Small intestinal regulation of iron absorption in the rat

Ultrastructural, biochemical, and immunologic studies of the small intestinal mucosa of rats were undertaken to investigate factors associated with the regulation of iron absorption. The quantity of iron within mucosal cells was proportionate to the degree of iron repletion. Although the quantity of iron-binding substances was similar in iron-deficient and iron-loaded animals, the unsaturated iron-binding capacity of mucosal cells varied inversely with the state of iron repletion of animals in ultrastructural, biochemical, and immunologic observations. Although only certain mucosal cells contained iron-binding substances, their number was not increased in iron-deficient animals. Greater quantities of iron and iron-binding substances were observed in duodenal mucosa than in ileal mucosa. These findings are consistent with the hypothesis that the quantity of unsaturated iron-binding substances within intestinal mucosal cells regulates iron absorption. These findings are only partially explained by changes observed in the concentration of ferritin and transferrin within intestinal mucosa and suggest that other iron-binding substances may also participate in the regulation of iron absorption.     

Enantioselective and nonlinear intestinal absorption of eflornithine in the rat

This study aimed to investigate if the absorption of the human African trypanosomiasis agent eflornithine was stereospecific and dose dependent after oral administration. Male Sprague-Dawley rats were administered single doses of racemic eflornithine hydrochloride as an oral solution (750, 1,500, 2,000, or 3,000 mg/kg of body weight) or intravenously (375 or 1,000 mg/kg of body weight). Sparse blood samples were obtained for determination of eflornithine enantiomers by liquid chromatography with evaporative light-scattering detection (lower limit of quantification [LLOQ], 83 microM for 300 microl plasma). The full plasma concentration-time profile of racemic eflornithine following frequent sampling was determined for another group of rats, using a high-performance liquid chromatography-UV method (LLOQ, 5 microM for 50 microl plasma). Pharmacokinetic data were analyzed in NONMEM for the combined racemic and enantiomeric concentrations. Upon intravenous administration, the plasma concentration-time profile of eflornithine was biphasic, with marginal differences in enantiomer kinetics (mean clearances of 14.5 and 12.6 ml/min/kg for L- and D-eflornithine, respectively). The complex absorption kinetics were modeled with a number of transit compartments to account for delayed absorption, transferring the drug into an absorption compartment from which the rate of influx was saturable. The mean bioavailabilities for L- and D-eflornithine were 41% and 62%, respectively, in the dose range of 750 to 2,000 mg/kg of body weight, with suggested increases to 47% and 83%, respectively, after a dose of 3,000 mg/kg of body weight. Eflornithine exhibited enantioselective absorption, with the more potent L-isomer being less favored, a finding which may help to explain why clinical attempts to develop an oral treatment have hitherto failed. The mechanistic explanation for the stereoselective absorption remains unclear.     

Role of cecal pH in intestinal oxalate absorption in the rat

Hyperoxaluria occurs with many gastrointestinal disorders complicated by malabsorption. This hyperoxaluria is known to be the result of increased colonic absorption of dietary oxalate. Proposed mechanisms for this effect include alterations in fecal fatty acids, alterations in fecal bile acids, and acidification of colonic pH. Using an animal model of lactulose-induced chronic colonic acidification, we examined the effect of pH on oxalate absorption in vivo. Rats were fed a diet containing 6.77 mg oxalate per day with and without lactulose. Cecal pH of the animals receiving lactulose was significantly lower than controls (4.90 +/- 0.42 vs 7.17 +/- 0.38; p less than 0.001). Urinary excretion of oxalate was significantly greater in animals receiving the lactulose diet than in controls (0.975 +/- 0.144 vs 0.844 +/- 0.172 mg oxalate per day; p less than 0.001). These results demonstrate that acidification of the colon results in a significant increase in urinary oxalate excretion. Thus, acidification of the colon may be an important factor in the pathogenesis of enteric hyperoxaluria.     

