Pancreatic B cells are bursting, but how?

Exendin-4 affects calcium signalling predominantly during activation and activity of beta cell networks in acute mouse pancreas tissue slices

Tight control of beta cell stimulus-secretion coupling is crucial for maintaining homeostasis of energy-rich nutrients. While glucose serves as a primary regulator of this process, incretins augment beta cell function, partly by enhancing cytosolic [Ca2+] dynamics. However, the details of how precisely they affect beta cell recruitment during activation, their active time, and functional connectivity during plateau activity, and how they influence beta cell deactivation remain to be described. Performing functional multicellular Ca2+ imaging in acute mouse pancreas tissue slices enabled us to systematically assess the effects of the GLP-1 receptor agonist exendin-4 (Ex-4) simultaneously in many coupled beta cells with high resolution. In otherwise substimulatory glucose, Ex-4 was able to recruit approximately a quarter of beta cells into an active state. Costimulation with Ex-4 and stimulatory glucose shortened the activation delays and accelerated beta cell activation dynamics. More specifically, active time increased faster, and the time required to reach half-maximal activation was effectively halved in the presence of Ex-4. Moreover, the active time and regularity of [Ca2+]IC oscillations increased, especially during the first part of beta cell response. In contrast, subsequent addition of Ex-4 to already active cells did not significantly enhance beta cell activity. Network analyses further confirmed increased connectivity during activation and activity in the presence of Ex-4, with hub cell roles remaining rather stable in both control experiments and experiments with Ex-4. Interestingly, Ex-4 demonstrated a biphasic effect on deactivation, slightly prolonging beta cell activity at physiological concentrations and shortening deactivation delays at supraphysiological concentrations. In sum, costimulation by Ex-4 and glucose increases [Ca2+]IC during beta cell activation and activity, indicating that the effect of incretins may, to an important extent, be explained by enhanced [Ca2+]IC signals. During deactivation, previous incretin stimulation does not critically prolong cellular activity, which corroborates their low risk of hypoglycemia.

Pulsatile insulin secretion, impaired glucose tolerance and type 2 diabetes

Type 2 diabetes (T2DM) results when increases in beta cell function and/or mass cannot compensate for rising insulin resistance. Numerous studies have documented the longitudinal changes in metabolism that occur during the development of glucose intolerance and lead to T2DM. However, the role of changes in insulin secretion, both amount and temporal pattern, has been understudied. Most of the insulin secreted from pancreatic beta cells of the pancreas is released in a pulsatile pattern, which is disrupted in T2DM. Here we review the evidence that changes in beta cell pulsatility occur during the progression from glucose intolerance to T2DM in humans, and contribute significantly to the etiology of the disease. We review the evidence that insulin pulsatility improves the efficacy of secreted insulin on its targets, particularly hepatic glucose production, but also examine evidence that pulsatility alters or is altered by changes in peripheral glucose uptake. Finally, we summarize our current understanding of the biophysical mechanisms responsible for oscillatory insulin secretion. Understanding how insulin pulsatility contributes to normal glucose homeostasis and is altered in metabolic disease states may help improve the treatment of T2DM.

Electrophysiology of pancreatic β-cells in intact mouse islets of Langerhans

When exposed to intermediate glucose concentrations (6-16 mol/l), pancreatic β-cells in intact islets generate bursts of action potentials (superimposed on depolarised plateaux) separated by repolarised electrically silent intervals. First described more than 40 years ago, these oscillations have continued to intrigue β-cell electrophysiologists. To date, most studies of β-cell ion channels have been performed on isolated cells maintained in tissue culture (that do not burst). Here we will review the electrophysiological properties of β-cells in intact, freshly isolated, mouse pancreatic islets. We will consider the role of ATP-regulated K⁺-channels (K(ATP)-channels), small-conductance Ca²⁺-activated K⁺-channels and voltage-gated Ca²⁺-channels in the generation of the bursts. Our data indicate that K(ATP)-channels not only constitute the glucose-regulated resting conductance in the β-cell but also provide a variable K⁺-conductance that influence the duration of the bursts of action potentials and the silent intervals. We show that inactivation of the voltage-gated Ca²⁺-current is negligible at voltages corresponding to the plateau potential and consequently unlikely to play a major role in the termination of the burst. Finally, we propose a model for glucose-induced β-cell electrical activity based on observations made in intact pancreatic islets.

Bursting and calcium oscillations in pancreatic beta-cells: specific pacemakers for specific mechanisms

Oscillatory phenomenon in electrical activity and cytoplasmic calcium concentration in response to glucose are intimately connected to multiple key aspects of pancreatic β-cell physiology. However, there is no single model for oscillatory mechanisms in these cells. We set out to identify possible pacemaker candidates for burst activity and cytoplasmic Ca(2+) oscillations in these cells by analyzing published hypotheses, their corresponding mathematical models, and relevant experimental data. We found that although no single pacemaker can account for the variety of oscillatory phenomena in β-cells, at least several separate mechanisms can underlie specific kinds of oscillations. According to our analysis, slowly activating Ca(2+)-sensitive K(+) channels can be responsible for very fast Ca(2+) oscillations; changes in the ATP/ADP ratio and in the endoplasmic reticulum calcium concentration can be pacemakers for both fast bursts and cytoplasmic calcium oscillations, and cyclical cytoplasmic Na(+) changes may underlie patterning of slow calcium oscillations. However, these mechanisms still lack direct confirmation, and their potential interactions raises new issues. Further studies supported by improved mathematical models are necessary to understand oscillatory phenomena in β-cell physiology.

Novel spliced variants of large-conductance Ca(2+)-activated K(+)-channel β2-subunit in human and rodent pancreas

Large-conductance Ca(2+)-activated K(+ )(BK) channel regulates action potential firing in pancreatic β-cells. We cloned novel spliced variants of the BK-channel β(2)-subunit (BKβ2b), which consisted of 36 amino acids including the N-terminal in the original human BKβ2 (BKβ2a), from human and rodent pancreas. Real-time PCR analysis showed the abundant expression of BKβ2b transcripts in human and rodent pancreas and also in the RINm5f insulinoma cell line. In addition, up-regulation of both BK-channel α-subunit (BKα) and BKβ2b transcripts was observed in pancreas tissues from diabetes mellitus patients. In HEK293 cells co-expressing BKα and BKβ2b, the inactivation of BK-channel currents, which is typical for BKα + BKβ2a, was not observed, and electrophysiological and pharmacological properties of BKα + BKβ2b were almost identical to those of BKα alone. In HEK293 cells stably expressing BKα, the transient co-expression of yellow fluorescence protein (YFP)-tagged BKβ2a proteins resulted in their distribution along the cell membrane. In contrast, the co-expression of YFP-tagged BKβ2b with BKα showed diffusely distributed fluorescence signals throughout the cell body. Taken together, the predominant splicing of BKβ2b versus that of BKβ2a presumably enhances the contribution of BK channels to membrane potential and may possibly be a factor modulating insulin secretion in a suppressive manner in pancreatic β-cells.

