Glucose transport during ageing by human intestinal brush-border membrane vesicles

The Lack of Bmal1, a Core Clock Gene, in the Intestine Decreases Glucose Absorption in Mice

The circadian clock network is an evolutionarily conserved system that regulates systemic metabolism, such as glucose homeostasis. Intestinal tissue is a pivotal organ for the regulation of glucose metabolism, mainly via glucose absorption into the circulation; however, the significance of the intestinal circadian clock network for glucose metabolism remains largely unclear. We herein utilized a mouse model in which Bmal1, a core clock gene, was deleted in an intestine-specific manner (Bmal1Int-/- mice) and demonstrated a rhythmic expression of Sglt1 with its peak at zeitgeber time (ZT) 10.7 ± 2.8 in control mice, whereas this was lost in Bmal1Int-/- mice. Mechanistically, chromatin immunoprecipitation analysis revealed rhythmic binding of CLOCK to the E-box elements in the Sglt1 gene in control mice; however, this was absent in Bmal1Int-/- mice. Accordingly, SGLT1 protein levels were decreased during the dark phase in Bmal1Int-/- mice and this was associated with impaired glucose absorption, leading to a decline in hepatic glycogen levels at ZT4, which was restored by ingestion of high-sucrose water. Additionally, when mice were starved from ZT0, greater expression of the lipolysis-related gene Pnpla2 was observed in adipose tissue of Bmal1Int-/- mice, and this was not noted when glycogen storage was restored by high-sucrose water prior to fasting, suggesting that higher Pnpla2 expression in Bmal1Int-/- mice was likely caused by lower glycogen storage. These results indicate that disruption of the intestinal circadian clock system impairs glucose absorption in the intestine and affects systemic glucose homeostasis.

Relative contributions of afferent vagal fibers to resistance to diet-induced obesity

BACKGROUND

We previously demonstrated vagal neural pathways, specifically subdiaphragmatic afferent fibers, regulate expression of the intestinal sodium-glucose cotransporter SGLT1, the intestinal transporter responsible for absorption of dietary glucose. We hypothesized targeting this pathway could be a novel therapy for obesity. We therefore tested the impact of disrupting vagal signaling by total vagotomy or selective vagal de-afferentation on weight gain and fat content in diet-induced obese rats.

METHODS

Male Sprague-Dawley rats (n = 5-8) underwent truncal vagotomy, selective vagal de-afferentation with capsaicin, or sham procedure. Animals were maintained for 11 months on a high-caloric Western diet. Abdominal visceral fat content was assessed by magnetic resonance imaging together with weight of fat pads at harvest. Glucose homeostasis was assessed by fasting blood glucose and HbA1C. Jejunal SGLT1 gene expression was assessed by qPCR and immunoblotting and function by glucose uptake in everted jejunal sleeves.

RESULTS

At 11-months, vagotomized rats weighed 19% less (P = 0.003) and de-afferented rats 7% less (P = 0.19) than shams. Vagotomized and de-afferented animals had 52% (P < 0.0001) and 18% reduction (P = 0.039) in visceral abdominal fat, respectively. There were no changes in blood glucose or glycemic indexes. SGLT1 mRNA, protein and function were unchanged across all cohorts at 11-months postoperatively.

CONCLUSIONS

Truncal vagotomy led to significant reductions in both diet-induced weight gain and visceral abdominal fat deposition. Vagal de-afferentation led to a more modest, but clinically and statistically significant, reduction in visceral abdominal fat. As increased visceral abdominal fat is associated with excess morbidity and mortality, vagal de-afferentation may be a useful adjunct in bariatric surgery.

Small intestinal disorders in the elderly

The topic of gastroenterology (GI) in the elderly has been extensively reviewed. It takes special skill, patience and insight to interview the elderly, as well as to appreciate their altered physiology and interpretation of their presenting symptoms and signs, often against an extreme background of complex medical problems. The maldigestion and malabsorption coupled with altered motility contributes to the development of malnutrition. There generally a decrease of function of the GI tract, but there may be loss of adaptability in response to changes in diet or nutritional stress. Pathological alterations which might lead to minor overall intestinal functional variations in the young because of a normal process of adaptation, may lead to much more serious events in the elderly.

