Chlorpropamide raises fructose-2,6-bisphosphate concentration and inhibits gluconeogenesis in isolated rat hepatocytes

The addition of chlorpropamide to hepatocytes isolated from fed rats raised the cellular concentration of fructose-2,6-bisphosphate (F-2,6-P2), a regulatory metabolite that plays a relevant role in the control of hepatic glucose metabolism. The effect of chlorpropamide was dose dependent; a statistically significant increase was already seen at 0.2 mM of the sulfonylurea. The accumulation of F-2,6-P2 caused by chlorpropamide (1 mM) was parallel to the stimulation of L-lactate production (36.6 +/- 4.8 versus 26.1 +/- 2.6 mumol of lactate/g of cells X 20 min; N = 5, P less than 0.05) and to the inhibition of gluconeogenesis (0.57 +/- 0.1 versus 0.94 +/- 0.09 mumol of [U-14C]pyruvate converted to glucose/g of cells X 20 min; N = 5, P less than 0.05). In addition, chlorpropamide enhanced the inhibitory action evoked by insulin on glucagon-stimulated gluconeogenesis. This combined effect of chlorpropamide and insulin seems to be correlated with the synergistic accumulation of F-2,6-P2 provoked by the simultaneous action of these two agents on glucagon-treated hepatocytes. Finally, neither 6-phosphofructo-2-kinase activity nor hepatocyte cyclic AMP levels were significantly changed by the presence of the sulfonylurea in the incubation medium. Our results support the concept that chlorpropamide, by a cyclic AMP-independent mechanism, increases the hepatic content of F-2,6-P2 and, in this way, enhances the glycolytic flux and inhibits glucose output by the liver.     

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.     

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.     

Glucose-induced accumulation of fructose-2,6-bisphosphate in pancreatic islets

Rat islets contain the acid-labile activator of phosphofructokinase, fructose-2,6-bisphosphate. The islet content in activator is higher in islets exposed to glucose (16.7 mM) than in islets deprived of glucose. The islets display fructose-6-phosphate, 2-kinase activity with a Km for fructose-6-phosphate close to 0.08 mM. Glucose fails to affect the activity of this enzyme. It is proposed that the effect of glucose to increase the islet content of fructose-2,6-bisphosphate is attributable, in part at least, to the glucose-induced increase in the concentration of fructose-6-phosphate in the islet cells.     

Assessment of liver graft function after cold preservation using 31P and 23Na NMR spectroscopy]

We investigated the functional damages of cytoplasm and cell membrane of liver grafts in male WKY rats after 24hr- or 48hr-cold preservation using a UW solution in vitro. 10mM fructose or 50mM NH4Cl was added to the perfusate, and synthesis of fructose-1-phosphate (F-1-P) and Na(+)- and H(+)-ion transports through the cell membrane were evaluated with 31P-MRS and 23Na-MRS. After 30 minutes of reperfusion, beta-ATP/(inorganic phosphate: Pi) of the 48hr-preserved liver was significantly lower than those of the other groups. In the control and the 24hr-preserved liver, the changes of F-1-P were almost the same, but F-1-P synthesis was lower in the 48hr-preserved liver than those of the other groups. Intracellular pH began to drop after cessation of NH4Cl loading, and then recovered to the previous level. At the same time Nain was increased in the control group. However, in the other two groups, the increasing rates of Nain were lower, and the recoveries of Nain were worse. In conclusion, the function of cell membrane was more fragile than that of mitochondria and cytoplasmic sugar metabolism in the liver graft.     

Assessment of liver graft function after cold preservation using 31P and 23Na magnetic resonance spectroscopy.

