Cytoplasmic Ca2+ oscillations in pancreatic ß-cells (original) (raw)
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Diabetes, 1999
Normal mouse islets were used to determine whether oscillations of these three signals are able and necessary to trigger oscillations of insulin secretion. The approach was to minimize or abolish spontaneous oscillations and to compare the impact of forced oscillations of each signal on insulin secretion. In a control medium, repetitive increases in the glucose concentration triggered oscillations in metabolism [NAD(P)H fluorescence], [Ca 2+ ] i (fura-PE3 method), and insulin secretion. In the presence of diazoxide, metabolic oscillations persisted, but [Ca 2 + ] i and insulin oscillations were abolished. When the islets were depolarized with high K + with or without diazoxide, [ C a 2 + ] i was elevated, and insulin secretion was stimulated. Forced metabolic oscillations transiently decreased or did not affect [Ca 2 + ] i and potentiated insulin secretion with oscillations of small amplitude. These oscillations of secretion followed metabolic oscillations only when [Ca 2+ ] i did not change. When [Ca 2+ ] i fluctuated, these changes prevailed over those of metabolism for timing secretion.
Journal of Endocrinology, 2000
Oscillation of insulin release by the pancreatic islets was evaluated under stringent Ca 2+-free conditions for the first time. Isolated single rat islets were exposed to 16•7 mM glucose in the presence of 1•9 mM Ca 2+ , or under the stringent Ca 2+-free conditions (Ca 2+ omission with 1 mM EGTA, 6 µM forskolin and 100 nM phorbol 12-myristate 13-acetate). Fifteen minutes after the initiation of glucose stimulation, effluent was collected at a 6-s interval, insulin was determined in duplicate by a highly sensitive insulin radioimmunoassay, and oscillation and pulsatility of release statistically analyzed. Significant oscillation of insulin release was observed in all islets irrespective of presence and absence of Ca 2+. Significant pulsatility of release was detected in 7 of 11 islets in the presence of Ca 2+ and three of six islets in the absence of Ca 2+. In conclusion, high glucose elicits oscillatory insulin release both in the presence and absence of extracellular Ca 2+ .
Cell Calcium, 1992
Glucose stimulation of individual the cytoplasmic Ca*+ ancreatic &cells is associated with a rise of concentration ([Ca* i) 5 manifested either as large amplitude oscillations (0.2-O.Umin) or as a sustained increase. Determinants for the transitions between the basal and the two stimulated states have now been studied using dual-wavelength fluorometric measurements on individual ob/ob mouse f&cells loaded with the Ca*+ indicator Fura-2. The transition from the basal state to large amplitude oscillations was induced by raising the glucose concentration to 7 mM or above. The frequencies and shapes of the [Ca*+]i cycles remained largely unaffected when raising glucose as high as 40 mM. However, in some cells the oscillatory pattern was transformed into a sustained increase of [Ca*+]i at high glucose concentrations. Although the peak values for the oscillations exceeded the steady-state increase, the time average [Ca*+]i was higher during the latter phase. Both types of glucose-induced transitions were facilitated by the presence of l-100 nM glucagon. Protein kinase C activation by IO nfvl of the phorbol ester TPA resulted in a transformation of the glucose-induced oscillations into a sustained increase of [Ca*+]i but the levels reached were considerably lower than obtained with glucose alone. It is concluded that the glucose sensing of the individual &cell is based on sudden transitions between steady-state and oscillating cytoplasmic Ca*+. It is these transitions rather than alterations of the oscillatory characteristics which determine the average [Ca*+]i regulating insulin release. Glucose stimulation of insulin release depends on a rise of the cytoplasmic Ca2+ concentration ([Ca2+]i)
Glucose-induced oscillations of cytoplasmic Ca2+ in the pancreatic β-cell
Biochemical and Biophysical Research Communications, 1988
The cytoplasmic calcium concentration (Ca2+i) was measured in individual mouse pancreatic G-cells loaded with fura-2 by recording the 340/380 nm fluorescence excitation ratio. An increase of the glucose concentration from 3 to 20 mM, caused initial lowering of Ca2+i followed by a rise with a peak preceding constant elevation at an intermediary level. However, at ii mM glucose there were large Ca2+i oscillations with a frequency of 1 cycle per 2-6 min. The results indicate that both first and second phase secretion depend on elevated Ca2+i, and that many electrically coupled cells collectively determine the pace of rhythmic depolarization.
