Polyethylene glycol 400 penetration of the colonic epithelial barrier of the rat.

Permeability changes of polyethylene glycol 400 have been seen in patients with inflammatory bowel diseases. Because the colon can be involved in inflammatory bowel disease, the mechanisms, kinetics, and influence of intraluminal factors on polyethylene glycol 400 permeation of perfused colonic segments of rats were studied. The absorption rate of polyethylene glycol 400 was linearly related to its luminal concentration (r = 0.94), suggesting that passive diffusion is a significant mechanism involved in polyethylene glycol 400 absorption. Changing the perfusate pH from 6.0 to 7.5 did not affect water absorption or polyethylene glycol 400 permeation. Increasing luminal osmolarity significantly decreased water and polyethylene glycol 400 absorption (P less than 0.01). The relationship between polyethylene glycol 400 and water absorption at different luminal osmolarities was linear (r = 0.97). At luminal osmolarity of 0.3 osm/L, 14.3% of polyethylene glycol 400 absorption was mediated by passive diffusion and 85.7% was mediated by convection. The solvent drag reflection coefficient for polyethylene glycol 400 in the colon was 0.03. Taurocholic acid (10 mmol/L) and chenodeoxycholic acid (5 mmol/L) decreased polyethylene glycol 400 and water absorption (P less than 0.01). Addition of 1 micrograms/mL of 16,16-dimethyl prostaglandin E2, 2 mmol/L of dibutyryladenosine-3',5'-cyclic monophosphate, or 10 mmol/L of aminophylline significantly decreased water and polyethylene glycol 400 absorption (P less than 0.01). These studies demonstrate that polyethylene glycol 400 permeation of the colon is mediated by both passive diffusion and solvent drag. Convective absorption is the major mechanism of polyethylene glycol 400 permeation of the colon. Polyethylene glycol 400 permeation is modified by bile acids, prostaglandins, and cyclic nucleotides through changes in water flux.

Regulation of polyethylene glycol 400 intestinal permeability by endogenous and exogenous prostanoids. Influence of non-steroidal anti-inflammatory drugs.

Polyethylene glycol 400 (PEG 400) is a clinically useful intestinal permeability probe whose rate of intestinal permeation is influenced in part by solvent drag. As mucosal prostanoids are increased in inflammatory bowel disease and affect water transport we examined the possible relationship between prostaglandin E2 (PGE2) and the inhibitors of endogenous prostaglandins--the non-steroidal anti-inflammatory drugs (NSAIDS)--on PEG 400 absorption in vivo using segmental perfusion of rat small intestine. We found that the addition of exogenous PGE2 in concentrations of 0.5, 1.0, and 1.5 micrograms/ml significantly (p less than 0.01) decreased PEG 400 and water absorption. Addition of 5 mmol/l of the cyclooxygenase inhibitors acetylsalicylic acid (ASA) or indomethacin in concentrations 2.5 or 5.0 mmol/l to the perfusate significantly (p less than 0.01) increased PEG 400 and water absorption. The simultaneous addition of 1.0 micrograms/ml of exogenous PGE2 to the perfusate with 5 mmol/l of ASA or with 2.5 mmol/l of indomethacin reversed the increase of PEG 400 and water transport (p less than 0.01). There were no differences in PEG 400 and water absorption when PGE2 was given alone or in combination with ASA or indomethacin. This study suggests that endogenous or exogenous prostanoids play an important role in the regulation of PEG 400 permeation. PGE2 and NSAIDS modify PEG 400 permeation in parallel with changes in water transport indicating that their effect on permeability is through changes in solvent drag. These findings provide a mechanism which might explain the increase in PEG 400 intestinal permeability in Crohn's disease patients and the increase in intestinal permeability found in patients receiving NSAIDS.

Mechanisms of polyethylene glycol 400 permeability of perfused rat intestine.

Abnormal permeability to polyethylene glycol 400 (PEG 400) has been demonstrated in various disorders with defective intestinal barrier functions. To understand the basic mechanisms of PEG 400 permeability, we compared PEG 400 permeation in different segments of the intestine and studied the kinetics and influence of intraluminal factors on PEG 400 absorption in vivo in perfused intestinal segments of the rat. The permeation rate of PEG 400 was dependent on the luminal concentration (y = 12.99x + 3.5; r = 0.97), indicating that passive movement is the mechanism involved in PEG 400 absorption. Changing the perfusate pH from 6 to 7.4 or modifying the unstirred water layer resistance by changing luminal flow rate did not affect PEG 400 absorption. When luminal osmolarity was varied from 0.225 to 0.6 osmol/L, higher osmolarity decreased both water and PEG 400 absorption (p greater than 0.01). The relationship between PEG 400 and water absorption at different osmolarities was linear (y = 0.9x + 5.7; r = 0.98). At a luminal osmolarity of 0.3 osmol/L 43% of PEG 400 permeation was mediated by passive diffusion and 57% was mediated by solvent drag. Increasing water absorption by decreasing luminal osmolarity resulted in proportional increase of PEG 400 permeation through solvent drag or convection. The solvent drag reflection coefficient (sigma f) for PEG 400 permeation of the jejunum was 0.1. Taurocholic acid (10 mM) alone or with oleic acid (2.5 mM) did not affect PEG 400 absorption. Permeabilities of 1 mM PEG 400 and water were similar in jejunum and ileum but were markedly increased in the colon (p greater than 0.01). These studies demonstrate that PEG 400 is absorbed by both passive diffusion and by solvent drag, with the latter accounting for a greater fraction of the absorptive drive under normal conditions. Polyethylene glycol 400 uses aqueous pathways for its permeation across the intestinal epithelium.

