Characterization and effect of phospholipid on bile acid absorption by villi isolated from hamster small intestine

The effects of phospholipid on absorption of bile acids by hamster small intestine were studied to determine if this compound inhibits absorption of bile acids. Absorption of taurocholic and cholic acids was studied using a new in vitro technique that relates uptake rates to the weight of the villi present on the intestinal sample rather than to the weight of the entire segment of intestine used for the study. This procedure removes from consideration various components of the intestinal wall that are not directly involved with the absorptive process. Using radioactive techniques absorption of each type of bile acid was determined over a broad range of concentrations both in the presence and absence of phospholipid in the incubation medium. Absorption of taurocholic acid by villi from jejunum was determined to be a passive process, as previously reported by others. Villi from ileum absorbed both bile acids by an apparent active process when initial concentrations of bile acids were below 2.0 mM. Above this concentration bile acid absorption by the ileum appeared to be mainly passive. Phospholipid was found to inhibit bile acid absorption by ileum when initial bile acid concentrations were moderately high. However, at low substrate concentration, phospholipid has no appreciable effect on bile acid transport.     

Characterization of p-aminobenzoic acid transport across the rat intestine.

The intestinal absorption of PABA, a member of the vitamin B complex, was characterized in vivo and in vitro in the rat by the segmental intestinal perfusion and everted gut sac technique. Net PABA absorption was directly proportional to substrate concentration, and saturation of absorption did not occur with increasing concentrations of PABA (1 to 50 mM), indicating a nonsaturable process. Jejunal and ileal absorption rates were similar and were not influenced by the presence of glucose or the absence of sodium in the test solution. Similarly, 14C-PABA transport in vitro was nonsaturable and proportional to PABA concentration (0.05, 0.5, 1, 10, and 50 mM). It was not inhibited by ouabain or other PABA analogues such as folic acid and benzoic acid. These studies indicate that PABA, a vitamin B cofactor, is absorbed by a nonsaturable, sodium-independent process, which characterizes passive diffusion and is similar to the absorption of other vitamin B members.     

In vitro determination of active bile acid absorption in small biopsy specimens obtained endoscopically or surgically from the human intestine

BACKGROUND: In the construction of a Kock reservoir for continent urinary diversion, 70 cm of the distal ileum are used. Impaired absorption of bile acids in these patients might cause diarrhoea. Data on the absorption of bile acids in different parts of the human intestine are limited. METHODS: Biopsies were taken during endoscopy from the duodenum, the terminal ileum or the right colon, and during surgery 10, 50, 100 and 150 cm proximally to the ileo-caecal valve using standard endoscopy biopsy forceps. The biopsy specimens were incubated in vitro with radio-labelled taurocholic acid at 37 degrees C for 22 or 45 min The radioactivity was determined using the liquid scintillation technique. RESULTS: A linear increase in the uptake was observed, with increased concentrations of taurocholic acid between 100 and 500 microm in all specimens tested, that represented passive uptake or unspecific binding. The active uptake could be calculated from the intercept of the line representing passive uptake with the ordinate. The active uptake in the terminal ileum was 3-4 times greater than 100 cm proximal to the valve. CONCLUSIONS: The active absorption of bile acids in humans can be determined in small biopsy specimens taken using standard biopsy forceps during endoscopy or surgery. This method is suitable for clinical studies of bile acid absorption. Active uptake of bile acids not only takes place in the very distal part of the ileum but also to a considerable degree 100 cm proximally to the ileo-colonic valve. This should be taken into account when selecting the ileal segment for continent urinary diversion.     

