Excitotoxins and amyotrophic lateral sclerosis

Recent data suggest that amiotrophic lateral sclerosis (ALS) could be the result of motoneuron damage induced by endogenous or exogenous excitotoxins, and especially by excitatory amino acids (EAA). Three main sources support this hypothesis: 1) The induction of experimental models of motor neuron disease by 2 excitotoxins (BOAA and BMAA). 2) Evidence of disordered glutamate metabolism in ALS. 3) Data suggesting that EAAs may be a factor in the pathogenesis of other degenerative neurologic diseases (Huntington disease and Alzheimer disease). This new "excitotoxin hypothesis" of ALS is of particular interest as several effective antiglutamate agents are now available for human therapeutic trials.

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.

Vascular and microvascular changes--key factors in the development of acetic acid-induced gastric ulcers in rats

The present study examined the time sequence and histologic and ultrastructural features of the formation and evolution of experimental, acetic acid-induced gastric ulcerations in rats. One hundred percent acetic acid was applied to the gastric serosa of 140 fasted male Sprague-Dawley rats through a polyethylene tube for 30 s. Gastric mucosal changes were evaluated at 1, 5, 15, and 30 min, 1 and 3 h, and 1, 2, 3, 5, 8, and 11 days after acetic acid application by visual inspection, by quantitative and qualitative light microscopy, and by transmission electron microscopy. Following exposure to acetic acid, the earliest morphologic changes occurred at 1 min and consisted of dilatation of large submucosal veins and arteries and mucosal collecting venules. Five to 15 minutes after injury, thrombi developed in submucosal veins and collecting venules, leading to microvascular stasis and mucosal necrosis. By 3 h, necrotic masses started to detach. By 24-48 h, necrotic changes penetrated the submucosa. By 72 h, most ulcers underwent transition into a "chronic" stage characterized histologically by the presence of granulation tissue at the bottom, and the appearance of a transitional healing zone at the margins. By 5 days, an increased amount of granulation tissue was observed and the gastric glands in transitional zones at the ulcer margin displayed cystic dilatation. Based on this study, we conclude that a key feature of acetic acid-induced ulcer formation is the early vascular and microvascular injury, which precedes glandular cell necrosis.

Influence of aging on vitamin A transport into the lymphatic circulation

Vitamin A is a lipid soluble essential dietary micronutrient. Because aging individuals have elevated serum levels of vitamin A, we tested the intestinal absorptive capacity of the vitamin in aging rats by measuring the appearance rate of vitamin A in the lymphatic circulation as a measure of intestinal absorption. We infused the vitamin in a physiological concentration of 350 nM into the proximal jejunum of groups of Sprague-Dawley rats 2 to 23 months of age. Lymph was collected for 6 h and vitamin A radioactivity was measured. Lymph flow rate over 24 h ranged from 16 +/- 3.1 ml in the oldest to 21.5 +/- 2.4 ml in the youngest animals. Vitamin A transport into lymph was highest at 23 months of age, reaching 7869 +/- 154 pmol/6 h as compared to 6732 +/- 106 pmol/h at 2 months (p less than .01). The increase in vitamin A absorption with aging represented an increase from 35.6% of the infused vitamin at 2 months to 41.6% at 23 months of age. Because of the cumulative storage of vitamin A in the liver this increase in absorption and lymphatic appearance is of nutritional and metabolic significance. Increased lymphatic appearance of the vitamin could explain its higher absorption and serum levels in aging individuals.

PEG 400, a hydrophilic molecular probe for measuring intestinal permeability

There is a widely held misconception that low-molecular-weight polyethylene glycols are "highly lipophilic" permeability probes and therefore are transported across lipid cell membranes. The relative lipophilicity of polyethylene glycols 400 and 600 were examined by determining their partition coefficients (Kd) in water and organic solvents of increasing relative polarity. The Kd of polyethylene glycol 414 between hexane and water was 0.000015, indicating that there are only 1.5 parts of polyethylene glycol 414 in hexane for 100,000 parts of polyethylene glycol 414 in water. When the Kd was determined in organic solvents with increasing relative polarity or "water character", there was a linear increase in Kd. The relative urinary recovery of individual molecular weight fractions of polyethylene glycol 400 in normal volunteers was analyzed. After oral ingestion, there was a progressive decrease in relative urinary recovery of increasing molecular weight fractions of polyethylene glycol 400 suggesting that increase in the molecular size limited polyethylene glycol intestinal permeability. There was excellent correlation between the relative urinary recovery and the hydrophilicity of the intravenously administered polyethylene glycol 400 fractions. It is concluded that polyethylene glycols 400 and 600 are strongly hydrophilic. Since partitioning of polyethylene glycol into lipid phase is negligible in lipid/water mixtures, they are unlikely to be transported via lipid pathways. The intestinal permeability of polyethylene glycols are governed by their molecular size, and once in circulation their urinary excretion appears to be governed in part by their plasma or water solubility.

