The stimulus-secretion coupling of glucose-induced insulin release. XXVIII. Effect of glucose on Na+ fluxes in isolated islets

A crosstalk between Na⁺ channels, Na⁺/K⁺ pump and mitochondrial Na⁺ transporters controls glucose-dependent cytosolic and mitochondrial Na⁺ signals

Glucose-dependent cytosolic Na(+) influx in pancreatic islet β cells is mediated by TTX-sensitive Na(+) channels and is propagated into the mitochondria through the mitochondrial Na(+)/Ca(2+) exchanger, NCLX. Mitochondrial Na(+) transients are also controlled by the mitochondrial Na(+)/H(+) exchanger, NHE, while cytosolic Na(+) changes are governed by Na(+)/K(+) ATPase pump. The functional interaction between the Na(+) channels, Na(+)/K(+) ATPase pump and mitochondrial Na(+) transporters, NCLX and NHE, in mediating Na(+) signaling is poorly understood. Here, we combine fluorescent Na(+) imaging, pharmacological inhibition by TTX, ouabain and EIPA, with molecular control of NCLX expression, so as to investigate the crosstalk between Na(+) transporters on both the plasma membrane and the mitochondria. According to our results, glucose-dependent cytosolic Na(+) response was enhanced by ouabain and was followed by a rise in mitochondrial Na(+) signal. Silencing of NCLX expression using siNCLX, did not affect the glucose- or ouabain-dependent cytosolic rise in Na(+). In contrast, the ouabain-dependent rise in mitochondrial Na(+) was strongly suppressed by siNCLX. Furthermore, mitochondrial Na(+) influx rates were accelerated in cells treated with the Na(+)/H(+) exchanger inhibitor, EIPA or by combination of EIPA and ouabain. Similarly, TTX blocked the cytosolic and mitochondrial Na(+) responses, which were enhanced by ouabain or EIPA, respectively. Our results suggest that Na(+)/K(+) ATPase pump controls cytosolic glucose-dependent Na(+) rise, in a manner that is mediated by TTX-sensitive Na(+) channels and subsequent mitochondrial Na(+) uptake via NCLX. Furthermore, these results indicate that mitochondrial Na(+) influx via NCLX is antagonized by Na(+) efflux, which is mediated by the mitochondrial NHE; thus, the duration of mitochondrial Na(+) transients is set by the interplay between these pivotal transporters.

Pancreatic β-cell Na+ channels control global Ca2+ signaling and oxidative metabolism by inducing Na+ and Ca2+ responses that are propagated into mitochondria

Communication between the plasma membrane and mitochondria is essential for initiating the Ca(2+) and metabolic signals required for secretion in β cells. Although voltage-dependent Na(+) channels are abundantly expressed in β cells and activated by glucose, their role in communicating with mitochondria is unresolved. Here, we combined fluorescent Na(+), Ca(2+), and ATP imaging, electrophysiological analysis with tetrodotoxin (TTX)-dependent block of the Na(+) channel, and molecular manipulation of mitochondrial Ca(2+) transporters to study the communication between Na(+) channels and mitochondria. We show that TTX inhibits glucose-dependent depolarization and blocks cytosolic Na(+) and Ca(2+) responses and their propagation into mitochondria. TTX-sensitive mitochondrial Ca(2+) influx was largely blocked by knockdown of the mitochondrial Ca(2+) uniporter (MCU) expression. Knockdown of the mitochondrial Na(+)/Ca(2+) exchanger (NCLX) and Na(+) dose response analysis demonstrated that NCLX mediates the mitochondrial Na(+) influx and is tuned to sense the TTX-sensitive cytosolic Na(+) responses. Finally, TTX blocked glucose-dependent mitochondrial Ca(2+) rise, mitochondrial metabolic activity, and ATP production. Our results show that communication of the Na(+) channels with mitochondria shape both global Ca(2+) and metabolism signals linked to insulin secretion in β cells.- Nita, I. I., Hershfinkel, M., Kantor, C., Rutter, G. A., Lewis, E. C., Sekler, I. Pancreatic β-cell Na(+) channels control global Ca(2+) signaling and oxidative metabolism by inducing Na(+) and Ca(2+) responses that are propagated into mitochondria.

Expression of TMEM16A and SLC4A4 in human pancreatic islets

BACKGROUND/AIMS

Stimulation of insulin release by D-glucose is accompanied by Cl(-) and HCO(3)(-) efflux from pancreatic islet cells. The efflux of these anions may involve volume-regulated anion channels, including possibly TMEM16A, and the Na(+)-HCO(3)(-)-cotransporter SLC4A4. The present study was designed to explore the expression of both TMEM16A and SLC4A4 in human pancreatic islets.

METHODS

Pancreases were obtained from human cadaveric donors. Immunodetection of TMEM16A and SLC4A4 was performed by immunohistochemistry on sections of fixed pancreas, while real-time PCR for the study of corresponding gene expression was performed on RNA extracted from both total pancreatic pieces and isolated pancreatic islets.

RESULTS

RT-PCR yielded lower levels of SLC4A4 in isolated islets than in the total pancreas, whilst a mirror image prevailed for TMEM16A mRNA. Immunohistochemistry of human pancreas, however, indicated comparable immunostaining of SLC4A4 in insulin-producing cells and exocrine pancreatic cells, whilst that of TMEM16A appeared less pronounced in insulin-producing cells than in exocrine cells.

CONCLUSION

The present findings support the view that, in humans like in rodent, the regulation of anion fluxes in insulin-producing cells may involve both SLC4A4 and TMEM16A.

Fatty acid pattern of pancreatic islet lipids in Goto-Kakizaki rats

Perturbations of fatty acid content and pattern were recently documented in epididymal and parametrial lipids, as well as plasma, liver, spleen, and brain phospholipids and triglycerides of Goto-Kakizaki rats (GK). This study extends such an investigation to pancreatic islets from both control and GK rats. Groups of 5,500-14,560 islets were obtained from either control or GK rats (n = 3 in each case) and examined for their lipid fatty acid content. In the islet triglycerides, the major difference between control and GK rats, i.e., a higher C18:2ω6 content in GK rats, was similar to that found in liver triglycerides. In the islet phospholipids, however, a number of differences between control and GK rats, concerning saturated, monodesaturated, and long-chain polyunsaturated ω3 and ω6 fatty acids, were often not similar to those found in liver phospholipids. The present study reveals a number of anomalies in the fatty acid pattern of islet phospholipids in GK rats, often differing from those encountered in liver phospholipids. Such a tissue specificity was borne out by the finding that, even in control animals, the situation found in islet phospholipids differed from that recorded in liver phospholipids.

