Inhibition of intestinal absorption and improvement of oral glucose tolerance by biguanides in the normal and in the streptozotocin-diabetic rat

Metformin may be associated with false-negative cancer detection in the gastrointestinal tract on PET/CT

OBJECTIVE

Concurrent therapy with the antihyperglycemic drug metformin can hinder the detection of malignancy in the abdominal and pelvic portions of 18F-fluordeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) imaging performed for the diagnosis or staging of malignancy, as well as for treatment response and radiation therapy planning. This is due to the metformin-induced increase in intestinal FDG radiotracer uptake. We aim to bring this potentially important interaction to the attention of clinicians who care for cancer patients with diabetes.

METHODS

We searched MEDLINE (from 1970 to January 2014) and Google Scholar for relevant English-language articles using the following search terms: "metformin and FDG/PET, metformin and bowel uptake, metformin, and cancer, metformin and the intestine, metformin pharmacokinetics, hyperglycemia and FDG/PET." We reviewed the reference lists of pertinent articles with respect to metformin gut physiology, impact on FDG uptake and the effect on diagnostic accuracy of abdominalpelvic PET/CT scans with concurrent metformin therapy.

RESULTS

We reviewed the action of metformin in the intestine, with particular emphasis on the role of metformin in PET/CT imaging and include a discussion of clinical studies on the topic to help refine knowledge and inform practice. Finally, we discuss aspects pertinent to the management of type 2 diabetes (T2D) patients on metformin undergoing PET/CT.

CONCLUSIONS

Metformin leads to intense, diffusely increased FDG uptake in the colon, and to a lesser degree, the small intestine, which limits the diagnostic capabilities of FDG PET/CT scanning and may mask gastrointestinal malignancies. We suggest that metformin be discontinued 48 hours before FDG PET/CT scanning is performed in oncology patients. More rigorous data are needed to support the widespread generalizability of this recommendation.

Metformin Alters Upper Small Intestinal Microbiota that Impact a Glucose-SGLT1-Sensing Glucoregulatory Pathway

The gut microbiota alters energy homeostasis. In parallel, metformin regulates upper small intestinal sodium glucose cotransporter-1 (SGLT1), but whether changes of the microbiota or SGLT1-dependent pathways in the upper small intestine mediate metformin action is unknown. Here we report that upper small intestinal glucose sensing triggers an SGLT1-dependent pathway to lower glucose production in rodents. High-fat diet (HFD) feeding reduces glucose sensing and SGLT1 expression in the upper small intestine. Upper small intestinal metformin treatment restores SGLT1 expression and glucose sensing while shifting the upper small intestinal microbiota partly by increasing the abundance of Lactobacillus. Transplantation of upper small intestinal microbiota from metformin-treated HFD rats to the upper small intestine of untreated HFD rats also increases the upper small intestinal abundance of Lactobacillus and glucose sensing via an upregulation of SGLT1 expression. Thus, we demonstrate that metformin alters upper small intestinal microbiota and impacts a glucose-SGLT1-sensing glucoregulatory pathway.

Metformin discontinuation less than 72 h is suboptimal for F-18 FDG PET/CT interpretation of the bowel

OBJECTIVE

Metformin-induced [F-18] fluorodeoxyglucose (FDG) bowel uptake can hinder positron emission tomography/computed tomography (PET/CT) evaluation of the bowel. This study aimed to investigate the segmental bowel uptake of FDG according to metformin discontinuation times up to 72 h.

METHODS

We retrospectively divided 240 diabetic patients into four groups: metformin discontinuation <24 h (group A; n = 86), 24-48 h (group B; n = 40), 48-72 h (group C; n = 12), and no metformin (control group; n = 102). Segmental FDG bowel uptakes were measured visually (four-point scale) and semi-quantitatively (maximum standardized uptake value).

RESULTS

Compared with the control group, FDG uptake increased significantly from the ileum to the rectosigmoid colon in group A, from the transverse to the rectosigmoid colon in group B, and from the descending colon to the rectosigmoid colon in group C in both visual and semi-quantitative analyses.

CONCLUSIONS

Metformin discontinuation for <72 h is likely suboptimal for PET/CT image interpretation, especially with respect to the distal segments of the colon.

