Enhanced glucose absorption in the jejunum of patients with cystic fibrosis

Deconstructing the physical processes of digestion: reductionist approaches may provide greater understanding

I provide a broad overview of the physical factors that govern intestinal digestion i.e. the admixture of food particles in digesta with secreted enzymes and the subsequent mass transfer of liberated nutrients from the surfaces of particles to the gut wall, with a view to outlining the quantitative work that is required to determine the relative importance of these factors in the digestion of particular foods. I first discuss what is known of the mechanical forces generated by contraction of the walls of the various segments of the gut and the level of diffusive, and advective mixing that it generates within the lumen. I then discuss the particular physical effects that may limit the digestion of solid, physically and/or chemically homogenous and heterogeneous food particles, notably capillarity, porosity, poro-elastic flow and compaction and their likely effects on diffusive and convective mass transfer at particulate surfaces. Similarly, I discuss mucins and morphology on mass transfer of nutrients to the gut wall i.e. the mucosa.

A Practical Approach to Glucose Abnormalities in Cystic Fibrosis

Cystic fibrosis is a common genetic condition and abnormal glucose handling leading to cystic fibrosis-related diabetes (CFRD) is a frequent comorbidity. CFRD is mainly thought to be the result of progressive pancreatic damage resulting in beta cell dysfunction and loss of insulin secretion. Whilst Oral Glucose Tolerance Testing is still recommended for diagnosing CFRD, the relationship between glucose abnormalities and adverse outcomes in CF is complex and occurs at stages of dysglycaemia occurring prior to diagnosis of diabetes by World Health Organisation criteria. Insulin remains the mainstay of treatment of CF-related glucose abnormalities but the timing of insulin commencement, optimum insulin regime and targets of glycaemic control are not clear. These complexities are compounded by common issues with nutritional status, need for enteral feeding, steroid use and high disease burden on CF patients.

Morphological and Functional Assessment of Oesophageal Mucosa Integrity in Children With Cystic Fibrosis

OBJECTIVES

The aim of the study was to investigate morphological and functional characteristics of oesophageal epithelial barrier in children with cystic fibrosis (CF) with or without gastro-oesophageal reflux disease (GORD) in comparison to healthy controls.

METHODS

Oesophagogastroduodenoscopy with oesophageal biopsies and combined oesophageal multichannel intraluminal impedance-pH monitoring was performed in 17 children with CF (CFtot) with (CFgord) or without GORD (CFnorm). Histological combined severity score was calculated and widths of spaces between epithelial cells were measured. Basal impedance value was used to assess functional integrity of epithelial barrier. Results of each investigation were compared with a group of children without oesophageal disease.

RESULTS

CFtot, but also CFnorm, had more severe pathohistological changes included in the compound severity score than controls (0.75 ± 0.32 and 0.75 ± 0.20 vs 0.27 ± 0.25; P < 0.001 and P = 0.001, respectively). They also had more dilated intercellular spaces (2.6 μm ± 0.6 and 2.7 μm ± 0.5 vs 1.9 μm ± 0.2; P = 0.001 and P < 0.001, respectively). Baseline impedance values between proximal and distal pairs of electrodes were significantly lower in CFtot (2876 Ω ± 484, 2590 Ω ± 1013) and also in CFnorm (2922 Ω ± 363, 2844 Ω ± 457) than in controls (3703 Ω ± 859, 3753 Ω ± 1070) (P = 0.012 and P = 0.002; and P = 0.027 and P = 0.005, respectively). The treatment of CFgord with proton pump inhibitor increased, but did not normalise the baseline impedance values (2860 Ω ± 560 to 3355 Ω ± 750 and 2178 Ω ± 1564 to 3057 Ω ± 594).

CONCLUSIONS

Children with CF had morphological and functional changes of oesophageal mucosal integrity even in the absence of GORD.

Digestive system dysfunction in cystic fibrosis: challenges for nutrition therapy

Cystic fibrosis can affect food digestion and nutrient absorption. The underlying mutation of the cystic fibrosis trans-membrane regulator gene depletes functional cystic fibrosis trans-membrane regulator on the surface of epithelial cells lining the digestive tract and associated organs, where Cl(-) secretion and subsequently secretion of water and other ions are impaired. This alters pH and dehydrates secretions that precipitate and obstruct the lumen, causing inflammation and the eventual degradation of the pancreas, liver, gallbladder and intestine. Associated conditions include exocrine pancreatic insufficiency, impaired bicarbonate and bile acid secretion and aberrant mucus formation, commonly leading to maldigestion and malabsorption, particularly of fat and fat-soluble vitamins. Pancreatic enzyme replacement therapy is used to address this insufficiency. The susceptibility of pancreatic lipase to acidic and enzymatic inactivation and decreased bile availability often impedes its efficacy. Brush border digestive enzyme activity and intestinal uptake of certain disaccharides and amino acids await clarification. Other complications that may contribute to maldigestion/malabsorption include small intestine bacterial overgrowth, enteric circular muscle dysfunction, abnormal intestinal mucus, and intestinal inflammation. However, there is some evidence that gastric digestive enzymes, colonic microflora, correction of fatty acid abnormalities using dietary n-3 polyunsaturated fatty acid supplementation and emerging intestinal biomarkers can complement nutrition management in cystic fibrosis.

Status of fluid and electrolyte absorption in cystic fibrosis

Salt and fluid absorption is a shared function of many of the body's epithelia, but its use is highly adapted to the varied physiological roles of epithelia-lined organs. These functions vary from control of hydration of outward-facing epithelial surfaces to conservation and regulation of total body volume. In the most general context, salt and fluid absorption is driven by active Na(+) absorption. Cl(-) is absorbed passively through various available paths in response to the electrical driving force that results from active Na(+) absorption. Absorption of salt creates a concentration gradient that causes water to be absorbed passively, provided the epithelium is water permeable. Key differences notwithstanding, the transport elements used for salt and fluid absorption are broadly similar in diverse epithelia, but the regulation of these elements enables salt absorption to be tailored to very different physiological needs. Here we focus on salt absorption by exocrine glands and airway epithelia. In cystic fibrosis, salt and fluid absorption by gland duct epithelia is effectively prevented by the loss of cystic fibrosis transmembrane conductance regulator (CFTR). In airway epithelia, salt and fluid absorption persists, in the absence of CFTR-mediated Cl(-) secretion. The contrast of these tissue-specific changes in CF tissues is illustrative of how salt and fluid absorption is differentially regulated to accomplish tissue-specific physiological objectives.

