Different metabolic responses in alpha-, beta-, and delta-cells of the islet of Langerhans monitored by redox confocal microscopy

Abstract

Blood glucose homeostasis is mainly achieved by the coordinated function of pancreatic alpha-, beta-, and delta-cells, which secrete glucagon, insulin, and somatostatin, respectively. Each cell type responds to glucose changes with different secretion patterns. Currently, considerable information can be found about the signal transduction mechanisms that lead to glucose-mediated insulin release in the pancreatic beta-cell, mitochondrial activation being an essential step. Increases in glucose stimulate the mitochondrial metabolism, activating the tricarboxylic acid cycle and raising the source of redox electron carrier molecules needed for respiratory ATP synthesis. However, little is known about the glucose-induced mitochondrial response of non-beta-cells and its role in the stimulus-secretion coupling process. This limited information is probably a result of the scarcity of these cells in the islet, the lack of identification patterns, and the technical limitations of conventional methods. In this study, we used flavin adenine dinucleotide redox confocal microscopy as a noninvasive technique to specifically monitor mitochondrial redox responses in immunoidentified alpha-, beta-, and delta-cells in freshly isolated intact islets and in dispersed cultured cells. We have shown that glucose provokes metabolic changes in beta- and delta-cell populations in a dose-dependent manner. Conversely, no significant responses were observed in alpha-cells, despite the sensitivity of their metabolism to drugs acting on the mitochondrial function, and their intact ability to develop Ca2+ signals. Identical results were obtained in islets and in cultures of dispersed cells. Our findings indicate metabolic differences in glucose utilization among the alpha-, beta-, and delta-cell populations, which might be important in the signal transduction events that lead to hormone release.

FIGURE 1

Autofluorescence in intact islets. Images of the fluorescence emission of two islets excited with a 15-mW argon laser at 488 nm. Emission was collected with a bandpass filter at 505–550 nm. The laser intensity was adjusted to 3% in the case of the islet shown in A, and at 10% in B. A laser intensity of 3% allowed temporal series of well-contrasted images without detectable photobleaching (please see Materials and Methods). Optical section, 6 μm; scale bar, 100 μm.

FIGURE 2

FAD fluorescence changes in response to glucose monitored in the intact islet of Langerhans. (A) Transmitted-light image of an islet and confocal images of FAD fluorescence in the presence of 3 mM glucose (G), 25 mM glucose, 10 μM FCCP, and 2 mM CN (top left to bottom right). Optical section, 8 μm; scale bar, 100 μm. Areas from which fluorescence changes are represented in B are also indicated. (B) Changes in fluorescence from four different areas of 20 × 20 μm are plotted as the percentage of (F − F₀)/F₀, where F₀ is the fluorescence at the beginning of the record. Red line shows the average fluorescence from the whole optical section of the islet selected for these measurements. These results are representative of nine islets. Glucose concentrations are given in millimolars. FCCP and CN were applied at 10 μM and 2 mM, respectively.

FIGURE 3

Metabolic changes in response to glucose in immunoidentified pancreatic β-cells. (A) Once fluorescence records were acquired, the islets were treated with antiinsulin antibodies to identify pancreatic β-cells. The images show the immunofluorescence (1) for the same optical section selected for the autofluorescence (2) record. Scale bar, 20 μm. (B) Fluorescence changes from two immunoidentified β-cells referred to as β1 and β2 are shown. The position of these cells is also indicated in A. Glucose concentrations are given in millimolars. FCCP and CN were applied at 10 μM and 2 mM, respectively.

FIGURE 4

Glucose-induced metabolic responses in pancreatic α-cells. (A) Subsequent to fluorescence records, an immunocytochemistry with antiglucagon antibodies was performed to identify pancreatic α-cells in the intact islet. The images show the immunofluorescence (1) for the same optical section selected for the autofluorescence (2) record. Scale bar, 15 μm. (B) Fluorescence changes in response to glucose of two immunoidentified α-cells (α1 and α2). A typical β-cell record and the average signal from the whole optical section of this islet are also shown for comparison. The position of cells (α1 and α2) is indicated in A. (C) Two different fields showing the autofluorescence of dispersed islet cells. Scale bar, 30 μm. (D) In separate experiments, cells were loaded for 20 min with Fluo-3, a fluorescent Ca²⁺ reporter, after acquiring FAD fluorescence changes. Adrenaline (Adren.), which has been proved to induce Ca²⁺ signals in α-cells, was applied to the bath at 5 μM. Redox-associated fluorescence changes (black line) and adrenaline-induced Ca²⁺ signals (green line) from two immunoidentified α-cells (α1 and α2) are shown in the figure. These records represent the findings of seven different experiments. The images illustrate the incorporation of Fluo-3 in these two cells (green) and the immunofluorescence with antibodies against glucagon (red) along with the transmitted-light image of the same field after chemical fixation. A cell not positive for antiglucagon antibodies is also shown next to α2. Scale bar, 10 μm. (E) Glucose-induced responses of dispersed islet cells in culture. Records illustrate the typical patterns of immunoidentified pancreatic α- and β-cells. In all the experiments, glucose concentrations are given in millimolars. FCCP and CN were applied at 10 μM and 2 mM, respectively. The transmitted-light (1) and fluorescence (2) images on the top illustrate the immunoidentification of the α-cell whose record is shown in this figure. An image of the autofluorescence of these cells is also shown (3). Scale bar, 20 μm.

FIGURE 5

Glucose-induced metabolic responses in pancreatic δ-cells. (A) Once measurements of autofluorescence were performed, pancreatic δ-cells were identified by means of an immunocytochemistry with antisomatostatin antibodies. The image shows the immunofluorescence for the same optical slice selected for FAD fluorescence records. Scale bar, 15 μm. (B) Changes in fluorescence in response to glucose of an immunoidentified δ-cell. A typical β-cell record and the average signal from the islet whole optical section are also shown for comparison. The position of the represented cells is indicated in A. (C) Evolution of FAD fluorescence in dispersed islet cells in culture. These records illustrate the typical patterns of immunoidentified pancreatic δ- and β-cells. Glucose concentrations are given in millimolars.. FCCP and CN were applied at 10 μM and 2 mM, respectively. The transmitted-light (1) and fluorescence (2) images on the top illustrate the immunoidentification of the δ-cell whose record is shown in this figure. An image of the autofluorescence of these cells is also shown (3). Scale bar, 15 μm.

FIGURE 6

Percentage of FAD fluorescence change of each cell type in the intact islet (A) and in culture (B). The percentage was calculated as the change in fluorescence from the signal obtained with 0.5 mM glucose. Results are shown as the mean ± SE from 19 β-cells (three islets), 9 α-cells (four islets), and 7 δ-cells (four islets) and from 45 β-cells (nine cultures), 36 α-cells (six cultures), and 13 δ-cells (four cultures). All values were found to be significant except the fluorescence change obtained in α-cells with 3 mM glucose (p < 0.05, compared with basal values).