Effects of phosphoenolpyruvate, other glycolytic intermediates and methylxanthines on calcium uptake by a mitochondrial fraction from rat pancreatic islets

Phosphoenolpyruvate cycling via mitochondrial phosphoenolpyruvate carboxykinase links anaplerosis and mitochondrial GTP with insulin secretion

Pancreatic beta-cells couple the oxidation of glucose to the secretion of insulin. Apart from the canonical K(ATP)-dependent glucose-stimulated insulin secretion (GSIS), there are important K(ATP)-independent mechanisms involving both anaplerosis and mitochondrial GTP (mtGTP). How mtGTP that is trapped within the mitochondrial matrix regulates the cytosolic calcium increases that drive GSIS remains a mystery. Here we have investigated whether the mitochondrial isoform of phosphoenolpyruvate carboxykinase (PEPCK-M) is the GTPase linking hydrolysis of mtGTP made by succinyl-CoA synthetase (SCS-GTP) to an anaplerotic pathway producing phosphoenolpyruvate (PEP). Although cytosolic PEPCK (PEPCK-C) is absent, PEPCK-M message and protein were detected in INS-1 832/13 cells, rat islets, and mouse islets. PEPCK enzymatic activity is half that of primary hepatocytes and is localized exclusively to the mitochondria. Novel (13)C-labeling strategies in INS-1 832/13 cells and islets measured substantial contribution of PEPCK-M to the synthesis of PEP. As high as 30% of PEP in INS-1 832/13 cells and 41% of PEP in rat islets came from PEPCK-M. The contribution of PEPCK-M to overall PEP synthesis more than tripled with glucose stimulation. Silencing the PEPCK-M gene completely inhibited GSIS underscoring its central role in mitochondrial metabolism-mediated insulin secretion. Given that mtGTP synthesized by SCS-GTP is an indicator of TCA flux that is crucial for GSIS, PEPCK-M is a strong candidate to link mtGTP synthesis with insulin release through anaplerotic PEP cycling.

Phosphate depletion impairs insulin secretion by pancreatic islets

Phosphate depletion (PD) is associated with resistance to the peripheral action of insulin and with glucose intolerance. However, data on the effect of PD on insulin secretion are not consistent, and were derived indirectly by measurements of blood levels of insulin during intravenous glucose tolerance test (IVGTT) or with hyperglycemic clamp technique. Direct evidence for an effect of PD on insulin secretion by pancreatic islets is not available, and the potential mechanisms through which PD may affect insulin secretion are not known. We performed IVGTT, examined in vitro insulin secretion by pancreatic islets, and evaluated various factors involved in insulin secretion in PD and pair weighed (PW) rats. PD animals had fasting hyperglycemia and normal plasma insulin levels, and displayed abnormal IVGTT as compared to PW rats. Both initial and late phases of D-glucose-induced insulin secretion from islets were markedly and significantly (P less than 0.01) lower than from islets of PW rats. In contrast, D-glyceraldehyde-induced insulin release in PD rats was similar to that of PW rats. [H3]2-deoxyglucose uptake by islets and their cyclic AMP content after exposure to D-glucose, D-glyceraldehyde or forskolin were not different among the two groups of animals. Insulin content in PD islets was modestly but significantly (P less than 0.01) higher than PW islets. In PD islets, ATP content and the ATP/ADP ratio at basal state and after incubation with 16.7 mM D-glucose were significantly (P less than 0.01) lower and resting cytosolic calcium was significantly (P less than 0.01) higher than in PW islets.(ABSTRACT TRUNCATED AT 250 WORDS)

24-h blood glucose pattern in type I and type II diabetics after oral treatment with pentoxifylline as assessed by artificial endocrine pancreas

Based on the known action of xanthine derivatives on the insulin secretion, the effect of pentoxifylline on carbohydrate homeostasis of type I (IDDM) and type II (NIDDM) diabetics was investigated. Pentoxifylline is known to exert a favorable influence on hemorheological disturbances in such patients. Twenty-four hour blood glucose pattern and insulin requirements were evaluated in type I and type II diabetics by the use of the artificial pancreas before and after a 14-day treatment with pentoxifylline 400 mg p.o. (Trental 400) t.i.d. During the stabilization period before treatment with pentoxifylline, NIDDM patients required 10.1 +/- 3.8 U of insulin and the IDDM 35 +/- 13.7 U. After 2 weeks on pentoxifylline, NIDDM required only 6.3 +/- 2.8 U (p less than 0.05) and IDDM 28.5 +/- 9.7 U (n.s.). Average blood glucose during the 24h decreased by 15.8 +/- 3.5% in NIDDM and by 10.3 +/- 2.5% in IDDM. Moreover, a significant smoothing of glucose fluctuations during the 24h was noted in both groups. It is concluded that pentoxifylline administered concurrently to any antidiabetic type of treatment leads to better blood glucose control as well as to prevention or delay of vascular complications.

