Multivesicular exocytosis in rat pancreatic beta cells.

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