Effect of a high fructose diet on the activities of enzymes implicated in the conversion of fructose to glucose by the liver and the intestinal mucosa of the rat

The activity of enzymes implicated in the metabolic pathway of fructose to glucose conversion was shown in rat liver and intestine. In rats on normal diet, the specific activity of glucose-6-phosphatase, fructokinase, fructose-1,6-diphosphatase and triokinase was low in the intestine confirming that sugar conversion is not operative in this organ. In rats on a fructose diet, all the specific enzymatic activities tested were increased except for the hepatic triokinase and triose phosphate isomerase and for the intestinal triose phosphate isomerase. The intestine acquires the possibility to transform fructose to glucose by modifying the activities of enzymes implicated in the same metabolic pathway as that intervening in the liver.     

Changes in the shape of Leishmania major promastigotes in response to hexoses, proline, and hypo-osmotic stress

Leishmania major promastigotes in late-log phase are generally long and slender, and remain so during a 1 h incubation in buffer without exogenous substrate. When glucose, 2-deoxyglucose, fructose, mannose, or proline are added, the cells become shorter and more rounded. The shape change in response to glucose is complete within 20 min and is reversible upon incubating the cells without substrate. Galactose, 3-O-methylglucose, 6-deoxyglucose, sucrose, maltose, ribose, glycerol, alanine, glutamate or aspartate do not cause the shape change. Decreasing the osmolarity of the medium causes a rounding of the cells similar to that observed in the presence of glucose, and increasing the osmolarity inhibits the shape change in response to glucose. Inhibitors of glucose transport and 2nd messenger analogs do not affect the shape change.     

Intestinal transport of fructose and glycine in Ophiocephalus punctatus.

The regional variations in the absorption of fructose and glycine have been studied in the intestine and pyloric caeca of the fish, Ophiocephalus punctatus in vivo, by preparing intestinal sacs. Diffusion of amino acids out of the intracellular compartment of the intestine was observed in all the portions of the intestine. The pyloric caeca showed maximum transport of fructose. Absorption of fructose was slightly more in the posterior intestine than in the anterior intestinal sac. Maximum transport of glycine took place in the anterior intestinal sac. Pyloric caeca adsorbed more glycine than did the posterior sac. For both nutrients, the rate of uptake did not differ significantly in the upper and the lower portion of the intestine. In all the portions, the transport of fructose was more rapid than that of glycine. The experiments revealed that though the nutrients are absorbed in the entire length of the intestine, the quantity absorbed varies from one portion to another and is dependent on the nature of the compound absorbed.     

Changes in the Shape of Leishmania major Promastigotes in Response to Hexoses, Proline, and Hypo-osmotic Stress

Leishmania major promastigotes in late-log phase are generally long and slender, and remain so during a 1 h incubation in buffer without exogenous substrate. When glucose, 2-deoxyglucose, fructose, mannose, or proline are added, the cells become shorter and more rounded. The shape change in response to glucose is complete within 20 min and is reversible upon incubating the cells without substrate. Galactose, 3-O-methylglucose, 6-deoxyglucose, sucrose, maltose, ribose, glycerol, alanine, glutamate or aspartate do not cause the shape change. Decreasing the osmolarity of the medium causes a rounding of the cells similar to that observed in the presence of glucose, and increasing the osmolarity inhibits the shape change in response to glucose. Inhibitors of glucose transport and 2nd messenger analogs do not affect the shape change.     

Glucose metabolism in the rat small intestine: the effect of glucose analogues on hexokinase activity (Short Communication)

The effect of intubation of starved rats with solutions of glucose, 3-O-methylglucose, mannose, 2-deoxyglucose and sorbitol on the subcellular location of hexokinase in the mucosa of the small intestine was studied. Glucose, 3-O-methylglucose and mannose caused the soluble hexokinase activity to rise to a value similar to that found in fed animals, whereas sorbitol and 2-deoxyglucose caused smaller increases.     

The acute regulation of glucose absorption, transport and metabolism in rat small intestine by insulin in vivo.

The effect of acute changes in insulin concentrations in vivo on the absorption, transport and metabolism of glucose by rat small intestine in vitro was investigated. Within 2 min of the injection of normal anaesthetized rats with anti-insulin serum, lactate production and glucose metabolism were respectively diminished to 28% and 21% of normal and the conversion of glucose into lactate became quantitative. These changes correlated with the inhibition of two mucosal enzymes, namely the insulin-sensitive enzyme pyruvate dehydrogenase, and phosphofructokinase, which was shown by cross-over measurements to be the rate-limiting enzyme of glycolysis in mucosa. The proportion of glucose translocated unchanged from the luminal perfusate to the serosal medium was simultaneously increased from 45% to 80%. All the changes produced by insulin deficiency were completely reversed with 2 min when antiserum was neutralized by injection of insulin in vivo. The absorption and transport of 3-O-methylglucose were unaffected by insulin. It is concluded that glucose metabolism in rat small intestine is subject to short-term regulation by insulin in vivo and that glucose absorption and transport are regulated indirectly in response to changes in metabolism. Moreover, transport and metabolism compensate in such a way as to deliver the maximal 'effective' amount of glucose to the blood, whether as glucose itself or as lactate for hepatic gluconeogenesis.     