The electrophysiology of the beta-cell based on single transmembrane protein characteristics

The electrophysiology of beta-cells is at the origin of insulin secretion. beta-Cells exhibit a complex behavior upon stimulation with glucose including repeated bursts and continuous spiking. Mathematical modeling is most suitable to improve knowledge about the function of various transmembrane currents provided the model is based on reliable data. This is the first attempt to build a mathematical model for the beta-cell electrophysiology in a bottom-up approach that relies on single protein conductance data. The results of previous whole-cell-based models are reconsidered. The full simulation including all prominent transmembrane proteins in beta-cells is used to provide a functional interpretation of their role in beta-cell bursting and an updated vantage point of beta-cell electrophysiology. As a result of a number of in silico knock-out and block experiments the novel model makes some unexpected predictions: single-channel conductance data imply that large-conductance calcium-gated potassium currents acquire the potential of driving oscillations at supralarge glucose levels. A more complex burst interruption model is presented. It also turns out that, depending on the species, sodium currents may be more relevant than considered so far. Experiments are proposed to verify these predictions.

Pharmacological properties and functional role of Kslow current in mouse pancreatic beta-cells: SK channels contribute to Kslow tail current and modulate insulin secretion

The pharmacological properties of slow Ca²⁺-activated K⁺ current (K_slow) were investigated in mouse pancreatic β-cells and islets to understand how K_slow contributes to the control of islet bursting, [Ca²⁺]_i oscillations, and insulin secretion. K_slow was insensitive to apamin or the K_ATP channel inhibitor tolbutamide, but UCL 1684, a potent and selective nonpeptide SK channel blocker reduced the amplitude of K_slow tail current in voltage-clamped mouse β-cells. K_slow was also selectively and reversibly inhibited by the class III antiarrythmic agent azimilide (AZ). In isolated β-cells or islets, pharmacologic inhibition of K_slow by UCL 1684 or AZ depolarized β-cell silent phase potential, increased action potential firing, raised [Ca²⁺]_i, and enhanced glucose-dependent insulin secretion. AZ inhibition of K_slow also supported mediation by SK, rather than cardiac-like slow delayed rectifier channels since bath application of AZ to HEK 293 cells expressing SK3 cDNA reduced SK current. Further, AZ-sensitive K_slow current was extant in β-cells from KCNQ1 or KCNE1 null mice lacking cardiac slow delayed rectifier currents. These results strongly support a functional role for SK channel-mediated K_slow current in β-cells, and suggest that drugs that target SK channels may represent a new approach for increasing glucose-dependent insulin secretion. The apamin insensitivity of β-cell SK current suggests that β-cells express a unique SK splice variant or a novel heteromultimer consisting of different SK subunits.

Crosstalk between membrane potential and cytosolic Ca2+ concentration in beta cells from Sur1-/- mice

AIMS/HYPOTHESIS

Islets or beta cells from Sur1(-/-) mice were used to determine whether changes in plasma membrane potential (V(m)) remain coupled to changes in cytosolic Ca(2+) ([Ca(2+)](i)) in the absence of K(ATP) channels and thus provide a triggering signal for insulin secretion. The study also sought to elucidate whether [Ca(2+)](i) influences oscillations in V(m) in sur1(-/-) beta cells.

METHODS

Plasma membrane potential and ion currents were measured with microelectrodes and the patch-clamp technique. [Ca(2+)](i) was monitored with the fluorescent dye fura-2. Insulin secretion from isolated islets was determined by static incubations.

RESULTS

Membrane depolarisation of Sur1(-/-) islets by arginine or increased extracellular K(+), elevated [Ca(2+)](i) and augmented insulin secretion. Oligomycin completely abolished glucose-stimulated insulin release from Sur1(-/-) islets. Oscillations in V(m) were influenced by [Ca(2+)](i) as follows: (1) elevation of extracellular Ca(2+) lengthened phases of membrane hyperpolarisation; (2) simulating a burst of action potentials induced a Ca(2+)-dependent outward current that was augmented by increased Ca(2+) influx through L-type Ca(2+) channels; (3) Ca(2+) depletion of intracellular stores by cyclopiazonic acid increased the burst frequency in Sur1(-/-) islets, elevating [Ca(2+)](i) and insulin secretion; (4) store depletion activated a Ca(2+) influx that was not inhibitable by the L-type Ca(2+) channel blocker D600.

CONCLUSIONS/INTERPRETATION

Although V(m) is largely uncoupled from glucose metabolism in the absence of K(ATP) channels, increased electrical activity leads to elevations of [Ca(2+)](i) that are sufficient to stimulate insulin secretion. In Sur1(-/-) beta cells, [Ca(2+)](i) exerts feedback mechanisms on V(m) by activating a hyperpolarising outward current and by depolarising V(m) via store-operated ion channels.

Small-conductance calcium-activated K+ channels are expressed in pancreatic islets and regulate glucose responses

Glucose-stimulated insulin secretion is associated with transients of intracellular Ca(2+) concentration [Ca(2+)](i) in the pancreatic beta-cell. We identified the expression and function of specific small-conductance Ca(2+)-activated K(+) (SK) channel genes in insulin-secreting cells. The presence of mRNA for SK1, -2, -3, and -4 (intermediate-conductance Ca(2+)-activated K(+) 1 [IK1]) channels was demonstrated by RT-PCR in rodent islets and insulinoma cells. SK2 and -3 proteins in mouse islets were detected by immunoblot and immunocytochemistry. In the tTA-SK3 tet-off mouse, a normal amount of SK3 protein was present in islets, but it became undetectable after exposure to doxycycline (DOX), which inhibits the transcription of the tTA-SK3 gene. The SK/IK channel-blockers apamin, dequalinium, and charybdotoxin caused increases in average [Ca(2+)](i) levels and in frequency of [Ca(2+)](i) oscillations in wild-type mouse islets. In SK3-tTA tet-off mice, the addition of apamin with glucose and tetraethylammonium (TEA) caused a similar elevation in [Ca(2+)](i), which was greatly diminished after DOX suppression of SK3 expression. We conclude that SK1, -2, -3, and IK1 (SK4) are expressed in islet cells and insulin-secreting cells and are able to influence glucose-induced calcium responses, thereby regulating insulin secretion.

Regulation of human insulin gene transcription by the immunosuppressive drugs cyclosporin A and tacrolimus at concentrations that inhibit calcineurin activity and involving the transcription factor CREB

Cyclosporin A and tacrolimus are important immunosuppressive drugs. They share a diabetogenic action as one of their most serious adverse effects. In a single study, tacrolimus (100 nM) inhibited human insulin gene transcription in the beta-cell line HIT. Using transfections of a human insulin-reporter gene into HIT cells, the present study shows that this inhibition is seen only at high concentrations of tacrolimus and is not caused by cyclosporin A. However, after stimulation by the major second messengers in the regulation of the insulin gene, cAMP and depolarization-induced calcium influx, both tacrolimus and cyclosporin A inhibited human insulin gene transcription in a concentration-dependent manner with IC(50) values of 1 nM and 30 nM, respectively. A further analysis offers a mechanism for this effect by revealing that the activation by cAMP and calcium of human insulin gene transcription is mediated by the transcription factor cAMP-responsive element binding protein (CREB) whose activity is inhibited by the immunosuppressants. These data demonstrate for the first time that cAMP- and calcium-induced activity of the human insulin gene is mediated by CREB and blocked by both tacrolimus and cyclosporin A at concentrations that inhibit calcineurin phosphatase activity. Since also the immunosuppressive effects of cyclosporin A and tacrolimus are thought to be secondary to inhibition of calcineurin, the present study suggests that inhibition of human insulin gene transcription by the immunosuppressants is clinically important and may contribute to their diabetogenic effect.