"Healed" experimental gastric ulcers remain histologically and ultrastructurally abnormal

The present study was designed to assess histologic and ultrastructural features of gastric mucosa in the areas of grossly healed ulcers (acetic acid-induced gastric ulcers) in rats. The specific question we studied was whether the structure and cellular composition of the gastric mucosa in an area of grossly healed ulcer were fully restored. Eighty Sprague-Dawley rats underwent laparotomy; 100% acetic acid was applied to the lower gastric corpus serosa for 30 s and the abdomen was closed. The stomachs were reopened after 2 weeks or after 2, 3, or 4 months. Standardized gastric wall specimens from the area of grossly healed ulcers were obtained, processed, and evaluated by light microscopy and by transmission electron microscopy. The gastric mucosa of grossly healed ulcers demonstrated re-epithelialization at each study time but the mucosa beneath the surface epithelium displayed prominent histologic and ultrastructural abnormalities. Two different patterns of scar could be distinguished: (a) the mucosa in the area of healed ulcer was thinner (25-45% reduction vs. normal), with increased connective tissue and poor differentiation and/or degenerative changes in the glandular cells; or (b) the mucosa displayed ballooning dilatation of gastric glands, reduction in the microvascular network, and poor differentiation of glandular cells. We conclude that (i) the subepithelial mucosa of grossly healed gastric ulcer displays disorganized restoration of glandular and vascular structures and remains histologically and ultrastructurally abnormal; (ii) these abnormalities may interfere with oxygenation, nutrient supply, and with mucosal resistance and defense, and therefore could be the basis for ulcer recurrence.

Intestinal absorption of arachidonic acid in experimental azotemia

The effect of renal failure (RF) on intestinal absorption of dietary fatty acids is not known. We studied the intestinal absorption of arachidonic acid (AA) in rats with experimental short-term (2 weeks post-subtotal nephrectomy) and long-term (5-6 weeks post-subtotal nephrectomy) RF. The results were compared with those obtained in sham-operated animals on liberal food intake (NL) and in those pair-fed (PF) with the respective RF groups. In vivo perfusion and in vitro incubation experiments were performed at a wide range of AA concentrations. The rates of AA transport determined both in vivo and in vitro were significantly lower in the short-term RF group than those found in the NL controls and the PF animals who showed comparable values. In contrast animals with long-term RF exhibited an increased rate of AA transport as compared with the respective controls. The observed changes in the transport rates appeared to parallel directional changes in mucosal mass which was reduced in animals with short-term RF and restored in those with long-term RF.

The protective and therapeutic mechanisms of sucralfate

Sucralfate is a nonsystemic agent that is effective in protecting the gastroduodenal mucosa against injury. In addition, sucralfate is effective in the healing of acute duodenal and gastric ulceration, the therapy of esophagitis, and the prevention of ulcer recurrence. The mechanisms responsible for sucralfate's successful protective and therapeutic actions include the adsorption of pepsin and bile acids, the stimulation of bicarbonate and mucus secretion, and stimulation of endogenous synthesis of prostaglandins. When sucralfate is given to experimental animals or humans, it stimulates endogenous synthesis and release of prostaglandin E2 and inhibits thromboxane release. Pretreatment of animals with the cyclooxygenase inhibitor indomethacin results in a marked decrease in the protective effect of sucralfate against alcohol injury. Sucralfate also increases epidermal growth factor binding to ulcerated areas and stimulates macrophage activity. In addition, sucralfate stimulates endogenous sulfhydryl compounds. At the microscopic level sucralfate protects the vascular integrity of the mucosa and the mucosal proliferative zone. It also stimulates epithelial cell restitution and stimulates cell proliferation. The administration of sucralfate before acute injury results in decreased depth and extent of injury and in acceleration of healing. Because of sucralfate's ability to stimulate the protective and reparative mechanisms of the gastric and duodenal mucosa, it is an important nonsystemic agent for the therapy and prevention of peptic ulceration.

