Dietary regulation of fructose metabolism in the intestine and in the liver of the rat: time response and effect of cycloheximide

Sucrose induces fatty liver and pancreatic inflammation in male breeder rats independent of excess energy intake

Fructose induces metabolic syndrome in rats; but studies have been criticized for using high concentrations of fructose that are not physiologic, for using only pure fructose, and for not controlling for energy intake. We tested the hypothesis that a 40% sucrose diet (containing 20% fructose) might induce features of metabolic syndrome in male breeder rats independent of excess energy intake. Male Sprague-Dawley breeder rats were pair fed 40% sucrose or isocaloric starch diet for 4 months and evaluated for metabolic syndrome and diabetes. In vitro studies were performed in rat insulinoma cells (RIN-m5F) exposed to uric acid, and markers of inflammation were assessed. Rats fed a 40% sucrose diet developed accelerated features of metabolic syndrome with up-regulation of fructose-dependent transporter Glut5 and fructokinase. Fatty liver and low-grade pancreatic inflammation also occurred. Uric acid was found to stimulate inflammatory mediators and oxidative stress in islet cells in vitro. Sucrose, at concentrations ingested by a subset of Americans, can accelerate metabolic syndrome, fatty liver, and type 2 diabetes mellitus in male breeder rats; and the effects are independent of excess energy intake.

Cyclic AMP stimulates fructose transport in neonatal rat small intestine

Intestinal fructose transporter (GLUT5) expression normally increases significantly after completion of weaning in neonatal rats. Increases in GLUT5 mRNA, protein, and activity can be induced in early weaning pups by precocious consumption of dietary fructose or by perfusion of the small intestine with fructose solutions. Little is known about the signal transduction pathway of the dietary fructose-mediated increase in GLUT5 expression during early intestinal development. Recent microarray results indicate that key gluconeogenic enzymes modulated by cAMP are markedly upregulated by fructose perfusion; hence, we tested the hypothesis that cAMP plays an important role in regulating intestinal fructose absorption by simultaneously perfusing adenylyl cyclase, phosphodiesterase, or protein kinase A (PKA) inhibitors along with fructose. Intestinal fructose uptake rates increased by 100% in rat pups perfused with 8-bromo-cAMP. Simultaneous fructose and dideoxyadenosine (DDA; inhibitor of adenylyl cyclase) perfusion completely inhibited increases in fructose uptake rate induced by perfusion with fructose alone. Fructose perfusion increased intestinal mucosal cAMP concentrations by 27%, but simultaneous perfusion of fructose and DDA inhibited the fructose-induced increase in cAMP. However, GLUT5 and sodium-glucose cotransporter (SGLT1) mRNA abundance and glucose transport rates were each not significantly affected by 8-bromo-cAMP and DDA. Moreover, simultaneous perfusion of the small intestine with fructose and PKA inhibitor or N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamid. 2HCl, both inhibitors of PKA, did not prevent the fructose-induced increases in GLUT5 mRNA abundance and fructose uptake rate. Cyclic AMP appears to modulate fructose transport without affecting GLUT5 mRNA abundance, and without involving PKA.

Vanadate treatment rapidly improves glucose transport and activates 6-phosphofructo-1-kinase in diabetic rat intestine

The effect of oral vanadate on intestinal sodium-dependent glucose transport and 6-phosphofructo-1-kinase (EC 2.7.1.11) activity was examined in male Sprague-Dawley rats following a 30-day period of non-treated streptozotocin-induced diabetes. Non-treated diabetic rats were hyperglycaemic and demonstrated increased intestinal sodium-dependent glucose transport and Na,K-ATPase activity compared with controls. These increases were associated with a significant decrease in the total activity and activity ratios (activity at 0.5 mmol/l fructose 6-phosphate at pH 7.0/activity at pH 8.0) of intestinal 6-phosphofructo-1-kinase and decreased levels of fructose 2,6-bisphosphate. Supplementation of drinking water with vanadate (0.5 mg/ml) resulted in a rapid decline in blood glucose levels to a slightly hyperglycaemic level. Jejunal glucose transport and Na,K-ATPase activity were normalized after 48 h of vanadate treatment. In contrast, ileal glucose transport was significantly reduced 12 h following beginning vanadate treatment even though Na,K-ATPase activity did not normalize until 36 h later. Km was significantly decreased in both jejunum and ileum by vanadate treatment indicating an increased affinity of the sodium-dependent intestinal glucose transporter for glucose. 6-phosphofructo-1-kinase total activity and susceptibility to ATP inhibition was completely restored after 12 h of vanadate treatment. This increase was associated with a rise in fructose 2,6-bisphosphate levels. Fasting rats for 12 h had no effect on glucose transport or 6-phosphofructo-1-kinase activity, indicating the anorectic effect of vanadate was not responsible for changes in either parameter. In contrast, cycloheximide prevented both the rise in 6-phosphofructo-1-kinase activity and the rise in fructose 2,6-bisphosphate levels, and the subsequent reduction in glucose transport, indicating a requirement for protein synthesis. The removal of vanadate resulted in an immediate return to pre-treatment blood glucose levels. In contrast, intestinal glucose transport and 6-phosphofructo-1-kinase activity remained at treatment levels up until 72 h, indicating that oral vanadate treatment can have prolonged beneficial effects on intestinal function. In conclusion, the treatment of streptozotocin-induced diabetic rats with oral vanadate results in an activation of 6-phosphofructo-1-kinase coupled with a normalization of intestinal sodium-dependent glucose transport. Vanadate may thus have a beneficial effect on intestinal function and may prove useful as oral adjunctive diabetic therapy.