Antidiabetic properties and mechanism of action of Orthosiphon stamineus Benth bioactive sub-fraction in streptozotocin-induced diabetic rats

Orthosiphon stamineus is a popular folk medicine widely used to treat many diseases including diabetes. Previous studies have shown that the sub-fraction of chloroform extract was able to inhibit the rise of blood glucose levels in a glucose tolerance test. This study was carried out to evaluate the chronic effect and possible mechanism of action of the bioactive chloroform sub-fraction of O. stamineus using streptozotocin-induced diabetic rats and in vitro methods. Administration of the chloroform extract sub-fraction 2 (Cƒ2-b) at a dose of 1 g/kg twice daily on diabetic rats for 14 days showed a significant lowering (p < 0.05) of the final blood glucose level compared to the pretreatment level. However, there were no significant differences in the plasma insulin levels post-treatment compared to the pretreatment levels for all doses of Cƒ2-b. Conversely, Cƒ2-b at a concentration of 2 mg/mL significantly increased (p < 0.001) the glucose uptake by the rat diaphragm muscle. The increase in glucose uptake was also shown when the muscle was incubated in a solution containing 1 IU/mL of insulin or 1 mg/mL of metformin. Furthermore, the effect of this sub-fraction on glucose absorption in the everted rat jejunum showed that Cƒ2-b at concentrations of 0.5 mg/mL, 1 mg/mL and, 2 mg/mL significantly reduced the glucose absorption of the jejunum (p < 0.05-0.001). Similarly, the absorption of glucose was also inhibited by 1 mg/mL and 2 mg/mL of metformin (p < 0.001). These results suggest that the effect of Cƒ2-b may be due to extra-pancreatic mechanisms. There was no evidence that the plant extract stimulated the release of insulin in order to lower the blood glucose level.

Impact of medication discontinuation on increased intestinal FDG accumulation in diabetic patients treated with metformin

OBJECTIVE

We evaluated the impact of stopping medication for 2 days on reductions in the high intestinal FDG uptake induced by metformin.

SUBJECTS AND METHODS

One hundred thirty-eight diabetic patients were divided into two groups: one in which the antihyperglycemic drug regimen included metformin (group A; n = 107) and one in which the regimen did not include metformin (group B; n = 31). Fifty-two patients without diabetes mellitus served as the control group (group C). Group A was divided into two subgroups: 77 patients (group A1) were taking metformin at the time of FDG PET/CT scans, whereas the remaining 30 patients (group A2) were asked to stop taking metformin for 2 days before PET/CT scans. In addition, 10 diabetic patients underwent two consecutive PET/CT scans before and after the discontinuation of metformin. The intestinal FDG uptake and blood glucose levels were compared among the four groups, as well as before and after the discontinuation of metformin.

RESULTS

The high intestinal FDG uptake in group A1 was significantly reduced after the discontinuation of metformin (p < 0.001 vs group A2); thus, there were no significant differences among group A2, group B, and group C (p = 0.581-0.872). There were also no statistically significant differences in the blood glucose levels among the three groups of diabetic patients (p > 0.9). In 10 patients who underwent serial PET/CT scans, mean intestinal FDG uptake decreased by 64% without significant changes in the blood glucose level. Hidden colorectal malignancies were revealed in two patients after the discontinuation of medication.

CONCLUSION

The discontinuation of metformin for 2 days is feasible for reducing the high intestinal FDG uptake induced by metformin.

Glucose supply and insulin demand dynamics of antidiabetic agents

BACKGROUND

For microvascular outcomes, there is compelling historical and contemporary evidence for intensive blood glucose reduction in patients with either type 1 diabetes mellitus (T1DM) or type 2 diabetes mellitus (T2DM). There is also strong evidence to support macrovascular benefit with intensive blood glucose reduction in T1DM. Similar evidence remains elusive for T2DM. Because cardiovascular outcome trials utilizing conventional algorithms to attain intensive blood glucose reduction have not demonstrated superiority to less aggressive blood glucose reduction (Action to Control Cardiovascular Risk in Diabetes; Action in Diabetes and Vascular Disease: Preterax and Diamicron Modified Release Controlled Evaluation; and Veterans Affairs Diabetes Trial), it should be considered that the means by which the blood glucose is reduced may be as important as the actual blood glucose.

METHODS

By identifying quantitative differences between antidiabetic agents on carbohydrate exposure (CE), hepatic glucose uptake (HGU), hepatic gluconeogenesis (GNG), insulin resistance (IR), peripheral glucose uptake (PGU), and peripheral insulin exposure (PIE), we created a pharmacokinetic/pharmacodynamic model to characterize the effect of the agents on the glucose supply and insulin demand dynamic. Glucose supply was defined as the cumulative percentage decrease in CE, increase in HGU, decrease in GNG, and decrease in IR, while insulin demand was defined as the cumulative percentage increase in PIE and PGU. With the glucose supply and insulin demand effects of each antidiabetic agent summated, the glucose supply (numerator) was divided by the insulin demand (denominator) to create a value representative of the glucose supply and insulin demand dynamic (SD ratio).

