Biophysical properties of gap junctions between freshly dispersed pairs of mouse pancreatic beta cells

Insulin secretion from human beta cells is heterogeneous and dependent on cell-to-cell contacts

AIMS/HYPOTHESIS

We assessed the heterogeneity of insulin secretion from human isolated beta cells and its regulation by cell-to-cell contacts.

METHODS

Insulin secretion from single and paired cells was assessed by a reverse haemolytic plaque assay. The percentage of plaque-forming cells, the mean plaque area and the total plaque development were evaluated after 1 h of stimulation with different secretagogues.

RESULTS

Not all beta cells were surrounded by a haemolytic plaque under all conditions tested. A small fraction of the beta cell population (20%) secreted more than 90% and 70% of total insulin at 2.2 and 22.2 mmol/l glucose, respectively. Plaque-forming cells, mean plaque area and total plaque development were increased at 12.2 and 22.2 compared with 2.2 mmol/l glucose. Insulin secretion of single beta cells was similar at 12.2 and 22.2 mmol/l glucose. Insulin secretion of beta cell pairs was increased compared with that of single beta cells and was higher at 22.2 than at 12.2 mmol/l glucose. Insulin secretion of beta cells in contact with alpha cells was also increased compared with single beta cells, but was similar at 22.2 compared with 12.2 mmol/l glucose. Delta and other non-beta cells did not increase insulin secretion of contacting beta cells compared with that of single beta cells. Differences in insulin secretion between 22.2 and 12.2 mmol/l glucose were observed in murine but not in human islets.

CONCLUSIONS/INTERPRETATION

Human beta cells are highly heterogeneous in terms of insulin secretion so that a small fraction of beta cells contributes to the majority of insulin secreted. Homologous and heterologous intercellular contacts have a significant impact on insulin secretion and this could be related to the particular architecture of human islets.

The hyperbolic effect of density and strength of inter beta-cell coupling on islet bursting: a theoretical investigation

BACKGROUND

Insulin, the principal regulating hormone of blood glucose, is released through the bursting of the pancreatic islets. Increasing evidence indicates the importance of islet morphostructure in its function, and the need of a quantitative investigation. Recently we have studied this problem from the perspective of islet bursting of insulin, utilizing a new 3D hexagonal closest packing (HCP) model of islet structure that we have developed. Quantitative non-linear dependence of islet function on its structure was found. In this study, we further investigate two key structural measures: the number of neighboring cells that each beta-cell is coupled to, nc, and the coupling strength, gc.

RESULTS

BETA-cell clusters of different sizes with number of beta-cells nbeta ranging from 1-343, nc from 0-12, and gc from 0-1000 pS, were simulated. Three functional measures of islet bursting characteristics--fraction of bursting beta-cells fb, synchronization index lambda, and bursting period Tb, were quantified. The results revealed a hyperbolic dependence on the combined effect of nc and gc. From this we propose to define a dimensionless cluster coupling index or CCI, as a composite measure for islet morphostructural integrity. We show that the robustness of islet oscillatory bursting depends on CCI, with all three functional measures fb, lambda and Tb increasing monotonically with CCI when it is small, and plateau around CCI = 1.

CONCLUSION

CCI is a good islet function predictor. It has the potential of linking islet structure and function, and providing insight to identify therapeutic targets for the preservation and restoration of islet beta-cell mass and function.

Beta cells preferentially exchange cationic molecules via connexin 36 gap junction channels

AIMS/HYPOTHESIS

Pancreatic beta cells are connected by gap junction channels made of connexin 36 (Cx36), which permit intercellular exchanges of current-carrying ions (ionic coupling) and other molecules (metabolic coupling). Previous studies have suggested that ionic coupling may extend to larger regions of pancreatic islets than metabolic coupling. The aim of the present study was to investigate whether this apparent discrepancy reflects a difference in the sensitivity of the techniques used to evaluate beta cell communication or a specific characteristic of the Cx36 channels themselves.

METHODS

We microinjected several gap junction tracers, differing in size and charge, into individual insulin-producing cells and evaluated their intercellular exchange either within intact islets of control, knockout and transgenic mice featuring beta cells with various levels of Cx36, or in cultures of wild-type and Cx36-transfected MIN6 cells.

RESULTS

We found that (1) Cx36 channels favour the exchange of cations and larger positively charged molecules between beta cells at the expense of anionic molecules; (2) this exchange occurs across sizable portions of pancreatic islets; and (3) during glibenclamide (known as glyburide in the USA and Canada) stimulation beta cell coupling increases to an extent that varies for different gap junction-permeant molecules.

CONCLUSIONS/INTERPRETATION

The data show that beta cells are extensively coupled within pancreatic islets via exchanges of mostly positively charged molecules across Cx36 channels. These exchanges selectively increase during stimulation of insulin secretion. The identification of this permselectivity is expected to facilitate the identification of endogenous permeant molecules and of the mechanism whereby Cx36 signalling significantly contributes to the modulation of insulin secretion.

Investigating the role of islet cytoarchitecture in its oscillation using a new beta-cell cluster model

The oscillatory insulin release is fundamental to normal glycemic control. The basis of the oscillation is the intercellular coupling and bursting synchronization of β cells in each islet. The functional role of islet β cell mass organization with respect to its oscillatory bursting is not well understood. This is of special interest in view of the recent finding of islet cytoarchitectural differences between human and animal models. In this study we developed a new hexagonal closest packing (HCP) cell cluster model. The model captures more accurately the real islet cell organization than the simple cubic packing (SCP) cluster that is conventionally used. Using our new model we investigated the functional characteristics of β-cell clusters, including the fraction of cells able to burst f_b, the synchronization index λ of the bursting β cells, the bursting period T_b, the plateau fraction p_f, and the amplitude of intracellular calcium oscillation [Ca]. We determined their dependence on cluster architectural parameters including number of cells n_β, number of inter-β cell couplings of each β cell n_c, and the coupling strength g_c. We found that at low values of n_β, n_c and g_c, the oscillation regularity improves with their increasing values. This functional gain plateaus around their physiological values in real islets, at n_β∼100, n_c∼6 and g_c∼200 pS. In addition, normal β-cell clusters are robust against significant perturbation to their architecture, including the presence of non-β cells or dead β cells. In clusters with n_β>∼100, coordinated β-cell bursting can be maintained at up to 70% of β-cell loss, which is consistent with laboratory and clinical findings of islets. Our results suggest that the bursting characteristics of a β-cell cluster depend quantitatively on its architecture in a non-linear fashion. These findings are important to understand the islet bursting phenomenon and the regulation of insulin secretion, under both physiological and pathological conditions.

Are gap junctions truly involved in inhibitory neuromuscular interaction in mouse proximal colon?

