Acidification in the rat proximal jejunum

Immunoglobulin G receptors of intestinal brush borders from neonatal rats

Intestinal absorptive cells of the neonatal rat display on their brush border membranes receptors for immunoglobulin G (IgG) which function in selective transfer of maternal IgG. Our Scatchard analysis of [125I]IgG binding to isolated brush borders has corroborated the presence of two classes of specific binding sites (KA1 = 2.4 X 10(7) M-1 and KA2 = 3.7 X 10(5) M-1) and the increase in overall binding with decreased buffer concentration, as shown by Wallace & Rees 1980. However, our Scatchard analysis of binding at different buffer concentrations indicates that the only significant effect of lowered buffer concentration is to increase the number of low-affinity sites. Neither the number nor the affinity of the high-affinity sites is affected. Furthermore, brush borders from rats at 21 days have only the low-affinity sites and at this age the selective transfer of IgG has ceased. Morphological experiments with tracers for both light and electron microscopy suggest that the high-affinity sites correspond to the specific IgG receptors on the apical membrane. The majority of the low-affinity sites are found within the terminal web and are likely not to be involved in selective transport of IgG.

A reconsideration of the evidence for Escherichia coli STa (heat stable) enterotoxin-driven fluid secretion: a new view of STa action and a new paradigm for fluid absorption

A review of the evidence for Escherichia coli STa causing fluid secretion in vito leads to the conclusion that the concept of STa acting through enhanced chloride secretion in order to derange intestinal function is unproven. However, a consistent effect of STa in the small intestine is on Na+/H+ exchange, leading to interruption of luminal acidification. A model for the action of STa, involving inhibition of Na+/H+ exchange, is proposed which explains the ability of STa to reduce absorption in vito but its inability to cause secretion in vito in contrast to its apparent secretory effect in vitro. The apparent ability to demonstrate secretion in vitro is shown to derive from methodologies which do not involve measurement of mass transport of water but instead, infer it from in vitro and in vivo proxy measurements. The in vitro demonstration of notional secretion after STa exposure can be reconciled with the proposed new model for fluid absorption in that cell swelling is argued to arise as a transient consequence of STa challenge followed by regulatory volume decrease. Evidence for this derangement model is presented in the form of observations derived from acute in vivo physiological studies and clinical studies on patients without the exchanger. This process of appraisal of the evidence for the mechanism of action of STa has led to a new model for fluid absorption. This is based on the formation of hypotonicity at the brush border luminal surface rather than hypertonicity within the lateral spaces as required by the present standing gradient model of fluid absorption. Evidence from the literature is presented for this new paradigm of water absorption, which may only be relevant for small intestine and other tissues that have Na+/H+ exchangers in contact with HCO-3-containing solutions but which may also be generalizable to all mammalian absorbing epithelial membranes.

Review of theoretical passive drug absorption models: historical background, recent developments and limitations

In the drug discovery process the optimization of a promising lead to an orally bioavailable drug remains a difficult task. Recent progress in the understanding of the role of physicochemical properties in membrane permeability relevant to important processes such as drug absorption and blood-brain barrier crossing, brings rational drug delivery more within reach. In the last thirty years a number of theoretical transport and absorption models have been developed to describe mathematically how a drug is being passively transported from its site of administration to its site of action and how a compound passes a membrane. The goal of such models is to rationalize the physical significance of the observed non-linear structure-permeability relationships. The models are based on various views on the composition of the biological membranes and on the underlying diffusion and distribution mechanisms. Often simplifications reducing the mathematical complexity are made. We review here a selection of the most important models and discuss modern views on the role of lipophilicity and various pathways through membranes.

