Effects of vasopressin on electrolyte transport in lizard intestine

Summary Vasopressin applied serosally had no effect on electrical parameters and unidirectional Na and Cl fluxes across anin vitro short circuited preparation of lizard ileum. Short circuit current (Isc) and transmural potential difference (PD) across colon were decreased by vasopressin and increased by cyclic AMP. Vasopressin increased the mucosal-to-serosal flux of sodium and chloride across short circuited colon. Cyclic AMP had no effect on the rate of Na absorption but reversed Cl absorption to secretion. Vasopressin enhanced the net absorption of water across the colon but had no effect on absorption across ileum. Cyclic AMP activity in homogenates of colon was not altered by vasopressin but was increased by theophylline. It is concluded that the colonic response of the lizard colon to vasopressin is mediated by a noncyclic AMP mechanism.     

Characterization of small intestinal water, sodium, and potassium transport and morphology in the pig.

The suitability of the young pig for studies of intestinal transport was investigated by determination of net water, sodium, and potassium absorption, and morphometry in jejunum and ileum. Active net absorption was seen in both jejunum and ileum. There was no difference in absorption between segments and there was a good correlation between water and electrolyte transport. Heart rate, arterial pressure, and absorption were constant with time. Villus height and width were similar to those of human, but crypt depth was greater, and the ratio of villus height to crypt depth less. The mucosa was partially permeable to polyethylene glycol 4000 and this marker is not suitable for studies of transport in young pigs.     

Fluid secretory responses to enterotoxin STa and 8-bromo-cyclic GMP in fed and nutrionally-deprived gerbils: jejunum, ileum and colon in vivo

Fluid transport was measured gravimetrically in vivo in the jejunum, ileum and colon of fed, fasting (four days) and undernourished (50 % of control food intake for 21 days) gerbils (Gerbillus cheesmani). The effects of luminal enterotoxin Escherichia coli STa (50 ng/ml) and luminal 8-bromo-cyclic GMP (cGMP 1 mM) on fluid transport across jejunum, ileum and colon were also assessed. Fasting and undernourishment reversed the normal basal fluid absorption measured in fed ileum and colon into secretion. Neither fasting nor undernourishment had any effect on jejunal basal fluid absorption. In jejuna, ilea and colons of fed animals as well as in jejuna from fasting and undernourished gerbils STa (50 ng) reversed the normal absorptive "tone" to secretion but it had no significant effects on fluid secretion in either the ileum or colon from fasted gerbils. STa increased significantly the fluid secretion in ileum from undernourished gerbils. Luminal cGMP had no effect on basal absorptive tone in the jejunum of fed and fasted gerbils, but reversed absorption into secretion in the jejuna from undernourished gerbils. In the ilea taken from fed animals the small basal absorption was reversed to secretion by luminal cGMP. Although cGMP produced no significant changes in fluid secretion in the ilea taken from fasted gerbils, yet it caused a significant increase in those from undernourished gerbils. In the colon taken from fed animals cGMP decreased the basal fluid absorption significantly, but it had no significant effect on fluid secretion in the colon of fasted or undernourished gerbils. We conclude that fasting and undernourishment have no significant effects on fluid transport across the gerbil jejunum but reversed basal absorption in the fed ileum and colon into secretion. cGMP mimic the effects of STa in the jejunum taken from undernourished gerbils, in the ileum obtained under the three nutritional states and in the colon taken from fasting animals.     

Altered expression of the Na+/H+ exchanger isoform-3 in experimental colitis: Effect of garlic

The sodium-hydrogen exchanger isoform, NHE-3 is essential for the absorption of sodium and water from intestine. Whether this protein plays any role in inflammatory bowel disease is less understood. To address this issue, NHE-3 mRNA and protein levels were estimated in the terminal ileum and colon of the rats having colitis induced with trinitrobenzenesulphonic acid (TNBS). The effect of garlic (Allium sativum) was also evaluated on the expression of NHE-3. The animals were treated with garlic extract intraperitoneally starting 2 h before the TNBS administration until day 4 post-TNBS administration and were sacrificed on day 5. In control animals, the levels of NHE-3 in colon was higher than the ileum. As a result of colitis, the levels of NHE-3 protein and mRNA increased both in the colon and terminal ileum. Garlic treatment of the colitic animals resulted in a selective suppression of NHE-3 in the terminal ileum. Colitis caused an induction of the myeloperoxidase activity, the marker of inflammation in the colon but not in the ileum. These findings suggest that induction of NHE-3 is not primarily due to inflammation. Selective suppression of this protein in ileum by garlic may cause loss of sodium chloride and water during colitis.     

