The Pancreatic β-Cell Recognition of Insulin Secretagogues

Effects of external ATP on Ca(2+) signalling in endothelial cells isolated from mouse islets

External ATP is believed to initiate and propagate Ca(2+) signals co-ordinating the insulin release pulses within and among the different islets in the pancreas. The possibility that islet endothelial cells participate in this process was evaluated by comparing the effects on [Ca(2+)](i) of purinoceptor activation in these cells with those in beta-cells. beta-Cell-rich pancreatic islets were isolated from ob/ob mice and dispersed into single cells/aggregates. After culture with or without endothelial cell growth supplement (ECGS), the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) was measured with ratiometric fura-2 technique. Presence of ECGS or prolongation of culture (>5 days) resulted in proliferation of endothelial cells and altered their phenotype from rounded to elongated. Endothelial cells, preliminarily identified by attachment of Dynabeads coated with the Bandeiraea simplicifolia 1 lectin (BS-1), responded in a similar way as those stained with CD31 antibodies after measurements of [Ca(2+)](i). Spontaneous transients and oscillations of [Ca(2+)](i )were seen in beta-cells, but not in endothelial cells exposed to 20 mM glucose. Addition of ATP (10 microM) resulted in pronounced and more extended rise of [Ca(2+)](i) in endothelial cells than in beta-cells. The endothelial cells differed from the beta-cells by also responding with a rise of [Ca(2+)](i) to 10 microM UTP, but not to equimolar ADP and acetylcholine. The results support the idea of mutual interactions between islet endothelium and beta-cells based on ATP-induced Ca(2+) signals. It is suggested that the endothelial cells have a tonic inhibitory action on beta-cell P2 purinoceptors resulting in impaired synchronization of the insulin release pulses.

Phospholipase A2 is important for glucose induction of rhythmic Ca2+ signals in pancreatic beta cells

OBJECTIVES

Pancreatic beta cells respond to glucose stimulation with pulses of insulin release generated by oscillatory rises of the cytoplasmic Ca2+ concentration ([Ca2+]i). The observation that exposure to external ATP and other activators of cytoplasmic phospholipase A2 (cPLA2) rapidly induces rises of [Ca2+]i similar to ordinary oscillations made it important to analyze whether suppression of the cPLA2 activity affects glucose-induced [Ca2+]i rhythmicity in pancreatic beta cells.

METHODS

Ratiometric fura-2 technique was used for measuring [Ca2+]i in single beta cells and small aggregates prepared from ob/ob mouse islets.

RESULTS

Testing the effects of different inhibitors of cPLA2 in the presence of 20 mM glucose, it was found that N-(p-amylcinnamoyl)anthranilic acid (ACA) removed the oscillations at a concentration of 25 microM, arachidonyl trifluoromethyl ketone (AACOCF3) at 10 microM, and bromoenol lactone (BEL) at 10 to 15 microM. Withdrawal of ACA and BEL resulted in reappearance of the oscillations. Suppression of the arachidonic acid production by addition of 5 microM of the diacylglycerol lipase inhibitor 1,6-bis-(cyclohexyloximinocarbonylamino)-hexane (RHC 80267) effectively removed the [Ca2+]i oscillations, an effect reversed by removal of the inhibitor or addition of 100 microM tolbutamide. Suppression of the arachidonic acid production had a restrictive influence also on the transients of [Ca2+]i supposed to synchronize the beta-cell rhythmicity. Although less sensitive than the oscillations, most transients disappeared during exposure to 50 microM ACA or 35 microM RHC 80267.

CONCLUSIONS

The results support the idea that cyclic variations of cPLA2 activity are important for the generation and synchronization of the beta-cell [Ca2+]i oscillations responsible for pulsatile release of insulin.

Pulses of external ATP aid to the synchronization of pancreatic beta-cells by generating premature Ca(2+) oscillations

Pancreatic beta-cells respond to glucose stimulation with increase of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), manifested as membrane-derived slow oscillations sometimes superimposed with transients of intracellular origin. The effect of external ATP on the oscillatory Ca(2+) signal for pulsatile insulin release was studied by digital imaging of fura-2 loaded beta-cells and small aggregates isolated from islets of ob/ob-mice. Addition of ATP (0.01-100 microM) to media containing 20mM glucose temporarily synchronized the [Ca(2+)](i) rhythmicity in the absence of cell contact by eliciting premature oscillations. External ATP triggered premature [Ca(2+)](i) oscillations also when the sarcoendoplasmic reticulum Ca(2+)-ATPase was inhibited with 50 microM cyclopiazonic acid and phospholipase C inhibited with 10 microM U-73122. The effect of ATP was mimicked by other activators of cytoplasmic phospholipase A(2) (10nM acetylcholine, 0.1-1 micro M of the C-terminal octapeptide of cholecystokinin and 2 microg/ml melittin) and suppressed by an inhibitor of the enzyme (50 microM p-amylcinnamoylanthranilic acid). Premature oscillations generated by pulses of ATP sometimes triggered subsequent oscillations. However, prolonged exposure to high concentrations of the nucleotide (10-100 microM) had a suppressive action on the beta-cell rhythmicity. The early effects of ATP included generation of transients induced by inositol (1,4,5) trisphosphate and superimposed on the premature oscillation or on an ordinary oscillation induced by glucose. The results support the idea that purinergic activation of phospholipase A(2) has a co-ordinating effect on the beta-cell rhythmicity by triggering premature [Ca(2+)](i) oscillations mediated by closure of ATP-sensitive K(+) channels.

Ca(2+)-induced Ca(2+) release via inositol 1,4,5-trisphosphate receptors is amplified by protein kinase A and triggers exocytosis in pancreatic beta-cells

Hormones, such as glucagon and glucagon-like peptide-1, potently amplify nutrient stimulated insulin secretion by raising cAMP. We have studied how cAMP affects Ca(2+)-induced Ca(2+) release (CICR) in pancreatic beta-cells from mice and rats and the role of CICR in secretion. CICR was observed as pronounced Ca(2+) spikes on top of glucose- or depolarization-dependent rise of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)). cAMP-elevating agents strongly promoted CICR. This effect involved sensitization of the receptors underlying CICR, because many cells exhibited the characteristic Ca(2+) spiking at low or even in the absence of depolarization-dependent elevation of [Ca(2+)](i). The cAMP effect was mimicked by a specific activator of protein kinase A in cells unresponsive to activators of cAMP-regulated guanine nucleotide exchange factor. Ryanodine pretreatment, which abolishes CICR mediated by ryanodine receptors, did not prevent CICR. Moreover, a high concentration of caffeine, known to activate ryanodine receptors independently of Ca(2+), failed to mobilize intracellular Ca(2+). On the contrary, a high caffeine concentration abolished CICR by interfering with inositol 1,4,5-trisphosphate receptors (IP(3)Rs). Therefore, the cell-permeable IP(3)R antagonist 2-aminoethoxydiphenyl borate blocked the cAMP-promoted CICR. Individual CICR events in pancreatic beta-cells were followed by [Ca(2+)](i) spikes in neighboring human erythroleukemia cells, used to report secretory events in the beta-cells. The results indicate that protein kinase A-mediated promotion of CICR via IP(3)Rs is part of the mechanism by which cAMP amplifies insulin release.