Effects of aging on the microclimate pH of the rat jejunum

The acidic microclimate layer in the vicinity of the cell surface of mammalian jejunum is important for absorption of some nutrients, such as small peptides and folate. The present study was undertaken to investigate the effect of aging on the cell surface pH (microclimate pH) of the jejunum of rats. The microclimate pH was measured in vitro in superfused preparations using single-barreled pH-sensitive microelectrodes filled with a liquid ion exchanger. The thickness of the microclimate layer was estimated by reading the distance of microelectrode advancements. The existence of a microclimate pH in the jejunum was confirmed in the senescent rats, but the value of the microclimate pH was significantly higher in the senescent (24 mo) rats (6.52 +/- 0.02) than in the young-adult (6 mo) rats (6.09 +/- 0.01) (P < 0.01). Na+ removal from the perfusate or the addition of amiloride elevated the pH in the senescent rats as well as in the young-adult rats. The microclimate layer was slightly thinner in the senescent rats than in the young-adult rats. The acidity of the microclimate layer of intestinal surface is lower in senescent animals than in the young-adult ones. One of reasons for this is the thinner mucus layer in senescent animals.

Absorption and metabolism of procaine by the rat small intestine

The aim of this study was to obtain information about the absorption of procaine in the rat small intestine (Fisher-Parsons preparation). In the range from 0.25-10 mmol.l-1 procaine in the luminal perfusate, much more of the unchanged drug was absorbed in segments of the ileum than of the duodenum and jejunum. Besides procaine, two metabolites, p-aminobenzoic acid (PABA) and acetylated p-aminobenzoic acid (AABA), formed in the intestinal mucosa, appeared in the absorbate. With increasing substrate concentration in the perfusate the PABA in the absorbate increased considerably in all three segments; from 0.75 mmol.l-1 procaine upwards the PABA produced was highest in the jejunum. AABA formed in the mucosa and measured in the absorbate did not increase in the same manner with increasing substrate concentration; in the absorbate of jejunal segments the amount of AABA was significantly higher than in duodenal and ileal segments. Taking into account that in rats the microclimate of the ileum differs considerably from that of the upper part of the small intestine, the marked difference observed in the absorption of procaine between ileal segments on the one side, and duodenal and jejunal segments on the other, can be explained on the basis of the "non-ionic diffusion" theory.

Epidermal growth factor as a regulatory hormone maintaining a low pH microclimate in the rat small intestine

This study was designed to determine the effect of epidermal growth factor (EGF) in the lumen on the pH of the intestinal surface in the rat jejunum, which is referred to as "microclimate-pH". In the control experiment, a significant pH gradient was observed between the mucosal surface (approximately pH 6.8) and the bulk phase (approximately pH 7.3). The microclimate-pH was decreased by 0.2-0.6 pH units after addition of higher concentrations of EGF (3-100 nM) to the lumen. The microclimate-pH thus decreased recovers to the control value by replacing EGF with TES buffer, suggesting that the EGF effect is reversible. Considering that the Na+-H+ exchanger exists on the luminal membrane of the intestinal cells, the decrease in the microclimate-pH which was induced by EGF added to the luminal side may be due to the activation of Na+-H+ exchanger.

Effects of phenothiazine derivatives on the microclimate-pH in the rat jejunum

The effects of several phenothiazine derivatives (PTDs) and quinidine (QD) on the jejunal microclimate-pH in rats were studied using a microelectrode. Chlorpromazine, thioridazine, chlorpromazine sulfoxide (CPZSO), trifluoperazine, prochlorperazine, and QD at concentrations of 1 mM increased this microclimate-pH by 0.15-0.3 pH units, while 1 mM diethazine and 1 mM promethazine had little effect on it. The increases in the microclimate-pH caused by PTDs and QD were concentration dependent and reversible. We studied the effects of PTDs on the fluidity of intestinal brush border membranes and on the release of proteins from the intestinal tissue to the lumen. The PTD-induced changes in microclimate-pH could not be explained by either of these nonspecific effects on the membranes. Then, the effects of PTDs on Na+,K+-ATPase activity and Mg2+-ATPase activity were studied using the jejunal homogenate. Each PTD inhibited Na+,K+-ATPase and Mg2+-ATPase activity to some extent. The inhibitory effects on Na+,K+-ATPase and Mg2+-ATPase activity were compared with the PTD-induced increases in the microclimate-pH. No good correlation was obtained between the IC50 values of PTDs for Na+,K+-ATPase activity and the concentrations required to increase the microclimate-pH by 0.1 pH unit, while IC50 values of PTDs for Mg2+-ATPase activity showed a relatively good correlation, except for that of CPZSO. These findings suggest that the effects of PTDs on the microclimate-pH were not nonspecific, although the increases in the microclimate-pH caused by PTDs cannot be fully explained by the inhibitory effects of these compounds on either Na+,K+-ATPase activity or Mg2+-ATPase activity alone.