Expression of the electrogenic Na+-HCO3--cotransporters NBCe1-A and NBCe1-B in rat pancreatic islet cells

It was recently proposed that, in rat pancreatic islets, the production of bicarbonate accounts for the major fraction of the carbon dioxide generated by the oxidative catabolism of nutrient insulin secretagogues. In search of the mechanism(s) supporting the membrane transport of bicarbonate, the possible role of the electrogenic Na(+)-HCO(3) (-)-cotransporters NBCe1-A and NBCe1-B in rat pancreatic islet cells was investigated. Expression of NBCe1-A and NBCe1-B in rat pancreatic islet cells was documented by RT-PCR, western blotting, and immunocytochemistry. The latter procedure suggested a preferential localization of NBCe1-B in insulin-producing cells. Tenidap (3-100 microM), previously proposed as an inhibitor of NBCe1-A-mediated cotransport in proximal tubule kidney cells, caused a concentration-related inhibition of glucose-stimulated insulin secretion. It also inhibited 2-ketoisocaproate-induced insulin release and to a relatively lesser extent, the secretory response to L: -leucine. Tenidap (50-100 microM) also inhibited the metabolism of D: -glucose in isolated islets, increased (22)Na net uptake by dispersed islet cells, lowered intracellular pH and provoked hyperpolarization of plasma membrane in insulin-producing cells. This study thus reveals the expression of the electrogenic Na(+)-HCO(3) (-)-cotransporters NBCe1-A and NBCe1-B in rat pancreatic islet cells, and is consistent with the participation of such transporters in the process of nutrient-stimulated insulin secretion.

Differential effects of glucose and glyburide on energetics and Na+ levels of betaHC9 cells: nuclear magnetic resonance spectroscopy and respirometry studies

In the present study, noninvasive (31)P and (23)Na(+)-nuclear magnetic resonance (NMR) technology and respirometry were used to compare the effect of high glucose (30 mmol/l) with the effect of the antidiabetic sulfonylurea (SU) compound glyburide (GLY) on energy metabolism, Na(+) flux, insulin, and cAMP release of continuously superfused beta-HC9 cells encapsulated in microscopic agarose beads. Both high glucose and GLY increased oxygen consumption in beta-HC9 cells (15-30%) with a maximal effect at 8 mmol/l for glucose and at 250 nmol/l for GLY. At the same time, insulin release from beta-cells increased by 15- and 25-fold with high glucose or GLY, respectively. The P-creatine (PCr) level was greatly increased and inorganic phosphate (P(i)) was decreased with 30 mmol/l glucose in contrast to the decreased level of PCr and increased P(i) with GLY. ATP levels remained unchanged during both interventions. Studies on isolated mitochondria of beta-HC9 cells showed that GLY added to mitochondria oxidizing glutamine or glutamate abolished the stimulation of respiration by ADP (state 3) meanwhile leaving state 3 respiration unchanged during oxidation of other substrates. Exposure of beta-HC9 cells to 5 mmol/l glucose decreased intracellular Na(+) levels monitored by (23)Na(+)-NMR spectroscopy and 30 mmol/l glucose resulted in a further decrease in cytosolic Na(+). In contrast, Na(+) increased when 1 micro mol/l GLY was added to the perfusate containing 5 mmol/l glucose. These data support the hypothesis that glucose activates the beta-cell through a "push mechanism" due to substrate pressure enhancing fuel flux, energy production, and extrusion of Na(+) from the cells in contrast to SU receptor (SUR)-1 inhibitors, which may modify intermediary and energy metabolism secondarily through a "pull mechanism" due to higher energy demand resulting from increased ion fluxes and the exocytotic work load.

Na+ cotransport by metabolizable and nonmetabolizable amino acids stimulates a glucose-regulated insulin-secretory response

The involvement of Na+ in insulin-secretory responses to metabolizable and nonmetabolizable amino acids known to be cotransported with Na+, were examined using islet-derived BRIN-BD11 cells. At stimulatory (16.7 mM) glucose, 10 mM of l-alanine, alpha-aminoisobutyric acid (AIB) or l-proline stimulated 1.3- to 10. 4-fold (p < 0.01) insulin-secretory responses. In each case, these effects were significantly greater than those observed at nonstimulatory (1.1 mM) glucose (p < 0.01). While, tetrodotoxin blockade of voltage-dependent Na+ channels exerted no significant effect on insulin release, Na/K pump blockade with ouabain significantly promoted the amino acid-induced effects (p < 0.05). Replacement of extracellular Na+ with equimolar N-methyl-d-glucamine+ and omission of extracellular K+ or Ca2+ were all effective in removing the actions of each amino acid, confirming the critical role of ionic fluxes in the secretory responses to these amino acids. Collectively these results demonstrate that metabolizable and nonmetabolizable amino acids can induce glucose-dependent insulin-secretory responses by modulating electrogenic Na+ transport.

Glucose-induced mobilisation of intracellular Ca2+ in depolarised pancreatic islets

Perifused rat pancreatic islets, prelabelled with 45Ca, were exposed for 90 min to a medium containing 30 mM K+, 0.25 mM diazoxide and 0.5 mM EGTA, but deprived of CaCl2. Either verapamil (0.05 mM) or Cd2+ (0.05 mM) were also present in the perifusate. Under these conditions a rise in D-glucose concentrations from either 2.8 to 16.7 mM or zero to 8.3 mM increased both 45Ca outflow and insulin release, after an initial and transient decrease in effluent radioactivity. These findings suggest that, in islets depolarised by exposure to a high extracellular concentration of K+, D-glucose provokes an intracellular redistribution of Ca2+ ions and subsequent stimulation of insulin release. The functional response to D-glucose is apparently not attributable to either the closing of ATP-sensitive K+ channels, which were actually activated by diazoxide, or stimulation of Ca2+ influx, which was prevented by the absence of extracellular Ca2+. The present experimental design thus reveals a novel component of the glucose-induced remodelling of Ca2+ fluxes in islet cells. Such an effect might also be operative under physiological conditions, when the hexose leads to depolarisation of the islet B-cells.