Synchronization of pancreatic beta-cell rhythmicity after glucagon induction of Ca2+ transients

Pancreatic beta-cells are biological oscillators requiring a coupling force for the synchronization of the cytoplasmic Ca(2+) oscillations responsible for pulsatile insulin release. Testing the idea that transients, superimposed on the oscillations, are important for this synchronization, the concentration of cytoplasmic Ca(2+) ([Ca(2+)](i)) was measured with ratiometric fura-2 technique in single beta-cells and small aggregates prepared from islets isolated from ob/ob-mice. Image analyses revealed asynchronous [Ca(2+)](i) oscillations in adjacent beta-cells lacking physical contact. The addition of glucagon stimulated the firing of [Ca(2+)](i) transients, which appeared in synchrony in adjacent beta-cells. Moreover, the presence of glucagon promoted synchronization of the [Ca(2+)](i) oscillations in beta-cells separated by a distance <100 microm but not in those >200 microm apart. The results support the proposal that the repolarizing effect of [Ca(2+)](i) transients provides a coupling force for co-ordinating the pulses of insulin release generated by pancreatic beta-cells.

Stretch activation of Ca2+ transients in pancreatic beta cells by mobilization of intracellular stores

INTRODUCTION

Nonadrenergic, noncholinergic neurons have been proposed to synchronize pulsatile insulin release from the islets in the pancreas by triggering transient increases of the cytoplasmic Ca2+ concentration ([Ca2+]i) in beta-cells via an inositol trisphoshate-dependent mechanism.

AIMS

To test whether pancreatic beta-cells respond to stretch activation with similar types of transients and whether these Ca signals propagate to other beta-cells in the presence and absence of cell contacts.

METHODOLOGY

Single cells and small aggregates were prepared from beta-cell-rich islets from mice. After 2-5 days of culture, [Ca2+]i was measured with digital imaging and the indicator fura-2 during superfusion with a medium containing 20 mmol/L glucose and 50 micromol/L methoxyverapamil. Membrane stretch was induced by osmotic swelling or focal touch stimulation.

RESULTS

Lowering the medium osmolarity with 100-102 mOSM/L by removal of sucrose or by dilution resulted in a 2-3-fold increase in the number of transients during an initial 5-minute period. Sucrose omission was stimulatory also after isosmolar replacement with readily penetrating urea. The intracellular Ca2+-ATPase inhibitor thapsigargin suppressed both the spontaneously occurring transients and those initiated by volume expansion. Touch stimuli induced [Ca2+]i transients, which rapidly propagated to cells within the same aggregate or lacking contact.

CONCLUSION

The observations support the idea that beta-cells both receive and regenerate extracellular signals triggering [Ca2+]i transients. Touch stimulation is a useful tool for investigating the propagation of [Ca2+]i signals between pancreatic beta-cells lacking physical contact.

Isolated mouse pancreatic beta-cells show cell-specific temporal response pattern

The length of the silent lag time before elevation of the cytosolic free Ca2+ concentration ([Ca2+]i) differs between individual pancreatic beta-cells. One important question is whether these differences reflect a random phenomenon or whether the length of lag time is inherent in the individual beta-cell. We compared the lag times, initial dips, and initial peak heights for [Ca2+]i from two consecutive glucose stimulations (with either 10 or 20 mM glucose) in individual ob/ob mouse beta-cells with the fura 2 technique in a microfluorimetric system. There was a strong correlation between the lengths of the lag times in each beta-cell (10 mM glucose: r = 0.94, P < 0.001; 20 mM glucose: r = 0.96, P < 0.001) as well as between the initial dips in [Ca2+]i (10 mM glucose: r = 0.93, P < 0.001; 20 mM glucose: r = 0.79, P < 0.001) and between the initial peak heights (10 mM glucose: r = 0.51, P < 0.01; 20 mM glucose: r = 0.77, P < 0.001). These data provide evidence that the response pattern, including both the length of the lag time and the dynamics of the subsequent [Ca2+]i, is specific for the individual beta-cell.

Nitric oxide induces synchronous Ca2+ transients in pancreatic beta cells lacking contact

AIMS

To evaluate the role of nitric oxide (NO) in the coordination of the Ca2+ signals generating pulsatile insulin release in pancreatic beta cells isolated from ob/ob mice.

METHODOLOGY

Using ratiometric fura-2 technique for recording glucose-induced cytoplasmic Ca2+ transients, it was possible to demonstrate a synchronization of beta cells lacking contact.

RESULTS

The frequency of the transients increased 10-fold in the presence of 20 n M glucagon. Additional increase in frequency with maintenance of synchronization was observed when the beta cells were exposed to 100 microM of the NO donors sodium nitroprusside and hydroxylamine. Bolus additions of 0.1-10 microM gaseous NO resulted in prompt appearance of cytoplasmic Ca2+ transients. An activator of soluble guanylate cyclase (mesoporphyrin) increased the frequency of the transients, and inhibition of this enzyme with 1H-(1,2,4) oxadiazolo [4,3-a] quinoxalin-1-one had the opposite effect.

CONCLUSION

The results support the idea that nitrergic nerves generate beta-cell transients of Ca2+ synchronizing the activity of the numerous islets in the pancreas.

Modulation of islet ATP content by inhibition or stimulation of the Na(+)/K(+) pump

High (30 mM) K(+), known to cause beta-cell membrane depolarisation, significantly decreased the islet total ATP content, supporting the view that beta-cell membrane depolarisation can activate the ATP-consuming Na(+)/K(+) pump. Ouabain (1 mM) did not change the islet ATP content after 5-15 min of incubation in the absence or presence of 3 mM glucose but reduced it after 30 min, and in the presence of 20 mM glucose, the reduction by ouabain occurred already after 15 min. Incubation of islets with ouabain for 60 min decreased the islet ATP content in the presence of 3, 10 or 20 mM glucose or 30 mM K(+). Also, the islet glucose oxidation rate was decreased by ouabain. When K(+) deficiency was used to inhibit the Na(+)/K(+) pump, no change in ATP content was observed irrespective of glucose concentration, although K(+) deficiency caused a slight inhibition of the glucose oxidation rate. Diazoxide reduced the islet glucose oxidation rate and increased the islet ATP content in the presence of 20 mM glucose. There may exist a feedback mechanism decreasing the flow of glucose metabolism in response to reduced ATP consumption by the Na(+)/K(+) pump.

Microfluorometric analysis of Cl- permeability and its relation to oscillatory Ca2+ signalling in glucose-stimulated pancreatic beta-cells

The cytoplasmic concentrations of Cl-([Cl-]i) and Ca2+ ([Ca2+]i) were measured with the fluorescent indicators N-(ethoxycarbonylmethyl)-6-methoxyquinilinum bromide (MQAE) and fura-2 in pancreatic beta-cells isolated from ob/ob mice. Steady-state [Cl-]i in unstimulated beta-cells was 34 mM, which is higher than expected from a passive distribution. Increase of the glucose concentration from 3 to 20 mM resulted in an accelerated entry of Cl- into beta-cells depleted of this ion. The exposure to 20 mM glucose did not affect steady-state [Cl-]i either in the absence or presence of furosemide inhibition of Na+, K+, 2 Cl- co-transport. Glucose-induced oscillations of [Ca2+]i were transformed into sustained elevation in the presence of 4,4' diisothiocyanato-dihydrostilbene-2,2'-disulfonic acid (H2DIDS). A similar effect was noted when replacing 25% of extracellular Cl- with the more easily permeating anions SCN-, I-, NO3- or Br-. It is concluded that glucose stimulation of the beta-cells is coupled to an increase in their Cl- permeability and that the oscillatory Ca2+ signalling is critically dependent on transmembrane Cl- fluxes.

