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

The effects of insulin on glucose and fluid transport in the isolated small intestine of normal rats

Chronically administered insulin returns enhanced maximal glucose transport capacity induced by diabetes to its normal state. In this study, the direct and acute effects of insulin on glucose transport in different parts of isolated small intestine were investigated. Mucosal Fluid Transport (MFT), Mucosal Glucose Transport (MGT) and Serosal Glucose Transport (SGT) were measured in the presence and absence of insulin in averted sacs, prepared from female Wistar rats. This study shows that the presence of insulin in vitro (40 and 80 microU/mL) can reduce MGT and SGT in different segments of the small intestine (duodenum, jejunum and ileum) after 30 min whereas it had no effect on MFT. Mucosal glucose transfer rates in the duodenum, jejunum and ileum of the controls were 6.07+/-0.4, 6.34+/-0.62 and 6.43+/-0.47 mg/g tissue respectively which were significantly reduced to 3.82+/-0.93, 3.60+/-0.50 and 1.17+/-0.45 in the presence of 80 microU/mL of insulin. Serosal glucose transfer too was decreased significantly from 0.3+/-0.05, 0.57+/-0.07 and 0.43+/-.07 in the duodenum, jejunum and ileum to 0.16+/-0.03, 0.16+/-0.04 and .07+/-.02 respectively. Mucosal fluid transfer was not affected by insulin. Insulin was as effective whether it was added on the mucosal or the serosal side. The results of this study show that insulin can directly affect glucose transport in the small intestine; its physiological role must be examined. Direct effect of insulin deficiency on glucose absorption in diabetic patients may play a role in the pathophysiology of the disease.

Pharmacological activity of peroral chitosan-insulin nanoparticles in diabetic rats

The objective of the present study was to evaluate the effects of formulation parameters on the in vivo pharmacological activity of the chitosan-insulin nanoparticles. Chitosan-insulin nanoparticles were prepared by ionotropic gelation at pH 5.3 and 6.1 and denoted as F5.3 np and F6.1 np, respectively. F5.3 np and F6.1 np administered orally at insulin doses of 50 U/kg and/or 100 U/kg were effective at lowering the serum glucose level of streptozotocin-induced diabetic rats. The 100 U/kg-dose F5.3 np sustained the serum glucose at pre-diabetic levels for at least 11 h. In comparison, F6.1 np had a faster onset of action (2h versus 10h) but lower efficiency. The effectiveness of peroral F5.3 np and F6.1 np in lowering the serum glucose level of streptozotocin-induced diabetic rats was ascribed to the local effect of insulin in intestine. Confocal micrographs showed strong interaction between rat intestinal epithelium and chitosan nanoparticles 3h post-oral administration.

Experimental streptozotocin-reduced diabetes and intestinal glucose metabolism in the rat, in vivo and in vitro

The intestine has a high glycolytic activity, but its metabolic role could be altered in diabetes mellitus. The aim of the present work was to investigate in vivo the glucose retained and the lactate produced by the intestine of normal and diabetic rats and in vitro the effect of different arterial glucose concentrations on glucose utilization and lactate, alanine, and pyruvate production in normal and diabetic rats when the glucose is supplied to the intestine exclusively via the vascular route. In vivo, the normal and diabetic rats retained similar percentages of the arterially supplied glucose (14.7 +/- 2.4, respectively). In vitro, when the preparations were perfused under hyperglycemic conditions, the glucose consumed, as a fraction of the quantity infused, was significantly lower (P < 0.05) in the diabetic (247.0 +/- 22.8 mumol/mmol infused glucose) than in normal (315 +/- 16.3 mumol/mmol infused glucose rats) rats. The lactate produced was significantly higher in diabetic than in normal rats whether the preparations were perfused under isoglycemic (P < 0.01; 1916.4 +/- 124.0 vs vs 1284 +/- 67.7 mumol/mmol consumed glucose) or hyperglycemic (P < 0.05; 1356.4 +/- 199.7 vs 898.0 +/- 87.3 mumol/mmol consumed glucose) conditions. There was significantly (P < 0.05) greater alanine release from the diabetic (123.7 +/- 21.8 mumol/mmol consumed glucose) than from the normal (40.7 +/- 10.3 mumol/mmol consumed glucose) rat preparations perfused under isoglycemic conditions.