Antacids: new perspectives in cytoprotection

There is increasing evidence that aluminum-containing antacids are able to protect the gastric mucosa against various ulcerogenic and necrotizing agents including 0.6 M HCl, 0.2 M NaOH, and absolute alcohol. Since gastric mucosal necrosis produced by alcohol is independent of luminal acid and cannot be reduced by H2-receptor antagonists, the protective action of antacids is accomplished by mechanism(s) other than acid-neutralizing ability. In addition, since acidified antacids can protect the gastric mucosa even better than an antacid with intact neutralizing capacity, it is clear that such action is independent of acid-neutralizing ability and therefore has all the features of cytoprotection. Whereas the cytoprotective action of antacids in experimental conditions is well established, the mechanisms of antacid-induced mucosal protection are not known. The clinical relevance of antacid-induced protection also requires further elucidation. Antacids have advantages over the H2 blockers in protecting the gastric mucosa against alcohol-induced necrosis and in preventing stress-induced ulcers in critically ill patients. Although more work is needed to clarify the mechanisms of cytoprotective action of antacids, the recent experimental findings gave a new life to and new potential clinical applications for antacids.

Psychosis and destitution at Christmas 1985-88

Surveys of the destitute were conducted over four successive Christmas periods in Central London, UK. Functional psychosis with hallucinations and/or delusions, either current (19%) or in the past (22%), were associated with extreme social isolation and imprisonment.

Morphologic changes in gastric mucosa of aging rats

We examined histologic and ultrastructural changes in the gastric mucosa of aging rats. Standardized gastric specimens from Sprague-Dawley rats 3 months of age (young) and 24 months of age (old) were evaluated by qualitative and quantitative histology and transmission electron microscopy. Old rats had the following histologic changes: (1) partial atrophy of the gastric glands and their replacement with hyalinlike connective tissue; (2) cystic dilatation of the gastric glands at the bases with occasional squamous cell metaplasia; and (3) extensive perivascular depositions of PAS-positive material, negative for amyloid. The total mucosal thickness was 484 +/- 100 microns in young rats vs 1122 +/- 240 microns in old rats (P less than 0.01). Electron microscopy demonstrated degenerative changes in parietal and chief cells, hyperplasia of surface and foveolar mucous cells, and prominent accumulation of disorganized collagen fibrils in perivascular connective tissue. This study indicates that the gastric mucosa of aging rats that have not been exposed to damaging agents does show definite histologic and ultrastructural changes.

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.

Characterization of structural xenobiotic modifications in proteins by high sensitivity tandem mass spectrometry. Human hemoglobin treated in vitro with styrene 7,8-oxide

Alkylation of DNA by xenobiotic agents, or their electrophilic metabolites, is believed to be the major initiating process that may result ultimately in carcinogenesis. The study of hemoglobin alkylated in vivo by chemical carcinogens has previously been proposed as an indicator for DNA alkylation. Xenobiotically modified proteins, however, are not readily amenable to conventional methods for amino acid sequencing. Tandem mass spectrometry allows unambiguous structural elucidation of chemically modified proteins. Styrene is a widely used chemical in the plastics industry and its major metabolite, styrene 7,8-oxide, is both mutagenic and carcinogenic in rodents. Human hemoglobin was modified in vitro with styrene 7,8-oxide and digested with trypsin. Tryptic peptides from unmodified hemoglobin were isolated by high performance liquid chromatography, and their molecular weights were determined by liquid secondary ion mass spectrometry. This allowed confirmation of the known sequence of the protein and provided a reference for the identification of modified peptides. High performance tandem mass spectrometry of modified peptides allowed unambiguous assignment of specific residues modified. The externally accessible histidines were found to be the dominant sites for alkylation at high modification levels of the protein.

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 mucosal permeability and rheumatological diseases

Rheumatological disorders frequently have gastrointestinal manifestations and, conversely, intestinal disorders frequently have rheumatological manifestations. The possibility of altered intestinal permeability in arthritic patients may provide the bridge needed to link the two organ systems. The normal intestine absorbs nutrients and excludes the remaining material. If the intestine were less discriminating or 'leaky' then material normally excluded would be able to cross the intestinal mucosa into the lamina propria. An inflammatory response to these antigens, be they dietary, bacterial, or viral in origin, could produce either local or systemic disease. This would depend upon the type of immunological response and the cross-reactivity between the host's antigens and the absorbed antigens. This theory could account for the postulated relationship between intestinal abnormalities and the pathogenesis of some forms of arthritis.