Relationships between the Na(+)/K(+) pump and ATP and ADP content in mouse pancreatic islets: effects of meglitinide and glibenclamide

We have previously demonstrated that both D-glucose and glibenclamide stimulate the Na(+)/K(+) pump and suggested that this may be part of the membrane repolarization process, following the primary depolarization by these agents. The aim of this study was to investigate whether the non-sulphonylurea meglitinide (HB 699) exerts similar effects as glibenclamide or glucose on the islet Na(+)/K(+) pump and if effects of meglitinide or glibenclamide on this pump activity is paralleled by changes in islet ATP content and/or ATP/ADP ratio. The acyl-amino-alkyl benzoic acid derivative, meglitinide, stimulated the islet ouabain-sensitive portion of (86)Rb(+) influx (Na(+)/K(+) pump) by 53%, while the ouabain-resistant portion was inhibited by 70%. The stimulatory effect was not additive to that caused by D-glucose, suggesting that both agents may activate the Na(+)/K(+) pump via the same mechanism. Glibenclamide (10 microM) decreased the islet ATP and ADP content as well as the ATP/ADP ratio at 0 mM glucose. These effects were no longer observed at 10 mM glucose. Meglitinide (10 or 50 microM) lowered the islet ATP and ADP content at 0 mM glucose without affecting the ATP/ADP ratio. At 10 mM glucose, however, 10 microM of the drug reduced the islet ATP content but not the ATP/ADP ratio, while 50 microM of the drug, besides lowering the ATP content, also reduced the ATP/ADP ratio. Diazoxide (0.5 mM) increased the islet ATP content in the absence of glucose, an effect not seen in the presence of 10 mM glucose. The rate of glucose oxidation at 1, 10 or 20 mM of the sugar was not affected by glibenclamide (0.1 - 10 microM) and at 10 or 20 mM of the sugar not affected by meglitinide (1 - 100 microM). These results suggest that glibenclamide and meglitinide lower the islet ATP level by indirectly activating the beta-cell Na(+)/K(+) pump, which is a major consumer of ATP in the islets, while diazoxide increases the ATP level due to inhibition of the pump.

Role of voltage-dependent Na+ channels for rhythmic Ca2+ signalling in glucose-stimulated mouse pancreatic beta-cells

The putative role of voltage-dependent Na+ channels for glucose induction of rhythmic Ca2+ signalling was studied in mouse pancreatic beta-cells with the use of the Ca2+ indicator fura-2. A rise in glucose from 3 to 11 mM resulted in slow oscillations of the cytoplasmic Ca2+ concentration ([Ca2+]i). These oscillations, as well as superimposed transients seen during forskolin-induced elevation of cAMP, remained unaffected in the presence of the Na+ channel blocker tetrodotoxin. During exposure to 1-10 microM veratridine, which facilitates the opening of voltage-dependent Na+ channels, the slow oscillations were replaced by repetitive and pronounced [Ca2+]i transients arising from the basal level. The effects of veratridine were reversed by tetrodotoxin. The veratridine-induced [Ca2+]i transients were critically dependent on the influx of Ca2+ and persisted after thapsigargin inhibition of the endoplasmic reticulum Ca2+-ATPase. Both tolbutamide and ketoisocaproate mimicked the action of glucose in promoting [Ca2+]i transients in the presence of veratridine. It is suggested that activation of voltage-dependent Na+ channels is a useful approach for amplifying Ca2+ signals for insulin release.

Voltage-dependent entry and generation of slow Ca2+ oscillations in glucose-stimulated pancreatic beta-cells

The role of voltage-dependent Ca2+ entry for glucose generation of slow oscillations of the cytoplasmic Ca2+ concentration ([Ca2+]i) was evaluated in individual mouse pancreatic beta-cells. Like depolarization with K+, a rise of the glucose concentration resulted in an enhanced influx of Mn2+, which was inhibited by nifedipine. This antagonist of L-type Ca2+ channels also blocked the slow oscillations of [Ca2+]i induced by glucose. The slow oscillations occurred in synchrony with variations in Mn2+ influx and bursts of action currents, with the elevation of [Ca2+]i being proportional to the frequency of the action currents. A similar relationship was obtained when Ca2+ was replaced with Sr2+. Occasionally, the slow [Ca2+]i oscillations were superimposed with pronounced spikes temporarily arresting the action currents. It is concluded that the glucose-induced slow oscillations of [Ca2+]i are caused by periodic depolarization with Ca2+ influx through L-type channels. Ca2+ spiking, due to intracellular mobilization, may be important for chopping the slow oscillations of [Ca2+]i into shorter ones characterizing beta-cells situated in pancreatic islets.

Vagotomy in young obese hyperglycemic mice: effects on syndrome development and islet proliferation

Obese hyperglycemic mice have large pancreatic islets and high levels of serum insulin and blood glucose. Vagotomy was performed on 3-wk-old animals to investigate the role of gut cholinergic innervation in young Umea ob/ob mice. After vagotomy, obesity and hyperglycemia are dissociated. Weight increase in obese vagotomized mice was lower than in sham-operated controls during the 1st wk postoperatively but not thereafter. Blood glucose was lower up to 5 mo after vagotomy, but vagotomized mice showed reduced glucose tolerance. Islet cell proliferation rate was reduced 2 and 3 wk but not 5 mo after vagotomy. After 5 mo, islet volume was smaller in vagotomized mice. Serum insulin levels were the same in vagotomized animals as in sham-operated controls. The effects of reduced cholinergic innervation are probably caused both by direct effects of denervation and by lowered metabolic demand.

Origin of slow and fast oscillations of Ca2+ in mouse pancreatic islets

1. Pancreatic islets exposed to 11 mM glucose exhibited complex variations of cytoplasmic Ca2+ concentration ([Ca2+]i) with slow (0.3-0.9 min-1) or fast (2-7 min-1) oscillations or with a mixed pattern. 2. Using digital imaging and confocal microscopy we demonstrated that the mixed pattern with slow and superimposed fast oscillations was due to separate cell populations with the respective responses. 3. In islets with mixed [Ca2+]i oscillations, exposure to the sarcoplasmic-endoplasmic reticulum Ca2+-ATPase inhibitors thapsigargin or 2,5-di-tert-butylhydroquinone (DTBHQ) resulted in a selective disappearance of the fast pattern and amplification of the slow pattern. 4. In addition, the protein kinase A inhibitor RP-cyclic adenosine 3',5'-monophosphorothioate sodium salt transformed the mixed [Ca2+]i oscillations into slow oscillations with larger amplitude. 5. Islets exhibiting only slow oscillations reacted to low concentrations of glucagon with induction of the fast or the mixed pattern. In this case the fast oscillations were also counteracted by DTBHQ. 6. The spontaneously occurring fast oscillations seemed to require the presence of cAMP-elevating glucagon, since they were more common in large islets and suppressed during culture. 7. Image analysis revealed [Ca2+]i spikes occurring irregularly in time and space within an islet. These spikes were preferentially observed together with fast [Ca2+]i oscillations, and they became more common after exposure to glucagon. 8. Both the slow and fast oscillations of [Ca2+]i in pancreatic islets rely on periodic entry of Ca2+. However, the fast oscillations also depend in some way on paracrine factors promoting mobilization of Ca2+ from intracellular stores. It is proposed that such a mobilization in different cells within a tightly coupled islet syncytium generates spikes which co-ordinate the regular bursts of action potentials underlying the fast oscillations.

