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Address correspondence to Matthias Braun, Department of Physiological Sciences, Lund University, BMC B11 SE-22184 Lund, Sweden. Fax: (46) 46-222-7763; email: matthias.braun{at}mphy.lu.se
We have explored whether 
-aminobutyric acid (GABA) is released by regulated exocytosis of GABA-containing synaptic-like microvesicles (SLMVs) in insulin-releasing rat pancreatic ß-cells. To this end, ß-cells were engineered to express GABAA-receptor Cl--channels at high density using adenoviral infection. Electron microscopy indicated that the average diameter of the SLMVs is 90 nm, that every ß-cell contains 
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 Ca2+ was dialysed into the cell interior. Depolarization-evoked GABA release was suppressed when Ca2+ entry was inhibited using Cd2+. 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 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 ß-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.
Key Words: GABA • GAD65 • pancreatic islets • paracrine communication • SLMV
Abbreviations used in this paper: GABA,
-aminobutyric acid; LDCV, large dense-core vesicle; SLMV, synaptic-like microvesicle.
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