Intra-individual variability of the metabolic effect of inhaled insulin together with an absorption enhancer

OBJECTIVE: To study the metabolic effect and the variability of the effect elicited by inhalation of 87.2 U insulin powder combined with an absorption enhancer. The metabolic effect was compared with that of 10.2 U regular insulin injected subcutaneously and of 5.5 U regular insulin given intravenously RESEARCH DESIGN AND METHODS: In this single-center open euglycemic glucose clamp study 13 healthy male volunteers received 5 insulin administrations on separate study days: once as an intravenous dose, once as a subcutaneous injection, and 3 times by inhalation, in randomized order. Glucose infusion rates (GIRs) necessary to keep blood glucose concentrations constant at 5.0 mmol/l were determined over an 8-h period after administration. RESULTS: After inhalation of the insulin powder aerosol, the onset of action was substantially more rapid than after subcutaneous insulin injection, and maximal action was reached earlier (86+/-47 vs. 182+/-53 min, P<0.0001). The maximal glucose infusion rate after inhalation of insulin was comparable to that after subcutaneous insulin injection (9.2+/-2.6 vs. 8.8+/-2.8 mg x kg(-1) x min(-1), NS). The metabolic effect in the first 2 h after inhalation was significantly greater than that after subcutaneous insulin injection (amount of glucose infused: 0.88+/-0.25 vs. 0.59+/-0.20 g x kg(-1) x 120 min(-1), P<0.0001). However, the total metabolic effect after inhalation and subcutaneous injection was comparable (2.50+/-0.76 vs. 2.56+/-0.69 g x kg(-1) x 480 min(-1), NS). The relative bioefficacy of inhaled insulin calculated in relation to the data from the subcutaneous insulin application was 12.0+/-3.5% (absolute bioefficacy 10.1+/-3.1%) but was highest in the first 2 h after application (18.5+/-3.7%; absolute bioefficacy 8.2+/-4.1%). The intraindividual variability of the metabolic response induced by insulin inhalation was 14+/-9% for the maximal glucose infusion rate, 15+/-10% for the time-to-maximal effect, and 16+/-12% for the total amount of glucose infused. CONCLUSIONS: This feasibility study shows that inhaled insulin with an absorption enhancer has a pronounced metabolic effect compared with the results of a previous study of inhaled insulin without an enhancer. The intraindividual variability of the metabolic effect was comparable with that of inhaled and subcutaneously injected insulin.     

Use of laminar flow and unstirred layer models to predict intestinal absorption in the rat.

Carbon monoxide (CO) and [14C]warfarin were used to measure the preepithelial diffusion resistance resulting from poor luminal stirring (RL) in the constantly perfused rat jejunum at varying degrees of distension (0.05, 0.1, and 0.2 ml/cm). RL was much greater than epithelial cell resistance, indicating that poor stirring was the limiting factor in absorption and that an appropriate model of stirring should accurately predict absorption. A laminar flow model accurately predicted the absorption rate of both probes at all levels of gut distension, as well as the absorption of glucose when RL was the rate-limiting factor in absorption. In contrast, an unstirred layer model would not have predicted that gut distension would have little influence on absorption, and would have underestimated [14C]warfarin absorption relative to CO. We concluded that in the perfused rat jejunum, laminar flow accurately models luminal stirring and an unstirred layer should be considered to be a unit of resistance in laminar flow, rather than a model of luminal stirring.     

EUV photofragmentation study of hybrid nonchemically amplified resists containing antimony as an absorption enhancer

A detailed investigation to understand the mechanism of the resist action at a fundamental level is essential for future Extreme Ultraviolet Lithography (EUVL) resists. The photodynamics study of a newly developed hybrid nonchemically amplified 2.15%-MAPDSA–MAPDST resist using synchrotron radiation excitation at 103.5 eV (12 nm) is presented. Antimony was incorporated in the resist as a heavy metal absorption center in the form of antimonate (2.15%). The results showed the fast decomposition rate of the radiation sensitive sulfonium triflate. HR-XPS and sulfur L-NEXAFS spectra of the copolymer films revealed that after irradiation the Ar–S⁺–(CH₃)₂ sulfonium group bonded to the phenyl ring resisted the EUV excitation. Those results confirmed the polarity switching mechanism from hydrophilic sulfonium triflates to hydrophobic aromatic sulfides obtained in previous results. The inorganic component SbF₆⁻ included in the resist formulations as an EUV absorption enhancer was particularly illustrative of the photofragmentation process. F 1s and O 1s HR-XPS spectra showed that fluorine remains linked to the antimony, even after 15 min of irradiation. A change of the antimony oxidation state was also observed with an increase in irradiation time. The presence of the heavy metal may control the high energy deposited on the resist which finally led to very well resolved 20 nm isolated line patterns by EUVL. The 10 times improved sensitivity compared with previous poly-MAPDST resists studied in the past showed the potential of this class of hybrid resists for next generation semiconductor industry applications.

Understanding the EUV photofragmentation mechanism in a highly sensitive hybrid n-CAR containing SbF₆⁻ as an absorption enhancer.     

