Glucagon-like peptide I increases cytoplasmic calcium in insulin-secreting beta TC3-cells by enhancement of intracellular calcium mobilization

In the insulin-secreting beta-cell line beta TC3, stimulation with 11.2 mmol/l glucose caused a rise in the intracellular free Ca2+ concentration ([Ca2+]i) in only 18% of the tested cells. The number of glucose-responsive cells increased after pretreatment of the cells with glucagon-like peptide I (GLP-I)(7-36)amide and at 10(-11) mol/l; 84% of the cells responded to glucose with a rise in [Ca2+]i. GLP-I(7-36)amide induces a rapid increase in [Ca2+]i only in cells exposed to elevated glucose concentrations (> or = 5.6 mmol/l). The action of GLP-I(7-36)amide and forskolin involved a 10-fold increase in cytoplasmic cAMP concentration and was mediated by activation of protein kinase A. It was not associated with an effect on the membrane potential but required some (small) initial entry of Ca2+ through voltage-dependent L-type Ca2+ channels, which then produced a further increase in [Ca2+]i by mobilization from intracellular stores. The latter effect reflected Ca(2+)-induced Ca2+ release and was blocked by ryanodine. Similar increases in [Ca2+]i were also observed in voltage-clamped cells, although there was neither activation of a background (Ca(2+)-permeable) inward current nor enhancement of the voltage-dependent L-type Ca2+ current. These observations are consistent with GLP-I(7-36) amide inducing glucose sensitivity by promoting mobilization of Ca2+ from intracellular stores. We propose that this novel action of GLP-I(7-36)amide represents an important factor contributing to its insulinotropic action.

Co-localization of L-type Ca2+ channels and insulin-containing secretory granules and its significance for the initiation of exocytosis in mouse pancreatic B-cells

We have monitored L-type Ca2+ channel activity, local cytoplasmic Ca2+ transients, the distribution of insulin-containing secretory granules and exocytosis in individual mouse pancreatic B-cells. Subsequent to the opening of the Ca2+ channels, exocytosis is initiated with a latency < 100 ms. The entry of Ca2+ that precedes exocytosis is unevenly distributed over the cell and is concentrated to the region with the highest density of secretory granules. In this region, the cytoplasmic Ca2+ concentration is 5- to 10-fold higher than in the remainder of the cell reaching concentrations of several micromolar. Single-channel recordings confirm that the L-type Ca2+ channels are clustered in the part of the cell containing the secretory granules. This arrangement, which is obviously reminiscent of the 'active zones' in nerve terminals, can be envisaged as being favourable to the B-cell as it ensures that the Ca2+ transient is maximal and restricted to the part of the cell where it is required to rapidly initiate exocytosis whilst at the same time minimizing the expenditure of metabolic energy to subsequently restore the resting Ca2+ concentration.

Activation of protein kinases and inhibition of protein phosphatases play a central role in the regulation of exocytosis in mouse pancreatic beta cells

The mechanisms that regulate insulin secretion were investigated using capacitance measurements of exocytosis in single beta cells maintained in tissue culture. Exocytosis was stimulated by voltage-clamp depolarizations to activate the voltage-dependent Ca2+ channels that mediate Ca2+ influx into the beta cell. Under basal conditions, the exocytotic responses were small despite large Ca2+ currents. The exocytotic responses were dramatically increased (10- to 20-fold) by conditions that promote protein phosphorylation, such as activation of protein kinases A and C or inhibition of protein phosphatases. The stimulation of secretion was not due to an enhancement of Ca2+ influx and both peak and integrated Ca2+ currents were largely unaffected. Our data indicate that exocytosis in the insulin-secreting pancreatic beta cell is determined by a balance between protein phosphorylation and dephosphorylation. They further suggest that although Ca2+ is required for the initiation of exocytosis, modulation of exocytosis by protein kinases and phosphatases, at a step distal to the elevation of Ca2+, is of much greater quantitative importance. Thus an elevation of Ca2+ may represent a permissive rather than a decisive factor in the regulation of the insulin secretory process.