Influence of inorganic phosphate on glucose-induced insulin release in vitro

The effect of various extracellular phosphate concentrations on glucose induced insulin secretion has been investigated in two experimental systems the isolated perfused rat pancreas and the isolated rat pancreatic islets incubated in vitro. In both systems, total insulin secretion in response to 16.7 mmol/l glucose increased as extracellular phosphate concentration increased from 0 to 3.6 mmol/l. In perfusion experiments insulin secretion was significantly reduced at 4.8 mmol/l phosphate. In incubation experiments insulin secretion was inhibited only at 9.6 mmol/l phosphate concentration. When IBMX 1 mmol/l was used in incubation experiments glucose induced insulin secretion increased as extracellular phosphate increased from 0 to 3.6 mmol/l, but was also inhibited at 4.8 mmol/l.

Evidence for calcium enhanced phosphorylation of pyruvate kinase by pancreatic islets

Pancreatic islet cytosol contains a calcium-calmodulin dependent protein kinase that can mediate the phosphorylation of an endogenous protein that has an Mr of 57 000, as well as exogenous muscle pyruvate kinase (subunit Mr, 57 000). EGTA and trifluoperazine decreased the phosphorylation. Alkaline inactivation of pyruvate kinase made it a better substrate for the kinase. As in rat islet cytosol, rabbit islet cytosol catalyzed the phosphorylation of a 57 000 Mr protein in the presence of calcium and calmodulin. This phosphoprotein was immunoprecipitated with anti-pyruvate kinase antibody. This is consistent with the idea that the 57 000 Mr phosphoprotein in islet cytosol is the subunit of pyruvate kinase. The paper following this paper shows that the kinetic and immunologic properties of the islet pyruvate kinase indicate it is the M2 isoenzyme and that its phosphorylation does not affect its catalytic activity.

Evidence for phosphorylation of pancreatic islet pyruvate kinase

Rabbit pancreatic islet cytosol catalyzes the calcium-activated phosphorylation by [gamma 32P]ATP of a protein with a molecular weight of 57,000 that is precipitated with antipyruvate kinase antibodies. We were unable to demonstrate that phosphorylation in the presence of calcium or cAMP had any immediate effect on rat pancreatic islet pyruvate kinase activity. This finding is consistent with our inability to confirm the finding of others that pancreatic islets contain phosphoenolpyruvate carboxykinase activity (Diabetes, 34:246, 1985). Since the carboxykinase catalyzes phosphoenolpyruvate formation and pyruvate kinase catalyzes essentially the opposite reaction, if the carboxykinase were present in the beta cell, pyruvate kinase would need to be inhibited to prevent recycling of phosphoenolpyruvate.

Thiols and pancreatic beta-cell function: a review

In pancreatic islets insulin secretion in response to a variety of stimulators is sensitive to the redox state of extracellular and intracellular thiols. In this connection variations of plasma glutathione (GSH) may also be of importance. In the process of stimulus-secretion coupling, membrane thiols play an important role. One major localization of critical thiols appears to be related to the influx of calcium through the voltage-dependent channel. Other transmembranal ion movements and the cAMP system seem to be less sensitive to thiol oxidation than calcium influx via voltage-dependent Ca channels.

Substrates for cyclic AMP-dependent protein kinase in islets of Langerhans. Studies with forskolin and catalytic subunit