Calcium-activated K+ channels of mouse beta-cells are controlled by both store and cytoplasmic Ca2+: experimental and theoretical studies

A novel calcium-dependent potassium current (K(slow)) that slowly activates in response to a simulated islet burst was identified recently in mouse pancreatic beta-cells (Göpel, S.O., T. Kanno, S. Barg, L. Eliasson, J. Galvanovskis, E. Renström, and P. Rorsman. 1999. J. Gen. Physiol. 114:759-769). K(slow) activation may help terminate the cyclic bursts of Ca(2+)-dependent action potentials that drive Ca(2+) influx and insulin secretion in beta-cells. Here, we report that when [Ca(2+)](i) handling was disrupted by blocking Ca(2+) uptake into the ER with two separate agents reported to block the sarco/endoplasmic calcium ATPase (SERCA), thapsigargin (1-5 microM) or insulin (200 nM), K(slow) was transiently potentiated and then inhibited. K(slow) amplitude could also be inhibited by increasing extracellular glucose concentration from 5 to 10 mM. The biphasic modulation of K(slow) by SERCA blockers could not be explained by a minimal mathematical model in which [Ca(2+)](i) is divided between two compartments, the cytosol and the ER, and K(slow) activation mirrors changes in cytosolic calcium induced by the burst protocol. However, the experimental findings were reproduced by a model in which K(slow) activation is mediated by a localized pool of [Ca(2+)] in a subspace located between the ER and the plasma membrane. In this model, the subspace [Ca(2+)] follows changes in cytosolic [Ca(2+)] but with a gradient that reflects Ca(2+) efflux from the ER. Slow modulation of this gradient as the ER empties and fills may enhance the role of K(slow) and [Ca(2+)] handling in influencing beta-cell electrical activity and insulin secretion.

Cellular function in multicellular system for hormone-secretion: electrophysiological aspect of studies on alpha-, beta- and delta-cells of the pancreatic islet

We review a new method to explore the cellular functions in multicellular system by application of the perforated patch-clamp technique to intact pancreatic islet of Langerhans. Using this approach, the integrity of the islet is preserved and intercellular communication via gap junctions and paracrine processes are maintained. By using low-resistance patch electrodes, rapid current responses can be monitored under voltage-clamp control. We have applied this methodology to answer questions not resolved by patch-clamp experiments on isolated single insulin-secreting beta-cells. First, the role of a K(+)-current dependent on Ca(2+)-influx for the termination of burst of action potentials in beta-cells could be documented. Neither the current, nor the bursting pattern of electrical activity is preserved in isolated beta-cells. Second, the conductance of gap junctions (approximately 1 nS) between beta-cells was determined. Third, electrical properties of glucagon-producing alpha- and somatostatin-secreting delta-cells and the different mechanisms for glucose-sensing in these cells could be explored. The findings emanating from these experiments may have implications for neuroscience research such as the mechanism of oscillatory electrical activity in general and processes involved in the glucose-sensing in some neurons, which response to changes of blood glucose concentration.

Feedback control of the ATP-sensitive K(+) current by cytosolic Ca(2+) contributes to oscillations of the membrane potential in pancreatic beta-cells

During glucose stimulation, pancreatic beta-cells display membrane potential oscillations that correspond to intermittent Ca(2+) influx, leading to oscillations of the cytosolic free calcium concentration ([Ca(2+)](c)) and insulin secretion. The role of ATP-sensitive K(+) (K(+)-ATP) channels in the control of these oscillations was investigated by measuring the K(+)-ATP current (I(KATP)) with the perforated mode of the patch-clamp technique. No oscillations of I(KATP) were observed when glucose-stimulated beta-cells were kept hyperpolarized, thus with low and stable [Ca(2+)](c). However, increasing [Ca(2+)](c) by Ca(2+) influx (depolarizing pulses) or Ca(2+) mobilization (acetylcholine) transiently augmented I(KATP). This effect was abolished by tolbutamide, attenuated by increasing the glucose concentration in the medium, and prevented by abrogation of the [Ca(2+)](c) rise, which demonstrates that the current is really I(KATP) and that its increase is Ca(2+)-dependent. Injection of a current of a similar amplitude to that of the Ca(2+)-induced increase in I(KATP) was sufficient to repolarize glucose-stimulated beta-cells. These results suggest that, in the absence of [Ca(2+)](c) oscillations, no metabolic oscillations affect I(KATP) in pancreatic beta-cells. In contrast, [Ca(2+)](c) oscillations evoke I(KATP) oscillations. This mechanism may constitute the feedback loop controlling the glucose-induced oscillating electrical activity in beta-cells.

Metabolic oscillations in beta-cells

Whereas the mechanisms underlying oscillatory insulin secretion remain unknown, several models have been advanced to explain if they involve generation of metabolic oscillations in beta-cells. Evidence, including measurements of oxygen consumption, glucose consumption, NADH, and ATP/ADP ratio, has accumulated to support the hypothesis that energy metabolism in beta-cells can oscillate. Where simultaneous measurements have been made, these oscillations are well correlated with oscillations in intracellular [Ca(2+)] and insulin secretion. Considerable evidence has been accumulated to suggest that entry of Ca(2+) into cells can modulate metabolism both positively and negatively. The main positive effect of Ca(2+) is an increase in oxygen consumption, believed to involve activation of mitochondrial dehydrogenases. Negative feedback by Ca(2+) includes decreases in glucose consumption and decreases in the mitochondrial membrane potential. Ca(2+) also provides negative feedback by increasing consumption of ATP. The negative feedback provided by Ca(2+) provides a mechanism for generating oscillations based on a model in which glucose stimulates a rise in ATP/ADP ratio that closes ATP-sensitive K(+) (K(ATP)) channels, thus depolarizing the cell membrane and allowing Ca(2+) entry through voltage-sensitive channels. Ca(2+) entry reduces the ATP/ADP ratio and allows reopening of the K(ATP) channel.

Modulation of Ca(2+) signaling by K(+) channels in a hypothalamic neuronal cell line (GT1-1)

The pulsatile release of gonadotropin releasing hormone (GnRH) is driven by the intrinsic activity of GnRH neurons, which is characterized by bursts of action potentials correlated with oscillatory increases in intracellular Ca(2+). The role of K(+) channels in this spontaneous activity was studied by examining the effects of commonly used K(+) channel blockers on K(+) currents, spontaneous action currents, and spontaneous Ca(2+) signaling. Whole-cell recordings of voltage-gated outward K(+) currents in GT1-1 neurons revealed at least two different components of the current. These included a rapidly activating transient component and a more slowly activating, sustained component. The transient component could be eliminated by a depolarizing prepulse or by bath application of 1.5 mM 4-aminopyridine (4-AP). The sustained component was partially blocked by 2 mM tetraethylammonium (TEA). GT1-1 cells also express inwardly rectifying K(+) currents (I(K(IR))) that were activated by hyperpolarization in the presence of elevated extracellular K(+). These currents were blocked by 100 microM Ba(2+) and unaffected by 2 mM TEA or 1.5 mM 4-AP. TEA and Ba(2+) had distinct effects on the pattern of action current bursts and the resulting Ca(2+) oscillations. TEA increased action current burst duration and increased the amplitude of Ca(2+) oscillations. Ba(2+) caused an increase in the frequency of action current bursts and Ca(2+) oscillations. These results indicate that specific subtypes of K(+) channels in GT1-1 cells can have distinct roles in the amplitude modulation or frequency modulation of Ca(2+) signaling. K(+) current modulation of electrical activity and Ca(2+) signaling may be important in the generation of the patterns of cellular activity responsible for the pulsatile release of GnRH.