1. Gap junctions exist between circular muscle cells of the colon and between interstitial cells of Cajal (ICC) in the myenteric plexus of the gastrointestinal tract. They also probably couple intramuscular ICC with smooth muscle cells. Recent functional evidence for this was found in dye-coupling and myoelectrical experiments. 2. In the present study, we tested the hypothesis of gap junctions putatively being involved in neuromuscular interaction in mouse colon by using different classes of gap junction blockers. 3. Electrical field stimulation of the myenteric plexus elicited tetrodotoxin-sensitive and hexamethonium-independent fast and slow inhibitory junction potentials (fIJP and sIJP, respectively) in circular smooth muscle cells, as evaluated by intracellular recording techniques in impaled smooth muscle cells. Heptanol produced a time-dependent hyperpolarization of the membrane potential (MP) and abolished fIJP and sIJP. Octanol had no effect on the MP and abolished fIJP and sIJP. Carbenoxolone produced a time-dependent depolarization of the MP without any effect on fIJP or sIJP. The connexin 43 mimetic gap junction blocker GAP-27 had no effect on MP, fIJP or sIJP. 4. Based on the presently available gap junction blockers we found no evidence that gap junctions are involved in neuromuscular transmission in mouse colon, as suggested by morphological studies.

Size distribution of mouse Langerhans islets

Pancreatic beta-cells are clustered in islets of Langerhans, which are typically a few hundred micrometers in a variety of mammals. In this study, we propose a theoretical model for the growth of pancreatic islets and derive the islet size distribution, based on two recent observations: First, the neogenesis of new islets becomes negligible after some developmental stage. Second, islets grow via a random process, where any cell in an islet proliferates with the same rate regardless of the present size of the islet. Our model predicts either log-normal or Weibull distributions of the islet sizes, depending on whether cells in an islet proliferate coherently or independently. To confirm this, we also measure the islet size by selectively staining islets, which are exposed from exocrine tissues in mice after enzymatic treatment. Indeed revealed are skewed distributions with the peak size of approximately 100 cells, which fit well to the theoretically derived ones. Interestingly, most islets turned out to be bigger than the expected minimal size (approximately 10 or so cells) necessary for stable synchronization of beta-cells through electrical gap-junction coupling. The collaborative behavior among cells is known to facilitate synchronized insulin secretion and tends to saturate beyond the critical (saturation) size of approximately 100 cells. We further probe how the islets change as normal mice grow from young (6 weeks) to adult (5 months) stages. It is found that islets may not grow too large to maintain appropriate ratios between cells of different types. Our results implicate that growing of mouse islets may be regulated by several physical constraints such as the minimal size required for stable cell-to-cell coupling and the upper limit to keep the ratios between cell types. Within the lower and upper limits the observed size distributions of islets can be faithfully regenerated by assuming random and uncoordinated proliferation of each beta-cell at appropriate rates.

Do gap junctions play a role in nerve transmissions as well as pacing in mouse intestine?

Varicosities of nitrergic and other nerves end on deep muscular plexus interstitial cells of Cajal or on CD34-positive, c-kit-negative fibroblast-like cells. Both cell types connect to outer circular muscle by gap junctions, which may transmit nerve messages to muscle. We tested the hypotheses that gap junctions transmit pacing messages from interstitial cells of Cajal of the myenteric plexus. Effects of inhibitors of gap junction conductance were studied on paced contractions and nerve transmissions in small segments of circular muscle of mouse intestine. Using electrical field stimulation parameters (50 V/cm, 5 pps, and 0.5 ms) which evoke near maximal responses to nitrergic, cholinergic, and apamin-sensitive nerve stimulation, we isolated inhibitory responses to nitrergic nerves, inhibitory responses to apamin-sensitive nerves and excitatory responses to cholinergic nerves. 18beta-Glycyrrhetinic acid (10, 30, and 100 microM), octanol (0.1, 0.3, and 1 mM) and gap peptides (300 microM of (40)Gap27, (43)Gap26, (37,43)Gap27) all failed to abolish neurotransmission. 18beta-Glycyrrhetinic acid inhibited frequencies of paced contractions, likely owing to inhibition of l-type Ca(2+) channels in smooth muscle, but octanol or gap peptides did not. 18beta-Glycyrrhetinic acid and octanol, but not gap peptides, reduced the amplitudes of spontaneous and nerve-induced contractions. These reductions paralleled reductions in contractions to exogenous carbachol. Additional experiments with gap peptides in both longitudinal and circular muscle segments after N(G)-nitro-l-arginine and TTX revealed no effects on pacing frequencies. We conclude that gap junction coupling may not be necessary for pacing or nerve transmission to the circular muscle of the mouse intestine.

Effect of heptanol on noradrenaline-induced contractions in rat vas deferens

We have studied the effects of 1-heptanol and nifedipine on noradrenaline (NA)-induced contractions in order to explore the role of gap junctions and their interactions with L-type Ca2+ channel mediated [Ca2+]o entry in the generation of NA-induced contractions in the rat vas deferens. Application of 20 microM NA to rat vas deferens resulted in contractions with three different components, an initial phasic component followed by a tonic component overlapped with an oscillatory component. Heptanol (0.01-2 mM) induced a concentration dependent reduction of the contractions. 2 mM heptanol reduced the phasic component by 32.9 +/- 4.4% and the tonic component by 93.8 +/- 1.9% of control, while the oscillatory component was completely abolished (n=7). Nifedipine (2 microM) reduced the phasic component by 34.5 +/- 4.1% and the tonic component by 89.5 +/- 3.8% of control and abolished the oscillatory component (n=6). In the presence of heptanol and nifedipine together, the phasic component was reduced by 61.3 +/- 8.3% and the tonic component by 94.5 +/- 1.0% of control. The oscillatory component was completely abolished (n=6). These results allow the conclusion that phasic contraction is mainly due to the direct action of NA, independent of gap junctions, while the tonic and oscillatory contractions may depend significantly on cell-to-cell communication. These in turn may depend critically on the availability of extracellularly derived Ca2+.

Effects of Heptanol on Neurogenic Contractions of Vas Deferens: A Comparative Study of Stimulation Frequency in Guinea Pig and Rat

This study examines the role of gap junctional communication in smooth muscle in relation to the frequency of stimulation and the innervation density of the tissue in the generation of neurogenic contractions. Toward this end the effects of heptanol, a gap junctional blocker, on the neurogenic contractions of guinea pig and rat vas deferens at different frequencies of stimulation (single pulse, 5, 10, 20, 40, 60, and 80 Hz) were studied. In both the prostatic and epididymal halves of these tissues, heptanol abolished the neurogenic contractions at the lower frequencies of stimulation. At higher frequencies, contractions were resistant to heptanol action. The effect of heptanol on the neurogenic contractions was found to decrease with increasing stimulation frequency. The neurogenic contractions of rat vas deferens were more resistant to heptanol than those of guinea pig vas deferens. Our data indicate that gap junctional communication is significant in the generation of neurogenic contractions in both guinea pig and rat vas deferens in a frequency-dependent manner, and we discuss the mechanisms underlying these findings.

How noise and coupling induce bursting action potentials in pancreatic {beta}-cells

Unlike isolated beta-cells, which usually produce continuous spikes or fast and irregular bursts, electrically coupled beta-cells are apt to exhibit robust bursting action potentials. We consider the noise induced by thermal fluctuations as well as that by channel-gating stochasticity and examine its effects on the action potential behavior of the beta-cell model. It is observed numerically that such noise in general helps single cells to produce a variety of electrical activities. In addition, we also probe coupling via gap junctions between neighboring cells, with heterogeneity induced by noise, to find that it enhances regular bursts.