To investigate substrates for cyclic AMP-dependent protein kinase in intact islets of Langerhans, batches of islets were incubated with [32P]Pi for 1 h in the presence of 10 mM-glucose; the adenylate cyclase activator forskolin, which in parallel experiments was shown to increase islet cyclic AMP content and insulin release, was then added. Islets were homogenized and subcellular fractions prepared by differential centrifugation. Phosphopeptides were electrophoresed on sodium dodecyl sulphate/polyacrylamide gels and quantified by autoradiography and densitometry. Within 5 min forskolin caused increased labelling of Mr-25 000 and -30 000 cytosolic and Mr-23 000 and -32 000 particulate peptides; a rapid decrease in phosphorylation of Mr-18 000 and -34 000 cytosolic peptides was also observed. In addition, rather slower phosphorylation occurred of the Mr-15 000 peptide previously identified as histone H3 [Christie & Ashcroft (1984) Biochem. J. 218, 87-99]. When similar subcellular fractions were incubated with [gamma-32P]ATP and purified catalytic subunit of cyclic AMP-dependent protein kinase, peptides phosphorylated included cytosolic species of Mr 25 000 and 30 000 and particulate species of Mr 23 000 and 32 000. The distribution of RNA in the subcellular fractions suggested that the Mr-32 000 species could be a ribosomal protein. The 24 000 g pellet was heterogeneous, as judged by marker assays, and was therefore fractionated further by Percoll-density-gradient centrifugation. The peak containing the Mr-23 000 peptide was resolved from marker enzymes for plasma membranes, mitochondria and endoplasmic reticulum and coincided with a peak for insulin: hence the Mr-23 000 peptide is likely to be a secretory-granule component. The study demonstrates that the potentiation of insulin release that occurs when islet cyclic AMP is increased is accompanied by rapid phosphorylation of specific islet substrates for cyclic AMP-dependent protein kinase. The data are consistent with the hypothesis that protein phosphorylation is involved in the regulation of insulin secretion.

Glucose increases the synthesis of lipoxygenase-mediated metabolites of arachidonic acid in intact rat islets

Previous studies suggested that products of a 12-lipoxygenase pathway in the pancreatic islet may promote insulin release. To determine whether glucose augments the production of such metabolites, intact rat islets prelabeled with [3H]arachidonate were stimulated with glucose, and 12-hydroxy-5,8,10,14-icosatetraenoic acid (12-HETE) release was measured by using HPLC. D-Glucose (16.7 mM) augmented the enzymatic synthesis of 12-HETE by 271% above that seen with 0-1.7 mM glucose. The glucose effect was stereospecific and preferential for the alpha anomer; it was modestly potentiated by the cyclo-oxygenase inhibitor ibuprofen. Glucose-stimulated 12-HETE accumulation was abrogated by mannoheptulose and was reproduced by the trioses glyceraldehyde or dihydroxyacetone, suggesting that the metabolism of glucose to glucose 6-phosphate or triose phosphates (or both) is critical. Glucose also augmented [3H]arachidonate labeling of islets, suggesting an action at the level of substrate release or re-uptake (or both). These features of islet 12-HETE synthesis accord well with other known effects of glucose on beta cell function and suggest that lipoxygenase-mediated metabolites of arachidonate may be suitable candidates to mediate or amplify glucose's effects on insulin release.

The role of phosphoenolpyruvate in insulin secretion: the effect of L-phenylalanine

Incubation of rat islets with phenylalanine increased the tissue content of phosophoenolpyruvate, both in the presence and in the absence of glucose. At the same time, L-phenylalanine neither stimulated nor inhibited insulin release. It is unlikely that insulin secretion is tightly coupled to the availability of phosphoenolpyruvate in rat islets.

Phosphoenolpyruvate carboxykinase in mouse pancreatic islets. ATP-induced changes in sensitivity to Mn2+ activation

The presence of high phosphoenolpyruvate carboxykinase (EC 4.1.1.32) activity in mouse islet cytosol has been demonstrated. The enzyme was activated by Mn2+ with a Ka of 100 X 10(-6) mol/l. The mean total activity of the Mn2+-stimulated phosphoenolpyruvate carboxykinase in islet cytosol estimated at 22 degrees C with saturating concentrations of the substrates oxaloacetate and ITP was 146 pmol/min per micrograms DNA. Km was calculated to be 6 X 10(-6) mol/l for oxaloacetate and 140 X 10(-6) mol/l for ITP. The islet phosphoenolpyruvate carboxykinase activity was not increased after starvation of the animals for 48 h. Preincubation of the cytosol at 4 degrees C with Fe2+, quinolinate, ATP, Pi, glucose 6-phosphate, fructose 1,6-bisphosphate, NAD+, NADH, oxaloacetate, ITP, cyclic AMP and Ca2+ had no effect on the enzyme activity. However, preincubation of the cytosol at 37 degrees C with ATP-Mg inhibited the Mn2+-stimulated phosphoenolpyruvate carboxykinase activity progressively with time and in a concentration-dependent manner. A similar but weaker inhibitory effect was observed with p[NH]ppA, whereas p[CH2]ppA, ADP, AMP, adenosine and Pi had no effect. It is tentatively suggested that ATP and p[NH]ppA either by adenylation or otherwise affect the interaction between islet phosphoenolpyruvate carboxykinase and the recently discovered Mr = 29000 protein modulator of the enzyme in such a way - perhaps by causing a dissociation between them - that phosphoenolpyruvate carboxykinase loses its sensitivity to Mn2+ activation.

