Glipentide and glucose metabolism in isolated rat hepatocytes

Effects of brazilin on the production of fructose-2,6-bisphosphate in rat hepatocytes

Increased hepatic glucose output is one of the major mechanisms of hyperglycemia in diabetic patients. Fructose-2,6-bisphosphate (F-2,6-BP), a gluconeogenic intermediate, plays a critical role in hepatic glucose output by regulating gluconeogenesis and glycolysis in the liver. Brazilin, an active component of sappan wood (Caesalpinia sappan), decreases blood glucose in diabetic animals. In this study, the effect of brazilin on gluconeogenic intermediate production and enzyme activity were examined to investigate the hypoglycemic mechanism of brazilin. Brazilin increased the production of F-2,6-BP in hepatocytes by elevating intracellular levels of fructose-6-phosphate (F-6-P) and hexose-6-phosphate (H-6-P). Brazilin was also found to significantly increase the activity of 6-phosphofructo-2-kinase (PFK-2) and pyruvate kinase in glucagon-treated hepatocytes. However, glucose-6-phosphatase activity was not affected by brazilin. This data suggests that brazilin inhibits hepatic gluconeogenesis by elevating the F-2,6-BP level in hepatocytes, possibly by elevating cellular F-6-P/H-6-P levels and PFK-2 activity. Increased pyruvate kinase activity may also play a role in the anti-gluconeogenic action of brazilin.

Effect of a novel hypoglycemic agent, KAD-1229 on glucose metabolism and fructose-2,6-bisphosphate content in isolated hepatocytes of normal rats

The effects of a novel hypoglycemic agent, calcium(2s)-2-benzyl-3-(cis-hexahydro-2-isoindolinylcarbonyl) propionate dihydrate (KAD-1229), which is a benzyl succinate derivative, on liver metabolism were investigated using isolated hepatocytes from normal rats. In the presence of 10 mM glucose, KAD-1229 increased the L-lactate production (41.1 +/- 0.9 versus 60.9 +/- 2.6 mumol of lactate/g of cells/30 min; P < 0.05) and inhibited gluconeogenesis in hepatocytes (0.94 +/- 0.02 versus 0.70 +/- 0.03 mumol of [2-14C]-pyruvate converted to glucose/g of cells/20 min; P < 0.05). These effects by KAD-1229 were accompanied by an increase in the cellular content of fructose-2,6-bisphosphate (F-2,6-P2), which is one of the important regulators of hepatic glucose metabolism, in a dose-dependent manner (0.05-2.5 mM). KAD-1229 also stimulated the oxidation of [2-14C]-pyruvate and [6-14C]-glucose in the tricarboxylic acid cycle (+18 and +31%, respectively), indicating that stimulation of tricarboxylic acid cycle activity and/or enhancement of the glycolytic flux rate had occurred. Moreover, KAD-1229 did not modify the activities of 6-phosphofructo 2-kinase or fructose-2,6-bisphosphatase, but increased significantly the accumulation of fructose 6-phosphate in hepatocytes. These results suggest that KAD-1229 has extrapancreatic effects on hepatic glucose metabolism, that its actions are mediated through the inhibition of fructose-1,6-bisphosphatase and stimulation of both the 6-phosphofructo 1-kinase reaction and tricarboxylic acid cycle activity by increasing the F-2,6-P2 content in hepatocytes, and that these multiple effects may account in part for the ability of KAD-1229 to reduce blood glucose levels in vivo.

Modulation of glycogen phosphorylase activity and fructose 2,6-bisphosphate levels by glibenclamide and meglitinide in isolated rat hepatocytes: a comparative study