Electrical properties and fluxes of Na+ and Cl- across lizard intestine

Electrical parameters and unidirectional Na+ and Cl- fluxes were determined in vitro across the duodenum, ileum and colon of lizard (Gallotia galloti). Electrical potential difference (PD) and short circuit current (Isc) were low in the three segments studied, whilst tissue conductance (Gt) was high. A net active transport of Na+ and Cl- was observed in the three segments. Net Na+ absorption was higher across duodenum and ileum than across the colon, while net Cl- absorption was similar in duodenum, ileum and colon. Ouabain virtually abolished Isc, PD and net Na+ and Cl- fluxes in all the segments. Amiloride abolished net Cl- flux in duodenum, ileum and colon, whereas net Na+ flux was abolished in colon but decreased in duodenum and ileum. PD and Isc were not affected by the presence of the diuretic.     

Angiotensin II and norepinephrine antagonize the secretory effect of VIP in rat ileum and colon

Vasoactive intestinal polypeptide (VIP) induces intestinal secretion of water and electrolytes in experimental animals and man. We assessed the ability of angiotensin II (AII) and norepinephrine (NE) to block the secretion evoked by VIP, in vivo. Ileal and colonic segments in rats were perfused in situ for two hours with a physiological buffer containing [14C]-PEG-4000 as a volume marker. Saline (0.9% NaCl) was infused intravenously during the first hour and VIP or a combination of VIP plus AII or NE was infused during the second hour. All (0.7 ng/kg/min) alone enhanced water absorption significantly (p less than 0.01) in the ileum and an appreciable, although not a statistically significant, effect was observed in the colon. AII antagonized the secretory effects of VIP in the ileum as well as in the colon. Norepinephrine (5 micrograms/kg/min) also reversed the effect of VIP on the small intestine and colon. Although the mechanism by which AII antagonizes the secretory effects of VIP has not been identified, it is probable that AII promotes absorption, at least in part secondary to release of mucosal NE.     

Lithium absorption in tight and leaky segments of intestine

There is significant absorption of Li+ by human jejunum and ileum, but negligible absorption by human colon. Thus, a proximal-to-distal gradient of decreasing Li+ absorption and increasing junctional tightness exists in intestine as well as in renal tubule. For six leaky epithelia the relative permeabilities of K+, Na+, and Li+ by the junctional route are in the sequence PK greater than PNa greater than PLi and all fall within a factor of 2.5. In contrast, for tight epithelia PLi approximately PNa much greater than PK in the amiloride-sensitive channel of the apical membrane, but PK much greater than PLi approximately PNa in the basolateral membrane. The ability of several tight epithelia to sustain nonzero transepithelial Li+ absorption despite this basolateral barrier may be due to Na+/Li+ countertransport at the basolateral membrane, resulting in secondary active transport of Li+ across the epithelium.     

Proximal-distal absorptive gradients in the in vivo intestine of normal and infected (Hymenolepis diminuta: Cestoda) rats.

Using a single-pass perfusion technique, H2O, Na+, Cl-, HCO3-, and glucose absorption were studied in the jejunum and proximal and distal ileum of rats either uninfected or infected with a tapeworm parasite (Hymenolepis diminuta). The effect of parasitization, region of the intestine, type of buffer, and concentration of glucose in the perfusion fluids on the transport data were analyzed by univariate and multivariate techniques. Proximal-distal flux gradients were observed for water and all the solute species studied, as well as for glucose- and bicarbonate-stimulated salt and water transport; there was a decreasing sensitivity to low pH proceeding distally. The major regional differences occurred between the proximal and distal ileum, with the fluxes in the jejunum being similar to those in the proximal ileum. Na+, H2O, and glucose transport decreased, while Cl- absorption increased, proceeding distally. The parasites diminished the rates of absorption of glucose, salt, and water, and altered the flux gradients, particularly the Na+ and HCO3- transport gradients. The differences in the gradients between control and infected animals were related to differential sensitivity of the different transport systems in the various regions of the gut to parasitism.     