Aging and the intestine

Over the lifetime of the animal, there are many changes in the function of the body’s organ systems. In the gastrointestinal tract there is a general modest decline in the function of the esophagus, stomach, colon, pancreas and liver. In the small intestine, there may be subtle alterations in the intestinal morphology, as well as a decline in the uptake of fatty acids and sugars. The malabsorption may be partially reversed by aging glucagon-like peptide 2 (GLP2) or dexamethasone. Modifications in the type of lipids in the diet will influence the intestinal absorption of nutrients: for example, in mature rats a diet enriched with saturated as compared with polysaturated fatty acids will enhance lipid and sugar uptake, whereas in older animals the opposite effect is observed. Thus, the results of studies of the intestinal adaptation performed in mature rats does not necessarily apply in older animals. The age-associated malabsorption of nutrients that occurs with aging may be one of the several factors which contribute to the malnutrition that occurs with aging.

Age-associated changes in intestinal fructose uptake are not explained by alterations in the abundance of GLUT5 or GLUT2

A reduction in nutrient absorption may contribute to malnourishment in the elderly. The objectives of this study were to determine the effects of aging on the absorption of fructose in rats, as well as the mechanisms of these adaptive changes. Male Fischer 344 rats aged 1, 9, and 24 months were fed standard Purina chow for 2 weeks (PMI #5001, PMI Nutritionals, Brentwood, MO). The uptake of (14)C-labeled D-fructose was determined in vitro using the intestinal sheet method. Intestinal samples were taken for RNA isolation and for brush border membrane (BBM) and basolateral membrane (BLM) preparation. Northern blotting, Western blotting, and immunohistochemistry were used to determine the effects of age and diet on GLUT5 and GLUT2. When expressed on the basis of intestinal or mucosal weights, aging was associated with a decline in jejunal and ileal fructose uptake, whereas jejunal fructose uptake was increased when expressed on the basis of serosal or mucosal surface area. The alterations in fructose uptake were not paralleled by changes in GLUT5 or GLUT2 abundance. These results indicate that 1) the effect of age on fructose uptake depends on the method used to express results, and 2) the age-associated changes in uptake are not explained by alterations in GLUT5 and GLUT2.

Dietary lipids modify the age-associated changes in intestinal uptake of fructose in rats

Because reduced nutrient absorption may contribute to malnourishment in the elderly, age and diet modulate fructose uptake in mice, and alterations in fructose uptake may be paralleled by changes in the abundance of fructose transporters, the objectives of this study were to determine 1) the effects of aging on fructose absorption in rats, 2) the effect of feeding diets enriched with saturated fatty acids (SFA) vs. polyunsaturated fatty acids (PUFA), and 3) the mechanisms of these age-and diet-associated changes. Male Fischer 344 rats aged 1, 9, and 24 mo received isocaloric diets enriched with SFA or PUFA. The uptake of (14)C-labeled D-fructose was determined in vitro using the intestinal sheet method. Northern and Western blot analyses and immunohistochemistry were used to determine the abundance of sodium-independent glucose and fructose transporters (GLUT)2 and GLUT5. When expressed on the basis of mucosal surface area, jejunal fructose uptake was increased in 9 and 24 mo compared with 1-mo-old animals fed SFA. PUFA-fed animals demonstrated increased fructose uptake at 24 mo compared with younger animals. Ileal fructose uptake was increased with SFA vs. PUFA in 9-mo-old rats but was reduced with SFA in 1- and 24-mo-old rats. Variations in GLUT2 and GLUT5 abundance did not parallel changes in uptake. These results indicate that 1) age increases fructose uptake when expressed on the basis of mucosal surface area, 2) age influences the adaptive response to dietary lipid modifications, and 3) alterations in fructose uptake are not explained by variations in GLUT2 or GLUT5.

The age-associated decline in the intestinal uptake of glucose is not accompanied by changes in the mRNA or protein abundance of SGLT1

Studies performed using human and animal models offer conflicting results regarding the effect of age on nutrient absorption. The objectives of this study were to determine (1) the effects of aging on the in vitro uptake of glucose in rats; and (2) the molecular mechanisms of these age-associated changes. Male Fischer 344 rats aged 1, 9 and 24 months were fed a standard laboratory diet (PMI # 5001). The uptake of 14C-labelled D-glucose was determined in vitro using the intestinal sheet method. Northern blotting, Western blotting and immunohistochemistry were used to determine the effects of age on the BBM sodium-dependent glucose transporter, SGLT1, and the BLM Na+K(+)-ATPase. When expressed on the basis of intestinal weight, mucosal weight or surface area, there was a reduction in glucose uptake in the 24-month-old animals. SGLT1, GLUT2 and Na+K(+)-ATPase mRNA and protein abundance did not parallel the changes seen in glucose uptake. These results indicate that (1) age reduces in vitro intestinal glucose uptake in the rat; and (2) this age-associated decline in glucose uptake was not explained by alterations in SGLT1, GLUT2 or Na+K(+)-ATPase.