The influence of glibenclamide and meglitinide, or 4-[2-(5-chloro-2-methoxybenzamide)ethyl]-benzoic acid, a compound similar to the nonsulfonylurea moiety of glibenclamide, on glycogen phosphorylase a activity, fructose 2,6-bisphosphate (F-2,6-P2) level, and cytoplasmic free-Ca2+ concentration has been studied in isolated rat hepatocytes. Both glibenclamide and meglitinide caused a transient and dose-dependent activation of glycogen phosphorylase, with half-maximal effects corresponding to 3.7 +/- 1.6 and 9.6 +/- 3.3 mumol/L, respectively. This enzyme activation occurred without significant changes in hepatocyte cyclic adenosine monophosphate (cAMP) levels and was accompanied by an increase in cytoplasmic concentration of free Ca2+. Parallel to these effects, glibenclamide increased the cellular content of F-2,6-P2, with this effect being associated with a reduction in the rate of glucose formation from a mixture of [14C]lactate/pyruvate. Under similar conditions, meglitinide caused a significant reduction of F-2,6-P2 levels and accelerated the gluconeogenic flux. The mechanism by which meglitinide decreases hepatocyte F-2,6-P2 levels seems to be mediated by stimulation of fructose-2,6-bisphosphatase. This comparative study may help to elucidate which among the hepatic effects of glibenclamide are exerted specifically by the sulfonylurea moiety.

Sulfonylureas activate glycogen phosphorylase and increase cytosolic free-Ca2+ levels in isolated rat hepatocytes

Without causing significant changes in cellular levels of cyclic adenosine monophosphate (cAMP), the addition of either glibenclamide or gliquidone to isolated rat hepatocytes caused a transient dose- and Ca(2+)-dependent activation of glycogen phosphorylase. The calculated concentrations corresponding to half-maximal activation were 5 and 2 mumol/L, respectively. In connection with this, it was observed that glibenclamide provoked a dose-dependent increase in cytosolic free-calcium concentration ([Ca2+]i) in Fura-2-loaded hepatocytes. Moreover, the presence of glibenclamide in the incubation medium accelerated the rate of Ca2+ uptake by Ca(2+)-depleted hepatocytes. These findings suggest that an increase in [Ca2+]i could mediate some of the effects of sulfonylureas in liver metabolism.

Inhibition of gluconeogenesis by tolbutamide in isolated rat hepatocytes: modulation of glucose-6-phosphate substrate cycle

In hepatocytes isolated from 24-hour fasted rats, the oral hypoglycemic agent tolbutamide (1 mmol/L) inhibited glucose formation from different concentrations (1 to 20 mmol/L) of galactose, dihydroxyacetone, glycerol, and a mixture of L-lactate:pyruvate (molar ratio, 10:1). Parallel to the reduction of gluconeogenesis, tolbutamide stimulated L-lactate formation when cells were incubated with either galactose, dihydroxyacetone, or glycerol. All these tolbutamide effects occurred without significant modification of hepatocyte fructose-2,6-bisphosphate (F-2,6-P2) levels. Only when glucose was included in the incubation medium was the inhibition of gluconeogenesis caused by the sulfonylurea accompanied by a significant increment of the cellular F-2,6-P2 concentration. Under these conditions, tolbutamide potentiated the effect of glucose in promoting the increase of this regulatory metabolite, as well as the stimulation of glycolysis; in addition, tolbutamide increased the cellular pool of hexose-6-phosphates and the rate of tritium release from (2-3H)glucose. These results support the hypothesis that tolbutamide regulates hepatic glucose metabolism, at least, by modulating the glucose-6-phosphate substrate cycle.

Effect of sulfonylureas on hepatic glycogen metabolism: activation of glycogen phosphorylase

In hepatocytes isolated from fed rats, both tolbutamide and glipizide caused a dose-dependent activation of glycogen phosphorylase, possibly by a Ca2+-mediated mechanism. Maximal effects (about twofold) were already obtained when drugs were used at 0.5 mmol/L, the calculated concentrations of tolbutamide and glipizide responsible for the half-maximal effects being 60 and 30 mumol/L, respectively. The activation of glycogen phosphorylase caused the mobilization of glycogen and increased the cellular concentration of hexose 6-phosphates (glucose 6-phosphate plus fructose 6-phosphate) and that of fructose 2,6-bisphosphate. Under the influence of sulfonylureas, glucose formation was slightly stimulated while the rate of L-lactate production was more markedly incremented, indicating that sulfonylureas canalize the metabolic flux coming from glycogen mainly to the glycolytic pathway. These results suggest that a glycogenolytic action of sulfonylureas could collaborate to raise hepatic fructose 2,6-bisphosphate concentration in the fed animal.