Weanling, but not adult, rabbit colon absorbs bile acids: flux is linked to expression of putative bile acid transporters

Intestinal handling of bile acids is age dependent; adult, but not newborn, ileum absorbs bile acids, and adult, but not weanling or newborn, distal colon secretes Cl(-) in response to bile acids. Bile acid transport involving the apical Na(+)-dependent bile acid transporter (Asbt) and lipid-binding protein (LBP) is well characterized in the ileum, but little is known about colonic bile acid transport. We investigated colonic bile acid transport and the nature of the underlying transporters and receptors. Colon from adult, weanling, and newborn rabbits was screened by semiquantitative RT-PCR for Asbt, its truncated variant t-Asbt, LBP, multidrug resistance-associated protein 3, organic solute transporter-alpha, and farnesoid X receptor. Asbt and LBP showed maximal expression in weanling and significantly less expression in adult and newborn rabbits. The ileum, but not the colon, expressed t-Asbt. Asbt, LBP, and farnesoid X receptor mRNA expression in weanling colon parallel the profile in adult ileum, a tissue designed for high bile acid absorption. To examine their functional role, transepithelial [(3)H]taurocholate transport was measured in weanling and adult colon and ileum. Under short-circuit conditions, weanling colon and ileum and adult ileum showed net bile acid absorption: 1.23 +/- 0.62, 5.53 +/- 1.20, and 11.41 +/- 3.45 nmol x cm(-2) x h(-1), respectively. However, adult colon secreted bile acids (-1.39 +/- 0.47 nmol x cm(-2) x h(-1)). We demonstrate for the first time that weanling, but not adult, distal colon shows net bile acid absorption. Thus increased expression of Asbt and LBP in weanling colon, which is associated with parallel increases in taurocholate absorption, has relevance in enterohepatic conservation of bile acids when ileal bile acid recycling is not fully developed.     

Segmental action of aldosterone on water and electrolyte transport across rabbit colon in vivo

Water and electrolyte transport (Na, K, Cl) and the electrical potential difference were investigated simultaneously in five segments of rabbit colon in vivo, under control conditions and under the influence of aldosterone. A dialysis method was used. The solution was designed to mimic the electrolyte concentration of the lumen, and to simulate normal in vivo conditions. Under in vivo conditions (imposed gradients), marked segmental differences in water and electrolyte transport were present. The proximal colon secreted sodium and absorbed potassium, whereas the distal colon absorbed sodium and secreted potassium. Chloride was secreted in all segments. Aldosterone stimulated water and NaCl absorption in the proximal colon by an electroneutral mechanism, whereas in the distal colon a stimulation of electrogenic Na-absorption occurred. The results demonstrate the role of aldosterone in regulating rabbit colonic electrolyte transport. A uniform hormonal stimulation causes qualitatively and quantitatively different effects in the various segments.     

Distribution of aquaporins in the colon of Octodon degus,a South American desert rodent

Octodon degus is a desert rodent of northern Chile, adapted to survive with a limited supply of water. This rodent has a high degree of fecal dehydration, related to colon water absorption. With the hypothesis that aquaporins (AQPs) might be present in the colon epithelium of O. degus and involved in fluid absorption, we studied colon water absorption in vivo and the distribution of AQPs and Na(+) transporters by immunocytochemistry. AQP-1 was found in apical and basolateral membranes of surface-absorptive and crypt epithelial cells. AQP-8 was found in the cytoplasm of enterocytes of surface colon. AQP-3 immunolabeling, on the other hand, was absent from the epithelium but present in a subepithelial fibroblast layer, pericryptal cells, and muscularis mucosae. The hydration state did not modify the amount of immunostaining for any of the AQPs. Colon water absorption was markedly decreased by the mercurial agent p-chloromercuribenzenesulfonic acid and was not affected by water deprivation. The NHE3 isoform of Na(+)/H(+) exchanger and alpha-1 subunit of the Na(+)-K(+)-ATPase were found in apical and basolateral membranes of surface-absorptive cells, respectively. These results suggest that colon water absorption is mostly transcellular and mediated by water channels like AQP-1. Apical Na(+)/H(+) exchanger and basolateral Na(+)-K(+)-ATPase in surface cells could be part of the Na(+) absorption pathway. It is hypothesized that this transport is necessary to provide an osmotic gradient for water absorption. The roles of AQP-8 and AQP-3 in water absorption remain to be established.     