Glucose-sensitive conductances in rat pancreatic beta-cells: contribution to electrical activity

The perforated patch technique was used to assess the relative contribution of K(ATP) channel activity, assessed from input conductance (G(input)), and volume-sensitive anion channel activity to the induction of electrical activity in single isolated rat pancreatic beta-cells by glucose, 2-ketoisocaproate and tolbutamide. In cells equilibrated in the absence of glucose, the membrane potential was -71 mV and G(input) 3.66 nS. Addition of 8 mM glucose resulted in depolarisation, electrical activity and a reduction in G(input), reflecting an inhibition of K(ATP) channels. Cells equilibrated in 4 mM glucose had a membrane potential of -59 mV and a G(input) of 0.88 nS. In this case, a rise in glucose concentration to 8-20 mM again resulted in depolarisation and electrical activity, but caused a small increase in G(input). 2-Ketoisocaproate also evoked electrical activity and an increase in G(input), whereas electrical activity elicited by addition of tolbutamide was accompanied by reduced G(input). Increasing the concentration of glucose from 4 to 8-20 mM generated a noisy inward current at -70 mV, reflecting activation of the volume-sensitive anion channel. The mean amplitude of this current was glucose-dependent within the range 4-20 mM. Addition of 2-ketoisocaproate or a 15% hypotonic solution elicited similar increases in inward current. In contrast, addition of tolbutamide failed to induce the inward current. It is concluded that K(ATP) channel activity is most sensitive to glucose within the range 0-4 mM. At higher glucose concentrations effective in generating electrical activity, activation of the volume-sensitive anion channel could contribute towards the nutrient-induced increase in G(input).

Tetraethylammonium-evoked oscillatory contractions of rat tail artery: A K-K model

Spontaneously rhythmic contraction of peripheral blood vessels actively modulates the peripheral circulation and blood pressure. However, the underlying mechanisms for the complex rhythmic contraction patterns of various vascular tissues are not yet fully understood. In the present study, the tetraethylammonium (TEA)-induced spontaneously oscillatory contractions of isolated rat tail artery tissues were examined. It was found that TEA evoked arterial oscillatory contractions in a concentration-dependent, but endothelium-independent manner. The voltage- dependent K+ (Kv) channel specific blocker, 4-aminopyridine (4-AP), induced a sustained, but not oscillated, vascular contraction. The presence of 4-AP had no effect on the TEA-induced oscillatory contractions. The blockade of KCa channels with charybdotoxin or apamin did not affect the basal force of vascular tissues. Neither the TEA-induced oscillatory contraction was affected by these blockers. The opening of KATP channels by levcromakalim or their blockade by glybenclamide ceased or increased, respectively, the oscillation of TEA-induced contractions. The absence of Ca2+ or the presence of nifedipine in the bath solution completely abolished the effects of TEA. The inhibition of Ca2+-ATPase in the sarcoplasmic reticulum with micromolar concentrations of thapsigargin or cyclopiazonic acid either abolished or enhanced, respectively, the TEA-induced oscillatory contractions. Ryanodine did not affect the TEA-induced oscillatory contraction. In conclusion, the TEA-induced oscillatory contraction may be initiated by the blockade of the TEA-sensitive delayed rectifier K+ channels and maintained by the TEA-insensitive but ATP-sensitive K+ channels. This K-K model presents a novel mechanism for the depolarization-induced rhythmic contractions of small arteries.Key words: KATP channels, Kv channels, rhythmic contraction, tetraethylammonium, vascular smooth muscle.

Activation of Ca(2+)-dependent K(+) channels contributes to rhythmic firing of action potentials in mouse pancreatic beta cells

We have applied the perforated patch whole-cell technique to beta cells within intact pancreatic islets to identify the current underlying the glucose-induced rhythmic firing of action potentials. Trains of depolarizations (to simulate glucose-induced electrical activity) resulted in the gradual (time constant: 2.3 s) development of a small (<0.8 nS) K(+) conductance. The current was dependent on Ca(2+) influx but unaffected by apamin and charybdotoxin, two blockers of Ca(2+)-activated K(+) channels, and was insensitive to tolbutamide (a blocker of ATP-regulated K(+) channels) but partially (>60%) blocked by high (10-20 mM) concentrations of tetraethylammonium. Upon cessation of electrical stimulation, the current deactivated exponentially with a time constant of 6.5 s. This is similar to the interval between two successive bursts of action potentials. We propose that this Ca(2+)-activated K(+) current plays an important role in the generation of oscillatory electrical activity in the beta cell.

Molecular biology of adenosine triphosphate-sensitive potassium channels

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.

Modulation of the bursting properties of single mouse pancreatic beta-cells by artificial conductances

Glucose triggers bursting activity in pancreatic islets, which mediates the Ca2+ uptake that triggers insulin secretion. Aside from the channel mechanism responsible for bursting, which remains unsettled, it is not clear whether bursting is an endogenous property of individual beta-cells or requires an electrically coupled islet. While many workers report stochastic firing or quasibursting in single cells, a few reports describe single-cell bursts much longer (minutes) than those of islets (15-60 s). We studied the behavior of single cells systematically to help resolve this issue. Perforated patch recordings were made from single mouse beta-cells or hamster insulinoma tumor cells in current clamp at 30-35 degrees C, using standard K+-rich pipette solution and external solutions containing 11.1 mM glucose. Dynamic clamp was used to apply artificial KATP and Ca2+ channel conductances to cells in current clamp to assess the role of Ca2+ and KATP channels in single cell firing. The electrical activity we observed in mouse beta-cells was heterogeneous, with three basic patterns encountered: 1) repetitive fast spiking; 2) fast spikes superimposed on brief (<5 s) plateaus; or 3) periodic plateaus of longer duration (10-20 s) with small spikes. Pattern 2 was most similar to islet bursting but was significantly faster. Burst plateaus lasting on the order of minutes were only observed when recordings were made from cell clusters. Adding gCa to cells increased the depolarizing drive of bursting and lengthened the plateaus, whereas adding gKATP hyperpolarized the cells and lengthened the silent phases. Adding gCa and gKATP together did not cancel out their individual effects but could induce robust bursts that resembled those of islets, and with increased period. These added currents had no slow components, indicating that the mechanisms of physiological bursting are likely to be endogenous to single beta-cells. It is unlikely that the fast bursting (class 2) was due to oscillations in gKATP because it persisted in 100 microM tolbutamide. The ability of small exogenous currents to modify beta-cell firing patterns supports the hypothesis that single cells contain the necessary mechanisms for bursting but often fail to exhibit this behavior because of heterogeneity of cell parameters.

