Jejunal uptake of sugars, cholesterol, fatty acids, and fatty alcohols in vivo in diabetic rats

Previous in vitro studies have demonstrated enhanced active and passive intestinal uptake of nutrients in streptozotocindiabetic rats, but the effect of diabetes on the in vivo absorption of glucose and amino acids remains controversial, and the effect of diabetes on the in vivo uptake of lipids has not been reported. Accordingly, an in vivo perfusion technique was used in rats to examine the uptake of nutrients from the intestinal lumen, their transfer to the body, their mucosal and submucosal content, and the percentage of uptake transferred. Diabetes was associated with reduced uptake of fatty alcohols, indicating that the effective resistance of the unstirred water layer in vivo is higher in diabetic than in nondiabetic control rats. The mucosal and submucosal content of dodecanol was lower in diabetic than in control rats, but the percentage of the dodecanol uptake transferred to the body was higher. Although the uptake of varying concentrations of D-galactose was similar in diabetic and in control animals, kinetic analysis corrected for unstirred layer effects demonstrated lower mean values of the passive permeability coefficients (Pd) for galactose in diabetic than in control animals, with lower values of the Michaelis constant (Km) and higher values of the maximal transport rate (Jmd). The uptake of lauric acid was reduced in diabetic rats, whereas the uptake of deconoic acid and of cholesterol was unchanged. With correction for unstirred layer effects, it was apparent that the jejunum of diabetic rats was in fact more permeable to decanoic and lauric acid as well as to cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Feeding rats a diet enriched with saturated fatty acids prevents the inhibitory effects of acute and chronic ethanol exposure on the in vitro uptake of hexoses and lipids

Chow-fed rats were given 15% ethanol in their drinking water for 4 weeks, and then for the next 2 weeks of ethanol exposure they were fed isocaloric semisynthetic diets enriched in either saturated (S) or polyunsaturated (P, linoleic acid) fats. Food intake was lower in ethanol-fed (ETH) than in control (C) rats, but the average body weight gain was similar in ETH and C fed S or P. Intestinal dry weight and the percentage of the intestinal wall comprised of mucosa were more than 2-fold higher in ETH than C fed P, whereas these values were 50% lower in ETH than C fed S. The in vitro jejunal uptake of glucose and galactose was higher in ETH than C fed S, whereas the converse was true when feeding P. These effects were due to differences in the values of the maximal transport rate (Vmax), the Michaelis constant (Km), and the contribution of passive permeation. The relative permeability of the intestine to lipids was unchanged by giving ethanol or by feeding S or P, but the individual rates of uptake of most medium- and long-chain fatty acids and cholesterol were lower in ETH fed P as compared with S. In a second series of studies the acute effect of ethanol exposure was examined: animals were fed S or P for 2 weeks and the intestine was then removed: when 5% ethanol was added directly to the test solutions, there was lower in vitro jejunal and ileal uptake of glucose and higher jejunal uptake of 18:2 when rats were previously fed P, but not in those fed S. In summary; (1) feeding an isocaloric polyunsaturated fatty acid diet has a trophic effect on the intestinal mucosa of animals chronically drinking ethanol; and (2) feeding rats a diet enriched with saturated fatty acids prevents the inhibitory effects of acute and chronic ethanol exposure on the in vitro jejunal uptake of glucose, galactose and lipids observed in animals fed a polyunsaturated diet. Thus, the effect of chronic consumption of ethanol on the active and passive jejunal uptake of nutrients is influenced by the type of lipids in the animal's diet.     

Comparison of kinetic constants of hexose uptake in four animal species and man

1. The rate of in vitro uptake of glucose (gl), galactose (gal), 3-O-methyl glucose (MG), and fructose (fr) was determined using jejunal discs of hamster (ham), rat, rabbit (rab) and guinea pig (gp). 2. The maximal transport rates (Jdm) of each hexose were ham greater than rab greater than gp = rat; 3. The Michaelis constants were different for the four sugars; for gl the values were gp greater than rat greater than ham greater than rab. 4. The passive permeability coefficients (Pd) were similar for each sugar, and were greatest in ham and rat, and lowest in rat and gp. 5. The effective resistance of the intestinal unstirred water layer was lowest in ham and highest in gp and rat. 6. There were differences in the kinetic constants for gl estimated using discs and biopsies of jejunum, and the greater uptake into human biopsies than in the other species was due to the higher Jdm and Pd.     

