Low protein diet confers resistance to the inhibitory effects of interleukin 1beta on insulin secretion in pancreatic islets*

High protein content in the diet during childhood and adolescence has been associated to the onset insulin-dependent diabetes mellitus. We investigated the effect of interleukin-1beta (IL-1beta) on insulin secretion, glucose metabolism, and nitrite formation by islets isolated from rats fed with normal protein (NP, 17%) or low protein (LP, 6%) after weaning. Pretreatment of islets with IL-1beta for 1 h or 24 h inhibited the insulin secretion induced by glucose in both groups, but it was less marked in LP than in NP group. Islets from LP rats exhibited a decreased IL-1beta-induced nitric oxide (NO) production, lower inhibition of D-[U(14)C]-glucose oxidation to (14)CO(2) and less pronounced effect of IL-1beta on alpha-ketoisocaproic acid-induced insulin secretion than NP islets. However, when the islets were stimulated by high concentrations of K(+) the inhibitory effect of IL-1beta on insulin secretion was not different between groups. In conclusion, protein restriction protects beta-cells of the deleterious effect of IL-1beta, apparently, by decreasing NO production. The lower NO generation in islets from protein deprived rats may be due to increased free fatty acids oxidation and consequent alteration in Ca(2+) homeostasis.     

Amomum xanthoides extract prevents cytokine-induced cell death of RINm5F cells through the inhibition of nitric oxide formation

We previously showed that Amomum xanthoides extract prevented alloxan-induced diabetes through the suppression of NF-kappaB activation. In this study, the preventive effects of A. xanthoides extract on cytokine-induced beta-cell destruction were examined. Cytokines produced by immune cells infiltrating pancreatic islets are important mediators of beta-cell destruction in insulin-dependent diabetes mellitus. A. xanthoides extract completely protected interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma)-mediated cytotoxicity in rat insulinoma cell line (RINm5F). Incubation with A. xanthoides extract resulted in a significant reduction in IL-1beta and IFN-gamma-induced nitric oxide (NO) production, a finding that correlated well with reduced levels of the inducible form of NO synthase (iNOS) mRNA and protein. The molecular mechanism by which A. xanthoides extract inhibited iNOS gene expression appeared to involve the inhibition of NF-kappaB activation. Our results revealed the possible therapeutic value of A. xanthoides extract for the prevention of diabetes mellitus progression.     

Inhibition of fetal rat pancreatic beta-cell replication by interleukin-1 beta in vitro is not mediated through pertussis toxin-sensitive G-proteins, a decrease in cyclic AMP, or protease activation.

It has been proposed that the cytokine interleukin-1 beta (IL-1 beta), secreted by islet-infiltrating macrophages, may be involved in the pathogenesis of insulin-dependent diabetes mellitus by participation in beta-cell destruction. Addition of IL-1 beta to isolated pancreatic islets in vitro results in cytotoxic effects on beta-cell function, but there is little information on the intracellular events that convey the actions of the cytokine. In the present study, fetal rat pancreatic islets containing a high fraction of beta-cells were exposed in culture to IL-1 beta. It was found that IL-1 beta markedly decreased beta-cell DNA synthesis, insulin secretion and cyclic AMP content. In order to explore whether the decrease in cAMP resulted from IL-1 beta interaction with GTP-binding proteins coupled to adenylyl cyclase, islets were treated for 24 h with pertussis toxin prior to addition of cytokine. While this treatment restored the decrease in cAMP, the reduced DNA synthesis and insulin secretion persisted. Pertussis toxin treatment without the addition of IL-1 beta resulted in increases in cAMP, DNA synthesis and insulin secretion. Addition of the stimulatory cAMP analog Sp-cAMPS also increase DNA synthesis and insulin secretion, but failed to affect the decrease in these functions evoked by IL-1 beta. The protease inhibitor N alpha-p-tosyl-L-lysine chloromethyl ketone, recently shown to protect completely against IL-1 beta-induced suppression of insulin production and secretion, was found to markedly reduce DNA synthesis without affecting insulin secretion. When the protease inhibitor was combined with IL-1 beta, the suppressed secretion was counteracted while DNA synthesis inhibition was not. It is concluded that cAMP stimulates DNA synthesis and insulin secretion in beta-cells, but that the inhibitory effect of IL-1 beta on these functions cannot be ascribed to the decrease in cAMP evoked by the cytokine. However, the repressive effect of the cytokine on insulin secretion, but not DNA synthesis, may be prevented by protease inhibition.     