Cystic fibrosis-related diabetes in children--gaps in the evidence?

As the life span of patients with cystic fibrosis has increased, so has the prevalence of cystic fibrosis-related diabetes mellitus. However, screening practices for cystic fibrosis-related diabetes mellitus vary widely, which affects accurate estimates of the health burden of this comorbidity. The management of prediabetes and hyperglycemia is an increasingly important aspect of care in patients with cystic fibrosis, but few studies have specifically addressed the management of cystic fibrosis-related diabetes mellitus. Previous studies support the use of insulin for the treatment of patients with this disorder, but the evidence for its use in patients with cystic fibrosis and impaired glucose tolerance is poor. Nutritional management is currently guided by dietary recommendations for individuals with cystic fibrosis, with little evidence specific to the dietary management of patients with cystic fibrosis-related diabetes mellitus. Additionally, microvascular complications have become more frequent as a result of the rise in life expectancy of these patients, yet to date no intervention studies have addressed prevention or management of diabetic complications in patients with cystic fibrosis. A strong evidence base is needed to guide the management of patients with cystic fibrosis-related diabetes mellitus and its complications.

A multiscale lattice Boltzmann model of macro- to micro-scale transport, with applications to gut function

Nutrient absorption in the small intestine cannot occur until molecules are presented to the epithelial cells that line intestinal villi, finger-like protrusions under enteric control. Using a two-dimensional multiscale lattice Boltzmann model of a lid-driven cavity flow with 'villi' at the lower surface, we analyse the hypothesis that muscle-induced oscillatory motions of the villi generate a controlled 'micro-mixing layer' (MML) that couples with the macro-scale flow to enhance absorption. Nutrient molecules are modelled as passive scalar concentrations at high Schmidt number. Molecular concentration supplied at the cavity lid is advected to the lower surface by a lid-driven macro-scale eddy. We find that micro-scale eddying motions enhance the macro-scale advective flux by creating an MML that couples with the macro-scale flow to increase absorption rate. We show that the MML is modulated by its interactions with the outer flow through a diffusion-dominated layer that separates advection-dominated macro-scale and micro-scale mixed layers. The structure and strength of the MML is sensitive to villus length and oscillation frequency. Our model suggests that the classical explanation for the existence of villi--increased absorptive surface area--is probably incorrect. The model provides support for the potential importance of villus motility in the absorptive function of the small intestine.

Small intestinal glucose absorption in cystic fibrosis: a study in human and transgenic ΔF508 cystic fibrosis mouse tissues

Intestinal transport is disturbed in cystic fibrosis (CF), with both defective Cl− secretion and changes in absorption being reported. We have examined the effects of the disease on Na+-dependent glucose absorption by the small intestine. Active glucose absorption was monitored as changes in short-circuit current (SCC) in intact and stripped intestinal sheets from normal (Swiss) and transgenic CF (Cftrtm 1 Eur and Cftrtm 2 Cam) mice with the ΔF508 mutation, and in jejunal biopsies from children with CF and normal controls. Na+-dependent glucose uptake at the luminal membrane was measured in brush-border membrane vesicles (BBMVs). Intact and stripped sheets of jejunum and midintestine from Swiss mice exhibited a concentration-dependent increase in SCC with glucose. Apparent Km values were similar in the two preparations, but the apparent Vmax was greater in stripped sheets. This difference was not due to a loss of neural activity in stripped sheets as tetrodotoxin did not influence the glucose-induced SCC in intact sheets. Similar results were observed in stripped sheets of jejunum and mid-intestine from wild-type Cftrtm 1 Eur mice, but in tissues from CF mice the apparent Vmax value was reduced significantly. A lower Vmax was also obtained in intact sheets of mid-intestine from CF (Cftrtm 2 Cam) mice. Jejunal biopsies from CF patients however, exhibited an enhanced glucose-dependent rise in SCC. Na+-dependent uptake by BBMVs from CF (Cftrtm 1 Eur) mice was not reduced compared with wild-type and Swiss BBMVs. It was concluded that, in contrast to human intestine, intestinal glucose absorption was reduced in transgenic mouse models of CF with the ΔF508 mutation, but that this could not be detected in an isolated preparation of brush-border membranes. Transgenic mouse models of CF may not accurately reflect all aspects of intestinal dysfunction in the human disease.

Dislipidemia relacionada à fibrose cística

Esse artigo tem por objetivo rever o conhecimento atual sobre a fisiopatologia, o diagnóstico e a abordagem da dislipidemia relacionada à fibrose cística (DFC). A pesquisa bibliográfica utilizou os bancos de dados Medline e Literatura Latino-Americana e do Caribe em Ciências da Saúde (1987-2007), selecionando os artigos mais relevantes sobre o tema. A DFC é caracterizada por hipertrigliceridemia e/ou hipocolesterolemia e deficiência de ácidos graxos essenciais. Seus principais fatores de risco são: insuficiência pancreática, dieta rica em carboidratos, hepatopatias, estado inflamatório e corticoterapia. Não existem recomendações específicas sobre a triagem, que habitualmente é realizada a partir do diagnóstico e, em intervalos regulares, com maior freqüência, nos indivíduos pertencentes aos grupos de risco. O tratamento inclui: dieta balanceada, reposição de micronutrientes, vitaminas e fibras, além de exercício físico regular de acordo com a tolerância individual. Na grande maioria dos casos, a hipertrigliceridemia da DFC não atinge valores que indiquem o uso de hipolipemiantes. Conclui-se que existem poucos trabalhos na literatura sobre a freqüência, etiologia e manejo da DFC. A recomendações preventivas e terapêuticas para a hipertrigliceridemia são extrapoladas de diretrizes para indivíduos sem fibrose cística. Mais pesquisas são necessárias para investigar a associação da deficiência de ácidos graxos essenciais com a fisiopatologia da fibrose cística. Como a hipertrigliceridemia é um importante fator de risco para doença arterial coronariana, estudos prospectivos irão contribuir para o melhor entendimento da história natural dessa complicação bem como definir maneiras de preveni-la e tratá-la.

