Na+-H+ exchange in the process of glucose-induced insulin release from the pancreatic B-cell. Effects of amiloride on 86Rb, 45Ca fluxes and insulin release

Dimethyl amiloride improves glucose homeostasis in mouse models of type 2 diabetes

Dimethyl amiloride (DMA) enhances insulin secretion in the pancreatic beta-cell. DMA also enhances time-dependent potentiation (TDP) and enables TDP to occur in situations where it is normally absent. As we have demonstrated before, these effects are mediated in part through inhibition of neuronal nitric oxide synthase (nNOS), resulting in increased availability of arginine. Thus both DMA and arginine have the potential to correct the secretory defect in diabetes by enabling or enhancing TDP. In the current study we have demonstrated the ability of these agents to improve blood glucose homeostasis in three mouse models of type 2 diabetes. The pattern of TDP under different conditions indicates that inhibition of NOS is not the only mechanism through which DMA exerts its positive effects. Thus we also have explored another possible mechanism through which DMA enables/enhances TDP, via the activation of mitochondrial alpha-ketoglutarate dehydrogenase.

Glucose-induced cytosolic pH changes in beta-cells and insulin secretion are not causally related: studies in islets lacking the Na+/H+ exchangeR NHE1

The contribution of Na(+)/H(+) exchange (achieved by NHE proteins) to the regulation of beta-cell cytosolic pH(c), and the role of pH(c) changes in glucose-induced insulin secretion are disputed and were examined here. Using real-time PCR, we identified plasmalemmal NHE1 and intracellular NHE7 as the two most abundant NHE isoforms in mouse islets. We, therefore, compared insulin secretion, cytosolic free Ca(2+) ([Ca(2+)](c)) and pH(c) in islets from normal mice and mice bearing an inactivating mutation of NHE1 (Slc9A1-swe/swe). The experiments were performed in HCO(-)(3)/CO(2) or HEPES/NaOH buffers. PCR and functional approaches showed that NHE1 mutant islets do not express compensatory pH-regulating mechanisms. NHE1 played a greater role than HCO(-)(3)-dependent mechanisms in the correction of an acidification imposed by a pulse of NH(4)Cl. In contrast, basal pH(c) (in low glucose) and the alkalinization produced by high glucose were independent of NHE1. Dimethylamiloride, a classic blocker of Na(+)/H(+) exchange, did not affect pH(c) but increased insulin secretion in NHE1 mutant islets, indicating unspecific effects. In control islets, glucose similarly increased [Ca(2+)](c) and insulin secretion in HCO(-)(3) and HEPES buffer, although pH(c) changed in opposite directions. The amplification of insulin secretion that glucose produces when [Ca(2+)](c) is clamped at an elevated level by KCl was also unrelated to pH(c) and pH(c) changes. All effects of glucose on [Ca(2+)](c) and insulin secretion proved independent of NHE1. In conclusion, NHE1 protects beta-cells against strong acidification, but has no role in stimulus-secretion coupling. The changes in pH(c) produced by glucose involve HCO(-)(3)-dependent mechanisms. Variations in beta-cell pH(c) are not causally related to changes in insulin secretion.

Amiloride derivatives enhance insulin release in pancreatic islets from diabetic mice

BACKGROUND

Amiloride derivatives, commonly used for their diuretic and antihypertensive properties, can also cause a sustained but reversible decrease of intracellular pH (pHi). Using dimethyl amiloride (DMA) on normal rodent pancreatic islets, we previously demonstrated the critical influence of islet pHi on insulin secretion. Nutrient-stimulated insulin secretion (NSIS) requires a specific pHi-range, and is dramatically enhanced by forced intracellular acidification with DMA. Furthermore, DMA can enable certain non-secretagogues to stimulate insulin secretion, and induce time-dependent potentiation (TDP) of insulin release in mouse islets where this function is normally absent. The present study was performed to determine whether pHi-manipulation could correct the secretory defect in islets isolated from mice with type 2 diabetes.

METHODS

Using two mouse models of type 2 diabetes, we compared a) pHi-regulation, and b) NSIS with and without treatment with amiloride derivatives, in islets isolated from diabetic mice and wild type mice.

RESULTS

A majority of the islets from the diabetic mice showed a slightly elevated basal pHi and/or poor recovery from acid/base load. DMA treatment produced a significant increase of NSIS in islets from the diabetic models. DMA also enabled glucose to induce TDP in the islets from diabetic mice, albeit to a lesser degree than in normal islets.

CONCLUSION

Islets from diabetic mice show some mis-regulation of intracellular pH, and their secretory capacity is consistently enhanced by DMA/amiloride. Thus, amiloride derivatives show promise as potential therapeutic agents for type 2 diabetes.