Ion channels, electrical activity and insulin secretion

The insulin-secreting pancreatic beta cell is electrically excitable and changes in the membrane potential play an important role in coupling the metabolism of glucose (and other nutrient secretagogues) to the discharge of the insulin-containing granule. The application of the patch-clamp technique, which permits the recordings of the minute currents associated with the opening of individual ion channels, to pancreatic islet cells has revolutionized our understanding of the beta cell electrophysiology. Here we review some of the recent progress in the field. The properties of functionally important ion channels are described and their possible roles are discussed.

Stimulus-secretion coupling in pancreatic beta cells

Insulin secretion is triggered by a rise in the intracellular Ca2+ concentration that results from the activation of voltage-gated Ca2+ channels in the beta-cell plasma membrane. Multiple types of beta-cell Ca2+ channel have been identified in both electrophysiological and molecular biological studies, but it appears that the L-type Ca2+ channel plays a dominant role in regulating Ca2+ influx. Activity of this channel is potentiated by protein kinases A and C and is inhibited by GTP-binding proteins, which may mediate the effects of potentiators and inhibitors of insulin secretion on Ca2+ influx, respectively. The mechanisms by which elevation of intracellular Ca2+ leads to the release of insulin granules is not fully understood but appears to involve activation of Ca2+/calmodulin-dependent protein kinase. Phosphorylation by either protein kinase A or C, probably at different substrates, potentiates insulin secretion by acting at some late stage in the secretory process. There is also evidence that small GTP-binding proteins are involved in regulating exocytosis in beta cells. The identification and characterisation of the proteins involved in exocytosis in beta cells and clarification of the mechanism(s) of action of Ca2+ is clearly an important goal for the future.

Exocytosis elicited by action potentials and voltage-clamp calcium currents in individual mouse pancreatic B-cells

1. Measurements of membrane capacitance, as an indicator of exocytosis, and intracellular Ca2+ concentration ([Ca2+]i) were used to determine the Ca2+ dependence of secretion in single pancreatic B-cells. 2. Exocytosis was dependent on a rise in [Ca2+]i and could be evoked by activation of voltage-dependent Ca2+ currents. The threshold for depolarization-induced release was 0.5 microM [Ca2+]i. Once the [Ca2+]i threshold was exceeded, exocytosis was rapidly (< 50 ms) initiated. When individual pulses were applied, exocytosis stopped immediately upon repolarization and the Ca2+ channels closed, although [Ca2+]i remained elevated for several seconds. 3. During repetitive stimulation (1 Hz), when [Ca2+]i attained micromolar levels, exocytosis also took place during the interpulse intervals albeit at a slower rate than during the depolarizations. 4. Exocytosis could be initiated by simulated action potentials. Whereas a single action potential only produced a small capacitance increase, and in some cells even failed to stimulate release, larger and more consistent responses were obtained with > or = four action potentials. 5. Comparison of the rates of exocytosis measured in response to depolarization, mobilization of Ca2+ from intracellular stores or infusion of Ca2+ through the patch pipette suggests that [Ca2+]i at the secretory sites attains a concentration of several micromolar. This is much higher than the average [Ca2+]i detected by microfluorimetry suggesting the existence of steep spatial gradients of [Ca2+]i within the B-cell. 6. Inclusion of inhibitors of Ca2+/calmodulin-dependent protein kinase II in the intracellular solution reduced the depolarization-induced exocytotic responses suggesting this enzyme may be involved in the coupling between elevation of [Ca2+]i to stimulation of the secretory machinery. 7. The size of the unitary exocytotic event was 2 fF, corresponding to a secretory granule diameter of 250 nm. 8. Over short periods, exocytosis may be extremely fast (1 pF/s or 500 granules/s), which is much higher than the rate of endocytosis (18 fF/s or 9 granules/s). Since the latter is in better agreement with the maximum rate of insulin secretion from islets (approximately 2 granules/s), we suggest that membrane retrieval may set an upper limit on the rate of exocytosis during extended periods of secretion.

