Effect of hexoses and mannoheptulose on cyclic AMP accumulation and insulin secretion in rat pancreatic islets

Role of oscillations in membrane potential, cytoplasmic Ca2+, and metabolism for plasma insulin oscillations

A model for the relationship between ionic and metabolic oscillations and plasma insulin oscillations is presented. It is argued that the pancreatic beta-cell in vivo displays two intrinsic frequencies that are important for the regulation of plasma insulin oscillations. The rapid oscillatory activity (2--7 oscillations [osc] per minute), which is evident in both ionic and metabolic events, causes the required elevation in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) for the exocytosis of insulin granules. This activity is important for regulation of the amplitude of plasma insulin oscillations. The frequency of the rapid oscillatory ionic activities is regulated by glucose and allows the beta-cell to respond in an analogous way, with gradual changes in [Ca(2+)](i) and insulin release in response to the alterations in glucose concentration. The slower oscillatory activity (0.2--0.4 osc/min), which is evident in the metabolism of the beta-cell, has a frequency corresponding to the frequency observed in plasma insulin oscillations. The frequency is not affected by changes in the glucose concentration. This activity is suggested to generate energy in a pulsatile fashion, which sets the frequency of the plasma insulin oscillations. It is proposed that the slow oscillations in [Ca(2+)](i) observed in vitro are a manifestation of the metabolic oscillations and do not represent an in vivo phenomenon.

The role of cyclic AMP in the pathogenesis of glucose desensitization in rat pancreatic islets

Adenosine-3',5'-cyclic monophosphate (cyclic AMP) promotes exocytosis of insulin in pancreatic beta cells. This study was performed to investigate the role of cyclic AMP in the pathogenesis of glucose desensitization in rat pancreatic islets. In islets cultured with high glucose for 48 hours, 27 mmol/L glucose-induced insulin release was markedly impaired, while 3.3 mmol/L glucose-or arginine-induced insulin release was enhanced, indicating glucose desensitization. Islet cyclic AMP content was 190% enhanced in high glucose-culture islets for 48 hours. In islets cultured with dibutyryl-cyclic AMP (dbcAMP) or 3-isobutyl methy-xanthine (IBMX), islet insulin content or 27 mmol/L glucose-induced insulin release was deteriorated. In contrast, 3.3 mmol/L glucose- or arginine-induced insulin release was increased, which was similar to glucose-desensitized islets. Wash-out of dbc AMP for the last 24 hours of the 48-hour culture period restored impaired high glucose-induced insulin release in the same manner as wash-out of high glucose. Diazoxide, the KATP channel opener, also restored impaired high glucose-induced insulin release from dbcAMP-cultured islets. The data suggest that enhancement of cyclic AMP in high glucose-culture islets may be one of the pathogenesis of glucose desensitization.

The effect of fructose metabolism on the accumulation of inositol phosphates in rat pancreatic islets

The mechanism by which glucose recognition of B cells results in the release of inositol 1,4,5-trisphosphate is not known at present. In pancreatic islets, fructose shares a common metabolic pathway with glucose from the second step of glycolysis and can augment insulin secretion at stimulatory glucose levels. To evaluate the impact of glycolysis on the release of inositol 1,4,5-trisphosphate, we studied the effect of glucose and fructose metabolism on insulin secretion and the activation of inositol-specific phospholipase C, using collagenase digested rat pancreatic islets incorporated with 3H-labelled myo-inositol. Inositol phosphates, generated by the cleavage of phosphatidyl inositol by inositol phospholipase C, were analyzed using fast protein liquid chromatography. The islets were exposed to 3.3, 5.5 and 12 mmol 1(-1) glucose for 45 min in the absence or presence of 10, 20 or 30 mmol 1(-1) fructose, and the amount of insulin released into the medium was measured. Intracellular inositol phosphate accumulation was measured under the same glucose concentrations with 0, 10 and 30 mmol 1(-1) fructose. As expected, fructose alone had no insulinotropic effect, but potentiated the glucose-induced (5.5 and 12 mmol 1(-1)) insulin secretion at concentrations of 10-30 mmol 1(-1). Glucose (12 vs. 3.3 mmol 1(-1)) significantly increased both intracellular content of inositol 1,4,5-trisphosphate, as well as its metabolite inositol 1,3,4-trisphosphate. Fructose, however, had no potentiating effects on the accumulation of inositol phosphates. It is therefore supposed that glucose does not activate inositol-specific phospholipase C via the glycolysis. Further, since fructose did not activate inositol-specific phospholipase C, this stimulation is likely to be induced by glucose as such.