RESULTS

Alpha-glucosidase inhibitors (1.25), metformin (2.20), and thiazolidinediones (TZDs; 1.25-1.32) demonstrate a greater effect on glucose supply (SD ratio >1), while secretagogues (0.69-0.81), basal insulins (0.77-0.79), and bolus insulins (0.62-0.67) demonstrate a greater effect on insulin demand (SD ratio <1).

CONCLUSION

Alpha-glucosidase inhibitors, metformin, and TZDs demonstrate a greater effect on glucose supply, while secretagogues, basal insulin, and bolus insulin demonstrate a greater effect on insulin demand. Because T2DM cardiovascular outcome trials have not demonstrated macrovascular benefit with more aggressive blood glucose reduction when using conventional algorithms that predominantly focus on insulin demand, it would appear logical to consider a model that incorporates both the extent of blood glucose lowering (hemoglobin A1c) and the means by which the blood glucose was reduced (SD ratio) when considering macrovascular outcomes.

Aqueous extracts of husks of Plantago ovata reduce hyperglycaemia in type 1 and type 2 diabetes by inhibition of intestinal glucose absorption

Plantago ovata has been reported to reduce postprandial glucose concentrations in diabetic patients. In the present study, the efficacy and possible modes of action of hot-water extracts of husk of P. ovata were evaluated. The administration of P. ovata (0.5 g/kg body weight) significantly improved glucose tolerance in normal, type 1 and type 2 diabetic rat models. When the extract was administered orally with sucrose solution, it suppressed postprandial blood glucose and retarded small intestinal absorption without inducing the influx of sucrose into the large intestine. The extract significantly reduced glucose absorption in the gut during in situ perfusion of small intestine in non-diabetic rats. In 28 d chronic feeding studies in type 2 diabetic rat models, the extract reduced serum atherogenic lipids and NEFA but had no effect on plasma insulin and total antioxidant status. No effect of the extract was evident on intestinal disaccharidase activity. Furthermore, the extract did not stimulate insulin secretion in perfused rat pancreas, isolated rat islets or clonal beta cells. Neither did the extract affect glucose transport in 3T3 adipocytes. In conclusion, aqueous extracts of P. ovata reduce hyperglycaemia in diabetes via inhibition of intestinal glucose absorption and enhancement of motility. These attributes indicate that P. ovata may be a useful source of active components to provide new opportunities for diabetes therapy.

Inhibitory effect of metformin on intestinal glucose absorption in the perfused rat intestine

To investigate the effect of metformin on intestinal glucose absorption, a perfusion study of the intestine was performed in the rat. Male Wistar albino rats (8 weeks old) were used in the present study. The glucose absorption by the perfused intestine (788.1+/-81.9 micromol/30 min) was not changed significantly by the direct addition of metformin (90 microg/mL) to the perfusing medium (737.0+/-118.2 micromol/30 min) or by intraduodenal metformin (250 mg/kg in saline solution) infusion (772.8+/-106.3 micromol/30 min). In rats orally administered metformin (250 mg/kg) for 5 days, glucose absorption by the perfused intestine (375.0+/-164.3 micromol/30 min) was significantly (P<0.001) lower than that in control rats (811.0+/-83.1 micromol/30 min). These results indicate that metformin had a significant effect on the digestive tract, and that metformin treatment exerted an inhibitory effect on intestinal glucose absorption in the rat.

Decreased skeletal muscle phosphotyrosine phosphatase (PTPase) activity towards insulin receptors in insulin-resistant Zucker rats measured by delayed Europium fluorescence

In order to measure the phosphotyrosine phosphatase (PTPase) activity in small muscle biopsies, a sandwich-immunofluorescence assay was developed using the phosphorylated human insulin receptor as a substrate, a C-terminal insulin receptor antibody as catching antibody and Europium-labelled anti-phosphotyrosine as detecting antibody. Soluble and particulate muscle fractions were prepared from soleus muscle of obese, diabetic (fa/fa) Zucker rats and their lean littermates (Fa/-). In the soluble muscle fractions of the obese (fa/fa) rats PTPase activity was significantly reduced compared to control (Fa/-) rats (45.2 +/- 2.6% vs 61.3 +/- 4.7%, p < 0.02). This reduction was completely prevented by 24 days of metformin treatment which decreased plasma glucose and plasma insulin levels. In particulate muscle fractions, however, no difference in PTPase activity was found among any groups of rats examined. These results show that the alterations in soluble PTPase activity in the insulin-resistant, diabetic Zucker rat vary with the abnormality in glucose homeostasis.