Requirements of calcium fluxes and ERK kinase activation for glucose- and interleukin-1β-induced β-cell apoptosis

Increasing evidence indicates that beta-cell apoptosis and impaired secretory function were partly mediated by interleukin (IL)-1beta and/or high-glucose-induced beta-cell production of IL-1beta. However, the specific signal transduction pathways and molecular events involved in beta-cell dysfunction remain largely unresolved. In this study, we investigated whether Ca(2+) and extracellular signal-regulated kinase (ERK) activation plays a role for IL-1beta action in rat islets. Exposure of rat islets for 4 days to 33.3 mM glucose and 140 ng/ml IL-1beta- induced beta-cell apoptosis and impaired glucose-stimulated insulin secretion. By Western blotting with phosphospecific antibodies, glucose and IL-1beta were shown to activate ERK. Ca(2+) channel blocker nimodipine or ERK inhibitor PD98059 prevented glucose- and IL-1beta-induced ERK activation, beta-cell apoptosis, and impaired function. Furthermore, treatment with Ca(2+) ionophore ionomycin, or exposure to thapsigargin, an inhibitor of sarco(endo)plasmic reticulum Ca(2+) ATPase, all caused an amplification of IL-1beta-induced ERK activation in rat islet. On the other hand, a chelator of intracellular free Ca(2+) [bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid-acetoxymethyl] (BAPTA/AM) and an inhibitor of calmodulin (W7) diminished IL-1beta-induced phosphorylation of ERK. Finally, islet release of IL-1beta in response to high glucose could be abrogated by nimodipine, mibefradil, or PD98059. Together, these data suggest that glucose- and IL-1beta-induced beta-cell secretory dysfunction and apoptosis are Ca(2+) influx and ERK dependent in rat islets.     

Ca2+-secretion coupling is impaired in diabetic Goto Kakizaki rats

The Goto Kakizaki (GK) rat is a widely used animal model to study defective glucose-stimulated insulin release in type-2 diabetes (T2D). As in T2D patients, the expression of several proteins involved in Ca(2+)-dependent exocytosis of insulin-containing large dense-core vesicles is dysregulated in this model. So far, a defect in late steps of insulin secretion could not be demonstrated. To resolve this apparent contradiction, we studied Ca(2+)-secretion coupling of healthy and GK rat beta cells in acute pancreatic tissue slices by assessing exocytosis with high time-resolution membrane capacitance measurements. We found that beta cells of GK rats respond to glucose stimulation with a normal increase in the cytosolic Ca(2+) concentration. During trains of depolarizing pulses, the secretory activity from GK rat beta cells was defective in spite of upregulated cell size and doubled voltage-activated Ca(2+) currents. In GK rat beta cells, evoked Ca(2+) entry was significantly less efficient in triggering release than in nondiabetic controls. This impairment was neither due to a decrease of functional vesicle pool sizes nor due to different kinetics of pool refilling. Strong stimulation with two successive trains of depolarizing pulses led to a prominent activity-dependent facilitation of release in GK rat beta cells, whereas secretion in controls was unaffected. Broad-spectrum inhibition of PKC sensitized Ca(2+)-dependent exocytosis, whereas it prevented the activity-dependent facilitation in GK rat beta cells. We conclude that a decrease in the sensitivity of the GK rat beta-cell to depolarization-evoked Ca(2+) influx is involved in defective glucose-stimulated insulin secretion. Furthermore, we discuss a role for constitutively increased activity of one or more PKC isoenzymes in diabetic rat beta cells.     

Single-channel Ba2+ currents in insulin-secreting cells are activated by glyceraldehyde stimulation

Single-channel Ba2+ current recordings have been made from the insulin-secreting cell line RINm5F with the patch-clamp technique. Depolarization evokes opening of Ca2+ (Ba2+) channels with a relatively high conductance (30 pS) and during the 200 ms depolarizing pulses there is no inactivation. The threshold is high as 50 mV depolarization from the resting membrane potential of -70 mV is required for any channel opening to occur. Glyceraldehyde, a substance evoking insulin secretion from the RINm5F cells, enhances the voltage-activated Ca2+ channel opening by increasing the mean open time and decreasing the longer of the two mean shut times and also decreases the voltage threshold for channel opening.     

