Tolbutamide stimulates fructose-2, 6-bisphosphate formation in perfused rat liver

Effect of tolbutamide on liver fructose-2,6-bisphosphate (F-2,6-P2) was examined in isolated perfused rat liver in situ with a flow-through method. Tolbutamide (1 mM) gradually increased liver F-2,6-P2 level from 7.4 +/- 1.6 to 21.2 +/- 1.6 pmol/mg wet wt for 20 min perfusion. The increase of liver F-2,6-P2 induced by tolbutamide was dose dependent and was significantly observed at 10 min perfusion. The maximum plateau level of F-2,6-P2 induced by 16.7 mM glucose was further increased with 1 mM tolbutamide. Glucagon (10(-11) M) decreased the elevated level induced by 16.7 mM glucose, but this effect was completely inhibited with 2 mM tolbutamide. Cyclic AMP level of the liver throughout the perfusion with tolbutamide did not change. Carboxytolbutamide or gliclazide perfusion did not change significantly the liver F-2,6-P2 level; however, the results suggest that tolbutamide may increase the liver F-2,6-P2 level by affecting the phosphorylation state of fructose-6-phosphate, 2-kinase/fructose-2,6-bisphosphatase through cyclic AMP-dependent protein kinase, resulting in the stimulation of glycolysis and the inhibition of gluconeogenesis in the liver. Thus, the extrapancreatic action and the mechanism of action of different sulfonylureas may differ.     

Potentiation of the hepatic action of insulin by chlorpropamide.

In perfused livers of fed rats, chlorpropamide inhibits glucagon-stimulated glucose production by augmenting the action of insulin. This effect is associated with a decrease in cyclic AMP accumulation in liver and perfusate. Alterations in glucose production appear to correlate more closely with changes in the amount of cyclic AMP in the perfusate than with changes in intrahepatic concentration of nucleotide. Potenitation by chlorpropamide of the hepatic action of insulin does not require administration of the drug prior to perfusion. Further, it is demonstrable at concentrations of insulin and glucagon (10(-11M) that approximate the normal plasma levels of these hormones.     

No reduction in total hepatic glucose output by inhibition of gluconeogenesis with ethanol in NIDDM patients

Increased gluconeogenesis has been suggested to account for all of the increase in basal glucose production in patients with non-insulin-dependent diabetes mellitus (NIDDM). We studied the effect of inhibition of gluconeogenesis with ethanol on total hepatic glucose output (HGO) in patients with NIDDM. Fourteen patients with NIDDM (mean +/- SE age 61 +/- 2 yr, fasting plasma glucose 11.4 +/- 0.8 mM; body mass index 27 +/- 1 kg/m2) were studied twice after an overnight fast, once during ethanol administration (blood ethanol approximately 12 mM) and once during saline administration. Total HGO rate was measured with [3H]glucose. Inhibition of gluconeogenesis by ethanol was followed qualitatively with [U-14C]lactate (n = 8) and [U-14C]glycerol (n = 6) as tracers. Ethanol inhibited gluconeogenesis from lactate by 71 +/- 5% (0.5 +/- 0.2 vs. 1.8 +/- 0.1 mumol glucose.kg-1.min-1, 240-300 min, P less than 0.001; ethanol vs. saline, P less than 0.001) and from glycerol by 65 +/- 6% (0.8 +/- 0.2 vs. 2.3 +/- 0.6 mumol glucose.kg.min-1, P less than 0.001). Total HGO rate remained unchanged and averaged 12.8 +/- 1.8 and 11.8 +/- 2.1 mumol.kg-1.min-1 in the saline and ethanol studies, respectively (NS). We concluded that inhibition of gluconeogenesis by ethanol does not decrease total HGO in patients with NIDDM. Our results suggest the existence of a regulatory mechanism in the liver that maintains constant total HGO despite inhibition of gluconeogenesis.     

Magnitude of negative arterial-portal glucose gradient alters net hepatic glucose balance in conscious dogs.