Factors influencing the fecundity of Moniliformis moniliformis (Acanthocephala): constant dose and varied diet

The reproductive performance, including survival, growth and mature egg production, of Moniliformis moniliformis was studied experimentally during primary infections in rats given 10 cystacanths each. Four isoenergetic purified diets containing either 1, 3, 6 or 12% fructose (w/w) were used and the amounts of fructose in the intestinal lumen of rats fed on these diets were measured. It was concluded that, while dietary composition had no effect on parasite establishment, there were associations between diet and the survival, growth and fecundity of the parasite. The host diet containing 3% fructose was considered to be more favourable for the worms than the others; 12% fructose was associated with a curtailment of survival time without any compensatory production of eggs. Female worms from rats fed on diets containing 3 or 6% fructose grew larger, and consistently carried more ovaries and produced more eggs than those from rats fed on the 1% fructose diet. It was concluded that these and other findings might be mediated not only through the amount of available fructose in the intestine for worm metabolism, but also by the responses of the host's intestinal physiology to the varying concentrations of fructose in the diet.     

Dietary fructose vs glucose lowers copper solubility in the digesta in the small intestine of rats

The hypothesis was tested that dietary fructose vs glucose lowers copper solubility in the digesta in the small intestine of rats, which in turn causes a decreased copper absorption. Male rats were fed adequate-copper (5 mg Cu/kg) diets containing either fructose or glucose (709.4 g monosaccharide/kg) for a period of 5 wk. Fructose vs glucose significantly lowered copper concentrations in plasma and the liver, but did not alter hepatic copper mass. Fructose feeding resulted in a significantly lesser intestinal solubility of copper as based on either a smaller soluble fraction of copper in the liquid phase of small intestinal contents or a lower copper concentration in the liquid phase. The latter fructose effect can be explained by the observed fructose-induced increase in volume of liquid phase of intestinal digesta. After administration of a restricted amount of diet extrinsically labeled with⁶⁴Cu, rats fed fructose also had significantly lower soluble⁶⁴Cu fraction in the digesta of the small intestine. Although this study shows that fructose lowered intestinal copper solubility, only a slight reduction of apparent copper absorption was observed. It is suggested that the fructose-induced lowering of copper status in part counteracted the fructose effect on copper absorption at the level of the intestinal lumen.     

Effect of size of Trichinella spiralis (Nematode) infections on glucose and ion transport in the rat intestine

An in vivo perfusion technique, using 3 intestinal loops representing the anterior, mid and posterior regions of the rat small intestine, was used to determine intestinal glucose uptake 5 days after infection with Trichinella spiralis. At high levels of infection (3,000 and 6,000 larvae/rat) net glucose absorption by the intestinal mucosa was significantly impaired in all regions of the small intestine when compared to uninfected controls. At low levels of infection (50 larvae/rat) glucose uptake by the mucosa was significantly enhanced in all 3 regions of the small intestine. Intermediate levels of infections (200-1,000 larvae/rat) also enhanced glucose uptake, but only in the anterior regions of the small intestine. When washings from the small intestine of rats infected with 50 larvae/rat were added to the perfusion fluid used on uninfected rats, glucose uptake was also significantly enhanced. These results suggest that at low levels of infection the intestinal lumen contains a metabolite which may affect the mucosal transport of glucose and the related fluxes of H2O, Na+, Cl-, and K+, in the rat intestine. Luminal [H+] and pCO2 decreased from the proximal to distal regions of the small intestine following perfusion; pO2 was significantly decreased in the proximal and distal regions.     