Bursting calcium rotors in cultured cardiac myocyte monolayers

Rotating waves (rotors) of cellular activity were observed in nonconfluent cultures of embryonic chick heart cells by using a macroscopic imaging system that detected fluorescence from intracellular Ca2+. Unlike previous observations of rotors or spiral waves in other systems, the rotors did not persist but exhibited a repetitive pattern of spontaneous onset and offset leading to a bursting rhythm. Similar dynamics were observed in a cellular automaton model of excitable media that incorporates spontaneous initiation of activity, and a decrease of excitability as a consequence of rapid activity (fatigue). These results provide a mechanism for bursting dynamics in normal and pathological biological processes.

Neurotransmitters and their receptors in the islets of Langerhans of the pancreas: what messages do acetylcholine, glutamate, and GABA transmit?

Although neurotransmitters are present in pancreatic islets of Langerhans and can be shown to alter hormone secretion, their precise physiological roles in islet function and their cellular mechanisms of action are unclear. Recent research has identified specific neurotransmitter receptor isoforms in islets that may be important physiologically, because selective receptor agonists activate islet ion channels, modify intracellular [Ca2+], and affect secretion. This article focuses on the putative roles of acetylcholine, glutamate, and GABA in islet function. It has been hypothesized that acetylcholine potentiates insulin secretion by either promoting Ca release from cellular stores, activating a store depletion-activated channel, or activating a novel Na channel. GABA and glutamate, in contrast, have been proposed to mediate a novel paracrine signaling pathway whereby alpha- and beta-cells communicate within the islet. The evidence supporting these hypotheses will be critically evaluated.

A minimum of fuel is necessary for tolbutamide to mimic the effects of glucose on electrical activity in pancreatic beta-cells

Glucose stimulation of pancreatic beta-cells triggers electrical activity (slow waves of membrane potential with superimposed spikes) that is best monitored with intracellular microelectrodes. Closure of ATP-sensitive K+ channels underlies the depolarization to the threshold potential and participates in the increase in electrical activity produced by suprathreshold (>7 mM) concentrations of glucose, but it is still unclear whether this is the sole mechanism of control. This was investigated by testing whether blockade of ATP-sensitive K+ channels by low concentrations of tolbutamide is able to mimic the effects of glucose on mouse beta-cell electrical activity even in the absence of the sugar. The response to tolbutamide was influenced by the duration of the perifusion with the low glucose medium. Tolbutamide (25 microM) caused a rapid and sustained depolarization with continuous activity after 6 min of perifusion of the islet with 3 mM glucose, and a progressive depolarization with slow waves of the membrane potential after 20 min. In the absence of glucose, the beta-cell response to tolbutamide was a transient phase of depolarization with rare slow waves (6 min) or a silent, small, but sustained, depolarization (20 min). Readministration of 3 mM glucose was sufficient to restore slow waves, whereas an increase in the glucose concentration to 5 and 7 mM was followed by a lengthening of the slow waves and a shortening of the intervals. In contrast, induction of slow waves by tolbutamide proved very difficult in the absence of glucose, because the beta-cell membrane tended to depolarize from a silent level to the plateau level, at which electrical activity is continuous. Azide, a mitochondrial poison, abrogated the electrical activity induced by tolbutamide in the absence of glucose, which demonstrates the influence of the metabolism of endogenous fuels on the response to the sulfonylurea. The partial repolarization that azide also produced was reversed by increasing the concentration of tolbutamide, but reappearance of the spikes required the addition of glucose. It is concluded that inhibition of ATP-sensitive K+ channels is not the only mechanism by which glucose controls electrical activity in beta-cells.

Effects of extracellular calcium on electrical bursting and intracellular and luminal calcium oscillations in insulin secreting pancreatic beta-cells

The extracellular calcium concentration has interesting effects on bursting of pancreatic beta-cells. The mechanism underlying the extracellular Ca2+ effect is not well understood. By incorporating a low-threshold transient inward current to the store-operated bursting model of Chay, this paper elucidates the role of the extracellular Ca2+ concentration in influencing electrical activity, intracellular Ca2+ concentration, and the luminal Ca2+ concentration in the intracellular Ca2+ store. The possibility that this inward current is a carbachol-sensitive and TTX-insensitive Na+ current discovered by others is discussed. In addition, this paper explains how these three variables respond when various pharmacological agents are applied to the store-operated model.

Bursting behavior during fixed-delay stimulation of spontaneously beating chick heart cell aggregates

Spontaneously beating embryonic chick atrial heart cell aggregates were stimulated with depolarizing current pulses delivered at a fixed delay after each action potential. This preparation is an experimental model of a reentrant tachycardia. During fixed-delay stimulation, bursting behavior was typically observed for a wide range of delays. Episodes of bursting at a rate faster (slower) than control were followed by overdrive suppression (underdrive acceleration). We use a simple nonlinear model, based on the interaction between excitability and overdrive suppression, to describe these dynamics. A modified version of the Shrier-Clay ionic model of electrical activity of the embryonic chick heart cell aggregates that includes a simplified Na+ pump term is also presented. We show that the complex patterns during fixed-delay stimulation arise as a result of delicate interactions between overdrive suppression and phase resetting, which can be described in terms of the underlying ionic mechanisms. This study may provide a basis for understanding incessant tachycardias in the intact heart, as well as an alternative mechanism for the emergence of bursting activity in other biologic tissue.

A novel role for HERG K+ channels: spike-frequency adaptation

1. The regular firing of a Hodgkin-Huxley neurone endowed with fast Na+ and delayed K+ channels can be converted into adapting firing by appending HERG (human eag-related gene) channels. 2. The computer model predictions were verified by studying the firing properties of F-11 DRG neurone x neuroblastoma hybrid cells induced to differentiate by long-term exposure to retinoic acid. These cells, which express HERG currents (IHERG), show clear spike-frequency adaptation of their firing when current clamped with long depolarizations. 3. In agreement with the prediction, the selective blocking of IHERG by class III antiarrhythmic drugs always led to the disappearance of the spike-frequency adaptation, and the conversion of adapting firing to regular firing. 4. It is proposed that, in addition to their role in the repolarization of the heart action potential, HERG channels may sustain a process of spike-frequency adaptation, and hence contribute to the control of burst duration in a way that is similar to that of the K+ currents, IAHP, IC and IM. In addition to the known cardiac arrhythmia syndrome (LQT2), genetic mutations or an altered HERG expression could lead to continuous hyperexcitable states sustained by the inability of nerve or endocrine cells to accommodate to repetitive stimuli. This might help in clarifying the pathogenesis of still undefined idiopathic familial epilepsies.

Ascorbate availability and neurodegeneration in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease in which upper and lower motoneurons progressively deteriorate and die. Neuronal damage is most evident in the lower central nervous system, and death generally occurs following central respiratory failure. Proposed and demonstrated mechanisms for amyotrophic lateral sclerosis are diverse, and include altered superoxide dismutase and neurofilament proteins, autoimmune attack, and hyperglutamatergic activity. However, they do not account for the late onset of the disease, its earlier onset in males, and the differential vulnerability of neurons located in the brainstem and spinal cord. It is proposed here that, within the context of a specific defect such as altered superoxide dismutase, age-dependent decline in ascorbate availability triggers the disease. A role for ascorbate, which is found in millimolar levels in neurons, is suggested by a number of consistencies: 1) superoxide radicals being a common substrate for superoxide dismutase and ascorbate; 2) a close association between central nervous system ascorbate levels and injury tolerance; 3) a steady decline in ascorbate plasma levels and cellular availability with age; 4) plasma ascorbate levels being lower in males; 5) an association of ascorbate release with motor activity in central nervous system regions, in vivo; 6) the coupling of brain-cell ascorbate release with glutamate uptake; 7) possible roles for ascorbate modulation of N-methyl-D-aspartate receptor activity; 9) the ability of ascorbate to prevent peroxynitrite anion formation; and 10) evidence supporting the scorbutic guinea pig as a model for amyotrophic lateral sclerosis. Emphasis is placed on the probable competition between superoxide dismutase and ascorbate within the context of a primary defect of metal-binding or metal access in high-concentration proteins such as superoxide dismutase and human heavy neurofilaments. Finally, distinct features of alpha-motoneuronal physiology suggest that cell physiological characteristics such as high metabolic activity and extensive calcium dynamics may render neurons differentially vulnerable in amyotrophic lateral sclerosis.

