Augmentation of neutral sodium chloride absorption by increased flow rate in rat ileum in vivo

Reciprocating intestinal flows enhance glucose uptake in C. elegans

Despite its physiological and pathological importance, the mechanical relationship between glucose uptake in the intestine and intestinal flows is unclear. In the intestine of the nematode Caenorhabditis elegans, the defecation motor program (DMP) causes reciprocating intestinal flows. Although the DMP is frequently activated in the intestines, its physiological function is unknown. We evaluated the mechanical signature of enhanced glucose uptake by the DMP in worms. Glucose uptake tended to increase with increasing flow velocity during the DMP because of mechanical mixing and transport. However, the increase in input energy required for the DMP was low compared with the calorie intake. The findings suggest that animals with gastrointestinal motility exploit the reciprocating intestinal flows caused by peristalsis to promote nutrient absorption by intestinal cells.

Randomized double-blind crossover study to determine the effects of erythromycin on small intestinal nutrient absorption and transit in the critically ill

BACKGROUND

The gastrokinetic drug erythromycin is commonly administered to critically ill patients during intragastric feeding to augment small intestinal nutrient delivery. However, erythromycin has been reported to increase the prevalence of diarrhea, which may reflect reduced absorption and/or accelerated small intestinal transit.

OBJECTIVE

The objective was to evaluate the effects of intravenous erythromycin on small intestinal nutrient absorption and transit in the critically ill.

DESIGN

On consecutive days, erythromycin (200 mg in 20 mL 0.9% saline) or placebo (20 mL 0.9% saline) were infused intravenously between -20 and 0 min in a randomized, blinded, crossover fashion. Between 0 and 30 min, a liquid nutrient containing 3-O-methylglucose (3-OMG), [13C]triolein, and [(99m)Tc]sulfur colloid was administered directly into the small intestine at 2 kcal/min. Serum 3-OMG concentrations and exhaled (13)CO2 (indices of glucose and lipid absorption, respectively) were measured. Cecal arrival of the infused nutrient was determined by scintigraphy. Data are medians (ranges) and were analyzed by using Wilcoxon's signed-rank test.

RESULTS

Thirty-two mechanically ventilated patients were studied. Erythromycin increased small intestinal glucose absorption [3-OMG AUC360: 105.2 (28.9-157.0) for erythromycin compared with 91.8 (51.4-147.9) mmol/L · min for placebo; P = 0.029] but tended to reduce lipid absorption [cumulative percentage dose (13)CO2 recovered: 10.4 (0-90.6) compared with 22.6 (0-100) %; P = 0.06]. A trend to slower transit was observed after erythromycin [300 (39-360) compared with 228 (33-360) min; P = 0.07].

CONCLUSIONS

Acute administration of erythromycin increases small intestinal glucose absorption in the critically ill, but there was a tendency for the drug to reduce small intestinal lipid absorption and slow transit. These observations have implications for the use of erythromycin as a gastrokinetic drug in the critically ill. This trial was registered in the Australian New Zealand Clinical Trials Registry as ACTRN 12610000615088.

Glucose absorption and small intestinal transit in critical illness

OBJECTIVES

Although enteral nutrition is standard care for critically ill patients, nutrient absorption has not been quantified in this group and may be impaired due to intestinal dysmotility. The objectives of this study were to measure small intestinal glucose absorption and duodenocecal transit and determine their relationship with glycemia in the critically ill.

DESIGN

Prospective observational study of healthy and critically ill subjects.

SETTING

Tertiary mixed medical-surgical adult intensive care unit.

SUBJECTS

Twenty-eight critically ill patients and 16 healthy subjects were studied. MATERIALS AND MAIN RESULTS: Liquid feed (100 kcal/100 mL), labeled with Tc-sulfur colloid and including 3 g of 3-O-methylglucose, was infused into the duodenum. Glucose absorption and duodenocecal transit were measured using the area under the 3-O-methylglucose concentration curve and scintigraphy, respectively. Data are median (range).