Enhanced stimulus-secretion coupling in polyamine-depleted rat insulinoma cells. An effect involving increased cytoplasmic Ca2+, inositol phosphate generation, and phorbol ester sensitivity

To extend previous observations on the role of polyamines in insulin production, metabolism, and replication of insulin-secreting pancreatic beta cells, we have studied the role of polyamines in the regulation of the stimulus-secretion coupling of clonal rat insulinoma cells (RINm5F). For this purpose, RINm5F cells were partially depleted in their polyamine contents by use of the specific ornithine decarboxylase inhibitor difluoromethylornithine (DFMO), which led to an increase in cellular insulin and ATP contents. Analysis of different parts of the signal transduction pathway revealed that insulin secretion and the increase in cytoplasmic free Ca2+ concentration ([Ca2+]i) after K(+)-induced depolarization were markedly enhanced in DFMO-treated cells. These effects were paralleled by increased voltage-activated Ca2+ currents, as judged by whole-cell patch-clamp analysis, probably reflecting increased channel activity rather than elevated number of channels per cell. DFMO treatment also rendered phospholipase C in these cells more sensitive to the muscarinic receptor agonist carbamylcholine, as evidenced by enhanced generation of inositol phosphates, increase in [Ca2+]i and insulin secretion, despite an unaltered ligand binding to muscarinic receptors and lack of effect on protein kinase C activity. In addition, the tumor promoter 12-O-tetradecanoylphorbol 13-acetate, at concentrations suggested to be specific for protein kinase C activation, evoked an increased insulin output in polyamine-deprived cells compared to control cells. The stimulatory effects of glucose or the cyclic AMP raising agent theophylline on insulin release were not increased by DFMO treatment. In spite of increased binding of sulfonylurea in DFMO-treated cells, there was no secretory response or altered increase in [Ca2+]i in response to the drug in these cells. It is concluded that partial polyamine depletion sensitizes the stimulus-secretion coupling at multiple levels in the insulinoma cells, including increased voltage-dependent Ca2+ influx and enhanced responsiveness to activators of phospholipase C and protein kinase C. In their entirety, our present results indicate that the behavior of the stimulus-secretion coupling of polyamine-depleted RINm5F insulinoma cells changes towards that of native beta cells, thus improving the usefulness of this cell line for studies of beta cell insulin secretion.

Increased activity of L-type Ca2+ channels exposed to serum from patients with type I diabetes

Type I diabetes [insulin-dependent diabetes mellitus (IDDM)] is an autoimmune disease associated with the destruction of pancreatic beta cells. Serum from patients with IDDM increased L-type calcium channel activity of insulin-producing cells and of GH3 cells derived from a pituitary tumor. The subsequent increase in the concentration of free cytoplasmic Ca2+ ([Ca2+]i) was associated with DNA fragmentation typical of programmed cell death or apoptosis. These effects of the serum were prevented by adding a blocker of voltage-activated L-type Ca2+ channels. When the serum was depleted of immunoglobulin M (IgM), it no longer affected [Ca2+]i. An IgM-mediated increase in Ca2+ influx may thus be part of the autoimmune reaction associated with IDDM and contribute to the destruction of beta cells in vivo.

Stimulation of the KATP channel by ADP and diazoxide requires nucleotide hydrolysis in mouse pancreatic beta-cells

1. The mechanisms by which ADP and the hyperglycaemic compound diazoxide stimulate the activity of the ATP-regulated K+ channel (KATP channel) were studied using inside-out patches isolated from mouse pancreatic beta-cells maintained in tissue culture. 2. The ability of diazoxide and ADP to increase KATP channel activity declined with time following patch excision and no stimulation was observed after 15-40 min. 3. Activation of KATP channels by ADP required the presence of intracellular Mg2+. The stimulatory effect of ADP was mimicked by AMP but only in the presence of ATP. Replacement of ATP with the non-hydrolysable analogue beta, gamma-methylene ATP did not interfere with the ability of ADP to stimulate KATP channel activity. By contrast, enhancement of KATP channel activity was critically dependent on hydrolysable ADP and no stimulation was observed after substitution of alpha,beta-methylene ADP for standard ADP. 4. The ability of diazoxide to enhance KATP channel activity was dependent on the presence of both internal Mg2+ and ATP. Diazoxide stimulation of KATP channel activity was not observed after substitution of beta,gamma-methylene ATP for ATP. However, in the presence of ADP, at a concentration which in itself had no stimulatory action (10 microM), diazoxide was stimulatory also in the presence of the stable ATP analogue. 5. The stimulatory action of diazoxide on KATP channel activity in the presence of ATP was markedly enhanced by intracellular ADP. This potentiating effect of ADP was not reproduced by the stable analogue alpha,beta-methylene ADP and was conditional on the presence of intracellular Mg2+. A similar enhancement of channel activity was also observed with AMP (0.1 mM). In the absence of ATP, diazoxide was still capable of stimulating channel activity provided ADP was present. This effect was not reproduced by AMP. 6. In both nucleotide-free solution and in the presence of 0.1 mM ATP, the distribution of the KATP channel open times were described by a single exponential with a time constant of approximately 20 ms. Addition of ADP or diazoxide resulted in the appearance of a second component with a time constant of > 100 ms which comprised 40-70% of the total number of events. Under the latter experimental conditions, the open probability of the channel increased more than fivefold relative to that observed in the presence of ATP alone.(ABSTRACT TRUNCATED AT 400 WORDS)