Factors affecting the microclimate pH in rat jejunum

1. Single-barrelled pH-sensitive microelectrodes filled with liquid ion exchanger were used to study the layer of microclimate pH in the vicinity of the surface of rat jejunum in vitro. 2. During perfusion with a Na+-containing solution of pH 7.30, a layer having a pH gradient ranging from 7.30 (pH of the luminal bulk phase) to 6.05 +/- 0.03 (pH of the deepest region) was detected in eighteen different animals. The thickness of the layer was estimated to be 600-700 microns. No regional difference was seen along the height of the villus. 3. The addition of D-glucose to the perfusion solution significantly augmented the acidity of the deepest region without changing the thickness of the layer. On the other hand, the elimination of Na+ from the perfusion solution caused a significant reduction of the pH gradient. The lowest pH changed from a control value of 6.18 +/- 0.15 (n = 13 measurements from three animals) to 6.46 +/- 0.06 (n = 13). The gradient was sensitive to amiloride in the presence of Na+, K+, Ca2+ and Cl- had no significant effect on the microclimate pH. 4. Depletion of the surface mucus by treatment with dithiothreitol significantly raised the pH of the deepest region. 5. Glycylglycine and L-carnosine were found to reduce the microclimate pH gradient significantly, while glycine did not. 6. These results indicate that H+ secretion by the Na+-H+ antiport and the formation of mucus layer are important factors for maintaining the microclimate pH layer, and that H+-coupled co-transport, such as H+-dipeptide co-transport, causes a significant diminution of the microclimate pH gradient.

Relationship between antipyrine absorption and blood flow rate in rat jejunum, ileum, and colon

The appearance rate of antipyrine in intestinal venous blood was measured in anesthetized rats during perfusion (0.2 ml/min) of a buffered solution with 1 mmol/l labeled antipyrine through a jejunal, ileal, or colonic segment (length: 2-5 cm). When the blood flow rate was increased from 0.9-1.2 to 1.6-2.0 ml min-1 g-1 by raising the systemic blood pressure from 80 to 130 mm Hg, the absorption of antipyrine increased only in the colon. Stepwise reduction of the blood flow rate from 1.4-1.7 to 0.2-0.3 or stepwise raise from 0.2-0.3 to 1.4 ml min-1 g-1 by constriction or release of the mesenteric artery decreased or increased the absorption rate of antipyrine. The relation between absorption and flow rate can be described by curves which ascend at low and level off into a horizontal section at high flow rates. At the same blood flow rate the regional absorption rate decreased in the order jejunum, ileum, and colon with the largest step between ileum and colon. Model analysis yielded the following results for jejunum, ileum, and colon, respectively: permeability-surface area product 0.083, 0.074, and 0.037 ml min-1 g-1; fraction of absorptive site blood flow rate 0.24, 0.19, 0.08. The differences can be attributed mainly to the change of the surface area from jejunum to ileum and colon.

[Mechanisms of intestinal absorption of nutrients]

The nutrient uptake from the intestinal lumen into the distributing blood circulation is mediated by the epithelial cell of the small intestine. The transfer process through this distinctly polar cell consists of three partial events: entrance of substances through the brush-border membrane, traversal of a metabolic active intracellular space and exit through the baso-lateral membrane. The fundamental transfer mechanisms--simple diffusion, facilitated diffusion, antiport and symport systems, electroneutral and electrogenic processes--are described. The significance of nutrient metabolization for transport processes is discussed: proton secretion by the epithelial cell coupled to the glucose and lactate metabolization is quoted as an illustration. The "acid microclimate" resulting from this proton secretion on the mucosal surface has a significant influence on weak-electrolyte absorption. This effect was clearly demonstrated for in vitro uptake of nicotinic acid into the intestinal tissue. It can be assumed that--similar to the role of a Na+-gradient--the proton gradient on the surface of absorptive epithelia is highly significant as a driving force of nutrient absorption.

