Inhibition of glucose-induced electrical activity by 4-hydroxytamoxifen in rat pancreatic beta-cells

The glucokinase activator GKA50 causes an increase in cell volume and activation of volume-regulated anion channels in rat pancreatic β-cells

Glucokinase plays a key role in the metabolism of glucose by pancreatic β-cells. In this study the effects of the glucokinase activator GKA50 on cell volume and electrical activity in rat β-cells were examined. One micro molar GKA50 caused an increase in β-cell volume in the presence of 4mM glucose. GKA50 also caused a depolarisation of β-cell membrane potential and increased electrical activity. These changes were associated with the activation of inward whole-cell currents, and were attenuated by the anion channel inhibitor 5-nitro-2-(3-phenylpropylamino) benzoic acid. In single channel experiments, the open probability of volume-regulated anion channels (VRAC) was increased from 0.03±0.01 to 0.19±0.04 (n=3) by the GKA50. The data suggest that a GKA50-evoked increase in glucose metabolism causes an increase in β-cell volume. This in turn activates VRAC leading to a depolarisation of the cell membrane potential.

Upregulation of ventricular potassium channels by chronic tamoxifen treatment

AIMS

Tamoxifen is a selective oestrogen receptor modulator widely used in the prevention and treatment of breast cancer. Women receiving long-term tamoxifen therapy do not experience cardiac arrhythmias although acute perfusion of tamoxifen has been shown to inhibit cardiac K(+) currents. This observation suggests that chronic tamoxifen treatment does not negatively modulate cardiac K(+) currents. Therefore, we investigated the chronic effects of tamoxifen on K(+) currents and channels in mouse and guinea pig ventricles.

METHODS AND RESULTS

Female mice and guinea pigs were treated with placebo or tamoxifen pellets for 60 days. Voltage-clamp experiments showed that the density of the Ca²(+)-independent transient outward (I(to)), the ultrarapid delayed rectifier (I(Kur)), the steady-state (I(ss)), and the inward rectifier (I(K1)) K(+) currents were increased in tamoxifen-treated mice ventricle. Western blot analysis revealed that protein expression of the underlying K(+) channels Kv4.3 (I(to)), Kv1.5 (I(Kur)), Kv2.1 (I(ss)), and Kir2.1 (I(K1)) were significantly higher in the ventricle of tamoxifen-treated mice. Protein expression of the K(+) channel subunits encoding I(Kr) and I(Ks) (ERG1, KCNQ1, and KCNE1) was also increased in tamoxifen-treated guinea pig ventricle.

CONCLUSION

Conditions with high oestrogen levels are associated with reduced K(+) currents. Thus, conceivably, tamoxifen might prevent the inhibitory effects of oestrogen on K(+) channels by blocking the oestrogen receptors, which would explain the reported increase in K(+) currents. These findings could contribute to explain the absence of cardiac arrhythmia with long-term tamoxifen therapy.

Opposing effects of tenidap on the volume-regulated anion channel and K(ATP) channel activity in rat pancreatic beta-cells

Tenidap (5-chloro-2-hydroxy-3-(thiophene-2-carbonyl)indole-1-carboxamide) is a non-steroidal anti-inflammatory and anti-rheumatic drug with several cellular actions including inhibition of anion transport processes. Since other anion transport inhibitors have been shown to inhibit activity of the volume-regulated anion channel (VRAC), the present study investigated the effects of tenidap on activity of this channel in pancreatic beta-cells. Membrane potential, VRAC currents and input conductance were recorded from single rat beta-cells in primary culture using perforated patch, conventional whole-cell and cell-attached configurations of the patch-clamp technique. Relative cell volume was measured using a video-imaging method. Tenidap (0.1mM) was found to rapidly hyperpolarise the beta-cell membrane potential and terminate glucose-induced electrical activity. This effect was associated with a pronounced outward current shift at a holding potential of -65mV. Tenidap was found to inhibit activity of the volume-regulated anion channel with IC(50) values of 31 and 43microM for outward and inward currents respectively. Tenidap also markedly increased beta-cell input conductance, representing an activation of the K(ATP) conductance. beta-cell regulatory volume decrease following hypotonically-induced cell swelling was sensitive to inhibition by 50microM tenidap. Tenidap is a potent inhibitor of the volume-regulated anion channel and K(ATP) channel activator in rat pancreatic beta-cells. These actions could at least in part explain the recently reported inhibitory actions of the drug on electrical and secretory activity in the beta-cell, and could also underlie other pharmacological actions of the drug.