Metformin ameliorates diabetes but does not normalize the decreased GLUT 4 content in skeletal muscle of obese (fa/fa) Zucker rats

We studied the expression of the glucose transporter GLUT 4 in the soleus and red gastrocnemius muscles from obese, diabetic (fa/fa) Zucker rats compared to their lean littermates (Fa/-), with and without treatment with the antidiabetic drug metformin. In the untreated groups of rats, the GLUT 4 content in a crude membrane fraction of both the soleus and the red gastrocnemius muscles were significantly lower in the obese (fa/fa) rats (3.46 +/- 0.28 vs. 6.04 +/- 0.41, p < 0.001 and 6.0 +/- 0.24 vs. 9.1 +/- 0.48, p < 0.0001, respectively). Differences in GLUT 4 expression in soleus muscle from the same rats were confirmed by quantitative immunofluorescence microscopy, and the results were significantly correlated with the results obtained from quantitative immunoblotting (rho = 0.70, p < 0.0005). The decreased expression of GLUT 4 in fa/fa rats could contribute to the well-established insulin resistance in skeletal muscle of these animals. After 4 weeks of treatment with metformin, weight gain was not affected in either the diabetic (fa/fa) rats or the lean (Fa/-) rats. Improvement of glucose homeostasis by metformin was not associated with normalization of the GLUT 4 expression in the skeletal muscles studied, indicating (1) that the decreased GLUT 4 expression is not directly related to hyperinsulinaemia and diabetes mellitus and (2) that metformin does not normalize the expression of GLUT 4 in skeletal muscle of the diabetic (fa/fa) Zucker rats.

Effect of metformin on glucose metabolism in the splanchnic bed

1. Use of the antihyperglycaemic agent, metformin, is often associated with a small rise in circulating lactate. This study investigates the source of the lactate and examines the effect of metformin on glucose metabolism by the intestine and liver of rats. 2. Changes in plasma glucose and lactate were measured in the inferior vena cava (IVC), hepatic portal vein (HPV), hepatic vein (HV) and aorta (A) after intrajejunal administration of metformin (50 and 250 mg kg-1) without and with glucose (2 g kg-1). 3. Metformin 250 mg kg-1 reduced the hyperglycaemic response to a glucose challenge, associated with a greater reduction of glucose concentrations in the HPV (average decrease of 33% at 60 and 120 min) than at other sites. 4. Both doses of metformin increased lactate concentrations in the glucose-loaded state: the highest concentration (2.5 fold increase) was recorded in the HPV 60 min after administration of 250 mg kg-1 metformin, with a high lactate concentration persisting in the HV at 120 min. Metformin 250 mg kg-1 also increased lactate concentrations in the basal state, with highest concentrations (2 fold increase) in the HPV. 5. Two hours after intrajejunal administration of metformin, 50 mg kg-1, rings of tissue from the small intestine showed an average 22% decrease in glucose oxidation ([14C]-glucose conversion to 14CO2) and a 10% increase in lactate production. Since glucose metabolism in the gut is predominantly anaerobic, metformin caused an overall 9.5% increase of intestinal glucose utilization.6. Metformin, 10-6 and I0- mol 1', did not significantly alter glucose oxidation or lactate production by isolated hepatocytes, but a very high concentration of metformin (102 mol 1') increased lactate production by 60%.7. The results support the view that metformin increased intestinal glucose utilization and lactate production by the intestine. Under basal conditions there was net extraction of lactate by the liver but not after an enteral glucose load.

Reconsideration of inhibitory effect of metformin on intestinal glucose absorption

New evidence that metformin increases intestinal glucose metabolism has necessitated a re-examination of the effect of metformin on intestinal glucose absorption. Normal 18 h fasted mice received an intragastric bolus of metformin 2 h before preparation of everted gut sacs from the proximal, middle and distal regions of the jejunum and ileum. Net mucosal glucose transfer from the intestinal lumen into the tissue was reduced by 15 and 28% after 50 and 250 mg kg-1 metformin, respectively (ANOVA, P less than 0.05). Net glucose transfer into the serosal fluid was reduced by 12 and 70% after 50 and 250 mg kg-1 metformin respectively (ANOVA, P less than 0.05 and P less than 0.01). The inhibitory effect of metformin on both the mucosal and serosal glucose transfer mechanisms was greatest in the middle portion of the small intestine. The results suggest that metformin decreases intestinal glucose absorption in a dose-dependent manner by effects on mucosal and serosal glucose transfer.