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.

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.

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.

Glucose stimulation of ouabain-resistant efflux of Na+ from rat pancreatic islets

1. Integrating flame photometry was employed for measuring the mobilization of sodium from rat pancreatic islets after substitution of extracellular Na+ by N-methylglucamine. 2. Glucose accelerated the initial loss of sodium both in the absence and presence of ouabain (1 mM). In the latter case the effect was maximal at 5 mM of the sugar. 3. Amiloride (0.1 mM), an inhibitor of Na(+)-H+ exchange, prevented the effect of glucose on the ouabain-resistant Na+ efflux, increasing the rate of outward transport in the absence of the sugar. 4. Extracellular K+ and arginine (10 mM) mimicked the action of glucose in promoting a ouabain-resistant mobilization of sodium. 5. Whereas the hypoglycaemic sulphonylurea tolbutamide (100 microM) did not modify the outward transport of Na+, the ouabain-resistant component of this process was partially suppressed after bumetanide (100 microM) inhibition of the chloride-dependent co-transport of Na+ and K+. 6. It is suggested that the glucose-induced lowering of the steady-state content of islet sodium involves an increased outward transport mediated at least in part by mechanisms other than stimulation of the Na(+)-K+ pump.

Hexose metabolism in pancreatic islets. Regulation of D-[6-14C]glucose oxidation by non-nutrient secretagogues

In rat pancreatic islets, a rise in D-glucose concentrations increases the oxidation of hexose-derived acetyl residues relative to glycolytic flux, an effect possibly attributable, in part at least, to the activation of key mitochondrial dehydrogenases by Ca2+ accumulated in the mitochondria of glucose-stimulated islet cells. The effects of non-nutrient insulinotropic agents upon D-[6-14C]glucose oxidation and D-[5-3H]glucose utilization were investigated. At an intermediate concentration of D-glucose (6 mM), the oxidation of D-[6-14C]glucose was unaffected by hypoglycemic sulfonylureas, an organic Ca2+ agonist, a cholinergic agent, forskolin, theophylline and cytochalasin B. At a higher concentration of the hexose (17 mM), however, the 14CO2/3H2O production rate was decreased by organic and inorganic Ca(2+)-antagonists and by ouabain, whilst being increased by NH+4 (10 mM) and aminooxyacetate. These findings suggest that the preferential stimulation of oxidative events in the Krebs cycle is largely independent of the rate of insulin release, and not merely consequential to the stimulation of Ca2+ inflow into the B-cell. It might be regulated, in a feedback process, by the rate of ATP utilization and, both directly and indirectly, by the mitochondrial redox state. The glucose-induced mitochondrial accumulation of Ca2+ and subsequent activation of the Krebs cycle appear to require an increase in both cytosolic Ca2+ activity and ATP availability.

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.

Binding of ouabain to normal and tumoral pancreatic islet cells

The binding of [3H(G)]ouabain was examined in both rat pancreatic islets and tumoral islet cells (RINm5F line). In the islets, the specific binding was considered too low to allow for extensive investigations, taking into account the limited number of islets readily available. In the RINm5F cells, which bound more ouabain than normal islet cells, the time course for binding, its temperature dependency, its competitive inhibition by K+, the number of binding sites, their affinity, and the turnover of ATP per binding site were all similar to those found in other cell types. An incomplete dissociation of [3H(G)]ouabain by unlabelled glycoside and the stimulation of binding by D-glucose were noticed, both phenomena being possibly attributable, in part at least, to the functional response of the RINm5F cells to D-glucose with resulting activation of exocytosis-endocytosis coupling.

Effect of extracellular sodium removal upon 86Rb outflow from pancreatic islet cells

The present study was undertaken to characterize the effect of extracellular Na+ removal on 86Rb outflow from perifused rat pancreatic islets. Complete Na+ omission inhibited 86Rb outflow whether the islets were perifused in the presence or in the absence of extracellular Ca2+. Ouabain (1 mM) did not reduce the inhibitory effect of Na+ deprivation, whilst diphenylhydantoin (72.9 microM) mimicked the Na+-removal-induced fall in 86Rb outflow. Glucose (16.7 mM) lost its capacity to inhibit 86Rb outflow when the perifusate was deprived of extracellular Na+. These results indicate that Na+ omission reproduces the inhibitory effect of glucose on 86Rb outflow. The reduction in 86Rb outflow recorded after Na+ deprivation could be mediated by an intracellular acidification and/or a decrease in the intracellular Na+ activity. It is tempting to speculate that the capacity of glucose to reduce the B-cell Na+ content may participate in the process by which the sugar decreases K+ permeability.

Effects of depolarizing agents on the sodium content of rat pancreatic islets

Rat pancreatic islets were used for studying the effects of depolarization on their sodium content. The islet sodium was markedly affected by small variations of extracellular K+. As with increased K+, the presence of low concentrations of glucose (5 mM) and arginine (2 mM) decreased the sodium content. The latter substances did not lower the sodium concentration below the value obtained by depolarization with excessive K+, nor was it possible to obtain a further decrease when 10 mM arginine was combined with 5 mM glucose. The sodium content was also reduced in the presence of 10 mM L-leucine, 10 mM 2-ketoisocaproate and 0.1 mM Ba2+. Tolbutamide differed from the other depolarizing agents in that it increased the sodium concentration, an effect manifested also in the presence of excessive K+. The observation that depolarizing agents other than sulfonylureas do not increase but actually reduce sodium implies that islet cells are exceptional among electrically excitable cells. The observed reduction of sodium may reflect activation of a voltage-sensitive carrier mechanism for outward transport of Na+.

Sulphonamide modulation of sodium content in rat pancreatic islets

Sodium was measured in rat pancreatic islet exposed to tolbutamide, glipizide, diazoxide or sulfisomidine. When added to a medium with physiologically balanced cations these sulphonamides induced a significant rise of the islet content of sodium. The insulin-releasing compounds, tolbutamide and glipizide, had effects opposite to those of the hyperglycemic diazoxide in counteracting the increase of sodium obtained with removal of K+. The tolbutamide-induced increase in sodium was reversed to a decrease when Ca2+ was omitted from the incubation medium. The increase of sodium, which was also seen with non-hypoglycemic sulphonamides, is itself not sufficient for initiating insulin release. However, it may well represent an important mechanism contributing to the secretory response initiated by Ca2+ entry into the sulfonylurea-depolarized beta-cell.