We investigated the functional damages of the cytoplasm and the cell membrane of liver grafts in male Wister Kyoto rats after 24-hr and 48-hr cold preservations using the University of Wisconsin solution in vitro. Fructose (10 mM) or NH4Cl (50 mM) was added to the perfusate, and synthesis of fructose-1-phosphate (F-1-P) and Na- and H-ion transports through the cell membrane were evaluated by magnetic resonance spectroscopy (MRS), 31P-MRS and 23Na-MRS. After 30 min of reperfusion, beta-ATP/(inorganic phosphate: Pi) of the 48-hr preserved group was significantly lower than the control group and the 24-hr preserved group. The changes of F-1-P in the control group and the 24-hr preserved group were almost the same, but F-1-P synthesis was lower in the 48-hr preserved liver than those of the other groups. Intracellular pH began to drop after the cessation of NH4Cl loading, and then it recovered to the preloading level. At the same time Nain+ was increased in the control group. However, in the other two groups, the increasing rates of Nain+ were lower, and the recoveries of Nain+ were less. In conclusion, the function of cell membrane was more fragile than that of mitochondria and cytoplasmic sugar metabolism in the liver graft.     

Mechanisms of hyperglycemia-induced insulin resistance in whole body and skeletal muscle of type I diabetic patients.

To examine the mechanisms of hyperglycemia-induced insulin resistance, eight insulin-dependent (type I) diabetic men were studied twice, after 24 h of hyperglycemia (mean blood glucose 20.0 +/- 0.3 mM, i.v. glucose) and after 24 h of normoglycemia (7.1 +/- 0.4 mM, saline) while receiving identical diets and insulin doses. Whole-body and forearm glucose uptake were determined during a 300-min insulin infusion (serum free insulin 359 +/- 22 and 373 +/- 29 pM, after hyper- and normoglycemia, respectively). Muscle biopsies were taken before and at the end of the 300-min insulin infusion. Plasma glucose levels were maintained constant during the 300-min period by keeping glucose for 150 min at 16.7 +/- 0.1 mM after 24-h hyperglycemia and increasing it to 16.5 +/- 0.1 mM after normoglycemia and by allowing it thereafter to decrease in both studies to normoglycemia. During the normoglycemic period (240-300 min), total glucose uptake (25.0 +/- 2.8 vs. 33.8 +/- 3.9 mumol.kg-1 body wt.min-1, P less than 0.05) was 26% lower, forearm glucose uptake (11 +/- 4 vs. 18 +/- 3 mumol.kg-1 forearm.min-1, P less than 0.05) was 35% lower, and nonoxidative glucose disposal (8.9 +/- 2.2 vs. 19.4 +/- 3.3 mumol.kg-1 body wt-1min-1, P less than 0.01) was 54% lower after 24 h of hyper- and normoglycemia, respectively. Glucose oxidation rates were similar. Basal muscle glycogen content was similar after 24 h of hyperglycemia (234 +/- 23 mmol/kg dry muscle) and normoglycemia (238 +/- 22 mmol/kg dry muscle). Insulin increased muscle glycogen to 273 +/- 22 mmol/kg dry muscle after 24 h of hyperglycemia and to 296 +/- 33 mmol/kg dry muscle after normoglycemia (P less than 0.05 vs. 0 min for both). Muscle ATP, free glucose, glucose-6-phosphate, and fructose-6-phosphate concentrations were similar after both 24-h treatment periods and did not change in response to insulin. We conclude that a marked decrease in whole-body, muscle, and nonoxidative glucose disposal can be induced by hyperglycemia alone.     

Activation by vanadate of glycolysis in hepatocytes from diabetic rats

In hepatocytes from starved streptozocin-induced diabetic rats, vanadate increases the glycolytic flux because it raises the levels of fructose-2,6-bisphosphate (Fru-2,6-P2), the main regulatory metabolite of this pathway. This effect of vanadate on Fru-2,6-P2 levels is time and dose dependent, and it remains in cells incubated in a calcium-depleted medium. Vanadate is also able to counteract the decrease on Fru-2,6-P2 levels produced by glucagon, colforsin, or exogenous cAMP. However, vanadate does not modify 6-phosphofructo-2-kinase and pyruvate kinase activities, but it does counteract the inactivation of these enzymes induced by glucagon. Likewise, Fru-2,6-P2ase activity is also not affected by vanadate. In addition, vanadate is able to increase the production of both lactate and CO2 in hepatocytes from streptozocin-induced diabetic rats incubated in the presence of glucose in the medium. Vanadate behaves as a glycolytic effector in these cells, and this effect may be related to its ability to normalize blood glucose levels in diabetic animals.     