4-Hydroxytamoxifen inhibits K(+) currents in mouse ventricular myocytes

Tamoxifen is a widely used chemotherapeutic agent, which has been associated with prolongation of the QT interval. Other studies have reported that acute exposure to tamoxifen can reduce cardiac K(+) currents. However, in vivo tamoxifen is largely metabolized and most of its activity is attributable to its major metabolite, 4-hydroxytamoxifen (4OH-tamoxifen). Accordingly, in the present study, we performed voltage-clamp experiments to directly investigate the effects of 4OH-tamoxifen on the repolarizing K(+) currents in adult mouse ventricular myocytes in order to determine whether the effects of tamoxifen on repolarization could be ascribed to 4OH-tamoxifen. K(+) currents were recorded before and after acute exposure to 4OH-tamoxifen (0.5, 1 and 10microM). 4OH-tamoxifen reduced the density of the Ca(2+)-independent transient outward (I(to)), the ultrarapid delayed rectifier (I(Kur)) and the inward rectifier (I(K1)) K(+) currents (by up to 43%, 41% and 26%, respectively) but had no significant effect on the steady-state outward K(+) current (I(ss)). Voltage dependence of steady-state inactivation and reactivation time of I(to) and I(Kur) were not affected by 4OH-tamoxifen. Experiments using the pure estrogen receptor antagonist, ICI 182,780 and the inhibitor of gene transcription, actinomycin D, were undertaken to assess the involvement of estrogen receptor. Administered alone these compounds did not affect the density of K(+) currents. Moreover, pretreatment of the cells with ICI 182,780 or actinomycin D did not prevent the inhibitory response to 4OH-tamoxifen. Overall, 4OH-tamoxifen reduced K(+) currents in mouse ventricle and this effect is unrelated to gene transcription and does not involve interaction of the drug with estrogen receptor.

Effect of fenretinide and low-dose tamoxifen on insulin sensitivity in premenopausal women at high risk for breast cancer

The prevalence of metabolic syndrome is increasing along with breast cancer incidence worldwide. Because fenretinide improves insulin action and glucose tolerance in insulin-resistant obese mice and because tamoxifen has shown to regulate several markers involved in metabolic syndrome, we sought to investigate the effect of fenretinide or tamoxifen at low dose on features linked to insulin resistance in premenopausal women at risk for breast cancer. We randomized 235 women to low-dose tamoxifen (5 mg/daily), fenretinide (200 mg/daily), or their combination or placebo for 2 years. We used the homeostasis model assessment (HOMA; fasting insulin x glucose/22.5) to estimate insulin sensitivity. Women were considered to improve insulin sensitivity when they shifted from a HOMA >/=2.8 to <2.8. There was no effect of fenretinide or tamoxifen on HOMA overall, but overweight women (body mass index, >or=25 kg/m(2)) had a 7-fold greater probability to normalize HOMA after 2 years of fenretinide treatment [odds ratio (OR), 7.0; 95% confidence interval (95% CI), 1.2-40.5], with 25% of women improving their insulin sensitivity, whereas tamoxifen decreased insulin sensitivity by almost 7 times compared with subjects not taking tamoxifen (OR, 0.15; 95% CI, 0.03-0.88). In this group only, 5% improved their insulin sensitivity. Interestingly, women with intraepithelial or microinvasive neoplasia had higher HOMA (3.0) than unaffected subjects (2.8; P = 0.07). Fenretinide can positively balance the metabolic profile in overweight premenopausal women and this may favorably affect breast cancer risk. Furthermore, features of the metabolic syndrome should be taken into consideration before proposing tamoxifen for breast cancer prevention. The clinical implications of these results require further investigations.

