Published in:
Open Access
01-04-2012 | Article
Multivesicular exocytosis in rat pancreatic beta cells
Authors:
M. B. Hoppa, E. Jones, J. Karanauskaite, R. Ramracheya, M. Braun, S. C. Collins, Q. Zhang, A. Clark, L. Eliasson, C. Genoud, P. E. MacDonald, A. G. Monteith, S. Barg, J. Galvanovskis, P. Rorsman
Published in:
Diabetologia
|
Issue 4/2012
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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 [Ca2+]i from 0.2 to 2 μmol/l Ca2+. 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 μm) 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.