Jejunal nutrient sensing is required for duodenal-jejunal bypass surgery to rapidly lower glucose concentrations in uncontrolled diabetes

Gastrointestinal bypass surgeries restore metabolic homeostasis in patients with type 2 diabetes and obesity(1), but the underlying mechanisms remain elusive. Duodenal-jejunal bypass surgery (DJB), an experimental surgical technique that excludes the duodenum and proximal jejunum from nutrient transit(1,2), lowers glucose concentrations in nonobese type 2 diabetic rats(2–5). Given that DJB redirects and enhances nutrient flow into the jejunum and that jejunal nutrient sensing affects feeding(6,7), the repositioned jejunum after DJB represents a junction at which nutrients could regulate glucose homeostasis. Here we found that intrajejunal nutrient administration lowered endogenous glucose production in normal rats through a gut-brain-liver network in the presence of basal plasma insulin concentrations. Inhibition of jejunal glucose uptake or formation of long chain fatty acyl-coA negated the metabolic effects of glucose or lipid, respectively, in normal rats, and altered the rapid (2 d) glucose-lowering effect induced by DJB in streptozotocin (STZ)-induced uncontrolled diabetic rats during refeeding. Lastly, in insulin-deficient autoimmune type 1 diabetic rats and STZ-induced diabetic rats, DJB lowered glucose concentrations in 2 d independently of changes in plasma insulin concentrations, food intake and body weight. These data unveil a glucoregulatory role of jejunal nutrient sensing and its relevance in the early improvement of glycemic control after DJB in rat models of uncontrolled diabetes.     

Role and differential expression of calpastatin mRNA and protein in cultured cardiomyocytes exposed to hypoxic stress

This study investigated the beneficial effects and mechanism of action of the juice of Momordica charantia in streptozotocin (STZ)-induced diabetes mellitus in rats. Diabetes mellitus was associated with significant (p < 0.01) time course reductions in body weight, plasma insulin and the number of insulin positive cells per islet and significant (p < 0.01) time course elevation in blood glucose and osmolarity and systolic blood pressure compared to age-matched healthy controls. Oral intake of M. charantia juice by STZ-induced diabetic rats partially reversed all the diabetes-induced effects measured. Daily oral administration of M. charantia juice to STZ-induced diabetic rates significantly (p < 0.01) reduced the Na+- and K+-dependent absorptions of glucose by the brush border membrane vesicles of the jejunum compared to the responses obtained in STZ-induced diabetic rat. Either insulin (100 MM) or the fruit juice lyophilised extract (5 microg x ml(-1)) can stimulate 14C-D-glucose uptake in L6 myotubes. These effects were completely blocked by wortmannin, an inhibitor of phosphatidylinositol 3-kinase. High concentrations (10-200 microg x ml(-1)) of M. charantia juice extract inhibited 14C-D-glucose uptake in L6 myotubes compared to the control response. The effect of M. charantia treatment was also investigated on myelinated fibre abnormalities in the tibial nerve of STZ-induced diabetic and control rats. The results show that diabetes was associated with significant (p < 0.05) reduction in the mean cross-sectional myelinated nerve fibres, axonal area, myelin area and maximal fibre area compared to end controls. Treatment of STZ-induced diabetic rats with M. charantia juice normalised the structural abnormalities of peripheral nerves. The results indicate that M. charantia can exert marked beneficial effects in diabetic rats, and moreover, it can regulate glucose uptake into jejunum membrane brush border vesicles and stimulate glucose uptake into skeletal muscle cells similar to the response obtained with insulin.     

Adaptation of intestinal glucose transport in rats with diabetes mellitus occurs independent of hyperphagia.