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.

Effect of insulin and gliclazide on glucose utilization by a perfused intestine-pancreas preparation isolated from diabetic and non-diabetic rats

The effect of insulin (40 microU/ml) and gliclazide (200 micrograms/ml) on intestinal glucose metabolism was investigated by using an in vitro perfused intestine-pancreas preparation isolated from normal or streptozotocin-diabetic rats. Glucose, lactate and alanine were measured enzymatically in the portal effluent. The glucose retained by the perfused preparation was reduced (P < 0.05) in diabetic versus control rats. The portal lactate levels were not modified, but alanine portal levels were increased (P < 0.05) in diabetic versus control rats. In the diabetic rats, the level of glucose retained by the preparations was increased (P < 0.05) by the presence of insulin, and insulin plus gliclazide in the perfusate. In the presence of insulin and/or gliclazide, the portal lactate levels were not modified, but the alanine levels were reduced (P < 0.05) to normal values. In preparations from non-diabetic rats, the level of glucose retained was increased (P < 0.05) by gliclazide and insulin plus gliclazide, without modification of the portal lactate and alanine levels. In conclusion, the results show that both insulin and gliclazide increased glucose utilization by perfused intestine-pancreas preparations isolated from diabetic rats. The effect was enhanced when both substances were present simultaneously in the perfusion medium.

Rapid regulation of rat jejunal glucose transport by insulin in a luminally and vascularly perfused preparation

1. The regulation of glucose transport by physiological concentrations of insulin was investigated using a preparation of rat jejunum perfused in situ with 5 mM glucose on both sides. 2. Luminal uptake was 87% inhibited (P < 0.001) by 0.2 mM phlorizin, indicating that it occurred by means of the Na(+)-D-glucose cotransporter. Vascular uptake was completely abolished by 0.2 mM phloretin, indicating that it was facilitated in nature. 3. When infused into the vascular circuit, insulin (10(-11) to 10(-7) M) stimulated vascular, and inhibited luminal, glucose uptake to a similar extent. Maximal stimulation of vascular uptake was increased by 40% compared with control infusions (P < 0.01) and occurred at 10(-10) M insulin. These effects were independent of changes in metabolism and vascular glucose concentration. 4. The time taken for half-maximal stimulation of vascular uptake was 6.3 +/- 0.7 min and preceded that for inhibition of luminal uptake by 6.5 +/- 1.3 min (P < 0.02). 5. The rapid inhibition of luminal glucose uptake by the acute administration of insulin was also detected by perfusion of jejunal loops in vivo. 6. It is concluded that the transport steps involved in intestinal glucose uptake are subject to rapid regulation by physiological concentrations of insulin and that the initial site of action is on the vascular side.

Regulation of 6-phosphofructo-1-kinase from the epithelial cells of rat small intestine during pregnancy and lactation

The regulation of 6-phosphofructo-1-kinase (PFK) in the epithelial cells of rat small intestine was studied during pregnancy and lactation. The total activities and activity ratios (activity at 0.5 mM fructose 6-phosphate at pH 7.0/activity at pH 8.0 (nu 0.5/V] of the partially purified mucosal PFK were found to increase initially in early pregnant rats (11-12 days of gestation) and to fall back to normal in late pregnant rats (19-20 days of gestation). These changes in enzyme activity during pregnancy were associated with similar changes in the circulating levels of progesterone. The maximal activity and activity ratio (nu 0.5/V) were increased in male and female rats injected with progesterone. An increase in the total activity and activity ratio of mucosal PFK was also obtained in lactating rats. However, the enzyme was not strongly activated by inorganic phosphate, fructose 2,6-bisphosphate or glucose 1,6-bisphosphate either in early pregnant or lactating rats. These results indicate that mucosal PFK is already present as an active form during early pregnancy and lactation. Therefore, it is suggested that female sex hormones increase the circulating levels of insulin during early pregnancy which, in turn, positively affect the activity of mucosal PFK which could be also stimulated by the increased levels of fructose 2,6-bisphosphate. The increased activity of PFK in the peak lactating rats could be possible because of an increased demand for lactate production from glucose together with the stimulation of PFK by the increased concentrations of fructose 2,6-bisphosphate which therefore increases the rate of glycolysis.