Characterization of the glucose-induced lowering of sodium in mouse pancreatic beta-cells

The mechanisms for glucose regulation of the sodium content of the pancreatic beta-cells were examined using aggregates of cells prepared from ob/ob-mice of a local colony. Exposure to glucose rapidly resulted in a protracted lowering of the sodium content as estimated with integrating flame photometry. Sodium became decreased after addition of 1 mmol l-1 glucose, and this effect was maximal with 5 mmol l-1 of the sugar. The effects of low glucose concentrations on the sodium content could not be mimicked by the poorly metabolized 3-o-methyl-D-glucose, and it disappeared in the presence of the metabolic inhibitor antimycin A. The significance of the Na/K pump for maintaining low sodium was illustrated by a substantial increase of the element in the presence of ouabain. However, there was no indication that glucose-induced lowering of sodium reflected activation of this pump when measuring the ouabain-sensitive uptake of 86Rb+. Neither bumetanide nor the bromo derivatives of cyclic AMP or cyclic GMP modified the glucose action on the sodium content. In evaluating whether the effect of glucose was mimicked by other inhibitors of the K+ permeability it was observed that 100 mumol l-1 quinine, but not tolbutamide, decreased sodium. It is concluded that the beta-cell is exceptional among excitable cells in responding to its natural physiological stimulant (glucose) by reduction of sodium. Acting in this way glucose facilitates the removal of Ca2+ from the cytoplasm.

Effects of acute sodium omission on insulin release, ionic flux and membrane potential in mouse pancreatic B-cells

The effects of acute omission of extracellular Na+ on pancreatic B-cell function were studied in mouse islets, using choline and lithium salts as impermeant and permeant substitutes, respectively. In the absence of glucose, choline substitution for Na+ hyperpolarized the B-cell membrane, inhibited 86Rb+ and 45Ca2+ efflux, but did not affect insulin release. In contrast, Li+ substitution for Na+ depolarized the B-cell membrane and caused a Ca2+-independent, transient acceleration of 45Ca2+ efflux and insulin release. Na+ replacement by choline in the presence of 10 mM glucose and 2.5 mM Ca2+ again rapidly hyperpolarized the B-cell membrane. This hyperpolarization was then followed by a phase of depolarization with continuous spike activity, before long slow waves of the membrane potential resumed. Under these conditions, 86Rb+ efflux first decreased before accelerating, concomitantly with marked and parallel increases in 45Ca2+ efflux and insulin release. In the absence of Ca2+, 45Ca2+ and 86Rb+ efflux were inhibited and insulin release was unaffected by choline substitution for Na+. Na+ replacement by Li+ in the presence of 10 mM glucose rapidly depolarized the B-cell membrane, caused an intense continuous spike activity, and accelerated 45Ca2+ efflux, 86Rb+ efflux and insulin release. In the absence of extracellular Ca2+, Li+ still caused a rapid but transient increase in 45Ca2+ and 86Rb+ efflux and in insulin release. Although not indispensable for insulin release, Na+ plays an important regulatory role in stimulus-secretion coupling by modulating, among others, membrane potential and ionic fluxes in B-cells.

Quantitative energy dispersive X-ray microanalysis of eight elements in pancreatic endocrine and exocrine cells after cryo-fixation

Quantitative X-ray microanalysis of 8 elements was performed on ultrathin, freeze-dried sections of islets and pancreas pieces from non-inbred ob/ob-mice. Diffusion of elements was reduced to a minimum by rapidly freezing the tissue samples between nitrogen-cooled polished copper surfaces and avoiding the use of chemical fixatives and stains. The ultrastructural morphology was adequately maintained to allow measurements on secretory granules, mitochondria, cell nuclei, and cytoplasm free of these organelles. The distribution of the various elements between cellular compartments was similar in islet beta-cells and exocrine pancreas cells. However, the insulin secretory granules were outstanding in exhibiting the highest concentrations of zinc and calcium. In comparison with cytoplasm in the beta-cells, the insulin granules accumulated calcium 2-fold and zinc as much as 40-fold. As no correlation could be made for endoplasmic reticulum in the cytoplasmic measurements areas, the true accumulations above cytosol are likely to be even higher.

Glucose-induced reduction of the sodium content in beta-cell-rich pancreatic islets

The sodium contents of beta-cell-rich pancreatic islets from ob/ob-mice were measured with an integrating flame photometer. After washing to an apparent steady state with different types of ice-cold media, islets incubated in the absence of glucose contained 79-108 mmol sodium kg-1 dry weight. Exposure to glucose resulted in 25% reduction of the islet content of sodium. This effect became manifest in the presence of 5 mM glucose, there being no additional reduction with a further increase of glucose to 20 mM. Depression of Na+ activity may partially explain why glucose, under certain conditions, can lower cytoplasmic Ca2+ and even inhibit insulin release.

Sodium requirement for insulin release: putative role in regulation of intracellular pH

The effects on insulin release of Na+ removal, alteration of extracellular pH, and inhibition of acid extrusion processes were examined using freshly isolated and 46-h cultured islets of Langerhans. Na+ removal inhibited the secretory response to 16.7 mM glucose specifically in fresh islets but stimulated the low-glucose response in cultured islets. These divergent effects of Na+ depletion were perfectly mimicked by amiloride, an inhibitor of Na+-H+ exchange, and could be overcome in a raised HCO-3, pH 8.0 medium. Simply raising extracellular pH at constant HCO-3 had no effect. Na+ removal also inhibited the secretory responses to other nutrient secretagogues such as D-glyceraldehyde and L-leucine but not to agents like 3-isobutyl-1-methylxanthine or 12-O-tetradecanoylphorbol-13-acetate. The results show that Na+ is not itself essential for the secretory process but rather suggest that it plays a permissive role in the regulation of intracellular pH via Na+-H+ exchange. Such a regulation appears important mainly for nutrient-stimulated insulin release, which is associated with the generation of acidic metabolites.

Do hypoglycemic sulfonylureas inhibit Na+,K+-ATPase activity in pancreatic islets?