Diabetologia, 1994
Plasma insulin levels in healthy subjects oscillate and non-insulin-dependent diabetic patients display an irregular pattern of such oscillations. Since an increase in cytoplasmic free Ca 2+ concentration ([Ca 2+ ]i) in the pancreatic beta cell is the major stimulus for insulin release, this study was undertaken to investigate the dynamics of electrical activity, [Ca a+ ]i-changes and insulin release, in stimulated islets from subjects of varying glucose tolerance. In four patients it was possible to investigate more than one of these three parameters. Stimulation of pancreatic islets with glucose and tolbutamide sometimes resulted in the appearance of oscillations in [ Ca2+ ]i, lasting 2-3 rain. Such oscillations were observed even in some islets from patients with impaired glucose tolerance. In one islet from a diabetic patient there was no response to glucose, whereas that islet displayed [Ca 2+ ]i-oscillations in response to tolbutamide, suggesting that sulphonylurea treatment can mimic the complex pattern of glucose-in-duced [Ca 2+ ]i-oscillations. We also, for the first time, made patch-clamp recordings of membrane currents in beta-cells in situ in the islet. Stimulation with glucose and tolbutamide resulted in depolarization and appearance of action potentials. The islet preparations responded to stimulation with a number of different secretagogues with release of insulin. The present study shows that human islets can respond to stimulation with glucose and sulphonylurea with oscillations in [Ca 2+ ]i, which is the signal probably underlying the oscillations in plasma insulin levels observed in healthy subjects. Interestingly, even subjects with impaired glucose tolerance had islets that responded with oscillations in [Ca 2+ ]i upon glucose stimulation, although it is not known to what extent the response of these islets was representative of most islets in these patients. [Diabetologia (1994[Diabetologia ( ) 37: 1121[Diabetologia ( -1131
Glucoseminduced [Ca2+]i oscillations in single human pancreatic islets
Cell Calcium, 1996
Changes in cytosolic free calcium concentration ([Ca"],) in response to stimulatory glucose concentrations were investigated in human pancreatic islets, using Fura-fluorescence imaging. Increasing glucose concentration from 3 to 11 mM caused a triphasic [Ca*+], response in human islets: an initial decrease (phase l), a rapid and transient increase (phase 2) and periodic oscillations with a frequency of 1 + 0.3 min (phase 3). Raising the glucose concentration from 11 to 16.7 mM lowered the frequency of the glucose-induced [Ca'+], oscillations to 0.15 + 0.2 min, without changes in their amplitude. Human islet [Ca'+], response to stimulatory glucose concentrations is synchronous throughout the islet. Freshly isolated human islets responded to tolbutamide (50 uM) with a rise in [Ca"+],. An increase in glucose concentration, from 3 to 16 mM, in the presence of 100 uM diazoxide, produced a decrease in [Ca'+],. It is concluded that human islets respond to glucose with regular [Ca'+], oscillations that are synchronous throughout the islet and whose duration is modulated by glucose.
Diabetes, 2006
Homeostasis of blood glucose is mainly regulated by the coordinated secretion of glucagon and insulin from ␣and -cells within the islets of Langerhans. The release of both hormones is Ca 2؉ dependent. In the current study, we used confocal microscopy and immunocytochemistry to unequivocally characterize the glucose-induced Ca 2؉ signals in ␣and -cells within intact human islets. Extracellular glucose stimulation induced an opposite response in these two cell types. Although the intracellular Ca 2؉ concentration ([Ca 2؉ ] i) in -cells remained stable at low glucose concentrations, ␣-cells exhibited an oscillatory [Ca 2؉ ] i response. Conversely, the elevation of extracellular glucose elicited an oscillatory [Ca 2؉ ] i pattern in -cells but inhibited lowglucose-induced [Ca 2؉ ] i signals in ␣-cells. These Ca 2؉ signals were synchronic among -cells grouped in clusters within the islet, although they were not coordinated among the whole -cell population. The response of ␣-cells was totally asynchronic. Therefore, both the ␣and -cell populations within human islets did not work as a syncitium in response to glucose. A deeper knowledge of ␣and -cell behavior within intact human islets is important to better understand the physiology of the human endocrine pancreas and may be useful to select high-quality islets for transplantation.