Chronic diabetes enhances intestinal absorption of glucose and induces hyperphagia. We examined the enhanced intestinal absorption of glucose in ad libitum-fed rats with streptozocin-induced diabetes mellitus and compared these results with those obtained from pair-fed diabetic animals. Maximal transport capacity (Vmax) and carrier affinity (K0.5) were determined by measuring jejunal and ileal short circuit current (Isc) responses to varying concentrations of 3-O-methyl-D-glucopyranose and D-glucose. Pair-fed diabetic animals maintained the same body weight as animals fed ad libitum, although ad libitum-fed diabetic rats had an increased oral chow intake. Age-matched control rats maintained a constant jejunal and ileal Vmax and K0.5 throughout the study. Diabetic rats fed ad libitum demonstrated an enhanced Vmax and K0.5 in both jejunum and ileum. Pair feeding diabetic animals further enhanced jejunal Vmax while lowering jejunal K0.5 levels. In contrast, pair feeding diabetic animals delayed and blunted changes in ileal Vmax and prevented changes in ileal K0.5. In conclusion, signals other than those of hyperphagia regulate kinetic changes in glucose absorption during diabetes mellitus. Furthermore, these changes have differing effects on jejunum and ileum.     

Insulin replacement therapy for the rat model of streptozotocin-induced diabetes mellitus

OBJECTIVE: This study was conducted to compare various strategies for insulin replacement therapy in the streptozotocin-induced diabetic rat model. METHODS: Control and diabetic Sprague Dawley rats were fed ad libitum, blood glucose concentration was measured twice daily, and outcome was assessed over the final 5 days of a 10-day treatment period, with adjustment of insulin dosage toward the goal of normal glucose values. RESULTS: All insulin regimens induced weight gain at least comparable to that of controls, but glucose regulation differed. It was not possible to normalize glucose values by use of protamine zinc insulin (PZI) or Ultralente insulin given once daily. In contrast, PZI and neutral protamine Hagedorn (NPH) insulin given twice daily provided glucose values comparable to those in controls, whereas glucose values were modestly higher in response to a 70% human insulin isophane suspension and 30% soluble human insulin solution (70/ 30 insulin) given twice daily. Attempted normalization of glucose values was limited by hypoglycemia, which was most common after administration of PZI once daily, and least common after 70/30 insulin given twice daily. Dosage requirements for Ultralente insulin were four- to fivefold higher than those for all other insulins. CONCLUSION: In streptozotocin-diabetic rats, normal weight gain can be achieved by treatment with PZI insulin once daily, but attainment of near-normal glucose values requires administration of PZI, NPH, or 70/ 30 insulin twice daily. Ultralente insulin may have reduced bioeffectiveness in this animal model.     

Intestinal zinc transport: influence of streptozotocin-induced diabetes, insulin and arachidonic acid

The influence of arachidonic acid (AA) on the zinc flux rates of jejunal segments, isolated from streptozotocin-induced diabetic rats injected with saline or with insulin, was investigated using an Ussing chamber technique. Although the zinc flux rates from mucosa-to-serosa (Jms) of normal rats were inhibited by addition of 5 microM AA to the jejunal segment bathing medium (46.4 +/- 5.0 vs 32.6 +/- 4.3 nmol/hr/cm2), AA had no effect on the Jms of diabetic rats either with or without insulin treatment. Induction of diabetes also significantly reduced Jms (46.4 +/- 5.0 vs 22.1 +/- 4.9 nmol/hr/cm2), but 3 day insulin treatment (NPH 8 U/Kg/day subcutaneously) did not reverse this effect (29.2 +/- 5.1 nmol/hr/cm2). Addition of AA to the serosal side did not significantly alter the zinc flux rate from serosa-to-mucosa (Jsm) in either control, diabetic or diabetic rats treated with insulin. The net zinc absorption rate (Jnet) of jejunal segments was decreased in diabetic rats compared to controls (13.2 +/- 3.0 vs -0.7 +/- 2.1 nmol/hr/cm2), but normalization of blood glucose with 3 day insulin treatment did not increase Jnet. Addition of AA was associated with a tendency to increase zinc uptake capacity. This change reached statistical significance in insulin treated diabetic rats. Short-circuit current (Isc) for diabetic rats was increased compared to controls but addition of AA to the mucosal side bathing medium decreased Isc in all groups. The results indicate that the zinc flux rate in the small intestine of streptozotocin-induced diabetic rats is decreased, that zinc uptake capacity of the small intestine does not directly reflect the zinc flux rate across the small intestine, and that AA or one of its metabolites may play a significant role in the control of the zinc flux across the intestinal epithelium.     