Effects of potential damaging agents on the microclimate-pH in the rat jejunum

This study was designed to determine the pH of the intestinal surface in the rat jejunum in an attempt to investigate the effect of potential damaging agents on the microclimate-pH. A significant pH gradient was observed between the mucosal surface and the bulk phase; however, the microclimate-pH usually ranged from 6.5 to 7.5, irrespective of the wide range in the bulk pH. When the bulk pH was either 7.3 or 4, the microclimate-pH was approximately 6.7, while the microclimate-pH was approximately 7.4 when the bulk pH was raised to 9. The distance of the pH gradient was found to be approximately 900-1300 microns. After treatment with ouabain (10 mM) or amiloride (8 mM), or replacement of Na+ with Li+, the microclimate-pH significantly increased compared with the control (buffer solution). Chlorpromazine (2 mM) and ouabain also significantly increased the microclimate-pH, but at the same time, chlorpromazine decreased the bulk pH so that a more marked pH gradient was observed. Both aspirin (10 mM) and acetazolamide (80 mg/kg, iv) significantly decreased the microclimate-pH under the condition of the acidic bulk phase pH. N-Acetyl-L-cysteine (5%) induced marked changes in microclimate-pH, the direction depending upon the bulk phase pH. Neither taurocholic acid (10 mM) nor glucose (10 mM) significantly changed the microclimate pH.

Determination of acid surface pH in vivo in rat proximal jejunum

Surface pH of rat intestine was measured in vivo in exteriorised loops using pH-electrodes. Surface pH was approximately 6.1 in Krebs-phosphate buffer in proximal jejunum and was significantly more acid (p less than 0.01) than the bulk medium pH of 7.2. Values for the midgut were approximately 6.5, yet the distal ileum gave values of 7.3, which was marginally more alkaline than the buffer. These results agree with the known acidification phenomenon in the jejunum and the alkalinisation process in the ileum. No difference in surface pH was detected when bicarbonate buffer was used, nor when glucose was included in the buffer. The acid surface pH was almost completely inhibited when jejunal loops were made anoxic by completely occluding the blood supply, showing that the low surface pH is not itself a consequence of preparative anoxia. If glucose was included in the buffer, allowing glucose access from the luminal surface, surface pH was not significantly altered after occlusion. This indicates that previously reported in vitro results in the presence of glucose are similar to the present in vivo findings. The present experiments confirm the existence in vivo of the 'acid-microclimate' proposed to alter the absorption profiles of some dissociable drugs.

Drug absorption by the rat jejunum perfused in situ. Dissociation from the pH-partition theory and role of microclimate-pH and unstirred layer

In anaesthetized rats the rate of appearance of benzoic acid and aminopyrine in jejunal venous blood was measured; the pH of the luminal perfusion solution was varied between 4 and 10.5. The pH-absorption curves were less steep than predicted by the unmodified pH-partition theory. A reduction of the mucosal unstirred layer thickness by means of the segmented-flow technique considerably increased the absorption rate without essentially changing the shape of the pH-absorption curves. The pH at the surface of the jejunal mucosa was 6.0, 6.5, 6.6, and 8.0 for luminal solutions of pH 4.0, 6.0, 8.0, and 10.8, respectively. From the absorption data the microclimate-pH was calculated which would explain best the observed pH-absorption curves. These calculated pH-values correspond well to the values measured at the mucosal surface. Therefore, it was concluded that a microclimate-pH caused the deviation of the intestinal pH-absorption curves of benzoic acid and aminopyrine from the prediction of the unmodified pH-partition theory. The mucosal unstirred layer represented only a considerable permeation resistance and was not responsible for the deviating shape of the pH-absorption curves.