Intestinal permeability in patients with Crohn's disease and their healthy relatives.

The healthy relatives of patients with Crohn's disease were previously found to have increased intestinal permeability to polyethylene glycol 400. To determine whether the abnormal permeability is uniquely detectable by polyethylene glycol 400, we studied the intestinal permeability of three new probes (lactulose, rhamnose, and mannitol) in 25 patients with Crohn's disease, 41 of their healthy relatives, and 29 normal controls without a family history of inflammatory bowel disease. Patients with Crohn's disease had increased lactulose permeability when compared with relatives or controls. Lactulose absorption by patients with Crohn's disease was 0.41% +/- 0.07% (mean +/- SE), whereas that of their relatives and unrelated controls was 0.28% +/- 0.03% and 0.26% +/- 0.03%, respectively. There was no significant difference between the relatives and controls, but both groups differed from the patients (p less than 0.05 and p less than 0.025, respectively). The patients' lactulose/rhamnose ratio was 70.5% +/- 9.2% vs. 37.2% +/- 3.3% in relatives and 40.6% +/- 5.7% in unrelated controls (p less than 0.0005 and p less than 0.0025, respectively). The two intermediate-sized probes, rhamnose and mannitol, did not detect permeability differences among the three groups. The inability of lactulose, rhamnose, or mannitol to detect permeability abnormalities in healthy relatives of patients with Crohn's disease suggests that these probes penetrate the intestinal barrier by routes or mechanisms that are different from those of polyethylene glycol 400. Lactulose, in particular, detects permeability changes in patients with intestinal inflammation, and polyethylene glycol 400 is able to detect permeability changes in the health relatives of our patients. These data indicate that permeability may be abnormal as a secondary result of inflammation, or as a result of a primary genetic abnormality.

Intestinal absorption of free oleic acid in the unanesthetized rat: evidence for a saturable component?

The intestinal absorption of free oleic acid at low intraluminal concentrations and the influence of luminal factors on its absorption were studied in the unanesthetized rat. The relationship between oleic acid concentration (30-2500 microM) and its rate of absorptions fitted best to a rectangular hyperbola (y = x/(2.19 + 0.0015x), r = 0.94). Oleic acid's rate of absorption increased as the hydrogen ion and sodium taurocholate concentrations were increased or as the thickness and resistance of the unstirred water layer were diminished or following the addition of lysolecithin. The additions of the artificial detergent Tween-80, or lecithin and linoleic, linolenic and arachidonic acids to the perfusate decreased oleic acid's rate of absorption. It was concluded that oleic acid absorption in this range of concentrations displays apparent saturation kinetics which are due to unstirred layer effects, limited aqueous solubility of oleic acid and possible saturation of cytosol fatty acid binding proteins. Factors which increase oleic acid's protonated concentration or diminish the unstirred layer resistance, enhance its absorption rate, while factors which enhance its micellar solubility or interfere with its transfer out of the cell membrane decrease its overall rate of absorption.

Does essential fatty acid absorption change with aging?

Linoleic acid, an essential fatty acid, is a prostaglandin precursor. We investigated the maximal capacity of the proximal jejunum and distal ileum to absorb linoleic acid in the unanesthetized rat. Groups of rats 1, 3, 12, and 28 months of age were studied. As the rats aged, their maximal capacity to absorb linoleic acid increased fivefold both in the jejunum and ileum. Since the intestinal wall content of linoleic acid remained relatively constant, age-related changes in mucosal surface area could not account for our observations. A decrease in the unstirred water layer thickness with aging was detected by measuring potential difference changes across the bowel. The total surface area of the unstirred water layer increased some fourfold and its resistance to linoleic acid transfer decreased fivefold with aging. These changes in the dimensions and characteristics of the unstirred water layer with aging may account for the fivefold increase in the maximal capacity of the small bowel to absorb linoleic acid.

Aging-associated pancreatic exocrine insufficiency in the unanesthetized rat.