RESULTS AND DISCUSSION

Glucose absorption was reduced in critical illness when compared to health (area under the concentration curve: 16 [1-32] vs. 20 [14-34] mmol/L·min; p = .03). Small intestinal transit times were comparable in patients and healthy subjects (192 [9-240] vs. 168 [6-240] min; p = .99) and were not related to glucose absorption. Despite higher fasting blood glucose concentrations (6.3 [5.1-9.3] vs. 5.7 [4.6-7.6] mmol/L; p < .05), the increment in blood glucose was sustained for longer in the critically ill (Δ glucose at t = 60; 1.9 [-2.1-5.0] mmol/L vs. -0.2 [-1.3-2.3] mmol/L; p < .01).

CONCLUSIONS

Critical illness is associated with reduced small intestinal glucose absorption, but despite this, the glycemic response to enteral nutrient is sustained for longer.

Analysis of unstirred water layer in in vitro permeability experiments

In vitro permeability experiments are used widely in drug discovery and other areas of pharmaceutical research. Much effort has been expended in developing novel epithelial models but generally much less attention has been paid to the hydrodynamic barrier in the actual experiments. The restricted liquid flow in the vicinity of solid surfaces leads to a zone where the diffusional movement of molecules exceeds the convection. This leads to formation of a concentration gradient between the bulk solution and the surface. The formed unstirred water layer (UWL) reduces the apparent permeability (P(app)) of compounds that rapidly pass through the actual epithelial layer. This lowers the resolution of P(app) versus fraction-absorbed assay, complicates the structure-permeability analysis and skews apparent kinetic parameters of transporter substrates. This review describes the techniques that can be used to determine the UWL thickness in permeability experiments and apparatuses described in the literature to control the in vitro hydrodynamics.

Pressure and frequency dependent linkage between motility and epithelial secretion in human proximal small intestine

BACKGROUND

Motor disturbances are sometimes associated with diarrhoea by unknown mechanisms.

AIM

To determine if there is a quantitative link between intestinal motility and epithelial secretion.

SUBJECTS

Experiments were performed in 21 healthy volunteers and three patients with villus atrophy.

METHODS

Duodenal and jejunal motor activities were registered in the fasted state by open tip manometry. Secretion was measured directly by marker perfusion and indirectly by recording transmural potential difference (PD).

RESULTS

A significant correlation was found between "low pass filtered" pressure and PD, but no correlation was found between amplitudes of isolated contractions and PD changes. During repeated phasic contractions (phase III of migrating motor complex), PD increased at a rate that was higher in the duodenum than in the jejunum, and higher in patients with villus atrophy than in healthy controls. After reaching a peak, PD decreased despite continuing phasic motor activity, provided that there was no concomitant increase in mean pressure. Fluid secretion increased roughly in parallel with PD, except at the very end of the cycle.

CONCLUSIONS

To explain these findings, one has to postulate participation of at least two types of receptor: a slowly adapting pressure sensitive receptor and another mechanoreceptor, possibly a mucosal touch receptor, to account for the run down phenomenon. This model predicts that short lasting trains of contractions, so called discrete clusters, will be a particularly potent stimulus for activation of mucosal secretion.

Myoelectric activity and absorptive capacity of rat small intestinal isografts

The effect of transplantation on small intestinal absorption, digestive capacity, myoelectric activity, and morphology was assessed in inbred Lewis rats. Electrodes were sutured to the duodenum and isografted jejunoileum or to the native jejunoileum in controls. The frequency of migrating myoelectric complexes (MMCs) in the duodenum was 3.3 +/- 0.3/hr in controls and 1.8 +/- 0.4/hr in transplants (P < 0.05). MMC frequency in the jejunoileum was 5.1 +/- 1.3/hr in controls and 3.2 +/- 0.9/hr in transplants (P > 0.05). MMCs appeared to migrate from the duodenum to the jejunoileum 80 +/- 3% of the time in controls and 59 +/- 7% of the time in transplant rats (P < 0.05). Absorption in the transplanted jejunoileum demonstrated a 35-40% decrease in glucose and electrolytes absorption. Villus height and number of nuclei per villus was reduced. Intestinal length (dry) was 103 +/- 6 cm for controls and 51 +/- 3 cm for transplant rats (P < 0.05). Brush border sucrase activity was unchanged. We conclude that small intestinal isografts display similar myoelectric activity as controls, but the decreased absorptive capacity and villus height may require longer segments of intestine to be transplanted in order to support normal nutrition.