Effects of cholinergic m-receptor agonists on insulin release in islets from obese and lean mice of different ages: the importance of bicarbonate

OBJECTIVES

Decreased beta-cell function is often observed in older individuals and may predispose to the development of type 2 diabetes. We have studied the age-related effects of M-receptor agonism on insulin release in islets isolated from female ob/ ob and lean mice.

METHODS

Islets were challenged with 11.1 or 16.7 mmol/L glucose in media with HCO3/CO2 (KRBH) or without (KRH).

RESULTS

Acetylcholine (ACh) (10 micromol/L) increased glucose-induced insulin release in islets from 4- to 5-week-old ob/ob mice both in KRBH and KRH. In islets from 9- to 13-month-old ob/ob mice, 10 micromol/L ACh and 10 micromol/L carbachol enhanced insulin release in KRBH but not in KRH. ACh increased insulin release in islets from 4- to 5-week-old and 16-month-old lean mice incubated in KRH but not in islets from 24-month-old lean mice. The Na/H exchange inhibitor dimethylamiloride (100 micromol/L) did not affect insulin release stimulated by M-receptor agonists. Carbachol did not enhance glucose-induced insulin secretion in islets from 9- to 10-month-old ob/ob mice in the presence of low extracellular Na concentration. ACh stimulated cytoplasmic Ca mobilization in islets from 9- to 10-month-old mice also when bicarbonate was omitted. The results suggest that cholinergic signal transduction involving extracellular bicarbonate and Na is reduced with age in mouse pancreatic islets.

CONCLUSION

Chronic hyperglycemia may add to the age-related decrease in M-receptor-mediated insulin release by affecting the buffering capacity of the islets through mechanisms other than amiloride-sensitive proton exchange.

Nutrient-stimulated insulin secretion in mouse islets is critically dependent on intracellular pH

BACKGROUND: Many mechanistic steps underlying nutrient-stimulated insulin secretion (NSIS) are poorly understood. The influence of intracellular pH (pHi) on insulin secretion is widely documented, and can be used as an investigative tool. This study demonstrates previously unknown effects of pHi-alteration on insulin secretion in mouse islets, which may be utilized to correct defects in insulin secretion. METHODS: Different components of insulin secretion in mouse islets were monitored in the presence and absence of forced changes in pHi. The parameters measured included time-dependent potentiation of insulin secretion by glucose, and direct insulin secretion by different mitochondrial and non-mitochondrial secretagogues. Islet pHi was altered using amiloride, removal of medium Cl-, and changing medium pH. Resulting changes in islet pHi were monitored by confocal microscopy using a pH-sensitive fluorescent indicator. To investigate the underlying mechanisms of the effects of pHi-alteration, cellular NAD(P)H levels were measured using two-photon excitation microscopy (TPEM). Data were analyzed using Student's t test. RESULTS: Time-dependent potentiation, a function normally absent in mouse islets, can be unmasked by a forced decrease in pHi. The optimal range of pHi for NSIS is 6.4-6.8. Bringing islet pHi to this range enhances insulin secretion by all mitochondrial fuels tested, reverses the inhibition of glucose-stimulated insulin secretion (GSIS) by mitochondrial inhibitors, and is associated with increased levels of cellular NAD(P)H. CONCLUSIONS: Pharmacological alteration of pHi is a potential means to correct the secretory defect in non-insulin dependent diabetes mellitus (NIDDM), since forcing islet pHi to the optimal range enhances NSIS and induces secretory functions that are normally absent.

Intracellular pH plays a critical role in glucose-induced time-dependent potentiation of insulin release in rat islets

The underlying mechanisms of glucose-induced time-dependent potentiation in the pancreatic beta-cell are unknown. It had been widely accepted that extracellular Ca(2+) is essential for this process. However, we consistently observed glucose-induced priming under stringent Ca(2+)-free conditions, provided that the experiment was conducted in a HEPES-buffered medium as opposed to the bicarbonate (HCO(3)(-))-buffered medium used in previous studies. The critical difference between these two buffering systems is that islets maintain a lower intracellular pH in the presence of HEPES. The addition of HEPES to a HCO(3)(-)-buffered medium produced a dramatic decrease in the intracellular pH. If it is the lower intracellular pH in islets in a HEPES-buffered medium that is permissive for glucose-induced time-dependent potentiation (TDP), then experimental lowering of intracellular pH by other means should allow TDP to occur in a Ca(2+)-free HCO(3)(-)-buffered medium, where TDP normally does not occur. As expected, experimental acidification produced by dimethyl amiloride (DMA) allowed glucose to induce TDP in a Ca(2+)-free HCO(3)(-)-buffered medium. DMA also enhanced the priming normally present in HEPES-buffered media. Priming was also enhanced by transient acidification caused by acetate. Experimental alkalinization inhibited the development of priming. In the presence of Ca(2+), the magnitude of glucose-induced TDP was higher in a HEPES-buffered medium than in an HCO(3)(-)-buffered medium. In summary, glucose-induced priming was consistently observed under conditions of low intracellular pH and was inhibited with increasing intracellular pH, irrespective of the presence of extracellular Ca(2+). These data indicate that glucose-induced TDP is critically dependent on intracellular pH.