The view that the insulinotropic response to hypoglycemic sulfonylureas is somehow related to an inhibitory action of these drugs on either the Ca2+-activated K+ permeability or the Na+,K+-ATPase activity in pancreatic islet cells was investigated by measuring the effect of tolbutamide on 86Rb outflow and uptake, 45Ca outflow, and insulin release in rat pancreatic islets. Although tolbutamide inhibited 86Rb efflux from glucose-deprived islets, whether in the absence or presence of extracellular Ca2+ or ouabain, a primary action of the sulfonylurea on the Ca2+-responsive K+ channels appeared unlikely, because tolbutamide failed to suppress the increase in 86Rb outflow evoked by either the ionophore A23187 or ouabain. Glibenclamide also failed to suppress the increase in 86Rb outflow evoked by the ionophore. Moreover, tolbutamide itself stimulated quinine-sensitive 86Rb outflow from glucose-stimulated islets. Likewise, although tolbutamide inhibited an ouabain-resistant modality of 86Rb inflow into the islet cells, an inhibitory action of the sulfonylurea on the Na+,K+-ATPase appeared improbable, because tolbutamide failed to minimize and instead favored the cationic and secretory response to ouabain. It is concluded, therefore, that the capacity of tolbutamide to cause under suitable conditions continuous electrical activity in islet cells cannot be attributed to inhibition of either the Ca2+-sensitive K+ permeability or ouabain-sensitive Na+,K+-ATPase.

Mathematical modelling of stimulus-secretion coupling in the pancreatic B-cell. II. Calcium-stimulated calcium release

An attempt was made to simulate in a mathematical model one of the two major effects of glucose upon 45Ca fractional outflow rate from prelabelled pancreatic islets, namely the increase in effluent radioactivity which is currently ascribed to the displacement of 45Ca from intracellular sites, as resulting from a facilitated influx of unlabelled 40Ca into the islet cells. The occurrence of such a rise in effluent radioactivity and its suppression in the absence of extracellular Ca2+ could only be simulated if the release of Ca by the vacuolar system was assumed to be stimulated by a rise in the cytosolic Ca concentration. It is proposed therefore that, in islets like in muscle, a process of Ca-stimulated Ca release may participate in the regulation of intracellular Ca distribution.

Effects of monensin on metabolism of isolated rat islets of Langerhans

Monensin, a univalent ionophore, is a carboxylic acid produced by Streptomyces cinnamonensis. It will complex various alkali-metal ions, but most readily binds Na+. Because of interest in the possible role of Na+ in the regulation of insulin secretion, we examined its effects on several aspects of the metabolism of isolated rat islets of Langerhans. The ionophore inhibited glucose-stimulated insulin release in a concentration-dependent manner, completely inhibiting secretion evoked by 20 mM-glucose at concentrations as low as 0.1 microM in static incubations. In perifusion experiments, both phases of insulin release were equally affected. Monensin (0.1 microM) had no significant effect on glucose oxidation as measured by the generation of 14CO2 from [14C]glucose. Monensin increased the rate of 22Na+ efflux from preloaded islets and net 22Na+ uptake over 30 min, in the absence of changes in islet volume or extracellular space. The ionophore increased the Rb+/K+ permeability of islet cells, as shown by its inhibition of 86Rb+ retention and stimulation of 86Rb+ efflux. At 0.1 microM, monensin abolished glucose-stimulated 45Ca2+ uptake by islets during 5 min incubations, and stimulated 45Ca2+ efflux from preloaded islets perifused with Ca2+-free medium, even in the complete absence of extracellular Na+. Studies of the uptake of 14C-labelled 5,5-dimethyloxazolidine-2,4-dione showed that 0.1 microM-monensin increased net intracellular pH from 7.05 to 7.13. 7 Monensin has widespread, complex, effects on the secretory responses and ion handling by the B cells, which are difficult to interpret in terms solely of actions as a Na+ ionophore.

Interactions of Ca2+, Mg2+, and Na+ in regulation of insulin release from rat islets

The effects of glucose and ionic modifications on unidirectional Ca2+ efflux and insulin release has been studied. Rat pancreatic islets were isotopically equilibrated with 45Ca2+ for 2 days and then perifused at 10(-8) M Ca2+ to allow for strict interpretation of 45Ca2+ efflux. Under these conditions 16.7 mM glucose inhibited Ca2+ efflux but did not stimulate insulin release. Removal of Mg2+ from the buffer markedly stimulated Ca2+ efflux that was counteracted by glucose. The omission of Na+ decreased basal Ca2+ efflux by 30% at 10(-8) M Ca2+, thus demonstrating the importance of Na-Ca countertransport for Ca2+ extrusion. Like glucose, Na+ omission or the addition of ouabain attenuated Ca2+ efflux stimulated by Mg2+ removal. Glucose may interfere with Na-Ca countertransport because the actions of 16.7 mM glucose and Na+ omission were not additive. At 10(-8) M Ca2+, glucose elicited insulin release only when both 1) loss of cellular calcium was minimized by prior inhibition of Ca2+ efflux (Na+ omission or ouabain), and 2) Ca2+ mobilization was favored by Mg2+ removal. Under these conditions (in contrast to normal Ca2+), insulin release was not accompanied by increased Ca2+ efflux. Thus, unidirectional Ca2+ measurements do not permit the detection of Ca2+ mobilization in intact islets because glucose may concomitantly inhibit Ca2+ extrusion.

Ionic determinants of bioelectrical spiking activity in the pancreatic B-cell

A rise in extracellular glucose concentration from 8.3 to 16.7 mM stimulates both Ca2+ inflow and K+ exit in perfused rat pancreatic islets. These ionic changes are associated with an increase in bioelectrical spiking activity. From a quantitative analysis of 45Ca and 86Rb outflow from prelabelled islets, it is proposed that each electrical spike coincides, approximately, with the entry of 0.8 fmol of Ca2+ and exit of 3.6 fmol of K+ per mm2 of cell surface.

