Nature of Large Vesicle Exocytosis in Pancreatic β-cells: Release of ATP and GABA

Several high resolution electrophysiological techniques such as measurements of cell membrane capacitance and amperometry are used to detect exocytotic events. A novel method was developed in this study based on the expression of P2X2 receptor channel in secreting cells. The presence of ATP sensitive channel on the cell membrane permitted the electrical detection of minute (amoles) quantities of ATP and thus created a possibility to measure ATP released from a single insulin granule. The release of peptides and low molecular weight substances was explored by combining the detection of exocytotic events by P2X2 receptor method with imaging of secretory granules. It was found that in rat insulinoma cells these substances were released independently and with different time course: nucleotides - 280 ms and granular peptide marker IAPP - 2.2 s after the membrane depolarization. In 72% of all exocytotic events nucleotide release was not followed by the discharge of peptide cargo. The chief inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is also found in pancreatic β-cells though its role in coordinating processes in the pancreas is unknown. Sub-cellular localization has not been fully resolved too. Electrical measurements of ATP, GABA and 5-HT release in rat pancreatic β-cells allowed us to demonstrate that at least 75% of GABA release events were attributable to the exocytosis of large dense core vesicles in these cells and its selective release was regulated by the size of the fusion pore. The combination of the cell capacitance measurements, the electrical measurements of ATP release and amperometric detection of 5-HT release demonstrated that in rat pancreatic β-cells ATP is stored and released by large dense core vesicles. The contribution of small synaptic like vesicles is below the detection levels of methods used. The possibility of multi-vesicular exocytosis in pancreatic β-cells was explored by the cell-attached capacitance measurements, the total internal reflection microscopy and by electrical ATP release measurements. It was found that global increase in [Ca2+]i promotes the formation of complexes of interconnected granules within cell cytoplasm that subsequently undergo exocytosis as one unit. This may lead to exocytosis of up to 15 granules simultaneously (compound exocytosis)

Corelease and Differential Exit via the Fusion Pore of GABA, Serotonin, and ATP from LDCV in Rat Pancreatic β Cells

The release of γ-aminobutyric acid (GABA) and ATP from rat β cells was monitored using an electrophysiological assay based on overexpression GABA(A) or P(2)X(2) receptor ion channels. Exocytosis of LDCVs, detected by carbon fiber amperometry of serotonin, correlated strongly (∼80%) with ATP release. The increase in membrane capacitance per ATP release event was 3.4 fF, close to the expected capacitance of an individual LDCV with a diameter of 0.3 μm. ATP and GABA were coreleased with serotonin with the same probability. Immunogold electron microscopy revealed that ∼15% of the LDCVs contain GABA. Prespike “pedestals,” reflecting exit of granule constituents via the fusion pore, were less frequently observed for ATP than for serotonin or GABA and the relative amplitude (amplitude of foot compared to spike) was smaller: in some cases the ATP-dependent pedestal was missing entirely. An inward tonic current, not dependent on glucose and inhibited by the GABA(A) receptor antagonist SR95531, was observed in β cells in clusters of islet cells. Noise analysis indicated that it was due to the activity of individual channels with a conductance of 30 pS, the same as expected for individual GABA(A) Cl(−) channels with the ionic gradients used. We conclude that (a) LDCVs accumulate ATP and serotonin; (b) regulated release of GABA can be accounted for by exocytosis of a subset of insulin-containing LDCVs; (c) the fusion pore of LDCVs exhibits selectivity and compounds are differentially released depending on their chemical properties (including size); and (d) a glucose-independent nonvesicular form of GABA release exists in β cells

Corelease and differential exit via the fusion pore of GABA, serotonin, and ATP from LDCV in rat pancreatic beta cells

The release gamma-aminobutyric acid (GABA) and ATP from rat beta cells was monitored using an electrophysiological assay based on overexpression GABAA or P2X2 receptor ion channels. Exocytosis of LDCVs, detected by carbon fiber amperometry of serotonin, correlated strongly (similar to 80%) with ATP release. The increase in membrane capacitance per ATP release event was 3.4 fF, close to the expected capacitance of an individual LDCV with a diameter of 0.3 mu m. ATP and GABA were coreleased with serotonin with the same probability. Immunogold electron microscopy revealed that similar to 15% of the LDCVs contain GABA. Prespike "pedestals," reflecting exit of granule constituents via the fusion pore, were less frequently observed for ATP than for serotonin or GABA and the relative amplitude (amplitude of foot compared to spike) was smaller: in some cases the ATP-dependent pedestal was missing entirely. An inward tonic current, not dependent on glucose and inhibited by the GABAA receptor antagonist SR95531, was observed in beta cells in clusters of islet cells. Noise analysis indicated that it was due to the activity of individual channels with a conductance of 30 pS, the same as expected for individual GABA(A) Cl- channels with the ionic gradients used. We conclude that (a) LDCVs accumulate ATP and serotonin; (b) regulated release of GABA can be accounted for by exocytosis of a subset of insulin-containing LDCVs; (c) the fusion pore of LDCVs exhibits selectivity and compounds are differentially released depending on their chemical properties (including size); and (d) a glucose-independent nonvesicular form of GABA release exists in beta cells