Biophysical evidence that connexin-36 forms functional gap junction channels between pancreatic mouse beta-cells

Connexin-36 (Cx36) is the only gap junction protein that has been unambiguously identified in rodent pancreatic beta-cells. However, properties of gap junction channel unitary currents between beta-cells remain unrevealed. To address whether Cx36 forms functional channels in beta-cells, we characterized biophysical properties of macro- and microscopic junctional currents recorded from dual whole cell voltage clamp isolated pairs of dispersed mouse beta-cells. Electrical coupling was recorded in 80% of cell pairs with a junctional conductance (g(j)) of 355 +/- 45 pS (n = 20). Transjunctional voltage dependence was identified in three of seven cell pairs with high-input membrane resistances. Normalized steady-state g(j) (Gj) and transjunctional-voltage relation were well described by a two-state Boltzmann equation [maximal conductance (Gmax) = 1.0, voltage-insensitive conductance (Gmin) = 0.3 and 0.28, voltage gating sensitivity (A) = 0.21 and 0.23, and voltage at which one-half of the initial voltage-dependent conductance was reached (Vo) = -85 and 87 mV for negative and positive potentials, respectively]. Halothane reversibly uncoupled beta-cell pairs, and, during recovery, unitary conductances of 5-10 pS were recorded while using patch pipettes containing mainly CsCl. Although these properties are similar to those previously described for Cx36 channels in mammalian cell systems, we found that beta-cell junctional currents were insensitive to quinine. Cx36 transcript and protein expression in islets and freshly dispersed cell preparations was confirmed by RT-PCR and immunofluorescence. In conclusion, biophysical properties of junctional channels between beta-cells are similar but not identical to those previously described for homomeric Cx36 channels. Cell type-specific mechanisms that may account for these differences are discussed.

Large dense-core vesicle exocytosis in pancreatic beta-cells monitored by capacitance measurements

This article discusses the currently used methodologies for monitoring exocytosis as changes in cell capacitance. Details are given on composition of solutions, experimental protocols, and how the observed responses can be interpreted physiologically. The concepts are illustrated by examples from our own work on insulin-releasing pancreatic beta-cells. Finally, we consider the feasibility of applying capacitance measurements to endocrine cells in intact pancreatic islets, where the cells are electrically coupled to each other.

Three roads to islet bursting: emergent oscillations in coupled phantom bursters

Glucose-induced membrane potential and Ca(2+) oscillations in isolated pancreatic beta-cells occur over a wide range of frequencies, from >6/min (fast) to <1/min (slow). However, cells within intact islets generally oscillate with periods of 10-60 s (medium). The phantom bursting concept addresses how beta-cells can generate such a wide range of frequencies. Here, we explore an updated phantom bursting model to determine how heterogeneity in a single parameter can explain both the broad frequency range observed in single cells and the rarity of medium oscillations. We then incorporate the single-cell model into an islet model with parameter heterogeneity. We show that strongly coupled islets must be composed of predominantly medium oscillating single cells or a mixture of fast and slow cells to robustly produce medium oscillations. Surprisingly, we find that this constraint does not hold for moderate coupling, and that robustly medium oscillating islets can arise from populations of single cells that are essentially all slow or all fast. Thus, with coupled phantom bursters, medium oscillating islets can be constructed out of cells that are either all fast, all slow, or a combination of the two.

Electrical synapses coordinate activity in the suprachiasmatic nucleus

In the suprachiasmatic nucleus (SCN), the master circadian pacemaker, neurons show circadian variations in firing frequency. There is also considerable synchrony of spiking across SCN neurons on a scale of milliseconds, but the mechanisms are poorly understood. Using paired whole-cell recordings, we have found that many neurons in the rat SCN communicate via electrical synapses. Spontaneous spiking was often synchronized in pairs of electrically coupled neurons, and the degree of this synchrony could be predicted from the magnitude of coupling. In wild-type mice, as in rats, the SCN contained electrical synapses, but electrical synapses were absent in connexin36-knockout mice. The knockout mice also showed dampened circadian activity rhythms and a delayed onset of activity during transition to constant darkness. We suggest that electrical synapses in the SCN help to synchronize its spiking activity, and that such synchrony is necessary for normal circadian behavior.

Microfluidic glucose stimulation reveals limited coordination of intracellular Ca2+ activity oscillations in pancreatic islets

The pancreatic islet is a functional microorgan involved in maintaining normoglycemia through regulated secretion of insulin and other hormones. Extracellular glucose stimulates insulin secretion from islet beta cells through an increase in redox state, which can be measured by NAD(P)H autofluorescence. Glucose concentrations over approximately 7 mM generate synchronous oscillations in beta cell intracellular Ca2+ concentration ([Ca2+]i), which lead to pulsatile insulin secretion. Prevailing models assume that the pancreatic islet acts as a functional syncytium, and the whole islet [Ca2+]i response has been modeled in terms of islet bursting and pacemaker models. To test these models, we developed a microfluidic device capable of partially stimulating an islet, while allowing observation of the NAD(P)H and [Ca2+]i responses. We show that beta cell [Ca2+]i oscillations occur only within regions stimulated with more than approximately 6.6 mM glucose. Furthermore, we show that tolbutamide, an antagonist of the ATP-sensitive K+ channel, allows these oscillations to travel farther into the nonstimulated regions of the islet. Our approach shows that the extent of Ca2+ propagation across the islet depends on a delicate interaction between the degree of coupling and the extent of ATP-sensitive K+-channel activation and illustrates an experimental paradigm that will have utility for many other biological systems.

Synchronization and entrainment of cytoplasmic Ca2+ oscillations in cell clusters prepared from single or multiple mouse pancreatic islets

In contrast to pancreatic islets, isolated beta-cells stimulated by glucose display irregular and asynchronous increases in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)). Here, clusters of 5-30 cells were prepared from a single mouse islet or from pools of islets, loaded with fura-2, and studied with a camera-based system. [Ca(2+)](i) oscillations were compared in pairs of clusters by computing the difference in period and a synchronization index lambda. During perifusion with 12 mM glucose, the clusters exhibited regular [Ca(2+)](i) oscillations that were quasi-perfectly synchronized (Delta period of 1.4% and index lambda close to 1.0) between cells of each cluster. In contrast, separate clusters were not synchronized, even when prepared from one single islet. Pairs of clusters neighboring on the same coverslip were not better synchronized than pairs of clusters examined separately (distinct coverslips). We next attempted to synchronize clusters perifused with 12 mM glucose by applying external signals. A single pulse of 20 mM glucose, 10 mM amino acids, or 10 microM tolbutamide transiently altered [Ca(2+)](i) oscillations but did not reset the clusters to oscillate synchronously. On a background of 12 mM glucose, repetitive applications (1 min/5 min) of 10 microM tolbutamide, but not of 20 mM glucose, synchronized separate clusters. Our results identify a level of beta-cell heterogeneity intermediate between single beta-cells and the whole islet. They do not support the idea that substances released by islet cells serve as paracrine synchronizers. However, synchronization can be achieved by an external signal, if this signal has a sufficient strength to overwhelm the intrinsic rhythm of glucose-induced oscillations and is repetitively applied.