Cytosolic free Ca2+ in insulin secreting cells and its regulation by isolated organelles

The role of Ca2+ in secretagogue-induced insulin release is documented not only by the measurements of 45Ca fluxes in pancreatic islets, but also, by direct monitoring of cytosolic free Ca2+, [Ca2+]i. As demonstrated, using the fluorescent indicator quin 2, glyceraldehyde, carbamylcholine and alanine raise [Ca2+]i in the insulin secreting cell line RINm5F, whereas glucose has a similar effect in pancreatic islet cells. The regulation of cellular Ca2+ homeostasis by organelles from a rat insulinoma, was investigated with a Ca2+ selective electrode. The results suggest that both the endoplasmic reticulum and the mitochondria participate in this regulation, albeit at different Ca2+ concentrations. By contrast, the secretory granules do not appear to be involved in the short-term regulation of [Ca2+]i. Evidence is presented that inositol 1,4,5-trisphosphate, which is shown to mobilize Ca2+ from the endoplasmic reticulum, is acting as an intracellular mediator in the stimulation of insulin release.

Pancreatic islet metabolism and redox state during stimulation of insulin secretion with glucose and fructose

The mechanism of potentiation of insulin secretion by fructose was investigated. Twenty mM fructose + 3 mM glucose stimulated insulin secretion in a biphasic manner similar to what is found during stimulation with 20 mM glucose, whereas 20 mM fructose alone did not affect secretion. Fructose utilization was measured as formation of tritiated water from 5-3H-fructose. At 27.8 mM fructose the utilization rate was 258 pmol/2 h/10 islets, which is less than the utilization rate of 2.8 mM glucose. 20 mM glucose increased the islet NADH/NAD+ and NADPH/NADP+ redox ratios as well as islet concentration of ATP and PEP. 20 mM fructose + 3 mM glucose did not affect the concentration of ATP and PEP or the NADH/NAD+ redox ratio. The NADPH/NADP+ ratio was significantly decreased (60%) after 2.5 min incubation with .20 mM fructose + 3 mM glucose. It is concluded that fructose potentiation of insulin secretion is not primarily dependent on fructose metabolism and that any conceivable effect on plasma membrane ion fluxes as caused by a reduction of plasma membrane disulfides, may be caused by mechanisms other than a mere increase in the pyridine nucleotide substrates for the transhydrogenation process.

Oxygenation products of arachidonic acid: third messengers for insulin release

Although an association between membrane phospholipid turnover and exocytotic hormone release has long been recognized, a causal relationship has not been firmly established. Recent studies suggest that glucose (and probably other insulin secretagogues) activates phospholipases and thereby releases membrane-bound arachidonic acid (AA). AA is then converted through islet 12-lipoxygenase to mediators or modulators of insulin release (tentatively identified as peroxides and epoxides of arachidonate). These products may be critical links in stimulus-secretion coupling, since blockade of either AA release or lipoxygenation abrogates insulin release induced by glucose and many other (but not all) stimuli. Cogeneration of prostaglandins from AA through the cyclooxygenase pathway may directly or indirectly modulate the formation and/or effect of lipoxygenase products. A critical role for lipoxygenase products (and possibly metabolites of AA synthesized by other pathways, such as P-450-dependent monooxygenases) may extend to many secretory cells in addition to pancreatic beta cells. The phasic release of AA described in many cells could explain the biphasic pattern of insulin release induced by glucose. Since some phospholipases and lipoxygenases are Ca++ activated, the release of AA in conjunction with its oxygenation appears to be a concerted system generating "third messengers" for hormone release.

Effects of theophylline and dibutyryl cyclic adenosine monophosphate on the membrane potential of mouse pancreatic beta-cells