Evidence of cosecretion of islet amyloid polypeptide and insulin by beta-cells

Islet amyloid polypeptide (IAPP) has been identified as the major constituent of the pancreatic amyloid of non-insulin-dependent diabetes mellitus (NIDDM) and is also present in normal beta-cell secretory granules. To determine whether IAPP is a pancreatic secretory product, we measured the quantity of IAPP-like immunoreactivity (IAPP-LI), insulin, and glucagon released into 5 ml of incubation medium during a 2-h incubation of monolayer cultures (n = 5) of neonatal (3- to 5-day-old) Sprague-Dawley rat pancreases under three conditions: 1.67 mM glucose, 16.7 mM glucose, and 16.7 mM glucose plus 10 mM arginine and 0.1 mM isobutylmethylxanthine (IBMX). The quantity of IAPP-LI, insulin, and glucagon in the cell extract was also determined. Mean +/- SE IAPP-LI in the incubation medium increased from 0.041 +/- 0.003 pmol in 1.67 mM glucose to 0.168 +/- 0.029 pmol in 16.7 mM glucose (P less than 0.05) and 1.02 +/- 0.06 pmol in 16.7 mM glucose plus arginine and IBMX (P less than 0.05 vs. 1.67 or 16.7 mM glucose). Insulin secretion increased similarly from 4.34 +/- 0.27 to 20.2 +/- 0.6 pmol (P less than 0.05) and then to 135 +/- 5 pmol (P less than 0.05 vs. 1.67 or 16.7 mM glucose). Glucagon release tended to decrease with the increase in glucose concentration (0.39 +/- 0.01 vs. 0.33 +/- 0.02 pmol, P less than 0.1), whereas with the addition of arginine and IBMX to high glucose, glucagon release increased to 1.32 +/- 0.03 pmol (P less than 0.05 vs. 1.67 or 16.7 mM glucose).(ABSTRACT TRUNCATED AT 250 WORDS)     

Ion channels of glucose-responsive and -unresponsive beta-cells.

To assess whether different electrophysiological characteristics could account for the heterogeneous secretion of individual beta-cells in vitro, we used patch-clamp configurations to study currents in plaque-forming (insulin-secreting) and non-plaque-forming rat pancreatic beta-cells that were distinguished in a reverse hemolytic plaque assay (RHPA) after a 30-min stimulation by 16.7 mM glucose. RHPA showed that the population of single beta-cells under study was stimulated (P less than 0.01-0.001) to secrete insulin by 16.7 mM glucose, 100 microM tolbutamide, 20 microM glyburide, or 30 mM KCl but, under these conditions, also comprised beta-cells that did not secrete detectable amounts of insulin. Under current clamp conditions, secreting and nonsecreting beta-cells showed analogous resting membrane potentials (approximately 60 mV) and were similarly depolarized by 30 mm KCl and 100 microM tolbutamide. Under voltage-clamp conditions, total membrane conductance (approximately 6 nS) was also similar in the glucose-responsive and -unresponsive beta-cells, which, when monitored in the whole-cell configuration after RHPA, showed the following currents: a voltage-dependent Na+ current, a voltage-activated Ba2+ current, a voltage-dependent K+ delayed-rectifier current, a voltage-dependent Ca(2+)-activated K+ current, and a voltage-independent and tolbutamide-sensitive K+ current. In the cell-attached configuration and the presence of 2.8 mM glucose, secreting and nonsecreting beta-cells displayed a similar single-channel activity that was abolished when glucose concentration was raised to 16.7 mM. We conclude that beta-cells studied after RHPA have an electrically normal membrane whether they release insulin in response to 16.7 mM glucose or not.     

Induction of memory in rat pancreatic islets by tolbutamide. Dependence on ambient glucose level, calcium, and phosphoinositide hydrolysis