Blood-glucose-lowering activity of 2-(3-phenylpropoxyimido)-butyrate (BM 13.677)

A single oral or intraperitoneal application of 2-(3-phenylpropoxyimido)-butyrate (BM 13.677) resulted in a dose-dependent blood-glucose-lowering effect in fasted guinea-pigs. The threshold dose and the EC50 were estimated as 25 mg/kg and 63 mg/kg, respectively, which is between that of the biguanides phenformin and metformin. A rise in blood lactate concentrations was observed only at high doses of BM 13.677, but was not related to an irreversible metabolic inhibition. Among several rodent species studied the potency of the drug decreased in the order guinea-pig much greater than mouse greater than rat = rabbit. Inhibition of hepatic gluconeogenesis by the drug was demonstrated in the perfused liver or hepatocytes of guinea-pigs. Inhibition of glucose production by the perfused liver in the presence of 0.1 mM BM 13.677 was dependent on the substrate and decreased in the order: lactate greater than pyruvate greater than alanine much greater than propionate greater than glycerol = fructose. This suggests a specific interaction of the drug with a mitochondrial key reaction of gluconeogenesis. Stimulation of glucose oxidation in rat diaphragm by the compound (EC50 = 0.85 mM) suggests that besides inhibition of gluconeogenesis also extrahepatic effects contribute to the blood-glucose-lowering effects of the drug.

Effect of metformin treatment on insulin action in diabetic rats: in vivo and in vitro correlations

The mechanism (both at the whole body and cellular level) by which metformin improves insulin sensitivity has yet to be defined. In the present study, we examined in vivo insulin-mediated whole-body glucose disposal, glycogen synthesis, hepatic glucose production, and insulin secretion, as well as in vitro muscle insulin receptor tyrosine kinase activity in eight control, eight neonatal streptozotocin diabetic rats, and eight diabetic rats before and after treatment with metformin. Ten weeks after birth diabetic rats had higher fasting (132 + 5 v 101 + 2 mg/dL) and postmeal (231 + 10 v 133 + 3) plasma glucose levels compared with controls (P less than .001). Metformin treatment was followed by a significant decrease in the growth rate and normalized glucose tolerance without enhancing the deficient insulin response. Insulin-mediated glucose uptake in diabetic versus control rats was reduced (P less than .01) during the high-dose (15.4 + 0.6 v 18.3 + 1.0 mg/kg.min) insulin clamp study and was increased to values greater (P less than .05) than controls following metformin treatment. Muscle glycogen synthetic rate in vivo, measured by incorporation of 3H-3-glucose radioactivity, was diminished by 25% (P less than .01) in diabetic rats, restored to normal values with metformin, and correlated closely (r = .82, P less than .002) with total-body glucose uptake during the insulin clamp in all three groups. Insulin receptor tyrosine kinase activity, measured in partially purified insulin receptors, was reduced in diabetic rats and increased to supernormal levels after metformin. The decrease in muscle tyrosine kinase activity in diabetic versus control animals was entirely accounted for by a reduction in maximal velocity (Vmax) (32 v 45 pmol/mg.min, P less than .01) and increased to supernormal levels following metformin (91 pmol/mg.min, P less than .001) without any change in affinity (Km). Muscle tyrosine kinase activity was closely correlated with both the muscle glycogen synthetic rate (r = .82, P less than .002) and total-body insulin-mediated glucose disposal (r = .64, P less than .01) in vivo. The close correlation between in vivo insulin action, muscle glycogen synthesis, and muscle insulin receptor tyrosine kinase activity is consistent with an important role of the enzyme in the insulin resistance of diabetes and its improvement following metformin treatment.

Hypoglycaemic effect of metformin in genetically obese (fa/fa) rats results from an increased utilization of blood glucose by intestine

The insulin-resistant obese fa/fa rat is a convenient model in which to study a potential effect of metformin, a biguanide used in the treatment of non-insulin-dependent diabetes, on insulin-mediated glucose utilization. Female fa/fa rats were given metformin orally for 8 days. Studies were performed on anaesthetized post-absorptive rats 5 h after the last dose of metformin. Glucose production and utilization were enhanced 1.5-fold in metformin-treated rats. The enhanced glucose production was almost entirely due to increased glucose recycling. The digestive tract was the only tissue responsible for the enhanced glucose utilization.