Demonstration of a novel apamin-insensitive calcium-activated K+ channel in mouse pancreatic B cells

The whole-cell configuration of the patch-clamp technique was used to characterize the biophysical and pharmacological properties of an oscillating K(+)-current that can be induced by intracellular application of GTP[gamma S] in mouse pancreatic B cells (Ammälä et al. 1991). These K+ conductance changes are evoked by periodic increases in the cytoplasmic Ca2+ concentration ([Ca2+]i) and transiently repolarize the B cell, thus inhibiting action-potential firing and giving rise to a bursting pattern. GTP[gamma S]-evoked oscillations in K+ conductance were reversibly suppressed by a high (300 microM) concentration of carbamylcholine. By contrast, alpha 2-adrenoreceptor stimulation by 20 microM clonidine did not interfere with the oscillatory behaviour but evoked a small sustained outward current. At 0 mV membrane potential, the oscillating K(+)-current elicited by GTP[gamma S] was highly sensitive to extracellular tetraethylammonium (TEA; 70% block by 1 mM). The TEA-resistant component, which carried approximately 80% of the current at -40 mV, was affected neither by apamin (1 microM) nor by tolbutamide (500 microM). The current evoked by internal GTP[gamma S] was highly selective for K+, as demonstrated by a 51-mV change in the reversal potential for a sevenfold change in [K+]o. Stationary fluctuation analysis indicated a unitary conductance of 0.5 pS when measured with symmetric (approximately 140 mM) KCl solutions. The estimated single-channel conductance with physiological ionic gradients is 0.1 pS. The results indicate the existence of a novel Ca(2+)-gated K+ conductance in pancreatic B cells. Activation of this K+ current may contribute to the generation of the oscillatory electrical activity characterizing the B cell at intermediate glucose concentrations.

Cytoplasmic calcium transients due to single action potentials and voltage-clamp depolarizations in mouse pancreatic B-cells

Changes in the cytoplasmic free calcium concentration ([Ca2+]i) in pancreatic B-cells play an important role in the regulation of insulin secretion. We have recorded [Ca2+]i transients evoked by single action potentials and voltage-clamp Ca2+ currents in isolated B-cells by the combination of dual wavelength emission spectrofluorimetry and the patch-clamp technique. A 500-1000 ms depolarization of the B-cell from -70 to -10 mV evoked a transient rise in [Ca2+]i from a resting value of approximately 100 nM to a peak concentration of 550 nM. Similar [Ca2+]i changes were associated with individual action potentials. The depolarization-induced [Ca2+]i transients were abolished by application of nifedipine, a blocker of L-type Ca2+ channels, indicating their dependence on influx of extracellular Ca2+. Following the voltage-clamp step, [Ca2+]i decayed with a time constant of approximately 2.5 s and summation of [Ca2+]i occurred whenever depolarizations were applied with an interval of less than 2 s. The importance of the Na(+)-Ca2+ exchange for B-cell [Ca2+]i maintenance was evidenced by the demonstration that basal [Ca2+]i rose to 200 nM and the magnitude of the depolarization-evoked [Ca2+]i transients was markedly increased after omission of extracellular Na+. However, the rate by which [Ca2+]i returned to basal was not affected, suggesting the existence of additional [Ca2+]i buffering processes.