Regulation of 6-phosphofructo-1-kinase in the placenta and small intestine of pregnant streptozotocin-induced diabetic rats

Changes in the activities of 6-phosphofructo-1-kinase (PFK, EC 2.7.1.11) in the placenta and jejunal mucosa of pregnant rats during the onset of experimental diabetes induced by streptozotocin were investigated. The concentrations of fructose 2,6-bisphosphate were significantly decreased in the placenta and small intestine of the streptozotocin-induced diabetic rats. The total activities and the activity ratios (activity at 0.5 mM fructose 6-phosphate at pH 7.0/activity at pH 8.0 (v0.5/V] of placental and jejunal PFK of diabetic pregnant and virgin rats were markedly diminished as compared to normal control rats. Also the susceptibility of jejunal and placental PFK to inhibition by ATP was increased in the diabetic virgin and pregnant rats. Administration of insulin in vivo completely reversed the effects of diabetes on the regulatory properties and on the total activities of placental and jejunal PFK. It is suggested that the diminished activity of PFK in the placenta and small intestine of streptozotocin-induced diabetic rats could be the result of the decreased concentrations of fructose 2,6-bisphosphate as well as the effect of insulin on the activity of PFK.

Effect of streptozotocin diabetes on the glycolytic flux and on fructose 2,6-bisphosphate levels in isolated rat enterocytes

In epithelial cells isolated from rat small intestine and incubated in the presence of 1 mM glucose, streptozotocin-induced diabetes reduced, by 46 and 29%, respectively, the rates of both glucose utilization and L-lactate formation. These effects were accompanied by a significant decrease of enterocyte fructose 2,6-bisphosphate concentration (about 50%) and of the glycolytic flux through the reaction catalyzed by 6-phosphofructo 1-kinase. The diminution of enterocyte fructose 2,6-bisphosphate levels caused by diabetes occurred in spite of an increase of hexose 6-phosphate concentration, and was associated with a reduction in the amount of active form of 6-phosphofructo 2-kinase; total activity of this enzyme was not significantly modified. Diabetes also caused an acceleration in the rate of 3-O-methyl-D-(14C) glucose uptake and increased hexokinase activity in enterocytes. Lactate dehydrogenase, pyruvate kinase and 6-phosphofructo 1-kinase activities were not found to be significantly different in epithelial cells isolated from control or diabetic animals. Our results indicate that a reduction of the glycolytic flux in enterocytes could collaborate to increase intestinal glucose absorption in the diabetic state.

Short-term regulation of glycolysis by vasoactive intestinal peptide in epithelial cells isolated from rat small intestine

In epithelial cells isolated from rat small intestine, we have studied the influence of vasoactive intestinal peptide (VIP), a neurotransmitter which markedly increases enterocyte cyclic AMP, and of two cyclic AMP analogues (8-bromo cyclic AMP and N6,2'-O-dibutyryl cyclic AMP) on the rate of glycolysis, fructose 2,6-bisphosphate concentration and 6-phosphofructo-2-kinase activity, as well as on the rate of 3-O-methyl-D-[14C]glucose uptake. Our results show that, without affecting the rate of 3-O-methyl-D-[14C]glucose accumulation, VIP and cyclic AMP analogues were able to inhibit glucose consumption and L-lactate formation by isolated rat enterocytes. These effects occurred parallel to a significant decrease in the cellular concentration of fructose 2,6-bisphosphate and to a partial inactivation of 6-phosphofructo-2-kinase. These findings support the hypothesis that VIP inhibits glycolysis in rat enterocytes through a cyclic AMP-dependent mechanism.