The effects of metformin on adipocyte insulin action and metabolic control in obese subjects with type 2 diabetes

To investigate the mechanisms of action of metformin, insulin receptor binding and the activity of several insulin-controlled metabolic pathways were measured in adipocytes taken from 10 obese Type 2 diabetic patients treated for 4 weeks with either metformin (0.5 g x 3 daily) or matching placebo using a double-blind crossover design. Metformin therapy was associated with a significant fall in serum fructosamine levels (3.1 +/- 0.4 vs 2.8 +/- 0.4 mmol l-1, p less than 0.02) as well as fasting (10.8 +/- 2.4 vs 9.4 +/- 2.1 mmol l-1) and daytime (11.5 +/- 2.4 vs 10.0 +/- 2.2 mmol l-1) plasma glucose concentrations (p less than 0.05). Fasting and postprandial plasma levels of C-peptide and insulin were unchanged. While fasting plasma lactate concentrations remained unaltered after metformin, a rise was noted in response to meals (from 1.4 +/- 0.1 to 1.8 +/- 0.2 mmol l-1, p less than 0.05). Adipocyte insulin receptor binding was unaffected by drug treatment. Moreover, no insulin-like effects or post-binding potentiation of insulin action could be found on adipocyte glucose transport, glucose oxidation, lipogenesis, glycolysis or antilipolysis. A complementary in vitro study using adipocytes from non-obese healthy volunteers failed to show any direct effect of metformin on adipocyte insulin binding or glucose transport and metabolism, at media drug concentrations corresponding to therapeutic plasma levels.

Pharmacokinetic-pharmacodynamic relationships of oral hypoglycaemic agents. An update

Oral hypoglycaemic drugs, sulphonylureas and biguanides, occupy an important place in the treatment of Type II (non-insulin-dependent) diabetic patients who fail to respond satisfactorily to diet therapy and physical exercise. Although the precise mechanisms of action of these compounds are still poorly understood, there is sufficient agreement that sulphonylureas have both pancreatic and extrapancreatic effects, whereas biguanides have predominantly extrapancreatic actions. By using labelled compounds or measuring the circulating concentrations, the main pharmacokinetic properties of oral hypoglycaemic agents have been assessed and, in some cases, their pharmacokinetic-pharmacodynamic relationships have been evaluated. A correlation between diabetes control and plasma sulphonylurea or biguanide concentrations is generally lacking at the steady-state, with the possible exception of long-acting agents; after either oral or intravenous dosing, the reduction of plasma glucose is usually related to the increased circulating drug concentrations. The toxic effects of oral hypoglycaemic drugs are more frequent in the elderly and in the presence of conditions that may lead to drug accumulation or potentiation (increased dosage, use of long-acting compounds, hepatic and renal disease, interaction with other drugs); however, a relationship between toxic effects and drug plasma levels has been reported only for biguanides.

Treatment--metformin

The hyperglycaemia of NIDDM is associated with insulin resistance due, in part, to reduced insulin receptor binding and more especially postreceptor defects. Metformin is an antihyperglycaemic agent which can be used to ameliorate insulin resistance. It appears to act directly on insulin target cells to enhance insulin action. Although metformin may increase insulin-receptor binding, its main effect appears to be directed at the postreceptor level of insulin action. Accordingly the drug potentiates insulin-suppression of hepatic gluconeogenesis and increases insulin-mediated peripheral glucose uptake and metabolism. It does not stimulate insulin release, does not cause weight gain and does not cause clinical hypoglycaemia. The risk of lactate accumulation should be appreciated in patients with renal insufficiency, liver dysfunction and following acute illness with hypoxia, when therapy should be stopped. Although metformin is often bracketed with phenformin in the context of lactic acidosis, different pharmacodynamics and adherence to prescribing guidelines render such a comparison unwarranted.

Postreceptor effect of metformin on insulin action in mice

To investigate the possibility that metformin (dimethylbiguanide) modifies insulin-mediated glucose metabolism by an effect that is independent of insulin receptor binding, glycogenesis and insulin binding were measured in soleus muscles isolated from streptozocin diabetic mice after treatment with 60 mg kg-1 metformin daily for 10 weeks. This dose of metformin increased insulin-stimulated glycogenesis but did not affect insulin binding in the soleus muscles of streptozocin diabetic mice. The results suggest that metformin can influence postreceptor sites of insulin action independently of insulin receptor binding.