Temperature modulates the Ca2+ current of HIT-T15 and mouse pancreatic beta-cells

The effects of raising temperature on the Ca2+ currents of insulin-secreting HIT and mouse pancreatic beta-cells were studied. Currents were measured in 3 mM Ca2+ containing solutions using standard whole-cell techniques. Increasing temperature from 22 degrees C to 35 degrees C increased peak Ca2+ current amplitude, percent (fast) inactivation and decreased the time-to-peak of the current. Ca2+ currents in HIT and mouse beta-cells responded in the same manner to an imposed physiological burstwave with test-pulses: (i) application of the burstwave inactivated the test-pulse Ca2+ current at both high and low temperatures; (ii) Ca2+ current inactivation leveled off during the plateau phase at 20-22 degrees C whereas there was an apparent continual decay at 33-35 degrees C; and (iii) recovery from inactivation occurred during the interburst period at both temperatures. Application of a physiological burstwave without test-pulses to mouse beta-cells before and after addition of 0.2 mM Cd2+ resulted in a Ca2+ difference current. This current activated during the hyperpolarized interburst phase, activated, inactivated and deactivated rapidly and continually during the plateau phase, and recovered from inactivation during the interburst. Although raising temperature strongly modified HIT and mouse beta-cells Ca2+ current, our work suggests that other channels, in addition to Ca2+ channels, are likely to be involved in the control of islet bursts, particularly at different temperatures. In addition, the effect of temperature on islet cell Ca2+ current may be partly responsible for the well-known temperature dependence of glucose-dependent secretion.

Partial characterization of K(+)-induced increase in [Ca2+]cyt and GnRH release in GT1-7 neurons

Secretion of pituitary gonadotropins is regulated centrally by the hypothalamic decapeptide gonadotropin releasing hormone (GnRH). Using the immortalized hypothalamic GT1-7 neuron, we characterized pharmacologically the dynamics of cytosolic Ca2+ and GnRH release in response to K+-induced depolarization of GT1-7 neurons. Our results showed that K+ concentrations from 7.5 to 60 mM increased [Ca2+]cyt in a concentration-dependent manner. Resting [Ca2+]cyt in GT1 -7 cells was determined to be 69.7 +/- 4.0 nM (mean +/- S.E.M.; n = 69). K+-induced increases in [Ca2+]cyt ranged from 58.2 nM at 7.5 mM [K+] to 347 nM at 60 mM [K+]. K+-induced GnRH release ranged from about 10 pg/ml at 7.5 mM [K+] to about 60 pg/ml at 45 mM [K+]. K+-induced increases in (Ca2+]cyt and GnRH release were enhanced by 1 microM BayK 8644, an L-type Ca2+ channel agonist. The BayK enhancement was completely inhibited by 1 microM nimodipine, an L-type Ca2+ channel antagonist. Nimodipine (1 microM) alone partially inhibited K+-induced increases in [Ca2+]cyt and GnRH release. Conotoxin (1 microM) alone had no effect on K+-induced GnRH release or [Ca2+]cyt, but the combination of conotoxin (1 microM) and nimodipine (1 microM) inhibited K+-induced increase in [Ca2+]cyt significantly more (p < 0.02) than nimodipine alone, suggesting that N-type Ca2+ channels exist in GT1-7 neurons and may be part of the response to K+. The response of [Ca2+]cyt to K+ was linear with increasing [K+] whereas the response of GnRH release to increasing [K+] appeared to be saturable. K+-induced increase in [Ca2+]cyt and GnRH release required extracellular [Ca2+]. These experiments suggest that voltage dependent N- and L-type Ca2+ channels are present in immortalized GT1-7 neurons and that GnRH release is, at least in part, dependent on these channels for release of GnRH.

Cloning of the beta cell high-affinity sulfonylurea receptor: a regulator of insulin secretion

Sulfonylureas are a class of drugs widely used to promote insulin secretion in the treatment of non-insulin-dependent diabetes mellitus. These drugs interact with the sulfonylurea receptor of pancreatic beta cells and inhibit the conductance of adenosine triphosphate (ATP)-dependent potassium (KATP) channels. Cloning of complementary DNAs for the high-affinity sulfonylurea receptor indicates that it is a member of the ATP-binding cassette or traffic ATPase superfamily with multiple membrane-spanning domains and two nucleotide binding folds. The results suggest that the sulfonylurea receptor may sense changes in ATP and ADP concentration, affect KATP channel activity, and thereby modulate insulin release.

Block of non-L-, non-N-type Ca2+ channels in rat insulinoma RINm5F cells by omega-agatoxin IVA and omega-conotoxin MVIIC

The high-voltage-activated (HVA) Ba2+ currents of rat insulinoma RINm5F cells insensitive to dihydropyridines (DHP) and omega-conotoxin GVIA (omega-CTx-GVIA) have been studied for their sensitivity to omega-agatoxin-IVA (omega-Aga-IVA) and omega-CTx-MVIIC. Blockade of HVA currents by omega-Aga-IVA was partial (mean 24%), reversible and saturated around 350 nM (half block approximately 60 nM). Blockade by omega-CTx-MVIIC was more potent (mean 45%), partly irreversible and saturated above 3 microM. The effects of both toxins were additive with that of nifedipine (5 microM) and were more pronounced at positive potentials. omega-Aga-IVA action was additive with that of omega-CTx-GVIA (3 microM) but was largely prevented by cell pre-treatment with omega-CTx-MVIIC (3 microM). In contrast, omega-CTx-MVIIC block was attenuated by omega-CTx-GVIA treatment (approximately 15%), suggesting that omega-CTx-MVIIC blocks the N-type (approximately 15%) and the non-L-, non-N-type channel sensitive to omega-Aga-IVA (approximately 30%). Consistent with this, cells deprived of most non-L-type channels by pre-incubation with omega-CTx-GVIA and omega-CTx-MVIIC exhibited predominant L-type currents that activated at more negative potentials than in normal cells (-30 mV in 5 mM Ba2+) and were effectively depressed by nifedipine (maximal block of 95% from -30 mV to +40 mV). Our results suggest that, besides L- and N-type channels, insulin-secreting RINm5F cells possess also a non-L-, non-N-type channel that contributes significantly to the total current (approximately 30%). Although the pharmacology of this channel is similar to Q-type and alpha 1 class A channels, its range of activation (> -20 mV) and its slow inactivation time course resemble more that of N- and P-type channels. The channel is therefore referred to as "Q-like".