The aging gut. Nutritional issues

With improvements in health care, living standards, and socioeconomic status, more adults are living to old age. As the population ages, it is increasingly important to understand the factors that affect the nutritional status and thus the health status of older adults. Many factors contribute to inadequate nutrition, including health status, financial capacities, mobility, exercise, and physiologic needs. This article considered only the potential changes in nutritional needs because of alterations in the gastrointestinal tract owing to aging. One of the most remarkable changes with aging is the frequent development of atrophic gastritis and the inability to secrete gastric acid. This process affects approximately a third of older adults in the United States and only recently was recognized to be due to infection by H. pylori in the majority of cases. The lack of gastric acid in atrophic gastritis may lead to small intestinal bacterial overgrowth and influences the absorption of a variety of micronutrients, including iron, folate, calcium, vitamin K, and vitamin B12. Lactose maldigestion is a frequent condition in older adults and is extremely common worldwide. The intolerance of dairy products leads to avoidance of these foods and likely contributes to the development of osteopenia. Overall, the small intestine and pancreas undergo astonishingly few clinically significant changes with aging. The relative preservation of overall gastrointestinal function with aging is likely due to the large reserve capacity of this multiorgan system. Further research is needed to define the precise nutritional needs for older adults because simple extrapolation of values from younger adults is now recognized to be insufficient. In addition, it is no longer acceptable to define adequate nutriture in terms of amounts of vitamins needed to maintain serum levels of a nutrient. Further RDAs must consider the functional implications of adequate nutrition. Nutrients in the elderly will be measured as to whether they result in improvements in markers of chronic disease such as homocysteine or, most importantly, in the prevention of chronic disease such as osteoporosis and cardiovascular disease.

Changes in small-intestine permeability with aging

OBJECTIVE

To evaluate small-intestine mucosal integrity and permeability with advancing age as measured by the lactulose/mannitol absorption test in healthy subjects.

DESIGN

Prospective cohort study.

SETTING

Clinical research unit of the USDA Human Nutrition Research Center on Aging.

STUDY PARTICIPANTS

Fifty-six subjects were recruited in three age groups: 20 to 39 years (n = 20), 40 to 59 years (n = 19), and > or = 60 years (n = 17). Subjects were healthy, community-dwelling volunteers.

INTERVENTION

After an 8-hour fast, all subjects ingested 10 g of lactulose and 5 g of mannitol. Urine was collected for 6 hours and analyzed for lactulose and mannitol by high performance liquid chromatography. Twenty-four-hour urinary creatinine clearances were determined.

MAIN OUTCOME MEASURES

Percentages of lactulose and mannitol excreted, the lactulose/mannitol ratio (x100), and the 24-hour creatinine clearance.

RESULTS

With increasing age, both the percentage of lactulose excreted (P = 0.09) and the percentage of mannitol excreted (P = 0.05) tended to decrease progressively. The lactulose/mannitol ratio (x100) did not change with increasing age (P = 0.65) because both the percentages of lactulose and mannitol excreted declined. The creatinine clearance decreased markedly with advancing age (P < .001) and accounted for the decline in percentages of lactulose and mannitol excreted (P < .02).

CONCLUSIONS

There is a progressive decline in the ability to excrete lactulose and mannitol with increasing age. This is probably attributable to a decline in renal function with advancing age. However, because of parallel decreases in lactulose and mannitol excretion, the lactulose/mannitol ratio does not change. Thus small-intestine "leakiness" does not increase with aging as measured by the lactulose/mannitol absorption test.

Duodenal brush-border mucosal glucose transport and enzyme activities in aging man and effect of bacterial contamination of the small intestine

Duodenal biopsies were collected from 38 subjects (24 female and 14 male) ranging in age from 55 to 91 years. Evidence of bacterial contamination of the small bowel (BCSB) was sought at the same time by bacterial culture of duodenal aspirates and by hydrogen and [14C]glycocholic acid breath tests; subjects were considered to be positive for BCSB if any one of the three tests was abnormal. Biopsies were analyzed for six brush-border membrane enzyme activities: maltase, sucrase, lactase, alkaline phosphatase, leucine aminopeptidase, and alpha-glucosidase. Analysis of covariance with age as the covariate indicated no significant effect of age on the specific activities of these enzymes. Mucosal Na(+)-dependent glucose transport was quantified in brush-border membrane vesicles prepared from the biopsies. In all groups, glucose transport at 20-30 sec was greater (ranging from mean values of 2.45 to 3.66 times) than at 45 min, consistent with Na(+)-coupled glucose transport, and no significant effect of age was observed. BCSB had no significant effect on specific activities of any of the duodenal mucosal hydrolases but was associated with reduced (P = 0.05) brush-border glucose transport. None of the variables studied was significantly affected by the gender of subjects. In conclusion, these biochemical data do not support the contention that reduced capacity for carbohydrate absorption in the elderly is explained by reductions in duodenal brush-border mucosal disaccharidase activities or glucose transport.