Hansenula anomala killer toxin induces secretion and severe acute injury in the rat intestine

BACKGROUND & AIMS

The yeast Hansenula anomala has been associated with gastrointestinal symptomatology and damage to the intestinal wall in humans. In vitro and in vivo, H. anomala secretes a toxin, killer toxin, which is lethal to other microorganisms. In view of the very high rate of killer phenotype expression recorded for H. anomala strains in nature, this study aimed to investigate the hypothesis that H. anomala killer toxin plays a role in the pathogenesis of H. anomala-induced enteritis.

METHODS

Effects of active and heat-inactivated H. anomala killer toxin on intestinal fluid homeostasis and electrolyte balance were investigated in rat small intestine using a standard intestinal perfusion technique. Sections of the perfused jejunum tracts were examined histologically.

RESULTS

H. anomala killer toxin induced a significant secretion of water and electrolytes. No significant change was observed when either heat-inactivated H. anomala killer toxin or control growth medium were tested. Histological analysis showed ischemic degeneration of villi and sloughing of surface epithelium in 50% of active H. anomala killer toxin-perfused jejuna.

CONCLUSIONS

This paper presents original observations compatible with the hypothesis that H. anomala killer toxin plays a role in the pathogenesis of H. anomala-induced enteritis.

Physiological measurements of luminal stirring in the dog and human small bowel

The resistance to absorption resulting from poor stirring of luminal contents (RLum) is considered to be equivalent to an unstirred layer of greater than 600 microns in the human small intestine. We measured RLum in the jejunum of conscious dogs by assessing the absorption rate of two rapidly absorbed probes, glucose, and [14C]warfarin. When RLum was expressed as an unstirred layer, the maximal thickness of the unstirred layer (assuming negligible epithelial cell resistance) was only approximately 35 and 50 microns for perfusion rates of 26 and 5 ml/min, respectively. Maximal unstirred layer thickness for the human jejunum, calculated from previous studies of glucose absorption, yielded a mean value of only 40 microns (range: 23 to 65 microns). Since epithelial resistance appears to be negligible during absorption of low concentrations of glucose, the maximal unstirred layer of 40 microns should be close to the true value for glucose in the human small intestine. We conclude that the unstirred layer for rapidly absorbed compounds in dogs and man are less than one-tenth of previously reported values, but this layer still may remain the rate limiting step in absorption of rapidly transported compounds.

Relationship between distention and absorption in rat intestine. II. Effects of volume and flow rate on transport

Studies in intact animals have shown that intestinal solute absorption may be enhanced with increasing intraluminal volume and flow rate, perhaps because of increases in functional absorptive surface area or perturbation of unstirred layers. We used single-pass perfusions of rat ileum, performed by simultaneously infusing and withdrawing at equal rates, to determine the separate effects of volume and flow rate on solute absorption at pressures between 3.0 and 12.5 cmH2O. Distention enhanced the absorption of passive probes (3H2O, urea), had no effect on the absorption of solutes transported by carrier mechanisms (D-glucose, L-alanine), and led to decreases in the net absorption of sodium and water whenever intraluminal pressure exceeded 10 cmH2O. Increasing flow rate enhanced the absorption of both glucose and 3H2O. However, the effects of increasing flow rate and distention on 3H2O were not additive. In the presence of higher filling volume, faster flow rate led to no further increases in 3H2O absorption; vice versa, at faster flow rate, no further increases in 3H2O absorption were noted when luminal volume was increased. We conclude that increased intraluminal volume enhances the absorption of solutes transported by passive but not carrier-mediated mechanisms, perhaps via augmentation of functional absorptive surface area. Increased flow rate and volume may increase the absorption of passively absorbed probes, in part, by a similar mechanism.

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.