Cyclic AMP potentiates glucose-induced insulin release from mouse pancreatic islets without increasing cytosolic free Ca2+

The effects of various stimulants of insulin release on cytosolic free Ca2+, [Ca2+]i, in dispersed and cultured pancreatic beta-cells from ob/ob-mice were studied using the indicator quin-2, which in itself has only slight effects on the glucose-induced insulin release and the metabolism of the sugar. The resting [Ca2+]i was 158 +/- 7 nM. After increasing glucose to 20 mM there was a lag-period of 1-2 min before [Ca2+]i gradually rose, reaching a new plateau 60% higher after 5-6 min. Increasing intracellular cyclic AMP by adding forskolin did not further increase [Ca2+]i; on the contrary there was a slight temporary reduction despite a doubling of insulin secretion. The maintenance of the beta-cell function was evident from a marked increase of cytosolic [Ca2+]i after depolarization evoked by high extracellular K+. Also dibutyryl cyclic AMP and theophylline lacked the ability to raise [Ca2+]i beyond that obtained by glucose. The results suggest that cyclic AMP potentiates glucose-induced insulin release by sensitizing the secretory machinery to changes of [Ca2+]i rather than by increasing the cytosolic concentration of the ion.

Hexose metabolism in pancreatic islets. The phosphorylation of fructose

Fructose, like glucose, rapidly equilibrates across the plasma membrane of pancreatic islet cells, but is poorly metabolized and is a weak insulin secretagogue in rat pancreatic islets. A possible explanation for such a situation was sought by investigating the modality of fructose phosphorylation in islet homogenates. Several findings indicated that the phosphorylation of fructose is catalyzed by hexokinase, but not fructokinase. First, at variance with the situation found in liver homogenates, the phosphorylation of fructose in the islet homogenate was unaffected by K+ and inhibited by glucose, mannose, glucose 6-phosphate or glucose 1,6-bisphosphate. Second, the Km for fructose was much higher in islets than in liver. Third, in islet homogenates the Km and Vmax for fructose were much higher than those for glucose or mannose phosphorylation, at low aldohexose concentrations, in good agreement with the properties of purified hexokinase. In intact islets fructose augmented the islet content in glucose 6-phosphate sufficiently to cause marked inhibition of its own rate of phosphorylation. These findings may account, in part at least, for the low rate of fructose utilization by rat pancreatic islets.

Mediation of insulin release by cGMP and cAMP in a starved animal model

Islets isolated from fed rats released insulin in response to glucose, 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) and 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP), but not to 8-bromoinosine 3',5'-cyclic monophosphate. Starving rats for 48 h significantly diminished insulin release from islets in response to these agents, and lowered endogenous levels of cGMP and cAMP. The analogs of cGMP and cAMP potentiated the glucose response in a dose-dependent manner in islets from starved rats, whereas in fed rat islets the cyclic nucleotide analogs did not potentiate glucose-stimulated insulin release. Sodium nitroprusside, which enhances endogenous cGMP levels in islets, also enhanced the glucose response in islets from starved rats. Mannoheptulose inhibited glucose and 8-Br-cGMP-stimulated insulin release, but not 8-Br-cAMP-stimulated release. These results suggest that the impaired glucose response of islets from starved animals is in part due to diminished levels of cyclic nucleotides, and that the role(s) of cGMP in insulin secretion may include enhancement of glucose metabolism.