Intracellular degradation of insulin and crinophagy are maintained by nitric oxide and cyclo-oxygenase 2 activity in isolated pancreatic islets

BACKGROUND INFORMATION: Pancreatic beta-cells require an optimal insulin content to allow instantaneous secretion of insulin. This is maintained by insulin biosynthesis and intracellular degradation of insulin. Degradation may be effected by crinophagy, i.e. the fusion of secretory granules with lysosomes. IL-1beta (interleukin 1beta) induces distinct changes of beta-cell lysosomes. To study the mechanisms for intracellular insulin degradation and crinophagy, isolated mouse pancreatic islets were exposed to IL-1beta and known pathways for IL-1beta actions were blocked. Intracellular insulin degradation was determined by following the fate of radioactively labelled insulin. Crinophagy was studied by ultrastructural analysis. The effects of blocking pathways for IL-1beta were monitored by measurements of nitrite and PGE(2) (prostaglandin E(2)). RESULTS: IL-1beta caused an enhancement of islet intracellular insulin degradation and an increase in the lysosomal incorporation of beta-cell secretory granules. The effects of IL-1beta were abolished by aminoguanidine, a selective inhibitor of inducible NOS (nitric oxide synthase), or by rofecoxib, a specific inhibitor of COX-2 (cyclo-oxygenase 2). In the absence of IL-1beta, nitroarginine, which is a selective inhibitor of constitutive NOS, caused a decrease in intracellular degradation of insulin in parallel with a decreased production of NO and PGE(2) by the islets. CONCLUSIONS: The correlation between the enhanced intracellular insulin degradation and lysosomal changes caused by IL-1beta suggests that insulin degradation may be effected by crinophagy. Under physiological conditions, significant beta-cell degradation of insulin may depend on the activity of COX-2, possibly stimulated by endogenous NO.     

Beta-cells in type 2 diabetes: a loss of function and mass

Type 2 diabetes mellitus manifests itself in individuals who lose the ability to produce sufficient amounts of insulin to maintain normoglycaemia in the face of insulin resistance. The ability to secrete adequate amounts of insulin depends on beta-cell function and mass. Chronic hyperglycaemia is detrimental to pancreatic beta-cells, causing impaired insulin secretion and playing an essential role in the regulation of beta-cell turnover. This paper will address the effect of chronically elevated glucose levels on beta-cell turnover and function. In previous studies we have shown that elevated glucose concentrations induce apoptosis in human beta-cells due to an interaction between constitutively expressed Fas ligand and upregulated Fas. Human beta-cells produce interleukin (IL)-1beta in response to high glucose concentrations, independently of an immune-mediated process. This was antagonized by the IL-1 receptor antagonist (IL-1Ra), a naturally occurring anti-inflammatory cytokine also found in the beta-cell. Therefore the balance of IL-1beta and IL-1Ra may play a crucial role in the pathogenesis of diabetes. Inhibition of glucotoxicity represents a promising therapeutic stratagem in diabetes therapy to preserve functional beta-cell mass.     

Phorbol ester-stimulated insulin secretion by RINm5F insulinoma cells is linked with membrane depolarization and an increase in cytosolic free Ca2+ concentration

In studying the regulation of insulin secretion by phorbol esters, we examined their effects on the cytosolic free Ca2+ concentration ([Ca2+]i), using the Ca2+ indicator fura-2 in the rat insulin-secreting beta-cell line RINm5F. [Ca2+]i was measured in parallel with the rate of insulin release. 50 nM 12-O-tetradecanoylphorbol-13-acetate (TPA), which may act via protein kinase C, stimulated insulin release and caused an increase in [Ca2+]i. Ca2+-free conditions eliminated the increase in [Ca2+]i and resulted in a reduced stimulation of insulin release by TPA. The Ca2+ channel blocker nitrendipine (300 nM) inhibited both the increase in [Ca2+]i and the increased rate of insulin secretion. Another phorbol ester, 4 beta-phorbol 12,13-didecanoate, which activates protein kinase C, also induced an increase in [Ca2+]i and in the rate of insulin release, while 4 alpha-phorbol 12,13-didecanoate, which fails to stimulate protein kinase C, was without effect. Further studies with bis-oxonol as an indicator of membrane potential showed that TPA depolarized the beta-cell plasma membrane. From these results, it is concluded that TPA depolarizes the plasma membrane, induces the opening of Ca2+ channels in the RINm5F beta-cell plasma membrane, increases [Ca2+]i, and results in insulin secretion. The action of TPA was next compared with that of a depolarizing concentration of KC1 (25 mM), which stimulates insulin secretion simply by opening Ca2+ channels. TPA consistently elicited less depolarization, a smaller rise of [Ca2+]i, but a greater release of insulin than KC1. Therefore an additional action of TPA is suggested, which potentiates the action of the elevated [Ca2+]i on insulin secretion.     