Risk factors for small bowel bacterial overgrowth in cystic fibrosis

OBJECTIVES

The purpose of this study was to determine the prevalence of small bowel bacterial overgrowth in patients with pancreatic-insufficient cystic fibrosis (CF) compared with age-matched controls and to identify potential risk factors for small bowel bacterial overgrowth.

PATIENTS AND METHODS

Fifty patients, 25 pancreatic-insufficient CF study patients (mean age, 17 y) and 25 gastrointestinal clinic control patients (mean age, 15 y), completed a glucose-hydrogen breath test after an overnight fast. Study patients completed a quality-of-life questionnaire modified from the Cystic Fibrosis Questionnaire. The medical history of each patient was compared with breath test results. A positive breath test was defined as a fasting hydrogen > or =15 ppm or a rise of > or =10 ppm hydrogen over baseline during the test.

RESULTS

The prevalence of positive breath tests was higher in the CF study group (56%) than in the control group (20%) (P = 0.02). The mean fasting hydrogen levels of patients in the study and control groups were 22 and 5 ppm (P = 0.0001). The mean questionnaire scores were not significantly different between breath test-positive and -negative study patients. The use of azithromycin was associated with an increased risk of a positive breath test. Use of laxatives and inhaled ipratropium was associated with a decreased risk of a positive breath test.

CONCLUSIONS

Patients with CF were more likely to have elevated fasting hydrogen levels compared with controls. This suggests a high prevalence of small bowel bacterial overgrowth in CF patients. Medications commonly used by CF patients may influence intestinal health.

Abnormal intracellular lipid processing contributes to fat malabsorption in cystic fibrosis patients

A common feature of cystic fibrosis (CF) is the functional derangement of the exocrine pancreas, which affects output of pancreatic lipase. This condition results in severe dietary malabsorption due to the poor hydrolysis of triacylglycerol (TG) in the lumen of the small intestine. Despite the benefits of pancreatic enzyme supplements, patients with CF present with persistent intestinal fat malabsorption. The aim of the present investigation was to determine whether defects in the intracellular phase of lipid transport occur in this pathophysiology in addition to the known disturbed digestive processes. Our hypothesis was tested by incubating intestinal biopsies from six CF and six healthy subjects with radiolabeled lipid and protein precursors. Lipid esterification and secretion were markedly decreased by 22-31% and 38-42%, respectively, in CF samples, as noted by the low incorporation of [(14)C]palmitic acid into TGs, phospholipids, and cholesteryl esters in patients' duodenal explants and culture media compared with controls (100%). Accordingly, the output of TG-rich lipoproteins was substantially reduced (P < 0.05), and a similar trend was observed for high-density lipoproteins. Because intestinal lipoprotein assembly/secretion shows an absolute requirement for apolipoprotein (apo) B-48, radioactive labeling experiments were performed; these experiments demonstrated a significantly (P < 0.05) diminished synthesis of apoB-48 (40%) and apoA-I (30%). Given the critical role of microsomal triglyceride transfer protein in the formation of apoB-containing lipoproteins, its activity was determined and not found to be altered in CF intestinal tissue. Together, these results suggest that CF malabsorption may also be caused by defects in mucosal mechanisms leading to abnormal lipoprotein delivery into the blood circulation.

Abnormal passive chloride absorption in cystic fibrosis jejunum functionally opposes the classic chloride secretory defect

Due to genetic defects in apical membrane chloride channels, the cystic fibrosis (CF) intestine does not secrete chloride normally. Depressed chloride secretion leaves CF intestinal absorptive processes unopposed, which results in net fluid hyperabsorption, dehydration of intestinal contents, and a propensity to inspissated intestinal obstruction. This theory is based primarily on in vitro studies of jejunal mucosa. To determine if CF patients actually hyperabsorb fluid in vivo, we measured electrolyte and water absorption during steady-state perfusion of the jejunum. As expected, chloride secretion was abnormally low in CF, but surprisingly, there was no net hyperabsorption of sodium or water during perfusion of a balanced electrolyte solution. This suggested that fluid absorption processes are reduced in CF jejunum, and further studies revealed that this was due to a marked depression of passive chloride absorption. Although Na+-glucose cotransport was normal in the CF jejunum, absence of passive chloride absorption completely blocked glucose-stimulated net sodium absorption and reduced glucose-stimulated water absorption 66%. This chloride absorptive abnormality acts in physiological opposition to the classic chloride secretory defect in the CF intestine. By increasing the fluidity of intraluminal contents, absence of passive chloride absorption may reduce the incidence and severity of intestinal disease in patients with CF.

Abnormal lipid concentrations in cystic fibrosis

BACKGROUND

Concentrations of cholesterol and triacylglycerol are commonly believed to be low in persons with cystic fibrosis and thus not of concern.

OBJECTIVE

The goal was to determine whether concentrations of cholesterol and triacylglycerol are related to glucose tolerance or nutritional status in patients with cystic fibrosis.

DESIGN

Fasting lipid profiles were measured in 192 patients ( +/- SD age: 21 +/- 11 y) in conjunction with an oral-glucose-tolerance test.

RESULTS

Cystic fibrosis patients in all age groups had higher triacylglycerol (1.51 +/- 0.95 mmol/L) and lower cholesterol (3.57 +/- 0.96 mmol/L) concentrations than US population means. Thirty patients (16%) had hypertriglyceridemia (3.22 +/- 1.22 mmol/L), and 8 patients (4%) had elevated cholesterol (6.05 +/- 1.32 mmol/L). In most cases, hypertriglyceridemia was isolated; only 3 subjects had elevation of both cholesterol and triacylglycerol. Lipid concentrations were not related to body mass index, weight, glucose tolerance, the areas under the curve for glucose or insulin, or glycated hemoglobin. Lipid concentrations also did not correlate with cystic fibrosis genotype, use of systemic steroids, blood pressure, liver enzymes, C-reactive protein, or pulmonary function.

