Effect of buformin on splanchnic carbohydrate and substrate metabolism in healthy man

Extensive therapeutic effects, underlying molecular mechanisms and disease treatment prediction of Metformin: a systematic review

Metformin (Met), a first-line management for type 2 diabetes mellitus, has been expansively employed and studied with results indicating its therapeutic potential extending beyond glycemic control. Beyond its established role, this therapeutic drug demonstrates a broad spectrum of action encompassing over 60 disorders, encompassing metabolic conditions, inflammatory disorders, carcinomas, cardiovascular diseases, and cerebrovascular pathologies. There is clear evidence of Met's action targeting specific nodes in the molecular pathways of these diseases and, intriguingly, interactions with the intestinal microbiota and epigenetic processes have been explored. Furthermore, novel Met derivatives with structural modifications tailored to diverse diseases have been synthesized and assessed. This manuscript proffers a comprehensive thematic review of the diseases amenable to Met treatment, elucidates their molecular mechanisms, and employs informatics technology to prospect future therapeutic applications of Met. These data and insights gleaned considerably contribute to enriching our understanding and appreciation of Met's far-reaching clinical potential and therapeutic applicability.

Biguanide-induced mitochondrial dysfunction yields increased lactate production and cytotoxicity of aerobically-poised HepG2 cells and human hepatocytes in vitro

As a class, the biguanides induce lactic acidosis, a hallmark of mitochondrial impairment. To assess potential mitochondrial impairment, we evaluated the effects of metformin, buformin and phenformin on: 1) viability of HepG2 cells grown in galactose, 2) respiration by isolated mitochondria, 3) metabolic poise of HepG2 and primary human hepatocytes, 4) activities of immunocaptured respiratory complexes, and 5) mitochondrial membrane potential and redox status in primary human hepatocytes. Phenformin was the most cytotoxic of the three with buformin showing moderate toxicity, and metformin toxicity only at mM concentrations. Importantly, HepG2 cells grown in galactose are markedly more susceptible to biguanide toxicity compared to cells grown in glucose, indicating mitochondrial toxicity as a primary mode of action. The same rank order of potency was observed for isolated mitochondrial respiration where preincubation (40 min) exacerbated respiratory impairment, and was required to reveal inhibition by metformin, suggesting intramitochondrial bio-accumulation. Metabolic profiling of intact cells corroborated respiratory inhibition, but also revealed compensatory increases in lactate production from accelerated glycolysis. High (mM) concentrations of the drugs were needed to inhibit immunocaptured respiratory complexes, supporting the contention that bioaccumulation is involved. The same rank order was found when monitoring mitochondrial membrane potential, ROS production, and glutathione levels in primary human hepatocytes. In toto, these data indicate that biguanide-induced lactic acidosis can be attributed to acceleration of glycolysis in response to mitochondrial impairment. Indeed, the desired clinical outcome, viz., decreased blood glucose, could be due to increased glucose uptake and glycolytic flux in response to drug-induced mitochondrial dysfunction.

Effects of metformin on the pathways of glucose utilization after oral glucose in non-insulin-dependent diabetes mellitus patients

To analyze the effects of metformin (M) on the kinetics and pathways of glucose utilization after glucose ingestion, nine non-insulin-dependent diabetes mellitus (NIDDM) patients underwent two 5-hour oral glucose tolerance tests (OGTTs) preceded in random order by a 3-week treatment with either M (850 mg twice per day) or placebo. Each test included intravenous infusion of 3-3H-glucose and labeling of the oral dose (75 g) with 1-14C-glucose, with measurements of glucose kinetics, glycolytic flux (3H2O production), and glucose oxidation (indirect calorimetry and expired 14CO2). Basal glycemia was decreased by M (6.6 v 8.2 mmol/L, P < .01) with no changes in insulin levels, with the hypoglycemic effect correlating strongly (P < .001) with a decrease in glucose production. Mean 0- to 5-hour postprandial glycemia was also decreased by the drug (9.9 v 12.2 mmol/L, P < .04), lactate concentration was increased (1.79 v 1.44 mmol/L, P < .01), and absolute insulin levels were increased, but not to a significant extent. The rates of appearance (Ra) of exogenous and endogenous glucose were not modified, and the hypoglycemic effect of M in the postprandial state was entirely related to an increase in systemic glucose disposal (85.1 v 77.5 g/5 h, P < .001). Carbohydrate oxidation was unchanged, and glycolytic flux and nonoxidative glycolysis were increased by approximately 13 g/5 h (P < .01), with the excess lactate produced probably being converted to glycogen in the liver. Whole-body glycogen synthesis through the direct pathway tended to be reduced (-8 g/5 h, P > .05). Thus, M decreases postprandial glycemia by increasing glucose disposal and stimulates lactate production. The data also suggest that the drug increases the proportions of glycogen deposited through the indirect rather than the direct pathway.

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.

Possible role of intracellular Ca2+ in the toxicity of phenformin

Selective use of various mitochondrial Ca2+ transport inhibitors indicated that significant Ca2+ redistribution may occur during the isolation of mitochondria. Exposure of guinea-pig liver mitochondria to phenformin (beta-phenethylbiguanide) during the isolation procedure resulted in decreased mitochondrial Ca2+. Novel isolation conditions were developed to determine liver mitochondrial calcium content considered to reflect that in vivo. Administration of phenformin to rats and guinea-pigs resulted in decreased mitochondrial Ca2+. Decreased liver mitochondrial Ca2+ correlated inversely with raised blood lactate concentrations in the guinea-pig; 2-oxoglutarate, but not succinate oxidation, was inhibited in these mitochondrial preparations. A mechanism of action for phenformin-associated lactic-acidosis, attributable to impaired mitochondrial function arising from inactivation of Ca2+-sensitive, NAD+-dependent mitochondrial dehydrogenases (e.g. 2-oxoglutarate dehydrogenase) due to alteration in mitochondrial calcium content, is proposed.

