Nonrandom distribution of gap junctions between pancreatic beta-cells

The numerical and spatial distribution of gap junctions between insulin-containing cells (beta-cells) under resting and stimulated conditions of insulin secretion were quantitatively analyzed in freeze-fracture replicas of isolated rat islets of Langerhans. The results show that the beta-cells located at the periphery of the islet have twice as many gap junctions per unit membrane area as the beta-cells situated in the islet center. In both locations, gap junctions assumed a nonrandom clustering on the beta-cell membranes. During stimulation of insulin secretion, the gap junctions were found increased between the central and between the peripheral beta-cells. The degree of their clustering was also modified. The latter change depended both on the location of the gap junctions in the islet and on the type of stimulation used (high glucose or glibenclamide).

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Tetraethylammonium modifies gap junctions between pancreatic beta-cells

AJP Cell Physiology ◽

10.1152/ajpcell.1981.240.3.c116 ◽

1981 ◽

Vol 240 (3) ◽

pp. C116-C120 ◽

Cited By ~ 12

Author(s):

M. S. Sheppard ◽

P. Meda

Keyword(s):

Beta Cell ◽

Gap Junctions ◽

Beta Cells ◽

Insulin Release ◽

Pancreatic Beta Cells ◽

Freeze Fracture ◽

Potassium Conductance ◽

Median Number ◽

Rat Islets Of Langerhans ◽

Gap Junctional

Gap junctions between pancreatic beta-cells were quantitatively assessed in freeze-fracture replicas of isolated rat islets of Langerhans incubated for 90 min with or without the potassium conductance blocker tetraethylammonium (TEA). The results show that TEA increases the median number of particles per beta-cell gap junction but not the frequency of gap junctions at both nonstimulating and threshold-stimulating concentrations of glucose. TEA increased the relative gap junctional area at both concentrations of glucose. TEA had no effect on insulin release at a basal concentration of glucose but potentiated that release at the threshold glucose level. Thus TEA modifies beta-cell gap junctions independently of its effect on insulin release. However, the junctional changes observed were greater when insulin release was also elevated.

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Increase of gap junctions between pancreatic B-cells during stimulation of insulin secretion.

The Journal of Cell Biology ◽

10.1083/jcb.82.2.441 ◽

1979 ◽

Vol 82 (2) ◽

pp. 441-448 ◽

Cited By ~ 109

Author(s):

P Meda ◽

A Perrelet ◽

L Orci

Keyword(s):

Insulin Secretion ◽

B Cells ◽

Gap Junctions ◽

Freeze Fracture ◽

Secretory Activity ◽

Isolated Rat Islets ◽

Pancreatic B Cells ◽

Stimulation Of

The development of gap junctions between pancreatic B-cells was quantitatively assessed in freeze-fracture replicas of isolated rat islets under different conditions of insulin secretion. The results show that in resting B-cells, gap junctions are small and scarce but that these junctions increase when insulin secretion is stimulated. Both a short (90 min) stimulation by glucose in vitro and a prolonged (2.5 d) stimulation by glibenclamide in vivo raise the number of gap junctions; in addition, the glibenclamide stimulation causes an increase in the size of individual gap junctions. As a consequence, the total area occupied by gap junctions on the B-cell membrane and the ratio of this area to the cell volume were found significantly increased in the latter condition. The slight increase of these values observed after the glucose stimulation did not reach significance. These data indicate a change of gap junctions during the secretory activity of the pancreatic B-cells. The possibility that the coupling of the cells is affected by the treatment is discussed.

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Studies on the role of inositol trisphosphate in the regulation of insulin secretion from isolated rat islets of Langerhans

Biochemical Journal ◽

10.1042/bj2280713 ◽

1985 ◽

Vol 228 (3) ◽

pp. 713-718 ◽

Cited By ~ 36

Author(s):

N G Morgan ◽

G M Rumford ◽

W Montague

Keyword(s):

Insulin Secretion ◽

Islets Of Langerhans ◽

Islet Cells ◽

Inositol Trisphosphate ◽

Rat Islets ◽

Isolated Rat Islets ◽

Rat Islets Of Langerhans ◽

Isolated Rat ◽

Stimulation Of

Glucose (20 mM) and carbachol (1 mM) produced a rapid increase in [3H]inositol trisphosphate (InsP3) formation in isolated rat islets of Langerhans prelabelled with myo-[3H]inositol. The magnitude of the increase in InsP3 formation was similar when either agent was used alone and was additive when they were used together. In islets prelabelled with 45Ca2+ and treated with carbachol (1 mM), the rise in InsP3 correlated with a rapid, transient, release of 45Ca2+ from the cells, consistent with mobilization of 45Ca2+ from an intracellular pool. Under these conditions, however, insulin secretion was not increased. In contrast, islets prelabelled with 45Ca2+ and exposed to 20mM-glucose exhibited a delayed and decreased 45Ca2+ efflux, but released 7-8-fold more insulin than did those exposed to carbachol. Depletion of extracellular Ca2+ failed to modify the increase in InsP3 elicited by either glucose or carbachol, whereas it selectively inhibited the efflux of 45Ca2+ induced by glucose in preloaded islets. Under these conditions, however, glucose was still able to induce a small stimulation of the first phase of insulin secretion. These results demonstrate that polyphosphoinositide metabolism, Ca2+ mobilization and insulin release can all be dissociated in islet cells, and suggest that glucose and carbachol regulate these parameters by different mechanisms.