CONCLUSIONS

Isolated hypertriglyceridemia appears to be common in cystic fibrosis, whereas cholesterol concentrations are generally low. Hypertriglyceridemia may be related to chronic low-grade inflammation or to a dietary macronutrient imbalance with excessive simple carbohydrate absorption relative to fat absorption. Whether it is associated with a risk of cardiovascular disease in this population is uncertain, but the clinical significance of triacylglycerol elevation may become important as survival improves.

Ileal mucosal bile acid absorption is increased in Cftr knockout mice

BACKGROUND

Excessive loss of bile acids in stool has been reported in patients with cystic fibrosis. Some data suggest that a defect in mucosal bile acid transport may be the mechanism of bile acid malabsorption in these individuals. However, the molecular basis of this defect is unknown. This study examines the expression of the ileal bile acid transporter protein (IBAT) and rates of diffusional (sodium independent) and active (sodium dependent) uptake of the radiolabeled bile acid taurocholate in mice with targeted disruption of the cftr gene.

METHODS

Wild-type, heterozygous cftr (+/-) and homozygous cftr (-/-) mice were studied. Five one-cm segments of terminal ileum were excised, everted and mounted onto thin stainless steel rods and incubated in buffer containing tracer 3H-taurocholate. Simultaneously, adjacent segments of terminal ileum were taken and processed for immunohistochemistry and Western blots using an antibody against the IBAT protein.

RESULTS

In all ileal segments, taurocholate uptake rates were fourfold higher in cftr (-/-) and two-fold higher in cftr (+/-) mice compared to wild-type mice. Passive uptake was not significantly higher in cftr (-/-) mice than in controls. IBAT protein was comparably increased. Immuno-staining revealed that the greatest increases occurred in the crypts of cftr (-/-) animals.

CONCLUSIONS

In the ileum, IBAT protein densities and taurocholate uptake rates are elevated in cftr (-/-) mice > cftr (+/-) > wild-type mice. These findings indicate that bile acid malabsorption in cystic fibrosis is not caused by a decrease in IBAT activity at the brush border. Alternative mechanisms are proposed, such as impaired bile acid uptake caused by the thick mucus barrier in the distal small bowel, coupled with a direct negative regulatory role for cftr in IBAT function.

Diabetes mellitus and profound insulin resistance in Johanson-Blizzard syndrome

Pancreatic exocrine insufficiency in Johanson-Blizzard syndrome (JBS) is well described but only two previous patient reports document pancreatic endocrine insufficiency manifested as diabetes mellitus, and each patient required only a modest dose of insulin to control hyperglycemia. We report a patient with JBS and new-onset diabetes mellitus with profound insulin resistance, with no clinical or laboratory evidence of pancreatic exocrine insufficiency.

Pancreatic exocrine-endocrine interrelationship. Clinical implications

The exocrine and endocrine pancreata are anatomically and functionally interrelated. As a result, exocrine pancreatic dysfunction often accompanies endocrine pancreatic dysfunction and vice versa. This article delineates the nature of these exocrine and endocrine pancreatic interrelationships and their clinical significance in the management of pancreatic disorders.

Physiological basis of cystic fibrosis: a historical perspective

Physiological Basis of Cystic Fibrosis: A Historical Perspective. Physiol. Rev. 79, Suppl.: S3-S22, 1999. - Cystic fibrosis made a relatively late entry into medical physiology, although references to conditions probably reflecting the disease can be traced back well into the Middle Ages. This review begins with the origins of recognition of the symptoms of this genetic disease and proceeds to briefly review the early period of basic research into its cause. It then presents the two apparently distinct faces of cystic fibrosis: 1) as that of a mucus abnormality and 2) as that of defects in electrolyte transport. It considers principal findings of the organ and cell pathophysiology as well as some of the apparent conflicts and enigmas still current in understanding the disease process. It is written from the perspective of the author, whose career spans back to much of the initial endeavors to explain this fatal mutation.

Pathophysiology of gene-targeted mouse models for cystic fibrosis

Pathophysiology of Gene-Targeted Mouse Models for Cystic Fibrosis. Physiol. Rev. 79, Suppl.: S193-S214, 1999. - Mutations in the gene causing the fatal disease cystic fibrosis (CF) result in abnormal transport of several ions across a number of epithelial tissues. In just 3 years after this gene was cloned, the first CF mouse models were generated. The CF mouse models generated to date have provided a wealth of information on the pathophysiology of the disease in a variety of organs. Heterogeneity of disease in the mouse models is due to the variety of gene-targeting strategies used in the generation of the CF mouse models as well as the diversity of the murine genetic background. This paper reviews the pathophysiology in the tissues and organs (gastrointestinal, airway, hepatobiliary, pancreas, reproductive, and salivary tissue) involved in the disease in the various CF mouse models. Marked similarities to and differences from the human disease have been observed in the various murine models. Some of the CF mouse models accurately reflect the ion-transport abnormalities and disease phenotype seen in human CF patients, especially in gastrointestinal tissue. However, alterations in airway ion transport, which lead to the devastating lung disease in CF patients, appear to be largely absent in the CF mouse models. Reasons for these unexpected findings are discussed. This paper also reviews pharmacotherapeutic and gene therapeutic studies in the various mouse models.

Interrelationship between the Na+/glucose cotransporter and CFTR in Caco-2 cells: relevance to cystic fibrosis

Both the Na+-dependent glucose cotransporter (SGLT1) and the cystic fibrosis transmembrane conductance regulator (CFTR) modulate Na+ and fluid movement, although in opposite directions. Yet few studies have investigated a possible interrelationship between these two transporters. By using the Caco-2 human colon carcinoma cell line, we confirmed that the activities of these transporters increased with spontaneous differentiation to the enterocytic phenotype. We showed that SGLT1 was positively regulated by Cl- and that optimal activity of CFTR was dependent on the presence of glucose. We also demonstrated that inhibition of CFTR by glibenclamide or diphenylamine-2-carboxylate did not modify the activity of SGLT1 and inhibition of SGLT1 by phlorizin did not modify the activity of CFTR, although it resulted in inhibition of glycoconjugate synthesis. These results point to positive substrate-cross regulation of SGLT1 and CFTR and suggest that NaCl and glucose are important for not only Na+ absorption and fluid movement, but also for cAMP-dependent Cl- efflux, and glycoconjugate synthesis, functions that are known to be anomalous in cystic fibrosis.