Effect of chronic alcohol consumption on rat uterine spontaneous motility, prostaglandin production and triglyceride metabolism

The spontaneous isometric developed tension (IDT), the synthesis and release of prostaglandins (PGs) into the incubating medium and the metabolism of triglycerides (TGs) in uterine strips isolated from controls and chronic ethanol fed rats, were studied. In order to observe how the uterus of rats fed alcohol reacts during a situation of metabolic emergency, the above mentioned studies were done in the presence or in the absence of glucose in the incubating medium. The decrement of IDT as time progressed was significantly greater in strips obtained from rats which had been drinking 20% ETOH than in controls. Nevertheless, the absolute magnitude of the initial IDT was similar in both groups. On the other hand, the decline of the frequency of contractions (FC) of uterine strips isolated from controls and from ETOH-exposed rats, after 60 min of spontaneous activity was similar. When the uterine strips isolated from ETOH-exposed and from control rats were suspended in glucose-free solution they exhibited the same decrement of IDT and FC after 60 min of activity. The basal release of PGE1 and PGE2 was similar in control tissues incubated in medium containing glucose, but the output of PGE2 was significantly smaller than that of PGE1 in uterine strips isolated from ETOH-exposed rats. The production of PGE1 and PGE2 by uteri suspended in glucose-free medium was similar in control preparations. On the contrary the release of both PGs differs in uterine strips from ETOH-exposed rats, i.e. the output of PGE2 was significantly smaller than in controls and the release of PGE1 increased around 4-fold in comparison with controls.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of sugars on leucine and lysine uptake by intestinal cells from rats fed sucrose and stock diets.

The effect of various dietary sugars on the uptake of 1 mM leucine and 1 mM lysine by intestinal cells isolated from stock-fed and sucrose-fed rats was determined. Leucine uptake was activated by 10 mM fructose and inhibited by 10 mM glucose or 20 mM sucrose on both diets. The major dietary effect noted was a significant increase in the inhibition of leucine by glucose in the sucrose-fed rats. The uptake of lysine was minimally affected by the sugars irrespective of the diet fed. These results demonstrate an important dichotomy in the properties of glucose and fructose transport in the intestine and suggest that dietary fructose may increase the transport of certain amino acids.     

Glucose metabolism in the mucosa of the small intestine. The effect of glucose on hexokinase activity

1. The effect of three dietary components on hexokinase activity in the mucosa of rat small intestine was studied in vivo. Glucose, amino acids or an emulsion of monoglyceride with long-chain fatty acids were given by stomach tube to previously starved rats, and hexokinase activity was determined in the particle-free supernatant of mucosal homogenates. The formation of lactate from glucose and glucose 6-phosphate respectively was also measured. 2. When the three dietary components were given in isocaloric amounts, only glucose brought about an increase in hexokinase activity. 3. Intravenous injection of a similar amount of glucose to that given orally did not alter hexokinase activity. 4. An increase in the hexokinase activity of the particle-free supernatant prepared from mucosal homogenates was also observed after sacs of the small intestine of starved rats had been incubated in vitro in a medium containing glucose. Hexokinase activity increased to the values observed in corresponding preparations from fed rats, and this increase was strictly glucose-dependent.     

Steady-state metabolism and transport of d-glucose by rat small intestine in vitro

1. Conditions of incubation of everted sacs of rat small intestine were selected to ensure that absorption of d-glucose by mucosal tissue from the incubation medium, intracellular metabolism of the absorbed glucose and transport of glucose through the intact intestinal tissue proceeded linearly with respect to time of incubation within stated time intervals. 2. Under these experimental conditions, steady intracellular concentrations of glucose and lactate were demonstrated. 3. The quantitative translocational and metabolic fate of absorbed glucose was determined under these steady-state conditions. About 25% of glucose absorbed from the external mucosal solution was accumulated (temporarily) within mucosal tissue and about 25% transported through the intact tissue into the external serosal solution; the remainder (about 50%) of the absorbed glucose was metabolized, 90% to lactate and 10% to CO₂. Concomitant respiration rates were comparable with those reported for several other preparations of intestine and were stoicheiometrically in excess of the O₂ metabolism required to account for the production of CO₂ from the absorbed glucose. 4. Water transport through the everted sacs proceeded at an optimum rate under the experimental conditions selected. 5. Some other observations are recorded which influenced the design of the experiments and the interpretation of results; these include the initial physiological state of the animal, the anaesthetic used and the ionic composition of the incubation medium.     

Effect of phloretin and phloridzin on properties of digestion and absorption in the rat small intestine

Experiments in vitro on everted sacs of rat small intestine have shown that phloretin (an inhibitor of basolatheral glucose GLUT2 transporter) added from mucosal side of the sacs decreases release of glucose from enterocytes into serosal fluid without changing glucose accumulation in tissue of the preparations. Addition of phloridzin (an inhibitor of Na⁺ and glucose co-transporter SGLT1) from mucosal side inhibited both glucose accumulation in the tissue and its release into serosal fluid. Unspecific effects of phloretin and phloridzin on activities of several digestive enzymes (in particular, alkaline phosphatase, amino peptidase, and glycyl-L-leucine dipeptidase) has been revealed in homogenates of the rat small intestine mucosa. In chronic experiments on rats, absorption of glycine from the isolated small intestinal loop was inhibited in the presence of phloretin in perfusate. The obtained results indicate that the experimental approach of inhibition of glucose absorption by phloretin used from mucosal side in vitro appears to give a significant overestimation of contribution of facilitated diffusion (with participation of the GLUT2 transporter inserted in the apical enterocyte membrane) to glucose transport across this membrane. Thus, the role of the GLUT2 transporter in the mechanism of glucose absorption in the small intestine under its physiological conditions does not seem to be as great as it is thought by the authors of the recently proposed hypothesis.     