Pathways of gastrointestinal protection and repair: mechanisms of action of sucralfate

Protection, i.e., prevention of major lesions and the mechanisms of repair/healing of major tissue loss in the gastrointestinal mucosa, are multifactorial processes. Conceptually, it is useful to categorize the components and mechanisms of gastroprotection and distinguish between: (1) Preservation of existing cells either by enhanced resistance of cells or by decreased exposure to damaging agents that can be achieved by maintenance of proper blood flow, vascular permeability, motility, mucus and bicarbonate secretion. If these mechanisms fail and tissue necrosis ensues, (2) replacement of lost tissue is achievable by either the original cells (e.g., epithelia), by cell migration (restitution) and proliferation (regeneration), and/or by connective tissue repair (e.g., fibroblasts, collagen) through cell proliferation and production of extracellular matrix. For acute gastroprotection ("cytoprotection"), maintenance of blood flow in the upper mucosa and epithelial restitution are listed as key mechanisms. For the long-term safeguarding of the mucosa, proper mucus and bicarbonate secretion, as well as ability to respond by cell proliferation, are the proposed key mechanisms of mucosal defense. The mechanisms of action of sucralfate are also multifactorial. The acute gastroprotection by sucralfate is a prostaglandin- and sulfhydryl-sensitive process: after early protection of microvasculature and maintenance of blood flow, along with direct or indirect preservation of the proliferative zone, rapid restitution repairs the initial epithelial defect. The mechanisms of accelerated healing by sucralfate of chronic ulcers include enhanced mucus and bicarbonate secretion, increased ability of mucus to maintain pH gradient, stimulated binding of epidermal growth factor and other growth factors, and maintained or enhanced blood flow resulting in increased cell proliferation leading to granulation tissue formation and re-epithelialization. The slight direct antipeptic and bile-acid binding property of sucralfate might also contribute to its ability to accelerate ulcer healing.

Protection of the rat gastric mucosa against aspirin injury by arachidonic acid: a dietary prostaglandin precursor fatty acid

We studied aspirin-induced injury to the gastric mucosa in control rats pretreated with a solubilizer, pluronic F-68 (PL), and in rats pretreated with solubilized arachidonic acid (AA). Fasted male rats were pretreated intragastrically with 1 ml of either pluronic or AA and 1 h later acidified ASA (1 ml suspension of 200 mg kg-1 body weight) was administered intragastrically. Grossly apparent mucosal lesions developed 1 h after aspirin in pluronic-pretreated rats, but were significantly reduced in AA-pretreated rats. Histology, scanning and transmission electron microscopy demonstrated that AA pretreatment did not prevent aspirin-induced initial damage to the surface epithelium but did significantly reduce extent of aspirin-induced deep mucosal necrosis at 1, 4 and 18 h after aspirin. Initial aspirin-induced surface epithelial damage was rapidly restituted by two distinct types of re-epithelialization - vertical and horizontal. While the vertical type of re-epithelialization has been reported previously as the first stage of mucosal repair following injury by various noxious agents such as concentrated ethanol, the horizontal type of re-epithelialization, which is described for the first time in this paper, seems to be specific for the repair of aspirin-induced gastric mucosal injury. These studies suggest that dietary factors such as essential fatty acids may play a role in gastric mucosal protection against aspirin injury.

Lymphatic and portal absorption of vitamin E in aging rats

We studied the intestinal absorption and lymphatic versus portal distribution of vitamin E and its metabolites in vivo in young (4 month), "middle-aged" (14 month), and old (24 month) male Sprague-Dawley rats. Twenty-four hours after its surgical preparation, the rat's jejunum was infused with a physiological micellar solution containing 200 nM alpha-tocopherol. Its transport rate into lymph and bile as well as its accumulation in liver and intestinal tissues was measured. Lymphatic transport of vitamin E increased from 92 to 269 pmol/5 hr and bile appearance of vitamin E and its polar metabolites increased from 230 to 298 pmol/5 hr as rats aged. Tissue accumulation of the vitamin in the small intestine and liver increased significantly with aging (P less than 0.05). Total absorption of the vitamin and its intestinal metabolites increased from 5912 pmol/5 hr in young rats to 16,467 pmol/5 hr in old rats (P less than 0.01). Absorption expressed as a percentage of infused alpha-tocopherol increased from 19.7% at 4 months to 54.9% at 24 months. These studies demonstrate an aging-associated increase in the total amount of vitamin E absorbed and a shift from portal to lymphatic transport. These changes may represent a salutary adaptive increase in the systemic availability of vitamin E with aging.