Ion transport across the murine intestine in the absence and presence of CFTR

CF mice, i.e., mice without functional CFTR (cystic fibrosis transmembrane conductance regulator) exhibit a very low basal Isc in all regions of the intestinal tract. The low basal Isc in the intestinal epithelia of the CF mice appears to be a result of lack of spontaneous Cl- secretion (and possibly HCO3- secretion) mediated by neurotransmitter release from the enteric nervous system. In contrast to intestinal epithelia from normal mice, the intestinal epithelia of CF mice do not secrete Cl- in response to agents that increase cAMP (forskolin). Furthermore, as in human CF patients, agents that increase intracellular Ca2+ (bethanacol, ionomycin) failed to elicit Cl- secretion in the intestinal epithelia of CF mice. There was no difference in the electrogenic Na(+)-coupled glucose absorption in the CF murine jejuna compared to jejuna from normal mice. However, further studies are warranted to determine whether amiloride-sensitive Na+ absorption is upregulated in the murine CF colon. It was concluded that the intestinal epithelium of the CF mouse model exhibits some striking similarities to its human counterpart, and therefore should be very useful in further characterizing the ion transport defects in this disease.

Intestinal permeability in cystic fibrosis in relation to genotype

BACKGROUND

The purpose of this study was to investigate whether the increase intestinal permeability (IP) seen in patients with cystic fibrosis (CF) is correlated with the basic defect, as revealed by the patient's genotype, and/or whether the intestinal disturbance reflects secondary abnormalities such as essential fatty acid deficiency.

METHODS

Nineteen CF patients were compared with nine age- and sex-matched healthy controls. IP was evaluated by studying urinary excretion for 5 hours after a test meal containing lactulose, L-rhamnose and xylose in water. Urine was analyzed for carbohydrates, and blood samples were taken for determination of the fatty acid pattern of serum phospholipids. The CF patients were grouped according to genotype: homozygous for delta F508, heterozygous for alpha F508, or unidentified.

RESULTS

Patients who were homozygous (n = 9) or heterzygous (n = 6) for delta F508 had significantly higher lactulose/L-rhamnose excretion ratios (mean(range) values of 0.08(0.05-0.13) and 0.09(0.03-0.13), respectively) than patients (n = 4) with unidentified genotypes [0.03(0.02-0.05); p = 0.005] or healthy controls [0.02(0.003-0.06); p = 0.002]. CF patients with EFAD (n = 6) did not differ from those with a normal pattern of serum phospholipid fatty acids, the lactulose/L-rhamnose excretion ratio being 0.08(0.02-0.13) and 0.07(0.03-0.12), respectively.

CONCLUSIONS

These data show that the IP in CF was related to patient genotype; those homozygozous or heterozygous for delta F508 having a significantly increased IP compared with patients with unidentified genotypes, who had IP values within the normal range.

Gastrointestinal handling of [1-13C]palmitic acid in healthy controls and patients with cystic fibrosis

AIM

To examine the gastrointestinal handling of [1-13C]palmitic acid given as the free acid by measuring the excretion of 13C label in stool in 16 healthy children and 11 patients with cystic fibrosis on their habitual enzyme replacement treatment.

METHODS

After an overnight fast, each child ingested 10 mg/kg body weight [1-13C]palmitic acid with a standardised test meal of low natural 13C abundance. A stool sample was collected before the test and all stools were collected thereafter for a period of up to five days. The total enrichment of 13C in stool and the species bearing the 13C label was measured using isotope ratio mass spectrometry.

RESULTS

The proportion of administered 13C label excreted in stool was 24.0% (range 10.7-64.9%) in healthy children and only 4.4% (range 1.2-11.6%) in cystic fibrosis patients. The enrichment of 13C in stool was primarily restricted to the species consumed by the subjects (that is as palmitic acid).

CONCLUSIONS

There does not appear to be a specific defect in the absorption of [1-13C]palmitic acid in patients with cystic fibrosis. The reasons why cystic fibrosis patients appear to absorb more of this saturated fatty acid than healthy children is not clear and requires further investigation.

Fecal polyamine concentration in children with and without nutrient malabsorption

BACKGROUND

Fermentation products of malabsorbed nutrients are thought to be responsible for intestinal adaptation following small bowel resection in rats. It has been suggested that either short-chain fatty acids or polyamines (mainly putrescine and cadaverine) could be the fermentation products involved. There are no data available on fecal polyamine content in humans. The present study compared the fecal polyamine concentrations in children with and without malabsorption.

METHODS

Sixteen (8 girls, 8 boys) malabsorption patients (cystic fibrosis: 13, short bowel syndrome: 2, biliary atresia: 1) with a mean age of 8 years were compared to 17 (9 girls, 8 boys) sick children without malabsorption (mean age 5.7 years). Three-day fecal collections were performed and analyzed for fat and polyamine concentrations. High-performance liquid chromatography (HPLC) was used for the measurement of polyamine concentrations.

RESULTS

Mean and SEM for fecal fat excretion was 13.4 +/- 2.5 g/day and 1.5 +/- 0.3 g/day in the malabsorption and control group respectively. Median fecal cadaverine and putrescine concentrations were 3723 mumol.kg-1 feces and 4737 mumol.kg-1 feces for the malabsorption group and 114 mumol.kg-1 feces and 306 mumol.kg-1 feces for the control group (p < 0.007 and < 0.00001 respectively). No significant differences were found for fecal spermine and spermidine concentrations between the two groups.

CONCLUSIONS

Children with malabsorption show very high fecal putrescine and cadaverine concentrations. Our results support the hypothesis that fecal polyamines could be important.

