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Multivesicular exocytosis in rat pancreatic beta cells.
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ABSTRACT: Aims/hypothesis
To establish the occurrence, modulation and functional significance of compound exocytosis in insulin-secreting beta cells.Methods
Exocytosis was monitored in rat beta cells by electrophysiological, biochemical and optical methods. The functional assays were complemented by three-dimensional reconstruction of confocal imaging, transmission and block face scanning electron microscopy to obtain ultrastructural evidence of compound exocytosis.Results
Compound exocytosis contributed marginally (<5% of events) to exocytosis elicited by glucose/membrane depolarisation alone. However, in beta cells stimulated by a combination of glucose and the muscarinic agonist carbachol, 15-20% of the release events were due to multivesicular exocytosis, but the frequency of exocytosis was not affected. The optical measurements suggest that carbachol should stimulate insulin secretion by ?40%, similar to the observed enhancement of glucose-induced insulin secretion. The effects of carbachol were mimicked by elevating [Ca(2+)](i) from 0.2 to 2 ?mol/l Ca(2+). Two-photon sulforhodamine imaging revealed exocytotic events about fivefold larger than single vesicles and that these structures, once formed, could persist for tens of seconds. Cells exposed to carbachol for 30 s contained long (1-2 ?m) serpentine-like membrane structures adjacent to the plasma membrane. Three-dimensional electron microscopy confirmed the existence of fused multigranular aggregates within the beta cell, the frequency of which increased about fourfold in response to stimulation with carbachol.Conclusions/interpretation
Although contributing marginally to glucose-induced insulin secretion, compound exocytosis becomes quantitatively significant under conditions associated with global elevation of cytoplasmic calcium. These findings suggest that compound exocytosis is a major contributor to the augmentation of glucose-induced insulin secretion by muscarinic receptor activation.
SUBMITTER: Hoppa MB
PROVIDER: S-EPMC3296018 | biostudies-literature | 2012 Apr
REPOSITORIES: biostudies-literature
Publications
Multivesicular exocytosis in rat pancreatic beta cells.
Diabetologia 20111222 4
<h4>Aims/hypothesis</h4>To establish the occurrence, modulation and functional significance of compound exocytosis in insulin-secreting beta cells.<h4>Methods</h4>Exocytosis was monitored in rat beta cells by electrophysiological, biochemical and optical methods. The functional assays were complemented by three-dimensional reconstruction of confocal imaging, transmission and block face scanning electron microscopy to obtain ultrastructural evidence of compound exocytosis.<h4>Results</h4>Compound ...[more]