Glutamate inhibits protein phosphatases and promotes insulin exocytosis in pancreatic beta-cells

In human type 2 diabetes mellitus, loss of glucose-sensitive insulin secretion from the pancreatic beta-cell is an early pathogenetic event, but the mechanisms involved in glucose sensing are poorly understood. A messenger role has been postulated for L-glutamate in linking glucose stimulation to sustained insulin exocytosis in the beta-cell, but the precise nature by which L-glutamate controls insulin secretion remains elusive. Effects of L-glutamate on the activities of ser/thr protein phosphatases (PPase) and Ca(2+)-regulated insulin exocytosis in INS-1E cells were investigated. Glucose increases L-glutamate contents and promotes insulin secretion from INS-1E cells. L-glutamate also dose-dependently inhibits PPase enzyme activities analogous to the specific PPase inhibitor, okadaic acid. L-glutamate and okadaic acid directly and non-additively promote insulin exocytosis from permeabilized INS-1E cells in a Ca(2+)-independent manner. Thus, an increase in phosphorylation state, through inhibition of protein dephosphorylation by glucose-derived L-glutamate, may be a novel regulatory mechanism linking glucose sensing to sustained insulin exocytosis.     

Aspects of the involvement of interleukin-1 and nitric oxide in the pathogenesis of insulin-dependent diabetes mellitus

The possible involvement of the cytokine interleukin-1 (IL-1) and nitric oxide (NO) in the pathogenesis of insulin-dependent diabetes mellitus (IDDM) is reviewed and current and potential therapies are discussed. IDDM is a common disorder in the Western world and it is rising in incidence. In IDDM, islet-infiltrating macrophages produce IL-1 which is cytotoxic specifically to beta-cells in vitro. IL-1 increases beta-cell formation of NO, ceramide, prostaglandins, heat-shock proteins, and activates a protease. Additionally, IL-1 depresses beta-cell energy production, insulin gene expression and cyclic AMP synthesis, and impacts negatively on different parts of the insulin stimulus-secretion coupling, actions mimicked by NO. Conversely, blocking NO formation prevented many of these effects in most reports published. Also, changes in cyclic AMP and prostaglandins seem unlikely events in mediating the cytotoxicity of IL-1, while the role of ceramide remains less clear. Peptides capable of blocking beta-cell IL-1 receptors, and agents blocking NO synthesis may prove valuable in preserving beta-cell function in IDDM. Although IDDM causes immense morbidity and expense, uniformly effective preventive or beta-cell protective therapy is not currently available. If IL-1 is causing beta-cell dysfunction in human IDDM through NO production, several processes in the IL-1-NO connection are appropriate targets for agents protecting beta-cells from destruction and functional inhibition in IDDM.     

A K ATP channel-dependent pathway within alpha cells regulates glucagon release from both rodent and human islets of Langerhans