Duodenal-jejunal bypass protects GK rats from {beta}-cell loss and aggravation of hyperglycemia and increases enteroendocrine cells coexpressing GIP and GLP-1

Dramatic improvement of type 2 diabetes is commonly observed after bariatric surgery. However, the mechanisms behind the alterations in glucose homeostasis are still elusive. We examined the effect of duodenal-jejunal bypass (DJB), which maintains the gastric volume intact while bypassing the entire duodenum and the proximal jejunum, on glycemic control, β-cell mass, islet morphology, and changes in enteroendocrine cell populations in nonobese diabetic Goto-Kakizaki (GK) rats and nondiabetic control Wistar rats. We performed DJB or sham surgery in GK and Wistar rats. Blood glucose levels and glucose tolerance were monitored, and the plasma insulin, glucagon-like peptide-1 (GLP-1), and glucose-dependent insulinotropic polypeptide (GIP) levels were measured. β-Cell area, islet fibrosis, intestinal morphology, and the density of enteroendocrine cells expressing GLP-1 and/or GIP were quantified. Improved postprandial glycemia was observed from 3 mo after DJB in diabetic GK rats, persisting until 12 mo after surgery. Compared with the sham-GK rats, the DJB-GK rats had an increased β-cell area and a decreased islet fibrosis, increased insulin secretion with increased GLP-1 secretion in response to a mixed meal, and an increased population of cells coexpressing GIP and GLP-1 in the jejunum anastomosed to the stomach. In contrast, DJB impaired glucose tolerance in nondiabetic Wistar rats. In conclusion, although DJB worsens glucose homeostasis in normal nondiabetic Wistar rats, it can prevent long-term aggravation of glucose homeostasis in diabetic GK rats in association with changes in intestinal enteroendocrine cell populations, increased GLP-1 production, and reduced β-cell deterioration.     

Insulin substitution: new insulins, new modes of delivery

The demonstrated role of the tight control of hyperglycaemia for the prevention of long-term diabetic complications has reoriented the goals of insulin supply toward the search for restoration of the effects of physiological insulin secretion rather than the simple survival of insulin deficient patients and the reduction in the number of daily insulin injections to be performed. Normal blood glucose control requires the availability of a fast-acting insulin therapy at meal time in order to reduce hyperglycaemic excursions and a basal insulin therapy able to stabilize blood glucose between meals. Reduction of induced hypoglycaemic risk represents the secondary objective beside the main goal of avoiding hyperglycaemia. Fast-acting analogues, by a faster dissociation of their hexameric conformation after their injection or infusion in subcutaneous tissue, reduce post-meal hyperglycaemia, while their shortened duration of action versus regular insulin minimizes late post-absorptive risk of hypoglycaemia. Long-acting analogues, by their precipitation in subcutaneous tissue or their slowly reversible binding to albumin, provide a benefit on blood glucose stability versus NPH or zinc insulins. Continuous insulin therapy using pumps offers both a better blood glucose stability than multiple daily injections and a broader flexibility in life mode. Using the peritoneal route by implantable pumps is a mean to improve blood glucose stability in poorly controlled patients in spite of optimized subcutaneous insulin therapy. The development of glucose sensors provides reinforced information on blood glucose, versus self-monitoring by capillary blood measurements, that contributes to a better adaptation of insulin therapy. First trials of connections between blood glucose data and insulin delivery open a perspective toward glucose-modulated insulin therapy, at least in periods outside meals, leading to first models of semi-automated artificial endocrine pancreas. The alternative of a cellular insulin supply by pancreas or islet transplantation looked promising during recent years, but lack of transplants and adverse events related to immune suppression limit their use to very specific cases where benefit/risk ratio is positive.     