The electrogenic sodium-potassium pump of mouse pancreatic B-cells

1. The contribution of the sodium pump to the membrane potential of mouse pancreatic B-cells was studied with micro-electrodes.2. In 0 or 3 mM-glucose, ouabain rapidly (within 2 min) depolarized the B-cell membrane by an average of 7 mV, whereas K omission hyperpolarized it markedly.3. In 6 or 7 mM-glucose, ouabain still produced depolarization, but K omission had no consistent effect. Both induced electrical activity in certain cells.4. In 10 mM-glucose, withdrawal of ouabain or K re-introduction caused a transient hyperpolarization with suppression of electrical activity. Duration and amplitude of the hyperpolarization increased with the time of pump blockade and with the concentration of ouabain.5. The hyperpolarization following K re-admission was abolished by ouabain and that following ouabain withdrawal was prevented by K omission. Re-admission of various K concentrations showed that the hyperpolarization was not due to depletion of K just outside of the membrane.6. In 10 mM-glucose, the membrane potential of B-cells exhibited repetitive slow waves with bursts of spikes on the plateau. These electrical events were modified by ouabain in a dose-dependent manner. The frequency of the slow waves augmented markedly because of an increase in the slope of the pre-potential and a shortening of the intervals; the slope of their repolarization phase decreased, but their duration was not changed.7. Omission of K increased the slope of the pre-potential and the frequency of the slow waves. It also accelerated their repolarization phase and reduced their duration, likely because of the increase in driving force for K efflux. Increasing K concentration to 8 mM slowed the repolarization phase and lengthened the slow waves without changing their frequency.8. Even when K permeability of the B-cell membrane was increased by high extracellular Ca, ouabain and K omission augmented the frequency of the slow waves.9. In 0 or 10 mM-glucose, ouabain increased (86)Rb(+) efflux from perifused islets, whereas K omission decreased it. In 10 mM-glucose, a marked decrease in (86)Rb(+) efflux accompanied ouabain withdrawal and K re-introduction. The hyperpolarization is thus not due to an increase in K permeability.10. It is concluded that, in pancreatic B-cells, the sodium pump is truly electrogenic, contributes to the resting potential and modulates the slow waves of membrane potential induced by glucose. Rapid changes in insulin release occurring upon inhibition or activation of the sodium pump may thus be due to the changes in B-cell membrane potential.

Regulation of insulin secretion from islets of Langerhans rendered permeable by electric discharge

1. High-voltage electric discharge has been used to increase the permeability of B-cells of isolated islets of Langerhans to facilitate studies of the effects of normally impermeable substances on insulin secretion. 2. The application of an intense electric field increased the [(14)C]sucrose space of the islets from 37.8+/-3.1% to 86.2+/-5.2% of their total volume as assessed by (3)H(2)O content. The cells remained permeable for at least 40min. 3. Ultrastructural studies showed no deleterious changes in the structure of the B-cells after discharge. 4. Insulin secretion from normal islets was unaffected by increasing the medium [Ca(2+)] from 10nm to 10mum. In the islets that had been rendered permeable by discharge, insulin secretion was significantly increased under these conditions, without any alteration in the release of lactate dehydrogenase, a cytoplasmic marker enzyme. 5. Studies of the dynamics of insulin release during perifusion showed that the response to increased (10mum) Ca(2+) concentration was rapid and sustained over a period of at least 13min. 6. Secretion responses to Ca(2+) in perifusion established that maximum release in permeabilized islets occurs at approx. 1mum-Ca(2+) and half-maximum release occurs at approx. 0.6mum-Ca(2+). 7. The study of the effect of agents that interfere with the microtubular microfilamentous system in B-cells using a perifusion system revealed that cytochalasin B caused a considerable increase, whereas vinblastine sulphate caused a significant inhibition, in insulin release in response to 1mum-Ca(2+). 8. This technique should facilitate the study of the role of normally impermeable ions and metabolic intermediates in the regulation of insulin secretion.

Fluctuations of calcium, phosphorus, sodium, potassium, and chlorine in single alpha and beta cells during glucose perifusion of rat islets

To study the relationship between islet hormonal secretion and intracellular content of five elements, a rat islet perifusion technique was used in 24 paired experiments. Control and experimental chambers each containing 100 islets, received 2.8 and 16.7 mM D-glucose, respectively. Effluent was collected frequently for hormone measurements. At eight different time intervals form 0--30 min islets were fixed and prepared for scanning electron microscopy. Over 900 unobscured alpha and beta cells were selected by size and shape criteria. Energy dispersive x-ray analysis was applied to each single cell to determine relative content of calcium (Ca), potassium (K), sodium (Na), chlorine (Cl), and phosphorus (P). Experimental chambers exhibited typical acute (0--9 min) and second phase (10--30 min) insulin secretion in association with suppression of glucagon release after 10 min. At 2 min an abrupt upward K spike in both alpha and beta cells was followed at 3--4 min with a 1.5- to 2-fold rise of Ca and a reciprocal decrease in K, Na, Cl, and P. From 3 to 30 min biphasic insulin secretion. Reduced alpha cell calcium after 6 min preceded suppression of glucagon secretion. After 2 min K related inversely to Ca content in both alpha and beta cells. These results could not be reproduced when D-galactose was substituted for D-glucose. We conclude that sequential changes of Ca content that are reciprocally related to K are predictive of beta cell insulin release and suppression of alpha cell glucagon secretion.

Ionic mechanism of diphenylhydantoin action on glucose-induced insulin release

Diphenylhydantoin inhibits glucose-stimulated insulin release. The mode of action of diphenylhydantoin was investigated by characterizing its effect on 86Rb and 45Ca fluxes in isolated pancreatic islets. The inhibition of glucose-stimulated insulin release by diphenylhydantoin was apparently not attributable to activation of a Na+ + K+ATPase as diphenylhydantoin failed to affect 86Rb net uptake, at least in glucose-stimulated islets. Diphenylhydantoin decreased 45Ca net uptake by the islets, an effect possibly due to inhibition of Ca2+ entry into the islet cells. Diphenylhydantoin indeed markedly inhibited the glucose-induced increase in 45Ca outflow and decreased the process of 40Ca-45 exchange evoked by a rise in extracellular Ca2+ concentration. Diphenylhydantoin failed to affect the inhibitory action of glucose upon 45Ca outflow whether in the presence or absence of extracellular Ca2+, and did not impair the antimycin A-induced release of 45Ca from intracellular organelles. These findings suggest that the inhibitory effect of diphenylhydantoin upon glucose-stimulated insulin release is attributable mainly to a blockade of Ca2+ inflow into the beta-cell.