Store-operated Ca2+ entry in insulin-releasing pancreatic beta-cells

The fluorescent indicator Fura-2 was used to characterize the store-operated Ca2+ entry in insulin-releasing pancreatic beta-cells. To avoid interference with voltage-dependent Ca2+ entry, the cells were hyperpolarized with 400 microM diazoxide and the channel blocker methoxyverapamil was also present in some experiments. The cytoplasmic Ca2+ concentration ([Ca2+]j) of hyperpolarized mouse beta-cells was strikingly resistant to changes in external Ca2+. In cells exposed to 20 mM glucose, stimulation with 100 microM carbachol induced an initial [Ca2+]j peak followed by a sustained increase due to store-operated influx of the cation. Store-operated influx was also induced by the intracellular Ca(2+)-ATPase inhibitor thapsigargin. In the presence of store-operated influx, [Ca2+]j became markedly sensitive to variations in external Ca2+, but this sensitivity was blocked by La3+. In beta-cells exposed to both Ca2+ and Mn2+ there was slow Mn2+ quenching of the Fura-2 fluorescence, which was accelerated upon stimulation of store-operated influx. This acceleration was reversed by glucose-stimulated filling of the internal Ca2+ stores. The store-operated Ca2+ entry increased markedly during culture of the beta-cells. Activation of protein kinase C by the phorbol ester 12-O-tetradecanoylphorbol-13 acetate, inhibition of serine/threonine phosphatase by okadaic acid and inhibition of tyrosine kinase by genistein had little effect on the store-operated influx of Ca2+. In beta-cells equilibrated in 5 mM Sr2+, carbachol exposure resulted in a pronounced cytoplasmic Sr2+ ([Sr2+]j) peak due to intracellular mobilization, but little or no sustained elevation. Moreover, after activating the store-operated pathway by exposure to thapsigargin, variations in extracellular Sr2+ between 0-2 mM had only marginal effects on [Sr2+]j. Although the store-operated influx apparently accounts for a minor fraction of the Ca2+ entry, its depolarizing influence may under certain conditions be up-regulated with resulting distortion of the beta-cell function.

Biological and physicochemical characterization of recombinant human erythropoietins fractionated by Mono Q column chromatography and their modification with sialyltransferase

Ten erythropoietin (EPO) fractions differing in sialic acid content, ranging from 9.5 to 13.8 mol mol-1 of EPO, were obtained from baby hamster kidney cell-derived recombinant human EPO by Mono Q column chromatography. The mean pI values of the EPO fractions determined by IEF-gel electrophoresis systematically shifted from 4.11 to 3.31, coinciding with the sialic acid content, without a change in the constitution of asialo N-linked oligosaccharides of each fraction. Although a linear relationship between the in vivo bioactivity and the sialic acid content of the fractionated samples was observed until 12.1 mol mol-1 of EPO, there was no further increase in their activity over 12.4 mol mol-1 of EPO. On the other hand, an inverse relationship between the in vitro bioactivity and sialic acid content of EPO was observed. Also, we showed that the in vivo bioactivity of some fractions with low sialic acid contents was increased after treatment with alpha 2,6-sialyltransferase, but the in vivo bioactivity of the other fractions with high sialic acid contents was either decreased or not affected.

The role of plasma membrane K+ and Ca2+ permeabilities for glucose induction of slow Ca2+ oscillations in pancreatic beta-cells

In individual pancreatic beta-cells the rise of the cytoplasmic Ca2+ concentration ([Ca2+]i), induced by 11 mM glucose, is manifested either as oscillations (0.2-0.5 min-1) or as a sustained elevation. The significance of the plasma membrane permeability of Ca2+ and K- for the establishment of these slow oscillations was investigated by dual wavelength microfluorometric measurements of [Ca2+]i in individual ob/ob mouse beta-cells loaded with fura-2. Increasing the extracellular Ca2+ to 10 mM or the addition of Ca2+ channel agonist BAY K 8644 (1 microM) or K+ channel blocker tetraethylammonium+ (TEA: 10-20 mM) caused steeper rises and higher peaks of the glucose-induced oscillations. However, when extracellular Ca2+ was lowered to 0.5 mM the oscillations were transformed into a sustained suprabasal level. When the beta-cells exhibited glucose-stimulated sustained elevation of [Ca2+]i in the presence of a physiological Ca2+ concentration (1.3 mM), it was possible to induce slow oscillations by promoting the entry of Ca2+ either by raising the extracellular Ca2+ concentration to 10 mM or adding TEA or BAY K 8644. The results indicate that glucose-induced slow oscillations of [Ca2+]i depend on the closure of ATP-regulated K+ channels and require that the rate of Ca2+ influx exceeds a critical level. Apart from an inherent periodicity in ATP production it is proposed that Ca(2+)-induced ATP consumption in the submembrane space contributes to the cyclic changes of the membrane potential determining periodic entry of Ca2+.

Glucose induces oscillations of cytoplasmic Ca2+, Sr2+ and Ba2+ in pancreatic beta-cells without participation of the thapsigargin-sensitive store

Individual pancreatic beta -cells were used to study the glucose effects on the handling of Ca2+, Sr2+ and Ba2+. In extracellular medium containing one of these ions, single beta -cells responded to 11 mM glucose with large amplitude oscillations in cytoplasmic Ca2+, Sr2+ or Ba2+ with indistinguishable average frequencies (0.30-0.33/min). The oscillations disappeared after hyperpolarization with 400 microM diazoxide. Under such hyperpolarization, glucose stimulated the sequestration of Ca2+ and Sr2+ but not of repetitively mobilized by consecutive exposures to 100 microM carbachol. A 2-3 min exposure to 100 nM of the intracellular Ca(2+)-ATPase inhibitor thapsigargin also mobilized Ca2+ and Sr2+ and irreversibly abolished subsequent release by carbachol. However, thapsigargin did not prevent the large amplitude oscillations in Ca2+, Sr2+ or Ba2+ under non-hyperpolarizing conditions although the frequency of the Ca2+ oscillations was almost doubled. The results indicate that the slow oscillatory behavior of glucose-stimulated individual beta -cells does not depend on inositol 1,4,5-trisphosphate mediated release of intracellular Ca2+.

Alpha 2-adrenergic stimulation counteracts glucose-induced rise of sodium in pancreatic islets exposed to ouabain

The effects of alpha 2-adrenergic activation by clonidine on sodium handling were analysed in beta-cell-rich pancreatic mouse islets. In the steady-state situation, clonidine (1 microM) amplified lowering of sodium induced by 20 mM glucose, while the content remained unchanged in 3mM glucose. The loss of sodium in Na(+)-deficient medium was stimulated by glucose but was not affected by clonidine. This agonist also did not influence the ouabain-induced uptake of sodium at 3 mM glucose but partially counteracted additional uptake in response to 20 mM glucose. Although lacking effects of its own, 5 microM yohimbine completely counteracted the action of clonidine. The glucose amplification of the ouabain-induced uptake of sodium was suppressed also by 10 microM of the Ca(2+)-channel blockers methoxyverapamil and diltiazem. Both tolbutamide (100 microM) and dibutyryl cyclic AMP (1 mM) mimicked the action of glucose by promoting clonidine-sensitive uptake of sodium in the presence of ouabain. It is concluded that activation of alpha 2-adrenoceptors has profound effects on the sodium handling of pancreatic beta-cells exposed to glucose and other stimulators of insulin release.