The effects of phenformin on the transport and metabolism of sugars by the rat small intestine

The effects of 0.25-10 mM phenformin on sugar transport and metabolism have been studied in a preparation for the combined perfusion of the vascular bed and the lumen. At all concentrations the effects of vascular phenformin were more pronounced than those of luminal phenformin. Phenformin inhibited galactose transport across the intestine, the pattern of inhibition depending on whether the phenformin was added to the luminal or vascular compartments. The active accumulation of galactose in the mucosal epithelial cells was also abolished. There was a linear relationship between the percentage reduction in mucosal ATP levels and vascular phenformin concentration. Phenformin reduced the rate of glucose uptake from the lumen, and the proportion of this glucose which reached the vascular effluent. Most of the glucose which did not reach the vascular side could be accounted for by the formation of lactic acid. Vascular phenformin increased glucose uptake from the vascular medium by ca 88%, 97% of which could be accounted for by lactate formation. Phenformin was sequestered by the mucosa when added to the vascular, but not the luminal, perfusates. There was very little translocation of intact phenformin across the gut in either the mucosal or serosal directions. It is suggested that the effects of phenformin on the gut mainly derive from an inhibition of mitochondrial oxidative phosphorylation, with a small contribution from a direct effect on the brush border, more pronounced at high phenformin concentrations. The results are consistent with the idea that phenformin delays sugar absorption in man, and that the intestine may be a significant source of lactate production in lactic acidosis.

The mechanism of action of phenformin in starved rats

The ability of phenformin to lower the blood glucose concentration after an intraperitoneal glucose load, with a concomitant increase in blood lactate concentration, indicated that the drug was increasing the rate of anaerobic glycolysis. The results of experiments in which glucose and gluconeogenic precursors were given to starved rats were explained by a hypothesis for the mechanism of action of phenformin involving inhibition of certain NAD+-dependent dehydrogenases. Substrates with NAD+-linked oxidations could be discriminated from those, like succinate, with FAD-linked oxidations, and succinate may be of use in the treatment of clinical lacticacidosis caused by biguanide drugs.

Effects of two glucose absorption inhibitors: phenformin and 43-522 on hepatic gluconeogenesis

The glucose absorption inhibitors, Phenformin and 3-phenylpropylaminoguanide HCl (SaH 43-522) have different effects on hepatic gluconeogenesis. Unlike phenformin, 43--522 in vivo does not inhibit hepatic gluconeogenesis; inhibition occurred only when 43--522 was added directly to a liver perfusion system. This suggests that when 43--522 is administered in vivo it is metabolized to an agent that does not inhibit hepatic gluconeogenesis whereas phenformin is not metabolized, and does inhibit gluconeogenesis. This is an advantage of 43--522 since it thereby specifically inhibits intestinal glucose absorption without a potential hypoglycaemic effect due to the suppression of gluconeogenesis.

Oxidation of an exogenous glucose load using naturally labelled 13C-glucose. Effect of butylbiguanide therapy in obese mildly diabetic subjects

The effect of a 14 day-administration of butylbiguanide was investigated in a group of 10 obese patients with mild-to-moderate glucose intolerance. Glucose tolerance was significantly improved, while fasting blood glucose and plasma levels of free fatty acids, insulin and glucagon remained unchanged. The estimation of the amount of the oral glucose load oxidized into CO2 was performed by means of a recently described procedure using "naturally labelled 13C-glucose" as tracer. The curves depicting the oxidation of the exogenous glucose load were similar in shape and magnitude before and after administration of the biguanide; in the latter case, however, slightly higher rates of oxidation of exogenous glucose were recorded during the 2nd, 3rd and 4th hours of the test. These data do not provide evidence that the biguanide-induced improvement in glucose tolerance in patients with mild-to-moderate glucose intolerance is associated with any inhibiting or delaying effect of this type of drug on intestinal absorption (and subsequent oxidation) of the exogenous glucose load. On the contrary, a slight, but statistically significant, increase in the oxidation of exogenous glucose has been observed after butylbiguanide.

Alteration of bile acid metabolism and vitamin-B12-absorption in diabetics on biguanides

Since vitamin B12malabsorption has been described in diabetics on biguanides and inhibition of bile acid absorption found in rat ileum the effect of treatment with different biguanides (phenformin, buformin, metformin) on bile acid metabolism and vitamin B12 absorption was assessed in maturity onset diabetics. Biguanides did not alter faecal weight or faecal fat excretion, but they decreased faecal bile acid excretion. All biguanides tested increased deconjugation of glycocholic acid, as determined by a simple breath test technique. Vitamin B12 malabsorption was most prominent in patients on metformin. Discontinuation of biguanide treatment, or administration of antibiotics, normalized or improved the increased deconjugation of bile acids and the Schilling test. Decreased faecal bile acid excretion, positive 14C-glycocholate breath tests, pathological Schilling tests and the reversal of pathological tests by antibiotic treatment suggest that small intestinal bacterial overgrowth, leading to binding of the intrinsic-factor-vitamin B12-complex to bacteria, is responsible for the previously observed pathological Schilling tests in diabetics on biguanides. Bile acid malabsorption, possibly responsible for the cholesterol-lowering effect of biguanides, does not occur in diabetics on biguanides. Whether qualitative changes in small intestinal bile acid composition might affect cholesterol metabolism remains to be determined.