Glucose-induced electrical activity in the pancreatic beta-cell: effect of veratridine

Veratridine was used in the presence of glucose to assess the role of the Na pump in the regulation of glucose-induced burst activity. In the presence of 8.4 mM glucose, veratridine elicited a silent hyperpolarization, followed by burst activity. The magnitude of depolarization to plateau potential and the duration of the silent phase were increased. The addition of tetrodotoxin (TTX) restored the pattern of electrical activity to that observed in the absence of veratridine. Similar results were observed when veratridine was used in the presence of 16.7 mM glucose and tetraethylammonium (blocks voltage-dependent potassium permeability). TTX or ouabain blocked the effects of veratridine, and produced depolarization and continuous spike activity. Quinine (blocks Ca-dependent potassium permeability) elicited continuous spike activity in the presence of 16.7 mM glucose. The addition of veratridine induced only a transient return to burst activity, followed by a return to continuous spike activity. These results suggest that an electrogenic Na pump is an important factor in maintaining the transmembrane potential at an optimum level for operation of a voltage- and Ca-sensitive potassium permeability: changes in potassium permeability operating on a background of electrogenic current may be responsible for the voltage transitions associated with burst activity.

Regulation of calcium fluxes in rat pancreatic islets. Quinine mimics the dual effect of glucose on calcium movements

The effects of quinine and 9-aminoacridine, two blockers of potassium conductance in islet cells, on 45Ca efflux and insulin release from perifused islets were investigated in order to elucidate the mechanisms by which glucose initially reduces 45Ca efflux and later stimulates calcium inflow in islet cells. In the absence of glucose, 100 microM quinine stimulated 45Ca net uptake, 45Ca outflow rate and insulin release. Quinine also dramatically enhanced the cationic and the secretory response to intermediate concentrations of glucose, but had little effect on 45Ca net uptake, 45Ca fractional outflow rate and insulin release at a high glucose concentration (16.7 mM). The ability of quinine to stimulate 45Ca efflux depended on the presence of extracellular calcium, suggesting that it reflects a stimulation of calcium entry in the islet cells. In the absence of extracellular calcium, quinine provoked a sustained decrease in 45Ca efflux. Such an inhibitory effect was not additive to that of glucose, and was reduced at low extracellular Na+ concentration. At a low concentration (5 microM), quinine, although reducing 86Rb efflux from the islets to the same extent as a non-insulinotropic glucose concentration (4.4 mM), failed to inhibit 45Ca efflux. In the presence of extracellular calcium, 9-aminoacridine produced an important but transient increase in 45Ca outflow rate and insulin release from islets perifused in the absence of glucose. In the absence of extracellular calcium, 9-aminoacridine, however, failed to reduced 45Ca efflux from perifused islets. It is concluded that quinine, by reducing K+ conductance, reproduces the effect of glucose to activate voltage-sensitive calcium channels and to stimulate the entry of calcium into the B-cell. However, the glucose-induced inhibition of calcium outflow rate, which may also participate in the intracellular accumulation of calcium, does not appear to be mediated by changes in K+ conductance.

Regulation of calcium fluxes in rat pancreatic islets: calcium extrusion by sodium-calcium countertransport

The mechanisms by which glucose regulates calcium fluxes in pancreatic endocrine cells were investigated by monitoring the efflux of 45Ca from prelabeled and perifused rat pancreatic islets. In the absence of both extracellular calcium and glucose, partial or total removal of extracellular sodium decreases the efflux of 45Ca from prelabeled islets. Glucose also reduces the efflux of 45Ca from islets perifused in the absence of extracellular calcium. This inhibitory effect of glucose on 45Ca efflux is decreased by half when the extracellular concentration of sodium is lowered to 24 mM. In the absence of extracellular calcium but presence of glucose, partial or even total removal of extracellular sodium fails to decrease the efflux of 45Ca. At normal extracellular calcium concentration (1 mM) partial removal of extracellular sodium dramatically increases 45Ca efflux from pancreatic islets. This increase in 45Ca efflux is partially but not totally suppressed by either 16.7 mM glucose or cobalt. It is totally suppressed by 4.4 mM glucose or by the combination of 16.7 mM glucose and cobalt. At normal extracellular calcium concentration, glucose initially reduces and subsequently increases 45Ca efflux. The initial fall is unaffected by tetrodotoxin but decreased by 50% at low extracellular sodium concentration (24 mM). The present results suggest the existence in pancreatic endocrine cells of a glucose-sensitive process of sodium-calcium counter-transport. By inhibiting such a process, glucose may decrease the efflux of calcium from islet cells. The effect of glucose is not mediated by an increase in intracellular sodium concentration. It could contribute to the intracellular accumulation of calcium which is thought to trigger insulin release.

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.

The stimulus-secretion coupling of glucose-induced insulin release. XLIII. Na-Ca countertransport mediated by pancreatic islet native ionophores

Native ionophores extracted from isolated pancreatic islets were able to transport Ca2+ from one aqueous medium into another across an organic immiscible phase. In the presence of a K+, Na+, Li+, or H+ gradient, Ca2+ was transported against its own concentration gradient from the medium of low monovalent-cation concentration to the opposite medium. The transport of Ca2+ was abolished by the organic calcium-antagonist suloctidil. These findings provide a model for the process of Na-Ca countertransport in islet cells and its inhibition in response to the conversion of nutrient secretagogues to their acidic metabolites.

Stimulus-response coupling in the human neutrophil. Transmembrane potential and the role of extracellular Na+

Receptor-ligand interactions at the surface of the human neutrophil induce lysosomal enzyme release and the generation of O2.-, responses which are anteceded by changes in the membrane potential (delta psi) as measured by [3H]-triphenylmethylphosphonium ion distribution. Surface stimuli (immune complexes, concanavalin A) initiated a rapid (less than 10 s) hyperpolarization response by both normal and cytochalasin B-treated cells. Replacement of extracellular Na+ with either K+ or choline depressed O2.- generation and lysosomal enzyme release in neutrophils exposed to concanavalin A or immune complexes. Replacement of Na+ with K+ led to a substantial fall in resting membrane potential, whereas replacement of Na+ with choline did not. Thus, depression of O2.- generation and lysosomal enzyme release in Na+-free medium were specifically due to a lack of extracellular Na+ and not to depolarization of the membrane. Although it has been shown that extracellular Na+, and possibly an influx of Na+, is required for optimal neutrophil function, neither depolarization nor Na+ influx per se was sufficient to activate fully these cells, since the Na+ ionophore, monensin, was not an effective stimulus for beta-glucuronidase release or O2.- generation. The hyperpolarization response to neutrophils exposed to immune complexes and to concanavalin A was greatly diminished in both high [K+] and [choline] buffers. Thus, extracellular Na+ was required for an optimal membrane potential response to receptor-ligand interaction. Since O2.- generation and lysosomal enzyme release in response to the Ca2+ ionophore, A23187, were also reduced in the absence of extracellular Na+, it was concluded that extracellular Na+ was also required after induction of Ca2+ fluxes. Ouabain (1 mM) had no effect on O2.- generation, lysosomal enzyme release or the hyperpolarization response to immune complexes, indicating that the hyperpolarization observed on stimulation cannot be due to the action of the electrogenic pump, (Na+ + K+)-ATPase. The experiments indicate that extracellular Na+ is required (1) in the delta psi response triggered by receptor-ligand interaction, and (2) at a step(s) subsequent to Ca2+ fluxes and common to O2.- generation and lysosomal enzyme release.