The relationship between intestinal crypt cell production and intestinal water absorption measured in vitro in the rat

In studies on intestinal adaptation it is often important to obtain contemporaneous data on the rate of cell production and the functional status of the intestine. The measure of the rate of accumulation of vincristine arrested metaphases in microdissected intestinal crypts to determine the crypt cell production rate (CCPR), is one of the most effective methods of estimating intestinal epithelial cell proliferation which is robust enough to withstand scrutiny. However, studies in the field of intestinal adaptation could be much more informative if a valid measure of intestinal function could also be included. One such method is the water absorption capacity in vitro. The intraperitoneal injection of vincristine sulphate (1 mg/kg) had no significant effect on the water absorption capacity of the small intestine, as measured by the segmented flow single pass perfusion method; thus the CCPR of the jejunum and intestinal water absorption were both measured in 19 groups of hypo- and hyper-proliferative rats which should have been in a relatively 'steady state' of cell production and turnover. The minimum values were obtained after hypophysectomy and the maximum values were observed in lactation. CCPR and absorption were significantly correlated (P less than 0.001) with each other. There was a significant (P less than 0.001) correlation between both CCPR and absorption and dry weight of the intestinal segment studies with food intake. Body weight was a poor predictor of either CCPR or absorption.(ABSTRACT TRUNCATED AT 250 WORDS)     

Correction: EUV photofragmentation study of hybrid nonchemically amplified resists containing antimony as an absorption enhancer

Correction for ‘EUV photofragmentation study of hybrid nonchemically amplified resists containing antimony as an absorption enhancer’ by Cleverson Alves da Silva Moura et al., RSC Adv., 2018, 8, 10930–10938.

The authors regret that Kenneth E. Gonsalves’ name was spelled incorrectly in the original article. The correct author names are as presented above.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Nonlinear intestinal absorption kinetics of cefuroxime axetil in rats.

Cefuroxime is commercially available for parenteral administration as a sodium salt and for oral administration as cefuroxime axetil, the 1-(acetoxy)ethyl ester of the drug. Cefuroxime axetil is a prodrug of cefuroxime and has little, if any, antibacterial activity until hydrolyzed in vivo to cefuroxime. In this study, the absorption of cefuroxime axetil in the small intestines of anesthetized rats was investigated in situ, by perfusion at four concentrations (11.8, 5, 118 and 200 microM). Oral absorption of cefuroxime axetil can apparently be described as a specialized transport mechanism which obeys Michaelis-Menten kinetics. Parameters characterizing absorption of prodrug in free solution were obtained: maximum rate of absorption (Vmax) = 289.08 +/- 46.26 microM h-1, and Km = 162.77 +/- 31.17 microM. Cefuroxime axetil transport was significantly reduced in the presence of the enzymatic inhibitor sodium azide. On the other hand, the prodrug was metabolized in the gut wall through contact with membrane-bound enzymes in the brush border membrane before absorption occurred. This process reduces the prodrug fraction directly available for absorption. From a bioavailability point of view, therefore, the effects mentioned above can explain the variable and poor bioavailability following oral administration of cefuroxime axetil. Thus, future strategies in oral cefuroxime axetil absorption should focus on increasing the stability of the prodrug in the intestine by modifying the prodrug structure and/or targeting the compound to the absorption site.     

Intestinal absorption of alpha-methyldopa: in vitro mechanistic studies in rat small intestinal segments

The mechanism of intestinal uptake of alpha-methyldopa (Aldomet) was investigated using isolated segments of rat small intestine. Incubations were limited to 2 to 5 min as histological examination of the tissue showed significant loss of structural integrity after 10 to 20 min at 37 degrees C. alpha-Methyldopa in the tissue was extracted and assayed by high-pressure liquid chromatography. Corrections were applied for uptake into extracellular spaces using inulin (14C). Uptake was temperature- and concentration-dependent (Km congruent to 10 mM), was dependent on the location in the small intestine and was inhibited by ouabain (2 mM) or lack of sodium or glucose in the incubation medium. alpha-Methyldopa uptake also was inhibited by other neutral amino acids. The mucosal cell layer accounted for approximately 50% of the total drug accumulated in the tissue. Uptake was less when the serosal surface was exposed than when the tissue was everted. The similarity in uptake parameters between alpha-methyldopa and L-phenylalanine (parallel experiments) suggests that alpha-methyldopa is principally absorbed into rat small intestinal mucosal cells via an amino acid transport system.     

Fetal bile salt metabolism. The intestinal absorption of bile salt.