To examine the relationship between the magnitude of the negative arterial-portal glucose gradient and net hepatic glucose uptake, two groups of 42-h fasted, conscious dogs were infused with somatostatin, to suppress endogenous insulin and glucagon secretion, and the hormones were replaced intraportally to create hyperinsulinemia (3- to 4-fold basal) and basal glucagon levels. The hepatic glucose load to the liver was doubled and different negative arterial-portal glucose gradients were established by altering the ratio between portal and peripheral vein glucose infusions. In protocol 1 (n = 6) net hepatic glucose uptake was 42.2 +/- 6.7, 35.0 +/- 3.9, and 33.3 +/- 4.4 mumol.kg-1.min-1 at arterial-portal plasma glucose gradients of -4.1 +/- 0.9, -1.8 +/- 0.4, and -0.8 +/- 0.1 mM, respectively. In protocol 2 (n = 6) net hepatic glucose uptake was 26.1 +/- 2.8 and 12.2 +/- 1.7 mumol.kg-1.min-1 at arterial-portal plasma glucose gradients of -0.9 +/- 0.2 and -0.4 +/- 0.1 mM, respectively. No changes in the hepatic insulin or glucose loads were evident within a given protocol. Although net hepatic glucose uptake was lower in protocol 2 when compared with protocol 1 (26.1 +/- 2.8 vs. 33.3 +/- 4.4 mumol.kg-1.min-1) in the presence of a similar arterial-portal plasma glucose gradient (-0.9 vs. -0.8 mM) the difference could be attributed to the hepatic glucose load being lower in protocol 2 (i.e., hepatic fractional glucose extraction was not significantly different) primarily as a result of lower hepatic blood flow. In conclusion, in the presence of fixed hepatic glucose and insulin loads, the magnitude of the negative arterial-portal glucose gradient can modify net hepatic glucose uptake in vivo.     

Predominant role of gluconeogenesis in increased hepatic glucose production in NIDDM

Excessive hepatic glucose output is an important factor in the fasting hyperglycemia of non-insulin-dependent diabetes mellitus (NIDDM). To determine the relative contributions of gluconeogenesis and glycogenolysis in a quantitative manner, we applied a new isotopic approach, using infusions of [6-3H]glucose and [2-14C]acetate to trace overall hepatic glucose output and phosphoenolpyruvate gluconeogenesis in 14 postabsorptive NIDDM subjects and in 9 nondiabetic volunteers of similar age and weight. Overall hepatic glucose output was increased nearly twofold in the NIDDM subjects (22.7 +/- 1.0 vs. 12.0 +/- 0.6 mumol.kg-1.min-1 in the nondiabetic volunteers, P less than .001); phosphoenolpyruvate gluconeogenesis was increased more than threefold in the NIDDM subjects (12.7 +/- 1.4 vs. 3.6 +/- 0.4 mumol.kg-1.min-1 in the nondiabetic subjects, P less than .001) and was accompanied by increased plasma lactate, alanine, and glucagon concentrations (all P less than .05). The increased phosphoenolpyruvate gluconeogenesis accounted for 89 +/- 6% of the increase in overall hepatic glucose output in the NIDDM subjects and was significantly correlated with the fasting plasma glucose concentrations (r = .67, P less than .01). Glycogenolysis, calculated as the difference between overall hepatic glucose output and phosphoenolpyruvate gluconeogenesis, was not significantly different in the NIDDM subjects (9.9 +/- 0.06 mumol.kg-1.min-1) and the nondiabetic volunteers (8.4 +/- 0.3 mumol.kg-1.min-1). We conclude that increased gluconeogenesis is the predominant mechanism responsible for increased hepatic glucose output in NIDDM.     