Basolateral K-Cl cotransporter regulates colonic potassium absorption in potassium depletion

Active potassium absorption in the rat distal colon is electroneutral, Na(+)-independent, partially chloride-dependent, and energized by an apical membrane H,K-ATPase. Both dietary sodium and dietary potassium depletion substantially increase active potassium absorption. We have recently reported that sodium depletion up-regulates H,K-ATPase alpha-subunit mRNA and protein expression, whereas potassium depletion up-regulates H,K-ATPase beta-subunit mRNA and protein expression. Because overall potassium absorption is non-conductive, K-Cl cotransport (KCC) at the basolateral membrane may also be involved in potassium absorption. Although KCC1 has not been cloned from the colon, we established, in Northern blot analysis with mRNA from the rat distal colon using rabbit kidney KCC1 cDNA as a probe, the presence of an expected size mRNA in the rat colon. This KCC1 mRNA is substantially increased by potassium depletion but only minimally by sodium depletion. KCC1-specific antibody identified a 155-kDa protein in rat colonic basolateral membrane. Potassium depletion but not sodium depletion resulted in an increase in KCC1 protein expression in basolateral membrane. The increase of colonic KCC1 mRNA abundance and KCC1 protein expression in potassium depletion of the rat colonic basolateral membrane suggests that K-Cl cotransporter: 1) is involved in transepithelial potassium absorption and 2) regulates the increase in potassium absorption induced by dietary potassium depletion. We conclude that active potassium absorption in the rat distal colon involves the coordinated regulation of both apical membrane H,K-ATPase and basolateral membrane KCC1 protein.     

Intestinal adaptation to zinc absorption following massive small bowel resection and colon interposition in rhesus monkeys

The intestinal uptake of zinc was studied in Rhesus monkeys, after 80% small bowel resection and colon interposition in between the remnant jejunum and the ileum. The hair and serum zinc levels decreased in the early postoperative period and gradually attained normal values after three months postoperatively. A significant increase in the zinc uptake was observed in vitro, in the remnant jejunum, ileum and the grafted colon, both in six months and one year old operated monkeys, suggesting a compensatory intestinal adaptation for zinc absorption after massive small bowel resection and antiperistaltic colon interposition.     

Expression of the peptide transporter hPepT1 in human colon: a potential route for colonic protein nitrogen and drug absorption

Substrates of the proton-coupled peptide transporter, hPepT1, include dietary di- and tripeptides plus therapeutically important drugs such as the beta-lactam antibiotics and angiotensin-converting enzyme inhibitors. Expression and function of hPepT1 in the small bowel is well established. We have compared levels of hPepT1 mRNA expression in regions of human gut by RT-PCR methods and examined the expression of hPepT1 in normal human colon using an anti-hPepT1 antipeptide antibody. hPepT1 mRNA was expressed in the large intestine, although at lower levels than in the small intestine. Quantitatively, expression in ileum was 4.6-fold greater than in sigmoid colon. Immunoreactive hPepT1 was detected in human colon at lower levels than in ileum. The pattern of expression differed between the two tissues: whilst expression in the ileum was localised to the apical enterocyte membrane along the length of the crypt-villus axis, expression in the colonocyte was detected at the apical membrane towards the luminal surface but predominantly at the basal membrane towards the base of the crypt. We conclude that distal regions of the bowel express hPepT1, which may provide a mechanism for colonic protein-nitrogen absorption and for absorption of therapeutically important peptidomimetic drugs.     

Application of the general fluid circuit theory to the chloride-bromide-deuterium oxide experiment

The general fluid circuit theory of active chloride absorption is applied to an experiment in which chloride, bromide, and deuterium oxide were simultaneously absorbed from the lower ileum, chloride only moving against a concentration gradient. The conclusion is reached that the constant for active absorption of deuterium oxide is equal, within reasonable limits of error, to the constant for active chloride absorption in accordance with the assumption that the absorption of water carries chloride out of the intestinal lumen without changing its concentration.     

Oxytocin is expressed by both intrinsic sensory and secretomotor neurons in the enteric nervous system of guinea pig

Single- and double-immunostaining techniques were used systematically to study the distribution pattern and neurochemical density of oxytocin-immunoreactive (-ir) neurons in the digestive tract of the guinea pig. Oxytocin immunoreactivity was distributed widely in the guinea pig gastrointestinal tract; 3%, 13%, 17%, 15%, and 10% of ganglion neurons were immunoreactive for oxytocin in the myenteric plexuses of the gastric corpus, jejunum, ileum, proximal colon, and distal colon, respectively, and 36%, 40%, 52%, and 56% of ganglion neurons were immunoreactive for oxytocin in the submucosal plexuses of the jejunum, ileum, proximal colon, and distal colon, respectively. In the myenteric plexus, oxytocin was expressed exclusively in the intrinsic enteric afferent neurons, as identified by calbindin 28 K. In the submucosal plexuses, oxytocin was expressed in non-cholinergic secretomotor neurons, as identified by vasoactive intestinal polypeptide. Oxytocin-ir nerve fibers in the inner circular muscle layer possibly arose from the myenteric oxytocin-ir neurons, and oxytocin-ir nerve fibers in the mucosa possibly arose from both the myenteric and submucosal oxytocin-ir neurons. Thus, oxytocin in the digestive tract might be involved in gastrointestinal tract motility mainly via the regulation of the inner circular muscle and the balance of the absorption and secretion of water and electrolytes.     