Inhibition of L-type calcium channels by internal GTP [gamma S] in mouse pancreatic beta cells

Pretreatment of pancreatic beta cells with pertussis toxin resulted in a 30% increase in peak whole-cell Ca2+ currents recorded in the absence of exogenous intracellular guanine nucleotides. Intracellular application of 90 microM GTP[gamma S], by liberation from a caged precursor, resulted in 40% reduction of the peak Ca2+ current irrespective of whether the current was carried by Ca2+ or Ba2+. Effects on the delayed outward K+ current were small and restricted to a transient Ca(2+)-dependent K+ current component. Inhibition by GTP[gamma S] of the Ca2+ current was not mimicked by standard GTP and could not be prevented either by pretreatment with pertussis toxin or by inclusion of GDP[beta S] or cyclic AMP in the intracellular medium. The inhibitory effect of GTP[gamma S] could be counteracted by a prepulse to a large depolarizing voltage. A similar effect of a depolarizing prepulse was observed in control cells with no exogenous guanine nucleotides. These observations indicate that inhibition of beta cell Ca2+ current by G protein activation results from direct interaction with the channel and does not involve second-messenger systems. Our findings also suggest that the beta cell Ca2+ current is subject to resting inhibition by G proteins.

Inositol trisphosphate-dependent periodic activation of a Ca(2+)-activated K+ conductance in glucose-stimulated pancreatic beta-cells

Glucose-stimulated insulin secretion is associated with the appearance of electrical activity in the pancreatic beta-cell. At intermediate glucose concentrations, beta-cell electrical activity follows a characteristic pattern of slow oscillations in membrane potential on which bursts of action potentials are superimposed. The electrophysiological background of the bursting pattern remains unestablished. Activation of Ca(2+)-activated large-conductance K+ channels (KCa channel) has been implicated in this process but seems unlikely in view of recent evidence demonstrating that the beta-cell electrical activity is unaffected by the specific KCa channel blocker charybdotoxin. Another hypothesis postulates that the bursting arises as a consequence of two components of Ca(2+)-current inactivation. Here we show that activation of a novel Ca(2+)-dependent K+ current in glucose-stimulated beta-cells produces a transient membrane repolarization. This interrupts action potential firing so that action potentials appear in bursts. Spontaneous activity of this current was seen only rarely but could be induced by addition of compounds functionally related to hormones and neurotransmitters present in the intact pancreatic islet. K+ currents of the same type could be evoked by intracellular application of GTP, the effect of which was mediated by mobilization of Ca2+ from inositol 1,4,5-trisphosphate (InsP3)-sensitive intracellular Ca2+ stores. These observations suggest that oscillatory glucose-stimulated electrical activity, which is correlated with pulsatile release of insulin, results from the interaction between the beta-cell and intraislet hormones and neurotransmitters. Our data also provide evidence for a close interplay between ion channels in the plasma membrane and InsP3-induced mobilization of intracellular Ca2+ in an excitable cell.

Alpha 2-adrenoreceptor stimulation does not inhibit L-type calcium channels in mouse pancreatic beta-cells

The effects of alpha 2-adrenergic stimulation on the Ca(2+)-current in mouse pancreatic beta-cells were investigated using the patch-clamp technique. When using the conventional whole-cell recording configuration (dialysis of cell interior with pipette solution), addition of adrenaline (1 microM) or the alpha 2-adrenergic agonist clonidine (5 microM) failed to reduce the Ca(2+)-current, irrespective of whether intracellular GTP (or GTP gamma S) was present or not and at both physiological (1.3 mM) and elevated (10.2 mM) Ca(2+)-concentrations. In fact, in the absence of added guanine nucleotides, the agonists tended to increase the Ca(2+)-current amplitude in the presence of the higher Ca(2+)-concentration. Ca(2+)-channel activation measured at 1.3 mM Ca2+ was not affected by clonidine. Half-maximal activation was observed at approximately -20 mV. In addition, when Ca(2+)-currents were recorded from intact beta-cells, using the perforated patch whole-cell configuration, clonidine (1 microM) also failed to detectably affect the Ca(2+)-current. It is therefore suggested that the inhibition of beta-cell electrical activity and insulin-secretion produced by alpha 2-adrenoreceptor stimulation does not result from suppression of the L-type Ca(2+)-current.