The ability of the sulfonylurea tolbutamide to induce insulin output, increase phosphoinositide (PI) hydrolysis, and modulate the insulin response to other agonists was assessed. At 200 microM, tolbutamide increased both insulin release and the efflux of 3H from [3H]inositol-prelabeled islets only in the presence of 5.5 or 7 mM glucose. When the glucose level was maintained at 2.75 mM, tolbutamide (200 microM) had no positive impact on either parameter. The calcium-influx inhibitor nitrendipine (200 nM) blocked the effects of 200 microM tolbutamide (with 7 mM glucose) on 3H efflux and insulin output. Prior exposure of islets to tolbutamide (200 microM) in the presence of 7 mM glucose amplified their subsequent insulin response to 10 mM glucose and 5 mM glyceraldehyde. The effect of 200 microM tolbutamide (with 7 mM glucose) was blocked by nitrendipine. Furthermore, the effect of 200 microM tolbutamide was not observed with 2.75 mM glucose; however, if the level of tolbutamide was increased to 1 mM, both PI hydrolysis and potentiated release to subsequent stimulation with 10 mM glucose were observed. Tolbutamide (200 microM with 7 mM glucose) stimulation for 20 min resulted in an increase in 3H efflux from [3H]inositol-prelabeled islets. Despite the rapid fall in insulin secretion, elevated rates of 3H efflux persisted long after the removal of the sulfonylurea from the medium. The duration of the 3H-efflux response paralleled the duration of potentiation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pulsatile insulin secretion in isolated rat islets

The pancreas secretes insulin in an oscillatory fashion, but the precise site of the pacemaker for pulsatile insulin secretion has not been identified. These studies were designed to determine whether islets also secrete insulin in a pulsatile fashion if they are isolated from their pancreatic milieu. Isolated rat islets (80-100) were perifused 8 h in culture medium after overnight incubation, and samples were collected at 3.3-min intervals. Insulin secretion was evaluated for pulsatility with the Clifton Cycle Detection Program. Perifusion of islets was associated with a spontaneous, persistent, and regular pulsatility of insulin secretion, which was observed in all conditions tested. Perifusion with medium containing 5.5 mM glucose (n = 11) demonstrated oscillations with a mean periodicity of 17.6 +/- 1.1 min and a mean amplitude of 4.8 +/- 0.4 microU/ml when overall mean insulin concentration was 16.7 +/- 2.4 microU/ml. When the glucose concentration was 16.7 mM (n = 9), overall mean insulin concentration was 54.4 +/- 2.6 microU/ml, with increases in periodicity (22.0 +/- 1.3 min) and amplitude (10.7 +/- 0.5 microU/ml). All measurements were significantly different from those observed during perifusion with 5.5 mM glucose (P less than 0.02-0.001). Theophylline (1 mM) also enhanced the overall mean insulin concentration and amplitude (69.4 +/- 10.4 and 14.2 +/- 1.2 microU/ml, respectively) compared with control studies without theophylline (16.7 +/- 5.3 and 4.3 +/- 0.5 microU/ml) (P less than 0.01). The period of the cycle was also increased from 17.5 +/- 1.1 to 26.4 +/- 6.3 min, but this was not significantly different from the control group.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sulfonylurea compounds uncouple the glucose dependence of the insulinotropic effect of glucagon-like peptide 1

Glucagon-like peptide (GLP)-1 mimetics have been reported to cause hypoglycemia when combined with sulfonylureas. This study investigated the impact of tolbutamide on the glucose dependence of the GLP-1-mediated effects on insulin, glucagon, and somatostatin secretion in the in situ perfused rat pancreas. At 3 mmol/l glucose, GLP-1 alone did not augment insulin secretion, whereas tolbutamide alone caused a rapid increase in insulin secretion. However, when GLP-1 and tolbutamide were administered simultaneously, insulin secretion increased significantly to 43.7 +/- 6.2 pmol/min (means +/- SE), exceeding the sum of the responses to GLP-1 (2.0 +/- 0.6 pmol/min; P = 0.019) and tolbutamide (11.3 +/- 3.8; P = 0.005) alone by a factor of 3.3. At 11 mmol/l glucose, co-infusion of GLP-1 and tolbutamide augmented insulin secretion to 141.7 +/- 10.3 vs. 115.36 +/- 14.1 (GLP-1) and 42.5 +/- 7.3 pmol/min (tolbutamide). Interestingly, increases in somatostatin secretion, both by glucose and GLP-1, were consistently paralleled by suppression of glucagon release. In conclusion, we demonstrate uncoupling of GLP-1 from its glucose dependence by tolbutamide. This uncoupling probably explains the tendency of GLP-1 to provoke hypoglycemia in combination with sulfonylureas. The results suggest that closure of ATP-sensitive K(+) channels by glucose might be involved in the glucose dependence of GLP-1's insulinotropic effect and that somatostatin acts as a paracrine regulator of glucagon release.     