Glucose promotes turnover of Na+ in pancreatic beta-cells

Ouabain-induced changes of the free cytoplasmic Na+ concentration ([Na+]i) were monitored in aggregates of cells prepared from beta-cell-rich pancreatic mouse islets and the results were compared with the total islet content of sodium. The steady-state [Na+]i was lower in 20 mM glucose (11 mM) than in 3 mM glucose (14 mM). In the presence of 3 mM glucose the addition of 1 mM ouabain resulted in a rise in [Na+]i with an initial rate of 1.5 mM/min. However, the increase of total sodium corresponded to 2.8 mM/min, suggesting that rapid binding and/or sequestration of Na+ are prominent features for pancreatic beta-cells. Elevation of the glucose concentration to 20 mM increased the rate of ouabain-dependent rise of [Na+]i. The effect of glucose was mimicked by 1 mM tolbutamide or 100 microM carbachol and was counteracted by 100 nM of the alpha 2-adrenergic agonist clonidine. Glucose also accelerated the lowering of [Na+]i after withdrawal of ouabain. In promoting not only the entry but also the extrusion of Na+, glucose actually enhances the turnover of the ion in pancreatic beta-cells.

Supramaximal decrease of sulphonylurea-induced accumulation of sodium in pancreatic islets

Sodium was measured in beta-cell-rich pancreatic islets of mice under steady state conditions or after 6 min of exposure to 1 mM ouabain. The islet content of sodium increased when 100 microM tolbutamide or 1 microM glipizide were added to an albumin-containing (1 mg ml-1) medium, but remained unaffected at 10-fold higher concentrations. Both sulphonylurea compounds promoted the uptake of Na+ in the presence of ouabain. Whereas tolbutamide was stimulatory at 10 microM or above in a medium containing 10 mg ml-1 albumin, only 0.1 microM was required in the absence of albumin. In the latter situation there was a reduction of the stimulatory action with increase of the tolbutamide concentration from 100 to 1000 microM. The inhibitory component in the sulphonylurea action on the Na+ uptake was particularly impressive with glipizide, maximal stimulation being reached at 10 microM in the presence of 1 mg ml-1 albumin. Diazoxide (400 microM) modified the glipizide action on Na+ uptake, making 1000 microM stimulatory instead of 1 microM. The latter concentration of glipizide became inhibitory after removal of K+. Glipizide stimulated the Na+ uptake both at low and high concentrations in a medium deficient in Ca2+ or when the cotransport of Na+, K+ and Cl- was blocked by 20 microM bumetanide. The observation that the sulphonylurea-induced islet accumulation of sodium is diminished at supramaximal concentrations reinforces existing arguments for additional effects of high concentrations of hypoglycemic sulphonylureas.

Impaired Ca2+ response to glucose in mouse beta-cells infected with coxsackie B or Echo virus

Five strains of Coxsackie B4 virus and one of Echo 11 virus were tested with regard to their ability to replicate in pancreatic mouse beta-cells and interfere with the alterations of the cytoplasmic Ca2+ concentration ([Ca2+]i) induced by glucose. All strains except one both multiplied and caused cytopathic effect. In a control group 68% of the beta-cells responded to 11 mM glucose with large amplitude oscillations of [Ca2+]i. After inoculation with the infectious strains these oscillations appeared in only 5% of the beta-cells, whereas the non-infectious strain did not modify the glucose effect on [Ca2+]i. Despite the virus interference with the glucose response, [Ca2+]i was increased after depolarization with excessive extracellular K+ and the oscillations were induced in most beta-cells when glucose was combined with the insulin-releasing sulfonylurea tolbutamide.

Sulfonylureas mimic glucose in stimulating the uptake of Na+ in pancreatic islets exposed to ouabain

The increase of sodium in response to ouabain inhibition of the Na+/K+ pump was measured in beta-cell-rich pancreatic islets from ob/ob mice using integrating flame photometry. D-Glucose promoted the uptake of sodium when added at a concentration of 6 mM or above. The hypoglycemic sulfonylurea compound, tolbutamide, mimicked the action of D-glucose in stimulating the sodium uptake at concentrations of 10 microM or above. There was no stimulation beyond that obtained with 20 mM glucose during exposure to 100 microM tolbutamide. Other test substances also affected sodium uptake in a way reflecting known effects on insulin release. Accordingly, the sodium uptake was stimulated with glibenclamide, glipizide, HB 699 (4-[2-(5-chloro-2-methoxybenzamido)ethyl]benzoic acid) and high K+. Sulphonamides (sulfamethazole, sulfadiazine and sulfadoxine) had practically no effect when added at a concentration of 1 mM. Sodium uptake in response to glucose and tolbutamide was antagonized by 400 microM diazoxide or 4 microM tetrodotoxin. It is concluded that both glucose and hypoglycemic sulfonylureas stimulate Na+ entry into the pancreatic beta-cells, a process presumably involving depolarization.

Inhibition by hydrochlorothiazide of insulin release and calcium influx in mouse pancreatic beta-cells

1. The effect of hydrochlorothiazide on insulin release, 36Cl- fluxes and 45Ca2+ uptake was tested in beta-cell-rich mouse pancreatic islets. 2. At high glucose concentrations (10 and 20 mmol l-1), low concentrations of hydrochlorothiazide (0.1-1.0 mumol l-1) reduced insulin release by 22-42%. At lower glucose concentrations (3-8.5 mmol l-1) insulin release was not affected by the drug. 3. Neither short-term influx (3 min) nor net accumulation (60 min) of 36Cl- in the islets was affected by hydrochlorothiazide (0.1-500 mumol l-1). 4. Glucose-stimulated 45Ca2+ uptake was significantly reduced by hydrochlorothiazide (1-10 mumol l-1). 5. The data suggest that the diabetogenic effect of hydrochlorothiazide, at least in part, can be mediated by direct inhibition of insulin release from the pancreatic beta-cells. The inhibition is not mediated by reduced chloride fluxes but may rather be caused by inhibition of calcium uptake.

Evidence for separate Na+, K+, Cl- and K+, Cl- co-transport systems in mouse pancreatic beta-cells

The effects of H 25 and H 74 on ouabain-resistant 86Rb+ influx were tested in beta-cell-rich mouse pancreatic islets. Both H 25, which is considered to be a specific inhibitor of Na+, K+, Cl- co-transport, and H 74, a specific inhibitor of K+, Cl- co-transport, reduced the ouabain-resistant 86Rb+ influx. The specific inhibitory effects of H 25 and H 74 on 86Rb+ influx did not overlap. The data suggest that mouse pancreatic beta-cells are equipped with separate systems for Na+, K+, Cl- and K+, Cl- co-transport.