Studies on the interaction of staphylococcal delta-haemolysin with isolated islets of Langerhans

delta-Haemolysin, a small surface-active polypeptide purified from the culture media of Staphylococcus aureus, was observed to stimulate the release of insulin from isolated rat islets of Langerhans. This effect was dose-dependent and saturable, with the half-maximal response elicited by a delta-haemolysin concentration of 10 micrograms/ml. Stimulation of insulin release by delta-haemolysin (10 micrograms/ml) was not dependent on the presence of glucose in the incubation medium, but was augmented by increasing concentrations of the sugar. The release of insulin in response to delta-haemolysin could be inhibited by depletion of extracellular Ca2+ or by adrenaline (epinephrine) (10 microM) and was readily reversible when delta-haemolysin was removed from the medium. In addition, the response was potentiated by incubation with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.2 mM). These observations suggest that delta-haemolysin induced a true activation of the beta-cell secretory mechanism. Stimulation of islets of Langerhans with delta-haemolysin was found to be associated with a modest increase in intracellular cyclic AMP levels, although the adenylate cyclase activity of islet homogenates was not increased by delta-haemolysin. delta-Haemolysin was observed to induce a dose-dependent net accumulation of 45Ca2+ by islet cells and to stimulate the efflux of 45Ca2+ from preloaded islets. The efflux of 45Ca2+ was modest in size and short-lived, but dramatically increased in medium depleted fo 40Ca2+. Incubation in the presence of verapamil augmented delta-haemolysin-induced 45Ca2+ efflux and insulin secretion. delta-Haemolysin was found to be a potent 45Ca2+-translocating ionophore in an artificial system. This response was dose-dependent and could be augmented by verapamil. In addition, phosphatidylcholine (25 micrograms/ml) was found to inhibit both delta-haemolysin induced 45Ca2+ translocation and insulin release in a precisely parallel manner. These studies suggest that the ability of delta-haemolysin to stimulate insulin release may be due, in part, to the facilitation of Ca2+ entry into the beta-cells of islets of Langerhans, mediated directly by an ionophoretic mechanism.

Metabolic control of potassium permeability in pancreatic islet cells

The K(+) permeability of pancreatic islet cells was studied by monitoring the efflux of (86)Rb(+) (used as tracer for K(+)) from perifused rat islets and measuring the uptake of (42)K(+). Glucose markedly and reversibly decreased (86)Rb(+) efflux from islet cells and this effect was antagonized by inhibitors of the metabolic degradation of the sugar, i.e. mannoheptulose, iodoacetate, glucosamine and 2-deoxyglucose. Among glucose metabolites, glyceraldehyde reduced the K(+) permeability even more potently than did glucose itself; pyruvate and lactate alone exhibited only a small effect, but potentiated that of glucose. Other metabolized sugars, like mannose, glucosamine and N-acetylglucosamine, also decreased (86)Rb(+) efflux from islet cells. Fructose was effective only in the presence of glucose. Non-metabolized sugars like galactose, 2-deoxyglucose and 3-O-methylglucose had no effect. The changes in K(+) permeability by agents known to modify the concentrations of nicotinamide nucleotides, glutathione or ATP in islet cells were also studied. Increasing NAD(P)H concentrations in islet cells by pentobarbital rapidly and reversibly reduced (86)Rb(+) efflux; exogenous reduced glutathione produced a similar though weaker effect. By contrast, oxidizing nicotinamide nucleotides with phenazine methosulphate or Methylene Blue, or oxidizing glutathione by t-butyl hydroperoxide increased the K(+) permeability of islet cells. Uncoupling the oxidative phosphorylations with dicumarol also augmented (86)Rb(+) efflux markedly. In the absence of glucose, but not in its presence, methylxanthines reduced (86)Rb(+) efflux from the islets; such was not the case for cholera toxin or dibutyryl cyclic AMP. Glucose and glyceraldehyde had no effect on (42)K(+) uptake after a short incubation (10min), but augmented it after 60min; the effect of glucose was suppressed by mannoheptulose and not mimicked by 3-O-methylglucose. The results clearly establish the importance of the metabolic degradation of glucose and other substrates for the control of the K(+) permeability in pancreatic islet cells and support the concept that a decrease in the K(+) permeability represents a major step of the B-cell response to physiological stimulation.

