteins for cholesterol and cholesterol absorption inhibitors in the enterocyte brush border membrane

Absorption of cholesterol from the intestine is a central part of body cholesterol homeostasis. The molecular mechanisms of intestinal cholesterol absorption and the proteins mediating membrane transport are not known. We therefore aimed to identify the proteins involved in intestinal cholesterol absorption across the luminal brush border membrane of small intestinal enterocytes. By photoaffinity labeling using photoreactive derivatives of cholesterol and 2-azetidinone cholesterol absorption inhibitors, an 80-kDa and a 145-kDa integral membrane protein were identified as specific binding proteins for cholesterol and cholesterol absorption inhibitors, respectively, in the brush border membrane of small intestinal enterocytes. The 80-kDa cholesterol-binding protein did not interact with cholesterol absorption inhibitors and vice versa; cholesterol or plant sterols did not interfere with the 145-kDa molecular target for cholesterol absorption inhibitors. Both proteins showed an identical tissue distribution and were exclusively found at the anatomical sites of cholesterol absorption-duodenum, jejunum and ileum. Neither stomach, cecum, colon, rectum, kidney, liver nor fat tissue expressed the 80- or 145-kDa binding proteins for cholesterol and cholesterol absorption inhibitors. Both proteins are different from the hitherto described candidate proteins for the intestinal cholesterol transporter,-SR-BI, ABC G5/ABC G8 or ABC A1. Our data strongly suggest that intestinal cholesterol absorption is not facilitated by a single transporter protein but occurs by a complex machinery. Two specific binding proteins for cholesterol (80 kDa) and cholesterol absorption inhibitors (145 kDa) of the enterocyte brush border membrane are probable protein constituents of the mechanism responsible for the intestinal absorption of cholesterol.