Interaction between perchlorate and nifedipine on insulin secretion from mouse pancreatic islets

In order to elucidate the mechanisms responsible for the stimulatory effect of perchlorate (ClO4-) on insulin secretion, we have investigated the interaction between this chaotropic anion and the organic calcium antagonist nifedipine. This drug, known as a blocker of L-type calcium channels, was chosen as a tool to test the idea that ClO4- acts on insulin secretion by stimulating the gating of voltage-controlled Ca2+ channels. ClO4- amplified the stimulatory effect of D-glucose on insulin release from perfused pancreas (first and second phases) as well as from isolated islets incubated in static incubations for 60 min. This indicates that ClO4- amplifies physiologically regulated insulin secretion. Nifedipine reduced D-glucose-induced (20 mM) insulin release in a dose-dependent manner with half-maximum effect at about 0.8 microM and apparent maximum effect at 5 microM nifedipine. In the presence of 20 mM D-glucose, the inhibitory effects of 0.5, 1 or 5 microM nifedipine were only slightly, if at all, counteracted by perchlorate. When 12 mM ClO4- and 20 mM D-glucose were combined, calculation of the specific effect of ClO4- revealed that nifedipine produced almost maximum inhibition already at 0.05 microM. Thus, the perchlorate-induced amplification of D-glucose-stimulated insulin release shows higher sensitivity to nifedipine than the D-glucose-effect as such. This supports the hypothesis that perchlorate primarily affects the voltage-sensitive L-type calcium channel in the beta-cell.

Glycine transformation of Ca2+ oscillations into a sustained increase parallels potentiation of insulin release

Increase of the glucose concentration from 3 to 11 mM resulted in a triphasic release of insulin from perifused ob/ob-mouse beta-cells. A slight inhibition was followed after 2 min by a marked peak and a less pronounced sustained response. At the lower glucose concentration glycine had only marginal effects. However, in the presence of 11 mM glucose, 1-10 mM glycine triggered an immediate and dose-dependent response with an initial peak of insulin release followed by sustained stimulation. In individual beta-cells, rise of the glucose concentration from 3 to 11 mM induced initial lowering of the cytoplasmic Ca2+ concentration ([Ca2+]i) followed by large amplitude oscillations from a level of 50-90 nM to peak values exceeding 300 nM. Already at a concentration of 1 mM, glycine transformed the oscillatory pattern into a sustained level with increase of time-average [Ca2+]i. This elevation became more pronounced in the presence of 10 mM glycine. The effects of glycine on insulin release and [Ca2+]i required extracellular Na+ and were reproduced with the N-methyl analogue sarcosine. It is suggested that glycine potentiation of secretion reflects the elevation of time-average [Ca2+]i both by increased entry and reduced elimination of the cation from the cytoplasm.

Glucose sensing of individual pancreatic beta-cells involves transitions between steady-state and oscillatory cytoplasmic Ca2+

Glucose stimulation of individual pancreatic beta-cells is associated with a rise of the cytoplasmic Ca2+ concentration ([Ca2+]i) manifested either as large amplitude oscillations (0.2-0.5/min) or as a sustained increase. Determinants for the transitions between the basal and the two stimulated states have now been studied using dual-wavelength fluorometric measurements on individual ob/ob mouse beta-cells loaded with the Ca2+ indicator Fura-2. The transition from the basal state to large amplitude oscillations was induced by raising the glucose concentration to 7 mM or above. The frequencies and shapes of the [Ca2+]i cycles remained largely unaffected when raising glucose as high as 40 mM. However, in some cells the oscillatory pattern was transformed into a sustained increase of [Ca2+]i at high glucose concentrations. Although the peak values for the oscillations exceeded the steady-state increase, the time average [Ca2+]i was higher during the latter phase. Both types of glucose-induced transitions were facilitated by the presence of 1-100 nM glucagon. Protein kinase C activation by 10 nM of the phorbol ester TPA resulted in a transformation of the glucose-induced oscillations into a sustained increase of [Ca2+]i but the levels reached were considerably lower than obtained with glucose alone. It is concluded that the glucose sensing of the individual beta-cell is based on sudden transitions between steady-state and oscillating cytoplasmic Ca2+. It is these transitions rather than alterations of the oscillatory characteristics which determine the average [Ca2+]i regulating insulin release.

Carbachol has opposite effects to glucose in raising the sodium content of pancreatic islets

Integrating flame photometry was used for measuring sodium in single pancreatic islets from ob/ob mice. Exposure to 100 microM carbachol resulted in a 25-40% increase in sodium without any effect on potassium during incubation with 0-5 mM glucose in media deficient or not in Ca2+. This action of carbachol was abolished by 10 microM atropine or by raising the glucose concentration to 20 mM. A minor increase of the steady state content of sodium occurred in the presence of 200 microM ATP or 10 nM tetradecanoylphorbol 13-acetate (TPA). Carbachol differed from TPA in markedly stimulating sodium accumulation after ouabain inhibition of the Na/K pump. The results indicate that muscarinic receptor activation has opposite effects to glucose in inducing a rise of the islet content of sodium. It is suggested that the cholinergic control of the endocrine pancreas involves entry of Na+ in addition to the Na+ entry mediated by protein kinase C activation of Na+/H+ countertransport.

Insulin release from isolated mouse islets in vitro: no effect of physiological levels of melatonin or arginine vasotocin

Some data in the literature suggest that heightened activity of the pineal gland may be diabetogenic. The onset of insulin-dependent diabetes mellitus is highest during the winter months and at puberty when melatonin levels are also greatest. To study the direct effects of pineal hormones on insulin release, hand-dissected ob/ob-mouse islets of Langerhans were incubated in vitro with melatonin (1 nmol/l to 100 mumol/l) or arginine vasotocin (1 pmol/l to 10 mumol/l) and D-glucose (3 or 20 mmol/l for 1 hr. Melatonin did not affect basal or glucose-stimulated insulin release. Arginine vasotocin (AVT) did not affect basal insulin release, but at presumably pharmacological levels (1 and 10 mumol/l) the peptide significantly increased glucose-stimulated insulin release. We conclude that melatonin and AVT at physiological concentrations have no direct effect on islet insulin release, and that any diabetogenic effect of the pineal gland must occur via suppression of insulin action or via production of a metabolite or hormone that suppresses insulin release.

Antigenic studies on an enzymatically sialylated carbohydrate: NeuAc(alpha 2-3)Gal(beta 1-3)GalNAc

Sialic acid residues are often the end moiety of the carbohydrate chain of biologically important glycoconjugates. It is difficult to study sialylated glycoconjugates because the purification of these compounds is often laborious yielding only very small amounts of oligosaccharides for study. Chemical synthesis of sialylated compounds is complicated by the labile nature of the sialic acid bond. In both of these cases the sialylated compounds would need to be conjugated to a polypeptide to be an effective immunogen, and again, such conjugation is fraught with problems due to the instability of the sialic acid linkage. We have developed a combined enzymatic and synthetic route for obtaining quantities of sialylated carbohydrates conjugated to a protein carrier in amounts sufficient for antigenic studies. The notable novelty of this protocol is the addition of sialic acid after the carbohydrate-protein conjugation step. Antiserum to the compounds was developed and after absorption, antibodies that demonstrate a requirement for sialic acid for their binding were produced and studied. CA 125 has been shown to be a prognostically significant marker for ovarian adenocarcinoma. The nature of the epitope involved has been analyzed with conflicting results. To attempt to resolve this conflict, we initiated studies on sialylated antigens with NeuAc alpha 2-3Gal beta 1-3GalNAc. This trisaccharide occupies the terminal region in a series of complex carbohydrates which have been suggested to be involved as the epitope. Hanisch et al. reported that the neuraminic acid was important for the reaction.