Metabolic effects of IGF-I in diabetic rats

Insulinlike growth factor I (IGF-I) stimulates glucose utilization (GU) in nondiabetic rats. We compared the effects of IGF-I and insulin on glucose metabolism in control (fed plasma glucose 7.7 +/- 0.1 mM, n = 30) and partially (90%) pancreatectomized diabetic (plasma glucose 18.4 +/- 0.8 mM, n = 30) awake unstressed rats. IGF-I was infused at 0.65 or 1.96 nmol.kg-1.min-1 and insulin at 22 or 29 pmol.kg-1.min-1 in combination with [3-3H]glucose while euglycemia was maintained by a variable glucose infusion. In controls, GU during the 0.65- and 1.96-nmol.kg-1.min-1 IGF-I infusions (127 +/- 7 and 168 +/- 4 mumol.kg-1.min-1, respectively) was similar to rates observed during the 22- and 29-pmol.kg-1.min-1 insulin infusions (121 +/- 2 and 156 +/- 5 mumol.kg-1.min-1). Whole-body glycolytic rate (3H2O generation) and muscle glycogen synthetic rate were identical during insulin and IGF-I infusions. In diabetic rats, GU was reduced by 30% versus control rats (P less than 0.01) during both the low-dose (88 +/- 7 vs. 121 +/- 7 mumol.kg-1.min-1) and higher-dose (109 +/- 4 vs. 156 +/- 5 mumol.kg-1.min-1) insulin clamps. The defect in insulin action involved both muscle glycogen synthesis and glycolysis. In diabetic rats, IGF-I elicited rates of GU similar to controls (115 +/- 10 and 164 +/- 12 mumol.kg-1.min-1 during the 0.65- and 1.96-nmol.kg-1.min-1 infusions, respectively) and corrected the intracellular defects in glycogen synthesis and glycolysis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tolbutamide inhibits cAMP-dependent phosphorylation of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase.

The activity of a bifunctional enzyme, liver 6-phosphofructo-2-kinase (PFK-2)/fructose-2,6-bisphosphatase (F-2,6-Pase), which regulates the level of liver fructose-2,6-bisphosphate (F-2,6-P2), the most potent activator of PFK, is modulated by its phosphorylation rate mainly catalyzed by cAMP-dependent protein kinase A (PKA). To elucidate the action mechanism of sulfonylurea on liver F-2,6-P2 production, effects of tolbutamide on PKA-dependent phosphorylation of purified liver PFK-2/F-2,6-Phase protein and on kinase and phosphatase activities of the purified enzyme were examined in vitro. The purified enzyme was phosphorylated in the presence of the catalytic subunit of PKA, and tolbutamide inhibited the enzyme phosphorylation catalyzed by PKA in a dose-dependent manner. By adding the same dosages of tolbutamide used in the phosphorylation experiment, reduced activity of PFK-2 and increased activity of F-2,6-Pase in the presence of PKA were restored to the levels observed in the absence of PKA. On the other hand, carboxytolbutamide, an inactive metabolite of tolbutamide, had little effect on enzyme phosphorylation and activity. Our results indicate that tolbutamide inhibits a phosphorylation of the liver PFK-2/F-2,6-Pase catalyzed by PKA along with an activation of PFK-2 and an inactivation of F-2,6-Pase, leading to liver F-2,6-P2 production.     

Hepatic effects of chlorpropamide: inhibition of glucagon-stimulated gluconeogenesis in perfused livers of fasted rats.

In perfused livers of rats fasted for 24 h, glucagon (5 x 10(-10) M) significantly elevated tissue and perfusate levels of cyclic AMP and caused a twofold increase in glucose formation from lactate. Chlorpropamide (0.8 x 10(-3) M) consistently blocked these effects. Measurements of metabolic intermediates suggest that chlorpropamide may inhibit gluconeogenesis by antagonizing the action of glucagon on the phosphoenolpyruvate cycle. In the experiments described, chlorpropamide did not lower hepatic ATP concentration or energy charge, and exerted its effects at perfusate concentrations comparable to serum concentrations reported in patients on maintenance doses of the drug.     