Role of cytosolic Ca2+ in impaired sensitivity to glucose of rat pancreatic islets exposed to high glucose in vitro.

Sustained exposure to high concentrations of glucose selectively impairs the ability of pancreatic islets to secrete insulin in acute glucose stimulation. In order to evaluate the interrelationship between impaired insulin secretion and the dynamics of the cytosolic free Ca2+ level ([Ca2+]i), we have investigated the effect of high glucose exposure on both [Ca2+]i dynamics in single rat beta-cells and insulin release from rat pancreatic islets. Islets cultured at a high glucose concentration (16.7 mM) for 24 h showed significant reductions of the 16.7 mM GSIR compared with islets cultured at a normal glucose concentration (5.5 mM) (3.38 +/- 0.24 vs. 4.26 +/- 0.34%, respectively, P < 0.05). The capacity of glucose to raise the [Ca2+]i level also was significantly reduced in the beta-cells maintained for 24 h at 16.7 mM glucose (P < 0.001). An additional culture in the medium with 5.5 mM glucose for 16 h restored both the GSIR and the [Ca2+]i response of islets cultured at high glucose. On the other hand, insulin release and [Ca2+]i rise in response to 20 mM L-Arg were well preserved. These observations confirm that exposure of pancreatic beta-cells to high glucose concentrations induces a selective reduction of the GSIR and, further, shows that this impaired response is reversibly restored by an additional culture with normal glucose. We also suggest that the inability of glucose to provoke a [Ca2+]i rise, which is observed in the beta-cells exposed to high glucose, may be responsible for the selective impairment of the GSIR.     

Effects of fluoride on parathyroid hormone secretion and intracellular second messengers in bovine parathyroid cells

Fluoride ion (F-) alone or in conjunction with aluminum (Al3+) has been shown to stimulate the activity of guanine nucleotide-binding proteins (G proteins) in cell membrane preparations from a variety of cell types and in intact hepatic cells. Several studies have indicated that G proteins are involved in the regulation of parathyroid hormone (PTH) secretion. Intracellular second messengers which modulate PTH secretion (e.g., cAMP) have also been found to be regulated by G proteins. We have, therefore, employed F- as a probe to investigate the possible role of G proteins in the modulation of PTH release and the intracellular second messengers that have been implicated in the control of PTH secretion. F- produces a dose-dependent inhibition of PTH release with a maximal inhibitory effect (67%) at 5 mM. F- exerts its inhibitory effect within 5 min and the degree of suppression of PTH secretion gradually increases over 1 hr. F- (5 mM) inhibits PTH secretion at 0.5 mM Ca2+ to the level observed with 2 mM Ca2+ alone; moreover, the effects of F- and high Ca2+ are not additive. While 1 mM F- suppresses PTH secretion by only 21%, and 10 microM Al3+ has virtually no effect at all, together they inhibit PTH release approximately to the level (63% inhibition) observed with 5 mM NaF alone. In the presence of 10(-5) M dopamine, F- produces a concentration-dependent inhibition of cAMP accumulation (0.684 +/- 0.033 pmoles/10(5) cells at 0 mM F- vs. 0.256 +/- 0.048 at 5 mM F-). However, the F- -induced decrease in cAMP cannot account for the inhibition of PTH release by this agent, since addition of methylisobutylxanthine (10(-4) M) by F- -treated cells raises intracellular cAMP content above that of control cells but fails to reverse the inhibition of PTH release. The cytosolic calcium concentration in Fura-2-loaded cells increases from 210 +/- 20 nM to 340 +/- 44 nM after 5 mM F- was added to incubation media. Prior removal of extracellular Ca2+ by EGTA totally blocks the F- -induced rise in cytosolic Ca2+ without preventing the inhibition of PTH release by NaF. F- also produces a time- and dose-dependent increase in the accumulation of IP, IP2, and IP3 in cells prelabeled with [3H]inositol and incubated with 10 mM Li+, consistent with activation of phospholipase C. We conclude that F- is a potent inhibitor of PTH secretion.(ABSTRACT TRUNCATED AT 400 WORDS)     