The effects of theophylline and dibutyryl cyclic AMP on the membrane potential of mouse beta-cells were studied with micro-electrodes. They were compared to their effects on insulin release by perifused mouse islets. In 3 mM-glucose, theophylline (10 mM) depolarized the beta-cell membrane and stimulated insulin release, but did not induce electrical activity. Dibutyryl cyclic AMP (1 mM) was without effect. In 7 mM-glucose, theophylline (0.5-2 mM) and dibutyryl cyclic AMP (1 mM) slightly depolarized the beta-cell membrane, induced electrical activity in otherwise silent cells and increased insulin release. A higher concentration of theophylline (10 mM) hyperpolarized the beta-cell membrane, did not induce electrical activity, but also stimulated insulin release. In 10 mM-glucose, the membrane potential of beta-cells exhibited repetitive slow waves with bursts of spikes on the plateau. Under steady state, these slow waves were differently affected by low or high concentrations of theophylline. At 0.5-2 mM, theophylline shortened the intervals, lengthened the slow waves and slightly increased their frequency. On the other hand, 10 mM-theophylline markedly decreased the duration of both intervals and slow waves, and increased their frequency. The effects of 1 mM-dibutyryl cyclic AMP were similar to those of 2 mM-theophylline. With 2-10 mM-theophylline, two other effects were also observed: a transient hyperpolarization with suppression of electrical activity immediately after addition of the methylxanthine and an increase in electrical activity upon its withdrawal. Theophylline and dibutyryl cyclic AMP markedly potentiated insulin release induced by 10 mM-glucose. The magnitude of these changes did not correlate well with the importance of the changes in electrical activity. However, with 2-10 mM-theophylline the increase in release was also preceded by an initial transient inhibition, whereas withdrawal of the methylxanthine was accompanied by a further increase. When Ca influx was inhibited by D600, the slow waves were suppressed, the membrane was depolarized to the plateau level and only few spikes were present. Although theophylline markedly increased insulin release under these conditions, it did not affect the membrane potential. Several conclusions can be drawn from this study. Insulin release and electrical activity in beta-cells can be dissociated when intracellular Ca is used to trigger exocytosis. High concentrations of theophylline produce effects unrelated to cyclic AMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclic AMP-dependent protein phosphorylation and insulin secretion in intact islets of Langerhans

Effects on insulin release, cyclic AMP content and protein phosphorylation of agents modifying cyclic AMP levels have been tested in intact rat islets of Langerhans. Insulin release induced by glucose was potentiated by dibutyryl cyclic AMP, glucagon, cholera toxin and 3-isobutyl-1-methylxanthine (IBMX); the calmodulin antagonist trifluoperazine reversed these potentiatory effects. Inhibition by trifluoperazine of IBMX-potentiated release was, however, confined to concentrations of IBMX below 50 microM; higher concentrations, up to 1 mM, were resistant to inhibition by trifluoperazine. IBMX-potentiated insulin release was also inhibited by 2-deoxyadenosine, an inhibitor of adenylate cyclase. In the absence of glucose, IBMX at concentrations up to 1 mM did not stimulate insulin release and in the presence of 3.3 mM-glucose IBMX was effective only at a concentration of 1 mM; under the latter conditions trifluoperazine again did not inhibit insulin secretion. The maximum effect on insulin release was achieved with 25 microM-IBMX. Islet [cyclic AMP] was increased by IBMX, with the maximum rise occurring with 100 microM-IBMX. The increase in [cyclic AMP] elicited by IBMX was more rapid than that induced by cholera toxin. Trifluoperazine did not significantly affect islet cyclic AMP levels under any of the conditions tested. When islets were incubated with [32P]Pi, radioactivity was incorporated into islet ATP predominantly in the gamma-position. The rate of equilibration of label was dependent on medium Pi and glucose concentration and at optimal concentrations of these 100% equilibration of internal [32P]ATP with external [32P]Pi required a period of 3h. Radioactivity was incorporated into islet protein and, in response to an increase in islet [cyclic AMP], the major effect was on a protein of Mr 15 000 on sodium dodecyl sulphate/polyacrylamide gels. The extent of phosphorylation of the Mr-15 000 protein was correlated with the level of cyclic AMP: phosphorylation in response to IBMX was inhibited by 2-deoxyadenosine but not by trifluoperazine. Fractionation of islets suggested that the Mr-15 000 protein was of nuclear origin: the protein co-migrated with histone H3 on acetic acid/urea/Triton gels. In the islet cytosol a number of proteins were phosphorylated in response to elevation of islet [cyclic AMP]: the major species had Mr values of 18 000, 25 000, 34 000, 38 000 and 48 000. Culture of islets with IBMX increased the rate of [3H]-thymidine incorporation.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical coupling between cells in islets of Langerhans from mouse

Two microelectrodes have been used to measure membrane potentials simultaneously in pairs of mouse pancreatic islet cells. In the presence of glucose at concentrations between 5.6 and 22.2 mM, injection of current i into cell 1 caused a membrane potential change in this cell, V1, and, provided the second microelectrode was less than 35 micron away, in a second impaled cell 2, V2. This result establishes that there is electrical coupling between islet cells and suggests that the space constant of the coupling ratio within the islet tissue is of the order of a few beta-cell diameters. The current-membrane potential curves i-V1 and i-V2 are very similar. By exchange of the roles of the microelectrodes, no evidence of rectification of the current through the intercellular pathways was found. Removal of glucose caused a rapid decrease in the coupling ratio V2/V1. In steady-state conditions, the coupling ratio increases with the concentration of glucose in the range from 0 up to 22 mM. Values of the equivalent resistance of the junctional and nonjunctional membranes have been estimated and found to change with the concentration of glucose. Externally applied mitochondrial blockers induced a moderate increase in the junctional resistance possibly mediated by an increase in intracellular Ca2+.