In vivo measurement of chloride and water secretion in the jejunum of cystic fibrosis patients

In the present study, we have investigated the possible consequences of the chloride channel defect in the intestine of cystic fibrosis (CF) patients for electrolyte and water transport in the jejunum in vivo, using a multilumen, double occluding balloon catheter, and an Ag/AgCl intraluminal electrode. During a chloride-free perfusion, to optimize the sensitivity of our measurements, the transmural potential difference (PD) (lumen with reference to serosal side) was found to be significantly higher in the jejunum of CF patients (+8.0 +/- 2.1 mV; n = 5) than in healthy control subjects (-2.2 +/- 2.0 mV; n = 9). The chloride concentration measured in chloride-free jejunal perfusates of CF patients was significantly lower than in controls (10.9 +/- 2.3 and 41.4 +/- 8.2 mM, respectively). Possible differences in net chloride and water secretion did not reach statistical significance (chloride secretion controls: -2.1 +/- 0.9 mmol/10 cm/h; CF: -0.8 +/- 0.2 mmol/10 cm/h; water secretion controls: -0.8 +/- 2.5 mL/10 cm/h; CF: -11.7 +/- 8.9 mL/10 cm/h). In control subjects, intraluminally applied theophylline stimulated the secretion of water (delta 23.4 +/- 4.6 mL/10 cm/h) and chloride (delta 4.1 +/- 1.1 mmol/10 cm/h), but not in CF patients (respectively delta 3.6 +/- 3.3 mL/10 cm/h and delta 1.1 +/- 1.1 mmol/10 cm/h). In controls, theophylline caused a significant increase in lumen negativity (PD -10.2 +/- 2.6 mV), but no change could be seen in CF patient transmural PD. These observations provide in vivo evidence for a decreased chloride permeability in the jejunum in CF, resulting in a significant reduction in net electrolyte and water secretion in the presence, but not in the absence, of an intestinal secretagogue.

Use of maltose hydrolysis measurements to characterize the interaction between the aqueous diffusion barrier and the epithelium in the rat jejunum

Rates of intestinal absorption and surface hydrolysis are determined by the interaction of two barriers: poorly stirred fluid adjacent to the mucosa, and the epithelial cell. These two barriers commonly are modeled as a fixed, flat layer of epithelium covered by a fixed thickness of unstirred fluid. To more accurately simulate these barriers in a villous mucosa, maltase activity (measured in vitro) was distributed over an anatomically correct model of rat jejunal villi. We then determined what interaction of the aqueous and epithelial barriers best predicted in vivo maltose hydrolysis rates measured over a broad range of infusate concentrations. Hydrolysis was accurately predicted by a model in which unstirred fluid extended from 20 microm over the villous tips throughout the intervillous space. In this model, the depth of diffusion into the intervillous space is inversely proportional to the efficiency of epithelial handling of the solute. As a result, both the aqueous barrier and the functional surface area are variables rather than constants. Some implications of our findings (relative to the conventional model) include: higher predicted Vmax, efficient handling of low concentrations of a solute at the villous tips while high concentrations must penetrate thick aqueous barriers, and sensitive regulation of transport rates via ease of access to the intervillous space.

Tolbutamide causes a modest increase in insulin secretion in cystic fibrosis patients with impaired glucose tolerance

We examined the effect of intravenous (i.v.) tolbutamide administration on glucose and hormone levels in cystic fibrosis (CF) patients with impaired first-phase insulin secretion and oral glucose tolerance (oral glucose tolerance test [OGTT]) and compared them with CF patients with only an impaired first-phase insulin secretion and healthy control subjects. Five CF patients with an impaired OGTT, ie, a serum glucose value of 7.8 mmol/L or greater 120 minutes after an oral glucose load (group I), five CF patients with a normal OGTT, ie, a serum glucose not exceeding 7.8 mmol/L 120 minutes after oral glucose (group II), and five healthy control (CON) subjects underwent IV glucose tolerance tests with glucose alone (IVGTT) and glucose administered in conjunction with tolbutamide ([IVTTT] 25 mg/kg; maximum dose, 1 g). Serum glucose levels were measured using the glucose oxidase method; insulin, C-peptide, and glucagon levels were measured by the double-antibody radioimmunoassay (RIA) technique. Serum immunoreactive trypsin (IRT) and hemoglobin A1 (HbA1) levels and height and weight were measured for each subject, and in addition, pulmonary function was assessed in those with CF. There were no significant differences in the area under the curve (AUC) for glucose or glucose or glucagon levels or the serum glucose disappearance rate (k value) between group I, group II, or CON subjects during the IVGTT. First-phase insulin and C-peptide secretion was abnormal during IVGTT and IVTTT in the CF groups: in group I it was severely impaired, whereas in group II it was between group I and CON values. During the IVTTT serum glucose levels and glucose k values were not significantly altered in any of the three groups as compared with the IVGTT.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid metabolism and substrate oxidation during intravenous fructose administration in cirrhosis

We used isotope dilution techniques (constant intravenous [IV] infusion of 2-3H-glycerol and 1-14C-palmitate) and indirect calorimetry to measure lipid kinetics and substrate oxidation rates during IV fructose administration at 200 and then 500 mg/kg/h in eight cirrhotic patients and seven normal control subjects. Fasting plasma glucose, glycerol, and glycerol appearance rate (Ra) were similar in both groups, but insulin levels were fourfold higher in cirrhotics (P < .01). Fasting serum nonesterified fatty acid (NEFA) levels (cirrhotics, 869 +/- 124, controls, 717 +/- 90 mumol/L) and NEFA Ra (7.1 +/- 0.8 v 5.5 +/- 0.9 mumol/min/kg) were higher in cirrhotics, but the differences were not significant. Plasma fructose was similar in both groups at both fructose infusion rates. Fructose appeared to stimulate insulin secretion. With i.v. fructose, serum NEFA levels decreased, reaching similar low levels when 500 mg/kg/h was infused, due to a reduction in NEFA Ra and an increase in the NEFA metabolic clearance rate (MCR). Glycerol levels showed little change. As glycerol Ra decreased by less than 20% in both groups, the decrease in serum NEFA was primarily due to enhanced reesterification of fatty acids both within adipose tissue (preventing their release) and in other tissues (enhancing their removal from plasma). Although total fructose utilization was normal in cirrhotics, they oxidized more of the infused fructose; nonoxidative disposal was reduced (first step, 242 +/- 12 v 318 +/- 16 mg/kg in 2 hours, P < .002; second step, 657 +/- 32 v 786 +/- 21 mg/kg in 2 hours, P < .005). Although tissue fructose uptake is insulin-independent, insulin resistance in cirrhosis may influence the intracellular metabolism of fructose.