Regulatory role of fructose-2,6-bisphosphate in pancreatic islet glucose metabolism remains unsettled

Fructose-2,6-P2 was measured in perifused, isolated rat pancreatic islets. Fructose-2,6-P2 was present in pancreatic islets at low levels approximately equal to fructose-2,6-P2 content of liver from fasted rats. In islets perifused with glucose at physiologic concentrations, fructose-2,6-P2 was increased from 0.8 microM in the presence of 5.5 mM glucose to 1.0 microM at 10 mM glucose and 1.3 microM at 16.7 mM glucose, but did not increase further at higher glucose concentration. Therefore, only modest increases in the phosphofructokinase-1 activator, fructose-2,6-P2, occur at glucose concentrations stimulating insulin secretion.     

Evidence for dual control mechanism regulating hepatic glucose output in nondiabetic men

We previously reported a fall in hepatic glucose output (HGO) during sleep accompanied by reductions in glucose utilization (Rd) and free fatty acids (FFAs). This study was undertaken to determine the potential role of changes in Rd and FFA on HGO in nondiabetic men. To determine if the fall in HGO during sleep could be reversed by FFA elevation, seven nondiabetic men underwent [3-3H]glucose infusions from 2200 to 0800, with heparin (90 mU.kg-1.min-1) added at 0200. Glucose appearance (Ra) fell from 11.7 +/- 1.1 at 2430 to 8.9 +/- 0.8 mumol.kg-1.min-1 (P less than 0.05) at 0200. The fall in Ra was associated with decreases in FFA (0.57 +/- 0.10 to 0.48 +/- 0.07 mM) and glycerol (0.08 +/- 0.01 to 0.06 +/- 0.01 mM). Infusion of heparin significantly increased FFA and glycerol (1.09 +/- 0.21 and 0.11 +/- 0.01 mM, respectively, P less than 0.01) and resulted in a significant fall in plasma alanine, suggesting that gluconeogenesis had been increased. However, rates of glucose turnover were indistinguishable from overnight studies without heparin. In additional studies (n = 6), intralipid and heparin-induced FFA elevation (from 0.61 +/- 0.07 to 0.95 +/- 0.05 mM, P less than 0.01) stimulated gluconeogenesis ([U-14C]alanine to glucose) twofold (188 +/- 22% increase compared to 114 +/- 6% in saline control studies, P less than 0.01). However, despite increasing gluconeogenesis, overall HGO did not change (10.6 +/- 0.5 vs. 10.7 +/- 0.6 mumol.kg-1.min-1) during lipid infusion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Impact of insulin deficiency on glucose fluxes and muscle glucose metabolism during exercise.

Exercise in the insulin-deficient diabetic state is characterized by a further increase in elevated circulating glucose and NEFA levels and by excessive counterregulatory hormone levels. The aim of this study was to distinguish the direct glucoregulatory effects of insulinopenia during exercise from the indirect effects that result from the metabolic and hormonal environment that accompanies insulin deficiency. For this purpose, dogs underwent 90 min of treadmill exercise during SRIF infusion with (SRIF + INS, n = 8) or without (SRIF - INS, n = 6) intraportal insulin replacement. Glucagon was not replaced, thus allowing assessment of the direct effect of insulinopenia at the liver independent of the potentiation of glucagon action. Glucose was infused to maintain euglycemia. Hepatic glucose production (Ra); glucose utilization (Rd); and LGlcU, LGlcE, and LGlcO were assessed with tracers ([3H]glucose, [14C]glucose) and arteriovenous differences. With exercise, insulin fell from 66 +/- 6 to 42 +/- 6 pM in the SRIF + INS group, and was undetectable in the SRIF - INS group. Plasma glucose was 6.33 +/- 0.38 and 6.26 +/- 0.30 mM at rest in the SRIF + INS and SRIF - INS groups, respectively, and was unchanged with exercise. Ra rose from 7.5 +/- 2.3 to 16.5 +/- 2.2 mumol.kg-1.min-1 and 9.1 +/- 2.0 to 31.4 +/- 3.9 mumol.kg-1.min-1 with exercise in the SRIF + INS and SRIF - INS groups, whereas Rd rose from 19.5 +/- 2.0 to 46.8 +/- 3.9 mumol.kg-1.min-1 and 15.1 +/- 1.8 to 29.9 +/- 3.3 mumol.kg-1.min-1. LGlcU rose from 36 +/- 9 to 112 +/- 25 mumol/min and 15 +/- 4 to 59 +/- 13 mumol/min and LGlcO rose from 5 +/- 2 to 61 +/- 12 mumol/min and 5 +/- 3 to 32 +/- 9 mumol/min with exercise in the SRIF+INS and SRIF-INS groups, respectively. Arterial levels and limb balances of NEFAs and glycerol were similar in the two groups. In summary, during exercise: 1) marked insulinopenia attenuates the increases in muscle glucose uptake and oxidation by approximately 50%, independent of changes in circulating metabolic substrate levels; 2) substantial increases in muscle glucose uptake and oxidation are, however, still present even in the absence of detectable insulin levels; and 3) insulinopenia facilitates the increase in Ra, independent of the potentiation of basal glucagon action. In conclusion, marked insulinopenia contributes directly to the exacerbation of glucoregulation during exercise in the diabetic state by limiting the rises in glucose uptake and metabolism and by enhancing hepatic glucose production.     