Increased sensitivity to insulin-releasing and glucoregulatory effects of dynorphin A1-13 and U 50488h in ob/ob versus lean mice

The effects of two kappa-opiate agonists, U 50488h and dynorphin A1-13, on plasma insulin and glucose concentrations in vivo and insulin release in vitro were tested in fasted genetically obese (ob/ob) and lean (+/+) mice at 12-15 wk of age. Fasting plasma insulin concentrations in ob/ob and lean mice were 1.22 +/- 0.10 and 0.23 +/- 0.05 nM, and plasma glucose levels were 6.90 +/- 0.84 and 4.70 +/- 0.29 mM, respectively. Administration of U 50488h (1 mg/kg body wt i.p.) to ob/ob mice dramatically raised plasma insulin by 670 and 790 pM at 15 and 30 min. Plasma glucose was raised from 5 min onward to a maximum increment of 4.2 mM above baseline. These effects were blocked by simultaneous administration of naloxone (10 mg/kg). A higher dose of U 50488h (10 mg/kg body wt i.p.) was required to produce significant increases in lean mouse plasma insulin (81 pM at 15 min) and glucose (0.7, 1.1, and 1.7 mM at 5, 15, and 30 min, respectively). Dynorphin (1 mg/kg body wt i.p.) raised plasma insulin in ob/ob mice by 380 and 410 pM at 15 and 30 min and raised plasma glucose by 1.6 mM at 15 min. In lean mice, the same dose of dynorphin had no effect on plasma insulin concentrations but induced a small rise in glucose. In ob/ob mice, the agonist-induced rise in glucose did not cause the insulin response, because insulin levels were not elevated by a glucose challenge.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evolution of abnormal insulin secretory responses during 48-h in vivo hyperglycemia

Recent in vitro studies have shown that insulin release caused by continuous exposure to high glucose concentration markedly falls within a few hours. We wanted to determine if a similar effect occurs in vivo with chronic intravenous infusions in normal rats. Male CD rats (200-250 g) were infused with 50% glucose at 2 ml/h for 6, 14, 24, or 48 h, whereas controls received 0.45% NaCl, and insulin responses were tested with the in vitro isolated perfused pancreas. Plasma glucose averaged 352 +/- 20 mg/dl after 4 h and 396 +/- 11 mg/dl after 24 h versus 137 +/- 5 mg/dl in controls; plasma insulin at the same times was 8.94 +/- 1.44 and 12.1 +/- 2.62 ng/ml versus 1.69 +/- 0.19 ng/ml in controls. The incremental insulin response caused by an increase in perfusate glucose from 2.8 to 16.7 mM was not significantly reduced after 24 h of glucose infusion; in contrast, paradoxical suppression was seen after 48 h. A second protocol examined glucose potentiation by giving 10 mM arginine at 2.8 and 16.7 mM glucose; a hyperresponse to arginine at the lower glucose level was present after just 14 h of infusion. Therefore, these results do not support the hypothesis that beta-cells lose their sensitivity to glucose within hours of being exposed to higher than normal glucose concentrations.     

Insulin regulation of hepatic glucose transporter protein is impaired in chronic pancreatitis.