Glucagon, secreted from pancreatic islet alpha cells, stimulates gluconeogenesis and liver glycogen breakdown. The mechanism regulating glucagon release is debated, and variously attributed to neuronal control, paracrine control by neighbouring beta cells, or to an intrinsic glucose sensing by the alpha cells themselves. We examined hormone secretion and Ca(2+) responses of alpha and beta cells within intact rodent and human islets. Glucose-dependent suppression of glucagon release persisted when paracrine GABA or Zn(2+) signalling was blocked, but was reversed by low concentrations (1-20 muM) of the ATP-sensitive K(+) (KATP) channel opener diazoxide, which had no effect on insulin release or beta cell responses. This effect was prevented by the KATP channel blocker tolbutamide (100 muM). Higher diazoxide concentrations (>/=30 muM) decreased glucagon and insulin secretion, and alpha- and beta-cell Ca(2+) responses, in parallel. In the absence of glucose, tolbutamide at low concentrations (<1 muM) stimulated glucagon secretion, whereas high concentrations (>10 muM) were inhibitory. In the presence of a maximally inhibitory concentration of tolbutamide (0.5 mM), glucose had no additional suppressive effect. Downstream of the KATP channel, inhibition of voltage-gated Na(+) (TTX) and N-type Ca(2+) channels (omega-conotoxin), but not L-type Ca(2+) channels (nifedipine), prevented glucagon secretion. Both the N-type Ca(2+) channels and alpha-cell exocytosis were inactivated at depolarised membrane potentials. Rodent and human glucagon secretion is regulated by an alpha-cell KATP channel-dependent mechanism. We propose that elevated glucose reduces electrical activity and exocytosis via depolarisation-induced inactivation of ion channels involved in action potential firing and secretion.     

Lysophosphatidylcholine stimulates IL-1beta release from microglia via a P2X7 receptor-independent mechanism

IL-1beta released from activated macrophages contributes significantly to tissue damage in inflammatory, degenerative, and autoimmune diseases. In the present study, we identified a novel mechanism of IL-1beta release from activated microglia (brain macrophages) that occurred independently of P2X(7) ATP receptor activation. Stimulation of LPS-preactivated microglia with lysophosphatidylcholine (LPC) caused rapid processing and secretion of mature 17-kDa IL-1beta. Neither LPC-induced IL-1beta release nor LPC-stimulated intracellular Ca(2+) increases were affected by inhibition of P2X(7) ATP receptors with oxidized ATP. Microglial LPC-induced IL-1beta release was suppressed in Ca(2+)-free medium or during inhibition of nonselective cation channels with Gd(3+) or La(3+). It was also attenuated when Ca(2+)-activated K(+) channels were blocked with charybdotoxin (CTX). The electroneutral K(+) ionophore nigericin did not reverse the suppressive effects of CTX on LPC-stimulated IL-1beta release, demonstrating the importance of membrane hyperpolarization. Furthermore, LPC-stimulated caspase activity was unaffected by Ca(2+)-free medium or CTX, suggesting that secretion but not processing of IL-1beta is Ca(2+)- and voltage-dependent. In summary, these data indicate that the activity of nonselective cation channels and Ca(2+)-activated K(+) channels is required for optimal IL-1beta release from LPC-stimulated microglia.     

The role of ion channels in insulin secretion.

Ion channels in beta cells regulate electrical and secretory activity in response to metabolic, pharmacologic, or neural signals by controlling the permeability to K+ and Ca2+. The ATP-sensitive K+ channels act as a switch that responds to fuel secretagogues or sulfonylureas to initiate depolarization. This depolarization opens voltage-dependent calcium channels (VDCC) to increase the amplitude of free cytosolic Ca2+ levels ([Ca2+]i), which triggers exocytosis. Acetyl choline and vasopressin (VP) both potentiate the acute effects of glucose on insulin secretion by generating inositol 1,4,5-trisphosphate to release intracellular Ca2+; VP also potentiates sustained insulin secretion by effects on depolarization. In contrast, inhibitors of insulin secretion decrease [Ca2+]i by either hyperpolarizing the beta cell or by receptor-mediated, G-protein-coupled effects to decrease VDCC activity. Repolarization is initiated by voltage- and Ca(2+)-activated K+ channels. A human insulinoma voltage-dependent K+ channel cDNA was recently cloned and two types of alpha 1 subunits of the VDCC have been identified in insulin-secreting cell lines. Determining how ion channels regulate insulin secretion in normal and diabetic beta cells should provide pathophysiologic insight into the beta cell signal transduction defect characteristic of non-insulin dependent diabetes (NIDDM).     