Possible role of carbonic anhydrase in rat pancreatic islets: enzymatic, secretory, metabolic, ionic, and electrical aspects

The presence of carbonic anhydrase (type V) was recently documented in rat and mouse pancreatic islet beta-cells by immunostaining and Western blotting. In the present study, the activity of carbonic anhydrase was measured in rat islet homogenates and shown to be about four times lower than in rat parotid cells. The pattern for the inhibitory action of acetazolamide on carbonic anhydrase activity also differed in islet and parotid cell homogenates, suggesting the presence of different isoenzymes. NaN3 inhibited carbonic anhydrase activity in islet homogenates and both D-[U-14C]glucose oxidation and glucose-stimulated insulin secretion. Acetazolamide (0.3-10.0 mM) also decreased glucose-induced insulin output but failed to affect adversely D-[U-14C]glucose oxidation, although it inhibited the conversion of D-[5-3H]glucose to [3H]OH and that of D-[U-14C]glucose to acidic metabolites. Hydrochlorothiazide (3.0-10.0 mM), which also caused a concentration-related inhibition of the secretory response, like acetazolamide (5.0-10.0 mM), decreased H(14)CO3- production from D-[U-14C]glucose (16.7 mM). Acetazolamide (5.0 mM) did not affect the activity of volume-sensitive anion channels in beta-cells but lowered intracellular pH and adversely affected both the bioelectrical response to d-glucose and its effect on the cytosolic concentration of Ca2+ in these cells. The lowering of cellular pH by acetazolamide, which could well be due to inhibition of carbonic anhydrase, might in turn account for inhibition of glycolysis. The perturbation of stimulus-secretion coupling in the beta-cells exposed to acetazolamide may thus involve impaired circulation in the pyruvate-malate shuttle, altered mitochondrial Ca2+ accumulation, and perturbation of Cl- fluxes, resulting in both decreased bioelectrical activity and insulin release.

Curcumin induces electrical activity in rat pancreatic beta-cells by activating the volume-regulated anion channel

Curcumin, the principal active component of turmeric, is reported to exert a number of therapeutic actions, including a hypoglycaemic/antidiabetic action. The underlying mechanisms to this action are essentially unknown. We have investigated the hypothesis that a direct stimulatory action on the pancreatic beta-cell could contribute towards the hypoglycaemic activity of this compound. Electrical and ion channel activity were recorded in rat beta-cells using the patch-clamp technique. beta-Cell volume was measured using a video-imaging technique. Insulin release was measured from intact islets by radioimmunoassay. Curcumin (2-10 microM) activated the volume-regulated anion channel in beta-cells. Single channel studies indicated that activation was the result of increased channel open probability. This effect was accompanied by depolarisation of the cell membrane potential, the generation of electrical activity and enhanced insulin release. Curcumin also decreased beta-cell volume, presumably reflecting loss of Cl(-) (and hence water) as a result of anion channel activation. These findings are consistent with the suggestion that Cl(-) fluxes play an important role in regulating beta-cell function. The stimulation of beta-cell function by curcumin could contribute to the hypoglycaemic actions of this compound, and these findings identify a novel potential therapeutic target for the treatment of type 2 diabetes mellitus.

Glucose-induced swelling in rat pancreatic alpha-cells

Pancreatic beta-cells increase in volume when exposed to elevated concentrations of extracellular glucose. This study has examined the effects of glucose on the volumes of pancreatic alpha-cells, which like beta-cells are regulated by glucose, and intestinal epithelial Caco-2 cells which are unresponsive to glucose. Cell volume changes were monitored by a video-imaging method. Increasing the extracellular glucose concentration caused a concentration-dependent increase in alpha-cell volume over the range 1-20mM. Glucose-induced swelling was not, however, observed in Caco-2 cells. The glucose-induced swelling in both alpha- and beta-cells was abolished by 0.5mM phloretin, an inhibitor of the GLUT proteins, indicating that GLUT mediated glucose transport is a pre-requisite for swelling. Glucose metabolism also appears to be essential, as islet cell swelling was not observed with 16 mM 3-O-methyl glucose. These data suggest that glucose-induced swelling may be a property exclusive to glucose-regulated cells.

Stimulus-secretion coupling of hypotonicity-induced insulin release in BRIN-BD11 cells