Inhibition of ATP-regulated K(+)-channels by a photoactivatable ATP-analogue in mouse pancreatic beta-cells

The effects of a photoactivable (DMNPE-caged) ATP-analogue on ATP-regulated K(+)-channels (KATP-channel) in mouse pancreatic beta-cells were investigated using the inside-out patch configuration of the patch-clamp technique. The caged precursor caused a concentration-dependent reduction of channel activity with a Ki of 17 microM; similar to the 11 microM obtained for standard Mg-ATP. The small difference in the blocking capacity between the precursor and ATP is probably the reason why no change in channel activity was observed upon photolysis of the caged molecule and liberation of ATP. It is suggested that the part of the ATP molecule involved in the blocking reaction of the KATP-channel is not sufficiently protected in DMNPE-caged ATP making this compound unsuitable for studying the rapid kinetics of ATP-induced KATP-channel inhibition.

Separate processes mediate nucleotide-induced inhibition and stimulation of the ATP-regulated K(+)-channels in mouse pancreatic beta-cells

The mechanisms by which nucleotides stimulate the activity of the ATP-regulated K(+)-channel (KATP-channel) were investigated using inside-out patches from mouse pancreatic beta-cells. ATP produces a concentration-dependent inhibition of channel activity with a Ki of 18 microns. The inhibitory action of ATP was counteracted by ADP (0.1 mM) and GDP (0.2 mM) but not GTP (1 mM). Stimulation of channel activity was also observed when ADP, GDP and GTP were applied in the absence of ATP. The ability of ADP and GDP to reactivate KATP-channels blocked by ATP declined with time following patch excision and after 30-60 min these nucleotides were without effect. During the same time period the ability of ADP and GTP to stimulate the channel in the absence of ATP was lost. In fact, ADP now blocked channel activity with 50% inhibition being observed at approximately 0.1 mM. By contrast, GDP remained a stimulator in the absence of ATP even when its ability to evoke channel activity in the presence of ATP was lost. These observations show that nucleotide-induced activation of the KATP-channel does not involve competition with ATP for a common inhibitory site but involves other processes. The data are consistent with the idea that nucleotides modulate KATP-channel activity by a number of different mechanisms that may include both regulation of cytosolic constituents and direct interaction with the channel and associated control proteins.

Activation by adrenaline of a low-conductance G protein-dependent K+ channel in mouse pancreatic B cells

Insulin is produced and secreted by the B cells in the endocrine pancreas. In vivo, insulin secretion is under the control of a number of metabolic, neural and hormonal substances. It is now clear that stimulation of insulin release by fuel secretagogues, such as glucose, involves the closure of K+ channels that are sensitive to the intracellular ATP concentration (KATP channels). This leads to membrane depolarization and the generation of Ca2(+)-dependent action potentials. The mechanisms whereby hormones and neurotransmitters such as adrenaline, galanin and somatostatin, which are released by intraislet nerve endings and the pancreatic D cells, produce inhibition of insulin secretion are not clear. Here we show that adrenaline suppresses B-cell electrical activity (and thus insulin secretion) by a G protein-dependent mechanism, which culminates in the activation of a sulphonylurea-insensitive low-conductance K+ channel distinct from the KATP channel.

ATP-Regulated K+ Channels and Diabetes Mellitus

Glucose-stimulated insulin secretion from pancreatic Beta-cells is dependent on closure of ATP-regulated K+ channels. These channels are selectively blocked by hypoglycaemic sulfonylureas, compounds used in treatment of non-insulin-dependent diabetes mellitus (NIDDM). This suggests that NIDDM may result from defective K+-channel regulation.

Delayed rectifying and calcium-activated K+ channels and their significance for action potential repolarization in mouse pancreatic beta-cells