Similar dose responsiveness of hepatic glycogenolysis and gluconeogenesis to glucagon in vivo

This study was undertaken to determine whether the dose-dependent effect of glucagon on gluconeogenesis parallels its effect on hepatic glycogenolysis in conscious overnight-fasted dogs. Endogenous insulin and glucagon secretion were inhibited by somatostatin (0.8 micrograms X kg-1 X min-1), and intraportal replacement infusions of insulin (213 +/- 28 microU X kg-1 X min-1) and glucagon (0.65 ng X kg-1 X min-1) were given to maintain basal hormone concentrations for 2 h (12 +/- 2 microU/ml and 108 +/- 23 pg/ml, respectively). The glucagon infusion was then increased 2-, 4-, 8-, or 12-fold for 3 h, whereas the rate of insulin infusion was left unchanged. Glucose production (GP) was determined with 3-[3H]glucose, and gluconeogenesis (GNG) was assessed with tracer (U-[14C]alanine conversion to [14C]glucose) and arteriovenous difference (hepatic fractional extraction of alanine, FEA) techniques. Increases in plasma glucagon of 53 +/- 8, 199 +/- 48, 402 +/- 28, and 697 +/- 149 pg/ml resulted in initial (15-30 min) increases in GP of 1.1 +/- 0.4 (N = 4), 4.9 +/- 0.5 (N = 4), 6.5 +/- 0.6 (N = 6), and 7.7 +/- 1.4 (N = 4) mg X kg-1 X min-1, respectively; increases in GNG (approximately 3 h) of 48 +/- 19, 151 +/- 50, 161 +/- 25, and 157 +/- 7%, respectively; and increases in FEA (3 h) of 0.14 +/- 0.07, 0.37 +/- 0.05, 0.42 +/- 0.04, and 0.40 +/- 0.17, respectively. In conclusion, GNG and glycogenolysis were similarly sensitive to stimulation by glucagon in vivo, and the dose-response curves were markedly parallel.     

Relative importance of first- and second-phase insulin secretion in glucose homeostasis in conscious dog. II. Effects on gluconeogenesis