Modulation of active Ca2+ uptake by the islet-cell endoplasmic reticulum

The possible effects of calmodulin and cyclic AMP on active Ca2+ uptake by the islet-cell endoplasmic reticulum were investigated. Neither calmodulin nor cyclic AMP affected the rate of active Ca2+ uptake, or the steady-state filling capacity of the endoplasmic reticulum when measured in the absence of oxalate. Consistent with these results, calmodulin did not activate the Ca2+-stimulated ATPase activity associated with this cell fraction. During the course of these experiments., it was unexpectedly discovered that the rate of Ca2+ uptake, as well as the steady-state Ca2+ filling capacity of the endoplasmic reticulum, were markedly increased by unidentified factor(s) in the cytosol. This effect could be demonstrated by reconstitution of the membranes in cytosol, or by direct addition of fresh or dialysed cytosol to the Ca2+ uptake assays. The degree of activation by the cytosol indicates that the endoplasmic reticulum may play a prominent role in controlling beta-cell Ca2+ concentrations and that the unidentified activator(s) present in the cytosol may be involved in regulation of this function.

Effects of cyclic AMP on DNA replication and protein biosynthesis in fetal rat islets of Langerhans maintained in tissue culture

The regulatory role of cyclic AMP (cAMP) in the growth and insulin production of the islet organ in vitro has been investigated. The effects of dibutyryl cyclic AMP (dbcAMP), theophylline, and 3-isobutyl-1-methylxanthine (IBMX) on DNA replication and on the biosynthesis of RNA and insulin in fetal rat islets of Langerhans maintained in tissue culture have been studied. Raising the glucose concentration from 2.7 mM to 16.7 mM caused a two-fold increase in DNA replication. Both dbcAMP and theophylline markedly inhibited the DNA replication at all glucose concentrations studied. Low concentrations of IBMX stimulated DNA synthesis. However, at higher concentrations of this drug, known to considerably increase the islet cAMP levels, a marked inhibition of islet DNA replication was observed. Both (pro)insulin and total protein biosynthesis were stimulated by glucose, whereas dbcAMP stimulated only the (pro)insulin biosynthesis. Since glucose is known to raise islet intracellular levels of cAMP, which is known to be an inhibitor of cellular proliferation, the observed glucose stimulation of both islet-cell DNA replication and insulin production appeared conflicting. It is suggested that this dual effect of glucose may depend on a stimulation of proliferation in a limited pool of islet cells which may not exhibit an increase in cAMP.

Immunological and kinetic properties of pyruvate kinase in rat pancreatic islets

Pyruvate kinase in rat pancreatic islets was characterized immunologically and kinetically. It is concluded that this activity is predominantly if not totally of the M(2) type.

Effects of trifluoperazine and pimozide on stimulus-secretion coupling in pancreatic B-cells. Suggestion for a role of calmodulin?

The possible involvement of calmodulin in insulin release was evaluated by studying the effects on intact islets of trifluoperazine and pimozide, two antipsychotic agents known to bind strongly to calmodulin in cell-free systems. Trifluoperazine (10-100mum) produced a dose- and time-dependent inhibition of the two phases of glucose-stimulated insulin release. The effect was not reversible by simple washing of the drug, but could be prevented by cytochalasin B or theophylline. Trifluoperazine also inhibited the release induced by glyceraldehyde, oxoisocaproate, tolbutamide or barium, but not that stimulated by 10mm-theophylline or 1mm-3-isobutyl-1-methylxanthine. Pimozide (0.5-10mum) also produced a dose-dependent inhibition of insulin release triggered by glucose, leucine or barium, but did not affect the release induced by methylxanthines. Glucose utilization by islet cells was not modified by trifluoperazine (25mum), which slightly increased cyclic AMP concentration in islets incubated without glucose. The drug did not prevent the increase in cyclic AMP concentration observed after 10min of glucose stimulation, but suppressed it after 60min. Basal or glucose-stimulated Ca(2+) influx (5min) was unaffected by 25mum-trifluoperazine, whereas Ca(2+)net uptake (60min) was inhibited by 20%. Glucose-stimulated Ca(2+) uptake was almost unaffected by pimozide. In a Ca(2+)-free medium, trifluoperazine decreased Ca(2+) efflux from the islets and did not prevent the further decrease by glucose; in the presence of Ca(2+), the drug again decreased Ca(2+) efflux and inhibited the stimulation normally produced by glucose. In the absence of glucose, trifluoperazine lowered the rate of Rb(+) efflux from the islets, decreased Rb(+) influx (10min), but did not affect Rb(+) net uptake (60min). It did not interfere with the ability of glucose to decrease Rb(+) efflux rate further and to increase Rb(+) net uptake. The results show thus that trifluoperazine does not alter the initial key events of the stimulus-secretion coupling. Its inhibition of insulin release suggests a role of calmodulin at late stages of the secretory process.

