Regulated exocytosis of GABA-containing synaptic-like microvesicles in pancreatic beta-cells.

We have explored whether gamma-aminobutyric acid (GABA) is released by regulated exocytosis of GABA-containing synaptic-like microvesicles (SLMVs) in insulin-releasing rat pancreatic beta-cells. To this end, beta-cells were engineered to express GABA(A)-receptor Cl(-)-channels at high density using adenoviral infection. Electron microscopy indicated that the average diameter of the SLMVs is 90 nm, that every beta-cell contains approximately 3,500 such vesicles, and that insulin-containing large dense core vesicles exclude GABA. Quantal release of GABA, seen as rapidly activating and deactivating Cl(-)-currents, was observed during membrane depolarizations from -70 mV to voltages beyond -40 mV or when Ca(2+) was dialysed into the cell interior. Depolarization-evoked GABA release was suppressed when Ca(2+) entry was inhibited using Cd(2+). Analysis of the kinetics of GABA release revealed that GABA-containing vesicles can be divided into a readily releasable pool and a reserve pool. Simultaneous measurements of GABA release and cell capacitance indicated that exocytosis of SLMVs contributes approximately 1% of the capacitance signal. Mathematical analysis of the release events suggests that every SLMV contains 0.36 amol of GABA. We conclude that there are two parallel pathways of exocytosis in pancreatic beta-cells and that release of GABA may accordingly be temporally and spatially separated from insulin secretion. This provides a basis for paracrine GABAergic signaling within the islet.