The normal pancreatic response to an exogenous glucagon infusion is a biphasic release of insulin. In our study the ability of each component of insulin release to counter the effects of the glucagon on gluconeogenesis and alanine metabolism was assessed by mimicking first- and/or second-phase insulin release with infusions of somatostatin and intraportal insulin. When a fourfold increase in glucagon was brought about in the presence of fixed basal insulin release, there was a large increase in overall glucose production and gluconeogenesis. The increase in the conversion of [14C]alanine into [14C]glucose (169 +/- 42%, P less than .05) was accompanied by an increase in the fractional extraction of alanine by the liver (FEA 0.32 +/- 0.06 to 0.66 +/- 0.10, P less than .05) and net hepatic alanine uptake (NHAU 2.97 +/- 0.45 to 4.61 +/- 0.48 mumol . kg-1 . min-1, P less than .05). Simulated first-phase insulin release had no effect on the ability of glucagon to increase FEA (0.32 +/- 0.03 to 0.66 +/- 0.03, P less than .05) or NHAU (3.69 +/- 0.80 to 5.10 +/- 0.69 mumol . kg-1 . min-1, P less than .05) but did limit the increase in overall gluconeogenic conversion (114 +/- 37%). Second-phase insulin release had no effect on either the glucagon-induced increase in FEA (0.35 +/- 0.08 to 0.73 +/- 0.04) or NHAU (3.35 +/- 0.92 to 5.13 +/- 0.85 mumol . kg-1 . min-1) but completely inhibited the increase in overall gluconeogenic conversion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Whole body de novo amino acid synthesis in type I (insulin-dependent) diabetes studied with stable isotope-labeled leucine, alanine, and glycine

Dynamic aspects of whole body alanine and glycine metabolism have been explored in insulin-dependent (type I) diabetic subjects. Using a primed, continuous intravenous (i.v.) infusion of [2H3]alanine and [15N]glycine given simultaneously with [1-13C]leucine, whole body alanine and glycine fluxes and their rates of de novo synthesis were measured in 6 diabetic young men. Subjects were studied in the postabsorptive state, after blood glucose was clamped overnight at 15.2 +/- 0.3 mM, and then, on the following night, at 5.9 +/- 0.2 mM (insulin infusion rates of 0.24 +/- 0.09 and 1.65 +/- 0.20 U/h, respectively). In the normoglycemic state, leucine, alanine, and glycine fluxes averaged 88 +/- 4, 378 +/- 39, and 155 +/- 8 mumol X kg-1 X h-1, respectively. Based on the leucine flux, alanine and glycine de novo synthesis rates were 264 +/- 36 and 67 +/- 8 mumol X kg-1 X h-1. In the hyperglycemic state, leucine flux increased 23% (P less than 0.01), alanine flux rose slightly (+5%) but significantly (P less than 0.05), while alanine de novo synthesis and glycine flux remained unchanged and glycine de novo synthesis decreased by 33% (P less than 0.001). These results show that small alterations in peripheral alanine inflow in the hyperglycemic state reflect increased proteolysis and suggest that increased circulating plasma glucose does not contribute to de novo alanine synthesis in the absence of adequate insulin effect and/or augmented glucose tissue uptake. These observations also reveal the importance of insulin in the maintenance of whole body leucine economy, since a lower rate of insulin administration was associated with an increased rate of leucine oxidation.     

Lack of evidence for improvement in long-term glycemic control by pulsatile insulin infusion in streptozocin-induced diabetic baboon

To assess the potential therapeutic use of pulsatile intravenous insulin delivery, five streptozocin-induced diabetic baboons were treated with alternate 3- to 6-wk periods of pulsatile and continuous insulin infusion. Time-averaged insulin concentrations were matched during two pulsatile administration periods (P1 and P2) and an intervening period of continuous insulin administration (C). There were no significant differences among the overall means of four daily glucose determinations performed during the three periods (P1, 5.7 +/- 1 mM; C, 5.6 +/- 0.9 mM; P2, 5.3 +/- 0.9 mM); the mean M value, a measure of the stability of glycemic control (P1, 4 +/- 1.7; C, 3.9 +/- 1.8; P2, 3.6 +/- 1.5); the percentage of glucose values less than 2.8 mM (P1, 13 +/- 8.5%; C, 14 +/- 12%; P2, 13 +/- 9.1%); or the glycosylated hemoglobin levels determined at the end of the P1 and C (7.5 +/- 3.4 and 6.5 +/- 1.8%, respectively [all values are means +/- SD]). Fasting hepatic glucose production was suppressed to a similar degree during pulsatile and continuous insulin infusion (P1, 23 +/- 3 mumol.kg-1.min-1; C, 24 +/- 8 mumol.kg-1.min-1). Arterial glucagon levels were similar during pulsatile and continuous insulin infusion, both in the fasting state (84 +/- 29 and 84 +/- 31 ng/L, respectively) and postprandially (30 +/- 14 and 27 +/- 12 ng/L, respectively). Pulsatile insulin infusion failed to entrain a corresponding glucagon secretory rhythm. These data suggest that the metabolic consequences of long-term pulsatile and continuous insulin infusion in an animal model of human non-insulin-dependent diabetes are comparable.     