Metformin improved insulin resistance in type I, insulin-dependent, diabetic patients

The biguanides have been shown to reduce insulin requirements in type I (insulin-dependent) diabetic patients with an increase in insulin binding to insulin receptors. The aim of this was to measure the effect of metformin (850 mg/twice daily) on insulin sensitivity. Ten type I diabetic patients of normal weight received metformin or placebo in addition to their insulin therapy for seven days. On the last day of metformin or placebo treatment, tissue sensitivity was measured by the euglycemic hyperinsulinaemic clamp procedure using the artificial pancreas. An 18% improvement in glucose uptake was observed after metformin therapy (P less than 0.01). Metformin was therefore effective in improving the insulin action in type I diabetic patients, although its use in such circumstances requires consideration of several other factors.

Hepatic disposal of biosynthetic human insulin and porcine C-peptide in humans

To examine the fate of insulin across the liver bed, biosynthetic human insulin was infused in increasing amounts in six healthy men. Trapping of insulin by the liver was determined by means of the hepatic venous catheter technique. To minimize any possible error in the estimation of insulin removal as a result of endogenous insulin, pancreatic insulin secretion was suppressed by intravenous administration of somatostatin (500 micrograms/h). Infusion rates of human insulin were 30, 60, and 150 pmol/m2 X min (corresponding to 0.25, 0.5, and 1.25 U/m2 X h) for 70 minutes each. Steady-state insulin levels were within the physiologic range, ie, 69 +/- 2, 135 +/- 3, and 342 +/- 10 pmol/L, respectively. Euglycemia was maintained throughout the study by a variable glucose infusion. The output of C-peptide from the splanchnic bed was reduced by somatostatin by about 90%, indicating that endogenous insulin production only minimally contributed to total insulin levels achieved during infusion of exogenous human insulin. Fractional extraction of insulin by the liver (63 +/- 6%, 71 +/- 4%, and 74 +/- 5%) and hepatic insulin clearance (201 +/- 19, 235 +/- 23, and 245 +/- 29 mL/m2 X min) did not differ significantly during the three insulin infusion studies. The hepatic uptake of insulin rose with increasing insulin infusion rates, constituting 40% to 60% of total-body insulin removal. No change in the total metabolic clearance rate of insulin was observed between the groups (451 +/- 8 mL/m2 X min). To study the extraction rate of C-peptide by the liver, porcine C-peptide was also infused at the same increasing rates.(ABSTRACT TRUNCATED AT 250 WORDS)

The 75-g oral glucose tolerance test: effect on splanchnic metabolism of substrates and pancreatic hormone release in healthy man

To determine the effect of the 75 g oral glucose tolerance test on carbohydrate and lipid metabolism, the splanchnic exchange of glucose, lactate, pyruvate, non-esterified fatty acids, beta-hydroxybutyrate and acetoacetate as well as the release of insulin, C-peptide, glucagon and pancreatic polypeptide were evaluated in eight healthy male volunteers in the basal state and for 150 min following glucose ingestion. Oral glucose loading was followed by a rapid rise in splanchnic output of glucose (mean +/- SEM; 154 +/- 12 mmol/150 min), pyruvate (1.2 +/- 1.2 mmol/150 min) and lactate (8.6 +/- 2.0 mmol/150 min), whereas there were reductions in the splanchnic uptake of non-esterified fatty acids (-10.7 +/- 4.4 mmol/150 min) and the splanchnic output of beta-hydroxybutyrate (-4.8 +/- 3.3 mmol/150 min) and acetoacetate (-3.0 +/- 1.2 mmol/150 min). In parallel, splanchnic output of insulin (12.3 +/- 2.7 nmol/150 min), C-peptide (36.1 +/- 5.0 nmol/150 min) and transiently of pancreatic polypeptide rose, whereas that of glucagon fell (-0.58 +/- 0.21 nmol/150 min). Even at 150 min after glucose ingestion, splanchnic output and arterial concentrations of glucose, lactate, insulin and C-peptide were still above their respective basal values while those of non-esterified fatty acids and glucagon were reduced. Taking into account the partial suppression of endogenous glucose production by ingested glucose it is concluded that, in normal postabsortive man, only 49-63% of a 75 g oral glucose load is retained by the splanchnic bed during the first 150 min, the rest being available for non-hepatic tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Drug and chemical-induced metabolic acidosis

Metabolic acidosis produced by drugs and/or chemicals can be conveniently divided into those with an increase in the anion gap (anion gap = Na- (Cl + HCO3)) and those with a normal anion gap. The increase in the anion gap is due to the accumulation of unmeasured organic anions, such as lactate or acetoacetate and beta-hydroxybutyrate, as occurs in ketoacidosis and lactic acidosis, or the accumulation of toxic anions such as formate or glycolate, as occurs with the ingestion of methanol or ethylene glycol. Increased concentrations of lactic acid may also be present in the toxic forms of metabolic acidosis. The most common drugs and chemicals that induce the anion gap type of acidosis are biguanides, alcohols, polyhydric sugars, salicylates, cyanide and carbon monoxide. In normal anion gap acidosis the reduction in bicarbonate is balanced by a reciprocal increase in the chloride concentration so that the sum of the two remains unchanged. Normal anion gap acidosis is caused by carbonic anhydrase inhibitors, hydrochloride salts of amino acids, toluene, amphotericin, spironolactone and non-steroidal anti-inflammatory drugs. The mechanism by which these substances produce metabolic acidosis and the therapy are discussed.