Mechanism of action of ATP on intestinal epithelial cells. Cyclic AMP-mediated stimulation of active ion transport

ATP, ADP and AMP but not adenosine increased cyclic AMP in dispersed enterocytes prepared from guinea pig small intestine. This action of ATP was augmented by IBMX and was reproduced by App(NH)p or App(CH2)p. ATP also increased the formation of cyclic [14C]AMP in enterocytes that had been preincubated with [14C]adenine. Gpp(NH)p and NaF each caused persistent activation of adenylate cyclase in plasma membranes from enterocytes and ATP caused significant augmentation of this persistent activation. In addition to increasing cellular cyclic AMP and augmenting Gpp(NH)p and NaF-stimulated persistent activation of adenylate cyclase, ATP increased the Isc across mounted strips of small intestine and inhibited net absorption of fluid and electrolytes in segments of everted small intestine. These results indicate that intestinal epithelial cells possess a receptor that interacts with ATP and other adenine nucleotides and that receptor occupation by ATP causes activation of adenylate cyclase, increased cyclic AMP and changes in active ion transport across intestinal mucosa.

The effect of theophylline on intestinal bicarbonate transport measured by pH stat in Amphiuma

1. The influence of theophylline on the mucosa to serosa and serosa to mucosa fluxes of HCO3- were measured by the pH stat technique in isolated segments of proximal small intestine from Amphiuma maintained under short-circuited conditions. The mucosal or serosal fluid was exposed to media containing 25 mM-HCO3- (pH 7.4) while the pH of unbuffered media in the opposite compartment was maintained by addition of acid. 2. Theophylline significantly increased the secretory flux of HCO3- and significantly reduced the absorptive flux when measured in Cl- -free (SO4(2-)) media. 3. In normal media theophylline did not alter the secretory flux but significantly lowered the absorptive flux of HCO3-. 4. Acetazolamide (0.1 mM) inhibited the theophylline-stimulated secretory flux of HCO3- and reduced the effect of theophylline on the absorptive flux. 5. In normal intestine there was an inequality between the secretory or absorptive HCO3- flux and the short-circuit current (Isc) consistent with the presence of Cl- absorption. After addition of theophylline the Isc was more nearly equal to the net secretory or absorptive HCO3- flux. 6. Exogenous cyclic AMP had effects identical with theophylline. 7. The results provide strong evidence that elevation of cyclic AMP stimulates net HCO3- secretion in urodele small intestine and provide indirect evidence that Cl- absorption is simultaneously reduced.

Acid microclimate in coeliac and Crohn's disease: a model for folate malabsorption

The surface pH of human proximal jejunum was measured in biopsy samples and found to be more acid than the phosphate buffer in which they were incubated. The in vitro jejunal surface pH was 5.93 +/- 0.05 in control subjects and 6.19 +/- 0.09 in treated coeliac patients. A group of untreated coeliac patients with a surface pH of 6.56 +/- 0.14 had a significantly less acid surface pH compared to controls, as did a group of Crohn's patients with a surface pH of 6.21 +/- 0.04. These two groups with a significantly raised surface pH were subdivisible into 'high' and 'low'groups. Surface pH was found to remain low in the treated coeliac and control groups but became more acid over the incubation period reaching almost normal values in the Crohn's group and the untreated coelic initial surface pH. The raised surface pH in untreated coeliac disease and Crohn's disease would alter the amount of a weak acid available for non-ionic diffusion. Therefore the present results may help to explain the folate malabsorption known to occur in untreated coeliac disease and the frequently seen low serum folate levels in Crohn's disease.

An ATPase dependent, radiosensitive acidic microclimate essential for intestinal folate absorption