Metformin normalizes nonoxidative glucose metabolism in insulin-resistant normoglycemic first-degree relatives of patients with NIDDM.

Many first-degree relatives of patients with non-insulin-dependent diabetes mellitus (NIDDM) are characterized by insulin resistance. Because metformin improves peripheral insulin sensitivity, we examined the acute effect of metformin and placebo on glucose and lipid metabolism in nine insulin-resistant first-degree relatives of NIDDM patients with the euglycemic insulin-clamp technique combined with indirect calorimetry and infusion of [3-3H]glucose. Either placebo or 500 mg metformin was taken in random order twice the day before and once 1 h before the clamp. Nine healthy individuals without family history of diabetes served as control subjects. Basal plasma glucose was normal and did not differ between the metformin and the placebo study (4.8 +/- 0.2 vs. 5.0 +/- 0.2 mM) and neither did basal hepatic glucose production (10.59 +/- 0.54 vs. 10.21 +/- 0.80 mumol.kg-1.min-1). Insulin-stimulated glucose disposal was significantly increased by 25% after metformin compared with placebo (26.67 +/- 2.87 vs. 21.31 +/- 1.73 mumol.kg-1.min-1, P less than 0.05). The enhancement in glucose utilization was primarily due to normalization of nonoxidative glucose disposal (from 8.02 +/- 1.35 to 15.07 +/- 2.69 mumol.kg-1.min-1, P less than 0.01, vs. 15.65 +/- 2.72 mumol.kg-1.min-1 in control subjects). In contrast, glucose oxidation during the clamp was slightly lower after metformin compared with both placebo (11.59 +/- 0.83 vs. 13.30 +/- 1.00 mumol.kg-1.min-1, P = 0.06) and healthy control subjects (15.68 +/- 1.38 mumol.kg-1.min-1, P less than 0.05). We conclude that acutely administered metformin improves peripheral insulin sensitivity in insulin-resistant normoglycemic individuals primarily by stimulating the nonoxidative pathway of glucose metabolism.     

Insulinomimetic properties of trace elements and characterization of their in vivo mode of action

Lithium and vanadate have insulinomimetic actions in vitro. In this study, we examined the in vivo effects of lithium and vanadate on glucose metabolism in diabetic (90% partial pancreatectomy) rats. Four groups of chronically catheterized rats were studied: control, diabetic, diabetic treated with lithium (plasma concn 1.0 +/- 0.1 meq/L) and vanadate (0.05 mg/ml in drinking water), and diabetic treated with lithium, vanadate, zinc, and magnesium. Postmeal plasma glucose was increased in diabetic versus control rats (18.7 vs. 7.7 mM, P less than 0.01) and was normalized by addition of lithium and vanadate (8 mM) or lithium, vanadate, zinc, and magnesium (7.4 mM). Euglycemic insulin-clamp studies were performed 2 wk posttreatment; insulin-mediated glucose uptake was reduced in diabetic compared with control rats (142 +/- 4 vs. 200 +/- 5 mumol.kg-1.min-1, P less than 0.01), returned to normal with lithium and vanadate (206 +/- 6 mumol.kg-1.min-1), or increased to supranormal levels with lithium, vanadate, zinc, and magnesium (238 +/- 6 mumol.kg-1.min-1). During the insulin clamp, muscle glycogenic rate was severely impaired in diabetic versus control rats (18 vs. 70 mumol.kg-1.min-1) and was normalized by lithium and vanadate (91 mumol.kg-1.min-1) or lithium, vanadate, zinc, and magnesium (93 mumol.kg-1.min-1).(ABSTRACT TRUNCATED AT 250 WORDS)     

Insulin is required for the liver to respond to intraportal glucose delivery in the conscious dog.