Propagation of cytoplasmic Ca2+ oscillations in clusters of pancreatic beta-cells exposed to glucose

Digital image analysis was employed for resolving the temporal and spatial variations of the cytoplasmic Ca2+ concentration ([Ca2+]i) in pancreatic beta-cells loaded with the Ca(2+)-indicator Fura-2. Glucose-stimulated individual beta-cells exhibited large amplitude oscillations of [Ca2+]i with a mean frequency of 0.33 min-1. When Ca2+ diffusion was restricted by increasing the Ca2+ buffering capacity, the sugar-induced rise of [Ca2+]i preferentially affected the peripheral cytoplasm. When glucagon was present glucose also caused less prominent oscillations with about a 10-fold higher frequency superimposed on an elevated [Ca2+]i. In small clusters of 6-14 cells the average frequency of the large amplitude oscillations increased to 0.60 min-1. The clusters were found to contain micro-domains of electrically coupled cells with synchronized oscillations. After increasing the glucose concentration, adjacent domains became functionally coupled. The oscillations originated from different cells in the cluster. Also the fast glucagon-dependent oscillations were synchronized between cells and had different origins. The results indicate that coupling of beta-cells leads to an increased frequency of the large amplitude oscillations, and that the oscillatory characteristics are determined collectively among electrically coupled beta-cells rather than by particular pacemaker cells. In the light of these data it is necessary to reconsider the previous ideas that glucose-induced oscillations of membrane potential and [Ca2+]i require coupling between many beta-cells, and that the peak [Ca2+]i values reached during oscillations should increase with the size of the coupled cluster.

Volume regulation in mouse pancreatic beta-cells is mediated by a furosemide-sensitive mechanism

A possible role for loop diuretic-sensitive Cl-/cation cotransport in volume regulation in the pancreatic beta-cells was investigated by measuring 86Rb+ efflux from beta-cell-rich pancreatic islets as well as the size of isolated beta-cells under different osmotic conditions. Lowering the osmolarity to 262 mosM (83% of control) resulted in a rapid cell swelling which was followed by regulatory volume decrease (RVD). RVD was completely inhibited by furosemide (1 mM), an inhibitor of Cl-/cation co-transport. The hypotonic medium (262 mosM) induced a rapid and strong increase in 86Rb+ efflux from beta-cell-rich mouse pancreatic islets and the furosemide-sensitive portion of the efflux was significantly increased. A slightly less hypotonic medium (285 mosM, 90% of control) induced only cell swelling and no RVD. With this medium only a marginal increase in 86Rb+ efflux was observed. Increasing the osmolarity by adding 50 mM NaCl (final osmolarity: 417 mosM, 132% of control) induced a rapid cell shrinkage but no regulatory volume increase (RVI). When the osmolarity was increased from a slightly hypotonic medium (262 mosM) to an isotonic medium (317 mosM) an initial cell shrinkage was followed by RVI. This RVI was inhibited by 1 mM furosemide. The data suggest that RVD as well as RVI in the beta-cells are mediated by loop diuretic-sensitive cotransport of chloride and cations and that these cells show a threshold for hypotonic stimulation of RVD.

Barium mimics the effect of D-glucose on 86Rb+ fluxes in mouse pancreatic beta-cells

The interaction between Ba2+, furosemide and D-glucose on 86Rb+ fluxes in ob/ob mouse islets was investigated. Ba2+ (2 mM) significantly reduced the ouabain-resistant 86Rb+ influx, without affecting the ouabain-sensitive influx. D-Glucose (20 mM) reduced the 86Rb+ influx in the absence of Ba2+ (2 mM) but not in the presence of the cation. Furosemide, an inhibitor of Na+, K+, Cl- co-transport, reduced the 86Rb+ influx and the effect was partly additive to the effect of 2 mM Ba2+. When the islets were preincubated with Ba2+ (2 mM) the specific effect of 1 mM furosemide on the 86Rb+ influx was reduced, whereas, in acute experiments, Ba2+ (2 mM) did not affect the specific effect of furosemide on 86Rb+ influx. 86Rb+ efflux from preloaded islets was significantly reduced by 2 mM Ba2+ and during the first 5 min of ion efflux the effect of the combination of 2 mM Ba2+ and 1 mM furosemide was stronger than the effect of Ba2+ alone. The data show that Ba2+ reduces 86Rb+ fluxes in the beta-cells and suggest that this is mainly mediated by inhibition of K+ channels in the beta-cell plasma membrane. Long-term exposure to Ba2+ may also reduce the activity of the Na+, K+, Cl- co-transport system. The effect of Ba2+ on K+ channels may help to explain the stimulatory effect on insulin release in the absence of nutrient secretagogues.

Bumetanide reduces insulin release by a direct effect on the pancreatic beta-cells

The effect of the loop diuretic bumetanide on glucose-induced insulin release, 45Ca2+ uptake, 36Cl- fluxes and 86Rb+ (K+ analogue) efflux was tested in isolated beta-cell-rich mouse pancreatic islets. Low concentrations of bumetanide (0.1-10 microM) reduced glucose-induced insulin release as well as 45Ca2+ uptake. High concentrations (0.5-1 mM) augmented glucose-induced insulin release and an intermediate concentration (100 microM) had no effect. Bumetanide (0.01-1 mM) reduced the islet accumulation of 36Cl-. The net efflux of 36Cl- in the presence of 20 mM D-glucose was reduced by a concentration (10 microM) that lowered glucose-induced insulin release. Bumetanide (10 microM) did not affect the rate coefficient for 36Cl- efflux, which suggests that chloride permeability is not affected. Bumetanide (10 microM) reduced 86Rb+ efflux from preloaded islets. The data show that bumetanide reduces insulin release by a direct effect on pancreatic beta-cells and suggest that this may be due to reduced chloride accumulation by a Na+, K+, Cl- co-transport system. It is suggested that the reduced chloride level is responsible for the decrease in glucose-induced chloride efflux and insulin release.

The cytoplasmic Ca2+ response to glucose as an indicator of impairment of the pancreatic beta-cell function

The effects of glucose on the cytoplasmic Ca2+ concentration (Ca2+i) regulating insulin release were investigated using pancreatic beta-cells representative for the normal and diabetic situations. Increase of the glucose concentration resulted in a slight lowering of Ca2+i followed by a rise, often manifested as high amplitude oscillations. The Ca2+i-lowering component in the glucose action associated with suppression of insulin release became particularly prominent when the beta-cells were already depolarized by tolbutamide. Glucose-induced inhibition of insulin release was observed also in experiments with rats made diabetic with streptozotocin or alloxan. Other studies indicated lowering of plasma insulin after intravenous glucose administration in patients with insulin- and noninsulin-dependent diabetes mellitus. Brief exposure of beta-cells to 2.2 mmol l-1 streptozotocin resulted in impairment of the response to glucose, manifested as disappearance of the cyclic variation of Ca2+i. The results indicate that glucose-induced depolarisation is a vulnerable process, the disturbance of which may contribute to insulin secretory defects in diabetes mellitus.