Evidence for critical-period programming of intestinal transport function: variations in the dietary ratio of polyunsaturated to saturated fatty acids alters ontogeny of the rat intestine

2-week isocaloric modifications in the dietary ratio of polyunsaturated/saturated fatty acids (P/S) alters intestinal transport in rats. This study was undertaken to test the hypotheses that (1) the fatty acid composition of a nutritionally adequate diet in early life has lasting consequences for active and passive intestinal transport processes; and (2) early life feeding experiences with diets of varying fatty acid composition influence the intestines' ability to adaptively up- or down-regulate intestinal transport in later life. Female Sprague-Dawley rats were weaned onto S or P and were maintained on these diets for 2, 10 or 12 weeks. An in vitro uptake technique was used in which the bulk phase was vigorously stirred to reduce the effective resistance of the intestinal unstirred water layer. P decreased and S increased the uptake of glucose, and this effect was progressive from 2 to 12 weeks. Switching from a P to an S diet decreased jejunal but increased ileal uptake of glucose, whereas switching from an S to a P diet was associated with a decline in both the jejunal and the ileal uptake of glucose. The ileal uptake of galactose increased as the animals grew on either P or S. Switching from P to S resulted in a decline in ileal uptake of galactose, whereas the opposite effect was observed when switching from S to P. The effect of feeding P or S on hexose uptake was influenced by the animals' dietary history: ileal glucose and galactose uptake was lower in animals fed P at an early age (PSP) than in animals fed P for the first time in later life (SSP). Jejunal glucose and galactose uptake was also lower in animals fed S at an early age (SPS) than in those fed S for the first time in later life (PPS). The alterations in the uptake of long-chain saturated and unsaturated fatty acids and cholesterol did not progress with longer periods of feeding, and in the jejunum, lipid uptake did not change when switching from P to S or S to P. Early feeding with P (PSP vs. SSP) was associated with lower jejunal uptake of 18:3 and lower ileal uptake of 12:0, whereas previous feeding with S (SPS vs. PPS) was associated with lower ileal uptake of cholesterol. The changes in uptake of hexoses and lipids was not explained by differences in the animals' food consumption, body or intestinal weight or mucosal surface area.(ABSTRACT TRUNCATED AT 400 WORDS)     

Ileal resection and dietary content of sucrose influence the intestinal uptake of monosaccharides

The intestinal uptake of 0.5 and 40 mM glucose, galactose, and 3-O-methyl glucose (3-O-MG) was examined in vitro in rabbits fed a high (HS) or a low (LS) sucrose diet. In animals with an intact intestinal tract, the jejunal uptake of 0.5 mM 3-O-MG was unaffected by the dietary content of sucrose, whereas the uptake of 40 mM 3-O-MG was lower in LS than HS. The uptake of 40 mM galactose was higher in LS than HS and the uptake of 0.5 mM galactose was similar in HS and LS, whereas the uptake of 0.5 mM but not 40 mM glucose was lower in LS than HS. In animals subjected 6 weeks previously to an ileal resection, the adaptive changes in the jejunal uptake of the hexoses in response to alterations in the dietary content of sucrose differed from the changes observed in rabbits with an intact intestinal tract. For example, feeding HS to ileal resected animals was associated with increased jejunal uptake of 40 mM galactose, decreased uptake of 40 mM glucose, and unchanged uptake of 40 mM 3-O-MG; whereas in control animals with an intact intestinal tract, feeding HS resulted in increased uptake of 40 mM 3-O-MG, decreased uptake of 40 mM galactose, and no change in the uptake of 40 mM glucose. A similar adaptive pattern was noted in the jejunum and ileum for the effect of dietary sucrose on the uptake of 0.5 and 40 mM glucose.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hormone secretion and glucose metabolism in islets of Langerhans of the isolated perfused pancreas from normal and streptozotocin diabetic rats.