Electrical coupling and neuronal synchronization in the Mammalian brain

Certain neurons in the mammalian brain have long been known to be joined by gap junctions, which are the most common type of electrical synapse. More recently, cloning of neuron-specific connexins, increased capability of visualizing cells within brain tissue, labeling of cell types by transgenic methods, and generation of connexin knockouts have spurred a rapid increase in our knowledge of the role of gap junctions in neural activity. This article reviews the many subtleties of transmission mediated by gap junctions and the mechanisms whereby these junctions contribute to synchronous firing.

Replacement by a lacZ reporter gene assigns mouse connexin36, 45 and 43 to distinct cell types in pancreatic islets

Transcripts of three connexin isoforms (Cx36, Cx43 and Cx45) have been reported in rodent pancreatic islets, but the precise distribution of the cognate proteins is still unknown. We determined expression of Cx36 in a cell-autonomous manner using mice with a targeted replacement of the Cx36 coding region by a lacZ reporter gene. For cell-autonomous monitoring of Cx43 expression, we used the Cre/loxP system: Mice carrying the Cx43 coding region flanked by loxP sites (floxed) also carried an embedded lacZ gene that is activated after Cre-mediated recombination in cells with transcriptional activity of the Cx43 gene. Deletion of the Cx43 coding region in beta-cells did not result in the activation of the embedded lacZ reporter gene. Instead, Cx43 expression was found in endothelial cells of the islets of Langerhans in mice with endothelium-specific deletion. Ubiquitous deletion of Cx43 led to a similar endothelial lacZ expression, but again, activity of the reporter gene was not detected in beta-cells. Mice with targeted replacement of the Cx45 coding region by lacZ showed a vascular expression similar to Cx43. The data show that native insulin-producing cells express a connexin isoform (Cx36) which differs from those (Cx43 and Cx45) expressed by vascular islet cells.

On-line analysis of gap junctions reveals more efficient electrical than dye coupling between islet cells

Pancreatic beta-cells constitute a well-communicating multicellular network that permits a coordinated and synchronized signal transmission within the islet of Langerhans that is necessary for proper insulin release. Gap junctions are the molecular keys that mediate functional cellular connections, which are responsible for electrical and metabolic coupling in the majority of cell types. Although the role of gap junctions in beta-cell electrical coupling is well documented, metabolic communication is still a matter of discussion. Here, we have addressed this issue by use of a fluorescence recovery after photobleaching (FRAP) approach. This technique has been validated as a reliable and noninvasive approach to monitor functional gap junctions in real time. We show that control pancreatic islet cells did not exchange a gap junction-permeant molecule in either clustered cells or intact islets of Langerhans under conditions that allowed cell-to-cell exchange of current-carrying ions. Conversely, we have detected that the same probe was extensively transferred between islet cells of transgenic mice expressing connexin 32 (Cx32) that have enhanced junctional coupling properties. The results indicate that the electrical coupling of native islet cells is more extensive than dye communication. Dye-coupling domains in islet cells appear more restricted than previously inferred with other methods.

A model for glucose-induced wave propagation in pancreatic islets of Langerhans

A reaction-diffusion type model is constructed, describing the spatio-temporal dynamics of the basic intracellular variables assumed to be involved in the initiation of the insulin secretion process by beta -cells in the pancreatic islets of Langerhans. The model includes equations for the electric membrane potential of the cells, with respective kinetics for ionic currents, for concentrations of both free and stored intracellular calcium, and for the intra- and extracellular concentrations of glucose. An empirical expression connecting the equation for the intracellular glucose concentration to the electrical equation is introduced. The model reproduces the events observed in experiments in vitro upon external glucose application to the islets of Langerhans, such as usual bursting oscillations of the membrane potential and corresponding oscillations of the intracellular calcium concentration. It also allows simulation of electric wave propagation through the islet, initiated by the spatial gradient of glucose concentration within the islet. The gradient emerges due to glucose diffusing into the islets from the external medium, being high at the edges. The latter results show that glucose diffusion presents a means for wave initiation in the islets, which supports our previous assumption (Aslanidi et al., 2001).

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.

Blocking AMPA receptor desensitization prolongs spontaneous EPSC decay times and depolarizes H1 horizontal cells in carp retinal slices

Desensitization of H1 horizontal cell (H1 HC) glutamate receptors was investigated in carp retinal slices using cyclothiazide (CTZ), an inhibitor of AMPA receptor desensitization. 100 microM CTZ depolarized H1 HCs and increased the amplitude of light responses, without any prominent changes in their kinetics. Spontaneous EPSCs (sEPSCs) in H1 HCs were observed in the presence of 2.5 mM heptanol, an uncoupling agent of gap junctions. 20 microM GYKI52466 (an AMPA receptor antagonist) blocked the sEPSCs, consistent with the sEPSCs being mediated by AMPA receptors. 100 microM cobalt suppressed the frequency of sEPSCs without changing their mean peak amplitude, suggesting that calcium-dependent transmitter release from cones was not affected by heptanol. CTZ increased the total inward charge transferred per sEPSC by increasing the sEPSC decay time constant twofold, without any significant change in their frequency and mean peak amplitude. This suggests that the depolarizing effect of CTZ on H1 HCs was due to blocking desensitization of AMPA receptors, increasing the inward current induced by glutamate released from cone synaptic terminals. The desensitization of glutamate receptors may function to extend the dynamic range of H1 HC light responses.

Tolbutamide stimulation of pancreatic beta-cells involves both cell recruitment and increase in the individual Ca(2+) response

Individual pancreatic beta-cells are functionally heterogeneous. Their sensitivity to glucose is variable, so that the proportion of active cells increases with the glucose concentration (recruitment). We have investigated whether sulphonylureas also recruit beta-cells, by measuring cytoplasmic Ca(2+) ([Ca(2+)](i)) - the triggering signal of insulin secretion - in single cells and clusters of cells prepared from mouse islets. In 4 mM glucose, the threshold concentration of tolbutamide inducing a [Ca(2+)](i) rise was variable (5 - 50 microM). The proportion of responsive cells and clusters therefore increased with the tolbutamide concentration, to reach a maximum of 90% of the cells and 100% of the clusters. This recruitment occurred faster when the glucose concentration was increased from 4 to 5 mM (EC(50) of approximately 14 and approximately 4 microM tolbutamide respectively). Within responsive clusters little recruitment was observed; when a cluster was active, all or nearly all cells were active probably because of cell coupling. Thus, tolbutamide-induced [Ca(2+)](i) oscillations were synchronous in all cells of each cluster, whereas there was no synchrony between clusters or individual cells. Independently of cell recruitment, tolbutamide gradually augmented the magnitude of the [Ca(2+)](i) rise in single cells and clusters. This increase occurred over a broader range of concentrations than did recruitment (EC(50) of approximately 50 and 25 microM tolbutamide at 4 and 5 mM glucose respectively). Tolbutamide (10 microM) accelerated the recruitment of single cells and clusters brought about by increasing glucose concentrations (range of 3 - 7 mM instead of 4 - 10 mM glucose), and potentiated the amplification of the individual responses that glucose also produced. In conclusion, both metabolic (glucose) and pharmacologic (sulphonylurea) inhibition of K(+)-ATP channels recruits beta-cells to generate a [Ca(2+)](i) response. However, the response is not of an all-or-none type; it increases in amplitude with the concentration of either glucose or tolbutamide.