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Glucose-induced oscillatory insulin secretion in perifused rat pancreatic islets and clonal beta-cells (HIT)

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.1996.271.4.e702 ◽

1996 ◽

Vol 271 (4) ◽

pp. E702-E710 ◽

Cited By ~ 16

Author(s):

B. A. Cunningham ◽

J. T. Deeney ◽

C. R. Bliss ◽

B. E. Corkey ◽

K. Tornheim

Keyword(s):

Insulin Secretion ◽

Beta Cells ◽

Pancreatic Beta Cells ◽

Basal Secretion ◽

Oscillation Pattern ◽

Glucose Stimulation ◽

High Concentrations ◽

Perifused Islets ◽

Stimulation Of

Normal insulin secretion is oscillatory in vivo and from groups of perifused islets. Stimulation of rat islets with different glucose concentrations gave insulin oscillations of similar period (5-8 min) but increasing amplitude. It has been assumed that oscillatory secretion is due to oscillations in intracellular free Ca2+, as seen in single islets and single pancreatic beta-cells. However, when islets were perifused with diazoxide and high KCl to maintain high intracellular free Ca2+, insulin oscillations of similar amplitude and period still occurred on glucose stimulation, although superimposed on elevated basal secretion. Several likely possibilities for a diffusible synchronizing factor were tested, including pyruvate, lactate, ATP, and insulin itself; nevertheless, perifusion with high concentrations of these did not prevent insulin oscillations. Clonal pancreatic beta-cells (HIT) and dissociated islets also exhibited oscillatory insulin secretion, but with the 5- to 8-min period oscillations superimposed on 15- to 20-min period oscillations. These results indicate that the mechanisms for generating and synchronizing insulin oscillations reside in the beta-cell, although the structure of the islet may modulate the oscillation pattern.

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Effects of the calcium-channel agonist CGP 28392 on insulin secretion from isolated rat islets of Langerhans

Biochemical Journal ◽

10.1042/bj2310629 ◽

1985 ◽

Vol 231 (3) ◽

pp. 629-634 ◽

Cited By ~ 5

Author(s):

N G Morgan ◽

C D Short ◽

G M Rumford ◽

W Montague

Keyword(s):

Insulin Secretion ◽

Islets Of Langerhans ◽

A Value ◽

Isolated Rat Islets ◽

Rat Islets Of Langerhans ◽

Dose Dependent ◽

Dihydropyridine Derivatives ◽

Ca2 Channels ◽

Isolated Rat ◽

Stimulation Of

The rate of insulin secretion from isolated rat islets of Langerhans was affected by a number of dihydropyridine derivatives known to interact with voltage-sensitive Ca2+ channels in excitable cells. The channel antagonists nifedipine and nitrendipine were potent inhibitors of glucose-induced insulin secretion in response to both 8 mM- and 20 mM-glucose, although they did not lower the basal secretion rate observed in the presence of 4 mM-glucose. The Ca2+-channel agonist, CGP 28392, also failed to alter the basal rate of insulin secretion. In the presence of 8 mM-glucose, however, 1 microM-CGP 28392 enhanced the insulin-secretion rate to a value approximately double that with 8 mM-glucose alone. This effect was dose-dependent, with half the maximal response elicited by 0.1 microM-CGP 28392, and full enhancement at 10 microM. The response was rapid in onset, with an increase in insulin secretion evident within 2 min of CGP 28392 infusion in perifused islets. Stimulation of insulin secretion by CGP 28392 was correlated with a rapid enhancement of glucose-stimulated 45Ca2+ uptake into islets cells, and with a transiently increased rate of 45Ca2+ efflux from pre-loaded islets. Stimulation of insulin secretion by CGP 28392 was abolished in the presence of noradrenaline, although under these conditions the rapid stimulation of 45Ca2+ influx induced by CGP 28392 was only partially inhibited. In contrast with these results, when islets were incubated in the presence of 20 mM-glucose, CGP 28392 caused a dose-dependent inhibition of insulin secretion. Half-maximal inhibition required approx. 0.2 microM-CGP 28392, with maximal effects observed at 10 microM. Under these conditions, however, the extent of insulin secretion was still only decreased by about 50%, to a value which was similar to that seen in the presence of 8 mM-glucose and CGP 28392. These results suggest that dihydropyridine derivatives can alter the activity of voltage-dependent Ca2+ channels in islet cells, and are consistent with the possibility that gating of these channels plays an important role in regulating the rate of insulin secretion after glucose stimulation.

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Gap junctions in the prothoracic glands of the tobacco hornworm, Manduca sexta

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100123933 ◽

1992 ◽

Vol 50 (1) ◽

pp. 706-707

Author(s):

Ji-da Dai ◽

M. Joseph Costello ◽

Lawrence I. Gilbert

Keyword(s):

Gap Junctions ◽

Small Molecules ◽

Manduca Sexta ◽

Freeze Fracture ◽

Cell Communication ◽

Cellular Level ◽

Thin Sections ◽

Prothoracic Glands ◽

Polyhydroxylated Steroids ◽

Stimulation Of

Insect molting and metamorphosis are elicited by a class of polyhydroxylated steroids, ecdysteroids, that originate in the prothoracic glands (PGs). Prothoracicotropic hormone stimulation of steroidogenesis by the PGs at the cellular level involves both calcium and cAMP. Cell-to-cell communication mediated by gap junctions may play a key role in regulating signal transduction by controlling the transmission of small molecules and ions between adjacent cells. This is the first report of gap junctions in the PGs, the evidence obtained by means of SEM, thin sections and freeze-fracture replicas.

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