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.

The effect of thermal injury on the regulation of phosphofructokinase in the mucosa of rat small intestine

The effects of thermal injury (72 h post-injury), 72 h-partial (20% less food) or full starvation on the regulation of phosphofructokinase in the mucosa of rat small intestine were studied. Thermal injury and 72 h-partial or full starvation decreased the activity ratio v0.5/V, but the ratios obtained for thermally injured or fully starved rats were significantly lower than those of controls or partially starved rats. The susceptibility of phosphofructokinase to ATP inhibition was increased after thermal injury and 72 h-partial or full starvation compared to that of normal controls. However, these changes that occurred in the enzyme activities of the rat small intestine were mainly specific to injury per se but do not exclude the contribution of partial starvation during the same period of time.

Changes in the somatostatin, substance P and vasoactive intestinal polypeptide content of the gastrointestinal tract following streptozotocin-induced diabetes in the rat

Rats with streptozotocin-induced diabetes of 10 weeks' duration showed significant changes in the total content of somatostatin, substance P and vasoactive intestinal polypeptide in the stomach and small intestine compared with control animals. An increase (p less than 0.05) in the concentration and total content of gastric somatostatin and a decrease (p less than 0.05) in the concentration and content of gastric substance P were seen in the streptozotocin-treated rats. The increase in the vasoactive intestinal polypeptide (VIP) content (54%, p less than 0.05) and the decrease in the substance P content (35%, p less than 0.05) of the gut may contribute to the impaired intestinal motility observed in animals with experimentally produced diabetes. Both the diabetogenic effect of streptozotocin and the changes in regulatory peptide concentrations were prevented by injection of nicotinamide before streptozotocin suggesting that the changes did not arise from a non-specific toxic effect of streptozotocin upon gastrointestinal neurones and/or endocrine cells.

A protein inhibitor of phosphofructokinase from the mucosa of rat small intestine. A mechanism for the regulation of glycolysis that is independent of glucose

A protein inhibitor of phosphofructokinase was detected by the chromatography on Sephadex G-100 of crude mucosal extracts prepared from the small intestine of starved rats. Addition of the protein inhibitor to extracts from fed rats increased the K0.5 of phosphofructokinase for fructose 6-phosphate to that for starved rats. The protein inhibitor provides a mechanism for the hormonal regulation of glycolysis independently of glucose supply.

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.

The effect of glucose on the activity of phosphofructokinase in the mucosa of rat small intestine

In common with other phosphofructokinase isoenzymes, phosphofructokinase in the epithelial cells of rat small-intestinal mucosa is activated by fructose 2,6-bisphosphate. However, fructose 2,6-bisphosphate was found not to be present in mucosa as judged by three criteria: (1) chromatography on Sephadex G-25 of crude mucosal extracts from fed rats did not result in a decrease, or indeed any change, in the activity of phosphofructokinase under suboptimal conditions at pH7; (2) ultrafiltrates of mucosal extracts did not possess any acid-labile activating activity when tested against chromatographed liver phosphofructokinase; (3) phosphofructokinase-2 activity was not detectable in mucosal extracts. Furthermore, the perfusion in vitro of isolated loops of jejunum or the incubation of mucosal scrapings from either fed rats or rats starved for 48 h showed that the activity of mucosal phosphofructokinase is not subject to short-term regulation by glucose. These observations are consistent with the view that phosphofructokinase is the rate-limiting enzyme of glycolysis in intestinal mucosa and account for the fact that the rate of glucose utilization by rat small intestine is not very responsive to changes in the concentration of glucose in the lumen.