The contribution of Ca2(+)-activated and delayed rectifying K+ channels to the voltage-dependent outward current involved in spike repolarization in mouse pancreatic beta-cells (Rorsman, P., and G. Trube. 1986. J. Physiol. 374:531-550) was assessed using patch-clamp techniques. A Ca2(+)-dependent component could be identified by its rapid inactivation and sensitivity to the Ca2+ channel blocker Cd2+. This current showed the same voltage dependence as the voltage-activated (Cd2(+)-sensitive) Ca2+ current and contributed 10-20% to the total beta-cell delayed outward current. The single-channel events underlying the Ca2(+)-activated component were investigated in cell-attached patches. Increase of [Ca2+]i invariably induced a dramatic increase in the open state probability of a Ca2(+)-activated K+ channel. This channel had a single-channel conductance of 70 pS [( K+]o = 5.6 mM). The Ca2(+)-independent outward current (constituting greater than 80% of the total) reflected the activation of an 8 pS [( K+]o = 5.6 mM; [K+]i = 155 mM) K+ channel. This channel was the only type observed to be associated with action potentials in cell-attached patches. It is suggested that in mouse beta-cells spike repolarization results mainly from the opening of the 8-pS delayed rectifying K+ channel.

Effects of external tetraethylammonium ions and quinine on delayed rectifying K+ channels in mouse pancreatic beta-cells

1. The whole-cell and outside-out patch configurations of the patch-clamp technique were used to study the mechanisms of block produced by external tetraethylammonium ions (TEA+) and quinine on delayed rectifying K+ channels in mouse pancreatic beta-cells. 2. In whole-cell recordings, TEA+ blocks the delayed outward current (which reflects the activity of delayed rectifying K+ channels by greater than 85%) in a concentration-dependent manner. The block appeared to be 1:1 with a Kd of approximately 1.4 mM at a membrane potential of 0 mV. The value of Kd varied with the membrane potential and there was an e-fold increase for a 70 mV depolarization. 3. Single-channel recordings revealed that delayed rectifying K+ channels have a unitary conductance of 8.5 pS ([K+]1 = 155 mM; [K+]o = 5.6 mM) and a single-channel K+ permeability of 2.8 X 10(-14) cm3 s-1. 4. First latency histograms of channel openings during voltage pulses from -70 to 0 mV peaked after 4 ms. A reaction scheme involving two closed states adequately but not perfectly described the distribution of the first latencies. The openings of the channels were grouped in bursts and the distribution of the closed times required two exponentials with time constants of 2.0 and 13 ms, respectively. The distribution of the open times could be described by a single exponential with a time constant of 25 ms. 5. Channel block produced by TEA+ (1 mM) was associated with a 40% decrease of the single-channel current amplitudes and a reduction in single-channel K+ permeability to 1.9 X 10(-14) cm3 s-1 but did not measurably affect the single-channel kinetics suggesting that the blocking reaction is very rapid. 6. Quinine blocked the whole-cell delayed outward current in a concentration-dependent manner. Half-maximal inhibition was attained at approximately 4 microM and the binding appeared to be 2:1. 7. Single-channel recordings indicated that the inhibition produced by quinine (10 microM) resulted from a decrease in the duration of the openings to a mean value of 6.7 ms. The time constants for the distribution of the closures were increased by approximately 30%. Quinine did not affect the amplitude of the openings. The rate constant of the blocking reaction (kB) was 15 mM-1 ms-1 at 0 mV.

Block of ATP-regulated and Ca2(+)-activated K+ channels in mouse pancreatic beta-cells by external tetraethylammonium and quinine

1. The whole-cell and outside-out patch configurations of the patch-clamp technique were used to investigate the effects of extracellular tetraethylammonium ions (TEA+) and quinine on both Ca2(+)-activated and ATP-regulated K+ channels in mouse pancreatic beta-cells. 2. The Ca2(+)-activated K+ channel has a single-channel K+ permeability of 4.7 x 10(-13) cm3 s-1 when recorded with physiological ionic gradients. This value decreased to 2.9 x 10(-13) cm3 s-1 after addition of 0.3 mM-TEA+. 3. Two exponentials with time constants of 0.2 and 4.7 ms were required to describe the distribution of the channel openings suggesting that the Ca2(+)-activated K+ channel has at least two open states. The fast and slow components comprised 16 and 84% of the total number of openings respectively. 4. TEA+ caused a concentration-dependent decrease in the single-channel amplitude and open probability of the Ca2(+)-activated K+ channel. A Kd for the reduction in the mean current of 0.14 mM was observed. The stoichiometry was approximately 1:1. 5. Quinine blocked the Ca2(+)-activated K+ channel in a concentration-dependent manner. Half-maximal block was observed at 0.10 mM and binding was 1:1. Inhibition by 20 microM-quinine was not associated with a decrease in channel amplitude but markedly reduced the lifetime of the channel openings. Two exponentials, with time constants of 0.5 and 1.3 ms, were required to describe the channel openings. The rapid component contained 55% of the events. 6. TEA+ reduced the single-channel amplitude of the ATP-regulated K+ channel in a concentration-dependent manner. Kd for the block was 22 mM and the binding approximately 1:1. The block was not associated with changes in the open probability or channel kinetics. Two exponentials were required to describe the distribution of the open times. The time constants for the fast and slow components were approximately 2 and approximately 20 ms respectively. The rapid component accounted for approximately 35% of the events. 7. Quinine (10-20 microM) almost abolished activity of the ATP-regulated K+ channels. Inhibition was characterized by slow onset and reversibility but not associated with a change in the appearance of the single-channel events. Quinine-induced block could not be reversed by diazoxide. 8. We conclude that TEA+ produces rapid block of both Ca2(+)-activated and ATP-regulated K+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Endothelin has no positive inotropic effect in guinea-pig atria or papillary muscle