Polyethylene glycol 900 permeability of rat intestinal and colonic segments in vivo and brush border membrane vesicles in vitro

Increased intestinal absorption of medium-sized aqueous probes has been found in patients with a variety of disorders. We studied the physiologic control mechanisms, intestinal regions, and effects of lumenal factors on the intestinal absorption of polyethylene glycol (PEG) 900 in the rat in vivo and in rabbit brush border membrane vesicles (BBMVs). The kinetics of PEG 900 intestinal absorption were compatible with simple passive diffusion. Because transport across BBMVs was minimal, we concluded that transport of PEG 900 is mostly through the paracellular tight junctions. Absorption was highest in the midcolon (104.3 +/- 9.5 mumol/100 mg protein per hour vs 9.1 +/- 1.2 mumol/100 mg protein per hour in the jejunum). Absorption was decreased by higher lumenal osmolarity (greater than 400 mOsm/L) after the additions of 2.5 to 5.0 mmol/L chenodeoxycholate or 2.5 mmol/L lysolecithin, or at higher lumenal flow rates (greater than 1 ml/minute), higher lumenal pressure (7.5 cm H2O),or higher lumenal pH (8.0). Lipid solubility of PEG 900 was less than 0.00079%. Under all experimental conditions, PEG net absorption followed changes in water transport. When water transport changed from absorption to secretion, PEG absorption decreased. When water absorption increased, PEG 900 absorption increased in parallel. We conclude that PEG 900 is absorbed by passive diffusion that is modulated by solvent drag and is maximal in the midcolon. Transport directly across cell membranes is mimimal, but overall PEG 900 permeability is closely linked to water absorption by solvent drag and takes place primarily through the paracellular junctions. We propose that these features and mechanisms of PEG 900 transport make PEG 900 a suitable probe molecule for studying intestinal permeability changes.

Effect of experimental azotemia on intestinal transport of butyric acid

Earlier studies have revealed an impairment of jejunal absorption of long chain fatty acids in experimental uremia. We investigated the intestinal absorption of butyric acid which is a short chain fatty acid in experimental renal failure (RF). Sprague-Dawley rats were randomized into the RF group which had subtotal nephrectomy, a sham-operated control group, and a pair-fed group. In vivo recirculating perfusion (n = 5) and in vitro everted sac incubation (n = 8) were employed. The in vitro experiments were repeated substituting the serosal buffer by either predialysis or postdialysis sera from uremic individuals, or normal serum (n = 10). The rate of in vivo butyric acid absorption was significantly lower while the in vitro absorption was significantly higher in the RF group than those observed in the sham-operated and pair-fed groups which showed comparable values. The normality of butyric acid absorption in the pair-fed animals despite comparable weight loss with the RF group tends to exclude anorexia and weight loss as a cause of altered butyric acid transport in RF animals. The disparity between the in vivo and in vitro data is suggestive of an inhibitory influence of uremic environment which is present in vivo and absent in vitro. This viewpoint was corroborated by the observed fall in butyric acid absorption by sacs containing predialysis uremic serum as compared with those containing normal or postdialysis sera. The latter further suggests that the inhibitory factor(s) is dialyzable.

Mechanisms of linoleic acid uptake by rabbit small intestinal brush border membrane vesicles

We examined the initial transport of a long-chain unsaturated fatty acid, linoleic acid, by brush border membrane vesicles isolated from rabbit small intestine. This preparation allowed us to examine the transport of linoleic acid across the brush border membrane without the effect of the unstirred water layer or cytosol binding proteins. Linoleic acid was solubilized in a 2 mM taurocholate solution which did not compromise the functional integrity of the vesicles. Linoleic acid uptake in the range of 1 to 100 microM followed passive diffusion kinetics. Time course study showed that linoleic acid uptake reached maximal levels during the initial 15 seconds. Although the amount of linoleic acid accumulated in the vesicles diminished over the next 30 minutes, the molar quantity was still twentyfold higher than that of D-glucose (6.5 vs 0.33 nmol/mg protein). Uptake of D-glucose by the vesicles demonstrated typical osmotic responsiveness. We found no osmotic effect on linoleic acid uptake. Hypotonic lysis of membrane vesicles loaded with linoleic acid released 40% of the fatty acid. We concluded that a major portion of the accumulated fatty acid was bound to or incorporated into the membrane itself while ca. 40% did traverse the membrane and accumulated in the intravesicular space as nonmicellar aggregates. The known inhibitors of anion transport, diisothiocyanatostilbene and isothiocyanatostilbene did not change the transport of linoleic acid. We conclude that, in the absence of an unstirred layer or cytosol proteins, linoleic acid transport at up to 100 microM concentration is passive with rapid accumulation both by the cell membrane and the lumen of vesicles.