Evidence for the involvement of Na/Ca exchange in glucose-induced insulin release from rat pancreatic islets

Glucose-induced inhibition of Ca(++) extrusion from the beta-cell may contribute to the rise in cytosol Ca(++) that leads to insulin release. To study whether interference with Na/Ca exchange is involved in this inhibition the effects of glucose were compared to those of ouabain. This substance inhibits Na/K ATPase, decreases the transmembrane Na(+) gradient in islets, and thus interferes with Na/Ca exchange. Collagenase isolated rat islets were maintained for 2 d in tissue culture with a trace amount of (45)Ca(++). Insulin release and (45)Ca(++) efflux were then measured during perifusion. In Ca(++)-deprived medium (to avoid changes in tissue specific radioactivity) 16.7 mM glucose inhibited (45)Ca(++) efflux. Initially 1 mM ouabain inhibited (45)Ca(++) efflux in a similar fashion, the onset being even faster than that of glucose. The effects of 16.7 mM glucose and ouabain were not additive, indicating that both substances may interfere with Na/Ca exchange. In the presence of Ca(++), 16.7 mM glucose induced biphasic insulin release. Ouabain alone caused a gradual increase of insulin release. Again, the effects of ouabain and 16.7 mM glucose were not additive. In contrast, at a submaximal glucose concentration (7 mM) ouabain enhanced both phases of release. An important role for Na/Ca exchange is suggested from experiments in which Ca(++) was removed at the time of glucose-stimulation (16.7 mM). The resulting marked inhibition of insulin release was completely overcome during first phase by ouabain added at the time of Ca(++) removal; second phase was restored to 60%. This could be due to the rapid inhibitory action of ouabain on Ca(++) efflux thereby preventing loss of cellular calcium critical for glucose to induce insulin release. It appears, therefore, that interference with Na/Ca exchange is an important event in the stimulation of insulin release by glucose.

Calcium and pancreatic beta-cell function. The mechanism of insulin secretion studied with the aid of lanthanum

La3+ was used to study the involvement of Ca2+ in insulin secretion in beta-cell-rich pancreatic islets micro-dissected from non-inbred ob/ob mice. Ultrastructural studies revealed that the localization of La3+ was entirely restricted to the exterior of the cells. Consistent with a membrane action, exposure to La3+ failed to affect glucose oxidation and either the sucrose space or the general ultrastructure of the islets. In contrast, La3+ had marked effects on insulin release and 45Ca fluxes. Exposure to La3+ resulted in pronounced inhibition of insulin release irrespective of the presence or absence of Ca2+, 3-isobutyl-1-methylxanthine or glucose. Perifusion experiments revealed that the inhibitory action was prompt, sustained and readily reversible. Removal of La3+ was associated with a subsequent prolonged stimulatory phase of insulin release even in medium deficient in Ca2+. This action could not be attributed to an increase in cyclic AMP, but was potentiated by 3-isobutyl-1-methylxanthine and abolished by L-adrenaline. La3+ displaced 45Ca from superficially located binding sites and inhibited the uptake and efflux of 45Ca. The stimulatory and inhibitory actions of glucose on 45Ca efflux were also abolished in the presence of 2 mM-La3+ Removal of La3+ was associated with the preferential mobilization of 45Ca incorporated in response to glucose. The results indicate that binding of La3+ to superficial sites in the plasma membrane leads to inhibition of insulin release by suppression of transmembrane Ca2+ fluxes. It is suggested that accumulation of Ca2+ in the cytoplasm accounts for the stimulation of insulin release seen after removal of La3+ from inhibitory binding sites in the beta-cell plasma membrane.

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.