The mechanisms of sodium absorption in the human small intestine

The present studies were designed to characterize sodium transport in the jejunum and ileum of humans with respect to the effects of water flow, sodium concentration, addition of glucose and galactose, and variations in aniomic composition of luminal fluid. In the ileum, sodium absorption occurred against very steep electrochemical gradients (110 mEq/liter, 5-15 mv), was unaffected by the rate or direction of water flow, and was not stimulated by addition of glucose, galactose, or bicarbonate. These findings led to the conclusion that there is an efficiently active sodium transport across a membrane that is relatively impermeable to sodium. In contrast, jejunal sodium (chloride) absorption can take place against only the modest concentration gradient of 13 mEq/liter, was dramatically influenced by water movement, and was stimulated by addition of glucose, galactose, and bicarbonate. The stimulatory effect of glucose and galactose was evident even when net water movement was inhibited to zero by mannitol. These observations led to the conclusion that a small fraction of jejunal sodium absorption was mediated by active transport coupled either to active absorption of bicarbonate or active secretion of hydrogen ions. The major part of sodium absorption, i.e. sodium chloride absorption, appeared to be mediated by a process of bulk flow of solution along osmotic pressure gradients. The stimulatory effect of glucose and galactose, even at zero water flow, was explained by a model in which the active transport of monosaccharide generates a local osmotic force for the absorption of solution (NaCl and water) from the jejunal lumen, which, in the presence of mannitol, is counterbalanced by a reverse flow of pure solvent (H(2)O) through a parallel set of channels which are impermeable to sodium. Support for the model was obtained by the demonstration that glucose and bicarbonate stimulated the absorption of the nonactively transported solute urea even when net water flow was maintained at zero by addition of mannitol to luminal contents.     

Inhibition of active hexose and amino acid transport by conjugated bile salts in rat ileum

Abstract. The effect of conjugated trihydroxy bile salts, tauro- and glycocholate, and of deoxycholate on tissue uptake and mucosal to serosal transfer of actively transported hex-oses and amino acids has been examined in rat small intestine in vitro. Conjugated trihydroxy bile salts and deoxycholate markedly inhibited active transport of hexoses and amino acids in the ileum of rat small intestine, whereas in the jejunum, deoxycholate alone was inhibitory. The inhibitory effect of tauro- and glycocholate increased with incubation time. It persisted after washing of the tissue and reincubation with hexoses in a bile salt free medium, and could be observed with only 2 × 10^-4 M taurocholate. Taurocholate was able to evoke an increase of transmural potential difference (PD) in the ileum, but did not affect PD in the jejunum. Prein cubation of ileal small intestine with taurocholate depressed subsequent glucose-induced PD-increments. In the jejunum, however, taurocholate did not affect PD-increments induced by D-glucose. It is concluded that conjugated trihydroxy bile salts have to enter intestinal mucosal epithelial cells to an appreciable extent in order to affect other active, energy-requiring transport systems in rat small intestine. Previous results showing a failure of conjugated bile salts to inhibit active transport of hexoses and amino acids are explained by the fact that only jejunal transport had been examined.     

Enhancement by epinephrine of benzylpenicillin transport in rat small intestine

The perfusion of rat small intestinal lumen with epinephrine (0.1 mM) resulted in a significant increase in the amount of benzylpenicillin (BP) transported from the mucosal to the serosal side. In this study, the perfusion of the lumen with phenylephrine, clonidine, dobutamine, or salbutamol had no effect on BP transport. However, the combinations of phenylephrine and isoproterenol, clonidine and isoproterenol, and phenylephrine and salbutamol increased the BP transport to a similar extent as that observed with epinephrine alone. Tolazolin or propranolol inhibited the epinephrine-induced increase in BP transport. An increase in the intracellular concentration of cAMP in conjunction with specific activation of either alpha(1)- or alpha(2)-adrenoceptors induced an increase in BP transport similar to that observed in response to epinephrine alone. Staurosporine or N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide abolished the epinephrine-induced increase in BP transport. Peptides or either zwitterionic or anionic cephalosporins also blocked the effect of epinephrine on BP transport. The extent of BP uptake into brush border or basolateral membrane vesicles prepared from epinephrine-perfused intestinal loops was markedly greater than that into vesicles prepared from control loops. The perfusion of intestinal lumen with carbonyl cyanide p-trifluoromethoxy phenylhydrazone, amiloride, or ouabain inhibited epinephrine-induced BP transport. These results indicate that the interaction of epinephrine with both beta(2)-adrenoceptors and either alpha(1)- or alpha(2-)adrenoceptors markedly stimulates the BP transport, an effect likely mediated by the enhancement of the function in the brush border membrane of intestinal epithelial cells coupled with the generation of an H(+) gradient.     