The glucose responsiveness of alpha- and beta-cells of normal as well as untreated and insulin-treated streptozotocin diabetic rats was tested in the extracorporeal perfusion system. Also assessed was the possible in vitro effect of added insulin on the glucose sensitivity of islets from untreated diabetic animals. Insulin and glucose responsiveness of the two cell types. The rate of glucose entry islet tissue was estimated, and the effect of glucose on the tissue supply of ATP and lactate and the cyclic 3':5'-AMP level of islets was measured under the above in vitro conditions. It was demonstrated that beta-cells are more accessible to glucose than alpha-cells, that glucose entry into islet cells is not significantly modified by insulin and that glucose had no effect on ATP, lactate and cyclic 3':5'-AMP levels of islet tissue under any of the conditions investigated. High insulin in vitro elevated ATP levels of alpha-cell islets independent of extracellular glucose. Glucose caused insulin release from normal but not from diabetic islets and rapidly and efficiently suppressed stimulated glucagon secretion of the pancreas from normal and insulin treated diabetic rats. Glucose was less effective in inhibiting stimulated glucagon secretion by the pancreas from untreated diabetic rats whether insulin was added to the perfusion media or not. Therefore, profound differences of glucose responsiveness of alpha-cells fail to manifest themselves in alterations of basic parameters of glucose and energy metabolism in contrast to what had been postulated in the literature. It is however, apparent that the glucose responsiveness of alpha-cells is modified by insuling by an as yet undefined mechanism.     

Glucose metabolism in perfused skeletal muscle. Interaction of insulin and exercise on glucose uptake.

1. The interaction of insulin and isometric exercise on glucose uptake by skeletal muscle was studied in the isolated perfused rat hindquarter. 2. Insulin, 10 m-i.u./ml, added to the perfusate, increased glucose uptake more than 10-fold, from 0.3-0.5 to 5.2-5.4 mumol/min per 30g of muscle in hindquarters of fed and 48h-starved rats respectively. In contrast, it did not stimulate glucose uptake in hindquarters from rats in diabetic ketoacidosis. 3. In the absence of added insulin, isometric exercise, induced by sciatic-nerve stimulation, increased glucose uptake to 4 and 3.4 mumol/min per 30g of muscle in fed and starved rats respectively. It had a similar effect in rats with moderately severe diabetes, but it did not increase glucose uptake in rats with diabetic ketoacidosis or in hindquarters of fed rats that had been "washed out" with an insulin-free perfusate. Insulin, at concentrations which did not stimulate glucose uptake in resting muscle, restored the stimulatory effect of exercise in these situations. 4. The stimulation of glucose uptake by exercise was independent of blood flow and the degree of tissue hypoxia; also it could not be reproduced by perfusing resting muscle with a medium previously used in an exercise experiment. 5. At rest glucose was not detectable in muscle cell water of fed and starved rats even when perfused with insulin. In the presence of insulin, a small accumulation of glucose, 0.25 mM, was noted in the muscle of ketoacidotic diabetic rats, suggesting inhibition of glucose phosphorylation, as well as of transport. 6. During exercise, the calculated intracellular concentration of glucose in the contracting muscle increased to 1.1-1.6mM in the fed, starved and moderately diabetic groups. Insulin significantly increased the already high rates of glucose uptake by the hindquarters of these animals but it did not alter the elevated intracellular concentration of glucose. 7. In severely diabetic rats, exercise did not cause glucose to accumulate in the cell in the absence of insulin. In the presence of insulin, it increased glucose uptake to 6.1 mumol/min per 30g of muscle and intracellular glucose to 0.72 mM. 8. The data indicate that the stimulatory effect of exercise on glucose uptake requires the presence of insulin. They suggest that in the absence of insulin, glucose uptake is not enhanced by exercise owing to inhibition of glucose transport into the cell.     

Beneficial effect of pretreatment of islets with fibronectin on glucose tolerance after islet transplantation.