Oral versus intravenous administration of butylbiguanide: effect on oral glucose tolerance in normal humans

1. The effect of oral and intravenous administration of butylbiguanide on the oral glucose tolerance test was investigated in 12 normal human volunteers. Orally administered butylbiguanide lowered the oral glucose tolerance curve and the corresponding serum insulin values, whereas intravenously injected butylbiguanide had no effect. 2. The lactate/pyruvate ratio determined 180 min. after glucose ingestion increased after oral butylbiguanide administration. No effect was observed after intravenous administration. 3. The decrease in serum phosphate levels observed after oral glucose loading was not influenced by butylbiguanide administered by either route. In addition, no effect of this drug was observed on fasting levels of glucose, insulin, phosphate, bicarbonate, lactate and pyruvate. 4. These results are compatible with recent observations that biguanides inhibit active transport mechanisms in the intestine. In addition, the effect of biguanides on blood levels of glucose metabolites is discussed with respect to their known action on oxidative phosphorylation.

Effects of metformin on insulin resistance after injury in the rat

Hyperglycaemia and insulin resistance occur after injury. The effects of the antidiabetic biguanide metformin in injured rats have been studied in order to elucidate the cause of these effects. Metformin (120 mg/kg S. C.) produced a significant hypoglycaemic effect after a 20% dorsal scald but did not affect the blood glucose concentration in non-injured rats. The hypoglycaemic effect did not result from increased insulin secretion. It was associated with a reduction in liver glycogen and an increase in blood lactate concentrations, suggesting that the drug acted by promoting peripheral glucose utilization. This was confirmed by measuring the clearance rate coefficient of [5-3H]glucose. This rate coefficient was significantly increased by metformin treatment (140 mg/kg S. C.) in scalded rats, although it was not affected in non-injured rats. Intravenous glucose tolerance in scalded rats was not improved, probably because of the increased lactate concentration. Metformin (120-160 mg/kg) also produced a hypoglycaemic effect in rats after a 4 hrs period of bilateral hind-limb ischaemia, suggesting that similar metabolic changes occur after these two types of injury.

Inhibition of bile salt absorption by blood-sugar lowering biguanides

The effect of blood sugar lowering biguanides (phenethyl-, butyl- and dimethylbiguanide) upon jejunal and ileal transport of bile salts (tauro- and glycocholate) was tested in rat small intestine by an in vitro technique. Giguanides inhibited active transport of bile salts in the ileum, but did not affect diffusional absorption of bile salts in the jejunum. The inhibitory effect was time-dependent and not reversible under in vitro incubation conditions, suggesting that biguanides must enter intestinal mucosal cells in order to exert their inhibitory action on active transport of glucose analogues, amino acids, calcium and bile salts. Since biguanides achieve high tissue concentrations in the small intestine even after parenteral administration, inhibition of ileal bile salt reabsorption by biguanides could possibly explain the lipid- and cholesterol-lowering effect of these oral antidiabetic drugs.

The effect of phenformin upon the plasma pancreatic and gut glucagon-like immunoreactivity in diabetics

Five patients with mild maturity-onset diabetes were given 250 ml of a 20% glucose solution by intraduodenal infusion and eight other patients similarly received an amino acid solution in a dose of 0.5 g amino acids per kg body weight. The pancreatic and gut glucagon-like immunoreactivity (pancreatic GLI and gut GLI) in plasma were measured before and after the application of the two stimuli. Each person was tested twice; the first (control) test was followed by a second test after three days of treatment with phenformin 150 mg daily, plus the same 150 mg dose taken 60 min before the intubation. The plasma pancreatic GLI increased slightly during both infusions, but was not affected by phenformin. Intraduodenal infusion of both glucose and the amino acid solution induced a greater rise in plasma gut GLI. After treatment with phenformin, the fasting plasma gut GLI was higher than the control value in eleven of thirteen patients. In most cases higher gut GLI plasma levels were also found after duodenal administration of glucose and amino acids. These data furnish further evidence of the local action of antidiabetic biguanides on the intestinal wall, including its hormonal activity. The hypothesis is advanced that the phenformin-induced increase in gut GLI secretion may bring about competition of the latter with pancreatic glucagon for receptors in liver cell membranes, reducing the effect of glucagon on the liver, and thus contributing to a decrease in glycaemia.