The site-specific transport and metabolism of tacrolimus in rat small intestine

The objective of this study was to evaluate the absorption of tacrolimus by means of simultaneous perfusion of intestinal lumen and blood vessels in rats. In our previous report, the permeability of tacrolimus was found to be higher in the jejunum than in the ileum or colon, suggesting the site-dependent absorption after oral administration. However, in this article, simultaneous perfusion technique revealed that the extent of absorption into blood vessels was similar in the jejunum and the ileum regardless of the site difference in permeability as the absorption rate. In addition to the multidrug resistance-associated protein-mediated efflux, cytochrome P450 (P450)-mediated metabolism could be a possible mechanism to explain the inconsistencies in the site dependence of tacrolimus absorption. Two enzyme inhibitors, ketoconazole and midazolam, were coperfused in rat intestinal lumen with tacrolimus to specify the effect of P-gp and P450. In the jejunum, both inhibitors significantly enhanced the absorbed amount of tacrolimus, whereas the permeability was not affected. It was suggested that both inhibitors mainly suppress P450-mediated metabolism in the upper region of the intestine. In contrast, in the ileum, ketoconazole significantly enhanced both the absorbed amount and the permeability of tacrolimus. However, midazolam failed to enhance the absorption of tacrolimus, indicating the dominant role of P-glycoprotein (P-gp)-mediated efflux in the lower region. From these findings, it is concluded that the site-dependent differences in P-gp and/or P450 activity could be the prime cause of large intra- and interindividual variability in the oral absorption of tacrolimus.     

Cimetidine transport in brush-border membrane vesicles from rat small intestine

In previous studies, sulfoxide metabolite was observed in animal and human intestinal perfusions of cimetidine and other H2-antagonists. A sequence of follow-up studies is ongoing to assess the intestinal contributions of drug metabolism and drug and metabolite transport to variable drug absorption. An evaluation of these contributions to absorption variability is carried out in isolated fractions of the absorptive cells to uncouple the processes involved. In this report, data is presented on the drug entry step from a study on [3H]cimetidine uptake into isolated brush-border membrane vesicles from rat small intestine. A saturable component for cimetidine uptake was characterized with a Vmax and Km (mean +/- S.E.M.) of 6.1 +/- 1.5 nmol/30s/mg protein and 8.4 +/- 2.0 mM, respectively. Initial binding, and possibly intravesicular uptake, was inhibited by other cationic compounds including ranitidine, procainamide, imipramine, erythromycin, and cysteamine but not by TEA or by the organic anion, probenecid. Initial uptake was not inhibited by amino acids methionine, cysteine, or histidine, by the metabolite cimetidine sulfoxide, or by inhibitors of cimetidine sulfoxidation, methimazole, and diisothiocyanostilbene-2,2'-disulfonic acid. Equilibrium uptake was inhibited by ranitidine, procainamide, and cysteamine but not by erythromycin or imipramine. Initial cimetidine uptake was stimulated by an outwardly directed H+ gradient, and efflux was enhanced by an inwardly directed H+ gradient. Collapse of the H+ gradient as well as voltage-clamping potential difference to zero significantly reduced initial cimetidine uptake. The data is supportive of both a cimetidine/H+ exchange mechanism and a driving-force contribution from an inside negative proton or cation diffusion potential.     

Effect of renal insufficiency on the active transport of calcium by the small intestine

The intestinal absorption of calcium is often depressed in patients with chronic renal insufficiency. Furthermore, the malabsorption of calcium and the osteodystrophy which occur in association with chronic renal disease are often “resistant” to vitamin D; the basis for this resistance remains uncertain however. Recent studies by others have emphasized the role of an abnormality in the metabolism of vitamin D in accounting for the alterations in the calcium absorption and the apparent vitamin D-resistance which accompany the uremic syndrome.     

Effect of cortisone treatment on the active transport of calcium by the small intestine

It is generally recognized that glucocorticoid administration may diminish calcium absorption in vivo as well as the active transport of calcium by the intestine in vitro. Recent studies by others have emphasized the possibility of an alteration in the metabolism of vitamin D to 25-hydroxycholecalciferol in accounting for the steroid effects on calcium absorption. The results obtained in the present studies fail to support this hypothesis.The present studies confirm that the administration of cortisone or other glucocorticoids to the rat interferes with the active transport of calcium by duodenal gut sacs in vitro. This abnormality is not due to an alteration in the permeability of the intestine to calcium, and it cannot be corrected by the administration of either massive doses of vitamin D(2) or modest doses of 25-hydroxycholecalciferol. Experiments concerned with the effects of cortisone on the level of the vitamin D-dependent duodenal calcium-binding protein, the amount of bioassayable vitamin D activity in the mucosa, and the distribution and metabolism of (3)H-vitamin D(3), did not provide evidence in favor of a harmone-related defect in either the localization of vitamin D or its metabolism to 25-hydroxycholecalciferol. Alterations in the transport of iron and D-galactose, not dependent on vitamin D, suggest that cortisone treatment may be responsible for more than a simple antagonism to the effects of vitamin D.The results of the present studies indicate that cortisone administration affects the cellular mechanisms mediating calcium transport in a manner that is opposite to the effects of vitamin D, but seems to be independent of any direct interaction with the parent vitamin or its metabolites. If a disorder in vitamin D metabolism is at all involved, it is at a step subsequent to 25-hydroxylation.     