Correlations of rates of insulin release from islets and plateau fractions for beta-cells

Pancreatic beta-cells in intact islets of Langerhans perfused with various glucose concentrations exhibit periodic bursting electrical activity (BEA) consisting of active and silent phases. The fraction of the time spent in the active phase is called the plateau fraction and appears to be strongly correlated with the rate of release of insulin from islets as glucose concentration is varied. Here this correlation is quantified and a theoretical development is presented in detail. Experimental rates of insulin release are correlated with "effective" plateau fractions over a range of glucose concentrations. There are a number of different models for BEA in pancreatic beta-cells and a method is developed here to quantify the dependence of a glucose dependent parameter on glucose concentration. As an example, the plateau fractions computed from the Sherman-Rinzel-Keizer model are matched with experimental plateau fractions to obtain a relationship between the model's glucose-dependent parameter, beta, and glucose concentration. Knowledge of the relationships between beta and glucose concentration and between experimental measurements of rates of insulin release and plateau fractions permits the determination of theoretical rates of insulin release from the model.

Culture duration and conditions affect the oscillations of cytoplasmic calcium concentration induced by glucose in mouse pancreatic islets

The pattern of the increase in cytoplasmic Cai2+ that glucose produces in beta cells has been reported to be highly variable. Here, we evaluated the influence of the culture duration (1-4 days) and conditions (5-10 mmol/l glucose) on Cai2+ in normal mouse islets stimulated by glucose. After 1 day of culture in 10 mmol/l glucose, a rise of the glucose concentration from 3 to 15 mmol/l induced a triphasic change of Cai2+ in the islets. A small initial decrease was followed by a large peak increase and then by regular fast oscillations (approximately 2.5/min). When the culture was prolonged to 2, 3 and 4 days, the initial decrease became inconsistent and the peak occurred earlier, whereas the oscillations decreased in frequency, increased in duration and eventually disappeared; on day 4 the Cai2+ rise was sustained. After culture in 5 mmol/l glucose, the pattern of Cai2+ changes induced by 15 mmol/l glucose was different. The initial decrease was very pronounced, the first peak was delayed and clearly separated from the subsequent oscillations. These were of a mixed type (fast Ca2+ transients on top of slow ones) after 1 day, and of a slow type only after 4 days. These alterations in the Cai2+ oscillations triggered by glucose could not be ascribed to desynchronization of the signal between different regions of the islets. In conclusion, culturing normal mouse islets in 5 or 10 mmol/l glucose for 1-4 days, markedly alters the characteristics of the changes in Cai2+ produced by glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical activity and calcium channels in neuroendocrine cells

Similar to neuronal cells, neuroendocrine cells express voltage-dependent ion channels and fire action potentials. Ca2+ influx through voltage-dependent Ca2+ channels couples changes in membrane potential to Ca(2+)-dependent cellular processes, such as hormone release. Using the patch-clamp technique, we studied the spontaneous electrical activity as well as voltage-dependent Ca2+ channels in cholecystokinin-producing pancreatic cells (RIN 1056E cell line), in prolactin-secreting pituitary cells (GH3 cell line), and in calcitonin-secreting cells of the thyroid (rMTC 44-2 cell line). All three cell types displayed spontaneous electrical activity, that is, they spontaneously produced action potentials. RIN 1056E cells, GH3 cells, and rMTC cells exhibited (various types of) voltage-dependent Ca2+ channels that were regulated by various neurotransmitters and hormones, such as somatostatin.

Inactivation of HIT cell Ca2+ current by a simulated burst of Ca2+ action potentials

A novel voltage-clamp protocol was developed to test whether slow inactivation of Ca2+ current occurs during bursting in insulin-secreting cells. Single insulin-secreting HIT cells were patch-clamped and their Ca2+ currents were isolated pharmacologically. A computed beta-cell burst was used as a voltage-clamp command and the net Ca2+ current elicited was determined as a cadmium difference current. Ca2+ current rapidly activated during the computed plateau and spike depolarizations and then slowly decayed. Integration of this Ca2+ current yielded an estimate of total Ca influx. To further analyze Ca2+ current inactivation during a burst, repetitive test pulses to + 10 mV were added to the voltage command. Current elicited by these pulses was constant during the interburst, but then slowly and reversibly decreased during the depolarizing plateau. This inactivation was reduced by replacing external Ca2+ with Ba2+ as a charge carrier, and in some cells inactivation was slower in Ba2+. Experimental results were compared with the predictions of the Keizer-Smolen mathematical model of bursting, after subjecting model equations to identical voltage commands. In this model, bursting is driven by the slow, voltage-dependent inactivation of Ca current during the plateau active phase. The K-S model could account for the slope of the slow decay of spike-elicited Ca current, the waveform of individual Ca current spikes, and the suppression of test pulse-elicited Ca current during a burst command. However, the extent and rate of fast inactivation were underestimated by the model. Although there are significant differences between the data obtained and the predictions of the K-S model, the overall results show that as predicted by the model, Ca current slowly inactivates during a burst of imposed spikes, and inactivation is dependent on both Ca2+ influx and membrane depolarization. We thus show that clamping cells to their physiological voltage waveform can be readily accomplished and is a powerful approach for understanding the contribution of individual ion currents to bursting.

Calcium current regulation of depolarization-evoked calcium transients in beta-cells (HIT-T15)

Glucose-induced insulin secretion by beta-cells is linked to phasic increases in intracellular Ca2+ concentration ([Ca2+]i) arising from membrane depolarization. We examined the source of this Ca2+ in cultured beta-cells using rapid dual-wavelength spectroscopy of fura-2 under voltage-clamp conditions. Depolarization of the beta-cell initiated a sustained rise in [Ca2+]i that was dependent on the activation of L-type Ca2+ current that exhibited very slow inactivation. Neither release of internally stored Ca2+ nor Na(+)-Ca2+ exchange contributed significantly to this calcium rise, as evidenced by the suppressive effect of rapid application of Cd2+ and the lack of effect of elevations of intracellular Na+ concentration. Restoration of control Ca2+ levels was primarily dependent on Ca2+ channel closure, but both a voltage-dependent and voltage-independent Ca2+ efflux system also contributed. Both the fluorescence-based and charge-based estimates of the rise in [Ca2+]i were in broad agreement, indicating that Ca current activation was the primary source of the Ca2+ transient. The results suggest that nutrient-induced changes in beta-cell membrane potential tightly regulate [Ca2+]i, and thereby insulin release, primarily via alterations in the conductive state of slowly inactivating Ca2+ channels.

Ion channels

Insulin secretion by the pancreatic Beta cell is dependent upon transmembrane ion fluxes gated by the ATP-regulated potassium channel and the voltage regulated, L-type calcium channel. This work group examined major recent advances in the structure and modulation of ion channels and how those advances may pertain to the physiology of insulin secretion and the pharmacological treatment of Type 2 (non-insulin-dependent) diabetes mellitus. Structural studies have revealed that voltage gated ion channels are related, complex, and comprised of multiple components: sodium channels consist of three distinct subunits. L-type calcium channels, crucial to the insulin secretory response are structurally related to the sodium channel but contain additional subunits. Potassium channels are less closely related and appear to function as homotetramers. Modulation of ion channel activity is similarly complex: site specific phosphorylation by multiple protein kinases under the control of several intracellular second messenger systems may increase or decrease conductance. Subunit composition and relatively stable changes in the modal state of ion channels also appear to be critical to ion channel gating properties. Functional studies of the Beta-cell ATP-regulated potassium channel suggest two distinct nucleotide binding sites which link this channel to the metabolic state of the Beta cell. The multiple paths of ion channel modulation provide multiple targets for therapeutic intervention. Where detailed characterisation of ion channel structure has been achieved, those targets are being used for specific drug design. Such complete characterisation has not yet been achieved for Beta-cell ion channels and this presents a major goal for diabetes research.