The stimulus-secretion coupling for hypotonicity-induced insulin release was investigated in BRIN-BD11 cells. A 50 mM decrease in extracellular NaCl caused a twofold increase in insulin release. The release of insulin evoked by hypotonicity progressively decreased in an exponential manner. The response to extracellular hypotonicity displayed a threshold value close to 20 mOsmol/L and a maximal response at about 70 mOsmol/ L. Hypotonicity also caused a rapid increase in cell volume followed by a regulatory volume decrease (RVD), cell membrane depolarization with induction of spike activity, and a rise in cytosolic Ca2+ concentration. 5-Nitro-2-(3-phenylpropylamino)benzoate inhibited the secretory response to hypoosmolarity, failed to affect the early increase in cell volume but prevented the RVD, and suppressed the hypotonicity-induced plasma membrane depolarization. Insulin release provoked by hypotonicity was inhibited by verapamil, absence of Ca2+, thapsigargin, furosemide, tributyltin, and diazoxide. On the contrary, tolbutamide augmented modestly insulin release recorded in the hypoosmolar medium. Last, a rise in extracellular K+ concentration, while augmenting basal insulin output, failed to affect insulin release in the hypoosmolar medium. Thus, the insulin secretory response to hypotonicity apparently represents a Ca2+-dependent process triggered by the gating of volume-sensitive anion channels with subsequent depolarization and gating of voltage-sensitive Ca2+ channels.

Glucose induces anion conductance and cytosol-to-membrane transposition of ICln in INS-1E rat insulinoma cells

The metabolic coupling of insulin secretion by pancreatic beta cells is mediated by membrane depolarization due to increased glucose-driven ATP production and closure of K(ATP) channels. Alternative pathways may involve the activation of anion channels by cell swelling upon glucose uptake. In INS-1E insulinoma cells superfusion with an isotonic solution containing 20 mM glucose or a 30% hypotonic solution leads to the activation of a chloride conductance with biophysical and pharmacological properties of anion currents activated in many other cell types during regulatory volume decrease (RVD), i.e. outward rectification, inactivation at positive membrane potentials and block by anion channel inhibitors like NPPB, DIDS, 4-hydroxytamoxifen and extracellular ATP. The current is not inhibited by tolbutamide and remains activated for at least 10 min when reducing the extracellular glucose concentration from 20 mM to 5 mM, but inactivates back to control levels when cells are exposed to a 20% hypertonic extracellular solution containing 20 mM glucose. This chloride current can likewise be induced by 20 mM 3-Omethylglucose, which is taken up but not metabolized by the cells, suggesting that cellular sugar uptake is involved in current activation. Fluorescence resonance energy transfer (FRET) experiments show that chloride current activation by 20 mM glucose and glucose-induced cell swelling are accompanied by a significant, transient redistribution of the membrane associated fraction of ICln, a multifunctional 'connector hub' protein involved in cell volume regulation and generation of RVD currents.

Glucose-induced electrical activity in rat pancreatic beta-cells: dependence on intracellular chloride concentration

A rise in glucose concentration depolarizes the beta-cell membrane potential leading to electrical activity and insulin release. It is generally believed that closure of KATP channels underlies the depolarizing action of glucose, though work from several laboratories has indicated the existence of an additional anionic mechanism. It has been proposed that glucose activates a volume-regulated anion channel, generating an inward current due to Cl- efflux. This mechanism requires that intracellular [Cl-] is maintained above its electrochemical equilibrium. This hypothesis was tested in rat beta-cells by varying [Cl-] in the patch pipette solution using the Cl--permeable antibiotic amphotericin B to allow Cl- equilibration with the cell interior. Under such conditions, a depolarization and electrical activity could be evoked by 16 mM glucose with pipette solutions containing 80 or 150 mM Cl-. At 40 or 20 mM Cl-, a subthreshold depolarization was usually observed, whilst further reduction to 12 or 6 mM abolished depolarization, in some cases leading to a glucose-induced hyperpolarization. With a pipette solution containing gramicidin, which forms Cl--impermeable pores, glucose induced a depolarization and electrical activity irrespective of [Cl-] in the pipette solution. Under the latter conditions, glucose-induced electrical activity was prevented by bumetanide, an inhibitor of the Na+-K+-2Cl- co-transporter. This inhibition could be overcome by the use of amphotericin B with a high [Cl-] pipette solution. These findings suggest that the maintenance of high intracellular [Cl-] in the beta-cell is an important determinant in glucose-induced depolarization, and support the hypothesis that beta-cell stimulation by glucose involves activation of the volume-regulated anion channel and generation of an inward Cl- current.

Effect of 17beta-estradiol on insulin secretion and cytosolic calcium in Min6 mouse insulinoma cells and human islets of Langerhans

OBJECTIVE

Female gonadal steroids can exert an insulinotropic effect in vivo. The objective of this study was to investigate the effects in vitro of 17-beta-estradiol (17beta-E2) on changes in cytosolic calcium ([Ca]i) and on insulin secretion from the MIN6 mouse insulinoma cell line and human primary islets of Langerhans.