1. 5-Methyltetrahydrofolic acid transport was studied across everted sacs of rat jejunal segments from control and whole body X-irradiated (700 rad) rats at 10(-5)M concentrations (at which optimum transport occurs) at various pHs.2. The folate transport from mucosal to serosal compartment was inhibited by about 55% in irradiated rat at the pH of the intestinal chyme (6.5). Extraneous ATP in the incubation system could restore the defective transport of the irradiated intestine.3. The maximum folate transport which occurred at pH 4.0 was not adversely affected by whole body irradiation. An acidic, pH dependent, passive uptake of 5-methyltetrahydrofolic acid was observed.4. The normal absorption barrier of the small bowel was not disrupted by the acidification process as practically no uptake was observed with irradiated segments pretreated at pH 4.0 except in the presence of ATP.5. Leucine and serine transport at a zero concentration gradient indicated active transport mechanisms which were not affected by acidification. Their uptake was additively increased in the presence of glucose and ATP, further indicating that the normal physiology of the intestines was not affected by the acidification process.6. An intestinal mucosal cell surface ATPase was observed which was Mg(2+) dependent. It could hydrolyse solution phase ATP and thus generate the protons necessary for the acidification of a microenvironment where passive uptake of the neutral folate species could occur.7. The ATPase activity was inhibited about 90% by 50 mM-Na azide at pH 6.5. Below this concentration folate transport was also inhibited.8. Na azide did not inhibit folate transport at pH 4.0, suggesting that its inhibition of folate uptake at pH 6.5 is related to its inhibitory effect on ATPase, rather than on folate transport per se. ATPase activity was therefore essential for folate transport at the pH of the intestinal chyme.

The association between acidification and electrogenic events in the rat proximal jejunum

1. Simultaneous measurement of hydrogen ion production, transmural potential difference and intermittent short-circuit current (SCC) was made in the rat proximal jejunum in vitro: similarly, potassium and sodium ion movements were measured to investigate the relationship of acidification to electrogenic events and associated ion movements in the jejunum.2. Acidification correlated significantly with the short-circuit current and both were inhibited by 10 mM serosal ouabain or 10 mM mucosal aminophylline. Both inhibitors had effects on net potassium movement but not on net sodium movement. Moreover, in isotope studies whereas 10 mM serosal ouabain reduced the J(ms) sodium flux, 10 mM mucosal aminophylline had no effect, i.e. aminophylline can reduce both short-circuit current and acidification without perceptibly altering the serosally directed sodium flux.3. In low-sodium buffers in which acidification still occurs although reduced, transmural potential differences occur of reversed polarity that are apparently unrelated to sodium diffusion potential differences (as evidenced by isotopic sodium efflux experiments) and which could be caused by hydrogen ion production. In low sodium buffers however the inhibitors have opposing effects, ouabain causing an increase and aminophylline a decrease in the reversed potential differences.4. A model for acidification (that of potassium rather than sodium ion exchange for the hydrogen ion and hydroxyl for chloride ion exchange) is proposed to explain the present experimental findings and other diverse observations in the literature: although either step might be the electrogenic step, acidification must be also considered as a component of the jejunal short-circuit current.

Aspects of intestinal folate transport in the rat

1. In vitro preparations of rat jejunum were used to study the metabolic dependence, the structural specificity, and the pH sensitivity of the folic acid absorption process. 2. The presence of 2:4 dinitrophenol, and the absence of oxygen, in the muscosal bathing medium both led to a decrease in the tissue accumulation and serosal transfer of folic acid by everted sacs. 3. 10-formylfolic acid, present in the mucosal medium at a molar ratio of 10:1 with labelled folic acid, appears to compete for tissue accumulation sites but has no significant effect on serosal transfer. 4. The efflux of folic acid from pre-loaded jejunal sacs is stimulated by the presence in the mucosal medium of the folic acid and related compounds. Pteroyl-L-glutamic acid, 10-formylfolic acid and methotrexate elicit a significant increase in the efflux rate; pteroyl-D-glutamic acid was significantly less effective and pteroic acid had no effect. 5. The uptake of folic acid by isolated jejunal cells prepared by enzymic disaggregation of the mucosa, was found to be influenced by the pH of the bathing medium, uptake being enhanced at a pH of between 5 and 6. 6. It is concluded that the effect of metabolic inhibitors and acid pH conditions on the uptake of folic acid in vitro is consistent with a passive absorption mechanism, influenced in intact preparations and in vivo by the jejunal pH microclimate. However the occurrence of competitive inhibition and stimulated efflux may indicate the existence of a structurally specific accumulation process at some site, or sites, within the mucosa.