OBJECTIVE: The effect of chronic pancreatitis and insulin on the expression of the hepatic facilitative glucose transporter protein (GLUT-2) was determined in rats. SUMMARY BACKGROUND DATA: Chronic pancreatitis is associated with diabetes mellitus or impaired glucose tolerance. Suppression of hepatic glucose production (HGP) by insulin is impaired, although the mechanism is unknown. METHODS: Normal rats, rats with chronic pancreatitis induced 12 to 16 weeks earlier by oleic acid injection into the pancreatic ducts, and sham-operated rats were studied. Isolated, single-pass liver perfusion was performed, during which glucagon (1.2 pM) was infused, with or without insulin (0.6 or 1.2 nM). The suppression of HGP production by insulin was compared with changes in GLUT-2 in the membrane fraction of liver biopsies obtained before and after hormone perfusion. RESULTS: Glycogen-rich (fed) livers of normal rats (n = 16) demonstrated a dose-dependent suppression of hepatic glucose production by insulin (50 +/- 5% HGP induced by glucagon alone during 1.2-nM insulin perfusion) and a dose-dependent decrease in GLUT-2 (30 +/- 13% of basal level during 1.2-nM insulin perfusion). Sham-operated rats (n = 6) also showed reductions in HGP (51 +/- 4%) and GLUT-2 (14 +/- 10%) during 1.2-nM insulin perfusion. In contrast, rats with chronic pancreatitis (n = 6) showed no suppression of HGP during 1.2-nM insulin perfusion, and an increase in GLUT-2 (+20 +/- 6%) after insulin perfusion (p < 0.02 vs. sham). CONCLUSIONS: Insulin suppresses glucagon-stimulated HGP in normal and sham-operated rats, and this reduction in HGP is associated with a decrease in the membrane-bound quantity of GLUT-2. In chronic pancreatitis, insulin suppression of HGP is absent, and this is accompanied by an increase in GLUT-2 in the hepatocyte membrane. The authors conclude that the insulin-mediated change in the level of hepatocyte GLUT-2 is impaired in chronic pancreatitis, and may contribute to the altered glucose metabolism observed commonly in this disease.     

Morphometric studies of secretory granule distribution and association with microtubules in beta-cells of rat islets during glucose stimulation

Morphometric studies of beta-cell granule positions and interactions with microtubules in areas of exocytosis were performed on islets of rats injected with tolbutamide for 72 h. This treatment depleted granules 85% below normal control values and rendered mapping of their positions and visualization of microtubules more feasible during electron microscopy. Islets were perifused with 2.8 or 16.7 mM glucose and were fixed at 4 min (acute phase) or 20 min (second phase). Photomicrographs of beta-cell sections were digitized, and computer replicas were compared to computer-simulated cells of similar size and shape into which an identical number of granules was inserted randomly. Margination of granules into a zone within 1.5 micron of the plasma membrane was examined. At both 2.8 and 16.7 mM glucose, real cells marginated significantly more secretory granules than did simulated cell pairs. Within the zones of margination during 16.7 mM glucose perifusion, the number of secretory granules associated with microtubules was less than 6% in the acute phase and less than 1% in the second phase. These data suggest that glucose stimulation promotes movement of secretory granules into the beta-cell periphery to a greater extent than can be attributed to chance alone. The role of microtubules in peripheral granule movement into regions of exocytosis appears to be minimal.     

Comparison of insulin secretory patterns in obese nondiabetic LA/N-cp and obese diabetic SHR/N-cp rats. Role of hyperglycemia

Obese diabetic SHR/N-(cp/cp) rats are a genetic model for non-insulin-dependent diabetes mellitus. When SHR/N-cp rats are overtly diabetic, they are hyperinsulinemic and hyperglycemic in the fed state when consuming commercial chow or semipurified high-carbohydrate diets. Obese SHR/N-cp rats were hyperinsulinemic by 4 wk of age, although hyperglycemia did not appear until 3-4 wk later and was exacerbated by a high-sucrose diet (mean +/- SE 1488 +/- 238 microU/ml insulin and 425 +/- 51 mg/dl glucose). The control SHR/N-cp rats (+/?) on the sucrose diet remained lean and normoglycemic. The obese diabetic SHR/N-cp rats showed three alterations in pancreas perfusion data (not present in control rats): 1) paradoxically high insulin secretion at low glucose levels (2.5 mM), 2) secretion of insulin in response to arginine (10 mM) in the absence of glucose, and 3) impaired response of insulin secretion to high glucose (16.7 mM). To determine whether hyperglycemia was responsible for the abnormalities of insulin secretion, perfusion studies were conducted in obese nondiabetic LA/N-cp rats and compared with the SHR/N-cp rats. The obese LA/N-cp rats resembled the corpulent SHR/N-cp rats in every way, except that they were normoglycemic on the sucrose diet. The obese LA/N-cp rats had two of the three alterations in insulin secretion shown by obese SHR/N-cp rats, lacking only the impaired response to high glucose, suggesting that hyperglycemia was required for that defect to occur.(ABSTRACT TRUNCATED AT 250 WORDS)