Multiphasic action of glucose and alpha-ketoisocaproic acid on the cytosolic pH of pancreatic beta-cells. Evidence for an acidification pathway linked to the stimulation of Ca2+ influx

Glucose stimulation raises the pHi of pancreatic beta-cells, but the underlying mechanisms are not well understood. We have now investigated the acute effects of metabolizable (glucose and the mitochondrial substrate alpha-ketoisocaproic acid, KIC) and nonmetabolizable (high K+ and the K-ATP channel blocker tolbutamide) insulin secretagogues on the pHi of pancreatic beta-cells isolated from normal mice, as assessed by BCECF fluorescence from single cells or islets in the presence of external bicarbonate. The typical acute effect of glucose (22-30 mM) on the pHi was a fast alkalinization of approximately 0.11 unit, followed by a slower acidification. The relative expression of the alkalinizing and acidifying components was variable, with some cells and islets displaying a predominant alkalinization, others a predominant acidification, and others yet a mixed combination of the two. The initial alkalinization preceded the [Ca2+]i rise associated with the activation of voltage-sensitive Ca2+ channels. There was a significant overlap between the glucose-evoked [Ca2+]i rise and the development of the secondary acidification. Depolarization with 30 mM K+ and tolbutamide evoked pronounced [Ca2+]i rises and concomitant cytosolic acidifications. Blocking glucose-induced Ca2+ influx (with 0 Ca2+, nifedipine, or the K-ATP channel agonist diazoxide) suppressed the secondary acidification while having variable effects (potentiation or slight attenuation) on the initial alkalinization. KIC exerted glucose-like effects on the pHi and [Ca2+]i, but the amplitude of the initial alkalinization was about twice as large for KIC relative to glucose. It is concluded that the acute effect of glucose on the pHi of pancreatic beta-cells is biphasic. While the initial cytosolic alkalinization is an immediate consequence of the activation of H+-consuming metabolic steps in the mitochondria, the secondary acidification appears to originate from enhanced Ca2+ turnover in the cytoplasm. The degree of coupling between glucose metabolism and Ca2+ influx as well as the relative efficacies of these processes determines whether the acute pHi response of a beta-cell (or of a tightly coupled multicellular system such as an islet of Langerhans) is predominantly an alkalinization, an acidification, or a mixed proportion of the two.

Voltage-dependent Na+ and Ca2+ currents in human pancreatic islet beta-cells: evidence for roles in the generation of action potentials and insulin secretion

We describe three voltage-dependent inward currents in human pancreatic beta-cells. First, a rapidly inactivating Na+ current, blocked by tetrodotoxin (TTX) is seen upon brief depolarization to or beyond -40 mV. Second, a transient, low-voltage-activated (LVA), amiloride-blockable Ca2+ current is seen upon depolarization to or beyond -55 mV; it inactivates within less than 1s of sustained depolarization to -40 mV. Third, a more sustained, high-voltage-activated (HVA) Ca2+ current, which shows variable sensitivity to dihydropyridines is seen upon depolarization to or beyond -40 mV, and thereafter slowly inactivates over a time course of many seconds. Our pharmacological evidence suggests that all three currents contribute to action potential initiation and upstroke when the background membrane potential (Vm) is equal or negative to -45 to -40 mV, a situation often induced by glucose concentrations (5-6 mM) in the range of those seen post-prandially. Consistent with this, TTX drastically reduces both transient and sustained insulin secretion in the presence of 5-6 mM glucose, but has little effect in 10 mM glucose, at which concentration cells rapidly depolarize to approximately -35 mV, a Vm sufficient to rapidly inactivate Na+ and LVA Ca2+ currents.