Comparison of glucose and fructose transport into adipocytes of the rat.

The purpose of these studies was to define the properties of the systems that transport hexoses into adipocytes. Glucose appears to enter adipocytes on a single transport system whose maximum velocity is stimulated by insulin and which is competitively inhibited by cytochalasin B, 5-thioglucose, fructose, mannose and 3-O-methylglucose. In contrast, fructose enters adipocytes by at least two separate mechanisms, one an insulin-sensitive transporter (probably the glucose transporter) and the other a mechanism that is insensitive to insulin. The fructose concentration required for half-maximal rates of transport is at least an order of magnitude higher than that for glucose and the maximum velocity of fructose transport is more than double that for glucose.     

Ca2(+)-dependent glucose transport in chicken intestine

Transport of glucose was measured in the intestine of white leghorn layers in vivo using ligated upper small intestinal segment in the presence of Ca2+ and other ions either singly or in combination. Transport of glucose across the intestine was very significantly increased with Ca2+ than Na+, K+ and Po4(3-) individually, but when Ca2+ was combined with Na+, K+ and PO4(3-), the glucose absorption increased significantly over that achieved by Na+ ions alone. These data revealed that Ca2+ ions might be exerting the major influence on glucose transport processes of the chicken intestine.     

Mannitol and glucose movement across the Golgi membrane of lactating-rat mammary gland.

1. Purified Golgi-membrane vesicles of lactating-rat mammary gland were penetrated by glucose. 3-O-methylglucose, mannose, fructose, sorbitol and mannitol, but not by lactose or sucrose. 2. The kinetics of mannitol uptake and release were followed at 2-6 degrees C with the aid of fine filters (0.45 micrometers pore size) to separate the vesicles from the medium. 3. Mannitol efflux exhibited apparent first-order kinetics with k approximately 1 min-1. Neither saturability, nor inhibition by excess sorbitol or glucose, could be observed. 4. Mannitol efflux at 18 degrees C was about seven times faster than at 1 degrees C, and rates at higher temperatures were too fast to be measured. The rate of glucose efflux at 2-6 degrees C exceeded that of mannitol severalfold. 5. These findings imply a channel or carrier of definite, but limited, specificity straddling the Golgi membrane and able to supply glucose for lactose synthesis.     

Locally and systemically active glucocorticosteroids modify intestinal absorption of sugars in rats.

Glucocorticosteroids enhance digestive and absorptive functions of the intestine of weaning and adult rats. This study was undertaken to assess the influence of treatment of weaning male rats with budesonide (Bud), prednisone (Pred), or control vehicle on the in vitro jejunal and ileal uptake of glucose and fructose. Bud and Pred had no effect on the uptake of d-glucose by sodium glucose transporter-1. In contrast, the uptake of d-fructose by GLUT-5 was similarly increased with Bud and with Pred. The increases in the uptake of fructose were not due to variations in the weight of the intestinal mucosa, food intake, or in GLUT-5 protein or mRNA expression. There were no steroid-associated changes in mRNA expression of c-myc, c-jun, c-fos, proglucagon, or selected cytokines. However, the abundance of ileal ornithine decarboxylase mRNA was increased with Pred. Giving postweaning rats 4 wk of Bud or Pred in doses equivalent to those used in clinical practice increases fructose but not glucose uptake. This enhanced uptake of fructose was likely regulated by posttranslational processes.     

Resistance of adult mammalian intestinal mucosa to cytochalasin B

Cytochalasin B (CB), in concentrations previously shown to inhibit glucose and glucosamine uptake and to disrupt cytoplasmic microfilament structure in a variety of biological systems, did not alter 3-O-methyl- -glucose, glucosamine or oleic acid uptake by adult hamster small intestinal everted rings nor did CB alter microfilament structure in the microvilli, terminal web region or apical cytoplasm of intestinal absorptive cells. Moreover, glycoprotein, protein and secretory IgA synthesis and secretion were not inhibited in rabbit intestinal mucosa cultured for 24 h in CB-containing medium. Similarly, synthesis of glycerides from freshly absorbed oleic acid by everted sacs of hamster small intestine was not inhibited by addition of CB to the oxygenated buffer used for incubation. Thus the transport and synthetic functions of adult mammalian small intestine which were examined and the fine structure of adult hamster and rabbit intestinal absorptive cells seem remarkably resistant to CB.