Excitation wave propagation as a possible mechanism for signal transmission in pancreatic islets of Langerhans

In response to glucose application, beta-cells forming pancreatic islets of Langerhans start bursting oscillations of the membrane potential and intracellular calcium concentration, inducing insulin secretion by the cells. Until recently, it has been assumed that the bursting activity of beta-cells in a single islet of Langerhans is synchronized across the whole islet due to coupling between the cells. However, time delays of several seconds in the activity of distant cells are usually observed in the islets of Langerhans, indicating that electrical/calcium wave propagation through the islets can occur. This work presents both experimental and theoretical evidence for wave propagation in the islets of Langerhans. Experiments with Fura-2 fluorescence monitoring of spatiotemporal calcium dynamics in the islets have clearly shown such wave propagation. Furthermore, numerical simulations of the model describing a cluster of electrically coupled beta-cells have supported our view that the experimentally observed calcium waves are due to electric pulses propagating through the cluster. This point of view is also supported by independent experimental results. Based on the model equations, an approximate analytical expression for the wave velocity is introduced, indicating which parameters can alter the velocity. We point to the possible role of the observed waves as signals controlling the insulin secretion inside the islets of Langerhans, in particular, in the regions that cannot be reached by any external stimuli such as high glucose concentration outside the islets.

Channel sharing in pancreatic beta -cells revisited: enhancement of emergent bursting by noise

Secretion of insulin by electrically coupled populations of pancreatic beta -cells is governed by bursting electrical activity. Isolated beta -cells, however, exhibit atypical bursting or continuous spike activity. We study bursting as an emergent property of the population, focussing on interactions among the subclass of spiking cells. These are modelled by equipping the fast subsystem with a saddle-node-loop bifurcation, which makes it monostable. Such cells can only spike tonically or remain silent when isolated, but can be induced to burst with weak diffusive coupling. With stronger coupling, the cells revert to tonic spiking. We demonstrate that the addition of noise dramatically increases, via a phenomenon like stochastic resonance, the coupling range over which bursting is seen.

Cyclic AMP and LDL trigger a rapid enhancement in gap junction assembly through a stimulation of connexin trafficking

Given the rapid turnover of connexin proteins, gap junction (GJ) assembly represents an important means of regulating the extent of GJ communication between cells. This report describes an increase in the level of GJ assembly within one hour following treatment with cAMP-elevating reagents or low density lipoprotein (LDL). Dye transfer methods and freeze-fracture with electron microscopy were used to assay junctional permeability and structure, respectively, subsequent to the dissociation, recovery and reaggregation of Novikoff hepatoma cells. Reaggregating cells in the presence of agents that increase cAMP levels (8-Br-cAMP, forskolin and IBMX) enhanced both dye transfer rates between cells and the extent of GJ formation 2- to 3-fold. These data and studies with the protein kinase A inhibitor, H-89, indicate that cAMP signaling plays a key role in enhanced assembly. The response to LDL parallels that to cAMP and relies on the activity of both adenylyl cyclase and protein kinase A. Immunoblot analysis revealed no change in the level of connexin43 (Cx43) or its phosphorylation states over a period of 2.5 hours. However, three agents (brefeldin A, monensin and nocodazole), that inhibit intracellular membrane trafficking by different mechanisms, all blocked the enhanced assembly of GJs when triggered by either elevated cAMP or exposure to LDL. Related studies, which employed trafficking inhibitors at different stages in GJ assembly, suggested that Cx43 trafficking during enhanced assembly is regulated, in part, by cell contact. Intracellular sources of Cx43 were characterized by colabeling for several markers of cytoplasmic membrane systems. We conclude that an increase in GJ assembly: (i) occurs rapidly in the presence of elevated cAMP or LDL, (ii) does not require an increase in Cx43 levels or major changes in Cx43 phosphorylation and (iii) is dependent upon the trafficking of Cx43 from intracellular storage sites.

Emergent global oscillations in heterogeneous excitable media: the example of pancreatic beta cells

Using the standard van der Pol-FitzHugh-Nagumo excitable medium model, I demonstrate a generic mechanism, diversity, that provokes the emergence of global oscillations from individually quiescent elements in heterogeneous excitable media. This mechanism may be operating in the mammalian pancreas, where excitable beta cells, quiescent when isolated, are found to oscillate when coupled, despite the absence of a pacemaker region.

Synchronization of Ca(2+)-signals within insulin-secreting pseudoislets: effects of gap-junctional uncouplers

The secretory response of the intact islet is greater than the response of individual beta-cells in isolation, and functional coupling between cells is critical in insulin release. The changes in intracellular Ca(2+)([Ca(2+)](i)) which initiate insulin secretory responses are synchronized between groups of cells within the islet, and gap-junctions are thought to play a central role in coordinating signalling events. We have used the MIN6 insulin-secreting cell line, to examine whether uncoupling gap-junctions alters the synchronicity of nutrient- and non-nutrient-evoked Ca(2+)oscillations, or affects insulin secretion. MIN6 cells express mRNA species that can be amplified using PCR primers for connexin 36. A commonly used gap-junctional inhibitor, heptanol, inhibited glucose- and tolbutamide-induced Ca(2+)-oscillations to basal levels in MIN6 cell clusters at concentrations of 0.5 mM and greater, and it had similar effects in pseudoislets when used at 2.5 mM. Lower heptanol concentrations altered the frequency of Ca(2+)transients without affecting their synchronicity, in both monolayers and pseudoislets. Heptanol also had effects on insulin secretion from MIN6 pseudoislets such that 1 mM enhanced secretion while 2.5 mM was inhibitory. These data suggest that heptanol has multiple effects in pancreatic beta-cells, none of which appears to be related to uncoupling of synchronicity of Ca(2+)signalling between cells. A second gap-junction uncoupler, 18 alpha-glycyrrhetinic acid, also failed to uncouple synchronized Ca(2+)-oscillations, and it had no effect on insulin secretion. These data provide evidence that Ca(2+)signalling events occur simultaneously across the bulk mass of the pseudoislet, and suggest that gap-junctions are not required to coordinate the synchronicity of these events, nor is communication via gap junctions essential for integrated insulin secretory responses.

Pathophysiology of impaired pulsatile insulin release

Plasma insulin displays 5-10 min oscillations. In Type 2 diabetes the regularity of the oscillations disappears, which may lead to insulin receptor down-regulation and glucose intolerance and explain why pulsatile delivery of the hormone has a greater hypoglycemic effect than continuous delivery. The rhythm is intrinsic to the islet. Variations in metabolism, cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), other hormones, neuronal signaling and possibly beta-cell insulin receptor expression have been implicated in the regulation of plasma insulin oscillations. Most of these factors are important for amplitude-regulation of the insulin pulses. Although evidence exists supporting a role of both metabolism and [Ca(2+)](i) as pacemakers of the pulses, metabolic oscillations probably have a primary role and [Ca(2+)](i) oscillations a permissive role. Results from islets from animal models of diabetes suggest that altered plasma insulin pattern could be due to lowering of pulse amplitude of insulin oscillations rather than alterations in their frequency. Supporting a role of metabolism, altered plasma insulin oscillations were found in MODY2, MIDD and glycogenosis Type VII, which are linked to alterations in glucokinase, mitochondrial tRNALeu(UUR) and phosphofructokinase. Plasma insulin oscillations require coordination of islet secretory activities in the pancreas. The intrapancreatic ganglia have been suggested as coordinators. The diabetes-associated neuropathy may contribute to the deranged pattern as indicated by glucose intolerance in chagasic patients. Continued investigation of the role and regulation of pulsatile insulin release will lead to better understanding of the pathophysiology of impaired pulsatile insulin release, which could lead to new approaches to restore normal plasma insulin oscillations in diabetes and related diseases.