Small-intestinal abnormalities in cystic fibrosis patients

A survey is given of the pathophysiology of the main alterations in the small intestine of cystic fibrosis patients. Special attention is paid to the understanding of meconium ileus in the fetus and the newborn, the repercussion of duodenal acidity on the duodenal mucosa and the intraduodenal digestion, and primary and secondary biochemical alterations in the secretory-digestive-absorptive function of the small-intestinal mucosa. The meconium equivalent syndrome and its connection with the atypical course of intussusception and appendiceal perforation with silent pelvic abscess are also discussed.

A reappraisal of the magnitude and implications of the intestinal unstirred layer

Until recently, a variety of studies had suggested that luminal stirring in the jejunum is relatively poor, with unstirred layers of about 600 microns reported for humans and 300-900 microns for animals. Unstirred layers of this magnitude would markedly retard the absorption of all solutes, and diffusion through this layer would be the rate-limiting step in the uptake of all rapidly absorbed compounds. As a result, luminal stirring, rather than epithelial transport, would be the major variable influencing absorption rate. However, recent studies in dogs and humans have shown that the unstirred layer has a maximal apparent thickness of only about 40 microns. This layer is far thinner than what can be achieved in vitro with vigorous stirring with a magnetic bar, suggesting that some unique stirring mechanism, perhaps villous contractions, is responsible for this extraordinarily efficient mixing. A 40-microns unstirred layer would produce only about 1/15 the resistance of the previously reported 600 microns value; with this thinner layer, alterations in either luminal stirring or epithelial function could readily influence the absorption rate of rapidly transported compounds.

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.

Evaluation of the efficacy of oral rehydration solutions using human whole gut perfusion

Whole gut perfusion in humans was used to compare the effect on intestinal water and electrolyte transport of the World Health Organisation oral rehydration solution (solution II, composition in mmol/l: glucose 111, sodium 90, bicarbonate 30, potassium 20; 308 mOsm/kg); a hypertonic commercial oral rehydration solution (solution III, glucose 188, sodium 50, bicarbonate 20, potassium 20 mmol/l; 335 mOsm/kg); and three experimental bicarbonate free, hypotonic oral rehydration solutions: solution IV (glucose 111, sodium 60, potassium 20 mmol/l; 260 mOsm/kg), solution V (glucose 80, sodium 60, potassium 20 mmol/l; 219 mOsm/kg), and solution VI (glucose 80, sodium 30, potassium 20 mmol/l; 177 mOsm/kg). Perfusion of the intestine with a standard cleansing solution (solution I, sodium 125, potassium 10, bicarbonate 20, sulphate 40, mannitol 80 mmol/l; 275 mOsm/kg) confirmed published data on minimal water and sodium absorption. Experimental solution VI produced maximum water absorption (mean (SE) +1660.0 (29.8) ml/h) significantly greater than solution II (+1195.3 (79.5) ml/h), III (+534.7 (140.3) ml/h), IV (+1498.0 (42.7) ml/h), and V (+1327.7 (24.4) ml/h; p less than 0.05). Sodium absorption was significantly greater with solution II (+97.4 (7.9) mmol/h) compared to VI (+43.3 (7.8) mmol/h; p less than 0.01) but not compared to IV (+67.2 (13.0) mmol/h). A hypotonic oral rehydration solution such as solution VI may provide optimal replacement treatment for patients with acute diarrhoea.

Enhanced intestinal glucose and alanine transport in cystic fibrosis

The rise in short-circuit current associated with the active transport of glucose and alanine was measured in intestinal biopsy samples from children with cystic fibrosis. The glucose-induced increase in the short-circuit current was greater in the cystic fibrosis tissues than in control samples over the whole range of concentrations tested (2.5-35 mM), a reflection of an increased maximum rate of transport. Similar results were obtained with alanine. These findings suggest that active Na(+)-linked nutrient transport is enhanced in cystic fibrosis.

Pharmacokinetics of drugs in cystic fibrosis

Considerable work has been conducted on patients with CF to clearly demonstrate enhanced clearances of many, but not all drugs. The precise mechanisms for enhanced drug clearance in CF remain to be elucidated, and it will be important to examine the two major organs of drug elimination, the kidney and liver, for answers to these questions. Although it has not been studied to date, the role of the lung as a contributor to enhanced drug clearance in CF needs to be considered as well. In the liver it appears that both hepatic biotransformation as well as biliary excretion may be enhanced. In the kidney, there is enhanced CL of acidic drugs, but variable findings for cationic and zwitterionic drugs. The latter finding can be explained on the basis of the pKa of the drugs and the urine pH. It would be most appealing if enhanced renal and nonrenal clearance of drugs in CF could be attributed to a common single defect. At this time, there is no obvious candidate, although altered transmembrane transport would represent a place to start.

Abnormal jejunal potential difference in cystic fibrosis

Transmucosal potential difference (PD) and intraluminal pressure were recorded from the same jejunal site in 15 healthy adult controls and 4 adults with cystic fibrosis. In the controls, runs of contractions were associated with wave-like changes in PD with, during phase III-like activity, a mean peak amplitude of -9.7 mV. Although there were no obvious differences in motor patterns, wave-like changes in PD were absent in patients with cystic fibrosis. Intraluminal boluses of 4 mg pilocarpine, or 0.1 mg prostaglandin E2, caused changes of -4.6 mV and -4.5 mV, respectively, in controls; these responses were not seen in patients with cystic fibrosis. There were no significant differences in basal PD and PD changes caused by altered concentrations of infused saline or glucose between patients and controls.

Duodenal instillation of pancreatin does not abolish steatorrhea in patients with pancreatic insufficiency

Many believe that intragastric deactivation of lipase accounts for the frequent failure of orally ingested pancreatic enzymes to normalize fat absorption in patients with pancreatic insufficiency. To test this hypothesis, we measured fat absorption from a large test meal in six patients with pancreatic insufficiency after we had instilled Viokase directly into the postcibal duodenum in two doses, one to deliver lipase at about 10% of normal secretory rates and the other at four times this rate. Direct duodenal instillation of neither the low nor the high dose of Viokase, nor the low dose of Viokase plus sodium bicarbonate, normalized fat absorption from the test meal; none of these duodenal instillations significantly improved fat absorption over that after the test meal plus orally ingested Viokase. Despite these various treatments, the patients excreted an average of 25.5 g of dietary fat as opposed to 2.1 g excreted by six normal subjects after the same meal. We conclude that more than just intragastric destruction of lipase underlies the frequent failure of orally ingested pancreatin to normalize fat absorption in pancreatic insufficiency.