The effect of calcium on somatostatin inhibition of insulin release and cyclic AMP production in mouse pancreatic islets

The effect of somatostatin on glucose-induced insulin secretion and cyclic AMP accumulation in isolated islets from obese, hyperglycemic ob/ob mice was studied in a microperifusion system. The normal biphasic pattern of insulin release as well as the inhibitory pattern of insulin release produced by somatostatin (0.5--1 microgram/ml) was matched by similar changes in the intracellular concentration of cyclic AMP. When islets were stimulated by glucose (3 mg/ml) plus 3-isobutyl-1-methylxanthine (0.1 mM), somatostatin (0.5 microgram/ml) failed to inhibit insulin secretion or cyclic AMP formation in the second phase whereas in the first phase both parameters were significantly reduced by somatostatin (0.5 microgram/ml). In batch-type incubations it was shown that addition of excess calcium (to 6 mM) reversed this inhibition. In the second phase calcium potentiated the (glucose + 3-isobutyl-1-methylxanthine)-stimulated insulin secretion without affecting the cyclic AMP production. This potentiation was inhibited by somatostatin (0.1 microgram/ml). Somatostatin (1 microgram/ml) inhibited adenylate cyclase activity in islet homogenates. No effect of somatostatin on islet glucose utilization could be demonstrated. The results indicate a dual action of somatostatin in the inhibition of insulin release, one involving the islet adenylate cyclase and one affecting the islet uptake of calcium.

The metabolism of cyclic AMP and glucose in isolated islets from Acomys cahirinus

Glucose-induced cyclic (3H) AMP accumulation, insulin secretory responses and the metabolism of glucose were studied in pancreatic islets from Acomys cahirinus. 27.7 mmol/l of glucose stimulated neither islet cyclic (3H) AMP accumulation nor insulin release during the first 5 min of incubation. Stimulation by glucose of cyclic (3H) AMP was observed after 15 min of incubation and insulin release was markedly stimulated between 15 and 30 min. The utilization of glucose, measured as the production of (3H)2O from (5--3H) glucose was stimulated by glucose after 10 min and proceeded at an apparently linear rate during a 20 min incubation period. In incubations of 5 min, glibenclamide, glucagon or chloromercuribenzene-p-sulphonic acid failed to stimulate islet cyclic (3H) AMP accumulation. 3-isobutyl-l-methylxanthine in a concentration of 1.0 mmol/l was the only agent tested that elevated rapidly (1 min) islet cyclic (3H) AMP. None of the agents tested elicited an insulin secretory response in 5 min incubations. It is concluded that 1) no gross defect is apparent in the utilization of glucose by Acomys islets, 2) the secretory derangement of the Acomys is associated with a delayed cyclic AMP response to glucose, 3) however a decreased level of cyclic AMP cannot be the sole explanation for the delayed insulin secretion in the Acomys.

Cyclic AMP and insulin release

The role of cyclic adenosine-3',5'-monophosphate (cAMP) for insulin secretion has been investigated. In isolated islets of Langerhans from the rat, glucose increases cAMP concomitant with insulin secretion. Stimulation of these two parameters is likewise reversible in parallel. The minimal and maximal concentrations of glucose eliciting cAMP and insulin responses are similar. Isomers and epimers of glucose influence insulin and cAMP in a parallel fashion as do sulfonylurea compounds (tolbutamide and glibenclamide). On the contrary, the time-dependent potentiation of glucose-induced insulin secretion is not accompanied by gross changes in cAMP. Reciprocally, in the absence of glucose islet cAMP can be markedly elevated by other agents (methyl xanthines, cholera toxin) without major insulin responses. The results indicate that metabolism of cAMP in the beta-cell is intimately linked to the glucose (and sulfonylurea) action on insulin secretion, although other factors influenced by the hexose are also necessary for the release process. The finding that the cAMP response is impaired in fasting, during the neonatal period and in diabetes mellitus (in the Chinese hamster) suggests an important role for the nucleotide in physiological and pathophysiological states characterized by decreased insulin release.