Voltage-gated and resting membrane currents recorded from B-cells in intact mouse pancreatic islets

1. The perforated patch whole-cell configuration of the patch-clamp technique was applied to superficial cells in intact pancreatic islets. Immunostaining in combination with confocal microscopy revealed that the superficial cells consisted of 35 % insulin-secreting B-cells and 65 % non-B-cells (A- and D-cells). 2. Two types of cell, with distinct electrophysiological properties, could be functionally identified. One of these generated oscillatory electrical activity when the islet was exposed to 10 mM glucose and had the electrophysiological characteristics of isolated B-cells maintained in tissue culture. 3. The Ca2+ current recorded from B-cells in situ was 80 % larger than that of isolated B-cells. It exhibited significant (70 %) inactivation during 100 ms depolarisations. The inactivation was voltage dependent and particularly prominent during depolarisations evoking the largest Ca2+ currents. 4. Voltage-dependent K+ currents were observed during depolarisations to membrane potentials above -20 mV. These currents inactivated little during a 200 ms depolarisation and were unaffected by varying the holding potential between -90 and -30 mV. 5. The maximum resting conductance in the absence of glucose, which reflects the conductance of ATP-regulated K+ (KATP) channels, amounted to approximately 4 nS. Glucose produced a concentration-dependent reduction of KATP channel conductance with half-maximal inhibition observed with 5 mM glucose. 6. Combining voltage- and current-clamp recording allowed the estimation of the gap junction conductance between different B-cells. These experiments indicated that the input conductance of the B-cell at stimulatory glucose concentrations ( approximately 1 nS) is almost entirely accounted for by coupling to neighbouring B-cells.

Voltage-gated and resting membrane currents recorded from B-cells in intact mouse pancreatic islets

1. The perforated patch whole-cell configuration of the patch-clamp technique was applied to superficial cells in intact pancreatic islets. Immunostaining in combination with confocal microscopy revealed that the superficial cells consisted of 35 % insulin-secreting B-cells and 65 % non-B-cells (A- and D-cells). 2. Two types of cell, with distinct electrophysiological properties, could be functionally identified. One of these generated oscillatory electrical activity when the islet was exposed to 10 mM glucose and had the electrophysiological characteristics of isolated B-cells maintained in tissue culture. 3. The Ca2+ current recorded from B-cells in situ was 80 % larger than that of isolated B-cells. It exhibited significant (70 %) inactivation during 100 ms depolarisations. The inactivation was voltage dependent and particularly prominent during depolarisations evoking the largest Ca2+ currents. 4. Voltage-dependent K+ currents were observed during depolarisations to membrane potentials above -20 mV. These currents inactivated little during a 200 ms depolarisation and were unaffected by varying the holding potential between -90 and -30 mV. 5. The maximum resting conductance in the absence of glucose, which reflects the conductance of ATP-regulated K+ (KATP) channels, amounted to approximately 4 nS. Glucose produced a concentration-dependent reduction of KATP channel conductance with half-maximal inhibition observed with 5 mM glucose. 6. Combining voltage- and current-clamp recording allowed the estimation of the gap junction conductance between different B-cells. These experiments indicated that the input conductance of the B-cell at stimulatory glucose concentrations ( approximately 1 nS) is almost entirely accounted for by coupling to neighbouring B-cells.

Influence of cell number on the characteristics and synchrony of Ca2+ oscillations in clusters of mouse pancreatic islet cells

1. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in single cells and cell clusters of different sizes prepared from mouse pancreatic islets. 2. During stimulation with 15 mM glucose, 20 % of isolated cells were inert, whereas 80 % showed [Ca2+]i oscillations of variable amplitude, duration and frequency. Spectral analysis identified a major frequency of 0.14 min-1 and a less prominent one of 0.27 min-1. 3. In contrast, practically all clusters (2-50 cells) responded to glucose, and no inert cells were identified within the clusters. As compared to single cells, mean [Ca2+]i was more elevated, [Ca2+]i oscillations were more regular and their major frequency was slightly higher (but reached a plateau at approximately 0.25 min-1). In some cells and clusters, faster oscillations occurred on top of the slow ones, between them or randomly. 4. Image analysis revealed that the regular [Ca2+]i oscillations were well synchronized between all cells of the clusters. Even when the Ca2+ response was irregular, slow and fast [Ca2+]i oscillations induced by glucose were also synchronous in all cells. 5. In contrast, [Ca2+]i oscillations resulting from mobilization of intracellular Ca2+ by acetylcholine were restricted to certain cells only and were not synchronized. 6. Heptanol and 18alpha-glycyrrhetinic acid, two agents widely used to block gap junctions, altered glucose-induced Ca2+ oscillations, but control experiments showed that they also exerted effects other than a selective uncoupling of the cells. 7. The results support theoretical models predicting an increased regularity of glucose-dependent oscillatory events in clusters as compared to isolated islet cells, but contradict the proposal that the frequency of the oscillations increases with the number of coupled cells. Islet cell clusters function better as electrical than biochemical syncytia. This may explain the co-ordination of [Ca2+]i oscillations driven by depolarization-dependent Ca2+ influx during glucose stimulation.

Slow wave and spike action potentials recorded in cell cultures from the muscularis externa of the guinea pig small intestine

Intracellular recordings were obtained to investigate whether slow wave and spike type action potentials are present in cell cultures of the muscularis externa from the guinea pig small intestine. The muscularis externa of the small intestine was dissociated by using specific purified enzymes and gentle mechanical dissociation. Cells were plated on cover slips and maintained in culture for up to 4 weeks. Dissociated cells obtained in this way reorganized themselves in a few days to form small cell clumps showing spontaneous movements. Intracellular recordings of these clumps displayed both spike and slow wave type action potentials. Spikes were observed on top of some slow waves and were abolished by the addition of nifedipine or the removal of extracellular calcium. Slow waves, however, were nifedipine insensitive and temperature sensitive, and were abolished by octanol (a gap junction blocker) and forskolin (an adenyl cyclase activator). Slow waves were never observed in small clumps (<50 µm), suggesting that a critical mass of cells might be required for their generation. These observations demonstrated for the first time the presence of nifedipine-insensitive slow waves in cell cultures of the muscularis externa from the guinea pig small intestine. Cell cultures allow rigorous control of the immediate environment for the cells and this should facilitate future studies on the molecular and cellular mechanisms responsible for the slow waves in the gastrointestinal tract.Key words: smooth muscle, slow waves, spiking activity, gastrointestinal tract, gut, small intestine, electrophysiology, pacemaker activity, guinea pig.