The role of metabolism, cytoplasmic Ca2+, and pH-regulating exchangers in glucose-induced rise of cytoplasmic pH in normal mouse pancreatic islets

Intact mouse islets were loaded with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein to study the effects of glucose on cytoplasmic pH (pHi) in pancreatic B-cells. In HCO3- buffer, glucose produced a steady-state increase in pHi that required metabolism of the sugar and was concentration-dependent between 0 and 10 mM (Km approximately 5 mM) before plateauing at a maximum value of approximately 0.2 pH units. In HEPES buffer, glucose concentrations above 7 mM caused an initial rise followed by a secondary decrease and an eventual return to about initial values. Inhibition of Ca2+ influx had little effect on the pHi changes produced by glucose in HCO3- medium, but unmasked an alkalinizing effect in HEPES buffer. Raising cytoplasmic Ca2+ by 30 mM potassium caused a larger acidification in HEPES than in HCO3- buffer, but a subsequent rise in glucose now increased pHi in both types of buffer. In the presence of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS; inhibitor of HCO3-/Cl- exchange), the effect of glucose on pHi in HCO3- buffer became similar to that in HEPES buffer. After inhibition of the Na+/H+ exchanger by dimethylamiloride, glucose produced a marked and sustained fall in pHi in HEPES buffer. A similar fall was seen in HCO3- buffer only when DIDS and dimethylamiloride were present together. However, if Ca2+ influx was prevented when both exchangers were blocked, glucose increased pHi. In conclusion, the metabolism of glucose tends to increase pHi in B-cells, whereas the concomitant rise in [Ca2+]i exerts an acidifying action. In HEPES buffer, this acidifying effect of Ca2+ is offset by the operation of the Na+/H+ exchanger. In physiological HCO3- buffer, the activity of the HCO3-/Cl- exchanger overcompensates and leads to an increase in pHi.

Inhibition of Na/Ca exchange in pancreatic islet cells by 3',4'-dichlorobenzamil

Na/Ca exchange may play a role in Ca2+ extrusion from the pancreatic B cell. The role played by the exchanger was examined by characterizing the effects of 3'-4'-dichlorobenzamil on ionic fluxes and insulin release in normal rat pancreatic islet cells. 3',4'-Dichlorobenzamil potently inhibited 45Ca uptake mediated by reverse Na/Ca exchange (IC50: 18 microM) in islet cells. The drug failed to decrease intracellular pH but reduced 86Rb outflow from perifused islets. The effects of glucose and 3',4'-dichlorobenzamil on 86Rb outflow were not additive. The drug potently blocked 45Ca uptake through voltage-sensitive Ca2+ channels (IC50: 7.5 microM). In the presence of extracellular Ca2+ and 3',4'-dichlorobenzamil, glucose lost part of its ability to reduce 45Ca outflow. The drug failed to affect the secondary rise in 45Ca outflow induced by the sugar. In the absence of extracellular Ca2+, 3',4'-dichlorobenzamil induced a delayed inhibition of 45Ca outflow, the effect of the sugar and the drug being not additive. This effect of 3',4'-dichlorobenzamil and its ability to impair the inhibitory effect of glucose were reproduced by the removal of extracellular Na+ and disappeared under the latter experimental condition. 3',4'-Dichlorobenzamil did not affect insulin release in the absence of glucose but significantly increased glucose-induced insulin release when used at a high concentration. It is concluded that 3',4'-dichlorobenzamil is a potent inhibitor of the process of Na/Ca exchange in the pancreatic B cell. Unfortunately, the drug is of poor specificity and blocks, in the same range of concentrations, both K+ channels and voltage-sensitive Ca2+ channels. The data also indicate that glucose inhibits 45Ca outflow from pancreatic islets to a great extent (at least 75%) by inhibiting Na/Ca exchange. The type of Na/Ca exchange that is inhibited by glucose, remains to be elucidated.

Calcium mobilization, prostaglandin E2 and alpha 2-adrenoceptor modulation of glucose utilization and insulin secretion in pancreatic islets

alpha 2-Adrenoceptor agonists inhibit glucose-stimulated insulin release and glucose utilization in pancreatic islets. In isolated pancreatic islets of the rat, the Ca2+ channel agonists CGP-28392 and BAY-K-8644 increased insulin release in the presence of clonidine. Neither CGP-28392 nor BAY-K-8644 antagonized the effect of clonidine on glucose utilization. The Ca2+ ionophore, ionomycin, also did not affect glucose utilization in the presence or absence of clonidine. Glucagon partly reversed the effects of clonidine on insulin release, and it potentiated glucose-stimulated insulin release in the absence of clonidine. Glucagon reversed the effects of clonidine on glucose utilization. Amiloride antagonized the effects of clonidine on insulin secretion but did not enhance markedly glucose utilization in the presence or absence of clonidine. Carbamylcholine and arecoline reversed the effects of clonidine on glucose utilization and partly reversed the effects on insulin release in the absence of extracellular Ca2+. Prostaglandin (PG) E2, but not PGF2 alpha, inhibited glucose utilization in a time- and concentration-dependent manner. PGE2 also inhibited glucose-stimulated insulin release. Pertussis toxin blocked both actions of PGE2. The cyclooxygenase inhibitor indomethacin did not affect insulin release or glucose utilization in the presence of clonidine. Thus, elevated intracellular Ca2+ levels antagonize the effects of clonidine on insulin release, whereas other mediators appear to be required to alter glucose utilization.