Dependency of cyclic AMP-induced insulin release on intra- and extracellular calcium in rat islets of Langerhans

Calcium and cyclic AMP are important in the stimulation of insulin release. The phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) raises islet cAMP levels and causes insulin release at nonstimulatory glucose concentrations. In isolated rat pancreatic islets maintained for 2 d in tissue culture, the effects of IBMX on insulin release and 45Ca++ fluxes were compared with those of glucose. During perifusion at 1 mM Ca++, 16.7 mM glucose elicited a biphasic insulin release, whereas 1 mM IBMX in the presence of 2.8 mM glucose caused a monophasic release. Decreasing extracellular Ca++ a monophasic release. Decreasing extracellular Ca++ to 0.1 mM during stimulation reduced the glucose effect by 80% but did not alter IBMX-induced release. Both glucose and IBMX stimulated 45Ca++ uptake (5 min). 45Ca++ efflux from islets loaded to isotopic equilibrium (46 h) was increased by both substances. IBMX stimulation of insulin release, of 45Ca++ uptake, and of efflux were not inhibited by blockade of Ca++ uptake with verapamil, whereas glucose-induced changes are known to be inhibited. Because IBMX-induced insulin release remained unaltered at 0.1 mM calcium, it appears that cAMP-stimulated insulin release is controlled by intracellular calcium. This is supported by perifusion experiments at 0 Ca++ when IBMX stimulated net Ca++ efflux. In addition, glucose-stimulated insulin release was potentiated by IBMX. These results suggest that cAMP induced insulin release is mediated by increases in cytosolic Ca++ and that cAMP causes dislocation of Ca++ from intracellular stores.

Effects of calcium manipulation and glucose stimulation on a histochemically detectable mobile calcium fraction in isolated rat pancreatic islets

Pancreatic B-cell calcium as histochemically detectable with glyoxal bis (2-hydroxyanil) = GBHA was studied in isolated islets of fed rats. GBHA has previously been shown by us to detect an ionized or readily ionizable Ca-fraction (GBHA-Ca). In the presence of Ca++ (2.5 mM), high glucose (15 mM) induced a rapid decrease (30%) of islet GBHA-Ca followed by a rise between 30 and 60 min to levels above the initial value. At low glucose (0 or 2.5 mM) GBHA-Ca showed a slight and gradual decline under these conditions. Omission of Ca++ at low glucose rapidly decreased (30%) islet GBHA-Ca. This decrease was markedly inhibited by high glucose, although glucose did not induce insulin secretion under these conditions. Preincubation in the absence of Ca++ (15 min) depleted islet GBHA-Ca, but partial restoration occurred during subsequent incubation with Ca++ at low glucose. By contrast, high glucose completely restored GBHA-Ca within 5 min, followed by a decline and a subsequent rise. Reintroduction of Ca++ also rapidly restored the glucose-induced insulin secretion. These results indicate that islet GBHA-Ca represents a mobile Ca-fraction which is dependent on extracellular Ca++ and which responds very rapidly to glucose stimulation. It is suggested that changes of GBHA-Ca in the B-cells may reflect changes in the Ca pool involved in the insulin secretory mechanism.

Effect of phosphate on the arginine-induced insulin release by the isolated perfused rat pancreas

The isolated perfused rat pancreas was used to investigate the effect of extracellular phosphate on the arginine-induced insulin release. In the absence of any metabolic substrate, the insulin response to arginine was monophasic. In the absence of phosphate in the medium, the insulin release as unaffected until the 15th minute of the stimulation period, but was significantly augmented from that time onward. In the presence of oleic acid in the perfusate, the insulin response to arginine was also monophasic but occurred earlier than in controls. In this conditin, phosphate omission resulted in an increase of the insulin response to arginine from the 3rd minute of the stimulatory period onward. In the presence of glucose 5.5 mM in the medium the insulin response to argnine was biphasic and was not affected by extracellular phosphate omission.

Glucoreceptor mechanisms and the control of insulin release and biosynthesis

The models proposed for the means whereby the B-cell recognises glucose and related compounds as signals for insulin release and biosynthesis are discussed. The observed correlations between rates of metabolism and insulin release and biosynthesis are consistent with the substrate-site hypothesis. For glucose itself, the enzymes catalysing the phosphorylation of the sugar provide an explanation for the major characteristics of the islet responses, but for N-acetylglucosamine evidence is presented that the sugar transport system fulfils this discriminatory role. Possible mechanisms whereby sugar metabolism may be linked to changes in Ca2+-handling are considered and evidence is given supporting a role for the cytosolic NADPH/NADP+ ratio and the islet content of phosphoenolpyruvate. The nature of the targets for cyclic AMP and Ca2+ is discussed and some properties of islet cAMP-dependent protein kinase are summarised. Evidence is presented for the presence of calmodulin in islets and the possible involvement of calmodulin in stimulus-secretion coupling. On the basis of these considerations a speculative hypothesis for the mechanisms involved in the B-cell responses to glucose is outlined.