Metabolic, cationic and secretory effects of hypoglycemic sulfonylureas in pancreatic islets

Tolbutamide, gliclazide and glibenclamide failed to stimulate glucose oxidation in rat pancreatic islets. Tolbutamide also failed to stimulate pyruvate and palmitate oxidation and decreased the islet content of NAD(P)H and ATP. Tolbutamide stimulated 45Ca net uptake, inhibited 86Rb net uptake and tended to increase 22Na net uptake by the islets. The effect of theophylline upon islet function differed from that of tolbutamide by the magnitude of its insulinotropic action as a function of the glucose concentration, by its stimulant action upon the utilization of endogenous nutrients in islets deprived of glucose and by the lack of gross alteration in 45Ca and 86Rb net uptake. It is concluded that the insulinotropic effect of hypoglycemic sulfonylureas cannot be merely equated to a facilitation of nutrient metabolism or inactivation of phosphodiesterase in the islet cells. The primary action of these drugs could be to affect cations transport across the B-cell membrane.

Influence of the murine diabetes gene on rubidium ion efflux from perifused islets

Islets from diabetic C57BL/KsJ db/db mice and normal C57BL/KsJ +/+ mice were loaded with 86Rb+ and micro-perfused with nonradioactive medium for 25 min. The appearance of 86Rb+ in the effluent could be described as the sum of two exponential functions with difference proportionality constants. The rapid efflux component may have represented washout from the extracellular space, and had about the same proportionality constant in normal and diabetic mice. The slow efflux component probably reflected efflux across the islet cell plasma membranes. At 3 mmol/l D-glucose in the medium, the slow efflux was significantly retarded in diabetic as compared with normal mice. In normal mice, but not in diabetics, 20 mmol/l D-glucose inhibited the slow efflux component. It is concluded that the basal K+ permeability is decreased in KsJ db/db mouse islet cells, and that this abnormality may explain their persistant depolarization at low glucose concentrations.

Regulation of calcium fluxes in rat pancreatic islets: dissimilar effects of glucose and of sodium ion accumulation

1. Removal of extracellular K+ provoked a dramatic increase in 43Ca efflux from pancreatic islets prelabelled in the presence of glucose. Such an increase in 45Ca efflux was preceded by a modest and transient reduction in 45Ca efflux. The increase in 45Ca efflux was reduced when the islets were perifused either in the presence of glucose 16 . 7 mM or in the absence of extracellular Ca2+. It was abolished when the islets were perifused in the absence of extracellular Na+. 2. When the islets had been prelabelled in the absence as distinct from presence of glucose, the increase in 45Ca efflux observed on removal of extracellular K+ was of smaller amplitude. It was completely abolished when, in addition, Ca2+ was replaced by Co2+ in the perifusate. 3. Veratridine also provoked a dramatic increase in 45Ca efflux. This increase was slightly reduced when the perifusate contained glucose 5 . 6 mM and markedly reduced in the absence of extracellular Ca2+. 4. Both the removal of extracellular K+ or addition of veratridine had little or no effect on insulin release in the absence of glucose. A significant increase in insulin release was observed, however, in the presence of glucose. The increase in insulin release, due to removal of extracellular K+, was completely abolished at low extracellular Na+ concentration. Such as ionic manipulation failed to affect veratridine-induced insulin release. Tetrodotoxin failed to inhibit glucose-stimulated insulin release. 5. It is concluded that Na+ accumulation in islet cells due to either veratridine or removal of extracellular K+ provokes the release of Ca2+ from intracellular stores but fails to reproduce the effect of glucose to reduce 45Ca efflux and to stimulate insulin release.

The stimulus-secretion coupling of glucose-induced insulin release. Metabolism of glucose in K+-deprived islets

1. When pancreatic islets were exposed to a K(+)-free medium, the intracellular concentration of K(+) was decreased and that of Na(+) increased. 2. In the K(+)-deprived islets, the utilization of [5-(3)H]glucose, output of lactic acid and oxidation of [U-(14)C]-glucose were decreased by about 30-40% below the control values found at normal extracellular K(+) concentration (5.0mm). However, the oxidation of [U-(14)C]pyruvate was unaffected. 3. The omission of extracellular K(+) little affected the production of (14)CO(2) from islets prelabelled with [U-(14)C]palmitate and incubated in the absence of glucose, despite the fact that K(+) deprivation significantly increased the ATP concentration and ATP/ADP concentration ratio in the glucose-deprived islets. 4. At normal K(+) concentration, glucose increased the concentrations of phosphoenolpyruvate, NAD(P)H and ATP in the islets. In the glucose-stimulated islets, the concentration of phosphoenolpyruvate, but not that of either NAD(P)H or ATP, was higher in the absence than in the presence of extracellular K(+). In islet homogenates, the activity of pyruvate kinase (EC 2.7.1.40) was stimulated by K(+) (optimal activity at 100-150mm-K(+)) and inhibited by Na(+) (except at very low K(+) concentrations). 5. K(+) could be replaced by NH(4) (+), Rb(+), Cs(+) or Na(+) to maintain, at least to some extent, pyruvate kinase activity in islet homogenates. Addition of Rb(+) or Cs(+), but not NH(4) (+), to K(+)-deprived media also increased [U-(14)C]glucose oxidation by intact islets. 6. The omission of K(+) did not cause any obvious anomaly in the apparent dependency of (45)Ca(2+) net uptake on NAD(P)H concentration in the islets. 7. These data suggest that the coupling between metabolic and ionic events in the islet cells involves feedback mechanisms through which glucose oxidation may be modulated by cationic factors.