Two sites for adenine-nucleotide regulation of ATP-sensitive potassium channels in mouse pancreatic beta-cells and HIT cells

ATP-inhibited potassium channels (K(ATP)) were studied in excised, inside-out patches from cultured adult mouse pancreatic beta-cells and HIT cells. In the absence of ATP, ADP opened K(ATP) channels at concentrations as low as 10 microM and as high as 500 microM, with maximal activation between 10 and 100 microM ADP in mouse beta-cell membrane patches. At concentrations greater than 500 microM, ADP inhibited K(ATP) channels while 10 mM virtually abolished channel activity. HIT cell channels had a similar biphasic response to ADP except that more than 1 mM ADP was required for inhibition. The channel opening effect of ADP required magnesium while channel inhibition did not. Using creatine/creatine phosphate solutions with creatine phosphokinase to fix ATP and ADP concentrations, we found substantially different K(ATP)-channel activity with solutions having the same ATP/ADP ratio but different absolute total nucleotide levels. To account for ATP-ADP competition, we propose a new model of channel-nucleotide interactions with two kinds of ADP binding sites regulating the channel. One site specifically binds MgADP and increases channel opening. The other, the previously described ATP site, binds either ATP or ADP and decreases channel opening. This model very closely fits the ADP concentration-response curve and, when incorporated into a model of beta-cell membrane potential, increasing ADP in the 10 and 100 microM range is predicted to compete very effectively with millimolar levels of ATP to hyperpolarize beta-cells. The results suggest that (i) K(ATP)-channel activity is not well predicted by the "ATP/ADP ratio," and (ii) ADP is a plausible regulator of K(ATP) channels even if its free cytoplasmic concentration is in the 10-100 microM range as suggested by biochemical studies.

Potassium channels of the insulin-secreting B cell

Ionic and electrical events play a central role in the stimulus-secretion coupling of the pancreatic B cell. Potassium permeability is critically involved in the regulation of B cell membrane potential and insulin secretion. In the absence of glucose, membrane potential remains stable, around -65 mV. This resting potential is mainly determined by the high potassium conductance of the membrane. The ATP generated by glucose metabolism in B cells blocks the K+(ATP) channels controlling resting membrane potential. Thus, glucose metabolism leads to closure of the ATP-dependent potassium channels; the resulting decrease in K+ permeability induces depolarization and opening of voltage-activated Ca-channels. The subsequent increase in Ca2+ influx raises the cytoplasmic concentration of free Ca2+, which in turn triggers exocytosis of secretory granules. Other types of K+ channels have also been identified in the B cell, such as voltage- and Ca(2+)-dependent K+ channels, which are not a target for the action of glucose, but may play a role in the repolarization of spikes. The modulation of insulin release by some hormones and neurotransmitters involves, among other mechanisms, an interference with the plasma membrane K+ conductance. Thus, galanine, somatostatin and adrenaline, which inhibit insulin release, increase K+ conductance by a G protein-dependent mechanism; both peptides were reported to open ATP-sensitive K+ channels in insulin-secreting cell line RINm5F. It was also observed that extracellular purine nucleotides could interfere with K+ channels. Among the various drugs interfering with insulin secretion, sulfonylureas, such as tolbutamide and glibenclamide, directly inhibit ATP-dependent K+ channels in the B cell membrane and thereby initiate insulin release. In contrast, potassium channel openers such as diazoxide, antagonize the effects of glucose by increasing K+ permeability of the B cell membrane. Furthermore, other classes of drugs have recently been shown to interact with K+ (ATP) channels. Thus, K+ channels of the pancreatic B cell, particularly ATP-dependent ones, play a crucial role in the electrophysiology of insulin secretion; they are an important target for pharmacological agents designed to modulate this secretion.

Cloning of the alpha 1 subunit of a voltage-dependent calcium channel expressed in pancreatic beta cells

The isoforms of the alpha 1 subunits of voltage-dependent Ca2+ channels expressed in human pancreatic islets were identified by using a pair of degenerate oligonucleotide primers and the polymerase chain reaction (PCR) to amplify mRNAs encoding alpha 1 subunit-like sequences. The sequences of the PCR products indicate that islets express the heart-type alpha 1 subunit as well as a second isoform whose complete sequence has not been previously reported. The sequences of cloned cDNAs encoding the human beta-cell, or neuroendocrine-type, alpha 1 subunit indicate that it is composed of 2181 amino acids. It shares 68%, 64%, and 41% identity with the sequences of the alpha 1 subunits of rabbit heart, skeletal muscle, and brain, respectively, and is predicted to have a similar structure including four homologous domains composed of six membrane-spanning segments each. RNA blotting studies indicate that the beta-cell-type alpha 1 subunit is also expressed in brain as well as in the insulin-producing cell lines RINm5F and beta TC-3; however, it could not be detected by RNA blotting in a third cell line, HIT-T15. In situ hybridization studies revealed expression of beta-cell-type alpha 1 subunit mRNA in beta cells of rat pancreatic islets, implying that this protein may play a role in the regulation of insulin secretion.

Role of voltage-dependent ionic currents in coupling glucose stimulation to insulin secretion in canine pancreatic islet B-cells

Glucose-induced electrical activity in canine pancreatic islet B cells is distinct from that in rodent islets, though both display Ca(2+)-dependent insulin secretion. Rodent islet B cells undergo regular bursts of Ca(2+)-dependent action potentials, while canine islet B cells generate isolated Na(+)-dependent action potentials which often give way to a plateau depolarization. Here we present evidence to reconcile the species difference in electrical activity with the similarity of Ca2+ dependence of secretion. (i) In canine B cells increasing glucose concentrations produce membrane depolarization and increasing frequency of Nao-dependent action potentials until a background membrane potential (approximately -40 mV) is reached where Na+ currents are inactivated. (ii) Voltage-dependent Ca2+ currents are present which are activated over the voltage excursion of the action potential (-50 to +20 mV) and inactivate slowly, (over seconds) in the range of the plateau depolarization (-40 to -25 mV). Hence, they are available to contribute to both phases of depolarization. (iii) Tetrodotoxin (TTX) reduces by half an early transient phase of glucose-stimulated insulin secretion but not a subsequent prolonged plateau phase. The transient phase of secretion often corresponds well in time to the period of initial high frequency action potential activity. These latter results suggest that in canine B cells voltage-dependent Na+ and Ca2+ currents mediate biphasic glucose-induced insulin secretion. The early train of Na(+)-dependent action potentials, by transiently activating Ca2+ channels and allowing pulsatile Ca2+ entry, may promote an early transient phase of insulin secretion. The subsequent sustained plateau depolarization, by allowing sustained Ca2+ entry, may permit steady insulin release.

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.