Fructose transport by rat intestinal brush border membrane vesicles. Effect of high fructose diet followed by return to standard diet

1. D-Fructose uptake in isolated rat intestinal brush border membrane comprised a simple diffusional component and a saturable, carrier-mediated, component. The latter did not follow typical Michaelis-Menten kinetics and appeared as a low affinity, high capacity process (Kt = 110 mM, Jmax = 160.5 nmol/min.mg protein). 2. Carrier-mediated D-fructose uptake was highly specific, Na independent and unaffected by phloridzin and metabolic inhibitors. 3. Fructose feeding for 3 days resulted in an activation of D-fructose uptake after a latent period superior to 3 days. The increase was of relatively short duration since it was not further observed 6 days after the return to the standard diet. 4. The activation in D-fructose uptake was due to a slight decrease in Kt and a marked increase in Jmax (from 160.5 to 306 nmol/min.mg protein), suggesting a rise in the number of transporters.

Effects of starvation and streptozotocin-induced diabetes on the activity of phosphofructokinase in the epithelial cells of rat colon

The regulation of phosphofructokinase in the colonic mucosa of 48 h-starved and streptozotocin-induced diabetic rats was investigated. The specific activities of phosphofructokinase from colonic mucosa of starved and diabetic rats were found to be diminished compared with normal controls. The enzyme obtained from the colonic mucosa of normal, diabetic and starved rats showed sigmoidal velocity curves with respect to fructose-6-phosphate, with apparent Km values of 0.6, 0.62 and 0.7 mM, respectively. However, the present results indicated that phosphofructokinase from the epithelial cells of rat colon is not regulated in a manner similar to that of the intestinal enzyme, which was shown to be highly regulated.

Absorption and metabolism of fructose by rat jejunum

The absorption and metabolism of fructose was investigated in the vascularly perfused jejunum of fructose-fed rats. With 10 mM-glutamate and 10 mM-fructose in the lumen, the viability of the tissue is maintained and fructose is absorbed and utilized at high rates. With 28 mM-fructose in the lumen, glucose appears in the vascular bed. With 10 mM- or 28 mM-fructose in the presence of 10 mM- or mM-glucose in the lumen, the fructose absorption is decreased. From 10 mM- or 28 mM-sucrose in the lumen, fructose uptake is also less than from the equivalent concentration of free fructose. The rate of appearance of fructose in the vascular bed is independent of the source of fructose from which it is derived. In the presence of glucose, either free or as sucrose, there is a marked decrease in the utilization of fructose, defined as the difference between that absorbed by the jejunum and that transported unchanged into the vascular bed. In all cases about half of the carbohydrate absorbed from the lumen is converted into lactate, most of which is secreted into the blood. The absorption of glucose and the rate of vascular appearance of glucose from glucose in the lumen are about 1.5 times greater than those of fructose from fructose in the lumen. It is concluded: firstly, that fructose uptake from the lumen of rat jejunum is determined by its concentration and by the demand for it as a fuel for the intestine, a demand that is severely decreased in the presence of glucose; secondly, that in the vascularly perfused jejunum there is no evident kinetic advantage for uptake of fructose or glucose from sucrose rather than from free monosaccharide in the lumen; thirdly, that some fructose can be converted into glucose.

Regulation of mucosal phosphofructokinase in the small intestine of the streptozotocin-diabetic rat

The total activity of mucosal phosphofructokinase in the proximal jejunum of the streptozotocin-diabetic at was found to be diminished compared with normal from 13.4 to 10.4 units/g wet weight, mucosa and the activity ratio of the enzyme at pH 7 was diminished from 0.40 to 0.23, indicating that the enzyme is more susceptible to inhibition by ATP. The combination of these changes is, in principle, more than sufficient to account for the 29% decrease in glucose utilisation observed in the jejunum of diabetic rats. Insulin, when injected in vivo, has the ability to restore both the total activity and the regulatory properties of phosphofructokinase to normal in streptozotocin-diabetic rats. The action of insulin is blocked by the protein synthesis inhibitor cycloheximide, and is not mediated by changes in blood glucose levels.