METHODS

Stimulus-induced changes in [Ca]i were measured in Fura-2-loaded cells by single cell microfluorimetry. The effects of 17beta-E2 on insulin secretion were measured in static incubation experiments, and the rate and pattern of secretory responses were studied in multi-channel perifusion experiments.

RESULTS

17Beta-E2 (1-100 nmol/L) enhanced basal (2 mmol/L glucose) insulin secretion but had no effect on secretory responses to 20 mmol/L glucose or to depolarizing stimuli (100 micromol/L tolbutamide, 20 mmol/L KCl). Approximately 60% of MIN6 cells responded to 17beta-E2 (1-100 nmol/L) with a small but sustained increase in [Ca]i, whereas 98% of MIN6 cells responded to tolbutamide (100 micromol/L). Similar effects were observed in experiments using human primary beta cells. In contrast, 17beta-E2 had no detectable effect on the increases in [Ca]i evoked by tolbutamide (100 micromol/L) or glucose (20 mmol/L).

CONCLUSIONS

Our observations are consistent with a rapid effect of 17beta-E2 to depolarize beta cells leading to an influx of extracellular Ca and the initiation of insulin secretion by the consequent elevations in [Ca]i. We suggest that this may offer a mechanism through which circulating estradiol can influence beta-cell responsiveness to other signals.

Estrogen reduces carbachol-induced constriction of asthmatic airways by stimulating large-conductance voltage and calcium-dependent potassium channels

Both the incidence and severity of asthma in women are influenced by fluctuations in estrogen (E(2)) levels, raising the possibility that E(2)s may reduce the hyperresponsiveness that is characteristic of asthma. We examined the effect of E(2) and its downstream signaling pathways in isolated mouse bronchial and tracheal rings passively sensitized either with serum from patients with atopic asthma (ATR) or with serum from control subjects (CTR). ATR exhibited significantly higher sensitivity to carbachol than CTR. Pretreatment of ATR with E(2) shifted the carbachol concentration-response curve (CCRC) toward that of CTR. The E(2) effect was abolished by the nitric oxide synthase inhibitor, L-nitroarginine methyl ester, the soluble guanyl cyclase inhibitor, quinoxalin-1, or the protein kinase G inhibitor, KT5823. Inhibition of the large-conductance, calcium-activated potassium (BK(Ca)) channel activity with iberiotoxin also attenuated the E(2) effect on ATR. In patch-clamp studies, E(2) increased by 50-fold the BK(Ca) channel activity in freshly isolated airway smooth muscle cells. This increase was completely blocked by KT5823. These studies suggest that, at physiologic concentrations, E(2) can prevent cholinergic-induced constriction of asthmatic tracheal rings by activating the nitric oxide-cGMP-protein kinase G pathway to increase BK(Ca) channel activity.

Inhibition of glucose-induced electrical activity in rat pancreatic beta-cells by DCPIB, a selective inhibitor of volume-sensitive anion currents

We have investigated the effects of the ethacrynic acid derivative 4-(2-butyl-6,7-dichloro-2-cyclopentyl-indan-1-on-5-yl) oxobutyric acid (DCPIB), an inhibitor of the volume-sensitive anion channel (VSAC), on electrical activity and insulin secretion in rat pancreatic beta-cells. DCPIB inhibited whole-cell VSAC currents in beta-cells with IC50 values of 2.2 and 1.7 microM for inhibition of outward and inward currents, respectively. DCPIB also inhibited the VSAC at the single channel level in cells activated by glucose. In intact cells, DCPIB caused a net increase in beta-cell input conductance and evoked an outward current that was sensitive to inhibition by tolbutamide, suggesting KATP channel activation. However, no KATP channel activation was evident under conventional whole-cell conditions, suggesting that the drug might activate the channel in intact cells via an indirect mechanism, possibly involving nutrient metabolism. DCPIB suppressed glucose-induced electrical activity in beta-cells, hyperpolarised the cell membrane potential at a substimulatory glucose concentration and prevented depolarisation when the glucose concentration was raised to stimulatory levels. The suppression of electrical activity by DCPIB was associated with a marked inhibition of glucose-stimulated insulin release from intact islets. It is concluded that DCPIB inhibits electrical and secretory activity in the beta-cell as a combined result of a reciprocal inhibition of VSAC and activation of KATP channel activities, thus producing a marked hyperpolarisation of the beta-cell membrane potential.