To determine whether insulin is essential for the augmented hepatic glucose uptake observed in the presence of intraportal glucose delivery, SRIF was used to induce acute insulin deficiency in 5 conscious dogs, and glucose was infused into the portal vein or a peripheral vein in two sequential, randomized periods. Insulin and C-peptide levels were below the limits of detection after SRIF infusion, and the load of glucose presented to the liver was approximately doubled and equivalent during the portal and peripheral periods. Net hepatic glucose output was 2.9 +/- 0.9 and 2.1 +/- 1.1 mumol.kg-1.min-1 during portal and peripheral glucose delivery, respectively. In an additional set of protocols, pancreatectomized dogs were used to investigate the effects of prolonged insulin deficiency (n = 5) and acute insulin replacement (n = 4) on the hepatic response to intraportal glucose delivery. In the prolonged insulin deficiency protocol, SRIF was used to lower glucagon and thereby reduce circulating glucose levels, and glucose was infused into the portal or peripheral circulations in two sequential, randomized periods. As with acute insulin deficiency, net hepatic glucose output was still evident and similar (3.6 +/- 1.1 and 4.2 +/- 1.3 mumol.kg-1.min-1) during portal and peripheral glucose delivery, respectively. When the pancreatectomized dogs were restudied using a similar protocol, but one in which insulin was replaced (4X-basal), and the glucose load to the liver was matched to that which occurred in the prolonged insulin deficiency protocol, net hepatic glucose uptake was 23.6 +/- 6.1 mumol.kg-1.min-1 during portal glucose delivery but only 10.3 +/- 3.5 mumol.kg-1.min-1 during peripheral glucose delivery. These results suggest that the induction of net hepatic glucose uptake and the augmented hepatic response to intraportal glucose delivery require the presence of insulin.     

Basal glucagon replacement in chronic glucagon deficiency increases insulin resistance

To evaluate the role of glucagon in insulin-mediated glucose metabolism, we studied four men and four women, ranging in age from 30-73 yr (mean +/- SEM, 54 +/- 5) who had undergone complete pancreatic resection for cancer or chronic pancreatitis 16-58 mo previously. The patients had undetectable C-peptide levels and established lack of biologically active 3500 mol wt glucagon. Euglycemic insulin clamp studies were performed with a 40 mU X m-2 X min-1 insulin infusion in the basal, post-absorptive, insulin-withdrawn state, before and during the last 3 h of a 72-h glucagon replacement-dose infusion (1.25 ng X kg-1 X min-1). In four patients, hepatic glucose production was determined by a primed-constant infusion of 3-[3H]glucose. Monocyte insulin-binding studies, pre- and postglucagon, were performed in all patients. The 72-h glucagon infusion, resulting in mean plasma glucagon levels of 124 +/- 7 pg/ml, caused a significant rise in the mean plasma glucose level (249 +/- 8 versus 170 +/- 13 mg/dl preglucagon) and a sixfold increase in mean 24-h glucose excretion. Both with and without glucagon, euglycemic hyperinsulinemia achieved identical and complete suppression of hepatic glucose production. The mean glucose utilization rate (4.70 +/- 0.36 mg X kg-1 X min-1 preglucagon) was significantly decreased by glucagon replacement (3.83 +/- 0.31 mg X kg-1 X min-1, P less than 0.02). Mean glucose clearance was also diminished with glucagon (4.49 +/- 0.32 versus 5.73 +/- 0.45 ml X kg-1 X min-1 preglucagon, P less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)