The absorption of insulin from various regions of the rat intestine

The absorption of intact, biologically active insulin from the ileum, or the ascending colon was measured by the resulting changes in blood glucose concentration. One hour after injection of the ascending colon with a 1 ml volume containing 12 u insulin and 2 mg DOC the blood glucose level was reduced to 50% of the initial value, i.e. 31±2.0 mg%.When insulin was injected directly into the lumen of the ileum, the addition of 3 mg soybean trypsin inhibitor boosted the insulin effect. Direct injection of the ileum with 12 u insulin and 3 mg soybean trypsin inhibitor resulted in a significant drop in blood glucose: 69±5.0 and 85±8.1% of the initial concentration, following 1 and 2 hours, respectively.In the presence of soybean trypsin inhibitor, it was found that the endogenous bile salts in the ileum aid in the absorption of biologically active insulin.     

Na, Cl, and Water Transport by Rat Colon

Segments of the colon of anesthetized rats have been perfused in vivo with isotonic NaCl solutions and isotonic mixtures of NaCl and mannitol. Unidirectional and net fluxes of Na and Cl and the net fluxes of water and mannitol have been measured. Net water transport was found to depend directly on the rate of net Na transport. There was no water absorption from these isotonic solutions in the absence of net solute transport, indicating that water transport in the colon is entirely a passive process. At all NaCl concentrations studied, the lumen was found to be electrically negative to the surface of the colon by 5 to 15 mv. Na fluxes both into and out of the lumen were linear functions of NaCl concentration in the lumen. Net Na absorption from lumen to plasma has been observed to take place against an electrochemical potential gradient indicating that Na is actively transported. This active Na transport has been interpreted in terms of a carrier model system. Cl transport has been found to be due almost entirely to passive diffusion.     

Effect of D ala2 metenkephalinamide on feline jejunal and ileal water and electrolyte transport

The aim of this investigation was to compare the effect of an opioid, D ala2 metenkephalinamide (DAMA), on net jejunal and ileal water and electrolyte fluxes using the gut perfusion technique in the anesthetized cat. Intestinal transport was measured during intravenous infusion of serial doses of 2, 6, and 18 micrograms.kg-1.h-1 of DAMA in 6 cats. Each cat was its own control during an intravenous infusion of 150 mmol/l NaCl preceding the first dose of peptide and following the last dose of DAMA. Both jejunal and ileal segments were isolated by inflated balloons and were studied at the same time. Fifteen ml of an iso-osmolar test solution with hypo-osmolar ion contents and complementary mannitol were administered in the upstream tube and collected 1 h later in the downstream tube. In the jejunum, water secretion was dose-dependently reversed to an absorption from a control value of +0.5 +/- 0.4 to -0.83 +/- 0.5 ml.h-1.10 cm-1; in the ileum, water absorption was increased from -0.5 +/- 0.3 to -1.5 +/- 0.2 ml.h-1.10 cm-1. The net absorption of all electrolytes, ie sodium, chloride, bicarbonate, potassium and calcium also increased during peptide administration. However, a qualitative difference in the ion transport was observed between the jejunum and the ileum.     

Functional changes in the rat distal colon after whole-body irradiation: dose-response and temporal relationships

The aim of this study was to determine the acute radiation response of the rat distal colon by in vivo and in vitro measurements of the functions of the colon over a range of radiation doses. Rats received a whole-body irradiation of 2 to 12 Gy and were studied from 1 to 7 days after exposure. In vivo water and electrolyte absorption was measured by insertion of an agarose cylinder in the colon of anesthetized rats. In vitro transepithelial electrical parameters (potential difference, short-circuit current, transepithelial conductance) were measured in Ussing chambers in basal and agonist-stimulated conditions. In vivo and in vitro functional studies were completed by standard histological analyses. The majority of functional modifications appeared at 4 days after exposure. At this time, a dose-dependent decrease in absorption of water and sodium/chloride ions in the colon was noted. In contrast, a twofold increase in potassium secretion was observed for every radiation dose studied. The response to secretagogues was attenuated at doses >8 Gy. Modifications of basal transepithelial electrical parameters together with marked histological alterations were observed at 4 days with the higher doses (>/=10 Gy). In conclusion, these results show that functions of the colon are affected by irradiation and may contribute to diarrhea induced by ionizing radiation.