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.

Modulation by cytosolic pH of calcium and rubidium fluxes in rat pancreatic islets

Cytosolic pH (pHi) of pancreatic islet cells was assessed using the fluorescent dye 2'7'-biscarboxyethyl-5'(6')-carboxyfluorescein (BCECF). pHi was rapidly lowered by addition of the sodium salt of a weak acid or by treatment with amiloride. In the latter case, no recovery of pHi occurred. NH4Cl produced a rise in pHi. Stimulation of islet cells with glyceraldehyde produced a sustained fall in pHi, whereas glucose and alpha-ketoisocaproate caused a small, gradual rise in pHi. Intracellular acidification, particularly with amiloride, resulted in an immediate potentiation of glucose-induced insulin secretion from perifused islets. In the case of weak acid treatment, subsequent removal of the weak acid produced a paradoxical stimulation of insulin release which was not observed upon removal of amiloride. NH4Cl produced a transient stimulation followed by a reduction in the rate of glucose-induced insulin secretion. A reduction in pHi, either in response to weak acid or amiloride treatment, was associated with a diminution in the rate of efflux of 86Rb+ and of 45Ca2+. Removal of weak acid produced a marked "rebound" stimulation of 86Rb+ and 45Ca2+ efflux. Treatment of islets with NH4Cl, either in the presence or absence of glucose or Ca2+, resulted in a marked stimulation of efflux of 86Rb+ and 45Ca2+. The stimulatory effect of NH4Cl on 45Ca2+ efflux was markedly impaired in the absence of Na+. It is concluded that pHi can influence the secretory activity of pancreatic islets, possibly via effects on potassium permeability and sodium-calcium exchange across the plasma membrane, resulting in altered mobilisation of calcium in the islet cell. However, it is unlikely that glucose or other nutrient stimuli activate islets solely via an effect on pHi.

The gating of nucleotide-sensitive K+ channels in insulin-secreting cells can be modulated by changes in the ratio ATP4-/ADP3- and by nonhydrolyzable derivatives of both ATP and ADP

The 31P-NMR technique has been used to assess the intracellular ratios and concentrations of mobile ATP and ADP and the intracellular pH in an insulin-secreting cell line, RINm5F. The single-channel current-recording technique has been used to investigate the effects of changes in the concentrations of ATP and ADP on the gating of nucleotide-dependent K+ channels. Adding ATP to the membrane inside closes these channels. However, in the continued presence of ATP adding ADP invariably leads to the reactivation of ATP-inhibited K+ channels, even at ATP4-/ADP3- concentration ratios greater than 7:1. Interactions between ATP4- and ADP3- seem competitive. An increase in the concentration ratio ATP4-/ADP3- consistently evoked a decrease in the open-state probability of K+ channels; conversely, a decrease in ATP4-/ADP3- increased the frequency of K+ channel opening events. Channel gating was also influenced by changes in the absolute concentrations of ATP4- and ADP3-, at constant free concentration ratios. ADP-evoked stimulation of ATP-inhibited channels did not result from phosphorylation of the channel, as ADP-beta-S, a nonhydrolyzable analog of ADP, not only stimulated but enhanced ADP-induced activation of K+ channels, in the presence of ATP. Similarly, ADP was able to activate K+ channels in the presence of two nonhydrolyzable derivatives of ATP, AMP-PNP and beta gamma methylene ATP.

Glucose-induced changes in cytosolic ATP content in pancreatic islets

The cytosolic and mitochondrial contents in ATP, ADP and AMP were measured in islets incubated for 45 min at increasing concentrations of D-glucose and then exposed for 20 s to digitonin. The latter treatment failed to affect the total islet ATP/ADP ratio and adenylate charge. D-Glucose caused a much greater increase in cytosolic than mitochondrial ATP/ADP ratio. In the cytosol, a sigmoidal pattern characterized the changes in ATP/ADP ratio at increasing concentrations of D-glucose. These findings are compatible with the view that cytosolic ATP participates in the coupling of metabolic to ionic events in the process of nutrient-induced insulin release.