The intestinal absorption of sodium taurocholate was studied in the near-term fetal and neonatal dog. Absorption rates were measured in vivo in isolated loops of fetal jejunum and ileum. Absorption was also measured in vitro in everted sacs and rings of fetal and neonatal jejunum and ileum. The maximal rates of taurocholate absorption observed after instillation of 1 micronmol taurocholate into closed segments of fetal jejunum and ileum with intact blood supply were not significantly different (P less than 0.2), and equalled 0.282+/-0.026 (mean+/-SEM) and 0.347+/-0.051 micronmol/h per 10-cm segment length jejunum and ileum, respectively. Similarly, the rates of absorption from open segments of jejunum and ileum perfused with 0.4 and 1.0 mM taurocholate were nearly identical (0.232+/-0.040 and 0.255+/-0.039, respectively at 0.4 mM, and 0.470+/-0.065 and 0.431+/-0.013, respectively at 1.0 mm) (P greater than 0.2). At perfusate concentrations of 4.0 mM, moreoever, jejunal absorption exceeded ileal absorption (1.490+/-0.140 and 0.922+/-0.200, respectively (P less than 0.05). As expected, concentration of taurocholate by the mucosa was readily demonstrated in adult ileal, but not in adult jejunal everted rings. In contrast, there were no significant differences in mucosal uptake of taurocholate by fetal jejunal and ileal rings. Fetal ileal mucosal concentrations were not significantly above those in the incubation medium after 1-h exposure of the mucosa to 0.003, 0.03, and 0.3 mM taurocholate. Uptake was proportional to incubation medium concentration over the full range of values. This was also true of tissues from 1-wk-old neonates. However, by 2 wk of age, ileal mucosal concentration of taurocholate was evident and adult levels were attained by 5 wk of age. It is concluded that taurocholate is absorbed by the fetal gut and that ileal absorption is no more efficient than jejunal absorption. Although active glucose transport was demonstrable in both jejunum and ileum, it was not possible to demonstrate an ileal mechanism for active transport of taurocholate in the fetus. Active ileal transport was not demonstrable in the newborn until at least 2 wk after birth.     

Inhibition of steady-state intestinal absorption of long-chain triglyceride by medium-chain triglyceride in the unanesthetized rat

Maximal steady-state intestinal absorption rates in unanesthetized rats for triolein, a long-chain triglyceride, and for trioctanoin, a medium-chain triglyceride, are known to differ. Both these lipids are hydrolyzed in the intestinal lumen but the products of hydrolysis are metabolized differently by the mucosal cell. Intraduodenal infusion of trioctanoin was found to reduce steady-state triolein absorption. Luminal lipolysis was shown not to be rate-controlling. High rates of trioctanoin infusion significantly lowered the pH of the luminal aqueous phase and altered the partition of oleic acid between aqueous and oil phases. Two possible mechanisms for the inhibition of triolein uptake are considered. In the intestinal lumen medium chain lipids might have lowered the activity of oleic acid monomers in the aqueous phase and reduced passive diffusion into mucosal cells. Alternatively, competition between long and medium chain fatty acids for some common receptor during transport into the intestinal mucosal cell may have occurred.Despite significant inhibition of triolein absorption by high levels of trioctanoin, the maximum number of calories absorbed from mixtures of triglycerides exceeded the maxima from either glyceride alone. The optimum proportion of triolein to trioctanoin in lipid infusion mixtures was about 3:4 by weight and the optimum dosages about half maximal for each triglyceride, which represented a caloric intake of 4 kcal/rat per 2 hr. The absorption coefficient for this lipid mixture was about 90%. It is suggested that in patients who have a limited intestinal absorptive capacity dietary fat intake might be doubled with a caloric supplement of medium-chain triglycerides without increase in steatorrhea of long-chain fat.     

Importance of bile acids for novel oral absorption system containing polyamines to improve intestinal absorption.

The synergetic improving effect of bile acids with spermine (SPM), a major polyamine, on the absorption of rebamipide, a poorly soluble and poorly absorbable drug (BCS Class IV), was evaluated in rats and beagle dogs. Although the absorption of rebamipide was improved by the addition of polyamines alone in normal rats, it was not improved in bile duct ligated (BDL) rats. The combinatorial use of sodium taurocholate (STC), a bile acid, with SPM improved the absorption of rebamipide even in BDL rats. In the beagle dogs, the oral administration of SPM alone did not enhance the absorption of rebamipide, but the combinatorial use of STC with SPM improved the absorption as well as in the BDL rats. These results indicate that bile acids are indispensable for the novel formulation containing SPM to improve the absorption of rebamipide after oral administration.