Heptanol exerts epileptiform effects in identified neurons of the buccal ganglia of Helix pomatia

1-Heptanol (0.2-5.0 mM) known to block electrical contacts was tested under epileptic and non-epileptic conditions in the buccal ganglia of Helix pomatia. Synchronicity of epileptiform activity was not affected. In concentrations below 1 mM, heptanol accelerated epileptiform activity induced by pentylenetetrazol. In concentrations above 1 mM, it evoked epileptiform activity without admixture of an epileptogenic drug. Coupling coefficient was increased and decreased in low and high concentration ranges of heptanol, respectively. The measured decrease of coupling is interpreted as a result of the activation of 'epileptiform' membrane conductances accompanied by decreased length constants of neuronal fibers.

Role of gap junctions in structural arrangements of interstitial cells of Cajal and canine ileal smooth muscle

We examined the structural and functional basis for pacemaking by interstitial cells of Cajal (ICC) in circular smooth muscle of the canine ileum. Gap junctions were found between ICC of myenteric plexus (MyP), occasionally between MyP ICC and outer circular smooth muscle cells, between individual outer circular smooth muscle cells, between them and ICC of the deep muscular plexus (DMP), and between DMP ICC. No visible gap junctions connected MyP ICC to longitudinal muscle cells or inner circular muscle cells. Occasionally contacts occurred between the two muscle layers. No special structures were found to connect MyP and DMP ICC networks. Octanol concentration dependently reduced the amplitude and frequency of, but did not abolish, slow waves in circular muscle in isolated ileum recorded near the MyP or the DMP. Slow waves triggered from MyP ICC by a current pulse also persisted. Contractile activity was abolished, cells were depolarized, and fast inhibitory junction potentials were reduced by octanol. We conclude that ICC pacemakers of the MyP and DMP utilize gap junctional conductances for pacemaking function but may not require them. Coupling between the two ICC networks may utilize the circular muscle syncytium.

Mechanisms for the coordination of intercellular calcium signaling in insulin-secreting cells

Insulin-mediated increases in cytosolic calcium are synchronized among the cells in a pancreatic islet, and result in pulsatile secretion of insulin. Pancreatic beta cells express the gap junction protein connexin43 and are functionally coupled, making gap junctional communication a likely mechanism for the synchronization of calcium transients among islet cells. To define the mechanism by which pancreatic islet cells coordinate calcium responses, we studied mechanically-induced intercellular calcium waves in the communication-deficient rat insulinoma cell line RINm5f, and in RINm5f cells transfected with the gap junction protein connexin43. Both RINm5f and RINm5f cells transfected with connexin43 propagated calcium waves that required release of calcium from intracellular stores, did not involve gap junctional communication, and appeared to be mediated by autocrine activity of secreted ATP acting on P2U purinergic receptors. Connexin43 transfectants also propagated calcium waves that required gap junctional communication and influx of extracellular calcium through voltage-gated calcium channels. Gap junction-dependent intercellular calcium waves were inhibited by preventing plasma membrane depolarization. These studies demonstrate two distinct pathways by which insulin-secreting cells can coordinate cytosolic calcium rises, and show that it is by ionic traffic that gap junctions synchronize calcium-dependent events in these cells.

Oscillation of gap junction electrical coupling in the mouse pancreatic islets of Langerhans

1. Pancreatic beta-cells oscillate synchronously when grouped in islets. Coupling seems essential to maintain this oscillatory behaviour, as isolated cells are unable to oscillate. This allows the islet to be used as a model system for studying the role of coupling in the generation of oscillatory patterns. 2. Pairs of beta-cells were intracellularly recorded in islets. beta-Cells oscillated synchronously. Propagated voltage deflections were observed as a function of glucose concentration and of the distance between the recording electrodes. Space constants were smaller in the silent than in the active phases, suggesting a higher intercellular connection in the active phases. 3. Coupling coefficients and estimated coupling conductances were larger in the active than in the silent phases. 4. Coupling coefficients and coupling conductances changed dynamically and in phase with the membrane potential oscillations, pointing to an active modulation of the gap junctions. 5. We hypothesize a role for coupling in the generation of the oscillatory events, providing different levels of permeability dependent on the state of conductance during the oscillatory phases.

Changes of intracellular free calcium following mechanical injury in a spinal cord slice preparation

Intracellular calcium ions are, in addition to free radicals, an important mediator of tissue destruction following traumatic injury to the spinal cord. In vivo measurements of calcium in the interstitial space and in the tissue suggest the occurrence of a posttraumatic shift of calcium from the extracellular to the intracellular compartment at the injury site. No information is, however, available on the posttraumatic changes of calcium in the intracellular compartment, where the ion exerts its crucial messenger function. We developed an in vitro model of local traumatic spinal injury, using a spinal cord slice preparation, allowing us to investigate injury-related changes of intracellular free calcium. The injury consisted of the impact of a small needle, and intracellular free calcium was measured with fura-2. Application of the injury at different places within the gray matter caused a transient and reproducible increase in the fura-2 fluorescence ratio. This injury-induced ratio increase was largely, but not completely, suppressed under zero extracellular calcium conditions. It was also largely depressed in the presence of high extracellular potassium and in the absence of extracellular sodium. It was modestly depressed by the calcium channel blocker nifedipin, by the calcium release channel blocker dantrolene, and by the gap junction blockers halothane and octanol. The calcium channel blocker flunarizine, the N-methyl D-aspartate (NMDA)-receptor-channel blocker MK-801 and the endoplasmic reticulum calcium-ATPase blocker thapsigargin had no effect. The experiments suggest that injury is associated with an increase in intracellular free calcium that is mediated by calcium influx, in part via L-type calcium channels. They furthermore give evidence that sodium influx and gap junctions are involved in these injury-associated changes of intracellular free calcium.

Quinine, intracellular pH and modulation of hemi-gap junctions in catfish horizontal cells

Quinine increases the conductance of hemi-gap junctions in horizontal cells. We investigated the mechanisms of alkalinization and the hypothesis that quinine-induced alkalinization produced these conductance increases. We found that quinine-induced alkalinizations were not blocked by cobalt, amiloride, or DIDS. Therefore, this suggests that the alkalinization is not likely due to net proton flux through opened hemi-gap channels nor is it likely due to an action on Cl-/HCO3- exchanger or Na+/H+ exchanger, both of which are known to regulate pHi in the horizontal cells. Quinine increased hemi-gap conductance even when cells were recorded with patch pipets containing up to 80 mM HEPES. We conclude that quinine-induced alkalinization cannot account solely for the hemi-gap junctional conductance increases.

Contributions of modeling to understanding stimulus-secretion coupling in pancreatic beta-cells

Mechanisms of ionic control of insulin secretion in beta-cells of the pancreatic islets of Langerhans are reviewed. The focus is on aspects that have been treated by mathematical models, especially those related to bursting electrical activity. The study of these mechanisms is difficult because of the need to consider ionic fluxes, calcium handling, metabolism, and electrical coupling with other cells in the islet. The data come either from islets, where experimental maneuvers tend to have multiple effects, or from isolated cells, which have degraded electrical activity and secretory sensitivity. Modeling aids in the process by integrating data on individual components such as channels and calcium handling and testing hypotheses for coherence and quantitative plausibility. The study of a variety of models has led to some general mathematical results that have yielded qualitative model-independent insights.