Bile acid active and passive ileal transport in the rabbit: effect of luminal stirring

The intestinal absorption of bile acids (BA) with different chemical structure has been evaluated in the rabbit, after intestinal infusion of different concentrations (0.25-30 mM) of BA, by mesenteric blood sampling. Cholic (CA), chenodeoxycholic (CDCA), ursodeoxycholic (UDCA) acid, free and taurine (T-) conjugated, together with glycocholic (GCA) acid and deoxycholic acid (DCA) were studied. The apparent uptake parameters were calculated. All conjugated BA showed active transport (T max, nmol min-1 cm-1 int.), with Tmax values in the following order: TCA > TUDCA > TCDCA; unconjugated BA showed passive uptake, with values in the following order: DCA > CDCA > UDCA > CA. GCA and CA showed both passive uptake and active transport. For all BA studied the % uptake in the ileal segment considered was less than 10%, BA uptake being thus limited by transport and/or diffusion kinetics, rather than by flow velocity. The liquid resistance to BA radial diffusion inside the lumen was evaluated, and the infusate-to-blood uptake parameters corrected for it, in order to get the uptake parameters from the epithelium-to-liquid interface to mesenteric blood: the apparent Km decreased, passive uptake coefficient increased, while Tmax was unchanged. The passive component of the uptake, corrected for the luminal resistance, correlated with the BA hydrophobicity (r = 0.963; P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Norepinephrine and alpha-2 adrenoceptors modulate active ion transport in porcine small intestine.

The effects of norepinephrine (NE) were examined on active ion transport in the porcine distal jejunum under base-line conditions and after electrical transmural stimulation (ETS). Serosal administration of NE decreased basal short-circuit current across sheets of muscle-stripped jejunal submucosa-mucosa in vitro. These effects were absent in tissues pretreated with the neuronal conduction blocker, tetrodotoxin. NE stimulated net Na secretion in tissues which displayed basal net Na and Cl absorption and increased net Cl absorption in tissues which displayed net Na absorption and Cl secretion under base-line conditions. Moreover, NE inhibited short-circuit current elevations produced by ETS (300 pulses at 10 Hz, 0.5 msec pulse duration, 2.8 mA cm-2), the ganglionic stimulant dimethylphenyl-piperazinium or the gut peptide neurotensin. In contrast, NE did not alter mucosal responses to the directly acting secretagogues forskolin and carbachol. The inhibitory action of NE on mucosal responses evoked by ETS were selectively antagonized by the alpha adrenoceptor blockers phentolamine and yohimbine. Moreover, the selective alpha-2 adrenoceptor agonists p-aminoclonidine, UK-14,304 and oxymetazoline and the NE releasing agent tyramine mimicked the inhibitory effects of NE on ETS-evoked mucosal responses. Desipramine, a blocker of neuronal NE uptake, produced a 1000-fold increase in the potency of NE at concentrations less than 1 nM. These results suggest that NE modulates active ion transport by interacting with alpha-2 adrenoceptors located on submucosal neurons of the porcine small intestine.     

Inhibition of ileal bile acid transport by cyclosporin A in rat

Abstract. Chronic administration of cyclosporin A may induce cholestasis and this effect has been attributed to impaired hepatic bile salt synthesis, metabolism and transport. We investigated the effect of cyclosporin A on intestinal absorption of bile acids in the ileum of rat. Ileal bile acid absorption was measured by in vivo intestinal perfusion with cyclosporin A and the solvent Cremophor EL. During ileal perfusion with 25 μM glycocholic acid, the concentration of 2.8 mm cyclosporin A inhibited intestinal bile acid absorption on average by 34%. Additional experiments were performed with everted gut sacs of the distal ileum to evaluate active absorption. A dose and time dependent inhibition of the active intestinal absorption of bile acids was found, with a 50% transport inhibition at an average cyclosporin A concentration of 2.69mmolL^-'. Thus, cyclosporin A inhibits the active intestinal absorption of bile acids which may influence bile acid synthesis, turnover and secretion and may contribute to cyclosporin A induced cholestasis.