Observations in individuals have suggested that pancreatic functions may be diminished with aging. In order to explore this possibility, we measured pancreatic fluid, protein and amylase output as a function of aging in the rat. Surgical preparations of the rats included external drainage of pancreatic secretions without obstructing normal bile flow and complete diversion of gastric acid out of the duodenum to prevent endogenous secretin release. Pancreatic secretions were collected in the basal state in unanesthetized animals 24 h after surgery. Protein and amylase output were highest at 3 months of age and decreased to less than 20% of maximal values by 27 months of age. For example, amylase output decreased from 4,354 to 105 U/h/kg and protein output fell from 33.5 to 1.7 mg/h/kg between 3 and 27 months of age. Our findings indicate that aging in the rat is associated with diminished pancreatic exocrine functions. Further careful assessment of pancreatic functions in aging individuals is advisable.

Aging: its influence on the intestinal unstirred water layer thickness, surface area, and resistance in the unanesthetized rat.

The intestinal absorption of some nutrients changes with aging. As the unstirred water layer (UWL) is an important rate limiting step in the absorption of nutrients in general and of lipid soluble nutrients in particular, we investigated possible changes in the UWL dimensions in the aging rat in vivo. We measured the thickness (d) of the UWL using rapid changes in the luminal sodium concentration to induce changes in the transmucosal potential differences. We assessed the surface area (Sw) and resistance (d/SwD) of the UWL at varying flow rates by using increasingly lipophilic medium chain saturated alcohols as probes. At high UWL resistance, d decreased from 318 to 268 microns between 1 and 29 months of age. As the animals aged, Sw changed from 114 to 106 cm2/100 cm and from 262 to 214 cm2/100 cm at low and high flow rates, respectively, using dodecanol as a probe. The resistance of the UWL (d/SwD) remained relatively stable at all ages studied. These experiments demonstrate that age-related changes in absorption are dependent on the aqueous diffusion coefficient and degree of lipid solubility of the specific nutrients. At low UWL resistance, absorption of compounds with higher diffusion coefficients and greater aqueous solubility is decreased with aging. In contrast, previous studies have demonstrated that the absorption of nutrients with low diffusion coefficients and high lipid solubility increases with aging especially when the resistance of the UWL is high.

Intestinal exsorption of oleic acid: influence of aging, bile, pH and ethanol.

The transfer of oleic acid from the circulation into the intestinal lumen perfusate (exsorption) was investigated in vivo in male Sprague-Dawley rats. Following intravenous infusion of oleic acid, its appearance in the intestinal perfusate was assessed over a 3-hour period. The relationship between exsorption rate and the amount of infused oleic acid delineated a linear plot. Oleic acid exsorption rate increased as the taurocholate concentration was raised to 10 mM in the intestinal perfusate. In contrast, oleic acid exsorption rate decreased following decrease in the intestinal perfusate pH, or increase in the intestinal perfusate's concentrations of oleic, butyric and octanoic acids or following the additions of increasing concentrations of ethanol to the intestinal perfusate. Increase in the age of the animals from 3 to 21 months resulted in a decrease of oleic acid exsorption. These data indicate that exsorption of oleic acid into the intestinal perfusate is not simply a process of "leakiness" of the intestinal epithelium. Rather, exsorption of this fatty acid is modified by factors that change the characteristics of the perfusate or modify the characteristics of the intestinal epithelium or the unstirred water layer at the luminal surface of the small intestine.

Increased intestinal absorption of oleic acid with aging in the rat.

Changes in nutrient absorption could be responsible for some of the disorders associated with aging. Oleic acid is the most common dietary fatty acid. Therefore, we investigated its absorption by the small intestine of aging rats in vivo. We used a single pass intestinal perfusion technique to study absorption in animals between 6 and 138 weeks of age. Rats less than 70 weeks of age absorbed 14,000 to 15,000 nmol of oleic acid per 100 cm/hr. In contrast, rats 94 weeks of age or older, absorbed 22,000 to 23,000 nmol of oleic acid per 100cm/hr. The 61% increase in oleic acid absorption as the rats aged correlated with a decrease in the resistance of the unstirred water layer from 0.8 to 0.5 min/cm3/100cm. Concomitantly, the surface area of the unstirred water layer increased from 97.1 to 141.2 cm2/100cm. The changes in unstirred water layer resistance and surface area explain the observed increase in absorption of oleic acid with aging. The increased surface area and decreased resistance of the unstirred water layer are both conducive to increased intestinal absorption of this lipid nutrient as the animals aged.

Intestinal absorption of aspirin. Influence of pH, taurocholate, ascorbate, and ethanol.

The small intestinal absorption of aspirin at pharmacological concentrations was studied in the unanesthetized rat by using a single-pass perfusion technique. The rate of aspirin absorption remained linear with its concentration (0.5 to 10 mM). Intestinal aspirin absorption increased as the concentration of hydrogen ion, sodium taurocholate, and ascorbic acid in the perfusate increased. Aspirin absorption did not change after ethanol addition. At pH 3.5 or 6.5, intestinal absorption of aspirin was greater than gastric absorption of the compound. Aspirin was not absorbed by the stomach at pH 6.5. These experiments indicate that aspirin can be absorbed to an appreciable extent in its ionized form by the small intestine but not by the stomach.