Regulated Exocytosis and Kiss-and-Run of Synaptic-Like Microvesicles in INS-1 and Primary Rat β-Cells

Abstract

We have applied cell-attached capacitance measurements to investigate whether synaptic-like microvesicles (SLMVs) undergo regulated exocytosis in insulinoma and primary pancreatic β-cells. SLMV and large dense-core vesicle (LDCV) exocytosis was increased 1.6- and 2.4-fold upon stimulation with 10 mmol/l glucose in INS-1 cells. Exocytosis of both types of vesicles was coupled to Ca²⁺ entry through l-type channels. Thirty percent of SLMV exocytosis in INS-1 and rat β-cells was associated with transient capacitance increases consistent with kiss-and-run. Elevation of intracellular cAMP (5 μmol/l forskolin) increased SLMV exocytosis 1.6-fold and lengthened the duration of kiss-and-run events in rat β-cells. Experiments using isolated inside-out patches of INS-1 cells revealed that the readily releasable pool (RRP) of SLMVs preferentially undergoes kiss-and-run exocytosis (67%), is proportionally larger than the LDCV RRP, and is depleted more quickly upon Ca²⁺ stimulation. We conclude that SLMVs undergo glucose-regulated exocytosis and are capable of high turnover. Following kiss-and-run exocytosis, the SLMV RRP may be reloaded with γ-aminobutyric acid and undergo several cycles of exo- and endocytosis. Our observations support a role for β-cell SLMVs in a synaptic-like function of rapid intra-islet signaling.