Protein kinase A- and C-induced insulin release from Ca2+ -insensitive pools

Insulin secretion is known to depend on an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)). However, recent studies have suggested that insulin secretion can also be evoked in a Ca(2+)-independent manner. In the present study we show that treatment of intact mouse islets and RINm5F cells with protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) or protein kinase A (PKA) activator forskolin promoted insulin secretion with no changes of [Ca(2+)](i). Moreover, insulin secretion mediated by PMA or forskolin was maintained even when extracellular or cytosolic Ca(2+) was deprived by treatment of cells with ethylene glycol bis(beta-amino ethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) or 1,2-bis(2-amino phenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxy methyl ester) (BAPTA/AM) in RINm5F cells. The secretagogue actions of PMA and forskolin were blocked by GF109203X and H89, selective inhibitors for PKC and PKA, respectively. PMA treatment caused translocation of PKC-alpha and PKC- epsilon from cytosol to membrane, implying that selectively PKC-alpha and PKC- epsilon isoforms might be important for insulin secretion. Co-treatment with high K(+) and PMA showed a comparable level of insulin secretion to that of PMA alone. In addition, PMA and forskolin evoked insulin secretion in cells where Ca(2+)-dependent insulin secretion was completed. Our data suggest that PKC and PKA can elicit insulin secretion not only in a Ca(2+)-sensitive manner but also in a Ca(2+)-independent manner from separate releasable pools.     

Physiological development of insulin secretion, calcium channels, and GLUT2 expression of pancreatic rat beta-cells

Insulin secretion in mature beta-cells increases vigorously when extracellular glucose concentration rises. Glucose-stimulated insulin secretion depends on Ca(2+) influx through voltage-gated Ca(2+) channels. During fetal development, this structured response is not well established, and it is after birth that beta-cells acquire glucose sensitivity and a robust secretion. We compared some elements of glucose-induced insulin secretion coupling in beta-cells obtained from neonatal and adult rats and found that neonatal cells are functionally immature compared with adult cells. We observed that neonatal cells secrete less insulin and cannot sense changes in extracellular glucose concentrations. This could be partially explained because in neonates Ca(2+) current density and synthesis of mRNA alpha1 subunit Ca(2+) channel are lower than in adult cells. Interestingly, immunostaining for alpha1B, alpha1C, and alpha1D subunits in neonatal cells is similar in cytoplasm and plasma membrane, whereas it occurs predominantly in the plasma membrane in adult cells. We also observed that GLUT2 expression in adult beta-cells is mostly located in the membrane, whereas in neonatal cells glucose transporters are predominantly in the cytoplasm. This could explain, in part, the insensitivity to extracellular glucose in neonatal beta-cells. Understanding neonatal beta-cell physiology and maturation contributes toward a better comprehension of type 2 diabetes physiopathology, where alterations in beta-cells include diminished L-type Ca(2+) channels and GLUT2 expression that results in an insufficient insulin secretion.     

Delayed-rectifier (KV2.1) regulation of pancreatic beta-cell calcium responses to glucose: inhibitor specificity and modeling

The delayed-rectifier (voltage-activated) K(+) conductance (K(V)) in pancreatic islet beta-cells has been proposed to regulate plasma membrane repolarization during responses to glucose, thereby determining bursting and Ca(2+) oscillations. Here, we verified the expression of K(V)2.1 channel protein in mouse and human islets of Langerhans. We then probed the function of K(V)2.1 channels in islet glucose responses by comparing the effect of hanatoxin (HaTx), a specific blocker of K(V)2.1 channels, with a nonspecific K(+) channel blocker, tetraethylammonium (TEA). Application of HaTx (1 microM) blocked delayed-rectifier currents in mouse beta-cells, resulting in a 40-mV rightward shift in threshold of activation of the voltage-dependent outward current. In the presence of HaTx, there was negligible voltage-activated outward current below 0 mV, suggesting that K(V)2.1 channels form the predominant part of this current in the physiologically relevant range. We then employed HaTx to study the role of K(V)2.1 in the beta-cell Ca(2+) responses to elevated glucose in comparison with TEA. Only HaTx was able to induce slow intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations in cells stimulated with 20 mM glucose, whereas TEA induced an immediate rise in [Ca(2+)](i) followed by rapid oscillations. In human islets, HaTx acted in a similar fashion. The data were analyzed using a detailed mathematical model of ionic flux and Ca(2+) regulation in beta-cells. The results can be explained by a specific HaTx effect on the K(V) current, whereas TEA affects multiple K(+) conductances. The results underscore the importance of K(V)2.1 channel in repolarization of the pancreatic beta-cell plasma membrane and its role in regulating insulin secretion.