Endothelin is a recently discovered, highly potent vasoconstrictor peptide. In isolated atria from rat and guinea-pig, endothelin has been reported to elicit a positive inotropic effect. The purpose of the present study was to compare the effects of endothelin on electromechanical coupling in guinea-pig atrial and ventricular muscle. In isolated, electrically driven specimens of atria and papillary muscle, action potentials and isometric contractions were recorded in the basal state and 30 min following non-cumulative exposure to endothelin (100 nM). In the atria, endothelin reduced action potential overshoot and relaxation velocity, and increased resting tension. In the papillary muscle the peptide slightly shortened the duration of the action potential. Endothelin did not affect peak tension, either in the atria or in the papillary muscle. These data contrast with earlier reports on a positive inotropic effect of endothelin in guinea-pig atria.

Glucose-inhibition of glucagon secretion involves activation of GABAA-receptor chloride channels

The endocrine part of the pancreas plays a central role in blood-glucose regulation. It is well established that an elevation of glucose concentration reduces secretion of the hyperglycaemia-associated hormone glucagon from pancreatic alpha 2 cells. The mechanisms involved, however, remain unknown. Electrophysiological studies have demonstrated that alpha 2 cells generate Ca2+-dependent action potentials. The frequency of these action potentials, which increases under conditions that stimulate glucagon release, is not affected by glucose or insulin. The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is present in the endocrine part of the pancreas at concentrations comparable to those encountered in the central nervous system, and co-localizes with insulin in pancreatic beta cells. We now describe a mechanism whereby GABA, co-secreted with insulin from beta cells, may mediate part of the inhibitory action of glucose on glucagon secretion by activating GABAA-receptor Cl- channels in alpha 2 cells. These observations provide a model for feedback regulation of glucagon release, which may be of significance for the understanding of the hypersecretion of glucagon frequently associated with diabetes.

Failure of glucose to elicit a normal secretory response in fetal pancreatic beta cells results from glucose insensitivity of the ATP-regulated K+ channels

Fetal pancreatic beta cells demonstrate a deficient insulin release in response to glucose, but the underlying mechanism at the cellular level is unknown. By using beta cells from 21-day fetal rats we made an attempt to clarify the mechanism(s) behind this reduced glucose response. In addition to measuring insulin release, glucose metabolism, and cellular ATP content, ATP-regulated K+ channels (G channels) and voltage-activated Ca2+ currents were investigated with the patch-clamp technique. It was thus demonstrated that the ATP-regulated K+ channels in fetal beta cells were not sensitive to glucose but otherwise had similar characteristics as those of adult beta cells. Also, the characteristics of the voltage-activated Ca2+ currents were similar in adult and fetal beta cells. However, as judged from measurements of both glucose oxidation and glucose utilization, glucose metabolism was impaired in fetal beta cells. In addition, there was no increase in the ATP content, even when the cells were stimulated for 30 min. It is therefore concluded that the attenuated glucose-induced insulin release in fetal pancreatic beta cells is due to an immature glucose metabolism resulting in impaired regulation of the ATP-sensitive K+ channels. These findings may be relevant to the understanding of the deficient stimulus-secretion coupling associated with non-insulin-dependent diabetes.