Clinical pharmacology of antibiotics and other drugs in cystic fibrosis

The disposition of many drugs in cystic fibrosis is abnormal. In general, changes in pharmacokinetics include: increased volume of distribution, decreased plasma concentration, and enhanced renal and sometimes non-renal elimination of drugs. Pathophysiology of the disease important for drug disposition includes: (a) hypersecretion of gastric acid and duodenal secretions which are of small volume, viscous and low in bicarbonate; (b) increased intestinal permeability to some sugars and probe substances; (c) hypergammaglobulinaemia and sometimes hypoalbuminaemia; (d) significant elevation of free fatty palmitoleic acid level and decreased low-density and high-density serum lipoproteins; (e) an average increase by 30 to 45% in plasma volume in patients with cystic fibrosis who have moderately severe pulmonary disease, right ventricle hypertrophy and dilatation, which occurs in 15 to 35% of patients with a Shwachman score of 81 to 100; (f) abnormal bile acid metabolism and enterohepatic recirculation; and (g) enlarged kidneys and glomerulomegaly with increased glomerular filtration rate, tubular clearance and urine flow rate in some patients with cystic fibrosis. Delayed absorption from the gastrointestinal tract has been reported in patients with cystic fibrosis for cloxacillin, epicillin, clindamycin, ciprofloxacin and probably for cephalexin, para-aminobenzoic acid and chloramphenicol. A possible increased absorption was reported for cimetidine. Of 7 drugs studied only theophylline had significantly decreased plasma protein binding. An increased volume of distribution and increased renal clearance reported for several drugs is caused mainly by increases in plasma volume and urine flow rate in many of these patients. Possible increased elimination of some drugs in bile (which probably results from bile acid malabsorption) and in bronchial secretions (which are abundant in some cystic fibrosis patients with acute pulmonary infection) may explain enhanced non-renal elimination of these drugs. The metabolism of cimetidine in cystic fibrosis was reported not to be changed significantly compared to control subjects.

Effect of acute Yersinia enterocolitica infection on in vivo and in vitro small intestinal solute and fluid absorption in the rabbit

The impact of acute Yersinia enterocolitica infection on jejunal and ileal solute and water transport was examined. New Zealand White rabbits (500-600 g) were infected with Y. enterocolitica and compared with unmanipulated controls and pair-fed controls. Transport studies were performed 6 days after infection on jejunum and ileum by an in vivo single-pass perfusion technique and in vitro under short-circuited conditions in Ussing chambers. When studied in vivo, Y. enterocolitica infection resulted in decreased water and electrolyte absorption in the jejunum under basal conditions and in both jejunum and ileum in the presence of glucose. Glucose absorption was also decreased in jejunum and ileum. When studied in vitro, net basal Na+ and Cl- transport was not altered in jejunal or ileal epithelium from infected animals. Glucose-stimulated Na+ absorption was decreased in ileal epithelium, and absorption of 3-O-methyl-D-glucose was decreased in both jejunum and ileum. Secretory capacity of Cl-, as assessed by isobutylmethyl xanthine stimulation, was unimpaired in both jejunum and ileum. Decreased food intake alone, in the pair-fed animals, had little effect on intestinal transport. The results indicate that malabsorption, rather than active intestinal secretion, is the major small intestinal transport defect in acute Y. enterocolitica infection. Furthermore, the abnormalities can be directly attributed to injury induced by the organism itself, rather than malnutrition.

Intestinal malabsorption in cystic fibrosis

An oral solution containing lactulose and L-rhamnose was administered to six patients with cystic fibrosis. Intestinal absorption of both sugars was assessed by measurement of their urinary excretion. The lactulose: L-rhamnose excretion ratio was raised, supporting the hypothesis of an intestinal component to the malabsorption of cystic fibrosis.

Absent postprandial duodenal motility in a child with cystic fibrosis. Correction of the symptoms and manometric abnormality with cisapride

A 19-mo-old boy with cystic fibrosis presented with a lifelong history of feeding problems and constipation, and an 8-mo history of episodes of repeated retching, diaphoresis, dehydration, and somnolence after eating. Tests of esophageal motility and gastric emptying of a 5% glucose meal were normal. Antroduodenal pressure recordings during fasting demonstrated the presence of all phases of the interdigestive motor complex. After consumption of a 240-ml complex liquid meal, however, the contractile pattern that generally accompanies eating was absent and gastric emptying was markedly delayed. When bethanechol or metoclopramide was given 10 min before the complex liquid meal, there was a paucity of contractile activity, gastric emptying was slow, and symptoms of lethargy, diaphoresis, and retching were present. When cisapride was given, there was frequent irregular contractile activity, faster gastric emptying, and no symptoms of lethargy. During the past year treatment with cisapride has been a requirement in order to prevent recurrence of the symptoms. Antroduodenal pressure studies proved helpful in the identification of a treatable manometric abnormality that was associated with symptoms of delayed gastric emptying.

Human intestinal motor activity and transport: effects of a synthetic opiate

Effects of opiates on intestinal motor activity and transport of water and electrolytes have been studied separately in previous investigations. The aim of these experiments was to evaluate simultaneously the effects of a synthetic opiate, loperamide, on motor activity and transport in the human intestine. Jejunal, ileal, and colonic perfusions were performed in 9 healthy volunteers. After application of loperamide (12 mg), cyclically recurring migrating motor complexes in the small intestine occurred at a significantly higher frequency than after application of placebo. This was primarily due to a decrease in the duration of irregular motor activity (phase II). Loperamide increased the transit time in the jejunum but not in the ileum or in the colon. Transport rates of water and electrolytes and transmural electrical potential differences were not significantly affected by the drug. These results suggest that opiates exert their constipating effect by inhibiting phase II-related irregular motor activity.