Age-related modifications of leucine uptake in brush-border membrane vesicles from rat jejunum

Leucine uptake in brush-border membrane vesicles purified from rat jejunum is sodium-dependent, sensitive to the membrane electrical potential difference and enhanced by the intravesicular presence of potassium. This last effect is not mediated by the genesis of an electrical potential difference, since potassium activation and electrical potential effects are additive. Sodium-dependent leucine Vmax (1568 +/- 91 pmol/mg per 3 s, is higher in young rats than in adult and old animals. The diffusion component of leucine transport decreases with increasing age. Preloading the vesicles with 100 mM KCl increases leucine Vmax 200% in young animals, 100% in adult and 44% in old animals. The potassium activation is a saturation function of the cation concentration. Leucine uptake in brush border membrane from old animals is less sensitive to the electrical potential difference than in membranes from adult and young animals.

d-glucose transport systems in rat jejunal brush border membrane: Influence of ageing

Jejunal brush border membranes were isolated from rats of different ages (very young, young, adult and old); the gamma-GT specific activity and the vesicle volumes were unaffected by ageing, whilst protein content was significantly reduced in brush border from old rats. Vesicles were used to investigate the kinetics of Na-glucose cotransport under voltage-clamped and zero-trans conditions over a wide range of D-glucose concentrations (0.005-70 mM). Results provide evidence that in all the ages tested D-glucose can cross the brush border membrane both by a passive diffusional component and by two Na-dependent saturable transport systems, namely one with high-affinity and low-capacity and the other with low-affinity and high-capacity. However, in some old rats only one saturable and a very small passive component occur. The two Na-dependent transport systems were analyzed to define the stoichiometry of coupling between Na and glucose fluxes. In all the ages tested the Na:glucose ratio is higher in the high-affinity system than in the low-affinity one. Accordingly the effect of a superimposed membrane potential is more evident for the high-affinity transport mechanism. In conclusion, D-glucose transport systems seem to be unaffected by ageing from very young to adult rats; only in old animals age-related alterations can be observed.

Aging and ATPase activities in rat jejunum

In addition to the well-known (Na,K)-ATPase activity, an ouabain-insensitive Na-ATPase has been evidenced in the basolateral membrane of intestinal and renal cells from different mammals. Basolateral membranes of jejunal enterocytes from rats of different ages, i.e., very young, young, adult and old were separated by self-orienting, Percoll-gradient centrifugation. The total protein content and both Na- and (Na,K)-ATPase activities in initial homogenate and final pellets were analyzed. The dry weight of homogenate and pellet was also determined. The two ATPase activities and the protein content of the basolateral membrane fraction decrease with age when referred to the dry weight of the pellet. This diminution is also evident in the initial homogenate. The activation curve of Na-ATPase, hyperbolic in shape, gives Km and Vmax values unaffected by aging. The same behaviour is true for the kinetic parameters of (Na,K)-ATPase, which has a sigmoidal velocity curve. From these results, it seems that both Na- and (Na,K)-ATPase have the same characteristics in the basolateral membrane of the enterocyte throughout the life span of the animal, but they decrease quantitatively with aging.

Changes in metabolite transport by small intestine and kidney of young and old rats

We have studied the influence of the ageing phenomenon on metabolite absorption by the small intestine and the kidney of the rat, using isolated brush-border membrane vesicles prepared from 2 groups, one composed of 2 month, and the other of 24 month, animals. "Overshoot", which is typical of Na(+)-glucose cotransporter activity, disappeared in the duodenum and decreased in the kidney of the old rats. Short-circuiting of vesicles with valinomycin showed that, in the presence of K+ and valinomycin, "overshoot" decreased in both groups by about the same percentage. The Na(+)-dependent uptake of aspartate and phenylalanine showed contrasting pictures in the jejunum and kidney of the aged animals: aspartate transport decreased only in the kidney, while phenylalanine uptake was negatively affected in the jejunum. Na(+)-dependent citrate uptake, studied in renal brush-border membrane vesicles, was lower in the old rats. The Km values determined for Na(+)-dependent D-glucose and citrate uptake in the kidney did not meaningfully differ between the two groups. A continuous decrease in Na(+)-dependent D-glucose and citrate uptake in the rat kidney, during ageing, was demonstrated.