Disappearance of glucose-induced oscillations of cytoplasmic Ca2+ in pancreatic beta-cells exposed to streptozotocin or alloxan

Dual wavelength microfluorometry and the indicator fura-2 were employed for measuring cytoplasmic Ca2+ (Ca2+i) in individual pancreatic beta-cells isolated from ob/ob-mice. In most beta-cells, a rise of external glucose from 3 to 20 mM resulted in large amplitude oscillations in Ca2+i, superimposed on a basal level of 60-90 nM. The diabetogenic agents streptozotocin and alloxan (1-4.4 mM) rapidly abolished the glucose-induced oscillations of Ca2+i. The presence of a high glucose concentration during the exposure to the drugs counteracted the action of alloxan but not that of streptozotocin. Perturbation of the cyclic variations of Ca2+i by streptozotocin did not interfere with a glucose-induced increase of the ion in mildly affected beta-cells. The most advanced lesions obtained with the exposure to the diabetogenic agents were manifested as uncontrolled and sustained increases of Ca2+i. Although disrupting the intracellular Ca2+ homeostasis by separate mechanisms, streptozotocin and alloxan may finally kill the beta-cells by activating a common suicidal process due to an excessive rise of Ca2+i.

Na+ participates in loop diuretic-sensitive Cl(-)-cation co-transport in the pancreatic beta-cells

In order to investigate whether Na+ participates in loop diuretic-sensitive Cl(-)-cation co-transport in the beta-cells, we tested the interaction between the effects of Na+ deficiency, furosemide and D-glucose on 86Rb+ fluxes in beta-cell-rich mouse pancreatic islets. Removal of extracellular Na+ slightly reduced the ouabain-resistant 86Rb+ influx and the specific effect of 1 mM furosemide on this influx was significantly smaller in Na(+)-deficient medium. The capacity of 20 mM D-glucose to reduce the ouabain-resistant 86Rb+ influx was not changed by removal of extracellular Na+. The 86Rb+ efflux from preloaded islets was rapidly and reversibly reduced by Na+ deficiency. Furosemide (1 mM) reduced the 86Rb+ efflux and the effect of the combination of Na+ deficiency and 1 mM furosemide was not stronger than the effect of furosemide alone. 22Na+ efflux was reduced by both ouabain and furosemide and the effects appeared to be additive. The data suggest that Na+ participates in loop diuretic-sensitive Cl(-)-cation co-transport in the pancreatic beta-cells. This adds further support to the idea that beta-cells exhibit a Na+, K+, Cl- co-transport system. Since some of the furosemide effect on 86Rb+ efflux persisted in the Na(+)-deficient medium, it is likely that also loop diuretic-sensitive K+, Cl- co-transport exists in this cell type.

Free and bound sodium in pancreatic beta-cells exposed to glucose and tolbutamide

The effects of glucose and tolbutamide on the sodium handling of the pancreatic beta-cells were evaluated by measuring the total sodium content in intact islets from ob/ob-mice by integrating flame photometry and the free ion in individual beta-cells by dual wavelength fluorometry. Whereas increasing the glucose concentration from 3 to 20 mM resulted in a lowering of sodium, the addition of 100 microM tolbutamide caused a rise. The above-mentioned effects were most marked (about 50%) for the physiologically significant free sodium. The data indicate a more important role for Na+ in the regulation of insulin release than so far acknowledged. Increase of Na+ may contribute to the secretory response to hypoglycemic sulfonylureas by providing an additional rise of cytoplasmic Ca2+.

Evidence that an L-fucose-containing component in the beta-cell plasma membrane is involved in the regulation of glucose-induced insulin release

The effect of the L-fucose-selective lectin Ulex Europeus I (UEA I), a blocker of the Na+, K+, Cl- co-transport system in the kidney, was tested on insulin secretion from isolated beta-cell-rich pancreatic islets. UEA I at doses from 50 to 100 micrograms ml-1 significantly reduced the glucose-induced (20 mmol l-1) insulin release whereas the basal (3 mmol l-1) release was unaffected. The inhibitory effect of 100 micrograms ml l-1 UEA I was completely abolished by 10 mmol l-1 L-fucose. The data suggest that an L-fucose-containing structure in the beta-cell plasma membrane participates in the regulation of glucose-induced insulin release. This structure may be similar to the L-fucose-containing glycoprotein in the kidney tubules that is believed to be the Na+, K+, Cl- cotransporter.

Furosemide reduces insulin release by inhibition of Cl- and Ca2+ fluxes in beta-cells

The effect of furosemide on insulin release, glucose oxidation, 36Cl- fluxes, and 45Ca2+ uptake was studied in isolated, beta-cell-rich pancreatic islets from ob/ob mice. Low concentrations of furosemide (0.01-0.1 mM) reduced the glucose-induced insulin release, whereas high doses (1-10 mM) increased basal and glucose-induced release. Furosemide at concentrations that reduced glucose-induced insulin release (0.01-0.1 mM) did not affect the islet production of 14CO2 from D-[U-14C]glucose. The influx rate and equilibrium content of 36Cl- were reduced by furosemide, whereas the basal and glucose-stimulated 36Cl- efflux rates were unaffected. The glucose-induced (10 mM) uptake of 45Ca2+ was inhibited by furosemide. It is suggested that the diabetogenic action of furosemide may be due, at least in part, to direct inhibition of insulin release from the pancreatic beta-cells. This may be caused primarily by inhibition of an inwardly directed Cl- pump, leading to a reduced transmembrane electrochemical gradient for chloride in the beta-cells. This reduced gradient in combination with unaltered Cl- permeability may lead to decreased total outward Cl- transport, a factor associated with stimulated calcium uptake and insulin release.

Diazoxide unmasks glucose inhibition of insulin release by counteracting entry of Ca2+

The interaction of diazoxide with the effects of glucose on the insulin-releasing mechanism was analyzed in beta-cell-rich pancreatic islets isolated from ob/ob mice. When added at a concentration of 400 microM to a medium containing 1.28 mM Ca2+, diazoxide converted glucose stimulation of insulin release into inhibition. Further addition of 2 mM theophylline restored the insulin secretory response to glucose. The paradoxical glucose inhibition of insulin release was accounted for by a diazoxide interaction with the entry of Ca2+, unmasking a capacity of the sugar to lower cytoplasmic Ca2+ below its resting concentration.

Furosemide and Ca2+ affect 86Rb+ efflux from pancreatic beta-cells by different mechanisms

The interaction between furosemide, calcium and D-glucose on the 86Rb+ efflux from beta-cell-rich mouse pancreatic islets was investigated in a perifusion system with high temporal resolution. Raising the glucose concentration from 4 to 20 mM induced an initial decrease in 86Rb+ efflux, which was followed by a steep increase and then a secondary decrease. Removal of extracellular calcium increased the 86Rb+ efflux at 4 mM D-glucose but reduced it at 20 mM. The initial biphasic changes in 86Rb+ efflux induced by 20 mM D-glucose were inhibited by calcium deficiency. Furosemide (100 microM) reduced the 86Rb+ efflux rate both at 4 and 20 mM D-glucose and the magnitudes appeared to be similar at either glucose concentration. Furosemide (100 microM) reduced the glucose-induced (10 mM) 45Ca+ uptake but did not affect the basal (3 mM D-glucose) 45Ca+ uptake. However, the ability of furosemide (100 microM) to reduce the 86Rb+ efflux at a high glucose concentration (20 mM) was independent of extracellular calcium. The inhibitory effects of furosemide and calcium deficiency on the 86Rb+ efflux rate appeared to be additive. It is concluded that the effect of furosemide on 86Rb+ efflux is not secondary to reduced calcium uptake and that the effects of furosemide and calcium deficiency are mediated by different mechanisms. The effect of furosemide is compatible with inhibition of loop diuretic-sensitive co-transport of Na+, K+ and Cl- and the effect of calcium deficiency with reduced activity of calcium-regulated potassium channels.