The scarcity of available islets is an obstacle for clinically successful islet transplantation. One solution might be to increase the efficacy of the limited islets. Isolated islets are exposed to a variety of cellular stressors, and disruption of the cell-matrix connections damages islets. We examined the effect of fibronectin, a major component of the extracellular matrix, on islet viability, mass and function, and also examined whether fibronectin-treated islets improved the results of islet transplantation. Islets cultured with fibronectin for 48 hours maintained higher cell viability (0.146 +/- 0.010 vs. 0.173 +/- 0.007 by MTT assay), and also had a greater insulin and DNA content (86.8 +/- 3.6 vs. 72.8 +/- 3.2 ng/islet and 35.2 +/- 1.4 vs. 30.0 +/- 1.5 ng/islet, respectively) than islets cultured without fibronectin (control). Absolute values of insulin secretion were higher in fibronectin-treated islets than in controls; however, the ratio of stimulated insulin secretion to basal secretion was not significantly different (206.9 +/- 23.3 vs. 191.7 +/- 20.2% when the insulin response to 16.7 mmol/l glucose was compared to that of 3.3 mmol/l glucose); the higher insulin secretion was thus mainly due to larger islet cell mass. The rats transplanted with fibronectin-treated islets had lower plasma glucose and higher plasma insulin levels within 2 weeks after transplantation, and had more favorable glucose tolerance 9 weeks after transplantation. These results indicate that cultivation with fibronectin might preserve islet cell viability, mass and insulin secretory function, which could improve glucose tolerance following islet transplantation.     

In vivo insulin responsiveness for glucose uptake and production at eu- and hyperglycemic levels in normal and diabetic rats.

Whole body glucose uptake (BGU) and hepatic glucose production (HGP) at maximal plasma insulin concentrations (+/- 5000 microU/ml) were determined by eu- (EC) (6 mM) and hyperglycemic (HC) (20 mM) clamps (120 min), combined with [3-3H]glucose infusion, in normal and streptozotocin-treated (65 mg/kg) 3-day diabetic, conscious rats. In normal rats, during EC, BGU was 12.4 +/- 0.4 mg/min and during HC, when urinary glucose loss was 0.54 +/- 0.09 mg/min, BGU was 25.5 +/- 1.6 mg/min. However, throughout the final 60 min of HC, glucose infusion rate (GIR) was not constant but a linear decline in time (r = -0.99) of 17%, P less than 0.0001, was observed indicating a hyperglycemia-induced desensitization process. In diabetic rats, during EC, BGU was 7.7 +/- 0.3 mg/min and during HC, BGU was 15.5 +/- 1.4 mg/min. Throughout the final 60 min of HC, GIR was constant, suggesting that the hyperglycemia-induced desensitization process was already completed. In normal and diabetic rats, HGP was similar: during EC 0.2 +/- 0.5 mg/min and 0.1 +/- 0.5 mg/min, and during HC 0.4 +/- 0.4 mg/min and 0.5 +/- 0.6 mg/min, respectively. In vitro adipocyte and muscle insulin receptor studies showed normal to increased receptor number and increased receptor autophosphorylation in diabetic compared to normal rats. In conclusion: (i) 3-day diabetic rats show, at maximal plasma insulin concentrations, insulin resistance to BGU, but not to HGP. The resistance to BGU is equally present (reduction of 38%) at eu- and hyperglycemic levels as compared to normal rats. (ii) 3-day diabetic rats reveal no defect in adipocyte and muscle insulin receptor function. These data indicate that the diabetes induced insulin resistance for BGU is at the post-receptor level and due to a decreased maximal capacity (Vmax) for glucose uptake, with no change in affinity, or Km.     