In vivo synchronous membrane potential oscillations in mouse pancreatic beta-cells: lack of co-ordination between islets

1. The properties of the oscillations in electrical activity of different beta-cells within the same islet of Langerhans, and of different islets within the same pancreas, recorded in vivo, are described. 2. Simultaneous recordings of two cells within the same islet showed that the oscillations were synchronous. A rapid increase in blood glucose led to the simultaneous appearance of a transitory phase of continuous electrical activity in both cells. These results indicate that under physiological conditions, the islets operate as a functional syncytium. 3. Simultaneous recordings of cells from two different islets within the same pancreas showed that the oscillations in the electrical activity were not synchronous, which suggests that each islet is a functionally independent unit. Rapid changes in blood glucose led to the appearance of a transitory phase of increased electrical activity in both islets, although of different duration. These results suggest that the endocrine pancreas lacks a pacemaker driving the electrical activity of all the islets. 4. The comparison of the degree of activation of different islets, simultaneously recorded at different glucose concentrations, indicated that all the islets had a similar sensitivity to glucose. Furthermore, when the glucose concentration was increased, the electrical activity in both islets increased in parallel, suggesting that the amount of insulin released due to the increase in glycaemia was produced by the simultaneous response of all the islets and not by the recruitment of islets with different sensitivities to glucose. 5. Our results predict that the synchronous electrical activity of all the cells within an islet will result in widespread intracellular calcium oscillations and pulsatile insulin secretion. The periodicity of the pulses of insulin secretion in different islets is suggested to be of slightly different length and asynchronous.

The intrinsic optical signal evoked by chiasm stimulation in the rat suprachiasmatic nuclei exhibits GABAergic day-night variation

Infrared light transmittance imaging was used in rat hypothalamic slices to record an intrinsic optical signal (IOS) of the cell ensemble in the suprachiasmatic nuclei (SCN), the locus of the endogenous circadian clock. Upon optic chiasm stimulation, a transient IOS was observed in an area conforming to the known retinohypothalamic tract innervation in the ventral SCN. An increase in extracellular Mg2+ concentration to 10 mM reduced th IOS, suggesting that the elicited IOS is dependent on synaptic transmission. D-2-amino-5-phosphonopentanoic acid and muscimol suppressed the elicited IOS, indicating that NMDA and GABAA receptor-mediated mechanisms were involved in cell ensemble activity reflected in the IOS. The extracellularly recorded spiking of SCN neurons located outside the IOS area remained largely unaffected by the afferent stimulus. Neurons located within the IOS area responded with a depressed electrical discharge, manifesting an inverse relationship between single-unit discharge and the optical measure. The influence of the endogenous circadian rhythm on the elicited IOS was assessed by carrying out daytime-dependent concentration-response experiments. NMDA and non-NMDA receptor specific compounds did not exhibit significant day-night differences, whereas GABA-specific ligands showed a significant day-night variation in activity. The competitive GABAA receptor antagonist bicuculline enhanced the IOS exclusively in the daytime SCN. 5alpha-Pregnane-3alpha,21-diol-20-one (allotetrahydrodeoxycorticosterone), a neuroactive steroid that potentiates GABAergic inhibition, suppressed the IOS in the night-time SCN more than in the daytime SCN. This suggests that in the rat the level of extracellular GABA is higher in night-time SCN compared to the daytime SCN.

Amperometric detection of quantal secretion from patch-clamped rat pancreatic beta-cells

Serotonin (5-HT) is taken up in insulin granules and co-released with insulin on stimulation of pancreatic islet beta-cells. Based on these observations, we have used microcarbon fiber amperometry to examine secretogogue-induced 5-HT release from rat beta-cells preloaded for 4-16 h with 5-HT and then exposed to a bath solution containing 10 microM forskolin. In response to local application of KCl (60 mM) or tolbutamide (50-200 microM), we recorded barrages of amperometric events. Each amperometric event consisted of a short pulse of current measurable at electrode voltages that catalyze 5-HT oxidation. With either secretogogue, release was calcium-dependent. On combining amperometry with perforated patch whole-cell recording, we found that barrages of such events were well coupled in time and graded in intensity with depolarization-induced Ca2+ currents and well correlated with increases in membrane capacitance. In cell-attached patch recording, amperometric events evoked by application of tolbutamide followed the closure of ATP-sensitive K+ channels and coincided with the onset of electrical activity. These experiments suggest that amperometry is a useful technique for studying, in real time, the dynamic aspects of stimulus-secretion coupling in beta-cells. Their performance was facilitated by the design of a new carbon fiber electrode (ProCFE) described within.

An ultrastructural analysis of the epithelial-fiber interface (EFI) in primate lenses

The purpose of this study was to conduct a comprehensive ultrastructural analysis of the epithelial-fiber interface (EFI) in normal adult primate (Macaque nemestrina and fascicularis; 6-9 years old, n = 10) lenses. Scanning electron microscopy (SEM) was used to initially characterize the gross size, shape and three-dimensional organization of central zone (cz) epithelial cells and the anterior ends of elongating fibers beneath these cells. This fiducial information was essential to properly orient lens pieces in freeze fracture specimen carriers for the production of replicas with unambiguously identifiable EFI. Transmission electron microscopy (TEM) of replicas and thin-sectioned material were used to ultrastructurally analyse the cz EFI. TEM thin-sectioned material was also used to ultrastructurally analyse the pregerminative (pgz), germinative (gz) and transitional zone (tz) EFI. Correlative SEM and TEM of cz EFI components revealed that the apical membrane of both epithelial and elongating fiber cells were irregularly polygonal in shape, and aligned in parallel as smooth, concave-convex surfaces. However, whereas epithelial cell apical surfaces had minimal size variation, elongating fibers were larger and considerably variable in size. Quantitative analysis of > 10000 micron2 cz elongating fiber apical surfaces failed to detect any gap junctions defined in freeze fracture replicas as complementary aggregates of transmembrane proteins (connexons) conjoined across a narrowed extracellular space. However, a comparable frequency of vesicular events was noted in this region as quantified previously in adult and embryonic chick lens. Correlative TEM analysis > 1500 linear micrometers of thin-sectioned EFI from this region confirmed the presence of epithelial-epithelial gap junctions, elongating fiber-elongating fiber gap junctions, and an extreme paucity of epithelial-elongating fiber gap junctions. In contrast, TEM analysis of > 1000 linear micrometers of thin-sectioned pgz, gz and tz EFI, confirmed the presence of epithelial-epithelial gap junctions, elongating fiber-elongating fiber gap junctions, numerous epithelial-elongating fiber adherens junctions and a few epithelial-elongating fiber gap junctions. Thus, the results of this and previous quantitative morphological and physiological studies (electronic and dye coupling) demonstrate that there is limited coupling between cz epithelial cells and underlying elongating fibers. Furthermore, the absence of gap junctional plaques in cz EFI freeze-fracture replicas and either pentalaminar or septalaminar profiles in correlative thin-sections, suggests that this limited coupling could be mediated via isolated gap junction channels. However, the results of this and previous quantitative studies further show that a greater degree of coupling exists across the pgz, gz and tz regions of the EFI and that this coupling is likely to be mediated by gap junction plaques. Finally, this and other studies continue to demonstrate that transcytotic processes play a role in lens physiology at the EFI.