Kinetics of uptake of glucose in rabbit jejunum: influence of sodium, unstirred layers, and passive permeation

Failure to account for the effect of the unstirred water layer and the contribution of passive permeation will lead to errors in the estimation of the kinetic constants of glucose uptake into the intestine. It is widely accepted that variations in the concentration of sodium in the bulk phase profoundly influence the rate of uptake of glucose in the intestine, but the kinetic basis for this effect remains in dispute. Accordingly, a previously validated in vitro technique was used to assess the effect of Na+ on the uptake of glucose into rabbit jejunum under conditions selected to reduce the unstirred layer resistance. Varying Na+ had no effect on the uptake of lauryl alcohol and therefore on unstirred layer resistance. The passive permeability coefficient for glucose uptake was estimated from the uptake of L-glucose, of D-glucose at 4 degrees C, or in the presence of 1 mM phlorizin or 40 mM galactose. The permeability for glucose increased as Na+ rose. The values of both the maximal transport rate and the Michaelis constant (Km) were influenced by Na+. A linear relationship was noted between Na+ and the maximal transport rate; the value of Km fell as Na+ was increased to 75 mequiv./L, but Km did not decline further with higher values of Na+. These results support the theoretical predictions of the presence of both an affinity and a velocity effect of the sodium gradient on the intestinal transport system for glucose.     

Postnatal amino acid uptake by the rat small intestine. Changes in membrane transport systems for amino acids associated with maturation of jejunal morphology.

The uptake of a number of amino acids by the developing small intestine of the rat was investigated in vitro. L-valine, L-leucine, L-methionine, L-phenylalanine, L-arginine and L-lysine were all taken up by active transport and concentrated within the jejunal mucosa. GABA was not actively transported by the jejunum. The kinetics of carrier transport of amino acids was determined from birth to maturity. The Michaelis constant (Km) of the L-leucine, L-methionine, L-arginine and l-lysine transport systems was found to be low postnatally and increased with age, particularly after the time of weaning. The rate of l-leucine, L-methionine, L-phenylalanine and L-lysine transport (Vmax) was high postnatally but decreased after weaning. Neutral amino acids were transported at higher rates than basic amino acids. l-arginine was poorly transported by the jejunum. The specificity of transport systems for amino acids was investigated in inhibition studies. Amino acid transport systems appeared to be polyfunctional in the postnatal period but were more specific in post-weaned animals. The changes in kinetics and specificity of amino acid transport in the small intestine are discussed with reference to their possible functional significance and to the maturational changes in the jejunum, particularly with the appearance of a functionally distinct absorptive cell lining the intestinal villi during the third postnatal week (the time of weaning).     

Treatment of the enhanced intestinal uptake of glucose in diabetic rats with a polyunsaturated fatty acid diet

Intestinal absorption of most nutrients is enhanced in diabetic rats. We wished to test the hypothesis that manipulation of dietary fatty acids will modify enhanced uptake of glucose in rats with established streptozotocin-diabetes. Chow-fed control rats or animals with one week of streptozotocin-diabetes were continued on chow or were fed ad libitum for three weeks with semisynthetic isocaloric diets containing a high content of either essential polyunsaturated or non-essential saturated fatty acids. The jejunal and ileal in vitro uptake of varying concentrations of glucose was much higher in diabetic than control rats fed chow or the saturated fatty acid diet. In contrast, the enhanced uptake of this sugar was reduced or normalized in diabetic rats fed the polyunsaturated fatty acid diet. Feeding the polyunsaturated fatty acid diet was associated with increased brush-border membrane activity of alkaline phosphatase in diabetic jejunum and ileum, but neither the saturated fatty acid diet nor the polyunsaturated fatty acid diet altered brush-border membrane cholesterol or phospholipids in control or in diabetic rats. Mucosal surface area was similar in diabetic rats fed the saturated fatty acid diet or the polyunsaturated fatty acid diet. Thus, (1) feeding the polyunsaturated fatty acid diet diminishes the enhanced jejunal and ileal uptake of glucose in diabetic rats, and (2) the influence of the polyunsaturated fatty acid diet on uptake in diabetic rats was not explained by alterations in intestinal morphology or brush-border membrane content of cholesterol or phospholipids. This study suggests that manipulation of dietary lipids may play a role in the normalization of the enhanced intestinal glucose uptake in rats with established diabetes.